Durability degradation mechanism of modified steel-fiber UHPC under coupled effect of salt freeze-thaw cycles and sustained flexural loading

In cold regions, fiber-matrix interfacial degradation under salt freeze-thaw (SFT) cycles and sustained loading critically governs the long-term performance of ultra-high-performance concrete (UHPC). This study investigates the frost resistance and mechanical evolution of ethylenediaminetetraacetic acid (EDTA)-modified steel fiber-reinforced UHPC (GUHPC) versus unmodified that is plain UHPC (PUHPC) under combined exposure to a 3 wt% NaCl + 3 wt% MgSO₄ solution and sustained flexural loading (at 0, 0.3, and 0.5 times the ultimate flexural strength, denoted as 0 f _t, 0.3 f _t, and 0.5 f _t, respectively). Relative dynamic elastic modulus (RDEM), mass loss, flexural and compressive strengths were measured, complemented by MIP, SEM/EDS, XRD, and TG to elucidate interfacial mechanisms. Results show that EDTA modification markedly enhances durability and load-bearing capacity of UHPC under SFT-loading coupling, however, both frost resistance and mechanical properties declined with increasing stress level For instance, under 0.5 f _t loading combined with 1500 SFT cycles, GUHPC exhibited reductions of 17.38 % in RDEM, 17.1 %% in compressive strength, and 69.6 % in flexural strength compared with 1500 SFT cycles alone, whereas PUHPC showed corresponding decreases of 21.02 %, 18.5 %, and 72.6 %. Microstructural evidence corroborated these trends: under 0.5 f _t with 1500 SFT cycles, EDTA treatment reduced the corrosion-layer thickness from 22.9 μm to 13.5 μm, forming an embedded-coated interface enriched in Ca and Si. The Ca(OH)₂ content was maintained at 1.7 %, whereas the unmodified counterpart measured 1.3 %, and the total porosity under coupling decreased from 9.32 % to 7.92 %. These findings provide mechanistic insight and engineering guidance for enhancing UHPC interfacial durability in harsh environments.