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.