GUIDE FOR PAVEMENT FRICTION
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
24 3.1 DEVELOPING PAVEMENT FRICTION MANAGEMENT POLICIES ............. 22
25 3.2 ESTABLISHING THE PAVEMENT FRICTION MANAGEMENT PROGRAM .. 24
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
34 APPENDIX A. TERMINOLOGY
36 APPENDIX B. STANDARDS RELEVANT TO PAVEMENT
1 LIST OF FIGURES
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
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
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.
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
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.
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1 CHAPTER 1. INTRODUCTION
4 1.1 BACKGROUND
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
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.
41 1.2 PURPOSE AND SCOPE OF GUIDE
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.
23 1.3 GUIDE ORGANIZATION AND USE
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.
1 CHAPTER 2. PAVEMENT FRICTION OVERVIEW
4 2.1. IMPORTANCE OF PAVEMENT FRICTION
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.
Total crashes, millions
47 Figure 1. Total crashes (from all vehicles types) on U.S. highways from 1990 to 2003
48 (NHTSA, 2004).
Total Fatalities, thousands
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
total number of crashes per 100
Mean Crash Risk, % (defined as
million vehicle km driven)
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.
8 2.2 PAVEMENT FRICTION PRINCIPLES
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).
20 μ= Eq. 1
23 Weight, FW
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.
29 of Friction Peak friction
40 Critical slip
43 0 100
(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
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
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
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
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
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.
32 Short stretch
33 of road
35 Amplification ca. 50 times
38 Amplification ca. 5 times
41 Contact Area
Amplification ca. 5 times
46 Figure 9. Simplified illustration of the various texture ranges that exist for a given
47 pavement surface (Sandburg, 1998).
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
9 Ext. Noise
11 Int. Noise
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.
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
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
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
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).
C = Shape factor which is closely related to the speed number SP in the
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
Rado Model(μmax,SMAX, C ) ≈ IFI(F(60),SP)
33 F(60) SP
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
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,
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
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
1 CHAPTER 3. PAVEMENT FRICTION MANAGEMENT
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.
1 3.1 DEVELOPING PAVEMENT FRICTION MANAGEMENT POLICIES
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
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
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
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
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
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
25 Yes Yes
28 Are Wet
34 Perform Detailed Site
Does Site Need
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.
4 3.2 ESTABLISHING THE PAVEMENT FRICTION MANAGEMENT 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
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).
5 Impending Skid
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.
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
can be used to adjust friction measurements to FN40:
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
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.
F(60) ,Sp or FN
34 Investigatory Level
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.
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
2 Inadequate Marginal Adequate
3 Macro-texture Macro-texture Macro-texture
inal F LEGEND
n/Mic Texture Investigatory Level
Texture Intervention Level
15 Friction Investigatory Level
16 Friction Intervention Level
19 te Fr
20 r o-tex
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-
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
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
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
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1 CHAPTER 4. PAVEMENT FRICTION DESIGN
4 4.1 INTRODUCTION
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
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
27 4.2 DEVELOPING FRICTION DESIGN POLICIES
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.
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.
Test Name Test Protocol Test Description Applications
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
Table 4. Test methods for characterizing aggregate frictional properties (continued).
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
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
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.
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
b Formerly AASHTO TP 56.
Table 4. Test methods for characterizing aggregate frictional properties (continued).
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.
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
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
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
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.
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
13 Table 5. Typical range of test values for aggregate properties.
Aggregate Aggregate Typical Property Range for
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
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
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
LA Abrasion, % Loss ≤ 35 to 45
Fine Acid Insoluble Residue (AIR), % ≥ 50 to 70
AIR, % ≥ 50 to 70
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
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
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.
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
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).
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
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.
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
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.
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
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.
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
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.
32 4.3 PROJECT-LEVEL DESIGN GUIDELINES
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
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-
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
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:
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.
12 60 1.524
14 40 4.343
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
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
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).
Design Vehicle Stopping Distance, m
39 MPD = 0.2 mm
42 1.2 mm 0.65 mm
43 50 2.0 mm
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
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
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
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.
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
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
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.
4 American Association of State Highway and Transportation Officials (AASHTO). 1976.
5 Guidelines for Skid Resistant Pavement Design, Prepared by the Task Force for Pavement
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11 American Association of State Highway and Transportation Officials (AASHTO). 2001. “A
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16 Factors,” Research Report KTC-96-13, University of Kentucky, Kentucky Transportation
17 Cabinet, Lexington, Kentucky.
19 Anderson, D.A., R.S. Huebner, J.R. Reed, J.C. Warner, and J.J. Henry. 1998. Improved
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21 Research Foundation (NCHRP), Washington, D.C.
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24 Austroads Publication No. AP-G83/05, Austroads Incorporated, Sydney, Australia.
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27 Pavement Properties Related to Friction and Wear Resistance, Report No. FHWA-RD-78-
28 209, Federal Highway Administration (FHWA), Washington, D.C.
30 Federal Highway Administration (FHWA). 1979. “Texturing and Skid Resistance of
31 Concrete Pavements and Bridge Decks,” Technical Advisory T 5140.10, U.S. Department of
32 Transportation, FHWA, Washington, D.C.
34 Federal Highway Administration (FHWA). 1980. “Skid Accident Reduction Program,”
35 Technical Advisory T 5040.17, U.S. Department of Transportation, FHWA, Washington,
38 Federal Highway Administration (FHWA). 1996. “PCC Surface Texture Technical
39 Working Group Findings,” FHWA, Washington, D.C.
41 Federal Highway Administration (FHWA). 2005. “Surface Texture for Asphalt and
42 Concrete Pavements,” Technical Advisory T 5040.36, U.S. Department of Transportation,
43 Federal Highway Administration, Washington, D.C.
45 Flintsch, G.W., E. de Leon, K.K. McGhee, and I.L. Al-Qadi. 2003. “Pavement Surface
46 Macro-texture Measurement and Application, Paper presented at 82nd Annual Meeting of
47 the Transportation Research Board, Washington, D.C.
1 Folliard, K.J. and K.D. Smith. 2003. “Aggregate Tests for Portland Cement Concrete
2 Pavements: Review and Recommendations,” September edition (No. 281) of NCHRP
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4 Washington, D.C.
6 Glennon, J.C. 1996. “Roadway Defects and Tort Liability,” Lawyers & Judges Publishing
7 Company, Inc., Tucson, Arizona.
9 Gransberg, D. and D.M.B. James. 2005. “Chip Seal Best Practices,” NCHRP Synthesis
10 342, National Cooperative Highway Research Program (NCHRP), Washington, D.C.
12 Hanson, D.I. and B.D. Prowell. 2004. “Evaluation of Circular Texture Meter for Measuring
13 Surface Texture of Pavements,” NCAT Report No. 04-05, National Center for Asphalt
14 Technology (NCAT), Auburn, Alabama.
16 Henry, J.J. 1983. “Comparison of Friction Performance of a Passenger Tire and the ASTM
17 Standard Test Tires, ASTM STP 793, ASTM, Philadelphia, Pennsylvania.
19 Henry, J.J. 2000. “Evaluation of Pavement Friction Characteristics—A Synthesis of
20 Highway Practice,” NCHRP Synthesis 291, Transportation Research Board, Washington,
23 Highways Agency (HA). 2005. Design Manual for Roads and Bridges, Volume 7, HD 28,
24 “Skid Resistance,” Department for Transport, London, United Kingdom.
26 Highway Innovative Technology Evaluation Center (HITEC). 2003. Evaluations—Italgrip
27 System, Civil Engineering Research Foundation (CERF) HITEC website
28 (www.cerf.org/hitec/evals.htm), Washington D.C.
30 Hoerner, T.E., K.D. Smith, R.M. Larson, and M.E. Swanlund. 2003. “Current Practice of
31 PCC Pavement Texturing,” Paper presented at 82nd Annual Meeting of the Transportation
32 Research Board, Washington, D.C.
34 Hoerner, T.E. and K.D. Smith. 2002. High Performance Concrete Pavement: Pavement
35 Texturing and Tire-Pavement Noise, FHWA Report No. FHWA-IF-02-020, Federal Highway
36 Administration, Washington, D.C.
38 Horne, W.B. and F. Buhlmann. 1983. “A Method for Rating the Skid Resistance and
39 Micro/Macro-texture Characteristics of Wet Pavements,” ASTM International STP No. 793,
40 W. E Meyers and J. D. Walter, Editors American Society for testing and materials,
41 Philadelphia, Pennsylvania.
43 Ivey, D.L., L.I. Griffin III, J.R. Lock, and D.L. Bullard. 1992. “Texas Skid Initiated
44 Accident Reduction Program,” Final Report, Research Report 910-1F, TTI: 2-18-89/910, TX-
45 92/910-1F, Texas Department of Transportation, Austin, Texas.
1 Kandhal, P.S., F. Parker Jr., and R.B. Mallick. 1997. Aggregate Tests for Hot Mix Asphalt:
2 State of the Practice, NCAT Report No. 97-6, National Center for Asphalt Technology
3 (NCAT), Auburn, Alabama.
5 Kandhal, P.S. and F. Parker Jr. 1998. “Aggregate Tests Related to Asphalt Concrete
6 Performance in Pavements,” NCHRP Report 405, National Cooperative Highway Research
7 Program (NCHRP), Washington, D.C.
9 Kuemmel, D.A., R.C. Sonntag, J. Crovetti, and Y. Becker. 2000. “Noise and Texture on
10 PCC Pavements: Results of a Multi-State Study,” Final Report No. WI/SPR-08-99,
11 Wisconsin Department of Transportation, Madison, Wisconsin.
13 Larson, R.M. 1999. “Consideration of Tire/Pavement Friction/Texture Effects on Pavement
14 Structural Design and Materials Mix Design,” Federal Highway Administration, Office of
15 Pavement Technology, Washington, D.C.
17 Liang, R.Y. 2003. “Blending Proportions of High Skid and Low Skid Aggregate,” Final
18 Report prepared for Ohio Department of Transportation (ODOT), Columbus, Ohio.
20 Liang, R.Y. and L.L. Chyi. 2000. “Polishing And Friction Characteristics Of Aggregates
21 Produced in Ohio,” Report No. FHWA/OH-2000/001, Ohio Department of Transportation,
22 Columbus, Ohio.
24 Mahone, D.C. and S.N. Runkle. 1972. “Pavement Friction Needs,” Highway Research
25 Record 396, Highway Research Board, Washington D.C.
27 McCullough, B.F. and K.D. Hankins. 1966. “Skid Resistance Guidelines for Surface
28 Improvements on Texas Highways,” Highway Research Record 131, Highway Research
29 Board, Washington D.C.
31 McGhee, K.K. and G.W. Flintsch. 2003. “High Speed Texture Measurement of
32 Pavements,” Report No. VTRC 03-R9, Virginia Transportation Research Council (VTRC),
33 Charlottesville, Virginia.
35 McNerney, M.T., B.J. Landsberger, T. Turen, and A. Pandelides. 2000. “Comparative Field
36 Measurements of Tire/Pavement Noise of Selected Texas Pavements,” Report No.
37 FHWA/TX-7-2957-2, Texas Department of Transportation, Austin, Texas.
39 Meegoda, J.N., C.H. Hettiarachchi, G.M. Rowe, N. Bandara, and M.J. Sharrock. 2002.
40 Correlation of Surface Texture, Segregation, and Measurement of Air Voids, Final Report
41 prepared for New Jersey Department of Transportation, Trenton, New Jersey.
43 Meininger, R. C. 1994. “Degradation Resistance, Strength, Toughness, and Related
44 Properties,” ASTM STP 169C, ASTM, West Conshohocken, Pennsylvania.
46 Meyer, W.E. 1982. Synthesis of Frictional Requirements Research, Report No. FHWA/RD-
47 81/159, Federal Highway Administration (FHWA), Washington, D.C.
1 Mockensturm, E.M., B.T. Kulakowski, and N.M. Hawk. 2002. “Measurement and
2 Evaluation of Roadside Noise Generated by Transit Buses,” Final Report, The Institute for
3 Safe, Quiet, and Durable Highways (ISQDH), Purdue University.
5 Moyer, R.A. 1959. “Historical Background of Skid Resistance Measurement—American
6 Experience,” 1st International Skid Prevention Conference, Charlottesville, Virginia.
8 National Highway Traffic Safety Administration [NHTSA]. 1998. “Report to Congress:
9 Update on the Status of Splash and Spray Suppression Technology for Large Trucks,”
10 NHTSA, Washington, D.C.
12 NHTSA. 2004. Traffic Safety Facts 2003 – A Compilation of Motor Vehicle Crash Data
13 from the Fatality Analysis Reporting System and the General Estimates System, NHTSA
14 website (http://www-nrd.nhtsa.dot.gov.)
16 Noyce, D.A., H.U. Bahia, J.M. Yambo, and G. Kim. 2005. Incorporating Road Safety into
17 Pavement Management: Maximizing Asphalt Pavement Surface Friction for Road Safety
18 Improvements,” Draft Literature Review and State Surveys, Midwest Regional University
19 Transportation Center (UMTRI), Madison, Wisconsin.
21 Ong, G.P. and T.F. Fwa. 2006. “Transverse Pavement Grooving Against Hydroplaning I:
22 Simulation Model,” Journal of Transportation Engineering, Volume 132, Number 6,
23 American Society of Civil Engineers (ASCE), Reston, Virginia.
25 Page, B.G. and L.F. Butas. 1986. Evaluation of Friction Requirements for California State
26 Highways in Terms of Highway Geometrics, Report No. FHWA/CA/TL 86/01, Federal
27 Highway Administration, Washington, D.C.
29 Permanent International Association of Road Congresses (PIARC). 1987. “Report of the
30 Committee on Surface Characteristics,” Proceedings of the 18th World Road Congress,
31 Brussels, Belgium.
33 Permanent International Association of Road Congresses (PIARC). 1995. International
34 PIARC Experiment to Compare and Harmonize Texture and Skid Resistance Measurements,
35 Report AIPCR- 01.040.T-1995, PIARC, Brussels, Belgium.
37 Prowell, B.D., J. Zhang, and E.R. Brown. 2005. “Aggregate Properties and the
38 Performance of SuperPave-Designed Hot Mix Asphalt,” NCHRP Report 539, National
39 Cooperative Highway Research Program (NCHRP), Washington, D.C.
41 Rado, Z. 1994. “Analysis of Texture Models,” PTI Report No. 9510, Pennsylvania
42 Transportation Institute, Penn State University, State College, Pennsylvania.
44 Radlinski, R.W. and S.F. Williams. 1985. “Stopping Capability of Air-Braked Vehicles,”
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32 Department of Transportation, Pierre, South Dakota.
34 Wallman, C.G. and H. Astrom. 2001. “Friction Measurement Methods and the Correlation
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42 Washington Department of Transportation (WSDOT). 2005. “Factors Affecting HMA
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21 APPENDIX A. TERMINOLOGY
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1 ABBREVIATIONS AND ACRONYMS
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
1 TERM DEFINITIONS
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
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
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
2) A reported friction value.
μmax Maximum friction value.
ω Angular velocity of tire.
20 APPENDIX B. STANDARDS RELEVANT TO
21 PAVEMENT FRICTION
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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
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
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
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