Development and validation of a novel testing procedure for the evaluation of rutting susceptibility of bituminous binders.

The research reported herein deals with evaluation of rutting susceptibility of different asphalt binders thorough various parameters and correlating them with parameters resulted from tests carried out on their corresponding mixtures. Several factors influence the occurrence of rutting in asphalt mixtures including aggregate properties, composition (aggregate gradation and binder content), and volumetric. However Asphalt binders play a major role in affecting rutting resistance of asphalt mixtures used in road pavements. In the first phase of research four different types of asphalt binders were analyzed by subjecting them to a new protocol combining Single Creep-Recovery (SCR) tests with Multiple Stress Creep (MSC) tests on DSR. Low shear viscosity (LSV) and Multiple Stress Creep Recovery (MSCR) tests were also performed for comparison purposes. LSV and MSCR tests are currently known as standard procedure for the evaluation of rutting susceptibility of binders. However, recent studies have shown the existence of significant drawbacks for LSV and MSCR method when applied to highly modified polymers products. Three shear stress levels (20, 100 and 500 kPa) were applied in case of MSC tests. In case of SCR test 100 kPa shear stress was applied. Two neat binders having same penetration grade 50/70 but different sources were tested at four temperatures (46°C, 52°C, 58°C and 64°C). Asphalt rubber and SBS modified binder were tested at 58°C, 64°C, 70°C and 76°C. The higher range of temperature was used because the aim of testing protocol was to reach steady state flow during creep phase at all temperatures. In case of MSCR tests on AR and PMB tests were started at 64 °C and for NA and NB same temperature range selected for SCR and MSC was also applied. In case of SCR-MSC creep compliance rate from creep phase and permanent compliance from recovery phase were calculated in order to rank binders for their rutting susceptibility. In case of LSV and MSCR tests EVT (Equi-viscous temperature) and non-recoverable compliances were calculated respectively. Single creep-recovery tests combined with multiple stress creep were found to be more accurate in ranking binders for their rutting susceptibility. In the 2nd phase of research the rutting resistance of asphalt mixtures containing above mentioned binders was evaluated. In particular three dense –graded and three gap-graded rubberized mixtures were investigated. Two rubberized gap graded mixtures were prepared in laboratory, one with 2 % filler and the second one with 5 % filler. The third gap graded rubberized mixture was supplied by a plant located in the center of Italy. Amount of binders in all gap graded asphalt mixture was 8 %. The first Dense-graded asphalt mixture was prepared in laboratory with neat binder NA. The second one was collected from local plant which employed neat binder NB, having same penetration grade as the first one but different source and slightly lower maximum design temperature. The third mixture was also prepared in laboratory with SBS polymer modified binder. The binder contents in dense graded asphalt mixtures were 5.5 %. All the three mixtures were analyzed by subjecting cylindrical specimen to Flow number tests on APMT. Flow number has been recognized as newly anti-rutting indicator of asphalt mixtures. Flow numbers for all the mixtures were measured at 3 different temperatures (46°C, 52°C and 58°C). These testing temperatures were selected keeping in mind diversity of pavement temperature across the country. The 2nd purpose of selecting different testing temperature was to study how Flow number varies with change in temperature. The results obtained from mixtures were compared with those obtained from binders. Considering three binders NA, NB and SBS and their corresponding mixtures, it was observed that strong correlation exists between binder parameters and Flow number, so parameters from binder testing can be used to predict rutting susceptibility of their corresponding mixtures. Rectangular specimens were prepared from all rubberized gap graded asphalt mixtures and one traditional mixture. These specimens were tested on wheel tracking test device in order to compare results from AMPT with WTD.