Stiffness, Strength, and Performance of Unbound Aggregate Material: Application of South African HVS and Laboratory Results to California Flexible Pavements

Caltrans specifies the use of a 75-mm Asphalt Treated Permeable Base (ATPB) layer as standard practice for all new flexible pavement designs in California.(1) The ATPB acts as a drainage layer beneath the asphalt concrete (AC) and is supported by an aggregate base layer. However, the potential exists for water to enter the unbound aggregate base layer from the ATPB layer through the prime coat that separates them. The stiffness, strength, and performance of unbound aggregate layers are largely influenced by moisture content.The assessment of the performance of two types of flexible pavement cross section, one with and one without an ATPB layer, were included in the strategic plan of the CAL/APT program (1994-2000). Goal 1 and Goal 5 of the CAL/APT project (continued in the Partnered Pavement Research Program after 2000) consist of the evaluation of these two sections under dry and wet base conditions, respectively.The evaluation includes accelerated pavement testing using the Heavy Vehicle Simulator (HVS) and laboratory testing. The test plan for Goal 5 (2) includes, the evaluation of the effects of compaction and water content on the stiffness of the aggregate base and subbase layers.The first objective of Goal 5 is to measure the effectiveness of the ATPB layer in the drained pavement in preventing a decrease in stiffness and strength of the unbound layers. Those results are included in References (3-6). It is, however, not only the degree of saturation of an unbound aggregate that influences the performance of the material but also the level of compaction of the material.The objective of this report is to illustrate the effect of the level of compaction and the degree of saturation on the stiffness, strength, and plastic deformation of unbound aggregate layers based on information obtained from HVS and laboratory testing in South Africa. The results will permit extrapolation of Goal 1 and Goal 5 results to other California materials and structures. The information presented in this report also contributes towards the improved understanding and modeling of the behavior of unbound aggregate material for mechanisticempirical design purposes.The scope of the information presented herein is limited to the aggregate used for pavement base and subbase layers. A comparison of California and South Africa specifications for base and subbase aggregate material is presented to identify possible similar material categories for which it is thought similar response and behavior will be exhibited. HVS tests that were performed on pavements with unbound aggregate base layers were identified and the deflection and permanent deformation responses of these test sections were extracted from the CSIR HVS database to be evaluated in terms of the objective of this report. Data from laboratory projects that were performed in association with the HVS program in South Africa in addition to the HVS data are presented in order to facilitate a more detailed evaluation of the effect of density and degree of saturation on the stiffness, strength, and plastic deformation of unbound aggregate material.