Tengfei Xu

This paper proposed a new method of exposing fibers using expandable polyethylene (EPE) foam to improve the interface performance. The flexural tensile behavior of the interface between precast steam-cured and cast-in-place non-steam-cured ultra-high performance concrete (UHPC) members was examined under four-point bending. The improvement effects of four non-destructive interface treatments—e.g., coating with epoxy glue and coating with a cement-based interface agent—were experimentally studied for comparison. The load-deflection curves of the specimens were measured, and the flexural tensile behavior of the interfaces was analyzed, including the failure mode, flexural tensile strength, ductility, fracture energy, and residual strength. The results indicate that the flexural tensile strength of an interface without any treatment is low and that coating with the cement epoxy glue or the cement-based interface agent has a limited effect on improving the flexural tensile strength. As a result of the bridging effect, the exposed steel fibers can transfer the tensile force across the interface. Therefore, exposing fibers using EPE foam can significantly improve the bond behavior of an interface. This enhancement effect depends on the number of exposed steel fibers and can be simulated based on the pullout load–slip relationship of the steel fiber embedded in the UHPC matrix. The proposed numerical model can be used to predict the bond behavior of an interface with the fiber-exposing treatment.

Both the linear and nonlinear compressive creep behaviors of ultra-high performance concrete (UHPC) are experimentally investigated and modeled using cylindrical specimens under sustained compression and reinforced UHPC beams under sustained bending. The 90-day creep tests on 16 groups of cylindrical specimens explore the influences of the curing procedure, initial loading age of the UHPC, and sustained stress levels. The tested creep coefficients are compared with predictions provided by existing creep models. The NF P18-710 model performs best for creep coefficient predictions of UHPC specimens under a sustained stress level of 0.3fc. However, all existing creep models for normal concrete overestimate the creep results. On the basis of the test results, a nonlinear creep model is proposed in which the sustained stress threshold of nonlinear creep for UHPC is 0.3fc. All existing nonlinear creep models for normal concrete underestimate the nonlinear creep of UHPC. The 180-day creep tests on the UHPC reinforced beams are used to investigate the influences of sustained stress levels and the reinforcement ratio. The creep strain and time-dependent displacement of seven reinforced UHPC beams are calculated using the proposed creep model. Comparisons between the calculated and measured results verify its accuracy.