Study on uniaxial tensile mechanical properties and damage constitutive model of SAP-PVA fiber-reinforced concrete after high-temperature exposure

To investigate the high-temperature tensile performance and damage characteristics of superabsorbent polymer (SAP)-internally cured polyvinyl alcohol (PVA) fiber-reinforced concrete (SAP-PVAC), concrete specimens subjected to four temperature gradients were designed and tested under static uniaxial tension. The tensile mechanical properties of SAP-PVAC under both ambient and elevated temperatures were analyzed. Based on the equivalent strain hypothesis, a thermomechanical coupled damage constitutive model and damage evolution equation were established. Experimental results revealed that at ambient temperature, SAP released water to promote hydration, reducing capillary pores and other defects, while PVA fibers formed chemical bonds and mechanical interlocking with the matrix, synergistically enhancing interfacial bonding and crack resistance. However, under high-temperature exposure, the mechanical properties of SAP-PVAC degraded significantly while tensile deformation capacity increased because SAP dehydration induced shrinkage pores, and PVA fiber melting (∼239°C) led to the loss of fiber bridging. Nevertheless, prior to melting, PVA fibers delayed crack propagation through plastic elongation, shifting the failure mode from brittle to ductile. Empirical equations for post-high-temperature residual strength, peak strain, and elastic modulus were proposed based on experimental data. And the developed thermomechanical coupled damage constitutive model effectively predicted the post-high-temperature mechanical behavior and post-peak softening of SAP-PVAC. This study elucidates the high-temperature degradation mechanisms of SAP-PVAC at both macro- and micro-scale levels, providing theoretical references for the design and application of SAP-PVAC structural elements in high-temperature environments.