Monotonic and Fatigue Flexural Behaviour of RC Beams Strengthened with Prestressed NSM CFRP Rods
Badawi, Moataz Assad
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The use of near surface mounted (NSM) carbon fibre reinforced polymer (CFRP) reinforcement is a recent and a promising technique for increasing the flexural capacity and the fatigue life of reinforced concrete (RC) flexural members. Prestressing the NSM CFRP rod may be utilized for a further enhancement in the monotonic and fatigue flexural response of RC beams. The aim of this study is to investigate the effectiveness of strengthening RC beams with non-prestressed and prestressed CFRP rods to increase the monotonic and fatigue flexural strength of the beams. Twenty-two RC beams were fabricated. Five beams were not strengthened and acted as control to simulate an existing structural member. The other beams were divided into groups that were strengthened with non-prestressed CFRP rod (0% prestressed), and prestressed CFRP rod (40%, or 60% prestressed of the CFRP rod tensile strength). A beam from each group was tested under monotonic load and acted as a reference beam for those tested under cyclic loads. The test results showed that strengthening the RC beams with NSM CFRP rods increased both the monotonic flexural capacity and the fatigue strength. An increase in the yield and ultimate load of 26% and 50% was achieved, when the beams were strengthened with non-prestressed CFRP rod compared to the control beam. Also, the flexural stiffness of the strengthened beam was slightly enhanced by 16% over that of the control beam. When the beams were strengthened with prestressed CFRP rod (40% and 60%), considerable improvements in the cracking, yield, and ultimate loads were achieved as well as the flexural stiffness (serviceability). In a comparison to the control beam, an increase up to 91% in the yield load and 79% in the ultimate load were obtained, in addition to 52.6% improvement in the flexural stiffness (pre-yielding) when a prestressed NSM CFRP rod was applied. A model to predict the flexural behaviour of the beams (control, non-prestressed, 40%, and 60% prestressed strengthened beams) under monotonic loading using section analysis is presented. It includes a model for flexural crack spacing considering the effect of the CFRP reinforcement, and the transfer length model. For an easy use, the monotonic flexural behaviour model is adopted in a computer language (Visual Basic 6). A model based on strain-life approach is also utilized to predict the fatigue life of the beams at various load ranges for all tested RC beams. For a given load range, by obtaining the nominal maximum and minimum stresses using the monotonic flexural model, the fatigue life of a beam is estimated by accounting for the effect of notch (ribs of the reinforcing bars), and the effect of mean stress. In summary, this study presents the first North American experience by using prestressed NSM CFRP rod for strengthening RC beams. Using such high prestressing levels of 40% and 60% with NSM strengthening method is considered the original contribution for monotonic flexural behaviour. Under cyclic loading, investigating the fatigue behaviour and constructing the fatigue life curves for RC beams strengthened with non-prestressed NSM CFRP rod is a considerable contribution to the very limited information available in the literature. This study also includes the inventiveness of testing the fatigue response of the RC beams strengthened with prestressed NSM CFRP rod. A monotonic flexural model of strengthened RC beams with non-prestressed and prestressed NSM CFRP strengthened beams was developed to predict load versus deflection, strain in the concrete, strain in the tension and compression steel reinforcement, and strain in CFRP rod. The model is verified with the experimental results with excellent agreement. A model using strain-life approach was also developed to predict the fatigue life of non-prestressed and prestressed beams with a reasonable accuracy.