Fatigue Performance and Pore Characteristics of SBS/Micro Carbon Fiber Composite-Modified Asphalt Concrete for Ultra-Thin Overlays

Durability deterioration and interlayer bonding failure of ultra-thin overlays remain critical challenges under coupled environmental and mechanical actions. Although environmental damage to asphalt mixtures has been widely investigated, the relationship between pore-structure evolution and interlayer fatigue deterioration in polymer-composite-modified ultra-thin overlays incorporating styrene–butadiene–styrene (SBS) and micro carbon fiber (MCF) remains insufficiently understood. This study therefore extends existing research by clarifying this relationship under freeze–thaw cycling and water immersion. Three-point bending and direct shear fatigue tests were conducted to evaluate bending and interlayer shear fatigue performance, respectively, while nanoindentation and X-ray computed tomography (CT) were used to characterize micromechanical properties and three-dimensional pore-structure evolution. The results showed that five freeze–thaw cycles reduced the bending fatigue life by 75.3% and the interlayer shear fatigue life by 49.6%, while six days of water immersion reduced the interlayer shear fatigue life by 55.1%. Freeze–thaw cycling promoted open-pore and pore-throat development and increased total porosity by 23.9%, contributing to aggregate displacement and redistribution of the internal skeleton. In contrast, immersion increased the proportion of small and closed pores, while isolated pores concentrated near the interlayer weakened interlayer shear resistance. Although immersion caused greater reductions in hardness and modulus, freeze–thaw-induced pore development was associated with greater deterioration in bending fatigue performance. Furthermore, an adaptive-network-based fuzzy inference system (ANFIS) was developed to predict pore tortuosity from equivalent diameter, shape factor, and porosity, with testing errors ranging from 0.102 to 0.129 for untreated, freeze–thaw, and immersed specimens. An exponential relationship was further identified between tortuosity and the pore comprehensive effect index (PCEI), providing a quantitative approach for characterizing pore connectivity and evaluating environmental deterioration in polymer-composite-modified asphalt concrete.