Abstract
The near surface mounted (NSM) technique has been shown to be one of the most promising methods for upgrading reinforced concrete (RC) structures. Many tests carried out on RC members strengthened in flexure with NSM fiber-reinforced polymer (FRP) systems have demonstrated greater strengthening efficiency than the use of externally-bonded (EB) FRP laminates. Strengthening with simultaneous pretensioning of the FRP results in improvements in the serviceability limit state (SLS) conditions, including the increased cracking moment and decreased deflections. The objective of the reported experimental program, which consisted of two series of RC beams strengthened in flexure with NSM CFRP strips, was to investigate the influence of a number of parameters on the strengthening efficiency. The test program focused on an analysis of the effects of preloading on the strengthening efficiency which has been investigated very rarely despite being one of the most important parameters to be taken into account in strengthening design. Two preloading levels were considered: the beam self-weight only, which corresponded to stresses on the internal longitudinal reinforcement of 25% and 14% of the yield stress (depending on a steel reinforcement ratio), and the self-weight with the additional superimposed load, corresponding to 60% of the yield strength of the unstrengthened beam and a deflection equal to the allowable deflection at the SLS. The influence of the longitudinal steel reinforcement ratio was also considered in this study. To reflect the variability seen in existing structures, test specimens were varied by using different steel bar diameters. Finally, the impact of the composite reinforcement ratio and the number of pretensioned FRP strips was considered. Specimens were divided into two series based on their strengthening configuration: series “A” were strengthened with one pretensioned and two non-pretensioned carbon FRP (CFRP) strips, while series “B” were strengthened with two pretensioned strips. Experimental tests illustrated promising results at ultimate and serviceability limit state conditions. A significant gain of the load bearing capacity, in the range between 56% and 135% compared to the unstrengthened beams, was obtained. Tensile rupture of the NSM CFRP strips was achieved, confirming full utilization of the material.
Highlights
There are two primary methods of strengthening reinforced concrete (RC) members in flexure with fiber reinforced polymer (FRP) materials: laminates externally bonded (EB) to the tensile surface of the concrete and the near surface mounted (NSM) method in which narrow, typically carbon FRP (CFRP)strips or bars are embedded into slots made in the concrete cover
All beams failed in flexure due to rupture of both pretensioned and passive NSM CFRP strips
All beams failed in flexure due to rupture of both pretensioned and passive NSM CFRP strips preceeded by sounds of gradually-breaking carbon fibers (Figure 6a)
Summary
There are two primary methods of strengthening reinforced concrete (RC) members in flexure with fiber reinforced polymer (FRP) materials: laminates externally bonded (EB) to the tensile surface of the concrete and the near surface mounted (NSM) method in which narrow, typically carbon FRP (CFRP)strips or bars are embedded into slots made in the concrete cover. The influence of many parameters on the strengthening efficiency of the NSM method have been investigated, including the CFRP pretensioning strain, internal reinforcement ratio, concrete strength, and the CFRP reinforcement length and elastic modulus [1,2,3,4,5,6,7,8,9,10,11,12,13]. Review of the available literature on flexural strengthening of RC members with passive and pretensioned NSM materials concludes that the strengthening efficiency depends on a number of factors, including the internal steel reinforcement ratio, the pretensioning limit, defined as the level of CFRP pretension as a ratio of the ultimate CFRP strain, and the pretensioning method. The greatest benefit of the NSM pretensioning technique is in preventing premature debonding of the FRP from the concrete surface [6], which is the dominant limit state for EB FRP laminates
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