Behaviour of Prestressed Ultra-High Performance Concrete I-Beams Subjected to Shear and Flexure

Ultra-high performance concrete (UHPC) is a new type of concrete developed by selecting the particle sizes and gradation in the nano- and micro-scales targeting the highest possible packing. The resulting concrete with very high density is called UHPC. UHPC has very low permeability and hence it is very highly durable compared to traditional or high performance concrete (HPC). Micro reinforcement of UHPC by random distributed steel-synthetic fibers results in superior mechanical properties such as very high compressive and tensile strengths, high ductility, and high fatigue resistance. The material selection and early age curing processes, use of fiber reinforcement, and very high quality in production resulted in a very high initial cost of UHPC structures. In order to enable the mass production and cost effective use of the material, performance based design and optimization of UHPC structural members are required. This study is part of an NRC Canada research project to develop innovative, cost effective, and sustainable bridge structural systems using UHPC and other innovative materials. In this study, the estimation of shear and flexural capacities using the available approaches of international design guidelines of UHPC structures are comprehensively compared to a proposed truss models, linear and nonlinear finite element models. Several design trials intended to allow for an optimized use of the materials and a maximum load capacity was conducted for simply supported beams with one or two external loads, and having rectangular or I cross sections. Linear and non-linear finite element models are developed and their results were compared to the available international design recommendations. Truss models are proposed to simplify the stress analysis in the shear zone of the prestressed UHPC beams. It is found that prestressed UHPC I-beam section gives the highest possible load capacity with minimum use of materials. The study shows that for the case of no stirrups, massive flexure and shear cracks initiate and propagate suddenly where a diagonal shear crack is fully developed and sudden collapse may expected. The proposed truss model gives very good match to nonlinear finite element analysis results for almost all the truss members. The results are significantly improved when additional struts are considered for both cases of beams with or without shear reinforcement. The study shows the importance of future experimental investigatinons to calibrate the proposed models