Material Laws of Frp Strengthened Reinforced Concrete under Uniaxial Tension and Biaxial Tension-Compression Stress Fields

Well established analytical models and design guidelines are already available for analyzing and designing FRP strengthened structures under flexural and axial-confinement actions. However, the understanding of the behavior of such members under in-plane stress field remains a subject of on-going discussion among several researchers and practitioners. Several analytical models have been proposed to predict the gain and upgrade of shear capacity due to FRP strengthening, among which, most models resulted in large discrepancies and produced large scatter when compared to experimental database. This is due to the lack of accurate constitutive models for strengthened reinforced concrete (RC) with FRP (FRP-RC) members. An efficient method to study the overall response of an RC member is to identify the characteristic behavior and the contribution of each material constituting the structure, the behavior of that specific element can be predicted by taking into account the inherent characteristics and material laws of the constituents that leads to understanding the global shear response of the structure. As a first step of developing a shear model of FRP-RC elements, constitutive laws of each material component, namely concrete, steel reinforcement, and FRP sheets were studied in this research project through experimental and analytical investigations. Thirteen full-scale prismatic specimens and six full-scale panels were tested using the Universal Panel Tester (UPT) to study the stress-strain relationships of concrete, steel and FRP in tension as well as concrete in compression and the Poisson effect resulting from the biaxial loading. The results indicate that compared to the un-strengthened RC element, the presence of the externally bonded FRP material typically alters the main characteristics of the stress-strain relationships for each components in FRP-RC element. These newly developed material laws will be used to further develop a model to predict the behavior of FRP strengthened RC elements subjected to shear and torsion. The results from both experimental and analytical study in this research project will provide a promising contribution to the prediction of the behavior of FRP-RC members under shear that will ultimately improve the accuracy of the available design guidelines.