Out-of-plane strengthening of unreinforced masonry walls using FRP.

Abstract

Unreinforced masonry (URM) structures constitute both a significant portion of the world’s heritage buildings and a significant component of the modern residential building stock, and are particularly susceptible to damage from out-of-plane loads such as those generated by earthquakes (Ingham and Griffith 2011). Consequently, there is a considerable need for the development of economical and effective seismic strengthening techniques for URM construction. This study investigates the performance of near surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strengthened clay brick masonry walls under monotonic and cyclic out-of-plane bending with particular attention to the FRP-to-masonry joint behaviour. Fourteen NSM carbon FRP-to-masonry pull tests were conducted to study the FRP-to-masonry bond behaviour and to investigate the effect that variables such as cyclic loading and FRP strip dimensions have on the debonding resistance of a NSM FRP-to-masonry joint. The pull tests results were then incorporated into a large database of FRP retrofitted masonry pull test results by various researchers over the past 10 years. An empirical model was derived for the intermediate crack (IC) debonding resistance of FRP-to-masonry joints using a large set of test data from the open literature (Kashyap et al. 2012). Further, in order to predict the global load-slip response of FRP-to-masonry pull tests using various local bond-slip relationships two analytical procedures, namely a new generic numerical procedure and a closed-form mathematical solution, were developed which account for the partial-interaction response at the FRP-masonry interface (Kashyap et al. 2011). Fifteen walls were tested in this study to investigate the behaviour of NSM CFRP retrofitted masonry walls under out-of-plane bending and investigate the IC debonding failure mechanism in them. Also, the effects of typical design variables such as reverse cyclic loading, axial pre-compression, FRP strip spacing and reinforcement ratio on the stiffness, displacement capacity and ultimate strength of FRP retrofitted masonry walls were studied. The test results demonstrated that NSM CFRP strips designed to fail by IC debonding can provide an increase in strength of up to 20 times the strength of the corresponding unreinforced wall highlighting the effectiveness of the retrofitting scheme used. With respect to the test variables under investigation it was found that FRP strip spacing and reinforcement ratio strongly affect wall performance whereas cyclic loading and vertical pre-compression had little effect. Finally, a simple design methodology has been developed for masonry walls retrofitted with vertical CFRP strips with IC debonding as the preferred failure mechanism. This design methodology will provide solutions for choosing the FRP strip dimensions (bρ and tρ) and spacing (S). Importantly, the methodology is generic in the sense that it can be used for any type of FRP material and both externally bonded (EB) and NSM retrofit techniques. It also enables the FRP retrofit to be optimised in terms of both the strip spacing and cross-section. Overall, the results of this study show that the proposed NSM technique is structurally efficient and viable for seismic retrofitting of URM structures. Moreover, implementation of the proposed technique could have a significant impact in strengthening of masonry structures including conservation of the heritage buildings with considerable historical importance.