Summary of Construction Activities and Results from Six Initial Accelerated Pavement Tests Conducted on Asphalt Concrete Pavement Section for Modified-Binder Overlay

This report summarizes the activities and data collected during the construction of a pavement section used for investigating the performance of asphalt concrete pavements under accelerated pavement testing. This report also presents the preliminary results of six accelerated pavement tests conducted on the test section. The pavement section was constructed in September 2001 at the Pavement Research Center, located at the University of California Richmond Field Station. The construction was performed by a highway contractor with the purpose of simulating highway paving operations. Under these conditions, the results from the tests can be translated into predicting the behavior of actual in-service pavements. The pavement was composed of 90 mm of asphalt concrete, and 410 mm of recycled aggregate base on top of a prepared 200 mm subgrade. The layer thicknesses were designed according to Caltrans design procedures and checked using mechanistic methods to ensure limited rutting in the subgrade. Preparation and construction of the subgrade, aggregate base, and asphalt concrete were completed according to Caltrans practice. Compaction of the asphalt concrete was controlled based on the maximum theoretical density of the mix. Average in-situ relative densities for the subgrade and aggregate base were above 95 percent. Average air-void contents in the asphalt concrete layer were between 7 and 10 percent. Average thickness was 79 mm. Asphalt extractions from two samples indicated binder content by weight of aggregate of between 4.3 and 5.7 percent. The target binder content was 5.0 percent. Deflection testing conducted during the construction of the pavement section showed the effect of the asphalt concrete layer on the behavior of the aggregate base and subgrade layers. The asphalt concrete provided an increase in confining pressure, which created an increase in the modulus of the aggregate base, as well as an additional cover that reduced the stresses on the subgrade and created an increase in the modulus of the subgrade. The intensive FWD testing conducted on the pavement section also helped identify portions of the section susceptible to premature failure. These areas were subsequently rejected as locations for HVS test sections. In general, FWD testing indicated that areas of soft subgrade translated into areas of soft or low aggregate base modulus. The FWD testing also revealed the effect of asphalt concrete modulus on the behavior of the aggregate base. The data indicated that aggregate base modulus increased with asphalt concrete modulus. FWD testing also revealed the effect of temperature on the modulus of the asphalt concrete, which is typical of asphalt concrete layer and important for the interpretation of the performance of asphalt concrete mixes. The Heavy Vehicle Simulator (HVS) was used to test the asphalt concrete under conditions of accelerated loading. HVS test sites were selected within the constructed test section to evaluate their performance. The results were compared in terms of fatigue cracking, rutting, and surface deflections. Results indicate that the sections tested during the dry/warm season lasted longer than those tested during the wet/cold season. The performance of the sections seems to have been controlled by the behavior of the aggregate base. Elevated moisture contents in the aggregate base were recorded during the wet/cold months with corresponding FWD results which indicated high aggregate base modulus values for the same period. The results suggest that the modulus of the aggregate base is not a good indicator of performance. The results of the HVS test sections are being used to analyze the performance of asphalt concrete pavements and to develop performance models for pavement life prediction as defined in Research Goals 4.1, 4.5, and 4.7 in the PPRC Strategic Plan for 2003/2004.