Stress-Strain Models for UHPFRC and Application in Seismic Design and Retrofit of Bridges

With the advent of Ultra-High-Performance Fiber-Reinforced Concrete (UHPFRC), most shortcomings of conventional concrete are mitigated, since UHPFRC has exceptional mechanical and durability properties. The behavior of the material in tension is the most important property that need be characterized with confidence for practical implementation of UHPFRC in construction. This is often extracted through reverse inverse analysis of flexural tests – existing inverse analysis methods are fraught with great uncertainty and scatter. In this thesis, several alternative characterization methods are explored and corroborated with test results, including an inverse analysis procedure that is consistent with first principles, as well as a practical, chart-based procedure for quality control by practitioners. Apart from the direct tension response, the tension stiffening property of UHPFRC when it interacts with embedded reinforcement was also studied both experimentally and through detailed finite element simulation. Parameters of the study included the volumetric ratio of fibers, casting methodology, loading protocol and the condition of the embedded reinforcement (corroded or uncorroded). Results quantify the amount of tension stiffening that UHPFC cover can provide to reinforcement. The emphasis on tensile stress and strain capacity of UHPFRC is of interest in seismic retrofitting of existing columns through jacketing. In this work a design framework was developed to design UHPFRC jackets by setting performance objectives for the retrofitted column, strain limits for the UHPFRC material in tension and compression, and by development of a constitutive relationship for the encased concrete under the influence of confinement imparted by the jacket. The study includes an illustrative example of a bridge pier confined with two alternative UHPFRC materials of different strength, indicating the effectiveness of the proposed methodology in estimating the performance limit states of the retrofitted component.