Analysis of Shear Failure in Ultra-High Performance Concrete Beams

Ultra-high performance concrete (UHPC) is an emerging cementitious material type currently undergoing research in different structural applications. This material is characterized by a compressive strength of 150 MPa (22 ksi), according to the Association Francaise de Genie Civil and the Federal Highway Administration, high tension strength, and high durability. Additionally, UHPC consists of secondary reinforcement in the form of steel or polymeric fibers, which enable UHPC to exhibit tension hardening behavior and tension ductility previously ignored in conventional concrete mixes. Previous research has focused on applications in the transportation and bridge design industries; however, applications in buildings and other heavy construction projects necessitate a better understanding of shear and flexural-shear failure modes in design recommendations and guidelines. The purpose of this research is to test a series of UHPC specimens to investigate the shear failure responses in beams. Additionally, this research will also investigate the tensile properties of UHPC to assist in determining the relevant shear properties. Two series of specimens were created for testing: small-scale beam specimens for load-displacement behavior analysis under four-point bending and UHPC prisms for determining the effects of tension stiffening. Both sets of specimens were created using ASTM Grade 60 steel and high strength steel for the primary reinforcement. Finally, numerical analyses using finite element analysis software, based on both continuum and macroelement models, of the UHPC specimens were carried out based on the modified compression field theory (MCFT) to verify and expand upon the experimental results. Results of the UHPC tension stiffening experiments were normalized to obtain an averaged tensile stress for UHPC in tension in the presence of primary reinforcement. The calibrated tension stiffening responses were used in the numerical analyses. The beam specimens exhibited the combined failure mode of shear-flexural failure in all of the specimens. Additionally, slight concrete crushing at the top of the specimens was noted at larger displacements. No purely flexural failures were observed, as would be expected in normal strength concrete. The numerical analyses were able to predict the load-displacement behavior and failure modes of the experimental UHPC beam specimens with acceptable accuracy. Further numerical analyses were then conducted to determine the effects of different UHPC strengths, aspect ratios, and reinforcement ratios. These analyses provide insight on the transition between flexural and shear failure modes for different design properties