## 1 Introduction and Research Significance  
Masonry buildings are characteristic of the architectural heritage throughout most of Europe, North Africa and Asia. However, masonry constructions are notoriously vulnerable to earthquakes due to the almost total lack of tensile strength of the masonry material. Traditional methods for retrofitting and restoration of masonry constructions have sometimes demonstrated to be inappropriate: Reinforced Concrete (RC) roofs and floors, concrete jacketing of wall panels can highly enhance the structural behavior of masonry buildings, but they also produce an increment in dead loads, increase the structural stiffness reducing ductility, present problems of oxidation of steel reinforcement. From the 1990s, there has been some movement away from traditional construction materials for retrofitting applications, toward lighter-weight solutions. The use of FRP (Fiber Reinforced Polymers)materials are particularly suited to strengthening pre-existing masonry constructions and can be used to extend the fatigue life or provide an increase in strength of a structure [1; 2; 3; 4]. However, there is limited guidance available on the use of FRPs in strengthening of masonry structures. The long term behavior of FRPs materials used in masonry strengthening needs to be investigated, especially when composite materials are in the form of thin sheets or grids. FRPs are usually bonded to the external surface of an existing masonry structure: bonding is often achieved using epoxy adhesives; recently, however, FRP materials has been employed also in combination with inorganic matrix such as lime mortars and cement, to fulfill new needs in retrofit of historical constructions [5; 6; 7; 8; 9; 10; 11]. In such situation, the analysis of the vulnerability of FRPs to environmental damage is critical and should be proved. The life cycle of FRPs must be competitive with traditional materials, because of the limited available resources to maintain and protect architectural heritage structures. There are many areas where knowledge of the behavior of FRPs for masonry applications is currently lacking. Perhaps one of the more important areas is the performance of these materials under combined effects of fatigue and ageing. In this paper, a durability study has been conducted in order to investigate the residual mechanical properties of GFRP grids subjected to exposure in alkaline bath and fatigue cycles. The effects of fatigue have been extensively studied for many composite products [12], mainly in areas outside of civil engineering structures (automotive and aerospace engineering). However there is little information about the products used in retrofitting of masonry structures. Notoriously, fatigue failure occurs due to the application of fluctuating stresses that are lower than the stress needed to produce failure during a monotonous loading application. The study reported in [13] considered the fatigue effects in bonded regions of FRP plates used for small bridge applications, but only few information are found in terms of residual strength of the fibers [14; 15; 16] and FRP-reinforced elements [17; 18]. In these studies the fatigue effects caused a reduction of the static strength as expected. An aggressive chemical environment such as high alkaline pore solution is another threat for the glass fibers, when Alkali Resistant (AR) solutions are not used. A significant degradation in modulus and strength can be produced by the chemical sensitivity of E-glass (alumino-borosilicate glass), ordinary basalt fibers and their organic matrices. According to the authors of [19; 20] this can be addressed to three different mechanisms.