This study presents an experimental program to evaluate the influence of the prestressing technique on the flexural behavior of reinforced-concrete (RC) beams strengthened with near-surface-mounted (NSM) carbonfiber-reinforced-polymer (CFRP) laminates. The experimental program was organized to highlight the benefits of this technique for flexural strengthening of RC beams with low reinforcement ratio, susceptible of not respecting serviceability-limit-state (SLS) conditions, namely the deflection limit. For this purpose, five RC beams were fabricated to be tested under monotonic four-point loading. One beam was kept unstrengthened as a control beam, and another one was strengthened with a non-prestressed NSM CFRP laminate. The remaining beams were reinforced with a NSM CFRP laminate prestressed at 20%, 30% and 40% of its nominal tensile strength. Based on the results, applying the prestress force provided an increase of load carrying capacity corresponding to the concrete cracking and steel yielding initiations compared to the non-prestressed strengthened beam. Moreover, the influence of prestress level on the prevailing failure mode of the tested beams was assessed, and the relevant results are presented and discussed. An advanced numerical strategy was also developed to simulate the tested beams, which was demonstrated capable of being used for the design of this type of structures.

a b s t r a c tEpoxy adhesives are nowadays being extensively used in Civil Engineering applications, mostly in the scope of the rehabilitation of reinforced concrete (RC) structures. In this context, epoxy adhesives are used to provide adequate stress transference from fibre reinforced polymers (FRP) to the surrounding concrete substrate. Most recently, the possibility of using prestressed FRPs bonded with these epoxy adhesives is also being explored in order to maximize the potentialities of this strengthening approach. In this context, the understanding of the long term behaviour of the involved materials becomes essential. Even when non-prestressed FRPs are used a certain amount of stress is permanently applied on the adhesive interface during the serviceability conditions of the strengthened structure, and the creep of the adhesive may cause a continuous variation in the deformational response of the element. In this context, this paper presents a study aiming to experimentally characterize the tensile creep behaviour of an epoxy-based adhesive currently used in the strengthening of concrete structures with carbon FRP (CFRP) systems. To analytically describe the tensile creep behaviour, the modified Burgers model was fitted to the experimental creep curves, and the obtained results revealed that this model is capable of predicting with very good accuracy the long term behaviour of this material up to a sustained stress level of 60% of the adhesive's tensile strength.

a b s t r a c tThe objective of this paper is to propose a simplified analytical approach to predict the flexural behavior of simply supported reinforced-concrete (RC) beams flexurally strengthened with prestressed carbon fiber reinforced polymer (CFRP) reinforcements using either externally bonded reinforcing (EBR) or near surface mounted (NSM) techniques. This design methodology also considers the ultimate flexural capacity of NSM CFRP strengthened beams when concrete cover delamination is the governing failure mode. A moment-curvature (M-v) relationship formed by three linear branches corresponding to the precracking, postcracking, and postyielding stages is established by considering the four critical M-v points that characterize the flexural behavior of CFRP strengthened beams. Two additional M-v points, namely, concrete decompression and steel decompression, are also defined to assess the initial effects of the prestress force applied by the FRP reinforcement. The mid-span deflection of the beams is predicted based on the curvature approach, assuming a linear curvature variation between the critical points along the beam length. The good predictive performance of the analytical model is appraised by simulating the force-deflection response registered in experimental programs composed of RC beams strengthened with prestressed NSM CFRP reinforcements.

Despite the extensive research that has been conducted on the debonding behaviour of FRP strengthening systems, no standard methodology has been yet established on its experimental characterization. In this context, to assess the performance and reliability of small scale testing Page 2 of 46 on NSM (near surface mounted) FRP strengthening systems, an experimental program was carried out on a series of nine NSM FRP strengthening systems, in the framework of an international Round Robin Testing (RRT). Eleven laboratories and seven manufacturers and suppliers participated in this extensive international exercise, which regarded both NSM and EBR FRP strengthening systems. Test results obtained for the NSM systems by the participating laboratories are discussed and compared in this paper to investigate the feasibility of the adopted single/double pulling shear test method, to investigate the mechanism of bond between NSM FRP reinforcement and concrete, and to investigate the level of variability obtained between the participating laboratories testing the same material batches. It is concluded that the tested variants in the adopted single/double shear pulling test have a significant influence, stressing the importance of the level of detail of standardized test protocols for bond verification. On overall, given the variants included in this study, the obtained variation in bond stress-slip behaviour between the laboratories remained fairly limited.

Near Surface Mounted (NSM) technique has proved to be a very effective technique for the flexural strengthening of RC beams. Due to the relatively small thickness of the concrete cover that several beams present, cutting the bottom arm of steel stirrups for the installation of NSM laminates might be a possible strategy, whose implications on the beam's load carrying capacity need to be assessed. When still stirrups are cut, however, the shear resistance can be a concern. This also happens when a strengthening intervention is carried out to increase the flexural resistance of a beam, since in certain cases it is also necessary to increase the shear resistance in order to avoid the occurrence of brittle shear failure. The present work assesses the effectiveness of a technique that aims to increase both the flexural and shear resistance of RC beams that have the bottom arm of the steel stirrups cut for the application of NSM laminates. This assessment is performed by experimental and numerical research. The main results of the experimental program are presented and analyzed, and the innovative aspects of a constitutive model implemented in a computer program are described, being their virtues and deficiencies discussed.

In this work, a new technique for the efficient confinement of reinforced concrete (RC) columns of rectangular crosssection is described and its effectiveness is assessed experimentally. This technique is based on the concept of applying strips of carbon fiber reinforced polymer (CFRP) wet layup sheets with a certain prestress level using a mechanical device. The influence of the cross-section aspect ratio of columns on the axial stress-strain response, strain field in the CFRP and strength increase provided by the different adopted strengthening configurations was investigated. All specimens had a height of 1100 mm, and three cross-sections were considered: 120 mm×120 mm, 240 mm×120 mm and 480 mm×120 mm, representing cross-section aspect ratios (large/small edge) equal to 1, 2 and 4, respectively. Four types of columns were tested: conventional RC columns (reference columns), fully-wrapped columns, partially-wrapped columns, and columns strengthened according to the new technique. All columns were subjected to axial compression loading until failure. The experimental results show that the cross-section aspect ratio has a significant effect on the confinement effectiveness that CFRP strengthened systems can provide to RC columns of rectangular cross-section. The maximum axial strength and axial strain at the peak load of all columns significantly decrease when the cross-section aspect ratio increases. Based on the obtained experimental results, it is shown that the proposed technique is more efficient in terms of increasing the load carrying capacity of rectangular RC columns than CFRP-based conventional strengthening techniques.

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