Impact of Binder and Aggregate Type on the Cracking, Rutting, and Moisture Susceptibility of Asphalt Mixes in Oklahoma: A BMD Approach

Several transportation agencies have moved toward the study and implementation of balanced mix design (BMD) in asphalt mixes. BMD is based on optimizing the mixture's mechanical properties using performance-related tests to achieve optimum rutting, cracking, moisture resistance, and durability. An important aspect of BMD implementation is to understand the effect of mix variables on different aspects of performance. In this study, the performance of five mixes prepared with different aggregate types (limestone and granite aggregate), different aggregate gradation (coarse and fine gradations), and different binder grades (a neat PG 64-22 and a polymer-modified PG 70-28) was evaluated using indirect tensile asphalt cracking (IDEAL-CT), the Hamburg wheel tracker (HWT) test, the modified Lottman test, and the moisture-induced stress tester (MiST). Using the IDEAL-CT, a cracking tolerance index (CTIndex) of 196 and 118 was obtained for the PG 70-28 and PG 64-22 mixes, respectively. The higher CTIndex for the PG 70-28 mixes was largely attributed to the increase in mix ductility as measured by the l75/|m75| parameter. The effect of aggregate type on the CTIndex was more pronounced for mixes with coarse gradation than those with fine gradation. Using the HWT test, it was shown that the corrected rut depth (CRD) provides a better measure of rutting compared to the total rut depth. All mixes met the CRD rutting criteria for high traffic except for the mix made with limestone, neat binder, and fine gradation, which only met the criteria for low traffic. Based on the tensile strength ratio (TSR), it was shown that the MiST procedure was less severe compared to the modified Lottman procedure for the mixes included in this study. All mixes showed good moisture susceptibility based on the MiST but when using the modified Lottman test, all mixes made with PG 64-22 showed either a marginal or poor moisture susceptibility. The different moisture tests provided different rankings of the mixes due to their different failure mechanisms.