Flexural strengthening of reinforced low strength concrete slabs using prestressed NSM CFRP laminates

Abstract

a b s t r a c tThe effectiveness of the Near Surface Mounted (NSM) technique with prestressed CFRP (carbon fiber reinforced polymer) laminates for the flexural strengthening of reinforced concrete (RC) slabs of low strength concrete was assessed. Four RC slabs were tested, a reference slab (without CFRP), and three slabs flexurally strengthened using NSM CFRP laminates with different prestress level of the ultimate tensile strength of the CFRP: 0%, 20% and 40%. The experimental program is described and the main results are presented and analyzed in terms of the structural behavior of the RC slabs, failure modes and performance of the NSM technique with prestressed CFRP laminates. The results show that prestressing CFRP laminates with NSM technique is an effective technology to increase cracking, service, yielding and maximum loads of RC slabs made by low strength concrete. By applying NSM CFRP laminates prestressed at 20%, the cracking, service and maximum loads have increased, respectively, 258%, 123% and 125%, when the corresponding values of the reference slab are taken for comparison purposes, while 400%, 190% and 134% were the increase when applying laminates prestressed at 40%. Using available experimental results obtained with the same test setup, but using RC slabs of higher strength concrete, it can be concluded that as minimum is the concrete strength as more effective is the NSM technique with prestressed CFRP laminates in terms of serviceability limit states. A numerical strategy was used to evaluate the load-deflection of the tested RC slabs and to highlight the influence of the percentage of CFRP, the percentage of tensile longitudinal bars and the elasticity modulus of the CFRP on the effectiveness of the NSM technique with prestressed CFRP laminates for the flexural strengthening of RC slabs, by performing a parametric study.

Fiber Reinforced Polymer Strengthening of Structures by Near-Surface Mounting Method

“…In order to further improve flexural performance and utilization of FRP strain, prestressing of NSM-CFRP components has been a recent area of study with excellent results [17,18,19,20,21,22,23,24]. A summary of testing conditions, presstressing levels, test variables and observed failure modes of NSM FRP presstressing experiments [17,18,19,20,21,22,23,24] are presented in Table 3.…”

“…The effectiveness of strengthening low concrete strength (15 MPa) RC slabs with prestressed NSM-CFRP was also explored by Hosseini et al [21]. Just like their earlier regular concrete strength RC slabs test, three of the slabs were strengthened with NSM-CFRP of 0%, 20% and 40% levels of prestressing and their behavior, failure modes and FRP performance were investigated.…”

“…For all tested prestressed NSM FRP RC slabs (normal and low concrete strength) by Hosseini et al [20,21], failure mode was the rupture of NSM FRP laminates, which is more favorable than FRP debonding failure. This was an indication that the FRP material was highly effective.…”

Preloading Effect on Strengthening Efficiency of RC Beams Strengthened with Non- and Pretensioned NSM Strips

“…Pretensioning the FRP effectively reduces existing crack widths, delays their further development, decreases existing deflections, and relieves stress in the internal reinforcement. 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.…”

“…An increase in the steel reinforcement ratio results in a decrease in the degree of strengthening that can be achieved. Another investigated parameter was the pretensioning level of the CFRP laminates [2,3,5,6,7,8,9,10,11]. The maximum pretensioning level studied was 60% of the tensile strength of the CFRP [2,5].…”