Effect of Fiber Characteristics on Cracking Resistance Properties of Stone Mastic Asphalt (SMA) Mixture

Cracking is a critical distress that reduces an asphalt pavement’s service life, and fiber reinforcement is an effective strategy to enhance anti-cracking capacity. However, the effects of fiber type, morphology, and length on key cracking modes remain insufficiently understood, limiting rational fiber selection in practice. This study systematically evaluated the influence of four representative fiber types on the anti-cracking performance of Stone Mastic Asphalt (SMA) mixture, combining mechanical testing and microstructural analysis. The fibers included lignin fiber (LF); polyester fiber (PF); chopped basalt fiber (CBF) with lengths of 3 mm, 6 mm, 9 mm; and flocculent basalt fiber (FBF). Key mechanical tests assessed specific cracking behaviors: three-point bending (low-temperature cracking), indirect tensile (tensile cracking), pre-cracked semi-circular bending (crack propagation), overlay (reflective cracking), and four-point bending (fatigue resistance) tests. A scanning electron microscopy (SEM) test characterized fiber morphology and fiber–asphalt interface interactions, revealing microstructural mechanisms underlying performance improvements. The results showed that all fibers improved anti-cracking performance, but their efficacy varied with fiber type, appearance, and length. PF exhibited the best low-temperature cracking resistance, with a 26.8% increase in bending strength and a 16.6% increase in maximum bending strain. For tensile and crack propagation resistance, 6 mm CBF and FBF outperformed the other fibers, with fracture energy increases of up to 53.2% (6 mm CBF) and CTindex improvements of 72.8% (FBF). FBF optimized reflective cracking resistance, increasing the loading cycles by 48.0%, while 6 mm CBF achieved the most significant fatigue life improvement (36.9%) by balancing rigidity and deformation. Additionally, SEM analysis confirmed that effective fiber dispersion and strong fiber–asphalt bonding were critical for enhancing stress transfer and inhibiting crack initiation/propagation. These findings provide quantitative insights into the relationship between fiber characteristics (type, morphology, length) and anti-cracking performance, offering practical guidance for rational fiber selection to improve pavement durability.