Near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement is one of the latest and most promising strengthening techniques for reinforced concrete (RC) structures. Research on this topic started only a few years ago but has by now attracted worldwide attention. Issues raised by the use of NSM FRP reinforcement include the optimization of construction details, models for the bond behaviour between NSM FRP and concrete, reliable design methods for flexural and shear strengthening, and the maximization of the advantages of this technique. This paper provides a critical review of existing research in this area, identifies gaps of knowledge, and outlines directions for further research.

Near-surface mounted FRP reinforcement: An emerging technique for strengthening structures

Near surface mounted (NSM) and externally bonded reinforcement (EBR) strengthening techniques are based on the use of carbon fiber reinforced polymer (CFRP) materials and have been used for the structural rehabilitation of concrete structures. In the present work, the efficacies of the NSM and EBR techniques for the flexural and shear strengthening of reinforced concrete beams are compared carrying out two experimental groups of tests. For the flexural strengthening, the efficacy of applying CFRP laminates according to NSM is compared to those resulting from applying CFRP laminates and wet lay-up CFRP sheets according to EBR technique. The influences of the equivalent reinforcement ratio (steel and laminates) and spacing of the laminates on the efficiency of the NSM technique for the flexural strengthening is also investigated. A numerical strategy is implemented to analyze the applicability of the FRP effective strain concept, proposed by ACI and fib in the design of FRP systems for the flexural strengthening. To assess the efficacy of the NSM technique for the shear strengthening of concrete beams, four beam series of distinct depth and longitudinal tensile steel reinforcement ratio are tested. Each series is composed of one beam without any shear reinforcement and one beam using the following shear reinforcing systems: conventional steel stirrups; strips of wet lay-up CFRP sheet of U configuration applied according to EBR technique; and laminates of CFRP embedded into vertical or inclined (45°) pre-cut slits on the concrete cover of the beam lateral faces, according to the NSM technique. Using the obtained experimental results, the performance of the analytical formulations proposed by ACI, fib and Italian guidelines is appraised.

Efficacy of CFRP-based techniques for the flexural and shear strengthening of concrete beams

A Near Surface Mounted (NSM) strengthening technique was developed to increase the shear resistance of concrete beams. The NSM technique is based on fixing, by epoxy adhesive, Carbon Fiber Reinforced Polymer (CFRP) laminates into pre-cut slits opened in the concrete cover of lateral surfaces of the beams. To assess the efficacy of this technique, an experimental program of four-point bending tests was carried out with reinforced concrete beams failing in shear. Each of the four tested series was composed of five beams: without any shear reinforcement; reinforced with steel stirrups; strengthened with strips of wet lay-up CFRP sheets, applied according to the externally bonded reinforcement (EBR) technique; and two beams strengthened with NSM precured laminates of CFRP, one of them with laminates positioned at 90° and the other with laminates positioned at 45° in relation to the beam axis. Influences of the laminate inclination, beam depth and longitudinal tensile steel reinforcement ratio on the efficacy of the strengthening techniques were analyzed. Amongst the CFRP strengthening techniques, the NSM with laminates at 45° was the most effective, not only in terms of increasing beam shear resistance but also in assuring larger deformation capacity at beam failure. The NSM was also faster and easier to apply than the EBR technique. The performance of the ACI and fib analytical formulations for the EBR shear strengthening was appraised. In general, the contribution of the CFRP systems predicted by the analytical formulations was slightly larger than the values registered experimentally. Performance of the formulation by Nanni et al. for NSM strengthening technique was also appraised. Using bond stress and CFRP effective strain values obtained in pullout bending tests with NSM CFRP laminate system, the formulation by Nanni et al. predicted a contribution of this CFRP system for the beam shear resistance of 72% the experimentally recorded values.

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Near surface mounted CFRP laminates for shear strengthening of concrete beams

Behavior and capacity of RC beams strengthened in shear with NSM FRP reinforcement

Properties and applications of FRP in strengthening RC structures: A review

https://repositorium.sdum.uminho.pt/bitstream/1822/5961/1/RI_2006_BarrosEtAl_ConstructionAndBuildingMaterials.pdf

Assessing the effectiveness of embedding CFRP laminates in the near surface for structural strengthening

Performance of reinforced concrete T beams strengthened in shear with NSM CFRP laminates

https://repositorium.sdum.uminho.pt/bitstream/1822/26409/4/IJ%28ISI%29_65.pdf

Salvador J. E. Dias

Papers

Efficacy of CFRP-based techniques for the flexural and shear strengthening of concrete beams

Near surface mounted CFRP laminates for shear strengthening of concrete beams

Assessing the effectiveness of embedding CFRP laminates in the near surface for structural strengthening

Performance of reinforced concrete T beams strengthened in shear with NSM CFRP laminates

Shear strengthening of RC beams with NSM CFRP laminates: Experimental research and analytical formulation