Low and intermediate temperatures fracture behavior of amorphous poly alpha olefin (APAO)-modified hot mix asphalt subjected to constant and variable temperatures

•Pure mode I and II fracture resistance of APAO-modified HMA under CT and VT cycles were evaluated.•SCB geometries including vertical and angular cracks were selected to evaluate the fracture toughness and fracture energy.•APAO improved the fracture energy and fracture toughness of HMA under CT and VT cycles.•Adding APAO to the HMA mixture increased the fracture brittleness under CT and VT cycles.•Mixtures containing 9 and 6% APAO were selected as the best samples based on the fracture mechanics results. Low-temperature cracks (LTC) and intermediate-temperature cracks (ITC) are two important types of cracks in asphalt mixtures that cause damage to pavements and incur high maintenance costs on roads. On the other hand, temperature cycles, in addition to the traffic load on the asphalt mixtures, increase the possibility of structural failures. Therefore, it is necessary to study the solutions to deal with and control this type of cracking using appropriate methods. In this study, an attempt was made to evaluate the effect of amorphous poly alpha olefin (APAO) additive on the LTC and ITC behavior of Hot Mix Aspahlt (HMA) under loading conditions of modes I and II under constant temperature (CT) and variable temperature (VT) cycles. For this purpose, two geometries called symmetric specimen SCB (contains a vertical crack with an angle of 90°) and classical-modified specimen SCB-2 (contains an angled crack with an angle of 35°) were selected to simulate the loading conditions of mode I, as well as two geometries called modified specimen SCB-1 (contains a vertical crack with a 90-degree angle) and modified specimen SCB-2 (contains a vertical crack with a 90-degree angle) were selected to simulate mode II loading conditions. A total of four HMA mixtures were made, including one mixture without additives and three mixtures containing 3, 6, and 9 % APAO additives. Next, in order to simulate and match the road conditions with the laboratory samples, CT and VT conditions were applied to all samples. At temperatures of ± 15 °C, two fracture resistance indices called fracture toughness (KIC and KIIC) and fracture energy (GF) were studied to investigate the fracture behavior of HMA mixtures. Also, two other fracture indices, namely the Tensile Stiffness Index (TSI) and the Tensile Strength (TS), which represent the fracture behavior of the HMA mixture before the peak load, were evaluated. Finally, the flexibility of the mixtures was evaluated using brittleness indices s