Shear performance of fluted interfaces between ultra-high-performance concrete and conventional concrete

Ultra-high-performance concrete (UHPC) is increasingly utilized for precast members and is combined in the field with cast-in-place conventional concrete (CC) to form composite members. The interface performance is crucial for ensuring the composite action in these structures. However, limited studies have examined the CC to UHPC interfaces, especially when fluted details are utilized. To address this gap, this study conducted push-off tests to investigate previously unexamined parameters affecting the interface behavior, including the interface reinforcement-to-flute base area ratio (RFR) and the distance from the flute edge to the interface reinforcement (FRD). The results indicate that damage initiates with shear failure of the concrete through the CC material and yielding of the interface reinforcement, followed by shear friction of the concrete interface, dowel action and fracture of the reinforcement. A theoretical stress model was modified for the non-prestressed interface, leading to the development of a new predictive capacity formula based on the experimental results. The average ratio and correlation coefficient between the experimental capacity and the capacity predicted by the formula are 1.22 and 0.928, respectively, indicating a good alignment with the test data. In terms of variables, the presence of FRD reduced the interface shear capacity by at least 19 %, and the higher RFR resulted in lower friction factor, ranging from 2.40 to 0.99, suggesting an upper limit on friction strength should be established.