Rapid early-age strength development in ultra-high-performance concrete through electro-thermal curing

Ultra-high-performance concrete (UHPC) delivers exceptional mechanical strength, yet its early-age gain often falls short of the tight schedules governing precast production and rapid on-site construction. Electro-thermal (EC) curing harnesses internal Joule heating without external heat sources or enclosed chambers, offering a field-deployable alternative; however, its efficacy is critically governed by the hydration age at activation. While EC has been explored for frost protection and functional self-heating, the optimal activation timing for strength-driven UHPC curing, specifically the activation window anchored to the conductive-network percolation state, remains unresolved. This study targets this gap by evaluating 3-hour EC curing applied to steel-fiber-reinforced UHPC at 24 h after casting, when the paste has entered the latter portion of the acceleration period, comparing constant 60 V (B3, high-intensity benchmark), 30→36 V stepped (B1, mild activation), and 45→60 V stepped (B2, intensified) protocols against a moist-cured reference. Through real-time thermal and electrical monitoring, mechanical testing at 3, 7, and 28 days, and coupled SEM/TGA characterization, we demonstrate that a 3-hour high-voltage activation increases 7-day compressive strength by 41% while preserving statistically comparable 28-day strength. The high-voltage EC flexural deficit diminishes under subsequent moist curing, with the 28-day value recovering to within 2–5% of the control; this trajectory is consistent with reactivated secondary hydration and points to a dehydration-mediated interfacial effect rather than irreversible chemical damage.