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Modification versus bound pavements

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					Modification versus bound pavements
George Vorobieff
Australian Stabilisation Industry Association (AustStab)


ABSTRACT

In Australia insitu road stabilisation is generally carried out by mixing a cementitious, lime or
bituminous binder with the existing pavement material to form a bound material. In some
instances, additional granular material may be used prior to stabilisation to increase the pavement
depth to be stabilised or to change the PI or other properties of the insitu material. The concept of
stabilisation is a simple process which involves the mixing of binder and materials to achieve a
more suitable pavement material to carry traffic loads over a long period of time. One aspect of
stabilisation that is often confused by practitioners is that of modified versus bound materials.

In 2004, pavement designers are at the cross-roads of using insitu stabilisation to modify the
material and ‘tailor’ a solution to the numerous input parameters and limitations of insitu pavement
materials. As the Austroads test methods for determining the modulus and fatigue properties of
modified materials becomes more widely used and more published data is made available, greater
confidence in the new generation design models will develop and road owners will ultimately
benefit.



CONTENTS

1.   INTRODUCTION
2.   DEFINITIONS
3.   DESIGNING FOR MODIFICATION
4.   CONSTRUCTION
5.   CONCLUSION
6.   REFERENCES




Modification versus bound pavements                                                       Page 1 of 8
NZIHT Stabilisation of Road Pavements Seminar 28 & 29 June 2004
1.    INTRODUCTION
In Australia insitu road stabilisation is generally carried out by mixing a cementitious, lime or
bituminous binder with the existing pavement material to form a bound material. In some
instances, additional granular material may be used prior to stabilisation to increase the pavement
depth to be stabilised or to change the PI or other properties of the insitu material. The concept of
stabilisation is a simple process which involves the mixing of binder and materials to achieve a
more suitable pavement material to carry traffic loads over a long period of time. One aspect of
stabilisation that is often confused by practitioners is that of modified versus bound materials.

This paper address the developments of the characterisation, design and construction of ‘modified’
stabilised materials.


2.    DEFINITIONS
The term stabilisation is typically applied to cover various binder contents such that the outcome
may be a modified or bound material. There is no internationally recognised and consistent
definition which clearly establishes the difference between a modified and bound material. In
2002, Austroads (Austroads, 2002) published definitions for modified and bound pavement
materials according to their strength measured in the laboratory (see Table 1). Note that the
strength values in Table 1 are based on 28-day laboratory values for samples prepared using
standard compaction and cured at 23°C. Some jurisdictions may use 7-day values or modified
curing techniques, and appropriate conversion factors from laboratory testing are required.

Figure 1 provides a diagrammatic representation of how a cementitious binder may increase in
UCS with increasing binder content and different types of pavement materials. Using the
Austroads definition, the different ranges of modified and bound materials can be sought.

The Austroads stabilisation guide (Austroads, 1998b) is currently being updated and the working
group has decided to nominate a new range of strength values to categorise modified and bound
pavement materials. Similar to previous descriptions documented by Austroads and industry
organisations, the definition of modification and bound materials is based on strength as the UCS
test is low cost and easy to perform. In addition, the primary role of the pavement material is to
support traffic loads and strength is an important criteria that will allow the determination of the
depth of layer to sustain repetitive loading.



  Table 1 Typical properties of modified, lightly bound and heavily bound materials. (Austroads,
                                               2002)
                 Degree of Binding          Design Strength1            Design Flexural
                                                 (MPa)                  Modulus (MPa)
                 Modified                        UCS < 1.0                   ≤ 1,000
                 Lightly bound                    UCS: 1-4               1,500 – 3,000
                 Heavily bound                    UCS > 4                    ≥ 5,000
                 Notes: 1. 28 day test results, standard compaction and moist curing to
                 AS 1141.51.
                 2. For slow setting binders the 28 day test results will be less than the
                 values shown but will continue to increase in the field for at least 6 to 12
                 months.




Modification versus bound pavements                                                             Page 2 of 8
NZIHT Stabilisation of Road Pavements Seminar 28 & 29 June 2004
                                         7
                                         6




                     UCS (28-days) MPa
                                                  Heavily bound
                                         5
                                         4

                                         3
                                                  Lightly bound
                                         2
                                         1
                                                                              Modified
                                         0
                                             0%   1%    2%     3%     4%      5%   6%    7%
                                                             Binder content

       Figure 1 Modification versus bound stabilised materials for various binder contents.


One of the limitations of using UCS strengths as a guide to modified materials is that when values
are about and less than 1 MPa, the reliability from the UCS test is more suited to quarried products
due to consistency in the materials source. For insitu stabilisation where the PI, particle size
distribution and content of bitumen from the existing seal may vary along and across the road, it is
harder to get a consistent strength result from samples of loose stabilised material which are
extracted from behind the reclaimer and then compacted and cured to laboratory standards.

In a contract environment, the typical compliance measures for insitu stabilisation are density,
stabilised layer thickness, application rate and geometrical limits. UCS outcomes are not used, as
waiting for an unfinished pavement layer for 28 days is not satisfactory for all parties in the
contract. Even if a 7-day accelerated test could be used, road owners may get very low
satisfaction levels from road users. From past experience, use of the above compliance measures
for bound materials has shown to be satisfactory. For modified materials, the compliance
measures should also be satisfactory and if density limits are not met due to incorrect moisture
content, then it would be reasonable to remix and compact the material on the subsequent day.


3.    DESIGNING FOR MODIFICATION
Bound materials have traditionally been produced from granular materials that fit the limits of road
specifications, especially particle size distribution (see Figure 2). Some of the materials used in
the past to construct our rural roads, do not meet road specifications however as they may be high
in PI, contain weaker aggregates or the particles size distribution is either too coarse or fine or gap
graded. To recycle these materials has required the additional use of a small proportion of new
quarried products or recycled material, such as crushed concrete from demolition sites.




Modification versus bound pavements                                                           Page 3 of 8
NZIHT Stabilisation of Road Pavements Seminar 28 & 29 June 2004
          Table 2   Proposed classification of stabilisation materials for Austroads guide.

       Type of Stabilisation        Typical binders               Performance attributes
                                         adopted
                                Blending other granular
                                                            Flexible pavement subject to
                                materials which are
             Granular                                       shear failure within pavement
                                classified as binders in
        40% < CBR < +120%                                   layers and/or subgrade
                                the context of this
                                                            deformation
                                Guide.
                                                            Flexible pavement subject to
                                                            shear failure within pavement
            Modified            Addition of lime.           layers and/or subgrade
       0.7 MPa < UCS* < 1.5     Addition of polymer or      deformation.
              MPa               chemical binders.           Can also be subject to erosion
                                                            by water penetration through
                                                            cracks.
                                Addition of small
                                quantities of               Lightly bound pavement which
                                cementitious binders.       may be subject to tensile fatigue
          Lightly Bound
                                Addition of small           and/or subgrade deformation.
       1.5 MPa < UCS* < 3.0
                                quantities of bituminous    Can also be subject to erosion
               MPa
                                or                          by water penetration through
                                bituminous/cementitious     cracks.
                                binders.
                                Addition of higher          Bound pavement subject to
                                quantities of               tensile fatigue cracking and
                                cementitious binder.        transverse drying shrinkage
             Bound
                                Addition of a               cracking.
          UCS* > 3.0 MPa
                                combination of              Less likely to be subjected to
                                cementitious and            erosion by water penetration
                                bituminous binders.         through cracks.
       Note: UCS test specimen prepared using standard compactive effort and 28 day normal
       curing.




                                                                   Preferred
                                                                    grading
                                         Fine
                                        material



                                                               Coarse
                                                               material




              Figure 2 Good particle size distribution is required to achieve strength
                              and durability (Austroads, 2002).



Modification versus bound pavements                                                          Page 4 of 8
NZIHT Stabilisation of Road Pavements Seminar 28 & 29 June 2004
The decision to modify or produce a bound pavement through the road stabilisation process is the
choice of the pavement designer and it is usually based on local experience. For instance, typical
Victorian practice is to carry out stabilisation by modifying the existing pavement material with the
addition of only 2% to 3% of a cementitious binder. The VicRoads specification guide notes
(VicRoads, 2000) that the rationale for modification of pavement materials is to:

       place less reliance on the binder for long term performance by improving grading and PI;
       minimise likelihood of shrinkage cracking by allowing 2% cementitious binder content to be
       used for all cases where the specified Unconfined Compression Strength (UCS) in Table
       307.052 is met;
       optimise the blend of cementitious binders to suit material type and available working time;
       avoid producing a bound layer within a highly flexible pavement that is more prone to
       flexural (fatigue) cracking.

Also of interest, the APRG Report 21 (Austroads, 1998a), written around Victorian experiences
with granular and bound materials, noted:

“Modified granular material - achieved by adding small amounts of cementitious binder and/or the
granular material. This process is undertaken to remedy deficiencies in the granular material (e.g.,
high plasticity, low shear strength, etc.). The amount of binder added is insufficient to convert the
granular material into a monolithic slab with significant tensile strength. Hence, for design
purposes, the modified material is considered to be (improved) granular material.”

Whilst the overall intent of this approach to modified granular material is satisfactory, the
information is too vague to be useful to a pavement designer. In addition, many local government
engineers in Sydney, Brisbane, and other parts of Australia, satisfactorily design their local roads
with sufficient cementitious binder to form a lightly bound stabilised material between 180 and
250 mm in thickness.

For granular materials, the empirical chart in the Austroads design guide (Austroads, 1992 & 2004)
is commonly used where a thin wearing surface is incorporated into the design. This chart is
based on the assumption that pavement failure is due to subgrade deformation. The proposed
new Austroads guide (Austroads, 2004) notes that similar lines can be generated if the top
granular moduli is 350 MPa and a SARs/ESA factor of 1.2 is used. In Appendix 7.2 of the guide,
the values of SARs/ESA vary from 1.07 to 4.13, which provides a possible explanation for one
cause of the poor performance of granular materials around Australia.

For modified materials where the modulus values exceed 350 MPa, Figure 3 will result in
conservative answers and the mode of failure may change from subgrade deformation to some
form of rutting in the upper layer of the material.

The new Austroads pavement design guide will permit the use of modulus values derived from
laboratory testing to AS1289.6.8.1 for granular materials (Section 6.2.3.3). This approach has to
be applauded as it allows suppliers of gramular and modified granular material the opportunity to
introduce new materials with a laboratory determined modulus.

AustStab has also been promoting research work in the area of modulus and fatigue determination
using indirect tensile loading. A project commissioned by Austroads in 2000, looks at many issues
including traffic loading, design modulus, modulus determination from laboratory, field and insitu
measurements, fatigue life determination and performance relationships, the failure mechanisms of
stabilised materials and subgrade materials (Foley et al, 2001). One of the outcomes from this
project was the recommendation to develop test procedures using indirect tension testing to
determine the resilient modulus and fatigue properties of stabilised materials with the same sample
and within one-day after curing.

Modification versus bound pavements                                                       Page 5 of 8
NZIHT Stabilisation of Road Pavements Seminar 28 & 29 June 2004
                        Figure 3 Empirical design chart for a wide range of
                         granular pavements with thin bituminous surfacing.



This recommendation was subsequently followed with a test procedure developed by ARRB in
conjunction with Butcher at Transport SA and AustStab. The draft Austroads test method
(Austroads 2003a) is currently being used on a range of modified materials by some Australian
laboratories (Figure 4). The method still requires further refinements but will be a great asset in the
determination of modulus and fatigue relationships in a mechanistic design model for modified,
bound and asphalt layers in a pavement configuration.




                      Figure 4 Indirect tensile testing of modified (stabilised)
                           materials being carried out by Transport SA.




Modification versus bound pavements                                                        Page 6 of 8
NZIHT Stabilisation of Road Pavements Seminar 28 & 29 June 2004
4.    CONSTRUCTION
In a deep-lift insitu pavement stabilisation process,1 sufficient binder is commonly added to ensure
a heavily bound pavement material which will provide a base layer able to carry heavy traffic for a
period of 20 years. It is well know that thin heavily bound materials, especially on weak
subgrades2, have a shorter design traffic life as they tend to form fatigue cracking and pumping of
fines on the surface when water is present in the road.

A modified material is less likely to form fatigue cracking from repetitive loading and stabilisation
depths are typically 200 to 250 mm, significantly less than the 350 to 400 mm used in deep lift
stabilisation works. In many cases, the designer selects modification over a bound layer due to the
shallow depth of existing pavement materials but normally not greater than 250 mm.

One of the benefits with modification is the ability to construct the materials in layers as the
reliance for a strong interface layer is not as critical as a bound layer. Whilst deep-lift insitu
stabilisation using bound materials has been carried out in two layers on the Dukes Highway in
South Australia (Austroads, 2003c), the performance data needs to be reviewed (see Figure 5).
Multi-layers of compacted modified materials to depths exceeding 400 mm are feasible provided
steps are taken to manage in-service moisture levels. Whilst plant mix and pavers may offer one
solution, insitu equipment and sound process control should deliver similar performance outcomes.




                      Figure 5 Typical two layered insitu stabilisation of heavily
                         trafficked highways being trialled in South Australia.


All binder types can be used for modification provided they have been shown to produce the
desired strength and other property limits specified by the designer. Modification of a pavement
will not overcome significant depth deficiencies. The decision to modify a material should be made
by an appropriately qualified person who will follow well established processes in determining the
most appropriate treatment.


5.    CONCLUSION
Pavement modification by stabilisation using a binder is likely to increase in Australia as lower cost
rehabilitation methods are considered in areas where there are expansive, weak and/or wet
subgrades and marginal insitu materials. With limited road maintenance funding, opportunities to
bring to site granular materials are constrained by the need for asset managers to find short term
solutions by addressing the ride quality of the network.


1
 Deep-lift is typically defined are those material stabilised in one layer of depth of more than 250 mm.
2
 Weak subgrade in this instance refers to subgrades having a CBR less than 5%.
Modification versus bound pavements                                                                 Page 7 of 8
NZIHT Stabilisation of Road Pavements Seminar 28 & 29 June 2004
This paper has highlighted recent developments in the characterisation of modified pavement
materials using insitu stabilisation. An added bonus discussed in this paper is the environmental
benefit of using insitu materials rather than importing new quarried products.

In 2004, pavement designers are at the cross-roads of using insitu stabilisation to modify the
material and ‘tailor’ a solution to the numerous input parameters and limitations of the insitu
pavement materials. As the Austroads test methods for determining the modulus and fatigue
properties of modified materials become more widely used and more published data is made
available, greater confidence in the new generation design models will develop and road owners
will ultimately benefit.



6.   REFERENCES
Austroads (1992) Pavement Design: A Guide to the Structural Design of Road Pavements
Sydney.
Austroads (1998a) A guide to the design of new pavements for light traffic Report No. APRG 21,
Sydney.
Austroads (1998b) Guide to Stabilisation in Roadworks Sydney.
Austroads (2001) 2001 Austroads Pavement Design Guide (Final Draft) for Public Comment
Report No. AP-T10, Sydney.
Austroads (2002) Mix design for stabilised pavement materials Report No. AP-T16, Sydney.
Austroads (2003a) Determination of the resilient modulus and fatigue properties of stabilised
materials – Indirect tensile method Draft Austroads Test Method, Sydney (30 March 2003 - Not
published)
Austroads (2003b) Guide to best practice for the construction of insitu stabilised pavements
Sydney.
Austroads (2004) Pavement Design: A Guide to the Structural Design of Road Pavements
Sydney (Not published).
Foley, G & Austroads Stabilisation Expert Group (2001) Mechanistic design issues for stabilised
pavement materials ARRB Transport Research Contract Report RC91022- 3, South Vermont.
Standards Australia (2002) Methods of Testing Soils for Engineering Purposes: DRAFT Method
6.8.1: Soil Strength and Consolidation Tests – Determination of the Resilient Modulus and
Permanent Deformation of Granular Unbound Pavement Materials Draft AS1289.6.8.1.
VicRoads (2000) Section 307 Insitu stabilisation of pavements with cementitious binders -
Specification guide notes Kew, December 2000.




Modification versus bound pavements                                                     Page 8 of 8
NZIHT Stabilisation of Road Pavements Seminar 28 & 29 June 2004

				
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