Influence of Fiber Mixing Process on the Cracking Resistance of Cold Recycled Asphalt Mixture

Abstract

Fiber reinforcement is often used to improve the road performance of cold recycled asphalt mixture (CRAM). The purpose of this research is to evaluate the impact of fiber mixing process on the cracking resistance of CRAM from multiple perspectives. Four kinds of fiber mixing processes, named A, B, C, and D, were designed by changing the order of fiber addition during the mixing process. Then, semicircle bending tests and indirect tensile tests were conducted to characterize the low-temperature cracking behavior of fiber CRAM. Freeze–thaw cycle tests under both dry and water-saturated conditions were performed to investigate the freeze–thaw damage behavior of fiber CRAM. Furthermore, the fiber dispersion in CRAM was observed using scanning electron microscopy (SEM). The results show that the fiber mixing process has a significant effect on the cracking resistance of CRAM. The CRAM specimens prepared by process C have the largest fracture energy, splitting strength, and fracture work, while the specimens made by process D have the smallest value. Specially, the fracture energy of the specimens prepared by process C is 77.23% larger than that of the specimens prepared by process A, while the fracture energy of the specimens prepared by process D is 5.6% smaller than that of the specimens prepared by process A. The reason for this phenomenon is that the fiber is well dispersed in the specimens prepared by process C, which contributes to obtain a better crack resistance. For all CRAM specimens, with the increase of freeze–thaw cycles, splitting strength and fracture work of fiber CRAM decrease. However, there is an obvious difference in the reduction rate of splitting strength and fracture work, especially for the specimens under the water saturation condition. The specimens made by process C have the smallest reduction rate, which indicates a better resistance to freezing and thawing damage. According to the analysis of fiber macro-distribution state in loose CRAM, the fiber dispersion is affected by the humidity conditions in the mixing environment. The best humidity conditions are obtained for fiber dispersion in process C. Based on the SEM observation, the overlapping bridging network structure can be observed in the microstructure of the specimens prepared by process C, allowing the mixture to better transfer and disperse stress.