Over the last decades, researchers have been studying fibre reinforced polymer (FRP) materials and their advantages in retrofitting of existing structures. The externally bonded reinforcement (EBR) technique is the most common practice in improving existing reinforced concrete (RC) structures with carbon FRP (CFRP) materials. In this regard, several additional advantages have been reported to the use of prestressed CFRP materials, mainly strips. However, the experience with RC strengthening using prestressed EBR-CFRP materials is still limited. Some concerns regarding the efficiency of the technique still exist, especially the durability and the long-term behaviour. This work aims at contributing to the knowledge on durability of RC slabs strengthened with prestressed CFRP laminate strips according the EBR technique. The durability was studied by exposing strengthened RC specimens to the following environments for approximately 8 months: (i) reference environment – specimens kept in a climatic chamber at 20 ºC; (ii) water immersion in tank at 20 ºC of temperature; (iii) water immersion in tank with 3.5% of dissolved chlorides at 20 ºC of temperature; and (iv) wet/dry cycles in a tank with a water temperature of 20 ºC. Additionally, half of the specimens were subjected to sustained loading at a load level of 1/3 of the ultimate load, with the occurrence of cracking. After the exposure period the slabs were monotonically tested up to failure by using a four-point bending test configuration. The results showed that the environmental conditions and the sustained loading, separately or combined, led in general to slight losses of performance and ductility. Although these losses were subtle, considering that the tests were carried out for 8 months, clear indications are given towards the importance of conducting similar tests for longer periods. The results obtained showed that the procedures implemented to assess the durability of the strengthening systems were sensitive to the most relevant deterioration mechanisms and their impact on the mechanical properties of the specimens. Therefore, these procedures may well contribute for the future establishment of standardized test programmes for the assessment of the durability of prestressed CFRP strengthening systems.This work was supported by the following programs: FEDER (European Funds for Regional Development) funds through the Operational Program for Competitiveness Factors – COMPETE, Operational Program for Competitiveness and Internationalization (POCI) and National Funds through FCT - Portuguese Foundation for Science and Technology under the projects FRPreDur FCOMP-01-0124-FEDER-028865 (FCT reference PTDC/ECM-EST/2424/2012), FRPLongDur POCI-01-0145-FEDER-016900 (FCT reference PTDC/ECM-EST/1282/2014) and POCI-01-0145-FEDER-007633. The authors also like to thank all the companies that have been involved supporting and contributing for the development of this study, mainly: S&P Clever Reinforcement Ibérica Lda, S&P Clever Reinforcement Company (Switzerland), Tecnipor - Gomes & Taveira Lda., Vialam – Indústrias Metalúrgicas e Metalomecânicas, Lda, Hilti Portugal-Produtos e Serviços, Lda. The first author wish also to acknowledge the grant SFRH/BD/98309/2013 provided by FCT. This paper is dedicated to Tiago Teixeira (1988–2015), former doctoral student at the ISISE R&D Research Centre at the University of Minho.info:eu-repo/semantics/publishedVersio

This work addresses the durability of structural epoxy adhesives and carbon fibre reinforced polymer (CFRP) laminates typically used in strengthening of existing reinforced concrete structures exposed to natural ageing. The experimental program included four natural (real) outdoor environments inducing ageing mainly caused by carbonation, freeze-thaw attack, elevated temperatures, and airborne chlorides from seawater. Moreover, a control (reference) environment (20 °C of temperature and 55% of relative humidity) and an environment involving water immersion of the materials under controlled temperature (20 °C of temperature) were also included in this investigation. The characterization involved the assessment of the physical, chemical and mechanical properties along a study period of up to two years. Furthermore, comparisons between the natural ageing tests developed in the scope of the present work and accelerated ageing tests existing in the literature were performed. Regarding to the epoxy adhesives, an increase in the glass transition temperature with the time was observed, while the tensile properties decreased, regardless of the outdoor environment. The CFRP laminates were marginally affected by the studied environments. Despite the remarkable dispersion of the results observed in the accelerated ageing tests for the period investigated, this testing protocol yielded higher mechanical degradation than under natural ageing.