Compressive self-sensing characteristics and mechanism of UHPC with hybrid steel fiber subjected to combined curing

Ultra-high performance concrete (UHPC) with intrinsic self-sensing capability is highly desirable for smart infrastructure, but existing studies mainly focus on standard or hot water curing (≤90 °C), neglecting the effect of high-temperature dry-air heating which is relevant to practical harsh environments. This study investigates the compressive self-sensing characteristics and mechanism of hybrid steel fiber reinforced UHPC under combined curing consisting of 90 °C hot water for 2 days followed by 200 °C dry-air heating for 3 days. Five UHPC mixtures were prepared: a reference with 2 vol.% hooked-end steel fibers, and four mixtures additionally containing 1 vol.% copper-coated straight steel fibers with different aspect ratios. Results show that the combined curing enabled all UHPC mixtures to achieve compressive strengths above 226 MPa, with the maximum of 248.8 MPa. Under cyclic loading, UHPC with 15 mm/0.2 mm fibers exhibited the best self-sensing capability, with FCR peak values of −52.5% (single-stage cycle) and −57.9% (three-stage cycle) at 60 MPa. Based on the FCR evolution, a three-stage model is proposed, and a safety evaluation indicator ( ISE ) is introduced to normalize FCR for damage warning. Mechanism analysis reveals that after combined curing, free water is nearly completely removed, eliminating ionic conduction; current is transmitted through tunnel conduction, local fiber contact, and the UHPC matrix until peak stress, where a continuous steel fiber conductive network forms due to fiber detachment from the matrix, causing a sharp resistance drop. This study demonstrates that UHPC with hybrid steel fiber under combined curing can achieve excellent self-sensing performance, providing a theoretical basis for structure‑function integrated UHPC in high‑temperature environments.