Characterizing the Shear Behavior of Ultra-High-Performance Concrete through the Universal Panel Tester
The limited understanding of the shear behavior of Ultra-High-Performance Concrete (UHPC), especially under the effects of axial loads and varying scales, poses challenges to the formulation of reliable design models. Although UHPC is being utilized in structural applications, its complex behavior under combined stress states and the influence of fiber alignment are yet to be characterized. This dissertation presents experimental and analytical investigations on the shear behavior of UHPC, considering axial load, size, and fiber alignment effects. The experimental program consists of three phases. In the first phase, pure shear and combined shear-axial load tests were conducted on rebar-free UHPC panels using the Universal Panel Tester (UPT) facility. The post-peak behavior and the effect of fiber alignment were evaluated. A series of material companion tests were performed along with large-scale Tension Strip Tests (TSTs). The second phase examined the contribution of bar reinforcement in UHPC shear resistance through UPT tests of bar-reinforced UHPC panels. Overall, the experimental results of rebar-free and bar-reinforced panels indicated that axial loads have a significant effect on the shear strength of UHPC, which is larger than conventional concrete. A correlation was observed between fiber alignment and UHPC’s shear and tensile behaviors. In the third phase, a set of shear panel tests was used to investigate size effects on the shear behavior of UHPC. Two sources of size effect were observed: (1) the classical depth size effect observed in conventional concrete and (2) a fiber alignment-related size effect which is dependent on formwork geometry and boundary layer effects in fresh UHPC flows at casting. Comparisons with uniaxial tension tests and beam shear datasets provided additional evidence for UHPC's scale-dependent behavior related to fiber alignment. The experimental findings were utilized to improve the Softened Membrane Model for UHPC (SMM-UHPC) by incorporating the effects of fiber alignment and axial loads. Combining experimental and analytical findings, a one-way shear design model for UHPC beams was developed. The design model integrates material, geometric, and fiber alignment parameters within a framework that is compatible with the current ACI 318 provisions.
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