Interfacial microstructure between ultrahigh-performance concrete–normal concrete in fresh-on-fresh casting

This study aimed to characterize the bond behavior and micro-properties of the fresh-on-fresh (FF) cast interface of ultrahigh-performance concrete (UHPC)–normal concrete (NC) layers, when compared with a fresh-on-harden (FH) cast one, and then reveal the strength of its mechanism. The investigations include interface spitting tensile strength test and micro property analysis using backscattered electron (BSE), nanoindentation, and energy dispersive spectroscopy (EDS). Two classic curing conditions are adopted, namely, standard curing and steam curing. The result shows that the bonding strength of the FF cast interface is 138% higher than that of the FH cast one. The interface zone (IZ) for the FF cast specimen is formed by the intimate contact and the staggered growth of UHPC and NC, which is an area with gradual variation of porosity and local indentation modulus. The width of IZ was 150 μm at the age of 1 d and decreased to below 100 μm as the hydration progressed. With content and packing, the density of the C–S–H gel increases gradually in the IZ, and bonding properties and density of the interface also increased gradually. This reveal the microstructure characteristics and strengthen mechanism of interface between UHPC and NC in FF casting.