Study On Flexural Fatigue Behavior Of RC Beams Strengthened With CFL Under Hot-Wet Cycling Environment

A mass of engineering practice shows that fiber reinforced polymer(FRP) strengthening RC members can significantly improve the load bearing capacity and durability of the structure or component. In recent years, FRP reinforcement technology has been more widely used, and has become a leading subject in the field of civil engineering at home and abroad.For a better understanding of the effect of FRP sheet, a lot of research on bearing capacity and durability of FRP reinforcing members has been made by domestic and foreign researchers,many of them in a variety of experiments to explore the harsh environmental conditions.However, the study on the fatigue behavior of FRP reinforcing member in the actual operating environment has not yet been found in the reports. Therefore, study on the impact on fatigue performance and durability of FRP reinforcing member in hot-wet cycling of natural environment, has important theoretical significance in practical engineering applications and also has great value for FRP reinforcement design and improving the performance of reinforcement of old bridge structures.Based on extensive research by our research group, combining features of Guangdong climate, a new kind of FRP sheet- Carbon Fiber Laminate(CFL) invented by our group was used to strengthen 9 test beams with size of 1850 mm × 200 mm × 100 mm. Then bending fatigue performance and interface characteristics in hot-wet cycling environment by theoretical analysis and experimental research were studied. The main contents and conclusions are as follows:(1)By using MTS810 material testing system and environmental simulation and control system invited by our research group,following the periodic the hot-wet environment of bridges in Guangdong, and on the method of the hot-wet cycling and fatigue load coupling,fatigue experiments on the 9 reinforcement test specimens at 27.5k N, 25 k N, 22.5k N and20 k N load levels were conducted with S~N experiment curve obtained. According to the S~N curve,we obtained fatigue limit of CFL strengthened RC beams under conditions of the hot-wet cycling. The results showed that the fatigue limit declines about 26% compared with fatigue test in atmospheric conditions at room temperature.(2)The dynamic secant stiffness evolution of beams strengthened at different levels wereanalyzed and a three-phase damage model of beams strengthened in hot-wet cycling environment on the basis of residual stiffness theory was built. The results showed that under hot-wet cycling, dynamic secant stiffness of tested beams showed sinusoidal cyclical downturn.At the end of the first stage of fatigue damage, the damage quantity of strengthened beam was 0.14; and when the damage quantity reached 0.37, the reinforced beam began to enter the unstable stage.(3)The finite element method was used to analyze the impact of hot-wet environment on CFL-concrete interfacial stresses of reinforced beams. The results showed that, ①under the influence of temperature and humidity, the ends of the reinforced beam interface will appear stress concentration; ② The hot-wet environment of bridges in Guangdong region could cause interfacial stress of reinforced beams changed periodically. Each year from November to February period, the shear stress of CFL-concrete interface decreases; and from March to October, CFL-concrete interface shear stress increases. The maximum shear stress may reach about 21% of interfacial shear strength. During this hot-wet environment will accelerate the interfacial shear failure.