Numerical Simulation And Theoretical Analysis Of Failure Modes For RC Beams Strengthened With FRP Sheets

Fiber Reinforced Polymer as a new material which has high strength-to-weight ratio, easy installation, and corrosion resistance generally replaces the outsourcing strengthening technology of steel in recent years. Although its application is wide, the study on theoretical analysis and numerical simulation is relatively delayed because the mechanical properties and failure modes are more complex compared with other common RC beams. The failure modes of RC beams strengthened with FRP sheets contain FRP rupture, the crushing of compressive concrete, the interfacial debonding by intermediate flexural crack and interfacial debonding of FRP sheets end. The paper describes the failure modes and simply introduces the experimental study, theory study and finite element analysis application in domestic and foreign. The experimental data of 24 beams have been collected according to the failure modes. Based on the experimental data of the tests beams, numerical simulations of all beams are carried out respectively by using ANSYS software. The primary research work and conclusions are summarized as follow:1. On the basis of bond-slip relationship of Lu Xinzheng between FRP sheets and concrete surface, the test beams are calculated by ANSYS finite element analysis according to the interface bond-slip. The results of numerical simulations shows that the yield and ultimate load born by the beams are obviously increased; Meanwhile, the deflections of the beams reduce a lot; regarding the specimens failed by tensile of FRP, when other conditions are the same, the enhancement of concrete intensity has a little influence on ultimate bearing capacity; regarding the specimens failed by FRP rupture and the crushing of compressive concrete, its section strain still meet the plane section assumption.2. The paper simulates the beams which have load history by setting the different load steps in solution. In the first load step FRP has no effect, the strain is 0; its strain has rapid growth after using FRP sheets, and it displays FRP great characteristic. The results indicates that the FRP sheets restrict upward expansion and development of cracks in tensile area and in vertical direction , and enhance the whole bending stiffness of the beams.3. The finite element analysis results point that section strain doesn't accord the plane section assumption when the beams are debonding failure, comparing with the load-strain curve of steel and FRP. Before the steel yields, the strain distribution is almost the same as the bending moment diagram. After the steel yields, the bending cracks grow in number, so FRP strain has mutation in the bending-shear and the pure-bending segment boundary. The quantity of bending cracks of strengthened beams is large and the gap is small in the pure bending segment.4. Theoretical analysis for the first two failure modes the RC beams strengthened with FRP sheets is also carried out in this dissertation. According to the fundamental theories of concrete and section analysis method, some formulas are derived for determining the ultimate loads that the strengthening beams can bear. Then It derivations the calculation formulas of the bond shear stress and normal stress according to the debonding failure mechanism of FRP sheets end. Regarding the specimens failed by interface debonding caused by bending crack in the mid-span, it deduces the calculation formula of crack according to existing criterion. It further yalidates the accuracy of formula according to the collected experimental datas and results of numerical simulation, so the theoretical formula may supply the reference to the actual engineering design.