Bond Behaviour of NSM FRP Strengthening Systems on Concrete Elements Under Sustained Load

During the last decades, Near Surface Mounted (NSM) Fibre Reinforced Polymer (FRP) strengthening system has been widely accepted as an efficient methodology in structural rehabilitation. The bond behaviour of the FRP-concrete joint has resulted to be governing the overall performance of the strengthening system. Moreover, due to the susceptibility of the materials used in the NSM strengthening system in front of high temperatures and sustained loads, their performance under service conditions can be affected. Although the short-term bond behaviour has been largely studied in the literature, there is still a lack of experimental data to better understanding the long-term performance. The paper aims to experimentally study the evolution of bond damage of the concrete-FRP bonded joint of an NSM FRP strengthened concrete element, and how it affects to the slip between the strengthening system and the concrete For this purpose, a total of eight NSM pull-out specimens with two different bonded lengths (150 and 225 mm) and two different groove thicknesses (7.5 and 10 mm) have been loaded with two different levels of sustained load (15 and 30% of the ultimate load) at room conditions up to 40 days. Specimens were previously characterized under a short-term test. Slip at the loaded end was measured to capture the evolution of slip during time. Results showed that the slip caused by creep increased with the sustained load level, whilst it decreased with the bonded length.