Evaluation of long-term fracture behavior of amorphous poly alpha olefin (APAO)-modified Hot Mix Asphalt (HMA) under modes I and II at low and intermediate temperatures

Abstract

Opening and in-plane sliding cracks are among the most important cracks in flexible pavements, resulting from tensile and sliding deformations, respectively. Loading and freeze–thaw damage (FTD) are the two main factors for the initiation and growth of these types of cracks, which may lead to two types of cracks, intermediate-temperature cracks (ITCs) and low-temperature cracks (LTCs), depending on the ambient temperature. Therefore, in order to simulate the tensile and sliding deformations of mixtures containing ITCs and LTCs, two geometries called symmetric sample SCB (containing vertical cracks) and classical-modified sample SCB-2 (containing angular crack) resulting from mode I loading, as well as two geometries named modified sample SCB-1 (containing vertical crack) and modified sample SCB-2 (containing vertical crack) resulting from mode II loading at temperatures of + 15 and −15 °C were evaluated. In order to control and reduce the cracking potential of Hot Mix Asphalt (HMA), the amorphous poly alpha olefin (APAO) additive was used in percentages of 3, 6, and 9 % and compared with the mixture without additives. Also, all specimens were under 0 and 3 FTD cycles. In order to measure and evaluate the fracture behavior results of unmodified and APAO-modified specimens, fracture toughness indicators (KIC and KIIC) and fracture energy (GF) were calculated. Eventually, to investigate the fracture behaviors of mixtures before the maximum loading, two indicators of tensile stiffness index (TSI) and tensile strength (TS) were evaluated. The results showed that at −15 °C and from the perspective of KIC (or KIIC) and GF (under modes I and II) indicators, the fracture resistance in specimens comprising 6 and 9 % APAO under 0 and 3 FTD cycles was higher compared to the base specimen. At + 15 °C and from the perspective of KIC (or KIIC) indicator, the fracture resistance of mixtures containing 6 and 9 % APAO in 0 and 3 FTD cycles was higher than the unmodified specimen; however, from the point of view of GF (under modes I and II) indicator, the mixture containing 9 % APAO represented greater fracture resistance. Also, it was indicated that mixtures modified with APAO represented higher TSI as well as TS values; therefore, APAO additive increased the resistance to elastic deformation by increasing the fracture performance before the maximum loading. Finally, mixtures containing 6 and 9 % APAO were chosen as the best mixture; however, the use of these mixtures in areas with temperatures lower than −15 °C should be done with caution due to the relative reduction in flexibility of the mixture containing APAO additive.