Interaction forces in RC beams strengthened with near-surface mounted rectangular bars and strips

The flexural performance of a reinforced concrete (RC) beam can be improved using the near-surface mounted (NSM) FRP strengthening technique. In the NSM technique, grooves are first cut in the concrete cover, and FRP bars are then placed in the grooves using a suitable bonding adhesive. In such an FRP-strengthened RC beam, debonding failure at the two bar ends may occur due to local high interaction forces in the bar-end regions between the RC beam and the FRP bars. This paper presents a theoretical investigation into these interaction forces in RC beams strengthened with NSM FRP rectangular bars (including FRP strips with a narrow rectangular section as a special case). By introducing and defining two interfacial stiffness parameters, an analytical solution for the tangential and the normal interaction forces between the beam and the NSM bars is first obtained based on the analytical solution developed by Smith and Teng for beams strengthened with an externally bonded FRP plate. The accuracy of the analytical solution is demonstrated by comparing its predictions with those from a 3-D linear elastic finite element (FE) model. The numerical results from both the analytical solution and the 3-D FE model confirms the existence of high interaction forces in the bar-end regions as the cause for bar-end debonding failure which has been commonly observed in tests. While the present paper is focused on RC beams strengthened with NSM rectangular bars, the proposed approach is general and is applicable to a range of similar problems such as RC beams strengthened with NSM bars of other cross-sectional shapes (e.g. circular and elliptical shapes) as well as timber beams strengthened with NSM bars.