EXPERIMENTAL AND NUMERICAL INVESTIGATION ON THE LONG- TERM BEHAVIOUR OF BOND BETWEEN FRP AND MASONRY

Modern composite materials like fibre reinforced polymers (FRP), steel reinforced polymers (SRP) and natural fibres have been increasingly accepted as effective repairing or strengthening materials for masonry structures due to high mechanical properties and light weight. The efficacy and reliability of FRPs intrinsically depends on bond between the composite and the masonry substrate. Hence, investigation of longterm durability is a key issue since failures due to inaccurate characterization of bond performance may result in costly repair or premature component replacement [1-2]. Despite of the extensive research in designing of accelerated laboratory tests and developing theoretical models in the field of polymeric materials [1-3], much efforts are required to give insight into long-term performance of FRP-masonry specimens exposed to various ageing factors, including temperature, humidity, salts, etc. [4-6]. Investigation on long-term durability of bond between composites and masonry makes using accelerated aging tests necessary. Studies on bond behaviour of FRP-masonry specimens subjected to accelerated freeze-thaw tests can be found in [4] and [6]. Different exposure paths were used and a change in bond behaviour was experienced after exposure. Moreover, results showed the decrease in shear strength with increasing the number of cycles. As for moisture exposure, tests were usually performed by exposing the samples to wet-dry cycles or immersing them into water for different time spans ([4], [5]). The main outcomes pointed out that significant reduction of bond strength is expected after wet-dry exposure. Experimental research on the UV exposure effect on bond behaviour was carried out in [6]. The maximum reduction in shear bond strength was of about 40 %. Moreover, the bond strength decay was quantified and related to the number of cycles. A review of experimental techniques for predicting durability of polymers and composites materials can be found in [8] and in [9-10], respectively. However, standardized accelerated ageing procedures to establish long-term performance of FRP-masonry components is still strongly required, as already recognized by the RILEM Technical Committee 223-MSC “Masonry strengthening with composite materials”. Furthermore, while methods for evaluating fundamental responses of composite materials are fairly well established for specific degradation mechanisms acting alone, the potential for synergistic effects among mechanisms is not completely understood. In this case, statistical experimental approaches should be used to establish dependencies between the various degradation mechanisms, as well as reliable numerical models should be implemented for an accurate simulation of the experimental behaviour. The ability to predict the interface time-dependent behaviour as a function of environmental changes over the structural components lifetime is a critical issue at design stage. The coupled effects on porous media due to e.g. heat, air, moisture is a very recent issue [11, 12]. Analytical studies for predicting the life expectancy of plastics and polymer composites able to take into account a plurality of degradation mechanisms demand further investigation [13-18]. The common approach to model the fundamental relationships among damage accumulation rates and structure and property metrics of such materials makes use of refined multi-scale modelling [19, 20]. The thesis deals with characterization of the long-term bond behaviour between composite materials and masonry supports through advanced laboratory-based tests and refined computational analysis. The aim of the study is to achieve insight into the effects of critical environmental conditions on FRP-masonry interface performance. A brief introduction of the main activities is presented in the following sections.