Investigation on carbon fibre reinforced polymer (CFRP) strengthened, out of plane curved concrete beam subjected to combined effects of shear and torsion

Abstract

Carbon Fibre Reinforced Polymer (CFRP) strengthening technique had been shown excellent performance in externally strengthening Reinforced Concrete (RC) elements due to their superior properties compared to the alternatives. A substantial amount of studies had been done to study the behaviour of externally CFRP strengthened RC elements. However, as per the authors’ knowledge, while most of the studies have been focused on the external strengthening of straight RC beams using CFRP, none of the studies had been focused on strengthening horizontally curved RC beams. Curved beams innately respond with flexure, shear and especially torsion under outof-plane

loads due to its curvature. Hence, it is important to study the effect of the additional torsional stress on the CFRP external reinforcement. A detailed experimental program, a theoretical analysis and a numerical analysis were conducted in order to reduce the aforementioned research gap. A series of testing was conducted on beam specimens with 2 m and 4 m radii and externally strengthened with normal modulus CFRP fabrics to enhance the effects; shear, torsion and their combination. From series one experimental results, it was found that the wrapping CFRP fabrics have enhanced the shear capacity of the curved RC beams by at least 30 % and the enhancement is higher for beams with lower curvatures. It showed that CFRP wrapping is a very effective method to enhance the shear capacity of horizontally curved beams. The series two experimental results showed that U-socketing of CFRP fabrics for shear has increased the shear strength by 15.61 kN (16.7 %) and 17.41 kN (18.2 %) respectively for 2 m and 4 m radius specimens. The main failure mode was crack induced intermediate debonding of CFRP U-sockets. However, the predicted shear enhancements according to theoretical investigation are 23.64 kN and 24.51 kN for 2 m and 4 m radius beams respectively. It can be observed that for 2 m and 4 m specimens recorded respectively 33.97 % and 28.97 % less than the predicted enhancement by the theoretical study. This can be explained by the additional torsional stresses contributing to the direct shear stresses which cause to reduce the shear capacities of RC beams. The results from numerical models showed excellent agreement with experimental results which was used to carry out a parametric study. Subsequently, a capacity reduction factor was defined as the ratio between numerical shear gain and theoretically predicted shear gain to quantify the effect of torsional stresses on the shear enhancement of U-socketed CFRP on RC beams with a 1.4 m support distance which can be used to modify the currently existing analytical model adopted by ACI 440.2R-17 guide to design of FRP as external reinforcement for RC beams.