CFRP flexural and shear strengthening technique for RC beams : experimental and numerical research

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 steel 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.The study reported in this paper forms a part of the research program "CUTINEMO - Carbon fiber laminates applied according to the near surface mounted technique to increase the flexural resistance to negative moments of continuous reinforced concrete structures" supported by FCT, PTDC/ECM/73099/2006. The authors wish to acknowledge the support also provided by the S&P, Casais and Artecanter Companies. The second Author acknowledges the grant under the aforementioned research project. The third author acknowledges the financial support of FCT, PhD Grant number SFRH/BD/23326/2005

Flexural strengthening of RC continuous slab strips using NSM CFRP laminates

To assess the effectiveness of the near surface mounted (NSM) technique, in terms of load carrying and moment redistribution capacities, for the flexural strengthening of continuous reinforced concrete (RC) slabs, an experimental program was carried out. The experimental program is composed of three series of three slab strips of two equal span length, in order to verify the possibility of increasing the negative (at the intermediate support region) resisting bending moment in 25% and 50% and maintaining moment redistribution levels of 15%, 30% and 45%. Though the flexural resistance of the NSM strengthened sections has exceeded the target values, the moment redistribution was relatively low, and the increase of the load carrying capacity of the strengthened slabs did not exceed 25%. This experimental program is analyzed to highlight the possibilities of NSM technique for statically indeterminate RC slabs in terms of flexural strengthening effectiveness, moment redistribution and ductility performance. Using a FEM-based computer program, which predictive performance was appraised using the obtained experimental results, a high effective NSM flexural strengthening strategy is proposed, capable of enhancing the slab’s load carrying capacity and maintaining high levels of ductility.The study reported in this paper forms a part of the research program "CUTINEMO - Carbon fiber laminates applied according to the near surface mounted technique to increase the flexural resistance to negative moments of continuous reinforced concrete structures" supported by FCT, PTDC/ECM/73099/2006. The authors wish to acknowledge the support also provided by the S&P, Casais and Artecanter Companies. The first Author acknowledges the financial support of National Council for Scientific and Technological Development (CNPq) - Brazil, Ph.D. Grant no. 200953/2007-9. The second Author wishes to acknowledge the support provided by FCT, by means of the SFRH/BSAB/818/2008 and SFRH/BSAB/913/2009 sabbatical grants

Effects of sustained load and freeze-thaw exposure on RC beams strengthened with prestressed NSM-CFRP strips

An experimental program was conducted to investigate the combined effects of sustained load and freeze-thaw cycling exposure on the flexural behaviour of reinforced concrete (RC) beams strengthened in flexure using prestressed Near-Surface Mounted (NSM) Carbon Fibre Reinforced Polymer (CFRP) strips. Two sets of three large-scale rectangular RC beams (sets BS-F and BS-FS) were tested. Each set consisted of one un-strengthened control beam, one strengthened beam with non-prestressed NSM-CFRP strips, and one strengthened beam using prestressed NSM-CFRP strips with 60% of the ultimate tensile strength of the CFRP strips. After strengthening, three beams from set BS-F were exposed to 500 freeze-thaw cycles (three cycles per day between +34oC to -34oC with a relative humidity of 75% for temperatures above +20oC). Also, the beams from set BS-FS were exposed to 500 freeze-thaw cycles (three cycles per day between +34oC to -34oC with fresh water spray for 10 minutes at a rate of 18 L/min at a temperature of +20oC while being subjected to a sustained load equal to 47% of the theoretical ultimate capacity of the non-prestressed NSM-CFRP strengthened beam. The beams were simply supported and tested under static monotonic loading in four-point bending configuration until failure. The damage done to the beams due to exposure was evaluated, and furthermore, the effects of sustained load and freeze-thaw exposure were elaborated on the load-deflection response, type of failure, ductility, and energy absorption. Analysis of the results revealed the significant effects of the sustained load and freeze-thaw cycling exposure on the flexural performance of RC beams strengthened with prestressed and non-prestressed NSMCFRP strips