Single-Side Shear Bond Strength and OTZ Microstructure of UHPC Repair Materials with Concrete Substrate

In this study, the effects of ultrahigh-performance concrete (UHPC) and normal-strength concrete (NSC) as repair materials on the bond strength and overlay transition zone (OTZ) microstructure were investigated. The single-side shear test was performed to obtain the shear bond strength at the interfaces between the repair materials and concrete substrates, and three types of substrate strength grades were adopted. OTZ crack width was measured and characterized using kernel density estimation, and the OTZ porosity was quantitatively characterized based on the gray-level value, for which 10-μm-wide strips successively extending from the substrate surface to the repair material were selected. The porosity of each strip was calculated as the percentage of the area of this component to the total area of the strip. The micromechanical properties were determined using a nanoindenter. The results show that the UHPC-combined substrate had a higher bond strength than the NSC-combined substrate for the three substrate strength grades. The UHPC decreased the OTZ crack width. Moreover, the fraction of pores in the OTZ of the UHPC was lower than that of the NSC. The modulus and hardness values of the OTZ for the UHPC exceeded those for the NSC. These superior OTZ microstructure properties of the UHPC repair materials explain the high bond strength of the UHPC-combined substrate. Therefore, UHPC combined with a concrete substrate featuring a rough surface represents a useful repair material.