Experimental study on flexural performance of fresh-on-fresh UHPC-NSC composites under combined chloride-sulfate exposure and dry-wet cycles with pouring delay

Fresh-on-fresh (FF) concrete casting technique is increasingly used in composite structural elements made from normal strength concrete (NSC) and ultra-high-performance concrete (UHPC) due to its efficient material utilization, and improved composite performance. However, the combined effects of pouring delays and harsh environmental conditions on flexural performance remain insufficiently explored. This study investigates the durability and flexural behavior of FF-cast UHP-NSC composites with pouring delays under severe ionic attacks and drying wetting cycles. Following curing, the specimens were immersed in chloride, sulfate, combined chloride-sulfate solutions, and freshwater for 180 days under drying-wetting cycles. Flexural strength, load-deflection response, stiffness, failure patterns, and energy absorption were evaluated, while deterioration mechanisms were studied using SEM-EDS and XRD. Results demonstrate that pouring delay significantly influences flexural performance, with the 60 min delay achieving the highest flexural strength, whereas the 90 min delay resulted in notable strength degradation. Sulfate exposure produced the most severe deterioration, while combined chloride-sulfate solutions caused complex interactions, where chloride partially mitigated sulfate-induced damage. Ionic exposure generally reduced stiffness, although energy absorption capacity increased for specimens with longer pouring delays. Microstructural analysis revealed that high sulfate promoted gypsum and ettringite formation, causing cracks in NSC and the interfacial zone, whereas chloride promoted Friedel's salt formation and reduced sulfate-related expansion. These findings elucidate the interaction between combined effects of pouring delay and ionic exposure, providing insights into the durability of FF-cast UHPC-NSC composite structures.