Bond Behaviour of FRP Strips Glued on Masonry: Experimental Investigation and Empirical Formulation

This paper investigates the bond behaviour of carbon and glass fibre reinforced polymers (FRPs) glued on various kinds of masonry supports. It presents the results of a wide experimental campaign carried out at the Laboratory of Structures of the University of Salerno (Italy). Four different types of masonry were considered. Clay-bricks as well as some natural masonry elements (i.e., tuff or limestone, that are rather common for existing buildings in the Mediterranean Area) were employed as supports for gluing FRP sheets and perform double-shear pull-out tests. A preliminary experimental work was carried out on samples of the above mentioned masonry blocks for quantifying their key mechanical properties. Thus, a wide series of masonry specimens were tested both in compression and bending for evaluating their compressive and tensile strength and determining the key statistical parameters which describe their distribution and scatter. Then, 29 double-shear pull-out tests were carried out for observing the bond behaviour of FRP strips on the various masonry substrates. Moreover, two sets of six coupon tests were performed for evaluating the actual axial stiffness of FRP plates glued on masonry samples. The paper reports the key results of the above mentioned tests with the aim of pointing out the influence of the various relevant parameters of both masonry and composite materials on the resulting bond strength. A significant influence of masonry properties was observed and quantified, whereas no significant effect induced by the two different kinds of composite materials were actually observed. The above mentioned experimental results are employed for assessing the theoretical formula adopted by Italian Guidelines for evaluating ultimate bond strength of composite laminates glued on masonry. Finally, an alternative fully empirical relationship is calibrated on pull-out test results presented in this paper.