Multi-scale fracture performance and mechanisms of basalt Fiber reinforced warm recycled asphalt mixtures under freeze-thaw cycles and various fracture modes

Asphalt pavements in frozen regions are highly susceptible to damage induced by freeze-thaw cycles, during which microcracks initiate, propagate, and ultimately evolve into macrocracks. However, warm-mixed recycled asphalt mixtures (WRAMs) exhibit limited resistance to multi-stage fracture cracking. To address this limitation, basalt fibers (BFs) with various morphologies and dosages were incorporated into WRAMs to enhance their crack-resistance mechanisms. WRAM, chopped basalt fiber-reinforced WRAM (WR-CBF), and flocculent basalt fiber-reinforced WRAM (WR-FBF) were investigated under Mode I, Mode III, and mixed Mode I/III fracture modes. Specimens were subjected to 10 and 20 water and salt freeze-thaw cycles, followed by macroscopic edge-notched disk bend (ENDB) tests, mesoscopic acoustic emission (AE) analysis, and microscopic scanning electron microscopy (SEM) observations. The results indicate that WR-FBF with a basalt fiber content of 0.4% exhibits optimal crack resistance. Fracture resistance decreases with increasing freeze-thaw cycles, with salt freeze-thaw cycles causing more severe deterioration than water freeze-thaw cycles. Chopped basalt fiber provides more stable resistance to crack propagation across different fracture modes, whereas flocculent basalt fiber primarily enhances resistance to crack initiation, particularly under fracture mode I loading. SEM observations further indicate that FBFs enhance interfacial bonding and stress redistribution through adsorption, anchoring, three-dimensional network formation, and fiber pull-out mechanisms, thereby significantly improving the overall crack resistance of WRAMs.