Experimental investigation on compressive and flexural behavior of composite concrete systems incorporating UHPC under chloride-sulfate attack

This study investigates the effect of various conjoint chloride–sulfate attacks on the compressive and flexural behavior of the fresh-to-hardened composite concrete system (FH-CCS) combining normal strength concrete (NSC) and ultra high-performance concrete (UHPC). Physical, mechanical, and microstructural degradation were thoroughly analyzed, including mass, visual condition and failure mode at 180 and 360 days of exposure. The digital image correlation (DIC) technique was utilized during mechanical testing to capture and analyze the failure mechanisms in the CCS visually. Moreover, scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) was applied to investigate the microstructural properties of the degraded bond zone. The results indicated a significant decline in the physical, mechanical, and microstructural properties of the CCS after the 360 days of attack. Microstructural analyses show that the conjoint attack induced varying levels of microcracking, and interfacial deterioration compared to the control group. The deterioration worsened with the increased sulfate in the conjoint attack, as the higher sulfate concentration showed the highest degree of microcracking in the NSC layer and interfacial area, while the increase in chloride caused a more substantial inhibition of sulfate action. The UHPC layers maintained their characteristically dense, consolidated microstructure in all exposure conditions.