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The State of Deformation and Stiffness Analysis of RC Beams Strengthened by Means of CFRP Strips at Different Load Levels

This work presented some selected results of laboratory tests and FEM analysis study of simple supported RC beams strengthened using carbon strips. The beams were examined to establish the effectiveness of this method of strengthening in terms of increasing their load-carrying capacity as well as flexural stiffness at different preloading states. A set of beams was divided into five groups, which differ in the level of the load applied before application of the composite strips on their bottom surfaces. Laboratory tests were supplemented with numerical analyses based on the finite-element method (FEM) using Abaqus software. The created numerical models were validated, and good agreement of the experimental results with the results obtained in the numerical analyses was observed. The deformation state existing in the main reinforcing bars, in concrete as well as in the strengthening composite strip, and its influence on the failure mechanism of the beams were analyzed. In the stiffness analysis, it was assumed that the stiffness of a beam strengthened with composite material after the elastic range (concrete and reinforcement steel) can be represented by the relation between the stiffness of the noncracked section B and beam curvature κ. The curvature–bending moment diagram as well as bending moment–stiffness diagram were prepared on the basis of laboratory and numerical results. Based on the results, it can be stated that the preload on the beam before strengthening affects the levels of deformation and the utilization rates of the composite material as well as reinforcing bars. The curvature and stiffness of the beams depend on the load level at which the CFRP strip strengthening is realized. The results of the analysis of preloaded beams before strengthening indicate that totally relieving them prior to strip application turns out to be the most beneficial solution.