Review of the Erosion Criteria for Bound Subbases in the Australian Rigid Pavement Design Procedure

The current Australian rigid road pavement design procedure considers both structural concrete slab fatigue and subbase erosion. The erosion component was introduced in the 1992 edition of the Austroads pavement design guide as nomographs and incorporated as algorithms in the 2004 edition of the guide. The erosion distress criteria were based on the USA 1984 Portland Cement Association (PCA) method. which used the AASHTO road test observations about pumping of subbases to address some modes of pavement distress unrelated to fatigue (flexural stresses). Lean mix concrete subbases were not covered in the assessment. Subbase erosion is defined as the removal of fines from the surface of the subbase under the concrete pavement slab and can lead to pumping, voids, and faulting. In addition, the voids, loss of support, and changes in the friction coefficient would result in an increase in slab stresses and subsequent premature cracking. Erosion is dependent on slab corner deflection, water between the slab and subbase, and the erodibility of the subbase. Erosion is more critical with thinner slabs where the slab corner deflection is higher, and therefore highly influenced by the presence of shoulders and dowels. A property not directly (indirectly through the effective CBR) incorporated in the Australian (and PCA) design procedure is the erodibility of the subbase material. Australian and international research findings since 1984 on the characterisation of the erodibility of subbase materials have not been included the Australian (and PCA) design procedures. The appropriate incorporation of these research findings could lead to more optimised designs, especially for pavements for lower traffic loading which would require thinner slabs. This paper discusses the principles of pumping and subbase erosion, presents relevant local and international research on subbase erosion, and demonstrates the effect of the inclusion of erosion on rigid pavement designs in Australia. The latter shows that substantial slab thickness reductions (and therefore costs savings and sustainability benefits) can be realised if subbase erosion is more accurately considered. Therefore, a more detailed review of the Austroads design procedure is warranted, which could lead to reduction in concrete slab thicknesses, lower costs, and sustainability benefits (reduced use of resources).