Laboratory characterisation of pavement foundation materials

Pavement foundations for major roads in the UK were historically designed on an empirical basis, related to a single design chart, restricting the incorporation of superior performing materials or materials for which the empirical data sets were not available. The adoption of performance based specifications was promoted as they are perceived to ‘open up’ the use of alternative materials (including ‘local’ sources of primary materials) or techniques, and allow for the incorporation of superior performance into the overall pavement design. Parallel developments to the performance based design of pavement foundations (allowing for superior performance) and in situ testing required the support of laboratory based performance tests. These laboratory based tests were required to determine material performance parameters (elastic modulus and resistance to permanent deformation) for both unbound and hydraulically bound pavement foundation materials. A review of the available laboratory apparatus indicated that they were either; unrealistic (and hence unable to provide the required material performance parameters), or overly complex and more suited to fundamental research. Therefore, the requirement for developmental research work was identified. The research reported herein details the development, manufacture and initial evaluation of simplified laboratory apparatus (the Springbox for unbound materials and static stiffness test for hydraulically bound mixtures) designed to produce the performance parameters of elastic modulus and relative resistance to permanent deformation for pavement foundation materials. The equipment and test procedure evaluation was undertaken across a range of materials, giving initial guidance on likely in situ performance. The innovative laboratory apparatus and materials guidance (including the potential to use recycled and secondary aggregates) was incorporated into key Highways Agency specification and guidance documents, which in turn influence construction practice outside of motorways and major trunk roads. This research concludes by outlining a number of recommendations for continued development and evaluation, including feeding back data sets from long term in situ performance testing for subsequent refinement of assumptions

LWD best practice guide

This Best Practice Guide has emerged from a working group (Pavement Foundations Group) to address the need for consistency in the implementation of LWD’s into UK practice. However, the guide does reflect best practice for a range of applications. It describes the industry best practice for using Lightweight Deflectometers to verify the construction quality of road foundations. The guide is seen as a statement of current knowledge, and includes recommendations for site operations. It is expected that this guidance will be updated periodically

Simplified laboratory assessment of subgrade performance parameters for mechanistic design of pavement foundations

With the increasing agenda for sustainability, the UK is attempting to move away from the empirical design of pavement foundations to develop a performance specification approach to facilitate analytical design. For analytical design the measurement of the subgrade performance parameters of resilient modulus and resistance to permanent deformation are required. These parameters ideally need to be assessed concurrently under loading and environmental conditions similar to those the materials will experience in the field. To date, measurement of these parameters is largely confined to research laboratories using cyclic triaxial testing with advanced on sample strain measurement. This apparatus is considered too complicated for routine commercial use, hence this potentially limits the implementation of laboratory performance evaluation for routine pavement foundation design. A previous program of cyclic triaxial testing on clay subgrades indicated a series of useful correlations between strength and permanent deformation behavior (via a threshold stress), and material resilient modulus at this threshold. This paper reviews the previous work and utilizing these correlations presents data from tests on three different clay materials performed to develop simplified equipment and procedures for the routine measurement of the required design parameters. It is shown that simple pseudo-static tests can measure a subgrade modulus for a simplified performance based design. It re-evaluates the previous data (in the light of the recent work) to show a boundary correlation that may allow a shear strength based parameter to control (in design) the onset of permanent deformation, and details how long-term subgrade water content changes can be accommodated

LWD best practice guide

This Best Practice Guide has emerged from a working group (Pavement Foundations Group) to address the need for consistency in the implementation of LWD’s into UK practice. However, the guide does reflect best practice for a range of applications. It describes the industry best practice for using Lightweight Deflectometers to verify the construction quality of road foundations. The guide is seen as a statement of current knowledge, and includes recommendations for site operations. It is expected that this guidance will be updated periodically

Pavement foundation stiffness testing: a new regime

Traditionally the construction of the foundations of paved infrastructure followed a recipe approach where specified materials are laid using specified plant in an approved way following a method specification. This approach is prescriptive and limits material use to those that meet the recipe assuming a given level of performance after completion. To encourage sustainability the UK Highways Agency launched new pavement and foundation design guidance that is moving away from this prescriptive approach (IAN 73/06 revised in 2009, and HA 26/06). The guidance aims to allow a more flexible design and assessment of the required foundation performance parameters of strength and resistance to permanent deformation. This also introduced a stiffness assessment of the constructed foundation to confirm compliance with design. In contrast to the previous regime, the actual performance of the foundation can influence (and provide savings to) the design of the structural pavement layers above. The new guidance permits the use of lightweight deflectometers to assess stiffness compliance. These are becoming increasingly common tools in the checking of foundations of paved infrastructure. This paper presents the background to the use of deflectometers within the new guidance, and elements of a recently completed good practice guide for their use

Testing of unbound materials in the Nottingham Asphalt Tester Springbox

The current trend in mechanistic (analytical) pavement design is to use the mechanistic properties of pavement materials to optimize design. This is compatible with the move toward performance-based specifications and away from traditional empirically based design methods and recipe specifications. Other drivers-such as the Europeanwide adoption of aggregate mixture standards, which no longer differentiate on source, but moves toward sustainable construction-mean that a wide range of recycled, secondary, and primary aggregate sources can potentially be used in highway construction. The requirement for accelerated, performance-based testing is therefore coming to the fore. The U.K. Highways Agency has funded an accelerated testing program across a range of unbound capping and subbase materials. The performance parameters assessed, over a range of moisture and soaking conditions, are resistance to permanent deformation and resilient stiffness. The apparatus used during the unbound mixture assessments is the newly developed Springbox, which uses the standard Nottingham Asphalt Tester loading frame and software. This follows a simplification of the K-mold test and facilitates repeated loading of 170-mm cubic specimens under variable confinement. Samples are compacted into stainless steel liners. If required, samples can then be soaked before placement within the Springbox apparatus for testing. The following are discussed: the aggregate mixture performance parameters being measured, sample preparation procedures, the Springbox equipment and test procedures, a testing program of unbound capping and subbase materials, and results. The performance of the Springbox apparatus is discussed, and conclusions on the relative performance of the aggregates are presented

Testing of unbound materials in the Nottingham Asphalt Tester Springbox

The current trend in mechanistic (analytical) pavement design is to use the mechanistic properties of pavement materials to optimize design. This is compatible with the move toward performance-based specifications and away from traditional empirically based design methods and recipe specifications. Other drivers-such as the Europeanwide adoption of aggregate mixture standards, which no longer differentiate on source, but moves toward sustainable construction-mean that a wide range of recycled, secondary, and primary aggregate sources can potentially be used in highway construction. The requirement for accelerated, performance-based testing is therefore coming to the fore. The U.K. Highways Agency has funded an accelerated testing program across a range of unbound capping and subbase materials. The performance parameters assessed, over a range of moisture and soaking conditions, are resistance to permanent deformation and resilient stiffness. The apparatus used during the unbound mixture assessments is the newly developed Springbox, which uses the standard Nottingham Asphalt Tester loading frame and software. This follows a simplification of the K-mold test and facilitates repeated loading of 170-mm cubic specimens under variable confinement. Samples are compacted into stainless steel liners. If required, samples can then be soaked before placement within the Springbox apparatus for testing. The following are discussed: the aggregate mixture performance parameters being measured, sample preparation procedures, the Springbox equipment and test procedures, a testing program of unbound capping and subbase materials, and results. The performance of the Springbox apparatus is discussed, and conclusions on the relative performance of the aggregates are presented