Study On Fatigue Performance Of RC Beams Strengthened With CFRP

In recent decades, the application of carbon fiber reinforced polymer (CFRP) in ternghtening concrete surtcuters developed rapidly. CFRP has been widely used to reinforce or repair the reinforced concrete (RC) bridges and other structures which need to withstand repeated loads. Therefore, there is a greater need to understand the fatigue performance of CFRP reinforced concrete beams. As the good bonding between CFRP and concrete is the basic prerequisite for the good working performance of CFRP reinforced members, the fatigue performance of the CFRP-concrete interface directly affects the working ability of CFRP reinforced members under fatigue loading. Accordingly, in this paper, the fatigue properties of CFRP-concrete interface and reinforced concrete beams strengthened with CFRP were studied using the methods of experiments, theoretical derivation, numerical simulation and so on. The main contents and achievements of this paper are as follows:Using beam specimens, the static and fatigue tests were performed on the CFRP sheet-concrete interface considering five variables, namely, the load level, concrete strength, CFRP sheet-to-concrete width ratio, bond length and cyclic loading frequency. The test results indicate that, the fatigue life of the CFRP sheet-concrete interface decreased with an increase in either the CFRP sheet-to-concrete width ratio or cyclic loading frequency, but increased with an increase in either the concrete strength or bonding length. Based on the test results and the proposed prediction method for fatigue lives of CFRP-concrete interfaces considering many factors, the empirical formula for â–³S-N relation (the relationship between load level and fatigue life) was obtained considering four variables, namely, the average stress amplitude, concrete strength, CFRP sheet-to-concrete width ratio, and bond length.The CFRP sheet-concrete interface fatigue crack propagation behavior was investigated based on the theory of fracture mechanics and the results of fatigue tests on CFRP sheet-concrete interface. The energy release rate was adopted to describe the growth of cracks. The theoretical method was proposed for calculating energy release rate of FRP-concrete interfacial crack. Based on this theoretical method, a growth-rate model for interface cracks induced by flexural cracks in CFRP sheet strengthened beams under cyclic loading was developed according to the results of fatigue tests on CFRP sheet-concrete interface.The evolution law for the bond-slip relationship of CFRP sheet-concrete interface was studied based on the results of fatigue tests on CFRP sheet-concrete interface. It was found that, cyclic loading leads to the degradation of bond-slip relationship of CFRP-concrete interface.This degradation is reflected in the reduced stiffness and shear strength of CFRP sheet-concrete interface adhesive layer. The degradation model of the bond-slip relationship of the CFRP-concrete interface was proposed based on the test results.The calculation method of interfacial shear stress of beams externally bonded by plates or sheets was studied. An improved analytical solution of interfacial shear stress was developed for a plated beam subjected to arbitrary loading. The effects of adherend shear deformations on longitudinal deformations caused by the bending moment and axial force have been included in the present theoretical analyses. The present analytical solution can more accurately predict the interfacial shear stresses in plated beams.The fatigue damage mechanism of reinforced concrete beams was studied based on the existing experimental results. The fatigue damage parameter was defined by using the residual bending rigidity of the beams. Based on this definition, the fatigue cumulative damage model was obtained according to the test results, and the expressions for correlation coefficients of the model were introduced. In addition, fatigue damage assessment method was proposed for reinforced concrete beams.To verify the feasibility of the models and methods proposed in this paper, fatigue tests were carried out on damaged RC beams strengthened with CFRP sheets. Using the models and methods proposed in this paper including the fatigue crack propagation model of CFRP sheet-concrete interface, evolution model of bond-slip relationship of CFRP sheet-concrete interface, interfacial shear stress calculation method for plated beams, the fatigue damage assessment of reinforced concrete beams, the interfacial shear stress of damaged RC beams strengthened with CFRP sheets was calculated and analysed, and the calculated results were compared with the results of fatigue tests on damaged RC beams strengthened with CFRP sheets. The calculated results were in good agreement with the experimental results, which indicated that the models and methods proposed in this paper were feasible to some extent. In addition, based on above researchs, fatigue design method were discussed for RC beams externally bonded with FRP, the checking method was proposed for fatigue stress in cross section material of RC beams strengthened with CFRP sheets.