Study On Flexural Behavior And Debonding Failure Of CFRP-strengthened RC Beams

It has been manifested both by experiment and engineering applications that FRP laminate (plate) used as a substitute for steel plate is very effective and efficient in strengthening concrete structures. Consequentaly a lot of achievements have been made in this field during the past two decades. However, as a new developing retrofit and rehabilitation technique, there are still many problems needed to be studied, such as, is there any effect of preload level on the flexural behavior of strengthened RC beam at the time of strengthening; how to evaluate the deflection of RC beams strengthened under different external load; what is the mechanism of intermediate crack (IC) induced debonding failure, and how to evaluate its initial debonding strength; and the flexural strengthening method of the beam at beam-column joint, and so on. Based on these problems, the following studies had been carried out in this dissertation.1. To investigate the flexural behavior of CFRP-strengthened RC beams under different preload level, 16 strengthened beams and 2 control beams were tested. The test variables were rebar ratio, number of plies of CFRP laminates, and preload level at the time of strengthening. Based on the test results, this dissertation presents a detailed analysis on the factors affecting the flexural performance of strengthened beams. Further analysis was carried on how and why the stiffness varied with the change of preload level. At last, an improved formula for evaluating the effective second inertia of moment was proposed and verified with test results reported in literature.2. Ten RC beams with beam-column joint were tested to investigate thier flexural behavior of the hogging moment region strengthened with CFRP laminate, and to find out a suitable anchorage system to anchor CFRP laminates at the joint. The test parameters included anchorage system, number of plies of CFRP laminates, and preload level. The flexural behavior of the strengthened beams was discussed and an appropriate anchorage system was proposed by comparison of different anchorage systems.3. The provisions for determining flexural strength of FRP-strengthened RC beams in ACI440.2R and CECS146, as well as the existing debonding strength models were discussed based on the test results of FRP-strengthened beamswith IC debonding failure. The objective is to assess whether these guides and models are reliable to prevent debonding failure, and to give some information for further research.4. Nine CFRP-strengthened RC beams were tested to examine the strain distribution along CFRP laminate and the crack pattern in the strengthened RC beam. The objective is to make best understanding of the mechanism of intermediate crack (IC) induced debonding. By analyzing the test results, the debonding mechanism had been made much clear, and a debonding strength model was proposed and verified by test results of 60 FRP-strengthened RC beams with IC debonding failure.5. Six cantilever RC beams of side bonding CFRP laminates and 1 control beam were tested to investigate the flexural behavior and appropriate anchorage system at the joint. The intention of side bonding was to overcome the restriction of serviceability as well as space limitation of the strengthened structure. The test variables included number of plies of CFRP laminate, configuration of CFRP laminates, and anchorage systems at the joint. Based on the test results, the flexural behavior and anchorage systems were analyzed.