Bond Performance between Ultrahigh-Performance Concrete and Normal-Strength Concrete

Ultrahigh-performance concrete (UHPC) exhibits several properties that make it appropriate for the rehabilitation of concrete structures. In this investigation, the application is focused on bridge deck overlays, but the study is equally applicable to other rehabilitation applications. Its negligible permeability makes this material suitable as a protective barrier that prevents any water or chemical penetration into the substrate. In addition, its ultra-high compressive strength and post-cracking tensile capacity could provide an improvement to the bearing capacity. However, for extensive acceptance, it has to be demonstrated that the bond between UHPC and normal strength concrete (NSC) offers a good long-term performance under a variety of operating conditions. The UHPC-NSC interface can experience high tensile, shear, and compressive stresses at both early and later life stages and the environmental conditions inherent to the operating environment. The success of the rehabilitation will depend on whether the bond interface can withstand the stress combinations subjected throughout its service-life owing to material incompatibilities or applied loads. This paper explores the bond characteristics between UHPC and NSC under varying stress configurations and environmental conditions. Variables, such as roughness degree of the concrete substrates, age of bond, exposure to freeze-thaw cycles and wetting conditions of the concrete substrate, were included in this study. The combination of splitting tensile test with 0, 300, 600, and 900 freeze-thaw cycles was carried out to assess the bond performance under severe environmental conditions. The slant-shear test was conducted with different interface angles to provide a broader understanding of the bond performance under several combinations of compression and shear stresses. In addition, measurements of the bond tensile strength, using the pull-off test, were used to provide data that can be correlated in the future with the other tests that only can be used in the laboratory. The experimental program showed that the bond performance between UHPC and NSC is adequate for bridge overlay applications, regardless of the degree of roughness of the concrete substrate, the age of the composite specimens, the exposure to freeze-thaw cycles, and the different loading configurations. The controlling factor was the strength gain of the UHPC at early ages relative to the strengths of the substrate.