Evaluation of the Shear Strength of Ultra-High Performance Concrete

Abstract

Ultra-High Performance Concrete (UHPC) is a cementitious concrete with reinforcing fibers that has exceptionally high compressive, tensile, flexural, and bond strengths, but not much is known about its shear strength. It is typically composed of cementitious material, fine sand, steel fiber reinforcement, admixtures, and water. The inclusion of steel fiber reinforcement and admixtures lead to higher costs than conventional concrete. Due to its high cost, UHPC applications in the United States have been associated with connections, in which its high strength is advantageous and the quantities are small enough to make it cost-effective. Proprietary UHPC mixes have been developed by private companies for use on large-scale projects, but these mixes are not always an option due to high cost. In an effort to make UHPC more accessible, proprietary mixes using local materials and varying percentages of fiber content have been developed. An experimental program was undertaken by the University of Washington, in collaboration with the University of Oklahoma (OU) and Florida International University (FIU), to study one such mix. Seven batches of UHPC were mixed and cast at UW, altering the material sourcing and fiber content. In total, five batches used UW materials and 2% fiber content, 1 batch used UW materials and 1% fiber content, and one batch used OU materials and 2% fiber content. Each batch of UHPC was subject to compression, modulus of elasticity, direct tension, and flexural beam tests. Additionally, they were tested in pure shear using the UW Panel Element Tester. After testing, the experimental data was analyzed to determine the effect of material sourcing and fiber content on UHPC behavior. It was concluded that the effect from material sourcing was negligible, at least partly because critical materials such as fibers and admixtures were obtained nationally and so were unchanged, and the effect from fiber content was little on compression and modulus of elasticity, but large on tension, flexural, and shear. The results were then compared to compression, modulus of elasticity, direct tension, and flexural beam test results from OU that used the same mix design but sourced materials locally and used different mixing equipment and mixing procedures. The conclusions from this comparison were consistent with the initial analysis. Finally, the UW test results were compared with strengths predicted by available equations. However, no such equations exist for UHPC panels tested in pure shear. Therefore, the shear test results were compared with mix parameters such as fiber content, compressive strength, and tensile strength. It was found that the results were best correlated with fiber content and tensile strength. Equations based on these correlations were proposed to estimate shear strength of UHPC.