Effects of Time and Consolidation Method on Bond Strength between Fresh UHPC Layers

Abstract

Full-scale ultra-high performance concrete (UHPC) members often require multiple UHPC batches to meet the required volume. UHPC's "elephant skin" forms quickly and could lead to a weakened interface between the placements. This study investigated placement best practices for multiple UHPC batches to develop procedural specifications that reduce the prevalence of weak interfaces. UHPC mixes staggered in intervals of 10–35 minutes were placed in layers. The flow of the concrete mixes and the resistance of the elephant skin to penetration were measured. A set of three samples from each time interval was rodded, one was externally vibrated, and one was not consolidated (control). After curing, cores were drilled through both layers of concrete and pull-off tests were used to determine bond strength and failure location. Computerized Tomography (CT) scans of select samples were taken to understand how consolidation method affected fiber orientation at the layer interface. Results demonstrate that rodded specimens produced the fewest percentage of interface failures and the highest average strengths, but external vibration also reduced interface failures and increased strength at lower time intervals. CT results showed very few fibers crossing through the concrete interfaces, but rodded specimens had small regions of perpendicular fibers at the locations of rodding.