Optimization Design and Fatigue Performance of Foamed Asphalt Cold Recycled Mixtures

Foamed asphalt cold recycling technology is one of the key engineering approaches to address the accumulation of large quantities of reclaimed asphalt pavement (RAP) in road maintenance and rehabilitation. However, a systematic design methodology that simultaneously accounts for long-term fatigue resistance and toughness has not yet reached a unified consensus or widespread application. Existing studies have investigated the effects of fine aggregate gradation or cement content on individual performance aspects of mixtures, but studies incorporating both factors into a unified experimental framework for parallel comparison of multiple performance indicators remain limited. To this end, this study designed three mineral aggregate gradations with significantly different fine aggregate contents and systematically evaluated the effects of gradation composition, foamed asphalt content, and cement dosage on the mechanical properties, moisture stability, high-temperature stability, and fatigue performance of the mixtures. Indirect tensile fatigue tests under a stress-controlled mode were conducted to determine the fatigue life of different gradations at four stress ratio levels. The results indicate that sufficient fine aggregate content, particularly particles smaller than 0.075 mm, is a key factor in enhancing mixture compactness, indirect tensile strength, and resistance to moisture damage. The effect of cement on fatigue performance exhibits stress-level dependency: at low stress ratios, the addition of cement improves fatigue life, whereas at high stress ratios, the increased brittleness of the material reduces fatigue resistance, which is consistent with findings reported in previous studies. Furthermore, this study provides comparative experimental data for different fine aggregate gradations. The optimal gradation scheme demonstrated superior overall performance across all evaluated indicators, verifying the feasibility of achieving a balance between strength and toughness through gradation optimization. Compared with conventional design methods guided by a single strength index, this study offers a more comprehensive basis for mix design optimization of foamed asphalt cold recycled mixtures and provides engineering references for their application in long-life pavement maintenance.