Pre-damaged reinforced concrete (RC) beams require significant enhancement of flexural capacity and ductility to maintain structural integrity under sustained loads. Although current strengthening techniques using fiber-reinforced polymer (FRP) sheets show promise, there is a limited understanding of how to control the failure modes and stress distribution around the beams. This study addresses this gap by experimentally testing RC beams strengthened with Carbon Fiber Reinforcement polymer (CFRP) sheets and analyzing how it affects the failure in the beams subjected under prolonged loads. During the experiment, four beams under different loading conditions were tested. Before the process, cracking was generated, followed by repair with U-shaped CFRP, and the beam was tested further to failure. Throughout the process, the service load remains constant. Using the standard code "ACI Committee 440–2002", the results for the potential damage due to the service loads were compared with theoretical outcomes met by the code. From the results, both the prior damage level and the applied CFRP have a substantial impact on the beams’ failure mode and strengthening effectiveness. The strengthened beams demonstrate higher capacity of withstanding sustained loads, with a capacity increase ranging from 21.8–66.4%. Additionally, cracking widths at failure load were reduced by 25.6–82.7% when compared to the control beam. The findings contribute to a better knowledge of the structural behavior of FRP-strengthened reinforced concrete beams, providing useful insights for engineers and researchers in the field of structural rehabilitation and retrofitting.