Structural Effectiveness of Externally Bonded CFRPs in Strengthening RC Beams

Abstract

The desirable characteristics of carbon fibre reinforced polymer (CFRP) composites such as their high stiffness-to-weight ratio, high environmental durability, high tensile strength, and ease of application have led to their use as an alternative to conventional materials used for shear and flexural strengthening of reinforced concrete (RC) members. Over the past two decades, many studies have been conducted in order to investigate the performance of FRP composites in civil engineering and structural applications. A primary concern of FRP structural strengthening systems is that FRP sheets may de-bond from the concrete surface at a force significantly lower than the strength of the FRP material. Debonding failures are often brittle and occur with little warning, therefore making them an undesirable failure mode. ACI 440.2R-08 mentions the use of transversely placed FRP sheets, or U-wraps, to delay debonding. There is, however, scarcity of research pertaining to strengthening beams in shear with FRP strips or continuous wraps.

The research work presented here focus on the strengthening efficiency of reinforced concrete beams with externally bonded CFRPs. A detailed experimental program provides evaluations of structural behaviour of the combined flexural and shear strengthening of RC beams by using CFRP sheets prebonded on the tension face of the beam for flexural strengthening, and then reinforced in shear by the CFRP sheets in various schemes. The variables investigated in this research study included CFRP amount and distribution (i.e., continuous wrap versus strips), bonded surface (i.e., lateral sides versus U-wrap) orientation of fibres (i.e., 90°/45°) and laminate width. During the experiments, the following aspects were evaluated regarding the response of the tested beams: load-deflection analysis, crack patterns, deformation behaviour, failure modes and ductility index of beams.

In order to attain these goals, fifteen beams were tested, out of which two beams were considered as controlled/ unstrengthened beams, whereas all other thirteen beams were strengthened with externally bonded CFRP sheets in various schemes. Research output shows that the flexural– shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength of the beam. In addition theoretical calculations on estimating the shear capacities of the strengthened beams based on debonding failure mode are presented and compared with the corresponding experimental results in a reasonably good agreement.