Evaluation of fracture behavior of Warm mix asphalt (WMA) modified with hospital waste pyrolysis carbon black (HWPCB) under freeze–thaw damage (FTD) at low and intermediate temperatures

• Pure mode I fracture resistance of HWPCB-modified WMA under FTD was evaluated. • The semi-circular bending and edge notched disc bend tests were performed on samples at ± 15 °C. • HWPCB improved the LTC and ITC behavior of WMA mixture under 0 and 3 FTD cycles. • HWPCB improved the fracture energy and fracture toughness of asphalt mixture under FTD. • ENDB geometry was critical than the other two geometries due to lower average fracture energy. Asphalt pavement, as an important engineering structure under various conditions such as freeze–thaw cycles and temperature fluctuations, suffers from failures such as low-temperature cracks (LTCs) and intermediate-temperature cracks (ITCs). Therefore, studying and providing a suitable solution is a concern of researchers in this field. On the other hand, the amount of hospital waste is increasing due to the Covid-19 pandemic; hence, one of the appropriate solutions is to recycle and use them in engineering structures such as asphalt pavement to reduce environmental pollution. In this study, a hospital waste pyrolytic carbon black (HWPCB) additive from recycled hospital waste resulting from the COVID-19 pandemic was used to improve the LTCs and ITCs resistance of Warm Mix Asphalt (WMA). For this purpose, three geometries called Symmetric specimen SCB (containing vertical cracks), Classical-Modified specimen SCB-1 (containing angular cracks), and Symmetric specimen ENDB (containing vertical cracks) were subjected to mode I loading conditions. In order to adapt the actual conditions of the pavement to the laboratory conditions, the samples were subjected to 0 and 3 cycles of freeze–thaw damage (FTD). They were fractured at 15°Celsius. The results showed that adding 18 % HWPCB to the WMA mixture increased the fracture energy and fracture toughness of all three geometries under mode I in both 0 and 3 FTD cycles at ± 15 °C. Also, it was concluded that the HWPCB additive was able to compensate for the reduction in resistance created by the 3 FTD cycles and increase it to some extent. On the other hand, the results of the Tensile Stiffness Index (TSI) and Tensile Strength (TS) indices showed that adding 9 and 18 % HWPCB increased the resistance to elastic deformation at ± 15 °C (under 0 and 3 FTD cycles) in addition to improving the crack resistance of the WMA mixture. The results of the Flexibility Index (FI), Toughness Index (TI), and Cracking Resistance Index (CRI) brittleness indices for WMA mixtures containing different percentages of HWPCB additive showed that this additive slightly reduced the flexibility of the mixture under 0 and 3 FTD cycles at temperatures of ± 15 °C. Finally, it was concluded that using HWPCB additive in the WMA mixture improved the behavior of LTC and ITC under mode I and FTD conditions.