Due to earthquake, wave erosion, freeze-thaw cycle, acid-alkali environment, ultraviolet radiation and overload, a large number of concrete bridges in China have local or overall damage, which needs to be repaired and strengthened. It is an effective reinforcement method to reinforce damaged concrete bridge with adhesive fiber reinforced polymer(FRP). At present, the main types of fiber used are carbon fiber fabric, glass fiber and aramid fiber.

The prominent feature of bridge structure is that it mainly bears fatigue loads such as vehicle loads and the environment is complex. Bending fatigue performance is one of the main performance of bridge structure. It is of great significance to study the flexural fatigue behavior of concrete bridges strengthened with fiber reinforced polymer(FRP) for ensuring the healthy development of transportation in China.

Flexural fatigue behavior of RC Beams Strengthened with fiber reinforced polymer(FRP)

In 1992, Meier et al. conducted a fatigue test of a reinforced concrete T-beam strengthened with carbon fiber reinforced polymer (CFRP) sheet with a span of 6m and four-point bending load in EMPA (Federal Building Materials Testing Laboratory of Rui-Tu). The results show that after strengthening with fiber reinforced polymer(FRP), a large number of micro-cracks appear in the beam due to the reduction of stress concentration in the bending cracks. The strengthened beams show good fatigue performance, which confirms the effectiveness of fiber reinforced polymer(FRP) in improving the flexural fatigue performance of concrete beams.

In 2001, Canadian scholar Masoud et al. studied the static and fatigue properties of corroded RC beams strengthened with CFRP sheets. Six beams were fastened with two different CFRP sheets for fatigue tests.

It is pointed out that the fatigue life of beams will be greatly reduced by corrosion of steel bars, and the fatigue life of beams strengthened by corrosion is 2.5-6 times higher than that of beams strengthened by corrosion, but still lower than that of beams strengthened without corrosion. It is concluded that carbon fiber reinforced polymer (CFRP) is an effective method to reinforce corroded RC beams, which can improve the performance of the members and then play a role in repairing and strengthening the structure.

In 2004, Heffernan and Erki jointly produced 20 reinforced concrete beams for static and fatigue tests. Three groups of specimens were subjected to fatigue tests under three stress amplitudes of 28.2/112.0,28.2/98.0,28.2/84.1kN. The fatigue life, mid span deflection and failure mode of reinforced beam and non reinforced beam are obtained. The failure mode of unreinforced beams is due to the fracture of reinforcement. The failure mode of the strengthened beam is due to the breakage of the reinforcement due to the carbon fiber sticking. The fatigue life of the strengthened beams is increased by 155%, 182% and 537% under three stress amplitudes, respectively, and S-N curves are obtained. However, the mathematical formula of S-N curves is not fitted in this paper.

Heffernan and Erki also studied the effect of CFRP layers on the fatigue properties of the beams. Four beams (0,2,4,6 layers respectively) were tested under 78.0/271.6 kN stress amplitude. The fatigue life of the beams is 335,31.2,627,1049 times respectively, which shows that the fatigue life of the beams can be improved by sticking more fiber cloth. The reason why the fatigue life of the 2 layer fiber cloth is reduced by 7.5% is not given.

In addition, Shahawy and SheifEl-Tawil have studied the static and fatigue properties of concrete T-beams strengthened with carbon fiber reinforced polymer (CFRP) respectively, and given the design suggestions of CFRP strengthening concrete beams.