Heavy Duty Pavement Design Guide[1]

Heavy Duty Industrial Pavement Design Guide Revision 1.035 19 March 2007

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Contents Foreword

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Introduction and Background

4

Scope of the Guide.........................................................................................................................5 Background: Design Methods ........................................................................................................7

Pavement Design Principles - General

8

Overview of Pavement Design System..........................................................................................9 Input Variables .....................................................................................................................9 Structural Analysis ...............................................................................................................9 Key Performance Indicators -– Level of Service (LOS) ...............................................................13

Pavement Materials

15

Asphalt .........................................................................................................................................16 Function – wearing surface................................................................................................16 Function – structural ..........................................................................................................16 Volumetric analysis ............................................................................................................17 Other Issues.......................................................................................................................20 Composite/ Resin Modified Asphalt .............................................................................................21 Granular Material..........................................................................................................................22 Stabilised Material ........................................................................................................................23 Subgrade......................................................................................................................................25

Traffic

27

Vehicle Types...............................................................................................................................28 Unequal Axle Loads...........................................................................................................29 Equal Axle Loads ...............................................................................................................29 Coordinate System for Vehicles...................................................................................................30 Vehicle Wander ............................................................................................................................32 Payload Distribution .....................................................................................................................33 Traffic Growth...............................................................................................................................35 Dynamic and Static Structural Loading ........................................................................................36 Modelling of Multiple Wheels and Axle Groups ...........................................................................38 Nature of Damage Pulses..................................................................................................39 Design Traffic Loading .................................................................................................................40

New Pavement Design

41

Design Period...............................................................................................................................42 Material Properties and Performance Models..............................................................................43 Subgrade Properties and Performance Models.................................................................43 Unbound Granular Material Properties ..............................................................................45 Asphalt Properties and Performance Models ....................................................................46

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Contents

Cement Stabilised Material Performance Models..............................................................50

Environment

52

Drainage (surface and subsurface)..............................................................................................53 Subgrade Volume Change...........................................................................................................54 Weathering / ageing .....................................................................................................................55

Construction Implications

57

General.........................................................................................................................................58 Compaction, Workability and Layer Bonding ...............................................................................59 Curing...........................................................................................................................................61 Opening to Traffic.........................................................................................................................62

Pavement Maintenance

63

Routine Maintenance ...................................................................................................................64 Major Maintenance.......................................................................................................................65 In-Service Monitoring ...................................................................................................................66

Pavement Rehabilitation

67

Site Investigation ..........................................................................................................................68 Functional and Structural Condition Assessment ........................................................................69 Treatment Types ..........................................................................................................................70 Functional Rehabilitation....................................................................................................70 Structural Rehabilitation.....................................................................................................71

Caveats

73

Life Cycle Costing

75

Analysis Period – Service Life......................................................................................................76 Present Worth Analysis................................................................................................................77

Case Studies

79

Case Study 1................................................................................................................................80 Loading ..............................................................................................................................80 Pavement Model ................................................................................................................80 Results ...............................................................................................................................81

Appendices

85

Material failure mode and implication...........................................................................................86 Improved asphalt material characterisation .................................................................................87

References

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Foreword

Foreword The purpose of this Guide is to assist pavement designers and managers with the planning, design, construction, maintenance and rehabilitation of heavy duty flexible pavements. Although the principles can be applied to various types of heavy duty pavements, this guide is primarily directed at port and container terminal pavements. The Guide covers the assessment of input parameters needed for design. Material properties, traffic factors, environmental considerations, pavement design methods, maintenance and rehabilitation treatments and life cycle costing are also discussed. At the end of the guide a few case studies are presented. The Guide is a collaborative effort currently involving: Dr. Leigh Wardle of Mincad Systems (Melbourne, Australia); Ian Rickards (Pioneer Road Services Pty Ltd, Melbourne, Australia) John Lancaster (formerly Pioneer Road Services) Dr. Susan Tighe (Dept. Civil and Environmental Engineering, University of Waterloo, Canada) The Guide presents the authors’ attempt to reflect best practice in the design, construction and rehabilitation of heavy duty flexible pavements. The Guide will steer the designer through all necessary design considerations and suggests external sources for research updates. It is intended to be supplementary to other published design guides with a focus on industrial pavements. The primary tool used in this guide to carry out the pavement design analysis is a program called HIPAVE that has been specifically developed for heavy duty flexible pavements. The Guide is a ‘living document’ that will be regularly updated to reflect advances in pavement technology and made freely available via the Internet at no charge. It is the author’s goal to preserve the relevance and currency of the Guide by in-house research and development and continuous liaison with international experts in pavement technology.

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Introduction and Background

Introduction and Background

Introduction and Background

Scope of the Guide This Guide addresses design of heavy duty flexible pavements for ports and container terminal pavements. The Guide focuses on the structural design of pavements rather than structural detailing or design detailing. The primary tool used in this design guide to reinforce the concepts is a program called HIPAVE, developed by Mincad Systems, which has been specifically developed for port and container terminal pavements. However, this is a stand alone document which can serve as a useful tool for highlighting key elements to the design, construction, maintenance and rehabilitation of heavy duty flexible pavements. The Guide covers the assessment of input parameters needed for design, design methods for flexible pavements and gives guidance on life cycle costing, construction, maintenance and rehabilitation issues. The guide is grouped into sections as briefly described herein. A brief overview of pavement design including the input variables and structural analysis is presented, followed by a brief discussion on key performance indicators, including the concept of level of service. Overall a pavement design system is presented in this section to assist with heavy duty flexible pavement design. The core of the design system is mechanistic structural analysis software such as layered elastic analysis. The next few sections of the Guide contain a detailed discussion of subgrade evaluation, pavement materials evaluation, analysis of traffic loading and structural design in addition to other factors relevant to pavement design. Various issues associated with construction of heavy duty flexible pavements are presented including compaction, workability and layer bonding, curing requirements, and the ability to open to traffic. Pavement maintenance in terms of typical routine maintenance and major maintenance are presented. Pavement rehabilitation including site investigation, condition assessment in terms of functional and structural considerations and the various typical treatment types are presented. The next section presents the concept of life cycle costing. The analysis period, service life and the present worth analysis are described in this section. The last section includes case studies. The procedures in this Guide are intended for the design of pavements for which the primary distress mode is load associated. If other modes of stress, for example environmental distress, have a significant effect on pavement performance, their effect should be separately assessed. It is emphasized that this document should be used as a guide only; it should not be referred to as a design specification. The designer must exercise judgment in choice of values for the parameters that are incorporated into particular designs. Pavement design is just one aspect associated with the achievement of sound pavement performance. Pavement performance also depends on other factors such as sound material quality control, adequate drainage, construction tolerances and pavement maintenance.

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Introduction and Background

Although, this guide is written with emphasis on Australian practices, it does have relevance to the design and construction of port and terminal container pavements around the world.

Introduction and Background

Background: Design Methods Many aspects of the design methods for highway/road pavements such as those presented in the new Austroads Pavement Design Guide (2004) are not appropriate for designing heavy duty flexible pavements for applications such as ports and container terminals. Traditionally, port pavements have been designed using chart-based, empirical processes such as the British Ports Association method (British Ports Association, 1996). In more recent times, designers have combined the full range of vehicles and shipping containers into a single number of repetitions of an ‘equivalent standard axle’. This equivalent axle would be applied in layered elastic design using tools such as CIRCLY (Wardle, 2004) and APSDS (Airport Pavement Structural Design System, Wardle, 1999). Alternatively, many designers prefer to use the actual wheel layouts of the vehicles and these can be used directly in CIRCLY and APSDS. While CIRCLY and APSDS have been used very successfully for the design of heavy duty industrial pavements, unwieldy data input makes it very difficult to model more than one or two payloads per vehicle. HIPAVE (Heavy Industrial PAVEment design), an outgrowth of CIRCLY and APSDS, was released in late 2005. HIPAVE has been designed to conveniently handle comprehensive details of the freight handling vehicles and the characteristics of the payload distribution for each vehicle. In recent years the ASCE have been developing a Port and Intermodal Yard Pavement Design Guide. Smallridge and Jacob (2001) give an outline of the Guide. At the time of writing, the Guide is close to becoming available in draft form (Jacob, 2006).

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Pavement Design Principles - General

Pavement Design Principles - General The goal of pavement design is to select the pavement design which is cost effective and provides a high level of service for the given traffic and environmental conditions. The designer must have sufficient knowledge of the available materials, the expected traffic loading, the local environment and their interactions. Ultimately all of these factors must be examined in order to predict the performance of a candidate pavement design. Furthermore the designer must have an understanding of the level of performance and pavement condition considered satisfactory for the operational conditions of the project. A systematic approach to pavement design is required as there are many variables and interactions which influence the outcome. HIPAVE facilitates the rapid evaluation of the variables and the user should, systematically, use this capability to examine “what if” scenarios to try and identify the level of risk associated with the various pavement options as illustrated for instance in case study 2.

Introduction and Background

Overview of Pavement Design System Input Variables Design Traffic The wheel layout, load distribution, loading rate (speed) and tyre pressures can all have a significant influence on pavement performance. In addition to the current traffic, attention need to be given to future traffic, including the change in volume, mass and composition during the design period. Detailed consideration of traffic is presented in the next section. The static load under stacked containers while considerable is not generally a structural pavement design issue as the magnitude of the load is generally less than under heavy vehicles and the loads are relatively widespread. The extreme stress at the surface under the container corner castings is however critical to the selection of the surfacing material.

Subgrade and Pavement Materials Details of the materials in the pavement structure should include: strength/stiffness measurements which can be used to quantify their load carrying properties; estimates of typical variations in material properties associated with changes in moisture, temperature, aging, shrinkage during the curing stage details on how pavement materials deteriorate due to fatigue under repeated loading and performance criteria including limiting value(s) of stresses or strains at which a given degree of distress will occur.

Structural Analysis The aim of structural analysis is to predict the critical strains and/or stresses which are induced by the traffic loading in the trial pavement design. Several trial pavement configurations or designs are analyzed and the most appropriate design is selected at the end of the analysis based on the technical and economic constraints. The traffic loading can be more generic ( I’m unsure what this means ) or it can include the details of each combination of vehicle model and payload.

Distress Prediction

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Pavement Design Principles - General

The structural analysis is used to estimate the allowable loading and associated distress of the trial pavement design. The performance criteria, in this case pavement distress prediction, assigned to pavement materials, and to the subgrade, are typically relationships between the strain induced by the single application of a load and the number of such applications which will result in the condition of the material, or the pavement, reaching an allowable limit. The allowable limit is related to a maximum distress or level of service. Generally most performance models may be represented graphically by a plot of tolerable strain versus load repetitions (generally by a straight line of 'best fit' on a log-log plot). Equation 1 below, shows the typical model format

⎡k ⎤ N=⎢ ⎥ ⎣ε ⎦

b

[1] where N

is the predicted life (repetitions)

k

is a material constant

b

is the damage exponent of the material

ε

is the induced strain (dimensionless strain)

Log-log relationships can be readily converted to the above form. For some material types the appropriate performance relationship may be in a different functional form but, the concept and intent is the same. A pavement structure consists of a variety of materials which have different distress modes. For example, a granular pavement surfaced with asphalt will have an allowable loading determined by the “weakest link”. The weakest link is the layer that has the highest Cumulative Damage Factor (CDF), that is the one for which the allowable loading is the first to be exceeded by the design traffic loading. If all loads applied to the pavement are of identical type and magnitude, then the number of repetitions to “failure” can be obtained directly from the limiting strain versus repetitions criteria. The service life is then determined as the amount of time (usually in years) during which the number of repetitions is just sufficient to cause failure.

Cumulative Damage Factor In reality the pavement is subjected to a range of loadings, and each magnitude of load produces its own level of strain and stress in the pavement. Determining the service life in these circumstances is more involved. There are two conventional ways of handling this issue. The first is to convert the numbers of loads of different magnitude to an equivalent number of loads of a standard magnitude – equivalent in the sense that they will cause the same amount of pavement damage. This involves estimating the approximate passes of different vehicle loads to passes of an ‘equivalent’ standard load or "design vehicle". This methodology is no longer necessary now that computer software such as layered elastic analysis is available.

Introduction and Background

The second method used to deal with loads of different magnitudes (i.e. actual traffic) is to use the concept of cumulative damage. The system explicitly accumulates the contribution from each loading in the traffic spectrum at each analysis point by using Miner's hypothesis. The damage factor for the i-th loading is defined as the number of repetitions (ni) of a given response parameter divided by the ‘allowable’ repetitions (Ni) of the response parameter that would cause failure. The Cumulative Damage Factor (CDF) for the parameter is given by summing the damage factors over all the loadings in the traffic spectrum as shown in equation 2 below:

Cumulative Damage Factor = Σ ni / Ni

[2]

The system is presumed to have reached its design life when the cumulative damage reaches 1.0. If the cumulative damage is less than 1.0 the system has excess capacity or remaining life and the cumulative damage represents the proportion of life consumed. If the cumulative damage is greater than 1.0 the system is predicted to ‘fail’ before all of the design traffic has been applied. The procedure takes account of: the design repetitions of each vehicle/load condition; and the material performance properties used in the design model. This approach allows analyses to be conducted by directly using a mix of vehicle or axle types. It is not necessary to approximate passes of different vehicles or axles to passes of an ‘equivalent’ standard load. In this method, the proportion of damage caused by loads of a given magnitude is equal to the ratio of the number of such loads in the design period to the number of such loads which will cause failure as derived from the performance criteria. The sum of these ratios for all load magnitudes indicates the total distress which will occur. If this sum is less than or equal to 1.0, then the pavement configuration being analyzed is assumed to be adequate. Conversely, if this is not the case, then the trial pavement configuration is deemed to be unacceptable and must be modified in the next trial so that the deficiency is overcome. The next trial will focus on the inadequacy and will adjust accordingly. For example, this might mean an increase in pavement thickness or a modification to stiffness. The process is repeated until a satisfactory result in achieved. The results of the mechanistic analysis are readily assessed by a number of graphical formats. For example, Figure 1 is a sample cumulative damage plot produced by the HIPAVE program.

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Pavement Design Principles - General

Figure 1: HIPAVE graph - Subgrade Damage Factor vs. container load. Note that on this “Spectral Damage Graph” there is a data point for each combination of vehicle model and payload – in this example the container weight distribution was specified at an interval of one tonne. HIPAVE can also generate graphs that show the variation of the damage factor across the pavement, as shown by:

Figure 2: HIPAVE cumulative damage graph - Damage Factor vs. lateral position

Introduction and Background

Key Performance Indicators -– Level of Service (LOS) The deterioration of a given pavement under traffic loading and environmental distress mechanisms can be characterized in terms of a number of distress modes such as rutting, cracking and roughness. Furthermore the progressive deterioration over the life cycle of the pavement can be quantified in terms of various parameters such as maximum rut depth, cracking and various measures of rideability and roughness. These indicators are commonly called Key Performance Indicators (KPIs) or Key Performance Measures (KPMs). From a pavement design viewpoint the choice of acceptable values of the KPIs will influence the selection of the relevant damage model or transfer function. The designer should understand the KPI’s on which the damage models are based. For instance the rut depth limit assumed in the Corp of Engineers subgrade strain criteria is 25 mm. If the designer considers a lesser value e.g. 15 mm is appropriate then the model must be modified. The damage model or transfer function, i.e. the relationship between the calculated stress/strain and life is a critical element in the design process and the designer should examine the background research used in the development of the models to ensure confidence in the outcomes.

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Pavement Materials

Pavement Materials The following sections detail typical pavement materials that are used in the various layers of the pavement structure and is directed to the design of heavy duty flexible pavements for ports and terminal container areas. For additional information, please refer to Chapter 6 of Austroads 2004, for a treatise of pavement materials or the appropriate local material pavement design practices. For more detailed information on the material properties and performance models to be used in the design process refer to the “New Pavement Design’ section.

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Pavement Materials

Asphalt The following additional considerations should be taken into account, for heavy duty pavement design:

Function – wearing surface The wearing course or surface layer is generally subjected to much greater forces in heavy duty pavement conditions, compared with traditional highway or road design. Typically the pavement located at a port or container terminal is subjected to highly channelised (straddle carriers especially) and extreme wheel loads. Vehicles execute tight turns and there is a tendency toward mechanical abrasion and indentation damage to the surface. The wearing surface design objective is therefore to maximise deformation resistance. With these loading conditions, it is necessary to design the wearing surface so it has the ability to provide both fatigue resistance and deformation resistance under industrial load conditions. Notwithstanding the extreme wheel loads, the empirical evidence in Australia suggests the use of conventional asphalt mixes, designed to meet heavy road traffic stress has given good performance in the context of heavy duty pavements, the exception being under highly channelised loading by straddle carriers and container corner castings. To put this into perspective, the significantly greater magnitude of loads in industrial pavements is to some extent balanced by significantly lower passages of load relative to many highway facilities with extensive truck traffic. The relatively higher stiffness of the heavy duty pavement, provided in order to protect the subgrade, results in greater support for the wearing surface. It is possible to enhance the functional performance of the wearing surface using polymer modified binder (PMB) or Multigrade bitumen (refer Austroads AP-T41/06), stiffer bitumen such as Class 600 (refer Australian Standard AS 2008, Standards Australia, 1997) or Gilsonite modified bitumen . Modern methods of asphalt characterisation (see appendix…) provide a rational measure of the benefit of mix modification to facilitate the selection of optimum mix components. It is evident however that under the extreme stress of container corner castings, some punching shear deformation and crushing is inevitable. This will adversely impact the performance of thin surfacing layers enabling water penetration and weakening base materials. Overall, special attention must be given to the design of the asphalt layer. A minimum asphalt thickness is necessary to ensure there is structural integrity and a bond with the underlayer. This is especially recommended in areas where heavy vehicles perform tight turning manoeuvres, where it is advisable to ensure a 50 mm minimum asphalt thickness for highway vehicles and 100 mm for heavy container handling equipment, always with a prime coat to ensure a good bond.

Function – structural

Pavement Materials

Asphalt base and subbase layers will contribute significantly to the structural adequacy of the heavy duty pavement design. The design objectives are to provide high stiffness and load spreading, and control fatigue cracking. Fundamentally both of these objectives can be met by selecting harder grades of bitumen, and increasing the bitumen content to improve fatigue performance (taking into account the support provided by base and foundation layers) The optimisation of the binder content is discussed in the following section. Research (Rickards, et al 2006) has shown that the selection of mix gradation, which is slightly fine of the theoretical maximum density, yields the highest stiffness, together with a higher filler content (material passing the 75 micron sieve) to stiffen the mortar. Experience has shown that while the selection of large stone mixes (e.g. > 20 mm nominal mix size) in theory yields higher stiffness, workability issues and the tendency to segregate will often jeopardize field performance reducing stiffness and a resulting in a propensity to moisture damage due to higher relative permeability. It is suggested that for practical purposes, a 20 mm nominal maximum aggregate size is used for these types of pavements. Historically, larger size mix has been used when thick asphalt layers were required. Conversely, French practice suggests that for a 14 mm nominal mix the layer thickness should be between 70 mm and 120 mm (5 – 8 times nominal mix size). A caution is provided about the potential loss of shape in the compaction of a layer at the maximum thickness but in multi-layer structures any loss of shape may be corrected by subsequent layers. For all practical purposes individual layers > 120 mm thick will not be required hence a 14 mm mix is a practical upper size. Certainly this mix will demand more binder than a larger stone mix but it is this factor that will benefit field performance both at a theoretical level (better fatigue performance) and practical level (improved homogeneity workability and impermeability).

Volumetric analysis It is critical to understand the importance of optimising the bitumen content to achieve optimum air void content in mix design. It is a fundamental requirement that the binder content be optimised at the in service mix density i.e. the design binder content must achieve the target air voids at a level of compaction in the laboratory that faithfully represents the level of compaction in the field. The consequence of optimisation at incorrect laboratory density is shown in Table 1. Table 1: Impact of Laboratory Density on Field Performance Laboratory density c/f in service density Lab >> in service density

Lab 150 mm cover)

Heavy channelised traffic

Va = 3% @ BS RD

Va = 2% @ BS RD

Va = 1% @ BS RD or Va = 4% @ 75 Blow Marshall

Heavy random traffic

Va = 1% @ BS RD or Va = 5% @ 75 Blow Marshall

Va 4% *@ 75 Blow Marshall

Va 3% *@ 75 Blow Marshall

The BS RD has its origins in compaction compliance testing for subbase asphalt with a minimum requirement of 96% BS RD for acceptance (on layers > 75 mm thick approximately). In the preceding table this would ensure ≤ 5% voids at construction – a desirable target. Further the evidence of good performance of 75 blow Marshall mixes suggests subsequent traffic compaction does not reduce voids to critical levels. It has been observed that well-compacted mixes containing thermoplastic rubber polymer binders do not compact significantly under traffic. Therefore target air voids could be reduced by approximately 1% when these materials are used in the asphalt. Note, these values are provided as a general guide and have had limited empirical verification. The user is advised to verify the design assumptions against field experience wherever possible. Complete laboratory testing on asphalt mixes should always be carried out and combined with field data whenever possible.

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Pavement Materials

Other Issues Asphalt manufactured with conventional bitumen or SBS based PMB can be prone to degradation on exposure to hydraulic fluid and fuel leaks. In short, these materials can soften the binder resulting in a significantly reduced resistance to deformation and mechanical damage. Other polymers may resist the softening effect and suppliers should be consulted. The Shell “FuelSafe” binder has exhibited substantially improved resistance to damage by hydrocarbon spills. The PRS Rigiphalte product referred to in the following provides significant resistance to both chemical and mechanical damage.

Pavement Materials

Composite/ Resin Modified Asphalt A number of composite products – known generically as Resin Modified Asphalt (RMA) – offer enhanced toughness which can make it a desirable wearing course for heavy duty pavements. These RMA materials consist of an asphalt carrier mix with high air voids. An extremely low viscosity highly modified cementitious grout is then pumped into the voids and vibrated to remove air pockets. This composite material has improved resistance to mechanical and chemical damage while the bituminous carrier mix has the ability to absorb shrinkage strains and inhibit cracking. Its high crushing strength makes it ideal for use in container stack areas to resist deformation under the highly channelised straddle traffic and to resist crushing under container corner castings. The RMA materials have higher stiffness relative to asphalt and may fatigue under repeated flexure. Their performance parameters (modulus and fatigue) can be entered into HIPAVE and evaluated as part of the design analyses. In the longer term it may also be prudent to conduct pavement deflection testing (see below) to establish tolerable limits to confirm the adequacy of the pavement foundation support to avoid premature fatigue failure of the RMA. A comparison of the dynamic modulus of the PRS Rigiphalte and a typical asphalt surfacing at a slow loading frequency (1Hz) is given in Figure 8 on page 47. It is observed that the Rigiphalte product has significantly higher modulus and elastic performance parameters over the temperature spectrum.

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Pavement Materials

Granular Material The depth and quality of unbound granular material is a critical parameter in the heavy duty pavement design process. This layer assists in providing adequate support for the surfacing materials and also provides resistance to rutting in the subgrade due to shear failure. The properties required in granular layers are a function of the applied traffic stress level and load frequency over the design period. The required depth of selected layers will vary with subgrade strength. The strength of granular materials varies with applied load stress which sets up mechanical interlock within the granular matrix and higher stress results in higher stiffness in the aggregate matrix. The stiffness of an unbound granular layer is also dependent on the stiffness of support layers and this diminishes with depth in the pavement. Hence, it is important to utilize unbound granular materials of quality appropriate to the position in the structure. Well compacted high strength aggregates are required for high stress locations close to the surface. At lower levels in the pavement, lesser quality aggregates may be used, provided they are of sufficient quality to mobilise the assigned layer stiffness. Examination of the stress distribution throughout the granular layer (e.g. by inspecting HIPAVE outputs), enables determination of material property needs (strength) throughout the pavement structure. A good starting point is to examine applicability of local State Road Agency specifications for highway pavements, for use in heavy duty off-road pavements. The specifications relate to material quality and compaction requirements. Attention must be paid to layer thickness, in relation to maximum particle size and density requirements. Close attention must also be given to ensure high construction standards as discussed in some detail in section…and experience has taught that premature failure is most often related to poor construction practice and less to material selection.

Pavement Materials

Stabilised Material Unbound pavement materials can be stabilized by either chemical and/or mechanical processes. Chemical stabilization involves mixing additives such as bitumen or cement in quantities and to layer depths as determined by the pavement design requirements. Granular materials treated with bitumen or hydraulic binders (such as cement) are generally referred to as “stabilised” if they are to act as a bound layer or “modified” if they are to act as an unbound layer with improved properties such as reduced plasticity. Engineering judgment needs to be exercised in modeling the resulting material. .A suggested delimiter between “stabilised” and “modified” conditions, is a UCS (7 day cured) of 0.8 MPa. Definition or determination of the degree of stabilisation is important, since a stiff, “stabilised” material will be prone to flexural fatigue and hence needs to be considered in the design. The material can be produced in a mixing plant or in-situ, using special equipment. The plant produced product, in general, should be of better quality due to enhanced product control in terms of uniformity of raw material and mixing. Conversely, the quality / variation of in-situ stabilised material may not be fully known, as it is a function of the random sampling regime. Refer to Austroads (2006b) for further reading on additives. Stabilised materials are usually described as ‘modified’ if only a relatively low level of binder is added (such as up to about 2% by mass). The addition of low quantities of lime or cement may serve to reduce the plasticity and improve marginal granular material such that it doesn’t act as a bound layer. If high quantities of cement are used (e.g. > 2% by mass) shrinkage cracking may ensue, which may reflect through to the surface. Experience in highway applications suggests that shrinkage cracks from cement treated subbase layers is substantially retarded when there is at least 175 mm cover. However the caution is noted that the rate of reflection may be related to the magnitude of vehicle loading. The type and quantity of stabilant affects the assigned modulus for the layer which should be determined by laboratory testing. The curing conditions and compaction in the field can have a significant affect on the modulus and fatigue performance of bound layers. Prudence also needs to be exercised in the adoption of the fatigue performance parameters especially for variable materials. Ideally, some laboratory fatigue characterization should be done, to gauge the material performance and check the validity of any assumed fatigue performance relationship. In the conduct of the flexure test an appropriate density must be replicated recognizing the effect of compaction density gradient and potential reduction at the bottom of the bound layer. Generally the Unconfined Compressive Strength (UCS) is used as a specification parameter. A number of empirical UCS modulus relationships exist (e.g. modulus equals 1000 UCS (MPa)) and the pavement designer should be aware of the substantial range in the scale of factors.

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Pavement Materials

The fatigue performance of stabilised material is a problematic design issue because of the change in material performance with time (curing),the effects of fluctuation in density and moisture content and the effect of shrinkage cracking. Great care needs to be taken in the pavement design especially where the stabilised material is a significant determinant of overall pavement design life. Refer to section 6 below, for further discussion. Materials can also be mechanically stabilised, by blending components without necessarily the need for binding agents (chemical additives). In such cases, the components are blended in proportions to achieve a target PSD and Atterberg Limits and ideally the product strength should then be assessed, using CBR &/or Repeated Load Triaxial (RLT) testing, which may also be valid for “modified” materials.

Pavement Materials

Subgrade The determination of an appropriate modulus of the subgrade layer for heavy duty pavements is similar to highway and road pavement structures. Designers are advised to refer to Chapters 4 and 5 of Austroads (2004), or the usual local standard, for advice on characterizing subgrade materials. However industrial pavements are often located in areas of extremely complex and very weak geological conditions with for instance extremely thick layers of saturated estuarine silts. The designer is cautioned that particularly in the case of extremely weak or saturated subgrade conditions the need for detailed and competent geotechnical exploration is essential to ensure a complete understanding of the conditions and the associated risks (refer Rollings and Rollings, 2005 and ASCE, 2001). While pavement thickness design may ensure the subgrade is adequately protected to limit deformation by shear failure, geotechnical advice is essential to prevent the potential for substantially greater loss of shape due to differential consolidation. It is noted that the subgrade stress distribution in heavy duty pavements is significantly different than that occurring normally in road pavements, due to the higher magnitude of loading and load duration. It is important, therefore to recognize that subgrade performance models used routinely for highway pavement design are generally not applicable for pavements subjected to loading by much heavier vehicles that impart far higher stresses in the pavement and with greater areas (depths) of influence on material behaviour. Refer to Section: Subgrade Properties and Performance Models on page 43 below for further details.

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Traffic

Traffic The following sections detail typical heavy duty traffic considerations for the design of heavy duty flexible pavements for ports and terminal container areas.

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Traffic

Vehicle Types In order to design a heavy duty pavement, it is important to have detailed information on the types of vehicles that will operate on the site. It is possible that both off-road and heavy road-use commercial vehicles, such as semi-trailers, may traffic the site. Initial contact should therefore be made with the facility operator, to obtain details of the type of vehicles using the site, including their load configurations and paths through the site. A wide range of vehicle types are used at intermodal/container terminals such as straddle carriers, forklifts, gantry cranes, and semi-trailers. For mechanistic pavement design, it is important to know what the typical wheel loads are for any given payload on the vehicle. Theoretically these loads can be calculated from the geometry and mass of the vehicle. A more practical approach is to use axle load values given in specifications provided by equipment manufacturers. This approach is used in HIPAVE. Container handling equipment can be broadly sub-divided into two categories according to the load transfer characteristics: •

unequal loads on each axle; and

•

equal loads on each axle.

Traffic

Unequal Axle Loads At ports and container terminals, there are many vehicles that have unequal axle loads. Examples of these vehicles are Fork Lifts and Reach Stackers. In this case, the vehicle loading characteristics are specified in terms of two load cases that express the axle loads as a function of Container Weight. For example this could be the Unladen case together with one specific Container Weight. Figure 3 below illustrates the concept of unequal axle loads. Axle loads for other container weights are obtained automatically by linear interpolation.

Figure 3: Load Distribution and Position of an Unequal Axle Load Using HIPAVE

Equal Axle Loads Vehicles such as straddle carriers are assumed to have equal loads on each axle. In this case the vehicle loading characteristics are specified in terms of the unladen and laden weights of the vehicle, the number of axle rows (i.e. the number of axles seen from one side of the vehicle), the total number of wheels on the vehicle and the tyre pressure. The traffic analysis should therefore consider the number of trips in the “design area” by both laden and unladen vehicles.

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Traffic

Coordinate System for Vehicles In the evaluation of vehicles, the X axis is taken as the direction transverse to the lane as shown in Figure 4. To ensure consistency between results for different vehicle types it is recommended that X = 0 correspond to the lane centreline. Usually all vehicles are assumed to have their centrelines at X=0.

Figure 4: Example of Coordinate Positioning Figure 5 illustrates the convention used to define the wheel locations. This example is for a Hyster Fork Lift -Model H40.00-16CH. HIPAVE will normally model the two axle loadings as separate components, with the front axle (assumed to be on Y=0) as component 1 and the rear axle as component 2. Modelling the two axles as separate components means that the two axles are modelled as two separate load cases, i.e. there is (assumed to be ?) no interaction between axle loads. In practice, it is usually only necessary to model the wheels on one side (X ≥ 0) of the vehicle, however, it may be prudent to model the whole axle, to verify whether there is interaction between the wheels . It may also be prudent to model all axle groups in one load case, where the distance between axles is similar to the width of the vehicle.

Traffic

Figure 5: Wheel Load Location for a Hyster Fork Lift – Model H40.00-16CH

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Traffic

Vehicle Wander Vehicle Wander is the design parameter representing the directional tracking width of the vehicle, which usually can be represented by a normal distribution, or wander width, around a notional centre-line along the vehicle path. It is important for the pavement designer to recognize that vehicles at ports and container terminals may not always travel along the confined wheel-paths due to the scale of the site and nature of the operations. Thus, the facility owner should be consulted about details of typical vehicle movements, including apparent wander width, which the designer can then use in the design model. One of the unique features of HIPAVE is that it is able to model vehicle wander, enabling economical pavement design It should be noted that vehicle wander is not normally considered in routine road pavement design, due to the narrow lane width, hindering any significant wander. However, in the design of heavy duty pavements, it should be considered as it can have a significant impact on long term performance of the pavement structure and hence, pavement construction cost. For example at ports, gantry crane areas may result in manouevres that are heavily channelised while in other areas where vehicles are not as restricted, there might be extensive wander.

Traffic

Payload Distribution Estimating the payload distribution is a critical component of the pavement design process. The relative proportions of each container weight in the overall spectrum are important for economical pavement design. A relatively small number of heavy loads may be more damaging than a higher number of smaller loads. It is also important to account for the fact that each vehicle will handle a range of container weights or payloads. Ideally, the designer should be able to to specify the detailed container weight distributions. For example, the British Ports Association Guide (1996) includes information on container weight frequency spectrum, based on data provided by United Kingdom (UK) ports. Figure 6 shows the container weight distribution for 40 foot containers. HIPAVE, in contrast to other existing techniques, does not force the designer to use a single design container weight, or to convert all vehicle characteristics to repetitions of an “equivalent” design vehicle or load. HIPAVE allows the designer to input detailed container weight distributions which ultimately provides a more realistic impact of payload distribution on the pavement structure.

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Traffic

Figure 6: Container weight distribution for 40 foot containers at UK ports (British Ports Association 1996). Care should be taken to ensure that the container load spectrum is reasonably up to date. For example, the summary data provided by the British Ports Association Guide (3rd edn, 1996), is the same as used in the second edition (1986) – so is at least 20 years old. Data provided by some major Australian port terminals suggest that the peak loads may be 4-5 tonnes higher than the BPA data.

Traffic

35

Traffic Growth The compound growth of traffic volume is commonly specified as a percentage increase in annual traffic volumes. If compound growth is constant throughout the design period, the cumulative growth factor over the design period can be calculated as shown in Equation 3. =

Cumulative Growth Factor (CGF) =

(1 + 0.01R)P − 1 0.01R

for R > 0

P

for R = 0

[3]

where R = Annual Growth Rate (%), and P = Design Period (years). Table 3 below provides values of CGF for a representative range of design periods and annual growth rates, P and R respectively. Table 3: Cumulative Growth Factor (CGF) Design Period (P)

Annual Growth Rate (R) (%)

(years)

0

1

2

3

4

6

8

10

5

5

5.1

5.2

5.3

5.4

5.6

5.9

6.1

10

10

10.5

10.9

11.5

12.0

13.2

14.5

15.9

15

15

16.1

17.3

18.6

20.0

23.3

27.2

31.8

20

20

22.0

24.3

26.9

29.8

36.8

45.8

57.3

25

25

28.2

32.0

36.5

41.6

54.9

73.1

98.3

30

30

34.8

40.6

47.6

56.1

79.1

113.3

164.5

35

35

41.7

50.0

60.5

73.7

111.4

172.3

271.0

40

40

48.9

60.4

75.4

95.0

154.8

259.1

442.6

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Traffic

Dynamic and Static Structural Loading The pavement designer also needs to have an accurate representation of the structural loading on the pavement. This data can often be obtained by the facility owner or from the vehicle equipment suppliers. Information on the typical range of possible loadings on the vehicles should also be incorporated into the design. Special attention should be given to high stress areas such as situations where vehicles conduct tight turns/ cornering manoeuvres, braking/acceleration, or in areas where the dynamic effects associated with rough surfacings can be of critical importance. In short, these areas result in higher stress and it is appropriate in those situations to apply a load multiplication factor. Table 4 provides some guidance on how to address this situation and is based on British Port Association (1986, 1996).

Traffic

Table 4: Suggested Load Factors to Address High Stress Areas Load Factor* Vehicle Braking Cornering Acceleration Uneven Surface Front Lift Truck

1.3

1.4

1.1

1.2

Straddle Carrier

1.5

1.6

1.1

1.2

Side Lift Truck

1.2

1.3

1.1

1.2

Tractor & Trailer

1.1

1.3

1.1

1.2

*Note: where conditions apply simultaneously, the factors should be multiplied together. The values provided in Table 4 are provided as guidance. However, engineering judgement / experience should be exercised in adopting load factors. For example: 1) In an area where a front lift truck is exposed to an uneven surface, a load factor of 1.2 could be applied. 2) In an area where the side lift truck is accelerating and also exposed to a corner, the load factor could be 1.1 *1.3 = 1.43 The pavement loading is usually represented in the design model, as circular loading, at constant tyre stress, as applied by the tyre ‘footprints’. It is likely, in reality, that the tyre footprint is more of an elliptical contact area, with non-uniform contact stress, but a circular contact area is adopted to simplify calculations. Furthermore, the design of thin asphalt surfaced pavements, under heavy point loads, may be considered problematic due to the size of the load footprint and magnitude of the load, in relation to the layer thickness (refer to further discussion on Asphalt Fatigue … page #). At this point in time, a simple approach to modeling the various effects is a reasonable assumption. However, it is important to note there are some shortcomings with current knowledge about the interaction of closely spaced axle groups and subsequent recommendations (Wardle et al, 1999), particularly in relation to assessing stresses & strains in the subgrade area.

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Modelling of Multiple Wheels and Axle Groups HIPAVE lets you model the pavement with the actual wheel layouts of the vehicles that operate on the pavement. Care needs to be taken to select which wheels to include in the model. Extensive research gives us guidance on choosing the "right" combination of wheels to include in the model. Using more wheels can lead to inaccurate model predictions (Wardle et al, 1999). Further details are given in the HIPAVE User Manual (Wardle, 2005). The recommended model for base/sub-base materials and subgrade performance relationship recommended for heavy duty loads is described below (Material Properties and Performance Models on page 43). This methodology was derived from full-scale aircraft pavement tests conducted by the US Army Corps of Engineers at their Waterways Experiment Station (WES). These 'WES' tests were essentially conducted using single gear assemblies. No tests were carried out to investigate the increased damage that might result due to interaction effects of adjacent gear assemblies. Considerable uncertainty exists with respect to prediction of damage for aircrafts that have main gears in close proximity. APSDS has been used to study multiple gear interaction effects for a Boeing 747 and 777 aircraft using a range of alternative damage models (Rodway 1995a, Wardle and Rodway 1998, Rodway, Wardle and Wickham 1999). Results from these studies show that the successful calibration of simplified design models against the full-scale test data does not create a capability to confidently extrapolate beyond the limits of the test data. The studies showed that simple damage models give unrealistic predictions for the damage caused by all sixteen main wheels of the aircraft when compared to that computed for a single isolated 4-wheel gear. Three different performance models, each of which gave a similar 'goodness of fit' to the full-scale test data, gave greatly different predictions of the damage caused by the interactions of the sixteen main wheels. The differences between the alternative predictions increased with increasing depth to subgrade. Given the above comments, as a general rule only groups of wheels that are within two metres of each other should be modeled as a single load case. For example, the most appropriate way of modeling a Fork Lift is described in the section Coordinate System for Vehicles.

Traffic

Nature of Damage Pulses The WES tests were performed on relatively thin pavements. In most of the test sections the elastic models predict a distinct strain pulse at subgrade level for each axle of a two-axle gear. For deep pavements (say 1.5 m or more) the models predict a single combined pulse resulting from the entire gear. In other words, a two-axle gear produces two strain pulses per pass for shallow subgrades and one strain pulse, of significantly different shape, for deep subgrades. HIPAVE uses strain repetitions as the basis for damage predictions, not passes or coverages. Pulse counts and pulse shapes both change with pavement thickness. There is significant uncertainty in the design of thick pavements because data must be extrapolated from thinner test pavements which have narrower pulses than those expected for the deeper subgrades. There is still no experimental data to show to what extent pavement damage depends on the transverse and longitudinal widths of the load pulse. The designer is referred to the US research at the National Airport Pavement Test Facility (NAPTF). This facility has conducted full scale pavement test loading under simulated B747 and B777 load gear hence loading and pavement configurations are of a similar dimension to heavy industrial pavements. Numerous researchers are analysing the performance of the test pavements and this will lead to improvements in the design models.

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Traffic

Design Traffic Loading Pavement damage is a function of the cumulative damage induced in the pavement and subgrade materials, by the traffic, over the design period. The duration of the design period affects the pavement composition and hence, construction (materials) cost. The predictive capacity of the design, is related to the accuracy of the design data, therefore, the facility operator should be consulted to advise on the planned traffic usage of the site, in terms of the number and types of vehicles, throughout the design period.

New Pavement Design

New Pavement Design

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42

New Pavement Design

Design Period The purpose of the pavement design, is to ensure with a high degree of confidence, that the pavement is structurally adequate to ensure it remains in serviceable condition, without significant maintenance expense, throughout the designated design period. Some suggested design periods are as follows : •

rehabilitation of existing in-service pavement : 10 – 15 years

•

new pavement construction or major pavement rehabilitation : 15 – 25 years

The design period refers to the serviceable life of the pavement structure. It can also be considered as the time when pavement distress, sufficient to render the facility practically dysfunctional, occurs over a significant proportion of the area. A distinction exists between the structural and functional performance parameters This differentiation is important and the designer must ensure the owner understands that that the surfacing may require cyclical rehabilitation to remedy deterioration of the functional performance parameters, i.e. roughness and rutting, within the design period. The mechanistic pavement design method is outlined in Austroads 2004. HIPAVE takes into account the effects of vehicle wander that is a much more prominent design consideration than with roads. Depending on the operational logistics the industrial pavement may have highly channelised traffic in tight lane configurations or more random and wider traffic paths in roadways. Industrial pavements could be significantly over or under designed if vehicle wander is ignored. Additionally HIPAVE enables the estimation of damage over the design container weight spectrum, to again avoid costly over or under design.

New Pavement Design

Material Properties and Performance Models A discussion of the factors that contribute to the performance of flexible pavement structures is given in the Appendix: Material failure mode and implication on page 86. The mode and consequence of failure of the different pavement materials will impact the selection of pavement components. By this process the intention of this discussion is to focus designers’ attention on the factors that may impact on the realisation of the properties used in the design. Experience has shown that poor pavement design methodology may reduce the performance of the pavement, but poor construction quality will devastate the performance. Each passage of a vehicle over a unit of pavement area (and indeed within an effective vicinity) causes damage to pavement material layers. The damage accumulates with each vehicle pass, resulting eventually in ultimate pavement failure. The mechanistic pavement design method attempts to determine the design life of the pavement, in terms of number of load passes until a defined failure of the pavement has occurred. In the mechanistic empirical design method two separate modes of failure are assumed; deformation due to subgrade shear failure, or fatigue cracking of layer(s) of bound pavement materials (which will ultimately result in subgrade shear failure due to consequent loss of load spreading). In reality both modes are a simplification of a complex environment. Deformation may occur as a consequence of consolidation and shear failure in pavement layers; cracking and loss of strength may occur in some pavement materials not as a consequence of fatigue. Care needs to be taken to ensure that realistic data is input and that material performance models used in design are valid for the load case under consideration. There currently is no widely accepted purely mechanistic pavement design undertaken and a semi-empirical / mechanistic approach is usually adopted. Subgrade performance models are currently empirically based on field trials, correlating axle group passes to deformation (usually 20 mm deep rut) in various subgrade soil conditions (strengths). The asphalt component is designed to resist premature deformation and premature flexural fatigue induced cracking, with the former catered for in the mix design process and the latter in the pavement thickness design process.

Subgrade Properties and Performance Models In general practice in Australia the stiffness or modulus of the subgrade ESG (MPa) is related to the CRB in the general relationship ESG (MPa) = 10.0 CBR

43

44

New Pavement Design

It must be understood that this is but one of numerous modulus/CBR relationships that have been derived by various researchers. However the value of assigned subgrade modulus is probably less critical to the outcome than the accuracy of the damage models used in the design. In the derivation of the following subgrade deformation model the above relationship was used. It must be understood that if a different relationship were used, a different damage model would be derived. Caution should be exercised before adopting road-based models for design in offroad situations, such as airports and ports, because of the much greater magnitude of loading with the latter cases and non-linearity of subgrade behavior. In general, the subgrade strain relationship, can be expressed as follows: N Where

N

=

= (k / ε)b

[4]

predicted design life (at strain level ε)

k

=

material (subgrade) constant

b

=

material damage exponent

ε

=

load induced strain in the material

Wardle et al (2001) report on the US Army Corps of Engineers CBR method (Method S77-1), for design of flexible aircraft pavements, which has yielded generally satisfactory pavement performance, when used for design of pavements over a range of subgrade strengths and vehicle loadings. Wardle et al (2001) report the results of back-analysis pavements on subgrades from CBR 3 – 15%, for aircraft masses ranging from 40 – 397 tonnes, using APSDS to derive the performance constants ‘k’ and ‘b’ (see above), below. Accordingly, the following subgrade performance model is suggested for aircraft between 40 – 400 tonnes (tyre pressures listed in Wardle et al (2001)), for subgrade design CBR ranging from 3 – 15 %, for 10,000 to 100,000 vehicle passes during the design period: k

=

(1.64 x 10-9 x E3) – (4.31 x 10-7 xE2) + (2.18 x 10-5 x E) + 0.00289

b

=

(-2.12 x 10-7 x E3) + (8.38 x 10-4 x E2) – (0.0274 x E) + 9.57

E

=

subgrade modulus (MPa; usually expressed as 10 x CBR )

The above performance relationships may be used with prudence, for design of pavements supporting heavy off-road vehicles, such as at ports / container terminals. More recently analyses of the performance data from the full scale trials at the National Airport Pavement Test Facility (NAPTF) has been carried out (Lancaster 2006) in an attempt to improve the empirical verification of our design models. The findings from those analyses were inconclusive because of the various failure mechanisms observed however the analyses did not indicate a need to change the current modeling practices.

New Pavement Design

Unbound Granular Material Properties The performance of the unbound granular layers is governed by the material specifications rather than thickness design. In other words it is assumed that the specification ensures that the materials used will resist crushing and shear failure under the applied stress. The adequacy of this approach is demonstrated empirically in the generally good performance of these materials in heavy duty industrial and airport pavements internationally. Historically failures in well specified granular base layers have been due to the effects of poor construction practice, excess moisture and other factors rather than inherent material inadequacy. The derivation of the subgrade performance model (previous section) resulted from analyses using the Barker-Brabston method derived at the US Corp of Engineers has been calculated and is shown in Figure 7 below. 1000

US CORP OF ENGINEERS (ARMY TM 5-825-2-1) SUBLAYERING FOR UNBOUND GRANULAR MATERIALS

MODULUS OF UPPER LAYER (MPa)

E2 500 MPa E2 320 MPa

E1 150 MPa

t = 200 mm Base t = 150 mm Base 100

t = 100 mm Base t = 200 mm Subbase t = 150 mm Subbase t = 100 mm Subbase

10 10

100 MODULUS OF SUPPORT LAYER (MPa)

1000

Figure 7: Sublayering of Unbound Granular Layers (after Barker and Brabston, 1975)

The stiffness of unbound granular layers varies with; 1

the quality of the aggregate (base or subbase material) soundness, durability, particle size distribution, angularity, etc.

2

the thickness of the layer, and

3

the stiffness of the supporting layer

4

moisture content (or saturation ratio) and PI

5

stress-state

6

relative density

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46

New Pavement Design

The Barker-Brabston model and the derived subgrade damage model assumes the granular layers to be isotropic. Preliminary analyses of the NAPTF trial data is being evaluated to test this model and results to date do not indicate a need for change. It is assumed that the specification limits of strength and durability will ensure the preservation of the layer stiffness. Empirical evidence suggests this is the case and there is no evidence of failures attributed to aggregate breakdown in compliant materials. To ensure the mobilisation of the Barker-Brabston base layer moduli the contract documents must specify compaction to be ≥100% modified compaction and dry back to