Experimental Investigation of the Natural Bonding Strength between Stay-In-Place Form and Concrete in FRP-Concrete Decks/Beams

The Fiber Reinforced Polymer (FRP)-concrete hybrid deck/beam is a structural system that combines the durable thin-walled FRP composite profiles and the cost-effective concrete by interfacial shear connections. The interfacial slip can reduce the composite action, thereby causing a degradation of flexural rigidity and capacity. Therefore, using stay-in-place (SIP) forms is a simple way to fully utilize the natural bonding between FRP and concrete, which plays a pivotal role in the structural design of FRP-concrete hybrid decks/beams. This paper presents an experimental study on the natural bonding provided by the SIP forms and the in situ cast concrete. First, four comparative push-out test specimens revealed that the use of SIP forms could improve the ultimate shear capacity of steel bolts by 11.1%. Moreover, it could provide an initial stage with nearly zero slip. The average natural bonding strength of FRP-concrete was evaluated as 0.27 MPa, which agreed well with previous tests in the literature. Second, the beam specimen also confirmed that there was a load response stage with nearly zero slip along the FRP-concrete interface when SIP forms were used as the permanent form. Third, the strain measurements on the steel bolts, FRP profile, and concrete revealed that the failure of the natural bonding was a brittle process. Finally, the flexural response of the FRP-concrete hybrid beam was analytically modeled as three distinct stages, namely the full composite action stage, the slipping stage caused by a natural bonding decrease, and the partial composite action stage.