Study on damage characteristics of early-age steel fiber shotcrete under high-temperature variable-temperature curing and cyclic impact loading

To clarify the cumulative damage mechanism of early-age steel fiber shotcrete under cyclic impact loads during the construction of high-ground-temperature tunnels, this study addresses the deficiency in existing research on the damage evolution of 1d-age specimens under the coupled action of variable-temperature curing and impact loading. Cyclic impact tests were conducted on 1d-age shotcrete specimens using a split Hopkinson pressure bar, in combination with ultrasonic testing and CT scanning to reveal their dynamic mechanical properties and macro-mesoscopic damage characteristics. The results show that variable-temperature curing affects the hydration process, leading to an abrupt thermal damage point, with 75℃ identified as the early warning temperature threshold. Variable-temperature curing at 28～75℃ accelerates hydration and improves early strength, but later pore coarsening reduces strength and hinders mechanical property improvement. When the temperature approaches 95℃, rapid internal moisture evaporation increases porosity and causes interfacial deterioration, weakening the material’s bearing capacity and toughness. Compared with traditional single characterization methods, this study integrates dynamic mechanical parameters, acoustic damage factors and CT 3D reconstruction to innovatively establish a multi-scale collaborative quantitative characterization system, revealing mesoscopic damage evolution. Based on thermal–mechanical coupling damage characteristics, a multi-scale coupled damage conceptual model of “curing regime-cyclic impact-interface degradation-structural deterioration” is proposed. The findings provide a theoretical basis for the optimal design of initial support materials and safety evaluation of support structures in high-ground-temperature tunnels.