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• We investigate the capacity of CFRP repaired RC beams under heavy pre-load level. • The results indicated that the CFRP sheets significantly affected increased the flexural capacity and flexural stiffness. • An equation is proposed herein to calculate the flexural capacity of pre-damaged reinforced concrete. This paper presents an experimental study to determine the effectiveness of carbon fiber reinforced polymer (CFRP) sheets as a flexural repair system for reinforced concrete (RC) beams. One reference beam and nine CFRP repaired RC beams under heavy pre-damaged level are designed and tested in this study. The effects of several parameters including pre-damaged level, CFRP sheet layer and reinforcement ratio on failure mode, flexural deflection, moment-curvature relation and stress-strain behavior of CFRP sheet are investigated. In the first scenario, a reference beam is tested to determine load levels that can be used as pre-damaged for the tests. With respect to the second experiment, the test beam is subjected to a pre-damaged load equivalent to 40% or 80% of the yield load of the reference beam, respectively. Two failure modes such as the debonding failure of CFRP sheet and the fracture failure of CFRP sheet are found for the strengthened RC beams. The flexural bearing capacity of strengthened RC beams increases as the CFRP sheet layer or reinforcement ratio increases, while it decreases with the pre-damaged level increases. The decrease of pre-damaged level or the improvement of CFRP sheet layer or reinforcement ratio will decrease the deflection of CFRP repaired RC beams. Considering the effect of pre-damaged level and strengthening layer of CFRP sheet, a reduction coefficient of the CFRP sheet layer and an effect coefficient of pre-damaged level are introduced and then a formula for predicting the flexural capacity of CFRP repaired RC beams under heavy pre-damaged level is proposed and it agrees well with the test data. [ABSTRACT FROM AUTHOR]