Synergistic Adsorption–Enhancement of Bamboo–Aramid Fibers in SMA-13 Asphalt Mixtures

The synergistic use of natural bamboo fiber and synthetic aramid fiber in asphalt mixtures has received limited research attention, particularly regarding the optimal blending ratio under a constant total fiber content and the underlying reinforcement mechanisms. This study systematically investigated the co-blending of bamboo and aramid fibers in SMA-13 asphalt mixtures with a fixed total fiber content of 0.3%. Five mixture groups were prepared, LF (0.3% lignin fiber, control), BF (0.3% bamboo fiber), as well as three hybrid groups: ABF-1 (0.27% bamboo + 0.03% aramid, 9:1), ABF-2 (0.24% bamboo + 0.06% aramid, 4:1), and ABF-3 (0.21% bamboo + 0.09% aramid, 7:3). The mixtures were evaluated using rutting tests, low-temperature flexural beam tests, moisture stability tests, and AMPT dynamic modulus testing. The results demonstrate that hybrid-fiber mixtures outperform single-fiber mixtures, with ABF-2 exhibiting the best overall performance. Compared with LF and BF, ABF-2 achieved a dynamic stability of 6921 passes/mm (increases of 97.7% and 52.7%, respectively); flexural tensile strength increased by 43.1% and 32.1%; maximum flexural tensile strain increased by 42.6% and 35.0%; and retained stability improved by 10.8% and 12.5%. AMPT results indicated a higher dynamic modulus and lower phase angle for the hybrid system, suggesting an enhanced elastic response. The superior performance of ABF-2 is attributed to the complementary adsorption–stabilization effect of bamboo fiber and bridging–reinforcement effect of aramid fiber. This study provides quantitative evidence for the beneficial combination of natural and synthetic fibers in asphalt mixtures and identifies key limitations that warrant future investigation.