Interface shear resistance of precast keyway UHPC and post-cast normal concrete

Use of permanent formwork can achieve rapid construction, shorten the construction period, and improve the bearing capacity and durability of concrete structure. Interfacial bond shear performance is the key to ensure the collaborative work of permanent formwork and concrete. Three groups of concrete specimens with different compressive strengths and ten groups of keyway ultra-high performance concrete (UHPC)-normal concrete (NC) composite specimens were prepared. The effects of keyway density, spacing, concrete strength, and curing condition on the shear property of keyway UHPC-NC were systematically studied through single-side shear tests and theoretical analysis. Results show that the crack of the concrete specimen is at a certain angle to the loading center axis. And the failure of the UHPC-NC joint surface occurs on the side of concrete. The load-displacement curve of UHPC-NC specimens can be subdivided into four approximately linear stages. And all of the displacements are within 0.9 mm. The total shear strength of the UHPC-NC composite specimen was provided by the contact friction of the bonding surface, matrix-aggregate interaction, keyway-aggregate interlocking mechanism and shear expansion. A reasonable interface of the UHPC-NC keyway can improve both the shear strength and the interface relative displacement of UHPC-NC specimens. After steam curing at 60℃, the interface of matrix-aggregate and the interface UHPC-NC are weakened. According to the results, a modified equation for the shear strength of the keyway UHPC-NC composite specimen was proposed. The calculated values agree well with the experimental results. Most of the calculated values fall within the 90% confidence interval. The values outside the confidence interval are affected by the curing time of the specimens. Therefore, to accurately calculate the shear strength of the keyway UHPC-NC composite specimen, the influence of curing time should also be considered by the curing time factor.