3                                    TABLE OF CONTENTS
6    LIST OF FIGURES.......................................................................... ii
8    LIST OF TABLES ........................................................................... iii
10   ACKNOWLEDGMENTS ................................................................ iv
12   ABSTRACT ........................................................................................v
14   1. INTRODUCTION.........................................................................1
15       1.1        BACKGROUND ........................................................................................... 1
16       1.2        PURPOSE AND SCOPE OF GUIDE .............................................................. 1
17       1.3        GUIDE ORGANIZATION AND USE .............................................................. 2
19   2. PAVEMENT FRICTION OVERVIEW .....................................3
20       2.1        IMPORTANCE OF PAVEMENT FRICTION ................................................... 3
21       2.2        PAVEMENT FRICTION PRINCIPLES .......................................................... 6
23   3. PAVEMENT FRICTION MANAGEMENT............................21
27   4. PAVEMENT FRICTION DESIGN..........................................39
28       4.1        INTRODUCTION ....................................................................................... 39
29       4.2        DEVELOPING FRICTION DESIGN POLICIES ........................................... 39
30       4.3        PROJECT-LEVEL DESIGN GUIDELINES ................................................. 52
32   REFERENCES ................................................................................60

 1                                         LIST OF FIGURES
 3                                                                                                                                 Page
 4   Figure 1.    Total crashes (from all vehicle types) on U.S. highways from 1990
 5                to 2003 (NHTSA, 2004)........................................................................................ 3
 6   Figure 2.    Total fatalities (from all vehicle types) on U.S. highways from 1990
 7                to 2003 (NHTSA, 2004)........................................................................................ 4
 8   Figure 3.    Relationship between wet-weather crash rates and pavement friction
 9                (Rizenbergs et al., 1973) ...................................................................................... 5
10   Figure 4.    Mean crash risk for roadway network in the United Kingdom
11                (Viner et al., 2004) ............................................................................................... 5
12   Figure 5.    Simplified diagram of forces acting on a rotating wheel ................................... 6
13   Figure 6.    Pavement longitudinal friction versus tire slip (Henry, 2000).......................... 7
14   Figure 7.    Dynamics of a vehicle traveling around a constant radius curve
15                at a constant speed, and the forces acting on the rotating wheel ..................... 8
16   Figure 8.    Key mechanisms of pavement–tire friction ........................................................ 9
17   Figure 9.    Simplified illustration of the various texture ranges that exist for a given
18                pavement surface (Sandburg, 1998) ................................................................. 11
19   Figure 10.   Texture wavelength influence on pavement–tire interactions
20                (adapted from Henry, 2000 and Sandburg and Ejsmont, 2002)...................... 12
21   Figure 11.   The IFI and Rado IFI models (Rado, 1994) ...................................................... 18
22   Figure 12.   Example PFM program ..................................................................................... 23
23   Figure 13.   Conceptual relationship between friction demand, speed, and friction
24                availability.......................................................................................................... 27
25   Figure 14.   Setting of investigatory and intervention levels for a specific friction
26                demand category using time history of pavement friction .............................. 32
27   Figure 15.   Setting of investigatory and intervention levels for a specific friction
28                demand category using time history of friction and crash rate history .......... 32
29   Figure 16.   Setting of investigatory and intervention levels for a specific friction
30                demand category using pavement friction distribution and crash
31                rate–friction trend.............................................................................................. 33
32   Figure 17.   Determination of friction and/or texture deficiencies using the IFI ............... 34
33   Figure 18.   Example illustration of matching aggregate sources and mix types/
34                texturing techniques to meet friction demand ................................................. 52
35   Figure 19.   Example of determining DFT(20) and MPD needed to achieve design
36                friction level........................................................................................................ 55
37   Figure 20.   Flowchart illustration of asphalt pavement friction design
38                methodology (Sullivan, 2005) ............................................................................ 56
39   Figure 21.   Illustration of vehicle response as function of PSV and MTD
40                (Sullivan, 2005) .................................................................................................. 56

 1                                       LIST OF TABLES
 3                                                                                                                        Page
 4   Table 1.   Factors affecting available pavement friction
 5              (Wallman and Astrom, 2001) ............................................................................ 10
 6   Table 2.   Summary of key issues to be considered in standardizing test conditions..... 29
 7   Table 3.   Assessment of hydroplaning potential based on vehicle speed and
 8              water film thickness .......................................................................................... 36
 9   Table 4.   Test methods for characterizing aggregate frictional properties .................... 41
10   Table 5.   Typical range of test values for aggregate properties...................................... 46
11   Table 6.   Asphalt pavement surface mix types and texturing techniques ..................... 48
12   Table 7.   Concrete pavement surface mix types and texturing techniques ................... 50
13   Table 8.   Pairs of MPD and DFT(20) needed to achieve design friction level of 40....... 55

 1                             ACKNOWLEDGMENTS
 4   The research described herein was performed under NCHRP Project 1-43 by the
 5   Transportation Sector of Applied Research Associates (ARA), Inc. Dr. Jim W. Hall, Jr., was
 6   the Principal Investigator for the study.
 8   Dr. Hall was supported in the research and in developing this Guide by ARA Research
 9   Engineers Mr. Leslie Titus-Glover, Mr. Kelly Smith, and Mr. Lynn Evans, and by three
10   project consultants—Dr. James Wambold (President of CDRM, Inc. and Professor Emeritus
11   of Mechanical Engineering at Penn State University), Mr. Thomas Yager (Senior Research
12   Engineer at the NASA Langley Research Center), and Mr. Zoltan Rado (Senior Research
13   Associate at the Pennsylvania Transportation Institute).
15   The authors gratefully acknowledge all of the individuals with state departments of
16   transportation (DOTs) who responded to the pavement friction survey conducted for this
17   project. The authors also express their gratitude for the valuable input provided by
18   knowledgeable representatives of DOTs, paving associations, academia, and manufacturers
19   of friction measuring equipment, vehicle tires, and trucks.

 1                                        ABSTRACT
 4   This report contains guidelines and recommendations for managing and designing for
 5   friction on highway pavements. The contents of this report will be of interest to highway
 6   materials, construction, pavement management, safety, design, and research engineers, as
 7   well as others concerned with the friction and related surface characteristics of highway
 8   pavements.
10   Information is presented that emphasizes the importance of providing adequate levels of
11   friction for the safety of highway users. The factors that influence friction and the concepts
12   of how friction is determined (based on measurements of surface micro-texture and macro-
13   texture) are discussed. Methods for monitoring the friction of in-service pavements and
14   determining appropriate actions in the case of friction deficiencies (friction management)
15   are described. Also, aggregate tests and criteria that help ensure adequate micro-texture
16   are presented, followed by a discussion of how paving mixtures and surface texturing
17   techniques can be selected so as to impart the macro-texture required to achieve the design
18   friction level.

1   This page intentionally left blank.
 1                       CHAPTER 1. INTRODUCTION
 6   Pavement–tire friction (or, simply, pavement friction) is one of the primary factors
 7   determining highway safety and, in particular, the probability of wet skidding crashes.
 8   Highway agencies have recognized this fact since the 1920's (Moyer, 1959). The probability
 9   of wet skidding crashes is reduced when friction between a vehicle tire and pavement is
10   high.
12   Skid-related crashes are determined by many factors, wet pavement friction being only one
13   of them. Other factors, such as road geometry, traffic characteristics, vehicle speed, and
14   weather conditions, must be considered together with friction data when evaluating the
15   safety of a particular section of roadway.
17   The Guide for Pavement Friction, Guidelines for Skid-Resistant Pavement Design,
18   published by the American Association of State Highway and Transportation Officials
19   (AASHTO) in 1976, recommended pavement specifications that would yield the desired
20   frictional properties upon completion of construction and that would maintain adequate
21   long-term friction. This Guide discussed the importance of aggregate selection and mixture
22   design for both asphalt- and concrete-surfaced pavements, and the role of micro-texture and
23   macro-texture in pavement surface friction.
25   Although much research has been conducted on pavement surface characteristics and
26   pavement–tire interactions since development of the 1976 Guide, the available information
27   is somewhat fragmented and has not been integrated into a comprehensive, systematic
28   approach for identifying friction needs and determining the optimum pavement strategy.
29   Exacerbating the problem are the changes that have taken place with time, including
30   changes in pavement construction materials and mixture design properties, construction
31   procedures and standards, vehicle and tire characteristics, traffic loading, and friction-
32   testing methods and equipment.
34   Continued introduction of new materials and technologies, coupled with the increasing
35   focus on the needs of the highway user (safer and more comfortable roads), has placed even
36   greater demands on highway engineers to design and build longer lasting, cost-effective
37   pavements. This Guide for Pavement Friction should help highway engineers accomplish
38   such a task.
43   This Guide for Pavement Friction was prepared under NCHRP Project 1-43 to provide
44   highway pavement practitioners with guidance in designing, constructing, and managing
45   pavement surfaces—as part of both new and rehabilitation projects—that meet the public’s
46   demand for safe friction levels, while recognizing and considering the effects of noise
47   generation and other pavement–tire interaction issues (e.g., splash and spray, tire wear).

 1   The Guide contains recommendations and tools for upper-level administrators and policy-
 2   makers, as well as front-line pavement designers and managers. These recommendations
 3   are intended to supplement but not replace an agency’s normal structural and/or mix
 4   design practices. The Guide covers the following topics:
 6      •   Characteristics of pavement materials and surfaces that contribute to adequate wet-
 7          weather friction.
 8      •   Friction-testing methods, equipment, and indices.
 9      •   Methods for establishing friction levels that signify (a) design of new pavement
10          surfaces, (b) increased potential for skid-related crashes, and (c) the immediate need
11          for friction restoration.
12      •   Guidance for aggregates, mixtures, and surface types that result in long-lasting,
13          high-quality friction surfaces, with proper consideration of noise, economics, and
14          other friction-related issues (e.g., splash and spray, hydroplaning, tire wear).
16   The Guide addresses both asphalt (i.e., flexible and semi-rigid) and concrete (i.e., rigid)
17   pavements associated with both original construction (i.e., new construction and
18   reconstruction) and maintenance and rehabilitation (M&R) treatments. It does not address
19   winter maintenance issues (i.e., snow and ice removal/treatment) and does not deal with
20   unpaved surfaces or non-highway pavements.
25   The Guide is divided into four chapters dealing with the importance of pavement friction,
26   the basic concepts of friction, how friction is measured and managed, and how to design for
27   friction. Following this introductory chapter, Chapter 2 discusses the importance of
28   providing adequate levels of friction for the safety of highway users and it provides an
29   overview of pavement friction (what it is, what influences it) and describes the equipment
30   and methods used to measure and report friction and texture.
32   Chapter 3 discusses friction from the management standpoint, covering both policy
33   development and the application of procedures for monitoring and restoring friction, based
34   on the principle of friction supply versus friction demand. Chapter 4 guides the user
35   through the surface friction design process. It discusses the development of design policies
36   that help promote long-term network-wide friction improvements, and provides project-
37   level how-to guidance for designing pavements with proper friction. Lastly, a glossary of
38   terms is included in Appendix A to facilitate understanding of the terminology and
39   nomenclature contained in the Guide, and a list of standards relevant to pavement friction
40   is provided in Appendix B.

 6   2.1.1 Highway Safety
 8   Between 1990 and 2003, an average of 6.4 million highway crashes (all vehicle types)
 9   occurred annually on the nation’s highways, resulting in 3 million injuries, 42,000
10   fatalities, and countless amounts of pain and suffering. This rate of fatality equates to 115
11   fatalities per day, or 1 death every 12 minutes (Noyce et al., 2005; National Highway
12   Traffic Safety Administration [NHTSA], 2004). In 2000, the cost of highway crashes was
13   estimated at $230.6 billion (Noyce et al., 2005; NHTSA, 2004).
15   Figures 1 and 2 present summaries of the total number of crashes and resulting fatalities
16   in the U.S. between 1990 and 2003. According to the National Transportation Safety Board
17   (NTSB) and the FHWA, approximately 13.5 percent of fatal crashes and 25 percent of all
18   crashes occur when roads are wet (Kuemmel et al., 2000).
20   One or more factors contribute to highway crashes. These factors fall under three main
21   categories: driver-related, vehicle-related, and highway condition-related (Noyce et al.,
22   2005). Of these three categories, only highway condition can be controlled by highway
23   agencies through design, construction, maintenance, and management practices and
24   policies. Although many highway-related conditions influence safety (e.g., geometric
25   design, intersection and roadside design, pavement surface conditions [friction, texture,
26   distress, smoothness]), this Guide focuses on the provision and maintenance of adequate
27   levels of friction.
30                                           7.0

32                                           6.8

34                                           6.6
                   Total crashes, millions

36                                           6.4
38                                           6.2
40                                           6.0
42                                           5.8
44                                           5.6













47      Figure 1. Total crashes (from all vehicles types) on U.S. highways from 1990 to 2003
48                                         (NHTSA, 2004).

 1                                                46
 3                                                45

 4                                                44
                    Total Fatalities, thousands
 7                                                42
 8                                                41
10                                                40
11                                                39
13                                                38

14                                                37













19            Figure 2. Total fatalities (from all vehicles types) on U.S. highways from
20                                  1990 to 2003 (NHTSA, 2004).
23   2.1.2 Crash Reduction
25   The friction developed between vehicle tires and a pavement surface is a critical factor in
26   controlling and reducing crashes (Henry, 2000; Ivey et al., 1992). Studies conducted in the
27   U.S. and elsewhere have generally shown that wet-weather crash rates increase as
28   pavement friction decreases (all other factors such as speed and traffic volumes remaining
29   the same). For instance, as seen in figure 3, crash and measured pavement friction data
30   obtained from mostly rural interstates and parkway roads in Kentucky (Rizenbergs et al.,
31   1973) showed increased wet crash rates at pavement friction values (SN40, skid/friction
32   number determined with a locked-wheel friction tester operated at 40 mi/hr [64 km/hr]) less
33   than 40 for low and moderate traffic levels.
35   In a study for the Texas Department of Transportation (TXDOT), McCullough et al. (1966)
36   found increasing fatal and injury crashes with decreasing coefficient of friction at 50 mi/hr
37   (80 km/hr). More recent research in this area (Agent et al., 1996; Wallman and Astrom,
38   2001) has provided similar trends, emphasizing the need to design for, monitor, and
39   expeditiously restore pavement surface friction properties. Recent research in the United
40   Kingdom also indicated an increase in crash risk as friction levels are reduced. Figure 4
41   provides the relationships determined in that study for tangent alignments in both wet and
42   dry conditions (Viner et al., 2004).
44   Although research has confirmed a basic relationship between pavement friction and wet
45   crash rates, it has not established an exact relationship nor identified any specific threshold
46   friction values below which wet crash rates increase substantially (Henry, 2000; Larson,
47   1999). This is because friction demand (i.e., the level of friction needed to prevent a vehicle
48   from slipping or sliding) varies with location and time due to changing site conditions,

25                   Figure 3. Relationship between wet-weather crash rates and pavement friction
26                                             (Rizenbergs et al., 1973).
                                                Motorway     Dual Carriageway non-event   Single Carriageway non-event
31                                        20
        total number of crashes per 100
        Mean Crash Risk, % (defined as

           million vehicle km driven)

33                                        15
36                                        10
39                                        5
41                                        0
                                               0.3         0.35        0.4        0.45      0.5         0.55         0.6
                                                            Skid Resistance, Sideways Force Coefficient (SFC)
46    Figure 4. Mean crash risk for roadway network in the United Kingdom (Viner et al., 2004)
47   (Note: dual carriageway = 4-lane divided highway, single carriageway = undivided highway,
48        non-event = segments with no junctions, crossings, or notable bends or gradients).

 1   traffic characteristics, and driver/vehicle characteristics. While a particular friction value
 2   may satisfy demand at one location and at one moment in time, the same value may not
 3   satisfy the demand at another location or at a different moment in time. Thus, control of
 4   friction at the network level must be based on periodic assessments of friction (and crashes)
 5   at the pavement segment/unit level.
10   2.2.1 Definition (of Pavement Friction)
12   Pavement friction is the force that resists the relative motion between a vehicle tire and a
13   pavement surface. This resistive force (illustrated in figure 5) is generated when the tire
14   rolls or slides over the pavement surface. A measure of the resistive force is the non-
15   dimensional coefficient of friction, μ, which as expressed in equation 1, is the ratio of the
16   tangential friction force (F) between the tire tread rubber and the horizontal traveled
17   surface to the perpendicular force or vertical load (FW).
19                                                      F
20                                              μ=                                             Eq. 1
23                                            Weight, FW
26                           Rotation
                                                                       of motion
                                                               Friction Force, F
38                Figure 5. Simplified diagram of forces acting on a rotating wheel.
41   Longitudinal Friction
43   For the longitudinal dynamic friction process between a rolling pneumatic tire and the road
44   surface, there are two modes of operation—free-rolling and constant-braked. In the free-
45   rolling mode (no braking), the relative speed between the tire circumference and the
46   pavement—referred to as the slip speed—is zero. In the constant-braked mode, the slip
47   speed increases from zero to a potential maximum of the speed of the vehicle. The following
48   mathematical relationship explains slip speed (Meyer, 1982):

 2                                       S = V − V P = V − ( 0.68 × ω × r )                           Eq. 2
 4   where:    S = Slip speed, mi/hr.
 5             V = Vehicle speed, mi/hr.
 6             VP = Average peripheral speed of the tire, mi/hr.
 7             ω = Angular velocity of the tire, radians/sec.
 8             r  = Average radius of the tire, ft.
10   A locked-wheel state is often referred to as a 100 percent slip ratio and the free-rolling state
11   is a zero percent slip ratio.
13   The coefficient of friction between a tire and the pavement changes with varying slip, as
14   shown in figure 6 (Henry, 2000). The coefficient of friction increases rapidly with
15   increasing slip to a peak value that usually occurs between 10 and 20 percent slip (critical
16   slip). The friction then decreases to a value known as the coefficient of sliding friction,
17   which occurs at 100 percent slip. The difference between the peak and sliding coefficients
18   of friction may equal up to 50 percent of the sliding value, and this disparity is much
19   greater on wet pavements than on dry pavements.
21   Vehicles with an anti-lock braking system (ABS) are designed to apply the brakes on and
22   off (i.e., pump the brakes) repeatedly, such that the slip is held near the peak. The braking
23   is turned off before the peak is reached and turned on at a set time or percent slip below the
24   peak. The actual timing is a proprietary design feature.
28              Coefficient
29              of Friction                      Peak friction
34                                                                    Intermittent
35                                                                                       Full
36                                                                                      sliding
40                                               Critical slip
43                            0                                                            100
                                                  Increased Braking
                        (free rolling)                                               (fully-locked)
45                                                     Tire Slip, %
47             Figure 6. Pavement longitudinal friction versus tire slip (Henry, 2000).

 1   Side Force Friction
 3   Another important aspect of friction relates to the lateral or side force friction that occurs
 4   as a vehicle changes direction or compensates for pavement cross-slope and/or cross-wind
 5   effects. The pavement–tire steering/cornering force diagram in figure 7 shows how the side
 6   force friction factor acts as a counter balance to the centripetal force developed as a vehicle
 7   performs a lateral movement. The basic relationship between the forces acting on the
 8   vehicle tire and the pavement surface as the vehicle steers around a curve, changes lanes,
 9   or compensates for lateral forces is as follows (AASHTO, 2001):
12                                             FS =     −e                                         Eq. 3
13                                                  15R
15   where:     FS = Side friction.
16              V = Vehicle speed, mi/hr.
17              R = Radius of the path of the vehicle’s center of gravity (also, the radius of
18                   curvature in a curve), ft.
19              e  = Pavement superelevation, ft/ft.
                                                 FS       W Weight of vehicle
           P                                              P Centripetal force (horizontal)
27                         α                              FS Friction force between tires and
28                                                           roadway surface (parallel to
29                             α                             roadway surface)
30                                                        α Angle of super-elevation (tan α = e)
31                                                        R Radius of curve
32                                    W
35                                      Side Friction Force
36                                       (Friction Factor)
38             Direction of Travel
                                                                     Friction Measuring Wheel
40                                                                            Drag Force
44             Figure 7. Dynamics of a vehicle traveling around a constant radius curve
45                 at a constant speed, and the forces acting on the rotating wheel.

 1   Combined Braking and Cornering
 3   With combined braking and cornering, a driver either risks not stopping as rapidly or losing
 4   control due to reduced lateral/side forces. The interaction of the longitudinal and lateral
 5   forces is such that as one force increases, the other must decrease by a proportional
 6   amount. Commonly referred to as the friction circle or friction ellipse (Radt and Milliken,
 7   1960), the vector sum of the two combined forces when depicted graphically (longitudinal
 8   force on the x-axis and lateral force on the y-axis, or vice versa) remains constant (circle) or
 9   near constant (ellipse). The degree of ellipse or circle depends on the tire and pavement
10   properties.
12   2.2.2 Mechanisms (of Pavement Friction)
14   Pavement friction is the result of a complex interplay between two principal frictional force
15   components—adhesion and hysteresis (figure 8). Although there are other components of
16   pavement friction (e.g., tire rubber shear), they are insignificant when compared to the
17   adhesion and hysteresis force components. Thus, friction can be viewed as the sum of the
18   adhesion and hysteresis frictional forces:
20                                            F = FA + FH                                      Eq. 4
22                                    Rubber Element                   F
24                                                                     V
36                                                             Hysteresis
                                                               Depends mostly on macro-
37                      Depends mostly on micro-
                                                               level surface roughness
38                      level surface roughness
41                       Figure 8. Key mechanisms of pavement–tire friction.
44   Adhesion is the friction that results from the small-scale bonding/interlocking of the vehicle
45   tire rubber and the pavement surface as they come into contact with each other. It is a
46   function of the interface shear strength and contact area. The hysteresis component of
47   frictional forces results from the energy loss due to bulk deformation of the vehicle tire.
48   The deformation is commonly referred to as enveloping of the tire around the texture.
49   When a tire compresses against the pavement surface, the stress distribution causes the

 1   deformation energy to be stored within the rubber. As the tire relaxes, part of the stored
 2   energy is recovered, while the other part is lost in the form of heat (hysteresis), which is
 3   irreversible. That loss leaves a net frictional force to help stop the forward motion.
 5   Surface texture influences both mechanisms. The adhesion force is proportional to the real
 6   area of adhesion between the tire and surface asperities. The hysteresis force is generated
 7   within the deflecting and visco-elastic tire tread material, and is a function of speed.
 8   Generally, adhesion is related to micro-texture, whereas hysteresis is mainly related to
 9   macro-texture. For wet pavements, adhesion drops off with increased speed, while
10   hysteresis increases with speed. Also, because tire rubber is a visco-elastic material, each
11   component is affected by temperature and sliding speed.
13   2.2.3 Factors Affecting Available Pavement Friction
15   The factors that influence pavement friction forces can be grouped into four categories—
16   pavement surface characteristics, vehicle operational parameters, tire properties, and
17   environmental factors. Table 1 lists the various factors comprising each category. Because
18   each factor in this table plays a role in defining pavement friction, friction must be viewed
19   as a process instead of an inherent property of the pavement. It is only when all these
20   factors are fully specified that friction takes on a definite value.
22   The more critical factors are shown in bold in table 1 and are briefly discussed below.
23   Among these factors, the ones considered to be within a highway agency’s control are micro-
24   and macro-texture, pavement material properties, and slip speed.
27       Table 1. Factors affecting available pavement friction (Wallman and Astrom, 2001).
       Pavement Surface         Vehicle Operating
        Characteristics            Parameters          Tire Properties                  Environment
     • Micro-texture            • Slip speed         • Foot Print           • Climate
     • Macro-texture               vehicle speed     • Tread design and        Wind
     • Mega-texture/               braking action      condition               Temperature
      unevenness                • Driving maneuver   • Rubber composition      Water (rainfall, condensation)
     • Material properties         turning             and hardness            Snow and Ice
     • Temperature                 overtaking        • Inflation pressure   • Contaminants
                                                     • Load                    Anti-skid material (salt, sand)
                                                                               Dirt, mud, debris
                                                     • Temperature
29   Note: Critical factors are shown in bold.
32   Pavement Surface Texture
34   Pavement surface texture is made up of the deviations of the pavement surface from a true
35   planar surface. These deviations occur at three distinct levels of scale, each defined by the
36   wavelength (λ) and peak-to-peak amplitude (A) of its components. The three levels of
37   texture, as established by the Permanent International Association of Road Congresses
38   (PIARC) (1987), are as follows:

 1      •   Micro-texture (λ < 0.02 in [0.5 mm], A = 0.04 to 20 mils [1 to 500 µm])—Surface
 2          roughness quality at the sub-visible/microscopic level. It is a function of the surface
 3          properties of the aggregate particles within the asphalt or concrete paving material.
 5      •   Macro-texture (λ = 0.02 to 2 in [0.5 to 50 mm], A = 0.005 to 0.8 in [0.1 to 20 mm])—
 6          Surface roughness quality defined by the mixture properties (shape, size, and
 7          gradation of aggregate) of an asphalt paving material and the method of
 8          finishing/texturing (dragging, tining, grooving; depth, width, spacing and orientation
 9          of channels/grooves) used on a concrete paving material.
11      •   Mega-texture (λ = 2 to 20 in [50 to 500 mm], A = 0.005 to 2 in [0.1 to 50 mm])—This
12          type of texture is the texture which has wavelengths in the same order of size as the
13          pavement–tire interface. It is largely defined by the distress, defects, or “waviness”
14          on the pavement surface.
16   Wavelengths longer than the upper limit (20 in [500 mm]) of mega-texture are defined as
17   roughness or unevenness (Henry, 2000). Figure 9 illustrates the three texture ranges, as
18   well as a fourth level—roughness/unevenness—representing wavelengths longer than the
19   upper limit (20 in [500 mm]) of mega-texture (Sandburg, 1998).
21   It is widely recognized that pavement surface texture influences many different pavement–
22   tire interactions. Figure 10 shows the ranges of texture wavelengths affecting various
23   vehicle–road interactions, including friction, interior and exterior noise, splash and spray,
24   rolling resistance, and tire wear. As can be seen, friction is affected primarily by micro-
25   texture and macro-texture, which correspond to the adhesion and hysteresis friction
26   components, respectively.
                                                                                 Reference Length
32                                                                                  Short stretch
33                                                                                    of road
35                                 Amplification ca. 50 times
36    Mega-texture
37                                                                                      Tire
38                                 Amplification ca. 5 times
39    Macro-texture
40                                                                                   Road–Tire
41                                                                                  Contact Area
42    Micro-texture
                                 Amplification ca. 5 times
43                                                                                    Single
44                                                                                   Chipping
46       Figure 9. Simplified illustration of the various texture ranges that exist for a given
47                               pavement surface (Sandburg, 1998).

                                               Texture Wavelength
 3         0.00001      0.0001       0.001        0.01          0.1           1             10      100
 4       10-6         10-5         10-4         10-3          10-2          10-1          100     101
 5                Micro-texture               Macro-texture          Mega-texture Roughness/Unevenness
 7                                           Friction
 9                                                     Ext. Noise
11                                                              Int. Noise
13                                        Splash/Spray
15                                                                       Rolling Resistance
17                      Tire Wear                                                  Tire/Vehicle
       Note: Darker shading indicates more favorable effect of texture over this range.
20              Figure 10. Texture wavelength influence on pavement–tire interactions
21                  (adapted from Henry, 2000 and Sandburg and Ejsmont, 2002).
24   At low speeds, micro-texture dominates the wet and dry friction level. At higher speeds,
25   the presence of high macro-texture facilitates the drainage of water so that the adhesive
26   component of friction afforded by micro-texture is re-established by being above the water.
27   Hysteresis increases with speed exponentially, and at speeds above 65 mi/hr (105 km/hr)
28   accounts for over 95 percent of the friction (PIARC, 1987).
30   Pavement Surface Material Properties
32   Pavement material properties (i.e., aggregate and mix characteristics, surface texturings)
33   influence both micro-texture and macro-texture. These properties also affect the long-term
34   durability of texture through their capacities to resist aggregate polishing and abrasion/
35   wear of both aggregate and mix under accumulated traffic and environmental loadings.
37   Slip Speed
39   The coefficient of friction between a tire and the pavement changes with varying slip. It
40   increases rapidly with increasing slip to a peak value that usually occurs between 10 and
41   20 percent slip. The friction then decreases to a value known as the coefficient of sliding
42   friction, which occurs at 100 percent slip.
44   Tire Tread Design and Condition
46   Tire tread design (i.e., type, pattern, and depth) and condition have a significant influence
47   on draining water that accumulates at the pavement surface. Water trapped between the
48   pavement and the tire can be expelled through the channels provided by the pavement
49   macro-texture and by the tire tread. Tread depth is particularly important for vehicles

 1   driving over thick films of water at high speeds. Some studies (Henry, 1983) have reported
 2   a decrease in wet friction of 45 to 70 percent for fully worn tires as compared to new ones.
 4   Tire Inflation Pressure
 6   High inflation pressure causes only a small loss of pavement friction, whereas low inflation
 7   pressure can significantly reduce friction at high speeds (Henry, 1983). This is because of
 8   the constriction of drainage channels within the tire tread and reduced contact pressure.
10   Temperature
12   Friction of natural rubber tires has a large dependence on temperature, especially when
13   going through the freezing point. Modern rubber compounds are formulated to remove the
14   effect of temperature on friction. Since hysteresis is affected by the visco-elastic property of
15   the tire, friction is reduced at high temperatures where the rubber becomes soft. However,
16   these temperatures are above the normal running speeds and temperatures. When a lot of
17   braking is performed, these higher temperatures can be reached, however the brakes would
18   fade first. Perhaps the biggest effect is under locked-wheel conditions, where the rubber
19   can melt and cause rubber hydroplaning.
21   Surface Water
23   The effect of surface water layer depth (or water film thickness [WFT]) on friction is
24   minimal at low speeds (<20 mi/hr [32 km/hr]) and quite pronounced at higher speeds (>40
25   mi/hr [64 km/hr]). The coefficient of friction of a vehicle tire sliding over a wet pavement
26   surface, decreases exponentially as WFT increases. The effect of WFT is influenced by tire
27   design and condition, with worn tires being most sensitive to WFT.
29   A particularly hazardous situation involving relatively thick water films and vehicles
30   traveling at higher speeds is hydroplaning. Hydroplaning occurs when a vehicle tire is
31   separated from the pavement surface by the water pressure that builds up at the
32   pavement–tire interface (Horne and Buhlmann, 1983), causing friction to drop to near zero.
33   This phenomenon is affected by several parameters, including water depth, vehicle speed,
34   pavement macro-texture, tire inflation pressure and tread depth, and tire contact area.
36   Snow and Ice
38   Ice and snow covering the pavement surface present the most hazardous condition for
39   vehicle braking or cornering. The level of friction between the tires and the snow- or ice-
40   covered pavement is such that almost any abrupt braking or sudden change of direction
41   results in locked-wheel sliding and loss of vehicle directional stability. This Guide does not
42   address winter-related friction issues.
44   2.2.4 Friction and Texture Measurement Methods
46   Overview
48   The measurement of pavement friction and texture has been of primary importance to state
49   highway agencies (SHAs) for at least 50 years. Many different types of equipment have

 1   been developed and used to measure these properties. Their differences, in terms of
 2   measurement principles and procedures and the way measurement data are processed and
 3   reported, can be significant.
 5   For friction-testing alone, there are several commercial devices that can operate at fixed or
 6   variable slip, at speeds up to 100 mi/hr (161 km/hr), and under variable test tire conditions,
 7   such as load, size, tread design and construction, and inflation pressure. Measurement of
 8   pavement surface texture can be done using a variety of laser devices, volumetric
 9   techniques, water drainage rates (outflow meter), and sliding rubber pad apparatus
10   (portable British Pendulum Tester [BPT]). This section provides an overview of the
11   different friction and texture measurement methods and available representative
12   equipment.
14   Methods and Equipment
16   AASHTO and ASTM have developed a set of surface characteristic standards and
17   measurement practice standards for both friction and texture. These standards ensure
18   comparability of the measurements for specific purposes; they are grouped according to
19   measurements performed at highway speeds (i.e., high-speed devices) and measurements
20   requiring lane closure (i.e., low-speed/walking and stationary devices).
22   In general, the measurement devices requiring lane closure are simpler and relatively
23   inexpensive, whereas the highway-speed devices are more expensive and require more
24   training to maintain and operate. With the recent development of technology in data
25   acquisition, sensor technology, and data processing power of computers, the once true
26   superiority of data quality for the stationary and low-speed devices is diminishing. The
27   resolution and accuracy of the acquired data for the measurement devices that are low-
28   speed or stationary can be still superseding that of the high-speed devices, but with smaller
29   and smaller margins.
31   The locked-wheel friction tester (AASHTO T 242) is the predominant high-speed device
32   used on U.S. roads. This device requires a tow vehicle and a locked-wheel skid trailer,
33   equipped with either a standard ribbed tire(AASHTO M 261) or a standard smooth
34   tire(AASHTO M 286). Friction measurements are obtained by locking the test tire (ribbed
35   or smooth) on a wetted pavement surface while traveling at a specified speed (40 mi/hr [64
36   km/hr] is the standard speed given inAASHTO T 242). The smooth tire is more sensitive to
37   pavement macro-texture, while the ribbed tire is more sensitive to micro-texture changes in
38   the pavement.
40   Portable friction measurement equipment requiring lane closures include the
41   BPT(AASHTO T 278) and the Dynamic Friction Tester (DFT) (ASTM E 1911).
43      •   The manually operated BPT provides an indicator of friction through the swinging of
44          a pendulum-based rubber slider and its contact with the pavement surface. The
45          elevation to which the pendulum swings after contact provides the basis for the
46          friction indicator, termed British Pendulum Number (BPN).

 2      •   The DFT is an electronic modular system that measures the torque necessary to
 3          rotate three small, spring-loaded rubber pads in a circular path over a wetted
 4          pavement surface at different speeds. Results are typically recorded at 12, 24, 36,
 5          and 48 mi/hr (20, 40, 60, and 80 km/hr), from which the speed–friction relationship
 6          is plotted.
 8   High-speed texture measuring equipment includes laser profilers, such as the FHWA Road
 9   Surface Analyzer (ROSANV). These non-contact devices use a combination of a horizontal
10   distance measuring device, a very high-speed (64 kHz or higher) laser triangulation sensor,
11   and a portable computer to collect and store pavement surface elevations (vertical
12   resolution usually 0.002 in [0.5 mm] or better) at intervals of 0.01 in (0.25 mm) or less.
13   From these elevations, the system calculates the mean profile depth (MPD), which is an
14   overall measure of macro-texture.
16   Texture measuring equipment requiring lane closures include the Sand Patch Method
17   (SPM) (ASTM E 965), the Outflow Meter (OFM) (ASTM E 2380), and the Circular Texture
18   Meter (CTM) (ASTM E 2157).
20      •   The SPM is a volumetric-based spot test method that assesses pavement surface
21          macro-texture through the spreading of a known volume of glass beads in a circle
22          onto a cleaned surface and the measurement of the diameter of the resulting circle.
23          The volume divided by the area of the circle is reported as the mean texture depth
24          (MTD).
26      •   The OFM is a volumetric test method that measures the water drainage rate
27          through surface texture and interior voids. It indicates the hydroplaning potential
28          of a surface by relating to the escape time of water beneath a moving tire. The
29          equipment consists of a cylinder with a rubber ring on the bottom and an open top.
30          Sensors measure the time required for a known volume of water to pass under the
31          seal or into the pavement. The measurement parameter, outflow time (OFT),
32          defines the macro-texture; high OFTs indicating smooth macro-texture and low
33          OFTs rough macro-texture.
35      •   The CTM is a non-contact laser device that measures the surface profile along an
36          11.25-in (286-mm) diameter circular path of the pavement surface at intervals of
37          0.034 in (0.868 mm). The texture meter device rotates at 20 ft/min (6 m/min) and
38          generates profile traces of the pavement surface, which are transmitted and stored
39          on a portable computer. Two different macro-texture indices can be computed from
40          these profiles: mean profile depth (MPD) and root mean square (RMS). The MPD,
41          which is a two-dimensional estimate of the three-dimensional MTD (Flintsch et al.,
42          2003), represents the average of the highest profile peaks occurring within eight
43          individual segments comprising the circle of measurement. The RMS is a statistical
44          value, which offers a measure of how much the actual data (measured profile)
45          deviates from a best-fit (modeled profile) of the data (McGhee and Flintsch, 2003).

 1   Friction Indices
 3   Friction indices have been in use for a long time. In 1965, ASTM started the use of the Skid
 4   Number (SN) (ASTM E 274) as an alternative to the coefficient of friction. In later years,
 5   AASHTO adopted the ASTM E 274 as AASHTO T 242 test method and changed the
 6   terminology from Skid Number to Friction Number (FN). In the early 1990s, PIARC
 7   developed the International Friction Index (IFI), based on the PIARC international
 8   harmonization study. A refined IFI model was developed shortly thereafter as part of a
 9   Ph.D. thesis (Rado, 1994).
11   The use of friction indices has allowed for harmonization of the different sensitivities of the
12   various friction measurement principles to micro-texture and macro-texture. Provided
13   below are brief discussions of these primary friction indices.
15   Friction Number
17   The Friction Number (FN) (or Skid Number [SN]) produced by the AASHTO T 242 locked-
18   wheel testing device represents the average coefficient of friction measured across a test
19   interval. It is computed as follows:
21                                       FN = 100×µ = 100×(F/W)                                Eq. 5
23   where: FN    =   Friction number at the measured speed.
24          µ     =   Coefficient of friction.
25          F     =   Tractive horizontal force applied to the tire, lb.
26          W     =   Vertical load applied to the tire, lb.
28   The reporting values range from 0 to 100, with 0 representing no friction and 100
29   representing complete friction.
31   FN values are generally designated by the speed at which the test is conducted and by the
32   type of tire used in the test. For example, FN40R = 36 indicates a friction value of 36, as
33   measured at a test speed of 40 mi/hr (64 km/hr) and with a ribbed (R) tire. Similarly,
34   FN50S = 29 indicates a friction value of 29, as measured at a test speed of 50 mi/hr (81
35   km/hr) and with a smooth (S) tire.
37   International Friction Index
39   Traditionally, pavement friction has been reported as a single number representing the
40   amount of friction available at the pavement-tire interface for a given pavement surface
41   condition (micro-texture and macro-texture) and test speed. The fundamental reason for
42   measuring pavement friction, however, is to estimate how much friction is available for
43   performing expected driving maneuvers under different speed conditions. In other words,
44   how much friction is available as the vehicle wheel rotation is gradually reduced from free
45   rolling to a locked state (i.e. as the slip speed of the wheel increases).
47   The International Friction Index (IFI), computed using ASTM E 1960, reflects the average
48   pavement friction over the typical range of vehicle tire free rolling and slip speeds. IFI is

 1   based on the PIARC international harmonization study conducted in 1992 and is composed
 2   of two numbers: a friction number, F(60), and a speed number or speed gradient, SP. The
 3   designation and reporting of this index is IFI(F(60),SP).
 5   F(60) indicates the friction at a slip speed of 37 mi/hr (60 km/hr) measured using any
 6   standardized friction test method. It is a harmonized friction value, which adjusts for the
 7   speed at which a particular friction test method is performed, as well as the type of
 8   measurement device used. Note that F(60) is the friction level for a test measurement at 37
 9   mi/hr (60 km/hr), which is close to the speed of the standard FN40 test measurement.
11   The speed number SP defines the relationship between measured friction and vehicle tire
12   free rotation or slip speed. It is calculated using the pavement macro-texture measured
13   using any standardized texture measurement method. The PIARC experiment strongly
14   confirmed that SP is a measure of the macro-texture influence on friction.
16   The IFI can be estimated (in Metric form, as outlined in ASTM E 1960) by following the
17   steps below.
19      1. Measure Pavement Friction and Macro-texture—Using a selected friction device,
20         measure pavement friction FR(S) at a given slip speed S (in km/hr). Also, using a
21         selected texture measuring device, measure pavement macro-texture and compute
22         MPD (ASTM E 1845) or MTD (ASTM E 965) (in millimeters).
24      2. Estimate the IFI Speed Number SP—Using the computed MPD or MTD, calculate SP
25         (in km/hr) as follows:
27                                    SP = 14.2 + 89.7×MPD                                  Eq. 6
28                                   SP = –11.6 + 113.6×MTD                                 Eq. 7
30      3. Convert Friction Measurement FR(S) at Slip Speed S to Friction at 60 km/hr—
31         Adjust the friction FR(S) measured by the selected friction device at slip speed S
32         using the following equation:
33                                                                 S − 60
                                                               (          )
34                                      FR(60) = FR( S ) × e        SP                      Eq. 8
36          where: FR(60) = Adjusted value of friction measurement FR(S)
37                          at a slip speed of S to a slip speed of 60 km/hr.
38                 FR(S) = Friction value at selected slip speed S.
39                 S      = Selected slip speed, km/hr.
41      4. Calculate the IFI Friction Number F(60)—Using the speed-adjusted friction value
42         FR(60) and the following equation, compute F(60):
44                                 F(60) = A + B×FR(60) + C×TX                              Eq. 9
46          where: A, B, C = Calibration constants for the selected friction measuring
47                           device. The values of A, B, and C for various devices are
48                           given in ASTM E 1960.

1                               TX               = The value of MPD or MTD, as determined in Step 1.
3    The IFI model describes friction experienced by a driver in emergency braking using
4    conventional brakes and deals with the friction from wheel lock-up to stop. The improved
5    IFI model developed by Rado considers the friction experienced by a driver in emergency
6    braking using an anti-lock braking system (ABS). This model takes the following form
7    (Henry, 2000):
                                                                                  ⎛ ⎛ S      ⎞⎞
                                                                                  ⎜ ln ⎜     ⎟⎟
                                                                                  ⎜ ⎜S       ⎟⎟
                                                                                 −⎜ ⎝ MAX    ⎠
                                                                                  ⎜            ⎟

                                                            μ (S ) = μ max × e
                                                                                  ⎜            ⎟
                                                                                  ⎝            ⎠
8                                                                                                                       Eq. 10
10   where: μ(S)                = Friction at slip speed S.
11          S                   = Slip speed of the measurement tire.
12           μmax               = Maximum friction value (a function of surface and tire properties,
13                                measuring speed, and slip speed).
14           SMAX               = Slip speed at maximum friction value (also known as the critical slip
15                                speed, which is when the tire is slipping on the pavement with SMAX slip
16                                speed while it develops μmax friction).
17           $
             C                  = Shape factor which is closely related to the speed number SP in the
18                                                        $
                                  original IFI equation ( C determines the skewed shape of the full
19                                friction curve).
21   Figure 11 presents graphically friction computed using IFI(F(60),Sp) and the Rado IFI
22   models.
27                                                                            ˆ
                                                        Rado Model(μmax,SMAX, C ) ≈ IFI(F(60),SP)

                    Friction Number

33                                                                      F(60)                            SP
34                                                         ˆ
                                                           C (shape)
40                                      0
41                                           0   Smax                         60
                                                                                                       Slip Speed (S)
43                                      Figure 11. The IFI and Rado IFI models (Rado, 1994).

 1   Index Relationships
 3   Over the years, many studies have been performed to correlate the different friction and
 4   texture measurement techniques. The established correlations are important in
 5   determining how micro-texture and macro-texture affect pavement–tire friction
 6   performance over a range of pavement conditions. Discussed below are some of the key
 7   relationships.
 9      •   Micro-Texture—Currently, there is no direct way to measure micro-texture in the
10          field. Even in the laboratory, it has only been done with very special equipment.
11          Because of this and because micro-texture is related to low slip speed friction, a
12          surrogate device is used for micro-texture.
14          In the past, the most common device was the BPT (AASHTO T 278), which produces
15          the low-speed wet friction number BPN. A newer testing device is the DFT (ASTM
16          E 1911), which measures friction as a function of slip speed from 0 to 55 mi/hr (0 to
17          90 km/hr). The DFT at 20 km/hr (DFT(20)) is now being used more and more
18          around the world as a replacement for the BPN. Testing at the NASA Wallops
19          Friction Workshops has shown DFT(20) to be more reproducible than the BPN
20          (Henry, 2000).
22      •   Macro-Texture—The primary indices used to characterize macro-texture are the
23          MTD and the MPD. While it was found in the international PIARC experiment that
24          the best parameter for determining the speed constant (SP) of the IFI is MPD, good
25          predictive capabilities were also observed for MTD (Henry, 2000). To allow for
26          conversions to either of these macro-texture indices, the following relationships
27          (given in both English and Metric form, respectively) have been developed (PIARC,
28          1995):
30          For estimating MTD from profiler-derived measurements of MPD (ASTM E 1845):
32                     Estimated MTD (or EMTD) = 0.79×MPD + 0.009           English (in)   Eq. 11
33                               EMTD = 0.79×MPD + 0.23                     Metric (mm)
35          For estimating MTD from CTM-derived measurements of MPD (ASTM E 2157):
37                              EMTD = 0.947×MPD + 0.0027                   English (in)   Eq. 12
38                              EMTD = 0.947×MPD + 0.069                    Metric (mm)
40          For estimating MTD from outflow time (OFT), as measured with the OFM device
41          (ASTM E 2380) (PIARC, 1995):
43                              EMTD = (0.123/OFT) + 0.026                  English (in)   Eq. 13
44                              EMTD = (3.114/OFT) + 0.656                  Metric (mm)

 2   •   Friction (Micro-Texture and Macro-Texture)—It has been shown that, using a
 3       combination of smooth (AASHTO M 286) and ribbed tires (AASHTO M 261) at
 4       highway speeds (i.e., >40 mi/hr [64 km/hr]), FN can be predicted from micro-texture
 5       and macro-texture. The relationships (equations 14 thru 16) are based on macro-
 6       texture measured using the SPM (ASTM E 965) and on BPN (AASHTO T 278), as a
 7       surrogate for micro-texture. Similar equations can be determined from other macro-
 8       texture measurement methods (such as MPD [ASTM E 1845]) and a surrogate for
 9       micro-texture (such as DFT(20) [ASTM E 1911]). The IFI provides a method to do
10       this through the following equations (Wambold et al., 1984):
12                       BPN = 20 + 0.405×FN40R + 0.039×FN40S                           Eq. 14
13                       MTD = 0.49 – 0.029×FN40R + 0.43×FN40S                          Eq. 15
15       where:    BPN        =   British pendulum number.
16                 FN40R      =   Friction number using ribbed tire at 40 mi/hr.
17                 FN40S      =   Friction number using smooth tire at 40 mi/hr.
18                 MTD        =   Mean texture depth, in.
20       The set of equations show that BPN (micro-texture) is an order of magnitude more
21       dependent on the ribbed tire than on the smooth tire. The reverse is true of MTD
22       (macro-texture). It should also be noted that these equations can be solved for as
23       follows:
25                         FN40R = 1.19×FN40S – 13.3×MTD + 13.3                         Eq. 16
27       So that a smooth tire friction and texture measurement made to determine IFI can
28       still be used to predict FN40R for reference. However, the BPN is not very
29       reproducible and the equations are only valid for the BPT used in the correlation.
30       For this reason, the following correlations with DFT(20) and the MPD (from the
31       CTM) were developed using NASA Wallops Friction Workshops data:
33                          FNS = 15.5×MPD + 42.6×DFT(20) – 3.1                         Eq. 17
34                         FNR = 4.67×MPD + 27.1×DFT(20) + 32.8                         Eq. 18
36       And, the correlation of FN40R, as a function of FN40S and MPD is as follows:
38                        FN40R = 0.735×FN40S – 1.78×MPD + 32.9                         Eq. 19

 4   Successful control of pavement friction suggests strategies at both the management and
 5   design levels of a highway pavement program. On the management end, policies and
 6   practices can be developed and administered that result in sufficient monitoring of friction
 7   and/or crashes, and proper and timely responses to potentially unsafe roadway surfaces.
 8   Where restorative treatments are needed, an evaluation of friction supply versus demand
 9   may be performed.
11   On the design end, policies and practices that focus on the provision of adequate levels of
12   micro-texture and macro-texture and ensures texture durability throughout the pavement
13   life should be encouraged. Using the established policies, an analysis of friction supply
14   versus demand should also be performed to design and construct the roadway.
16   Managing and designing for pavement friction within an agency should consider the
17   following (Austroads, 2005):
19      •   Policy.
20             Defining objectives and responsibilities.
21             Developing policies regarding friction demand and friction supply.
22             Developing materials standards (i.e., frictional and durability properties).
23             Developing standards for selection and construction of restoration activities.
24             Developing policies regarding investigatory and intervention friction levels, as
25             well as testing equipment and protocols (including calibration and maintenance).
26      •   Management.
27             Collection and processing of friction and/or crash data.
28             Identification and prioritization of sites for investigation and/or restoration.
29             Performance of detailed site investigation.
30      •   Design.
31             Determination of friction demand and optimum levels of micro-texture and
32             macro-texture to match friction demand and durability requirements.
33             Selection of remedial actions to restore pavement friction.
34      •   Research.
35             Monitoring, review, and improvement of policies.
37   Although these issues are considered key in the development of an agency strategy for
38   managing and designing for friction, they are neither comprehensive nor mandatory.
40   Friction management and friction design entail a host of different issues and are therefore
41   discussed in separate chapters (3 and 4) of this Guide. This chapter discusses the concept
42   of a pavement friction management (PFM) program and provides guidance in the
43   development of PFM policies and practices that could enhance highway safety. Section 3.1
44   covers the policy aspects of PFM, while section 3.2 describes a process that could be used to
45   establish a PFM program.

 3   A PFM program is a systematic approach to measuring and monitoring the friction
 4   qualities and wet crash rates of roadways, identifying those pavement surfaces and
 5   roadway situations that are or will soon be in need of remedial treatment, and planning
 6   and budgeting for treatments and reconstruction work that will ensure appropriate friction
 7   characteristics.
 9   The development of PFM policies within a highway agency requires a good understanding
10   of the agency’s current management/operational practices and resources (people,
11   equipment, materials). Detailed discussions of these items and how they might be used in
12   developing a customized PFM program follows.
14   It is important that a PFM program is practically achievable and that its implementation is
15   demonstrable. In other words, a PFM program shoud not create an unachievable goal that
16   cannot be reached and must provide a means of documenting the successful
17   implementation of the program. Anything less will create an atmosphere where potential
18   lilability outweighs any possible benefits of the program. To the extent possible, it should
19   be integrated with roadway safety and other highway management programs.
21   3.1.1 Federal Advisories Regarding Highway Safety
23   The FHWA has published technical advisories regarding skid-crash reduction and texturing
24   of asphalt and concrete pavements. Brief summaries of these are presented below.
26   FHWA Technical Advisory T 5040.17—Skid-Accident Reduction Program
28   This advisory (FHWA, 1980) presents a comprehensive guide for state and local highway
29   agencies in conducting skid-crash reduction programs. The purpose of the Skid-Accident
30   Reduction Program was to minimize wet-weather skidding crashes through (1) identifying
31   and correcting sections of roadway with a high or potentially high incidence of skid-crashes,
32   (2) ensuring that the new surfaces have adequate and durable friction properties, and (3)
33   utilizing resources available for crash reduction in a cost-effective manner.
35   FHWA Technical Advisory T 5040.36—Surface Texture for Asphalt and Concrete
36   Pavements
38   This advisory (FHWA, 2005) includes (a) information on state-of-the-practice for providing
39   surface texture/friction on pavements and (b) guidance for selecting techniques that will
40   provide adequate wet pavement friction and low-tire/surface noise characteristics. This
41   document replaced the 1979 Technical Advisory T 5140.10 on concrete pavement texturing
42   and friction.
44   3.2.2 Pavement Friction Management Approach and Framework
46   To develop PFM policies, an agency should identify an overall approach for managing
47   pavement friction and a process for implementing it. The comprehensive PFM program

 1   shown in figure 12 may be used. This program is comprised of the following key
 2   components:
 4      •   Network Definition—Subdivide the highway network into distinct pavement
 5          sections and group the sections according to levels of friction need.
 6              Define pavement sections.
 7              Establish friction demand categories.
 8      •   Network-Level Data Collection—Gather all the necessary information.
 9              Establish field testing protocols (methods, equipment, frequency, conditions, etc.)
10              for measuring pavement friction and texture.
11              Collect friction and texture data and determine overall friction of each section.
12              Collect crash data.
13      •   Network-Level Data Analysis—Analyze friction and/or crash data to assess overall
14          network condition and identify friction deficiencies.
15              Establish investigatory and intervention levels for friction. Investigatory and
16              intervention levels are defined, respectively, as levels that prompt the need for a
17              detailed site investigation or the application of a friction restoration treatment.
18              Identify pavement sections requiring detailed site investigation or intervention.
19      •   Detailed Site Investigation—Evaluate and test deficient pavement sections to
20          determine causes and remedies.
21              Evaluate non-friction-related items, such as alignment, the layout of lanes,
22              intersections, and traffic control devices, the presence, amount, and severity of
23              pavement distresses, and longitudinal and transverse pavement profiles.
24              Assess current pavement friction characteristics, both in terms of micro-texture
25              and macro-texture.
26              Identify deficiencies that must be addressed by restoration.
27              Identify uniform sections for restoration design over the project length.
28      •   Selection and Prioritization of Short- and Long-Term Restoration Treatments—Plan
29          and schedule friction restoration activities as part of overall pavement management
30          process.
31              Identify candidate restoration techniques best suited to correct existing
32              pavement deficiencies.
33              Compare costs and benefits of the different restoration alternatives over a
34              defined analysis period.
35              Consider monetary and non-monetary factors and select one pavement
36              rehabilitation strategy.

 2                                      Define Pavement Network & Identify Sites
                                         (Re-assess Site Categories Periodically)
 5                                           Perform Routine Friction Testing and
 6                                                   Collect Crash Data
           For all Sections Above
 7   Investigatory Level, Process and
 8    Evaluate Crash Data. Conduct
         Detailed Investigation of
 9   Sections with High Crash Rates                     Is Friction At
10      to Determine if High Crash      No                 or Below
      Rates are Due to (1) Setting of                   Investigatory
11   Inadequate Investigatory Levels                        Level?
12      or (2) Non-Friction Related
       Causes. Develop Appropriate
13      Recommendations Based on
14         Investigation Results                             Yes
15                                                                                                                      Pavement
                                         Process Crash Data for all Sections at or                                   Friction Testing
16                                              Below Investigatory Level                                               Frequency
20                                                       Is Friction                            Are Wet
21                                                      At or Below             No             Crash Rates           No
                                                        Intervention                              High?
22                                                         Level?
25                                                           Yes                                   Yes
26                                                                                                                        Lower
28                                                        Are Wet
                                                                                 No                                       Assess
                                                         Crash Rates
29                                                          High?

30                                                                                                                        Higher
32                                                           Yes
34                                                                                        Perform Detailed Site
                                                                                             Does Site Need
38                                                                         No
40                                                                                                 Yes
                                                                    1.   Shortlist Sites Requiring Restoration in Order of Priority
43                                                                  2.   Perform Short-Term Remedial Works, if Needed
44                                                                  3.   Identify Preferred Restoration Design Strategy
                                                                    4.   Develop Schedules for Restoration Activities
47                         Figure 12. Example of a Possible PFM program.

 6   The PFM program should consist of practical, well-defined work activities and be based on
 7   reliable information. This section describes issues relevant to each PFM program
 8   component and provides guidance on determining implementation approaches and defining
 9   activities and procedures.
11   3.2.1 Defining the Network
13   Pavement Section Definition
15   Section definition for a PFM network involves identifying a basic set of pavement
16   characteristics to help make informed management decisions. Without these
17   characteristics, a pavement friction number is virtually meaningless. To put an pavement
18   friction number in context, you must have information relating to the below described
19   characteristics. The main characteristic of interest is friction demand, which is defined as
20   the level of friction (micro- and macro-texture) needed to safely perform braking, steering,
21   and acceleration maneuvers. PFM sections can be established by reviewing the sectioning
22   in the PMS and identifying where changes in friction demand occur. A friction demand
23   category can then be assigned to serve as basis for monitoring friction adequacy throughout
24   the network.
26   Factors that affect friction demand can be grouped into four basic categories: highway
27   alignment, highway features/environment, highway traffic characteristics, and
28   driver/vehicle characteristics. Another category includes driver skills and age, vehicle tire
29   characteristics, and vehicle steering capabilities, not discussed herein. The specific factors
30   involved in the first three categories are discussed below.
32   Highway Alignment
34   Friction demand is significantly influenced by both the horizontal and vertical alignment of
35   a highway. The following are key considerations:
37      •   Horizontal Alignment—The horizontal alignment of a highway is defined by
38          tangents and curves (simple, compound, and spiral). The amount of friction required
39          on highway curves increases with increasing complexity of the curve (i.e., as the
40          alignment changes from a tangent to a horizontal curve). To counter increasing
41          friction demand in horizontal curves, highway designers increase the horizontal
42          radius of curvature and super-elevate the highway cross-section.
44          The lateral friction developed at the pavement–tire interface along a curve is
45          directly related to the square of the vehicle’s speed. As the speed increases, the force
46          required to maintain a circular path eventually exceeds the force that can be
47          developed at the pavement–tire interface and super-elevation. At this point, the
48          vehicle begins to slide in a straight line tangential to the highway alignment. The

 1       relationship between side-force friction for horizontal curves (the most critical
 2       horizontal alignment), vehicle speed, radius of curvature, and highway cross-section
 3       (super-elevation) is defined by the AASHTO Green Book equation (AASHTO, 2001).
 5   •   Vertical Alignment—Vertical alignment consists of a series of gradients (grades)
 6       connected by vertical curves. It controls how the highway follows existing terrain
 7       and its properties are mainly controlled by terrain, horizontal alignment, and sight
 8       distance.
10       The friction demand for vehicles traversing a highway is highly influenced by the
11       highway’s vertical alignment. This is because vertical alignment design policy is
12       based on the need to provide drivers with adequate stopping sight distance to enable
13       them to see an obstacle soon enough to perform evasive maneuvers. The ability to
14       perform evasive maneuvers successfully is highly dependent on friction availability.

 1   Highway Features/Environment
 3   Highway features/environment is an important characteristic of traffic flow that can
 4   influence pavement friction. This characteristic of traffic flow is defined largely by the level
 5   of interacting traffic situations (e.g., entrance/exit ramps, access drives,
 6   unsigned/unsignalized intersections), the presence of controlled (signed/signalized)
 7   intersections, the presence of specially designated lanes (e.g., separate turn lanes at
 8   intersections, center left-turn lanes, through versus local traffic lanes), the presence and
 9   type of median barriers, and the setting (urban versus rural) of the roadway facility. In
10   general, as the highway environment becomes more difficult and complex, significantly
11   higher levels of friction are required to help drivers perform the necessary maneuvers (e.g.,
12   sudden braking). Understanding the various features of this characteristic provides the
13   basis for determining how a friction number might provide useful information regarding
14   safety at a particular location.
16   Highway Traffic Characteristics
18   Traffic characteristics that influence friction demand include traffic volume, composition,
19   and speed. Key aspects of these factors are as follows:
21      •   Traffic Volume—As traffic volume increases, the number of driving maneuvers
22          taking place along any given segment increases. The risk associated with these
23          increased maneuvers is elevated, especially in high-speed areas. When traffic
24          volume is increased to the point that congestion occurs, the possibility of crashes is
25          aggravated if a highway facility is undivided and traffic speed is high (Page and
26          Butas, 1986; Mahone and Runkle, 1972).
28      •   Traffic Composition—For the same traffic volume, the composition of traffic vehicles
29          (i.e., the percentage of trucks in the traffic stream) can significantly affect highway
30          safety and thus friction demand for the following reasons:
32              Stopping distances of trucks are significantly longer than stopping distances of
33              passenger cars (AASHTO, 2001).
34              Trucks have inferior steering capability compared to passenger cars.
35              Truck tires produce less friction than passenger car tires.
37          Hence, for highway segments where a high percentage of trucks is anticipated,
38          friction demand will typically be higher than a corresponding highway having
39          predominantly passenger cars or lower percentage of trucks.
41      •   Traffic Speed—Vehicle speed is the most important factor influencing friction
42          demand. For wet pavement surfaces, for instance, an increase in truck speed on
43          tangents from 20 to 70 mi/hr (32 to 113 km/hr) results in an increase in truck
44          stopping distance from 50 to 1,200 ft (15 to 366 m) (Radlinski and Williams, 1985).
45          Such an increase in stopping distance significantly increases the risk of a crash.
47          Figure 13 shows the conceptual relationship between friction demand and friction
48          availability for wet pavements. This figure indicates that an increase in speed

 1          results in an increase in friction demand and a decrease in available surface friction
 2          (Glennon, 1996).
                                                                                            Speed of
 5                                                                                       Impending Skid
                            Pavement-Tire Friction
12                                                      Pavement-Tire
                                                     Frictional Capability
15                                                                               Friction Demand
16                                                                                   of Vehicle

18                                                                   Vehicle Speed
20               Figure 13. Conceptual relationship between friction demand, speed,
21                                   and friction availability.
24          Speed also contributes to the severity of impact when a collision occurs. For
25          passenger cars colliding with an impact speed of 65 mi/hr (105 km/hr), the likelihood
26          of death is 20 times greater than that associated with an impact speed of 20 mi/hr
27          (30 km/hr) (WHO, 2004). Finally, increasing speed (above 40 mi/hr [64 km/hr])
28          increases the likelihood of hydroplaning, which is a major cause of wet-weather
29          crashes (Glennon, 1996).
31          The speed of vehicles on the highway must therefore be considered in determining
32          friction demand. Highways with higher posted speed limits and overall travel
33          speeds (85th percentile of vehicle speed) require higher levels of pavement surface
34          friction than lower speed facilities.
36   Establishing Friction Demand Categories
38   Pavement friction demand categories should be established logically and systematically
39   based on highway alignment, highway features/environment, and highway traffic
40   characteristics. Ideally, friction demand categories should be established for individual
41   highway classes, facility types, or access types. Also, the number of demand categories
42   should be kept reasonably small, so that a sufficient number of PFM sections are available
43   for each category from which to define investigatory and intervention friction levels.
45   3.2.2 Network-Level Data Collection
47   Collection of Friction Data

 1   Measurements of pavement friction should consider (1) testing protocol and equipment, (2)
 2   testing frequency, (3) testing conditions, and (4) equipment calibration and maintenance.
 4   Testing Protocol
 6   At the network level, the locked-wheel friction tester (AASHTO T 242 is the most
 7   appropriate method of testing. The method is standardized (e.g., test speed, water flow
 8   rate), can be performed quickly and at high speeds, and is generally quite repeatable. The
 9   method can assess friction and texture by performing tests with both smooth and ribbed
10   tires or with a properly mounted texture laser.
12   Frequency of Testing
14   For a network-level evaluation, it is desirable to test all pavement sections annually
15   because of the year-to-year variation in pavement friction. However, the testing frequency
16   for each agency is determined by the length of network to be tested and available resources.
17   A practical approach is a rolling or cyclical testing regime, whereby portions of the network
18   are tested once every few years (e.g., for a rolling 3-year program, one-third of the network
19   is tested each year). Statistical sampling of pavement sections for network level analysis is
20   an acceptable option, as many agencies cannot test 100 percent of their pavement network
21   due to budgetary and/or other constraints.
23   Testing Conditions
25   Because pavement friction is influenced by various factors, such as pavement surface
26   temperature, test speed, and ambient weather conditions, testing should be performed
27   under standardized conditions to control the effect of these factors on test results.
28   Controlling testing conditions will minimize variability in test results and produce
29   repeatable measurements. The factors presented in table 2 should be considered along with
30   other relevant factors in establishing testing conditions (Highways Agency, 2005).
32   Equipment Calibration and Maintenance
34   Proper calibration and maintenance of the friction testing equipment is essential to the
35   collection of reliable friction data. To this end, agencies should follow the manufacturer-
36   specified regime or guidance for calibration and routine maintenance.
38   Collection of Crash Data
40   Crash data are generally available from an agency’s crash database or from other sources,
41   such as law enforcement agencies and statistical bureaus. Inputs to classify and describe
42   crashes may include (1) the location (route, milepost, direction) of each crash, (2) vehicles
43   involved along with their characteristics, (3) drivers and passengers involved along with
44   their characteristics, (4) ambient weather conditions at the time of the crash, and (5) injury
45   levels and property damage as a result of the crash.

1                   Table 2. Summary of issues relating to standardized test conditions.
        Factors                                                        Consideration
                         Because significant variations in measured friction may occur across seasons within a given year,
                         friction testing should be limited to a specific season or time of year when friction is typically lowest.
                         This will help maintain some consistency in year-to-year measurements and reduce variability in
                         measured data. For agencies that cannot perform all testing requirements within a given season, the
                         following can be considered to reduce test variability:
    Season for testing
                         • Develop correction factors, as needed, to normalize raw friction test data to a common baseline
                         • For a given pavement section, initial and subsequent testing must be done within a specific season
                           (e.g., pavement sections originally tested in fall should subsequently be tested in fall).
                         The standard speed recommended by AASHTO T 242 for pavement friction tests is 40 mi/hr (64
                         km/hr). However, since most agencies conduct friction tests without traffic control and because posted
                         or operational speeds vary dramatically throughout a network, it is very difficult for the operator to
                         conduct testing at just this speed. For such situations, the operator typically adjusts test speeds to
                         suit traffic conditions and to assure a safe operation. Thus, it is recommended that friction values
                         corresponding to testing done at speeds other than 40 mi/hr (64 km/hr) be adjusted to the baseline 40-
                         mi/hr (64-km/hr) value to make friction measurements comparable and useful.

                         To do this requires the establishment of correlations between friction measurements taken at 40
                         mi/hr (64 km/hr) and those taken at other speeds (i.e., speed gradient curves). The following equation
    Test speed
                         can be used to adjust friction measurements to FN40:
                                                                                             S −V
                                                                   FN ( S ) = FN V × e

                         where: FN(S) = Adjusted value of friction for a speed S.
                                FNV   = Measured friction value at speed V.
                                SP    = Speed number.

                       In order to produce accurate estimates of FN(s), SP must be established for a broad range of pavement
                       macro-textures and texture measuring devices.
                       Friction measurements must be done in the most heavily trafficked lane, as this lane usually carries
                       the heaviest traffic and is, therefore, expected to show the highest rate of friction loss (worst case
                       scenario). For 2-lane highways with a near 50-50 directional distribution of traffic, testing a single
                       lane will suffice; otherwise, the lane in the direction with heavier traffic should be tested. For multi-
                       lane highways, the outermost lane in both directions is typically the most heavily trafficked and
                       should be tested. Where the outermost lane is not the most heavily trafficked, a different lane or
    Test lane and line more than one lane should be tested.

                         Test measurements must be carried out within the wheelpath, as this is the location where friction
                         loss is greatest. Note that it is important to test along the same lane and wheelpath to maintain some
                         consistency between test results and to reduce variability. If it is necessary to deviate from the test
                         lane and wheelpath (e.g., to avoid a physical obstruction or surface contamination), the test data
                         should be marked accordingly.
                         Because ambient conditions can have an effect on pavement friction, it is important to standardize
                         ambient test conditions to the extent possible and document ambient test conditions so the
                         measurements can be corrected as needed. The following should be noted when setting ambient
                         conditions for testing:

    Ambient              • Testing in extremely strong side winds must be avoided because these can affect the measurements
    conditions             by creating turbulence under the vehicle that causes the water jet to be diverted from the correct
                         • Testing must be avoided in heavy rainfall or where there is standing water on the pavement
                           surface. Excess water on the surface can affect the drag forces at the pavement–tire interface and
                           influence the measurements.
                         • Measurements shall not be undertaken where the air temperature is below 41°F (5°C).
    Contamination        Contamination of the pavement surface by mud, oil, grit, or other contaminants must be avoided.

 1   3.2.3 Network-Level Data Analysis
 3   Establishing Investigatory and Intervention Friction Threshold Levels
 5   As pointed out in chapter 2, a general relationship exists between pavement friction and
 6   crashes. Because conditions and circumstances (previously defined as highway
 7   characteristics) along a highway change, there is no one “magic” friction number that
 8   defines the threshold between “safe” and “potentially unsafe.”
10   The ideal situation would be to identify a specific friction number that would meet or
11   exceed friction demand for the entire system, Such a practice would be prohibitively
12   expensive (as well as largely unnecessary) and would not generate the cost-benefits
13   associated with a better-targeted strategy. A more practical approach, would be to
14   maintain an appropriate level of pavement friction for pavement sections within the
15   highway network, based on each section’s friction demand. This approach seeks to provide
16   adequate friction levels for a variety of roadway (intersections, approaches to traffic signals,
17   tight curves) and traffic conditions.
20   The establishment of investigatory and intervention friction levels requires detailed
21   analyses of micro-texture and macro-texture data, and crash data, if available. Such
22   analysesshould be carried out separately for each friction demand category established by
23   the agency.
25   Presented in the sections below are three feasible methods for setting investigatory and
26   intervention friction levels, either in terms of FN or in terms of IFI(F(60),Sp). These
27   example methods are derived from many years of discussions at national and international
28   meetings and workshops on pavement friction (e.g., ASTM E 17, TRB AFD90, PIARC TC 1
29   [now T4.2], and the NASA Wallops Friction Workshops).
30   Establishing Thresholds Using Historical Pavement Friction Data Only (Method 1)
32   This method uses historical trends of friction loss determined by plotting friction loss
33   against pavement age or time for a specific friction demand category. An investigatory
34   level is set at the pavement friction value where friction loss begins to increase at a
35   significantly faster rate. An intervention level may then be set at a certain amount (e.g.,
36   five F(60),SP points or five FN points) or percentage (e.g., 10 percent) below the
37   investigatory level.
38   The friction value at which friction loss begins to increase rapidly can be determined
39   graphically or through the use of analytical/statistical methods. An example graphical
40   based method includes the following steps:
42      •   Step 1—Plot pavement friction versus age/time for a given friction demand category
43          (figure 14).
44      •   Step 2—Develop a friction loss deterioration curve based on the measured data.
45      •   Step 3—Graphically determine the slopes of the three stages of the S-shaped friction
46          loss versus pavement age/time relationship.
47      •   Step 4—Set the investigatory level as the friction value where there is a significant
48          increase in the pavement friction loss.

 1      •   Step 5—Set intervention level at a certain value or percentage below the
 2          investigatory level.
 4   Establishing Thresholds Using Both Historical Pavement Friction Data and Crash Data
 5   (Method 2)
 7   This method compares historical pavement friction and crash data for the given friction
 8   demand category for which levels are being set. Figure 15 shows a plot of friction and wet-
 9   to-dry crash trends for a specific friction demand category. An investigatory level may be
10   set corresponding to a large change in friction loss rate while the intervention level may be
11   set where there is a significant increase in crashes.
13   Establishing Thresholds Using Pavement Friction Distribution and Crash Rate–Friction
14   Trend (Method 3)
16   This method uses the distribution of friction data versus the crash rates that correspond
17   with the friction for the category of roadway for which the levels are being set. An example
18   of using this method includes the following steps:
20      •   Step 1—Plot a histogram of pavement friction for a given friction demand category,
21          based on current history. On the same graph, plot the current wet-to-dry crash ratio
22          for the same sections as the friction frequency distribution (figure 16).
23      •   Step 2—Determine the mean pavement friction and standard deviation for the
24          pavement friction frequency distribution.
25      •   Step 3—Set the investigatory level as the mean friction value minus “X” standard
26          deviations (say, 1.5 or 2.0) of the distribution of sections and adjust to where wet-to-
27          dry crashes begin to increase considerably.
28      •   Step 4—Set intervention level as the mean friction value minus “Y” standard
29          deviations (say, 2.5 or 3.0) of the distribution of sections and adjust the level to a
30          minimum satisfactory wet-to-dry crash rate or by the point where the amount of
31          money is available to repair that many roadway sections.

          F(60),Sp or FN

                            Investigatory Level
13                          Intervention Level
19                                                Pavement Age, years
22   Figure 14. Setting of investigatory and intervention levels for a specific friction demand
23                      category using time history of pavement friction.
                                                                                Crash Rates
       F(60) ,Sp or FN

34                         Investigatory Level

                                                                                                  Crash Rates
38                         Intervention Level
45                                                Pavement Age, years
47   Figure 15. Setting of investigatory and intervention levels for a specific friction demand
48                    category using time history of friction and crash rate.

 3                                                 Level
 4                                     Intervention
 5                                         Level      Mean – X*(Std Dev)

                                                                                              Wet to Dry Crashes, %
                                                  Mean – Y*(Std Dev)
                          Wet to Dry
        Number of Sites

20                                                      F(60) ,Sp or FN
21     Figure 16. Setting of investigatory and intervention levels for a specific friction demand
22           category using pavement friction distribution and crash rate–friction trend.
25   Method 3 is the most detailed approach. It has the advantage of discerning the number of
26   roadway sections below a certain level.
28   As in any engineering decision, a transportation agency considers the financial implications
29   of maintaining highway safety through managing pavement friction levels. Thus, an
30   agency should consider the effects of using different investigatory and intervention levels in
31   terms of the improvement in safety and the cost to achieve the level. Levels can be adjusted
32   to optimize the increase in safety within the agency’s budget.
34   Regardless of the method used, the investigatory and intervention levels selected should be
35   reviewed periodically and revised as needed. Improvements in highway safetyguidelines
36   may require changes in the levels set by an agency.
38   Identifying Pavement Sections Requiring Detailed Site Investigation or Intervention
40   Once a section has been identified as being at or below a friction threshold level, steps
41   should be taken to identify the cause(s) of the deficiency. An agency should consider
42   caution highway users by installing appropriate warning signs (e.g., slippery when wet,
43   reduced speed) and then proceed with plans for a detailed investigation of the section.
45   If the IFI is being used, a quick assessment can be made of the friction and texture
46   measurements to determine if micro-texture or macro-texture, or both, are inadequate and
47   in need of improvement. A graph similar to figure 17 can be developed and used, not only
48   as an aid to the detailed investigation, but for selecting the type of warning that should be
49   posted.

 2                          Inadequate               Marginal                      Adequate
 3                         Macro-texture           Macro-texture                  Macro-texture
 5                                         uate
 6                                                   ion/M
 7                                                                   re
      Pavement Friction

                                           inal F                                                 LEGEND
12                                               rictio
                                                       n/Mic                                               Texture Investigatory Level
13                                                           ro-te
                                                                                                           Texture Intervention Level
15                                                                                                         Friction Investigatory Level
16                                                                                                         Friction Intervention Level
18                                     Inad
19                                              te Fr
20                                                             r   o-tex
                                                                        t   ure
25                                         Pavement Macro-Texture
27                        Figure 17. Determination of friction and/or texture deficiencies using the IFI.
30   3.2.4 Detailed Site Investigation
32   A detailed site investigation of pavement sections at or below the investigatory or
33   intervention level is necessary to (a) identify other factors besides friction that are
34   adversely impacting safety and (b) determine specific causes of inadequate micro-texture
35   and/or macro-texture. The detailed investigation involves at a minimum two steps,
36   described below or of similar nature.
38   Step 1—Conduct Visual/Video Survey
40   Each identified section should be evaluated for highway characteristics (as previously
41   described)that may contribute to the safety issue, both in terms of available friction and
42   friction demand. Such items include the horizontal and vertical alignment, the layout of
43   lanes, intersections, and traffic control devices, the presence, amount, and severity of
44   pavement distresses (e.g., potholes, rutting, bleeding, deteriorated patches), longitudinal
45   pavement smoothness, and transverse pavement profile. Also of importance in the detailed
46   investigation are the issues of glare (as caused by the pavement or the lack of appropriate
47   traffic aids), splash and spray, and hydroplaning potential (often linked to rutting or
48   inadequate cross-slope). Discussion of these issues are provided below.

 1   Splash and Spray
 4   The occurrence of splash and spray is influenced by the drainage condition at the pavement
 5   surface. Providing positive drainage that quickly removes standing water from the
 6   pavement surface will reduce the occurrence of splash/spray significantly. Pavement
 7   surface drainage is enhanced by providing adequate amounts of macro-texture and cross-
 8   slope.
11   Suggestion here is to eliminate the entire section on hydroplaning as it is
12   not directly related to the Guide on Friction
14   Hydroplaning Potential
16   As discussed earlier in chapter 2, hydroplaning refers to the separation of the tire contact
17   from the pavement surface by a layer of water. It is a complex phenomenon that is affected
18   by (1) the water film thickness (WFT) on the pavement surface, (2) pavement macro-
19   texture, (3) tire tread depth, (4) tire inflation pressure, (5) tire contact area, and (6) vehicle
20   speed.
22   For a vehicle to experience hydroplaning, two things must occur simultaneously: there
23   must be a sufficient buildup of water on the pavement surface and the vehicle must be
24   traveling at a speed high enough to cause hydroplaning. Thus, the potential for
25   hydroplaning for a given highway segment can be assessed by determining (1) the
26   frequency of water buildup from precipitation (rainfall only) on the pavement surface and
27   (2) whether the traveling speeds of vehicles is high enough to result in hydroplaning for the
28   water buildup conditions.
30   A three-step procedure for determining hydroplaning potential is presented below.
32      •   Step 1—Estimate Critical Hydroplaning Speed (HPS): An approximate relationship
33          between the vehicle speed (in mi/hr) at which hydroplaning for both asphalt and
34          concrete pavements will occur and the tire inflation pressure (in lb/in2) is as follows
35          (Ong and Fwa, 2006):
37                                     HPS = 10.35 tire pressure                                Eq. 20
39          This equation assumes that WFT on the pavement surface exceeds the combined
40          capability of the surface macro-texture and tire design (i.e., tread depth) to remove
41          water from the pavement surface.
43      •   Step 2—Compute WFT using agency-established models or procedures or the WFT
44          prediction models (and accompanying software) developed in NCHRP Project 1-29
45          (Anderson et al., 1998).

1       •   Step 3—Determine Hydroplaning Potential: An example scheme for determining
2           hydroplaning potential according to four categories (none, low, moderate, and high)
3           is provided in table 3.
6                    Table 3. Assessment of hydroplaning potential based on vehicle speed
7                                         and water film thickness.
               Average Vehicle Speed (85th Percentile of                            WFT, in
                   Traveling Speed) minus Critical                    < 0.02      0.02 to 0.06       > 0.06
                 Hydroplaning Speed (HPS), mi/hr a
             Less than –5                                                 None          None          None
             Between +5                                                   None           Low        Moderate
             Greater than 5                                               None         Moderate        High
 9          1 mi/hr = 1.61 km/hr                                     1 in = 25.4 mm
10          a Guidelines for determining design speed based on highway functional classification, location

11            (i.e., rural versus urban), and terrain type (i.e., level, rolling, and mountainous) can be found
12            in the AASHTO Green Book (AASHTO, 2001).
15   Step 2—Evaluate Micro-Texture and Macro-Texture
17   The second step in the detailed site investigation involves testing the pavement surface for
18   micro-texture and macro-texture. These two properties can be evaluated using various
19   types of equipment, including:
21      •   Micro-texture, which can be evaluated using any of the following:
22             Locked-wheel friction tester.
23             British Pendulum Tester (BPT).
24             Dynamic Friction Tester (DFT).
25      •   Macro-texture, which can be evaluated using any of the following:
26             High-speed laser.
27             Circular Texture Meter (CTM).
28             Sand Patch Method (SPM).
30   Testing must be done in a manner that produces results that are representative of the
31   entire pavement section.
33   In addition to the micro-texture and macro-texture data, the following information must be
34   obtained from the records or through field testing:
36      •   Traffic applications, including truck percentages.
37      •   Pavement surface age.
38      •   Surface material type and/or finishing method.
39      •   Data on all materials used in the surface pavement (e.g., fine/coarse aggregate type),
40          including polishing/wear characteristics, structure, hardness, and so on, if available.
41      •   Other information, such as data from laboratory tests.

 1   Using the micro-texture and macro-texture results and the data listed above, the exact
 2   cause of friction loss can be determined. Common causes of friction loss include polishing of
 3   coarse aggregates and excessive wearing of the pavement surface resulting in a loss of
 4   macro-texture.
 6   3.2.5 Selection and Prioritization of Friction Restoration Treatments
 8   A next step in a PFM program could be to analyze the collected data to identify sites
 9   suggesting more frequent monitoring or forensic investigation, and sites that might be
10   considered for friction restoration. Highway agencies may use pavement friction, other
11   identified highway characteristic data and condition data to identify and prioritize sites to
12   be included in a program for:
14      •   Short-term remedial (maintenance) works.
15      •   Comprehensive restoration treatment (e.g., diamond grinding, cold milling, thin
16          overlays, chip seals) aimed directly at improving friction.
18   In analyzing pavement friction data, a desirable outcome is to ensure that the appropriate
19   sites are detected and given reasonable priority. The extent of the analysis and use of
20   pavement friction and other data is determined locally by the agency. Analysis can be
21   restricted to identifying all sites where the measured pavement friction is at or below any
22   investigatory or intervention level that has been set, followed by a detailed site
23   investigation to identify actions that might include:
25      1. Continuing to monitor the site: Such a decision typically would be reached where (a)
26         current crash rates are sufficiently low and an increase is not expected to
27         significantly impact safety and (b) the pavement surface does not require
28         maintenance because of other factors.
29      2. Listing the site for remedial action to improve pavement friction (e.g., resurface,
30         retexture),where such an increase in pavement friction would significantly impact
31         safety.

1   This page intentionally left blank.
 6   Although the design of pavement friction is a relatively small component of the overall
 7   pavement design process, it is critical because of its impact on highway safety. Its
 8   importance and complexity have increased over the years due to increased demands for
 9   safer roads and the desire for greater highway user comfort, which sometimes contradicts
10   friction.
12   Friction design requires a thorough understanding of the factors that influence friction and
13   knowledge of the materials and construction techniques (including equipment) that
14   ultimately dictate initial and long-term friction. It also requires an understanding of the
15   economic and engineering tradeoffs associated with different materials and techniques,
16   such as the costs/benefits of utilizing one friction strategy over another and how each
17   strategy impacts structural design and other functional aspects (e.g., noise, splash/spray).
19   This chapter provides guidance on the design of pavement friction, as determined by
20   surface micro-texture and macro-texture. The information provided can be used to (a)
21   develop and implement useful, effective friction design policies at the network level and (b)
22   formulate feasible, cost-conscious friction design strategies at the project level. This
23   guidance is intended to supplement and not replace existing agency standards and
24   procedures.
29   Friction design policies must focus on the selection and use of (a) aggregates for micro-
30   texture and (b) paving mixtures and surface texturing techniques for macro-texture. The
31   policies should effectively reduce occurrences of wet-weather friction hazards and vehicle
32   crashes. Specifically, they should be geared towards overcoming deficiencies in materials
33   and construction techniques through improvements in aggregate testing protocols and
34   standards, mix design methods and formulations, and construction specifications and
35   special provisions.
37   4.2.1 Aggregate Testing and Characterization
39   Aggregate properties are the predominant factor that determines frictional performance of
40   asphalt and concrete pavement surfaces. Aggregate makes up the bulk of both asphalt and
41   concrete mixtures, and therefore, for the surface of either pavement type, aggregate is the
42   primary contact medium with the vehicle tires.
44   Aggregate generally is viewed as two distinct fractions—coarse aggregate and fine
45   aggregate. Coarse aggregate pieces are greater than the No. 4 sieve (0.19 in [4.75 mm]),
46   with most pieces between 0.375 and 1.5 in (9.5 and 38 mm). Fine aggregate, on the other
47   hand, is the collection of natural or crushed/manufactured particles less than 0.19 in (4.75
48   mm), but greater than the No. 200 sieve (0.003 in [75 µm]).

 2   Aggregate testing and characterization must be targeted to the fraction(s) of aggregate in a
 3   mix that will control the frictional performance. In general, coarse aggregate controls the
 4   frictional properties of asphalt mixtures, while fine aggregate controls the frictional
 5   properties of concrete mixes. Exceptions include fine-graded asphalt mixes, where fine
 6   aggregates are in greater abundance, and concrete mixes in which coarse aggregates are
 7   either intentionally exposed at the time of construction (exposed aggregate concrete, porous
 8   concrete) or will become exposed in the future (diamond grinding, surface abrading).
10   In terms of friction design and performance, the important aggregate properties are:
12      •   Mineralogical and Petrographic Properties.
13             Aggregate composition/structure and mineral hardness.
14      •   Physical and Geometrical properties.
15             Angularity, shape, and texture.
16      •   Mechanical Properties.
17             Abrasion/wear resistance.
18             Polish characteristics.
19      •   Durability Properties.
20             Soundness.
22   Several test methods are available for characterizing aggregate frictional properties,
23   whether as part of a standard mix design process or an aggregate source rating program.
24   The extent of aggregate testing and characterization required as part of the friction design
25   process will vary from agency to agency, based on the types of aggregates available, the
26   variability of aggregate properties, the quality and historical performance of available
27   aggregates, and the anticipated applications (e.g., mix types, roadway functional class).
28   Laboratory material testing does not guarantee friction performance in the field. Thus, it is
29   essential that testing be used in conjunction with field performance history to identify
30   acceptable aggregate types.
32   The aggregate tests described in table 4 are considered the most relevant in characterizing
33   frictional properties. The test methods shown in this table are recommended for
34   consideration as part of agency construction and materials specifications pertaining to
35   pavement surface mixtures. Agencies are encouraged to establish test criteria
36   corresponding to varying levels of friction design. Such criteria should be based on a
37   correlation between (a) aggregate characteristics obtained from standardized laboratory
38   tests and actual field performance and/or (b) aggregate geologic type or petrography and
39   field performance.
41   A brief discussion of each important aggregate property, and its influence and impact on
42   pavement friction, is provided in the following subsections.
44   Aggregate Composition/Structure and Mineral Hardness
46   One of the most important properties of aggregate used in pavement surfaces is the
47   composition of the minerals that comprise the aggregate particles. Aggregates that exhibit
48   the highest levels of polish resistance and resistance to wear typically are composed of

                                            Table 4. Test methods for characterizing aggregate frictional properties.

      Aggregate          Aggregate
                                              Test Name              Test Protocol                        Test Description                                   Applications
      Property             Type
                                                                                        Rough measure of the resistance of a mineral’s surface
                                                                                        to scratching. Expressed using a 1-to-10 scale (1 being
                            Fine         Scratch Hardness test           Mohs           very soft, 10 being very hard), Mohs hardness is          • New concrete surfacings.
                                                                                        determined by observing whether its surface is
                                                                                        scratched by minerals of a known or defined hardness.
                                                                                                                                                  • New asphalt surfacings and
                                                                                                                                                    asphalt mixes used for friction
                           Coarse        Scratch Hardness test           Mohs           Same as above.                                              restoration.
                                                                                                                                                  • New concrete surfacings
                                                                                                                                                    (conventional and innovative).a
                                                                                        Provides brief descriptions of commonly occurring
                                             Descriptive                                natural or artificial aggregates from which mineral
                                          Nomenclature for                              aggregates are derived. The descriptions provide a
                                                                      ASTM C 294
                                       Constituents of Concrete                         basis for understanding the potential effects on
                                             Aggregates                                 pavement friction of using different aggregate
                            Fine                                                                                                                  • New concrete surfacings.
                                                                                        Used to assess aggregate (1) constituent minerals and
        Mineralogy                                                                      structure, (2) surface texture, and (3) mineralogy, and
     (i.e., Aggregate                                                                   to develop a petrographic database for aggregate
     Composition &                       Petrographic Analysis        ASTM C 295

                                                                                        sources to serve as a basis for linking aggregate sources
        Structure)                                                                      to pavement field performance (Folliard and Smith,
                                             Descriptive                                                                                          • New asphalt surfacings and
                                          Nomenclature for                                                                                          asphalt mixes used for friction
                                                                      ASTM C 294        Same as above.
                           Coarse      Constituents of Concrete                                                                                     restoration.
                                             Aggregates                                                                                           • New concrete surfacings
                                         Petrographic Analysis        ASTM C 295        Same as above                                               (conventional and innovative).a
      a   For conventional PCC surfaces, where coarse aggregates are expected to be exposed, and innovative surfaces, such as porous concrete and exposed
          aggregate concrete.
                                  Table 4. Test methods for characterizing aggregate frictional properties (continued).

      Aggregate        Aggregate
      Property           Type               Test Name               Test Protocol                         Test Description                                  Applications
                                                                                      Fine aggregate of prescribed gradation is allowed to
                                                                                      flow through orifice of a funnel and fill a 6.1-in3 (100-
                                                                                      cm3) cylinder. Excess material is struck off and
                                                                                      cylinder with aggregate is weighed. Uncompacted void
                                    Uncompacted Voids (UV)        AASHTO T 304
                           Fine                                                       content is computed using this weight and the bulk dry • New concrete surfacings.
                                      test for fine aggregates
                                                                                      specific gravity of the aggregate (Kandhal et al., 1997).
                                                                                      Higher uncompacted void contents are generally the
                                                                                      result of more fractured faces and rougher textures,
                                                                                      which are desirable for pavement friction.
                                                                                      Coarse aggregate angularity, shape, and texture can be • New asphalt surfacings and
        Shape, &
                                                                                      determined using principles similar to those described      asphalt mixes used for friction
                                         UV test for coarse                       b   above for fine aggregates. Again, higher uncompacted        restoration.
                                                                 AASHTO T 326
                                             aggregates                               void contents are generally the result of more fractured • New concrete surfacings
                                                                                      faces and rougher textures, which are desirable for         (conventional and innovative).a
                         Coarse                                                       pavement friction.
                                                                                      Determines the amount (percent) of fractured-faced (an • New asphalt surfacings and
                                                                                      angular, rough, or broken surface of an aggregate           asphalt mixes used for friction
                                     Fractured-Face Particles      ASTM D 5821
                                                                                      particle) aggregate particles, by visual inspection. The    restoration.
                                                 test          (or AASHTO TP 61)
                                                                                      fractured face of each aggregate particle must meet a     • New concrete surfacings
                                                                                      minimum cross-sectional area.                               (conventional and innovative).a

                                                                                      A fine aggregate sample is subjected to wet attrition by
                                                                                      placing it in a steel jar with 0.375-in (9.5-mm)
                                                               Canadian Standards diameter steel balls and water. The jar is rotated at
                                     Micro-Deval test for fine                                                                                  • New concrete surfacings
                           Fine                                  Association (CSA)    100 rpm for 15 minutes, after which aggregate damage
                                             aggregates                                                                                           (conventional).
                                                                     A23.2-23A        is assessed by mass loss using a No. 200 (75 μm) sieve.
                                                                                      Higher percentages of loss indicate greater potential
                                                                                      for aggregate breakdown (Folliard and Smith, 2003).
                                                                   AASHTO T 96        A dry aggregate sample is placed in a steel drum with • New asphalt surfacings and
                                                                                      six to twelve 420-gram steel balls, and the drum is         asphalt mixes used for friction
                                         LA Abrasion test           ASTM C 535        rotated for 500 to 1,000 revolutions. Degradation by        restoration.
                                                                  [for large-sized    impact of the aggregate sample is determined by the       • New concrete surfacings
                                                                coarse aggregates]) percentage passing the No. 12 (1.7-mm) sieve.                 (conventional and innovative)a
                                                                                      A coarse aggregate sample is subjected to wet attrition • New asphalt surfacings and
                                                                                      by placing it in a steel jar with 0.375-in (9.5-mm)         asphalt mixes used for friction
                                        Micro-Deval test for      AASHTO T 327
                                                                                      diameter steel balls and water. The jar is rotated at       restoration.
                                         coarse aggregates
                                                                                      100 rpm for 2 hours, after which aggregate damage is      • New concrete surfacings
                                                                                      assessed by mass loss using a No. 16 (1.18-mm) sieve.       (conventional and innovative)a
     a For conventional PCC surfaces, where coarse aggregates are expected to be exposed, and innovative surfaces, such as porous concrete and exposed

       aggregate concrete.
     b Formerly AASHTO TP 56.
                                 Table 4. Test methods for characterizing aggregate frictional properties (continued).

         Aggregate    Aggregate
         Property       Type             Test Name            Test Protocol                            Test Description                               Applications
                                                                               Estimates the percent by weight of insoluble, hard, non-
                                                                               carbonate residue in carbonate aggregates (e.g., limestone,
                                                                               dolomite), using hydrochloric acid solution to react the
                                    Acid Insoluble Residue
                         Fine                                 ASTM D 3042      carbonates. Higher acid insoluble residue (AIR) values          • New concrete surfacings.
                                          (AIR) test
                                                                               indicate larger percentages of siliceous minerals, which are
                                                                               considered more polish resistant than carbonate materials
                                                                               (Kandhal et al., 1997).
                                                                               Aggregate coupons (aggregates embedded in epoxy resin) are
                                                                               fabricated, subjected to accelerated polishing (using British
                                                                               polish wheel) for a specified time (usually 9 hrs), and then    • New asphalt surfacings and
                                                                               tested for frictional resistance (expressed as British            asphalt mixes used for
           Polish                                            AASHTO T 278 &
                                     Polished Stone Value                      Pendulum Number [BPN]) using the British Pendulum                 friction restoration.
         Resistance                                              T 279
                                          (PSV) test                           Tester. The BPN value associated with accelerated polishing     • New concrete surfacings
                                                                               is defined as the polished stone value (PSV), which is a          (conventional and
                                                                               quantitative representation of the aggregate’s terminal           innovative)a
                                                                               frictional characteristics. Higher values of PSV indicate
                                                                               greater resistance to polish.
                                                                                                                                               • New asphalt surfacings and
                                                                                                                                                 asphalt mixes used for
                                    Acid Insoluble Residue                                                                                       friction restoration.

                                                              ASTM D 3042      Same as above.
                                          (AIR) test                                                                                           • New concrete surfacings
                                                                                                                                                 (conventional and
                                                                               An aggregate sample is immersed in a solution of magnesium
                                                                               sulfate for a period of 16 to 18 hours at a temperature of 70°F
                                                                               (21°C). The sample is then removed, drained for 15 minutes,
                         Fine                                                  and oven-dried to a constant weight (5 cycles of immersion      • New concrete surfacings.
                                                                               and drying is typical). During the immersion process, the salt
                                                                               solution penetrates the permeable pore spaces of the
                                      Magnesium Sulfate      AASHTO T 104      aggregate. Oven drying dehydrates the sulfate salt
                                       Soundness test                          precipitated in the pores. The internal expansive force of the
                                                                               re-hydration upon re-immersion simulates the expansion of       • New asphalt surfacings and
                                                                               water upon freezing. Upon completion of the final cycle, the      asphalt mixes used for
                                                                               sample is sieved over various sieves and the maximum              friction restoration.
                                                                               weighted average loss is reported as the sulfate soundness      • New concrete surfacings
                                                                               loss. Higher percentages of loss indicate less sound or           (conventional and
                                                                               durable aggregate (Khandal et al., 1997).                         innovative)a
     a   For conventional PCC surfaces, where coarse aggregates are expected to be exposed, and innovative surfaces, such as porous concrete and exposed
         aggregate concrete.
 1   hard, strongly bonded, interlocking mineral crystals (coarse grains) embedded in a matrix
 2   of softer minerals (Folliard and Smith, 2003; Liang, 2003). The differences in grain size
 3   and hardness provide a constantly renewed abrasive surface because of differential wear
 4   rates and the breaking off of the harder grains from the softer matrix of softer minerals.
 6   The Mohs scratch hardness test is recommended for determining mineral hardness. While
 7   a visual inspection (using the descriptive nomenclature in ASTM C 294) of the aggregate
 8   can provide a basic understanding of mineral composition and structure, more detailed
 9   information can be obtained through advanced testing using petrographic analysis (ASTM
10   C 295). Some caution is advised with mineralogical tests due to the high variability
11   observed in the behavior of aggregates with similar mineralogy from different locations.
13   Aggregates made up of hard minerals (Mohs hardness > 6) alone typically resist wear and
14   other forms of degradation, yet may polish easily when subjected to traffic. Aggregates
15   made up of moderately soft minerals (Mohs hardness of 3 to 6) alone resist polishing, but
16   wear quickly when subjected to traffic. Agencies typically consider the ideal coarse
17   aggregate to consist of 50 to 70 percent coarse-grained and hard minerals embedded in a
18   matrix of 30 to 50 percent softer minerals. Coarse aggregates that contain larger and more
19   angular mineral grains or crystals exhibit higher levels of micro-texture and have a higher
20   frictional resistance.
22   The information presented in this section represents typical aggregate frictional properties
23   observed and reported through laboratory and field studies. However, because of the high
24   variability exhibited by aggregates frictional properties, it is recommended that agencies
25   guidance on the use of local aggregate sources be based on extensive laboratory testing
26   and/or field studies. This is the only way in which the recommendations provided will be
27   reliable and useful for pavement friction design.
29   Aggregate Angularity, Shape, and Texture
31   Aggregate angularity, shape, and texture are important parameters for defining pavement
32   surface micro-texture and macro-texture. Fine aggregates that exhibit angular edges and
33   cubical or irregular shapes generally provide higher levels of micro-texture, whereas those
34   with rounded edges or elongated shapes generally produce lower micro-texture. For coarse
35   aggregates, sharp and angular particles interlock and produce a deep macro-texture as
36   compared to more rounded, smooth particles. Moreover, in asphalt mixes, platy (i.e., flat
37   and elongated) aggregate particles tend to orient themselves horizontally, resulting in
38   lower macro-texture depth.
40   Recommended test methods for assessing angularity, shape, and texture are provided in
41   table 5. The uncompacted voids (UV) test (AASHTO T 304 ) is the most commonly used test
42   for assessing fine aggregate angularity, sphericity, and texture (Folliard and Smith, 2003).
43   As noted by Meininger (1994), this test does not require performing detailed petrographic
44   evaluations of shape and texture.
46   Two options are given for assessing coarse aggregates; the fractured-face particles test
47   (ASTM D 5821), which is very commonly used, and the UV test (AASHTO T 326 [formerly
48   AASHTO TP 56]), which is similar to the UV test for fine aggregate but is conducted with
49   proportionally larger equipment (Kandhal and Parker, 1998).

 2   Abrasion/Wear Resistance
 4   The use of abrasion-resistant aggregates is important for avoiding breakdown when
 5   subjected to traffic shear forces or during handling, stockpiling, mixing, placing, and
 6   compaction. The breakdown of fine and/or coarse aggregate particles can alter gradation
 7   significantly, thereby affecting asphalt mix volumetric properties, concrete mix strength,
 8   and overall mix porosity and macro-texture.
10   Table 4 lists the recommended test methods for both fine and coarse aggregates. While the
11   Micro-Deval test (AASHTO T 327, formerly AASHTO TP 58 ) for coarse aggregates have
12   been reported (Folliard and Smith, 2003; Kandhal and Parker, 1998) in recent years to be a
13   better indicator of the potential for aggregate breakdown, the LA Abrasion test is commonly
14   used with good success.
16   Polish Resistance
18   The resistance of fine and coarse aggregates to polishing under traffic wear is a major factor
19   in long-term frictional performance. Polish-resistant aggregates are those that retain their
20   harsh micro-texture under the grinding and shearing effects of repeated traffic loadings.
21   Polish-susceptible aggregates must be limited for use or blended with more polish-resistant
22   aggregates.
24   Recommended tests for aggregate polish resistance are provided in table 4. There are no
25   direct tests for assessing fine aggregate polish characteristics. Hence, the acid insoluble
26   residue (AIR) test (ASTM D 3042), which indicates the amount of softer polishing carbonate
27   material in an aggregate, is used.
29   For coarse aggregates, both the AIR test and the polished stone value (PSV) test (AASHTO
30   T 278 & T 279) have been used with good success and are recommended. It should be
31   noted, however, that other non-standard polish susceptibility tests exist and may be worthy
32   of further examination.
34   Soundness
36   Soundness refers to an aggregate’s ability to resist degradation caused by climatic/
37   environmental effects (i.e., wetting and drying, freezing and thawing). Similar to
38   abrasion/wear resistance, sound aggregate properties are important for avoiding
39   breakdown, particularly when used in harsh climates.
41   The test method considered to best characterize aggregate soundness is the sulfate
42   soundness test (AASHTO T 104). This widely used test was developed to simulate, without
43   the need for refrigeration equipment, the effects of freeze-thaw water action on aggregate
44   particles (Khandal and Parker, 1998).
46   Two options for sulfate solution are given in this test—sodium sulfate and magnesium
47   sulfate. The preferred option is the latter, as it has been reported to produce less variation
48   in mass loss (Folliard and Smith, 2003) and provide a better indication of good versus poor
49   aggregates (Kandhal and Parker, 1998).

 2   Aggregate Test Criteria
 4   Table 5 provides a range of typical test values for aggregate properties that will enhance
 5   pavement friction and friction durability (aggregate wear resistance). These values were
 6   obtained from various studies that related aggregate properties to frictional performance
 7   (Liang, 2003; Liang and Chyi, 2000; Dahir and Henry, 1978; FHWA, 2005; Kandhal and
 8   Parker, 1998; Wu et al., 1998; Prowell et al., 2005). The information presented pertains to
 9   typical virgin aggregates and may not apply to lightweight, heavyweight, or recycled
10   aggregates.
13                       Table 5. Typical range of test values for aggregate properties.
         Aggregate     Aggregate                                                 Typical Property Range for
                                            Test Type
         Property       Fraction                                                Good Friction Performancea,b
                          Fine   Mohs Scratch Hardness                                       ≥6

         Hardness                                                                      Hard minerals: ≥ 6
                         Coarse     Mohs Scratch Hardness                             Soft minerals: 3 to 5
                                                                         Differential hardness (hard minus soft): 2 to 3
                                    Visual Examination (Constituents
                          Fine      of Concrete Aggregates) and                 Hard siliceous mineral aggregate
                                    Petrographic Analysis
                                                                                    Percent of Hard Fraction
       Aggregate                                                                  Natural Aggregate: 50 to 70
     Composition &                                                                Artificial Aggregate: 20 to 40
       Structure                    Visual Examination (Constituents
                         Coarse     of Concrete Aggregates) and                    Hard Grain or Crystal Size
                                    Petrographic Analysis                        150 to 300 µm, average 200 µm

                                                                                  Hard Grain or Crystal Shape
                                                                                           Angular Tips
                          Fine      Uncompacted Voids content, %                               ≥ 45
                                    Uncompacted Voids content, %                               ≥ 45
                                                                          Agg. Particle Size: 0.12 to 0.5 in (3 to 13 mm)
          Shape, &                                                           Agg. Particle Shape: Conical, Angular
          Texture                   Fractured-Face Particles
                                                                          At least 90 percent by weight of the combined
                                                                       aggregates retained on No. 4 (4.75 mm) sieve should
                                                                         have two or more mechanically fractured faces.
                          Fine         Micro-Deval, % Loss                                  ≤ 17 to 20
                                      Micro-Deval, % Loss                                   ≤ 17 to 20
      Resistance           Coarse
                                      LA Abrasion, % Loss                                   ≤ 35 to 45
                            Fine       Acid Insoluble Residue (AIR), %                      ≥ 50 to 70
                                      AIR, %                                                ≥ 50 to 70
         Resistance        Coarse
                                      Polished Stone Value (PSV)                            ≥ 30 to 35
                                      Magnesium Sulfate Soundness
                            Fine                                                           ≤ 10 to 20
                                      (5 cycles), % Loss
                                      Magnesium Sulfate Soundness
                           Coarse                                                          ≤ 10 to 20
                                      (5 cycles), % Loss
15   a   Based on Liang, 2003; Liang and Chyi, 2000; Dahir and Henry, 1978; FHWA, 2005; Kandhal and Parker, 1998;
16       Wu et al., 1998; Prowell et al., 2005.
17   b   Property range descriptions given for Mohs Scratch Hardness and Visual Examination and Petrographic Analysis
18       pertain to individual aggregate particles.

 2   4.2.2 Surface Mix Types and Texturing Techniques
 4   Pavement surface drainage is in part a function of the surface macro-texture, which is
 5   defined largely by the aggregate gradation characteristics and finish quality of the surface
 6   mix. Surfaces with greater amounts of macro-texture provide greater resistance to sliding
 7   via hysteresis, and they help facilitate drainage, thereby reducing the potential for vehicle
 8   hydroplaning.
10   Several different surface mix types and finishing/texturing techniques are available for use
11   in constructing new pavements and overlays, or for restoring friction on existing
12   pavements. Tables 6 and 7 describe the more commonly used mix types and texturing
13   techniques, respectively, and they present the typical macro-texture levels achieved.
14   Pavement–tire considerations, such as noise, splash/spray, and hydroplaning, and general
15   considerations, such as constructability, cost, and structural performance, are not discussed
16   here, but they must be an integral part of any policies developed for these mixes and
17   texturing techniques.
19   4.2.3 Friction Design Categories
21   State highway agencies (SHAs) are encouraged to develop or update policies concerning the
22   friction design of new and restored pavements. Such policies should clearly define the
23   aggregate friction testing protocol (i.e., test types and criteria) and surface mix/texturing
24   techniques that are applicable for the friction demand categories established in the PFM
25   program.
27   As conceptually illustrated in figure 18, friction design categories should be established
28   that link combinations of rated aggregate sources and agency mix types/texturing
29   techniques with PFM sections having different levels of friction demand (defined by
30   investigatory/ intervention level). Each category should include a design friction level that
31   takes into consideration expected friction loss over time due to aggregate polishing and/or
32   macro-texture erosion.
34   As a minimum, friction design categories should be established according to highway design
35   speed and traffic (or design loadings in terms of equivalent single axle loads [ESALs]), since
36   these factors largely determine micro-texture and macro-texture needs. Other factors that
37   could be used in establishing categories include roadway facility type (i.e., functional or
38   highway class, access type), facility setting (rural, urban), climate (e.g., wet, dry), number of
39   lanes, and truck percentages.
41   Although several factors can be used in establishing friction design categories, the number
42   of categories should be limited to between three and five. When developing aggregate
43   source–texture options for a given design category, economics should be considered from the
44   standpoint that, if the local sources contain only low-polish aggregate, it may be justifiable
45   to use such aggregate for low friction demand situations. In addition, agencies should be
46   mindful of any existing classification schemes set forth in their wet-weather crash
47   reduction programs, materials and/or construction specifications, or other pavement-related
48   policies and systems, as they may by and large reflect the desired friction priorities.

                                         Table 6. Asphalt pavement surface mix types and texturing techniques.

                                      Mix/                                                                                                       Macro-texture
           Application            Texture Type                                           Description                                                Deptha
          New AC or AC         Dense Fine-Graded      Dense-graded HMA is a dense, continuously graded mixture of coarse and fine            Typically ranges from
            Overlay                  HMA              aggregates, mineral filler, and asphalt cement (5 to 6 percent). It is produced in a   0.015 to 0.025 in (0.4 to
                                                      hot-mix plant, delivered, spread, and compacted on site.                               0.6 mm)

                                                      Dense-graded HMA can be modified with polymers or crumb rubberb, and may
                                                      include recycled materials. Nominal maximum sizes for surfacing applications can
                                                      range from 0.38 in (9.5 mm) to 0.75 in (19.0 mm).

                                                  Fine HMA mixes contain gradations that pass above the maximum density line
                                                  (MDL) at the No. 8 (2.36-mm) sieve (WSDOT, 2005).
                              Dense Coarse-Graded Coarse HMA mixes have gradations that pass below the MDL at the No. 8 sieve                Typically ranges from
                                     HMA          (2.36-mm) (WSDOT, 2005).                                                                   0.025 to 0.05 in (0.6 to
                                                                                                                                             1.2 mm)
                               Gap-Graded HMA or SMA is a gap-graded mixture of course aggregate (typically, 0.4 to 0.6 in [10 to 15         Typically exceeds 0.04
                              Stone Matrix Asphalt mm]), filler, fibers and polymer-modified asphalt (typically, between 6 and 9             in (1.0 mm).
                                      (SMA)b          percent) produced in a hot-mix plant. Its primary advantage is resistance to

                                                      deformation, but its relatively coarse surface yields good frictional characteristics.
                              Open-Graded HMA or OGFC is an open-graded mixture of mostly coarse aggregate, mineral filler, and              Typically ranges from
                              Open-Graded Friction asphalt cement (3 to 6 percent). It is produced in a hot-mix plant, contains a high 0.06 to 0.14 in (1.5 to
                                 Course (OGFC)b       percentage of air voids (17-22 percent) in the mix, and is spread and compacted on 3.0 mm)
                                                      site. Friction, texture, and drainage properties can be controlled by the aggregate
                                                      gradation, size, angularity, and type. Open-graded HMA can be modified with
                                                      polymers, fibers, and/or crumb rubberc.
     a   Based in part on Hanson and Prowell, 2004; Meegoda et al., 2002; FHWA, 1996; FHWA, 2005; Richardson, 1999.
     b   Fine- and coarse-graded SMAs and OGFCs are being developed and increasingly used.
     c   Crumb rubber asphalt is a blend of 5 to 10 percent asphalt cement, reclaimed tire rubber, and additives in which the rubber component is 15 to 20
         percent by weight of the total blend. The rubber must react in the hot asphalt cement sufficiently to cause swelling of the rubber particles.
                                  Table 6. Asphalt pavement surface mix types and texturing techniques (continued).

                                      Mix/                                                                                                      Macro-texture
            Application           Texture Type                                            Description                                              Deptha
         Friction Restoration        Chip Seal      Thin surface treatment containing single-sized, high-quality, angular aggregates        Typically exceeds 0.04
            of Existing AC                          (0.38 to 0.63 in [9.5 to 15 mm]), spread over and rolled into a liquid asphalt or       in (1 mm).
               Pavement                             asphalt emulsion binder. Aggregates are sometimes pre-coated with asphalt
                                                    emulsion prior to spreading. Completed surface is somewhat coarse, yielding good
                                                    frictional characteristics.
                                  Slurry Seal       Slurry mixtures of fine aggregate, mineral filler, and asphalt emulsion. They are       Typically range from
                                                    similar to micro-surfacing, without interlocking aggregates. Polymers are not           0.01 to 0.025 in (0.3 to
                                                    always used in the emulsion. Their surface is typically gritty.                         0.6 mm).
                               Micro-Surfacing      A slurry mixture containing high-quality crushed, dense-graded aggregate,               Typically range from
                              (polymer-modified     mineral filler, and polymer-modified asphalt emulsion. It is placed over a tack coat 0.02 to 0.04 in (0.5 to 1
                                  slurry seal)      and is capable of being spread in variable thickness layers for rut-filling, correction mm).
                                                    courses, and wearing course applications.
                                HMA Overlay         See HMA surface mixes above.
                             Ultra-Thin Polymer- Thin gap-graded asphalt surfaces placed using specialized equipment immediately Typically exceeds 0.04
                            Modified Asphalt (e.g., over a thick polymer-modified asphalt emulsion membrane. Following slight               in (1 mm).
                                  NovaChip)         compaction the surface provides a semi-porous texture.

                              Epoxied Synthetic     A very thin surface treatment consisting of a two-part polymer resin placed on an       Typically exceeds 0.06
                               Treatment (e.g.,     existing pavement and covered with a man-made aggregate of re-worked steel slag in (1.5 mm).
                                    Italgrip)       (0.12 to 0.16 in [3 to 4 mm]). The surface is designed to substantially improve the
                                                    frictional characteristics of pavements.
          Retexturing of        Micro-Milling       Milling equipment, consisting of a self-propelled machine with carbide teeth            Typically exceeds 0.04
           Existing AC                              mounted on a rotating drum, typically removes 0.75 to 1.25 in (19 to 32 mm) from in (1 mm)
            Pavement                                the asphalt surface. Spacing of cuts is approximately 0.2 in (5 mm) versus 0.62-in
                                                    (6-mm) cut of conventional cold-milling machines. Resulting surface has a fine,
                                                    smooth pattern that gives smoother ride.
     a   Based in part on FHWA, 1996; FHWA, 2005; Hanson and Prowell, 2004; Mockensturm, 2002; Wade et al., 2001; McNerney et al., 2000; HITEC, 2003;
         Gransberg and James, 2005; Yaron and Nesichi, 2005.
                                        Table 7. Concrete pavement surface mix types and texturing techniques.

                                 Mix/                                                                                                            Macro-texture
          Application        Texture Type                                             Description                                                   Deptha
         New PCC or PCC        Broom Drag      A long-bristled broom is mechanically or manually dragged over the concrete surface in        Typically ranges from
            Overlay          (longitudinal or  either the longitudinal or transverse direction. Texture properties are controlled by         0.008 to 0.016 in (0.2 to
                                transverse)    adjusting the broom angle, bristle properties (length, strength, density), and delay          0.4 mm).
                                               behind the paver. Uniform striations approximately 0.06 to 0.12 in (1.5 to 3.0 mm) deep
                                               are produced by this method.
                             Artificial Turf   An inverted section of artificial turf is dragged longitudinally over a concrete surface      Typically ranges from
                           Drag (longitudinal) following placement. Texture properties are controlled by raising/lowering the support        0.008 to 0.016 in (0.2 to
                                               boom, adding weight to the turf, and delaying application to allow surface hardening.         0.4 mm), but a deep
                                               This method produces uniform 0.06 to 0.12 in (1.5 to 3.0 mm) deep surface striations.         texture (min depth of
                                                                                                                                             0.04 in [1.0 mm]) has
                                                                                                                                             been specifiedb.
                               Burlap Drag      One or two layers of moistened coarse burlap sheeting are dragged over the concrete          Typically ranges from
                              (longitudinal)    surface following placement. Texture properties are controlled by raising/lowering the       0.008 to 0.016 in (0.2 to
                                                support boom and adjusting the delay following concrete placement. This method               0.4 mm).
                                                produces uniform 0.06 to 0.12 in (1.5 to 3.0 mm) deep striations in the surface.
                           Longitudinal Tine A mechanical assembly drags a wire comb of tines (~ 5 in [127 mm] long and 10 ft [3 m]          Typically ranges from

                                                wide) behind the paver (and usually following a burlap or turf drag). Texture properties     0.015 to 0.04 in (0.4 to
                                                are controlled by the tine angle, tine length, tine spacing, and delay for surface curing.   1.0 mm).
                                                Grooves from 0.12 to 0.25 in (3 to 6 mm) deep and 0.12 in (3 mm) wide are produced by
                                                this method, typically spaced at 0.75 in (19 mm).
                            Transverse Tine Accomplished using methods similar to longitudinal tining, however, the mechanical               Typically ranges from
                                                assembly drags the wire comb perpendicular to the paving direction. Variations include       0.015 to 0.04 in (0.4 to
                                                skewing the tines 9 to 14° from perpendicular and using random or uniform tine spacing       1.0 mm).
                                                from 0.5 to 1.5 in (12 to 38 mm).
     a   Based in part on Hoerner et al., 2003; Hoerner and Smith, 2002; FHWA, 1996; FHWA, 2005.
     b   Minnesota Department of Transportation.
                                        Table 7. Concrete pavement surface mix types and texturing techniques.

                                  Mix/                                                                                                           Macro-texture
          Application         Texture Type                                               Description                                                Deptha
         New PCC or PCC     Diamond Grinding      A self-propelled grinding machine with a grinding head of gang-mounted diamond sawing         Typically ranges
            Overlay           (longitudinal)      blades removes 0.12 to 0.75 in (3 to 19 mm) of cured concrete surface, leaving a corduroy-    from 0.03 to 0.05
                                                  type surface. Blades are typically 0.08 to 0.16 in (2 to 4 mm) wide and spaced 0.18 to 0.25   in (0.7 to 1.2 mm).
                                                  in (4.5 to 6 mm) apart, leaving 0.08 to 0.16 in (2 to 4 mm) high ridges. This method is
                                                  most commonly used to restore surface characteristics of existing pavements, however, in
                                                  recent years, it has been used to enhance the surface qualities of new PCC pavements or
                                                  PCC overlays.
                                Porous PCC        Gap-graded, small-diameter aggregate are combined with cement, polymers, and water to         Typically exceeds
                                                  form a drainable surface layer (typically 8 in [200 mm] thick). That surface layer is         0.04 in (1 mm).
                                                  bonded to the underlying wet or dry dense concrete layer. Texture properties are
                                                  controlled by aggregate sizes and gradations. Air voids range from 15 to 25 percent.
                            Exposed Aggregate     A set retarder is applied to the wet concrete surface and the surface is protected for        Typically exceeds
                                  PCC             curing. After 12 to 24 hours, the unset mortar is removed to a depth of 0.04 to 0.08 in (1    0.035 in (0.9 mm).
                                                  to 2 mm) using a power broom. The large diameter aggregate is exposed by this process
                                                  leaving a uniform surface.
             Friction          HMA Overlay        See HMA surface mixes above.

          Restoration of
          Existing PCC
          Retexturing of    Diamond Grinding      See diamond grinding above.
          Existing PCC        (longitudinal)
            Pavement          Longitudinal        A self-propelled grooving machine saws longitudinal grooves in the road surface about       Typically ranges
                            Diamond Grooving      0.12 to 0.25 in (3 to 6 mm) deep and spaced 0.5 to 1.5 (13 to 38 mm) apart. This method     from 0.035 to
                                                  adds macro-texture for drainage but relies on the original surface for micro-texture.       0.055 in (0.9 to 1.4
                            Transverse Diamond Completed in a manner similar to longitudinal diamond grooving, except the grooves are         Typically ranges
                                 Grooving          sawn transverse to the travel direction. This method also adds macro-texture and positive from 0.035 to
                                                   drainage for surface water. It relies on the original surface for micro-texture.           0.055 in (0.9 to 1.4
                               Shot Abrading       An automated machine hurls recycled round steel abrasive material at the pavement          Typically ranges
                                                   surface, abrading the surface and/or removing the mortar and sand particles surrounding from 0.025 to 0.05
                                                   the coarse aggregate to a depth of up to 0.25 in (6 mm). Texture properties are controlled in (0.6 to 1.2 mm).
                                                   by adjusting the steel abrasive material velocity and approach angle and by modifying the
                                                   forward equipment speed.
     a   Based in part on Hoerner et al., 2003; Hoerner and Smith, 2002; FHWA, 1996; FHWA, 2005; HITEC, 2003; Rao et al., 1999.
     b   Other treatments, such as micro-surfacing, ultra-thin polymer-modified asphalt, epoxy-bonded laminates, and thin-bonded PCC overlays, have been used
         but often have structural performance and/or cost issues.
 1            Aggregate            Agency Aggregate  Agency Mix Types/                Friction Design
 2             Sources             Testing Protocol Texturing Techniques                 Category
 5              Source A                                                                  Category I
              (high polish)                                                              Low demand
 6                                                                                  α ≤ Design Friction < β
 8                                   Test   Criteria                          A–X         Category II
 9                                    1      >5                               A–Y      Moderate demand
                                                       Mix/Texture Type             β ≤ Design Friction < χ)
10             Source B
                                      2      < 25
                                                       X (low macro-t)        C–X
                                      3      > 50
11          (moderate polish)         .        .       Y (moderate macro-t)   B–Y
12                                    .        .       Z (high macro-t)

13                                    .        .
                                      .        .
14                                                                            C–Z          Category III
15                                                                                        High demand
                                                                                        Design Friction > χ
16              Source C
              (low polish)
18                                           Agg. Source–Texture Options
21     Figure 18. Example illustration of matching aggregate sources and mix types/texturing
22                              techniques to meet friction demand.
25   Once the design categories have been set, aggregate test protocols and mix/texture type
26   options can be developed for each category, along with design friction levels. The test
27   protocol should list the specific tests to be performed and the criteria/parameters to be used.
28   The criteria should be based on established links between historical friction performance
29   and laboratory test data.
34   This section provides information on state-of-the-practice for providing friction on new and
35   restored asphalt and concrete pavements. Although safety over the established pavement
36   design life is the paramount concern, the design process should target a surface that most
37   economically satisfies the following criteria:
39      •    Adequate levels of micro-texture over the life of the pavement, as produced by sharp,
40           gritty aggregate with low polish and high wear-resistance characteristics.
41      •    Adequate levels of macro-texture over the life of the pavement for efficient
42           displacement of water on the pavement surface.
43      •    Low levels of splash/spray, noise generation, glare, tire wear, and rolling resistance.
45   Project-level friction design entails selecting aggregates and mix types/texturing techniques
46   that satisfy both initial and long-term friction requirements. A five-step process for
47   designing surfaces for new asphalt or concrete pavement, as well as restoration treatments
48   of existing asphalt or concrete pavement, is as follows:

 2      1.   Determine design friction level.
 3      2.   Select aggregates.
 4      3.   Establish surface mix types and/or texturing techniques.
 5      4.   Develop construction specifications.
 6      5.   Formulate design strategies.
 8   These design steps are described in detail in the sections below.
10   4.3.1 Determining Design Friction Level (Step 1)
12   For each new construction or restoration project, a design friction level (expressed as F(60)
13   if IFI is used or as FN) must be selected to satisfy agency policy requirements. The selected
14   design level must ensure that adequate amounts of micro-texture and macro-texture are
15   available throughout the design period.
17   The selected design level should take into consideration the design levels of individual PFM
18   sections. Either one overall level can be established for the project corresponding to the
19   PFM section with the highest demand, or multiple levels can be used. In the latter case,
20   care must be taken such that the multiple levels do not result in an excessive number of
21   mix types and/or surface textures to be used along the project.
23   Once an agency sets the goal for friction for a particular project, the process of selecting
24   aggregates and mix types/texturing techniques that satisfy the design friction level can
25   begin. An initial list of aggregate source–texture options can be derived from the feasible
26   combinations identified previously for each design category (e.g., B-X, A-X, and A-Y for
27   design category I in figure 18). These, and other potential combinations, can be evaluated
28   more thoroughly for adequacy using the IFI model, as described below in step 3.
30   4.3.2 Selecting Aggregates (Step 2)
32   The most important factor in achieving long-lasting friction is aggregate selection.
33   Aggregates should have the physical, chemical, and mechanical properties needed to satisfy
34   both initial friction design levels and long-term friction requirements.
36   Aggregates must comply with the testing requirements established by the agency.
37   Aggregate samples should be tested early in a project to determine their suitability and
38   compliance with specifications. Frequently, two or more aggregate sources must be
39   combined in appropriate percentages to meet project gradation requirements.
41   As stated earlier, aggregates comprised of a matrix of both hard and soft minerals will
42   ensure friction durability. Aggregates not meeting the specified test parameters should be
43   rejected (prior to any mix design effort) and either new materials should be considered and
44   tested or a suitable blend of high- and low-polish susceptible aggregates should be
45   identified.
47   Micro-texture in asphalt surface mixes is provided by the coarse aggregate surface texture.
48   Coarse aggregates that exhibit “rough sandpaper” surface textures provide higher levels of
49   micro-texture than those with smooth “fine sandpaper” textures.

 2   Micro-texture in concrete surfaces is generally provided by the fine aggregates in the
 3   cement mortar/paste (for concrete mixes with exposed aggregates, the surface properties of
 4   the coarse aggregate will dictate micro-texture). Fine aggregates that exhibit angular
 5   edges and cubical or irregular shapes generally provide higher levels of micro-texture,
 6   whereas those with rounded edges or elongated shapes generally produce lower micro-
 7   texture.
 9   4.3.3 Establishing Surface Mix Types and/or Texturing Techniques (Step 3)
11   Framework for Achieving Design Friction Level
13   As discussed earlier in step 1, potential combinations of aggregate source and mix type/
14   texturing technique can be evaluated in detail using the IFI model (equations 6 through 9).
15   Using DFT(20) as a surrogate for micro-texture and the CTM to get MPD, FR(S) in
16   equation 8 can be set to DFT(20) at S equal to 20 km/hr. Furthermore, substituting
17   equation 6 into equation 8, one gets the following:
18                                                                    20 − 60
19                                  FR(60) = DFT (20) × e
                                                                14.2 + 89.7× MPD
                                                                                                                 Eq. 21
21   Inserting equation 21 into equation 9, adding in the A, B, and C calibration constants
22   (0.081, 0.732, and 0, respectively) for DFT(20) as given in ASTM E 1960, and re-arranging
23   to solve for DFT(20), the following equation is obtained:
24                                                                                                60 − 20
25                    DFT (20) = [(F (60) − 0.081 − 0 × MPD ) / 0.732] × e                                       Eq. 22
                                                                                        (                    )
                                                                                            14.2 + 89.7× MPD
27   Figure 19 is a plot of the above equation. As an example application, consider a project
28   where it is desired that a locked-wheel smooth-tire friction test give a friction number of 40
29   at a speed limit of 60 km/hr. Then F(60) becomes is 40 and equation 22 becomes as follows:
30                                                                                   40
31                           DFT ( 20) = [(40 − 0.081) / 0.732] × e                                              Eq. 23
                                                                         (                    )
                                                                             14.2 + 89.7× MPD
33   To achieve the design friction level of 40, the pairs of DFT(20) and MPD given in table 8 are
34   needed. The first pair includes a rather high DFT(20) and the last two pairs include high
35   MPD values. Therefore, the second and third pairs containing MPD values of 0.813 and
36   1.524 mm would need to be selected to give the F(60) or FN needed.
38   If the polishing characteristics have been measured or are already known, higher levels of
39   micro-texture and/or macro-texture should be selected to meet the required levels at the
40   end of the design life. For example, if the polished DFT(20) (i.e., PSV) and the MPD are
41   satisfactory, then the initial DFT(20) from the test would need to be specified. If the
42   polished DFT(20) is too low and thus requires a MPD that is too high to meet, then a higher
43   DFT(20) or different aggregate is needed to get the required polished DFT(20) at the end of
44   the design life.

 2                    160
 4                    140
 6                    120
 8                    100
                                                                                                MPD, mm

10                     80
11                                                                                                     0.813

12                     60                                                                              1.524

13                                                                                                     2.921
14                     40                                                                              4.343
16                     20
18                      0
                            20   25           30    35          40           45    50     55
21                                                  F(60) or FN
23     Figure 19. Example of determining DFT(20) and MPD needed to achieve design friction
24                                           level.
28        Table 8. Pairs of MPD and DFT(20) needed to achieve design friction level of 40.
                      MPD, mm         0.457        0.813             1.524        2.921        4.343
                      DFT(20)         112.5         86.3              71.1         63.0         60.2
32   This method is then a guide for evaluating the levels of micro-texture (DFT(20)) and macro-
33   texture (MPD) needed to achieve the design friction level established for a project. It can be
34   used directly in identifying a suitable combination(s) of aggregate and mix type/texturing
35   technique for a project or it can serve as a framework for agencies interested in developing
36   their own customized procedure. It should also be noted that a similar process utilizing the
37   combination of BPN (micro-texture) and MTD (macro-texture) could be established and
38   used.
40   During the mix design stage of an asphalt project, there may become the need to “fine-tune”
41   the gradation of a mix to satisfy the friction design requirement. A method for doing this
42   was developed by Sullivan (2005). This method, illustrated in figure 20, uses PSV and
43   MPD to compute IFI (as given in ASTM E 1960) and subsequently determine the design
44   vehicle stopping distance. Figure 21 shows an example vehicle response chart for a selected
45   speed of 50 mi/hr (80 km/hr).

29     Figure 20. Flowchart illustration of asphalt pavement friction design methodology
30                                      (Sullivan, 2005).
32                                                   350

               Design Vehicle Stopping Distance, m

34                                                   300

39                                                                             MPD = 0.2 mm
41                                                   100
                                                                                 0.4 mm

42                                                                         1.2 mm    0.65 mm

43                                                    50                             2.0 mm
45                                                    0
46                                                     10   20   30       40        50        60   70     80   90   100

47                                                                    Polished Aggregate Friction Value
48   Figure 21. Illustration of vehicle response as function of PSV and MPD (Sullivan, 2005).

 2   The Sullivan method uses an equation for computing the MPD based on key asphalt mix
 3   characteristics (maximum aggregate size, gradation, binder content). While historical data
 4   on asphalt surface mix textures can be used in this process, the MPD equation (derived
 5   using comprehensive mix design and surface texture data from the NCAT test track) gives
 6   the mix designer greater flexibility in establishing a mix design that will meet friction
 7   requirements.
 9   Although a similar process for conventional concrete mixes could be developed, it is not as
10   important, since the macro-texture is designed separately from the micro-texture.
11   However, agencies are encouraged to quantify the macro-texture (MPD or MTD) of both
12   newly applied and in-service surface texturings (e.g., tined, grooved, or ground surfaces
13   with different groove dimensions, spacings, and orientations), so as to ensure the right
14   supplement for the chosen fine aggregate,
16   Asphalt Mix Design
18   Mix design requirements for asphalt mixes may vary depending on the nature of the mix
19   type (dense-graded, OGFC, SMA, etc.). The standard agency procedures for mix design
20   should be followed for a given mix type and design requirement.
22   During the mix design process, the agency must ascertain the aggregate micro-texture,
23   either through aggregate source historical PSV test data or through testing of the chosen
24   aggregate. The agency must also ascertain the expected in-place macro-texture of the mix,
25   so that a determination can be made as to whether the mix will meet the friction design
26   requirements. Historical mean texture/profile depths (MTD/MPDs), theoretical
27   MTD/MPDs using established relationships, or laboratory-derived MTD/MPDs (using
28   molded samples and performing texture tests) are all possible means for identifying the mix
29   macro-texture.
31   Concrete Mix Design and Texturing Selection
33   The strength/abrasion properties of the cement mortar/paste largely determine the wearing
34   characteristics of new concrete surfaces, while the coarse aggregate polishing
35   characteristics define restored concrete macro-texture durability. Increasing the cement
36   content (or decreasing the water cement ratio) and implementing sound construction
37   practices maximizes cement paste/mortar strength and, thus, abrasion resistance.
39   Concrete surface macro-texture is determined by the texturing type applied. Designers
40   must select feasible surface types that meet macro-texture design requirements. Extensive
41   recommendations for applying the finishing methods listed in table 7 have been presented
42   in several references, including FHWA Technical Advisory T 5040.36 (FHWA, 2005). The
43   recommendations provided can be used to enhance macro-texture for the texturing method
44   selected.
46   4.3.4 Development of Construction Specifications (Step 4)
48   All agencies have standard specifications for construction of pavement surfaces that provide
49   guidance on requirements for aggregates, mixes, handling, placement, compaction, curing,

 1   and protection of new surfaces. For some agencies, these specifications do not specifically
 2   address friction properties of the wearing surface. To ensure quality friction on new or
 3   rehabilitated pavement surfaces, requirements for aggregate properties and test methods
 4   presented in this section may be included in project specifications, as needed.
 6   Special Provisions
 8   Each project has unique requirements because of the design and construction constraints
 9   and special demands. Items such as aggregate blending, noise mitigation, and QA should
10   be clarified in the special provisions of the construction documents and specifications.
12   Blending
14   Frequently, aggregates from two or more sources must be blended to meet the specification
15   limits. Several studies have reported that the blended aggregate properties tend to be the
16   same as the weighted average of the properties of the individual aggregates (Liang, 2003).
17   Thus, the goal of blending aggregate is to set the percentages of each aggregate used such
18   that the final blend has properties that lies within the specification limits of the tests to be
19   performed.
21   Quality Assurance
23   Among other things, a QA program often stipulates the frequency of testing aggregate
24   sources. While no specific guidance on the extent and frequency of testing is provided in
25   this Guide, it is strongly suggested that an aggregate source be tested extensively whenever
26   substantially new aggregate deposits are to be used for pavement surfacing. The extent
27   and frequency can be reduced as the agency becomes more familiar with the aggregate
28   source and there is a history of performance for aggregates from the given source (Folliard
29   and Smith, 2003).
31   Construction Issues
33   Construction deficiencies and poor construction practices can contribute to inadequate
34   friction. Construction issues involve control of aggregate and mix quality during
35   production, handling, stockpiling, mixing, placing, and finishing. Friction restoration
36   treatments in particular, such as chip seals, slurry seals, micro-surfacing, and proprietary
37   surfaces, are susceptible to providing less than expected friction, if poor construction
38   practices are employed.
40   4.3.5 Formulation of Design Strategies (Step 5)
42   Both monetary and non-monetary factors are considered in selecting preferred pavement
43   design strategy the various feasible alternatives. The main inputs required are (a)
44   estimates of costs, (b) estimates of benefits (if the benefit cost option is selected, not that
45   benefit cost analysis is required only if there is a significant difference in benefits between
46   alternatives, and (c) non-monetary factors.

 1   Important cost elements related to the inclusion of surface friction in the design strategy
 2   are:
 4      •   Agency costs.
 5             Additional design and engineering costs.
 6             Aggregate materials with required frictional properties.
 7             Additives, including polymers, to improve surface properties and performance.
 8             Frequency/duration of restoration activities.
 9                Design strategies involving frequent M&R are typically more costly overall
10                because of the effects of highway user delay costs, traffic control, and so on.
11                Timing of M&R can significantly escalate costs if M&R to restore surface
12                friction does not coincide with M&R to restore structural capacity.
14      •   User costs
15             Travel delays (time/delay) for friction restoration impact life cycle cost.
16             Friction can adversely influence pavement–tire factors such as tire wear, rolling
17             resistance, and fuel consumption.
18             Safety associated factors that impact crash costs.
19                 Frequency of crashes.
20                 Value of crashes.
22   Benefits from ensuring adequate levels of friction throughout the pavement life are
23   quantified through:
25      •   Improved highway safety (i.e., reduction in crash costs).
26             Value of lives saved.
27             Value of injuries avoided (medical, loss income, psychological damage).
28             Savings in pain and suffering of crash victims and their families due to a
29             reduction in crashes.
30             Reductions in property damage due to reduction in crashes.
32   Non-monetary factors can be included in the decision matrix and addressed through (a)
33   agency policies and criteria on these factors and (b) appropriate weights to these factors to
34   reflect the importance assigned to them by the agency. The non-monetary design
35   considerations include (AASHTO, 1993):
37      •   Service life.
38      •   Duration of construction.
39      •   Traffic control problems.
40      •   Reliability, constructability, and maintainability of design.
42   Non-monetary considerations associated with pavement friction include:
44      •   Pavement–tire noise.
45      •   Splash and spray.
46      •   Fuel consumption/rolling resistance.
47      •   Tire wear.
48      •   Reflectance and glare.

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36   Administration, Washington, D.C.
38   Horne, W.B. and F. Buhlmann. 1983. “A Method for Rating the Skid Resistance and
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17   Liang, R.Y. 2003. “Blending Proportions of High Skid and Low Skid Aggregate,” Final
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 4   ABS       Anti-lock Braking System
 5   AC        Asphalt Concrete
 6   ADT       Average Daily Traffic
 7   AIR       Acid Insoluble Residue test
 8   ASTM      American Society for Testing and Materials
 9   BPN       British Pendulum Number
10   BPT       British Pendulum Tester
11   CEN       European Committee for Standardization
12   CSA       Canadian Standards Association
13   CTM       Circular Texture Meter
14   DFT       Dynamic Friction Tester
15   ETD       Estimated Mean Texture Depth
16   FARS      Fatality Analysis Reporting System
17   FN        Friction Number
18   HMA       Hot-Mix Asphalt
19   HPS       (Critical) Hydroplaning Speed
20   ICM       Integrated Climatic Model
21   IFI       International Friction Index
22   ISO       International Standards Organization
23   IVS       Intelligent Vehicle System
24   LCCA      Life-Cycle Cost Analysis
25   MPD       Mean Profile Depth
26   MTD       Mean Texture Depth
27   MUTCD     Manual on Uniform Traffic Control Devices
28   M&R       Maintenance and Rehabilitation
29   OFM       Outflow Meter
30   OFT       Outflow Time
31   OGFC      Open-Graded Friction Courses
32   PCC       Portland Cement Concrete
33   PFM       Pavement Friction Management
34   PMS       Pavement Management System
35   PSV       Polished Stone Value
36   QA        Quality Assurance
37   QC        Quality Control
38   ROSANV    Road Surface Analyzer
39   SFT       Surface Friction Tester
40   SMA       Stone Matrix Asphalt
41   SPM       Sand Patch Method
42   SSD       Stopping Sight Distance
43   TDG       Texture Depth Gauge
44   WCR       Wet Crash Rate
45   WFT       Water Film Thickness
46   WSR       Wet Skidding Rate

 4   Adhesion—Frictional forces that result from the small-scale bonding/interlocking of the
 5   vehicle tire rubber and the pavement surface as they come in contact with each other.
 7   Anti-lock Braking System (ABS)—A collection of sensing and control hardware installed on
 8   a vehicle to prevent wheel lock-up during brake application.
10   Braking Force Coefficient—The ratio of tire braking force to normal force.
12   Braking Force Coefficient, Peak—The maximum value of tire braking force coefficient that
13   occurs prior to wheel lockup as the braking torque is progressively increased.
15   Braking Force Coefficient, Slide—The value of tire braking force coefficient obtained on a
16   locked wheel.
18   Coefficient of Friction—The ratio of the value of the tangential force between the tire tread
19   rubber and the horizontal traveled surface to the absolute value of normal force attainable
20   on a given traveled surface on a given rolling or locked wheel at specified test conditions.
22   Contact Area—The gross tire contact area that is loaded under static conditions against a
23   smooth flat surface.
25   Cornering Force—The horizontal force acting perpendicularly to the instantaneous motion
26   vector of the center of contact for a tire operating at a slip angle.
28   Critical Slip Angle—The value of the tire slip angle at the peak cornering force coefficient.
30   Friction Number (sometimes referred to as Skid Number)—The number that is used to
31   report the results of a pavement friction test conducted in accordance with ASTM Test
32   Method E 274; usually expressed as the friction coefficient multiplied by 100.
34   Hydroplaning—Phenomenon in which a vehicle tire is separated from the pavement surface
35   by the water pressure that builds up at the pavement–tire interface.
37   Hysteresis—Frictional forces resulting from the energy loss due to bulk deformation of the
38   vehicle tire.
40   International Friction Index (IFI)—Friction index defined by two parameters, a calibrated
41   friction value at 37 mi/hr (60 km/hr), F(60), and the speed gradient, SP.
43   Intervention Level—The point in a friction deterioration curve where an agency must either
44   take immediate corrective action, such as applying a restorative treatment, or provide
45   proper cautionary measures, such as posting “Slippery When Wet” signs and/or reduced
46   speed signs.

 1   Investigatory Level—The point in a friction deterioration curve where an agency should
 2   start more carefully monitoring the friction and/or crash levels at a particular site and
 3   begin the process of planning for some sort of restorative action.
 5   Pavement Friction Management (PFM)—A systematic approach to minimizing skid-crashes
 6   through friction and/or crash rate monitoring, timely application of friction restoration
 7   treatments, and utilization of good friction design and construction practices.
 9   Pavement–Tire Friction (or Pavement Friction)—The force that resists the relative motion
10   between a vehicle tire and a pavement surface. A measure of this resistive force is the non-
11   dimensional friction coefficient, µ.
13   Side-Force Friction—The friction that develops as a vehicle changes direction or
14   compensates for pavement cross-slope and/or wind effects.
16   Skid Resistance—The ability of the traveled surface to prevent the loss of tire traction.
18   Slip Angle—The angle between the X-axis and the direction of travel at the center of tire
19   contact.
21   Slip Speed—The difference between the speed of the axis of the measuring wheel, which is
22   equal to the traveling speed of the measuring device, and the tangential velocity of
23   measuring wheel with unloaded radius.
25   Splash—The large droplets of water that are thrown off the tire or squeezed out from
26   pavement–tire contact area. Splash is associated with large water depths or low vehicle
27   speeds.
29   Spray—The mist that is carried alongside and thrown behind a moving vehicle by the
30   turbulent airflow created by the vehicle, other nearby vehicles, and wind. Spray is
31   associated with shallow water depths or high vehicle speeds.
     A          Surface texture amplitude.
     BPN        British Pendulum Number
     CD         Coefficient of displacement drag.
     COV        Coefficient of variation.
     Ĉ          Shape factor in the Rado friction model (log normal).
     DFT        Dynamic Friction Test index.
     EMTD       Estimated mean texture depth.
     F          Friction force between vehicle tire and pavement surface.
     FA         Adhesion component of pavement friction.
     FH         Hysteresis component of pavement friction.

    FS         Side-force friction.
    FW         Applied vertical force on a wheel axle, equal to the device mass multiplied by the gravity
               constant, or a controlled vertically applied force.
    Fx         Brake force.
    Fy         Lateral force.
    FN(##)     Friction number
    FN##R,     Friction number/skid number determined at ## mi/hr using a ribbed tire.
    FN##S,     Friction number/skid number determined at ## mi/hr using a smooth tire.
    FN(##)R,   Friction number/skid number determined at ## km/hr using a ribbed tire.
    FN(##)S,   Friction number/skid number determined at ## km/hr using a smooth tire.
    F(60)      Friction number of the International Friction Index (IFI).
    HPS        Critical hydroplaning speed.
    IR         Excess rainfall (rainfall intensity minus pavement surface permeability).
    IFI        International Friction Index.
    L          Drainage path length.
    M          Mass of vehicle.
    MPD        Mean profile depth.
    MTD        Mean texture depth.
    P          Centripetal force (horizontal).
    R          Radius of curvature of highway curve.
    S          Slip speed.
    SMAX       Critical slip speed value in Rado friction model (log normal friction model).
    SP         Speed number of the International Friction Index (IFI).
    V          Travel speed.
    VP         Average peripheral speed of tire.
    W          Weight of vehicle
    WFT        Water film thickness.
    e          Pavement super-elevation
    i          Rainfall intensity.
    m          Mass of vehicle.
    n          Manning’s roughness coefficient.
    r          Average tire radius.
    α          Angle of super-elevation.
    λ          Surface texture wavelength.
    μ          1) Friction coefficient as the ratio of a horizontal force to a vertical force in the pavement–tire
                   contact area.
               2) A reported friction value.
    μmax       Maximum friction value.
    ω          Angular velocity of tire.

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    AASHTO TP 61       Standard Method of Test for Determining the Percentage of Fracture in Coarse Aggregate
    AASHTO T 96        Standard Method of Test for Resistance to Degradation of Small-Size Coarse Aggregate by
                       Abrasion and Impact in the Los Angeles Machine
    AASHTO T 104       Standard Method of Test for Soundness of Aggregate by Use of Sodium Sulfate or
                       Magnesium Sulfate
    AASHTO T 261       Standard Method of Test for Measuring Texture Depth of Portland Cement Concrete Using
                       a Tire Tread Depth Gauge
    AASHTO T 278       Standard Method of Test for Surface Frictional Properties Using the British Pendulum
    AASHTO T 279       Standard Method of Test for Accelerated Polishing of Aggregates Using the British Wheel
    AASHTO T 304       Standard Method of Test for Uncompacted Void Content of Fine Aggregate
    AASHTO T 327       Standard Method of Test for Resistance of Coarse Aggregate to Degradation by Abrasion in
    (formerly TP 58)   the Micro-Deval Apparatus
    AASHTO T 326       Standard Method of Test for Uncompacted Void Content of Coarse Aggregates (as
    (formerly TP 56)   influenced by particle shape, surface texture, and grading)
    ASTM C 88          Standard Test Method for Soundness of Aggregate of Use of Sodium Sulfate or Magnesium
    ASTM C 131         Standard Test Method for Resistance to Degradation of Small-Size Coarse Aggregate by
                       Abrasion and Impact in the Los Angeles Machine
    ASTM C 294         Standard Descriptive Nomenclature for Constituents of Concrete Aggregates
    ASTM C 295         Standard Guide for Petrographic Examination of Aggregates for Concrete
    ASTM C 535         Standard Test Method for Resistance to Degradation of Large-Size Coarse Aggregate by
                       Abrasion and Impact in the Los Angeles Machine
    ASTM C 1252        Standard Test Method for Uncompacted Void Content of Fine Aggregate (as Influenced by
                       Particle Shape, Surface Texture, and Grading)
    ASTM D 1155        Standard Test Method for Roundness of Glass Spheres (for Sand Patch Method)
    ASTM D 3042        Standard Test Method for Insoluble Residue in Carbonate Aggregates
    ASTM D 3319        Standard Practice for the Accelerated Polishing of Aggregates Using the British Wheel
    ASTM D 5821        Standard Test Method for Determining the Percentage of Fractured Particles in Coarse
    ASTM D 6928        Standard Test Method for Resistance of Coarse Aggregate to Degradation by Abrasion in
                       the Micro-Deval Apparatus
    ASTM E 274         Standard Test Method for Skid Resistance of Paved Surfaces Using a Full-Scale Tire
    ASTM E 303         Standard Test Method for Measuring Surface Frictional Properties Using the British
                       Pendulum Tester
    ASTM E 501         Standard Specification for Standard Rib Tire for Pavement Skid-Resistance Tests
    ASTM E 524         Standard Specification for Standard Smooth Tire for Pavement Skid-Resistance Tests
    ASTM E 670         Standard Test Method for Side Force Friction on Paved Surfaces Using the Mu-Meter
    ASTM E 965         Standard Test Method for Measuring Pavement Macro-texture Depth Using a Volumetric

    ASTM E 1845        Standard Practice for Calculating Pavement Macro-texture Mean Profile Depth (Laser
                       Profiler Method)

    ASTM E 1911     Standard Test Method for Measuring Paved Surface Frictional Properties Using the
                    Dynamic Friction Tester
    ASTM E 1960     Standard Practice for Calculating International Friction Index of a Pavement Surface
    ASTM E 2157     Standard Test Method for Measuring Pavement Macro-texture Properties Using the
                    Circular Track Meter
    ASTM E 2341     Standard Test Method for Determining the Stopping Distance Number by Initia1 Speed
                    and Stopping Distance at Traffic Incident Site
    ASTM E 2380     Standard Test Method for Measuring Pavement Texture Drainage Using an Outflow Meter
    ASTM WK 364
    CSA A23.2-23A   Resistance of Fine Aggregate to Degradation by Abrasion in the Micro-Deval Apparatus



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