Durability Testing of LCB and CTB Materials Supplied by Caltrans

Carbonation of cement-stabilized layers under concrete pavements in California was investigated as a potential reason for faulting of these pavements. In discussions with personnel from the Pavement Research Center, it was mentioned that a layer of loose material is often observed between the concrete slab and the supporting cement stabilized layer. Samples of materials commonly used under concrete slabs were sent to South Africa for durability testing using South African test methods. These test methods were primarily developed at the CSIR, Transportek, for the purpose of durability assessment of stabilized materials. The materials supplied by Caltrans consisted of samples of two aggregates used to make cement treated base (CTB) and lean concrete base (LCB). The aggregate sources for these materials were, named Mission Valley and Lake Herman. Details of gradings, water content, and cement content were provided with the samples. These materials were tested for durability using the Initial Consumption of Cement test (ICC), the mechanical and hand wet/dry brushing test, the erosion test, and the uncarbonated and carbonated unconfined compressive strength test (UCS). The test results were evaluated according to South African experience using these techniques and South African standards. Two compaction energy levels were also used to determine whether the density to which the materials were compacted had an effect on durability. Most of the specimens passed the test criteria at both compaction energy levels, suggesting that the amounts of cement used are excessive and may be reduced, thus creating the potential for cost saving in terms of cement costs. However, the erosion test indicated that at the lower compaction energy, the CTB materials are probably erodible under concrete slabs. This tendency toward erosion may be overcome by compacting the materials to higher densities.It is recommended that the in-service materials be compacted to higher densities and that further testing be done to determine the appropriate cement contents. The cement contents used here were those specified by Caltrans. A further recommendation is that site investigations be undertaken to enhance the knowledge gained from laboratory testing as the effects of other factors affecting layer performance such as drainage, poor compaction, material deficiencies, etc., can only be observed in-situ. The possibility of mechanical crushing of the cement-stabilized layer should also be investigated.