Parametric Study of the Physical Responses of NSM CFRP-Strengthened RC T-Beams in the Negative Moment Region

A non-linear finite element model (FEM) was constructed using a three-dimensional software (ATENA-3D) to investigate the effect of strengthening on the behavior of prestressed hollow-core (PHC) slabs with or without openings. The slabs were strengthened using near surface mounted (NSM)-carbon fiber reinforced polymer (CFRP) strips. The constructed model was validated against experimental results that were previously reported by the authors. The validated FEM was then used to conduct an extensive parametric study to examine the influence of prestressing reinforcement ratio, compressive strength of concrete and strengthening reinforcement ratio on the behavior of such slabs. The FEM results showed good agreement with the experimental results where it captured the cracking, yielding, and ultimate loads as well as the mid-span deflection with a reasonable accuracy. Also, an overall enhancement in the structural performance of these slabs was achieved with an increase in prestressing reinforcement ratio, compressive strength of concrete, external reinforcement ratio. The presence of openings with different dimensions along the flexural or shear spans reduced significantly the capacity of the PHC slabs. However, strengthening these slabs with 2 and 4 (64 and 128 mm2 that represent reinforcement ratios of 0.046 and 0.092%) CFRP strips was successful in restoring the original strength of the slab and enhancing post-cracking stiffness and load carrying capacity.

Near surface mounted CFRP laminates for shear strengthening of concrete beams

Tests and analyses were performed in this study to assess the flexural strengthening capacity of reinforced concrete (RC) members strengthened by the near surface mounted (NSM) technique, which is drawing attention as an alternative to the carbon fiber reinforced polymer (CFRP) bonding strengthening technique. Four-point bending tests were performed on 14 RC specimens. The test variables such as section geometry (250×400 and 400×200 mm), the compressive strength of concrete (27 and 34 MPa), tensile reinforcing bar ratio (0.43, 0.68, 0.98 and 1.42%), and a number of CFRP strips (one, two and three lines) were considered. Through the testing scenarios, the effect of each test variable on the failure mode and the flexural strengthening capacity of the RC members strengthened by the NSM technique were assessed. The test results show that the RC members strengthened by the NSM technique go to failure as a result of the partial CFRP fracture accompanied by crushing of the concrete in the compression zone, and that the CFRP reinforcement area as well as the section geometry have the greatest effect on the strengthening capacity. In addition, analyses confirm that the flexural strength prediction equation proposed by Yost et al.(2007) and the finite element method (FEM) analysis model proposed in this study appropriately predict the flexural strength and the load-displacement response of the RC members strengthened by the NSM technique, respectively. Key words: Near surface mounted (NSM) technique, flexural strengthening capacity,carbon fiber reinforced polymer (CFRP) strip, four-point bending test.

This paper presents an investigation of the bond mechanism between carbon fibre reinforced polymer (CFRP) laminates, concrete and steel in the near-surface mounted (NSM) CFRP-strengthened reinforced concrete (RC) beam-bond tests. The experimental program consisting of thirty modified concrete beams flexurally strengthened with NSM CFRP strips was published in. The effects of five parameters and their interactions on the ultimate load carrying capacities and the associated bond mechanisms of the beams are investigated in this paper with consideration of the following investigated parameters: beam span, beam depth, longitudinal tensile steel reinforcement ratio, the bond length of the CFRP strips and compressive concrete strength. The longitudinal steel reinforcement was cut at the beam mid-span in four beams to investigate a better assessment of the influence of the steel reinforcement ratio on the bond behaviour of CFRP to concrete bond behaviour. The numerical analysis implemented in this paper is based on a nonlinear micromechanical finite element model (FEM) that was used for investigation of the flexural behaviour of NSM CFRP-strengthened members. The 3D model based on advanced CFRP to concrete bond responses was introduced to modelling of tested specimens. The FEM procedure presents the orthotropic behaviour of the CFRP strips and the bond response between the CFRP and concrete. Comparison of the experimental and numerical results revealed an excellent agreement that confirms the suitability of the proposed FE model.

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

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.

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

In this study, a finite element (FE) model for nonlinear FE analysis was developed to evaluate the performance of reinforced concrete (RC) T-beams, which were strengthened in the negative moment region by near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods under low cyclic loading. Furthermore, the rods' depth of embedment was the research variable. Every component of the beam is considered in the model, including the concrete, steel rebars, CFRP rod, CFRP sheet, adhesive, and stirrups. The nonlinear properties of concrete, steel rebars, and adhesive were taken into account, while that of the CFRP was assumed to be linearly elastic till rupture. In addition, the user-programmable capabilities of ABAQUS were used to define the degradation of each material under low cyclic loading. The developed FE model was then compared to some experimental measurements comprising two specimens strengthened with NSM-CFRP rods and one un-strengthened control specimen. Overall, the predicted FE mid-span deflection responses were in line with the corresponding measured experimentally tested data. Finally, the research findings were summarized.

To assess experimentally the effect of flexural near surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) bars mounted on the sides of beams on the shear strength of reinforced concrete (RC) beams, 18 shear-deficient RC beams were built, strengthened with flexural NSM-CFRP bars, and tested under three-point bending until failure. The variables of the experimental program included the beam depth, the concrete compressive strength, and the flexural fiber-reinforced polymer (FRP) reinforcement ratio. It was observed that the strengthened beams exhibited up to 35% increase in shear capacity over the control beams. It was also observed that the increase in shear strength provided by concrete after strengthening was higher for beams with normal strength concrete when compared to those with higher strength concrete. The results have also revealed that the percent change in shear strength provided by concrete for the strengthened beams decreased with the increase in beam depth. Experimental data from this study showed that current standards become unconservative for beams with large depths. Five different beam shear strength models found in the literature were utilized to predict the shear strength of the tested beams. The models that exhibited the closest agreement with the experimental data were those of the University of Houston and the second order simplified modified compression field theory. It was concluded that flexural longitudinal NSM bars are a viable solution to enhance the shear strength of RC beams and that the shear strength provided by concrete can be accurately quantified using published models.

Engineers have proposed relocating externally bonded strengthening fiber reinforced polymer (FRP) material from the unprotected exterior of the concrete to the protected interior. This technology is known as near-surface mounted (NSM) strengthening. In NSM reinforcement, the FRP is surrounded by concrete on three sides so the bond and damage problems associated with externally bonded FRP strengthening systems are reduced or eliminated. This paper presents experimental results from 12 full-scale concrete beams strengthened with NSM carbon FRP (CFRP) strips. Three companion unstrengthened specimens were also tested to serve as a control. Experimental variables include three different ratios of steel reinforcement and two different ratios of CFRP reinforcement. Yield and ultimate strengths, flexural failure modes, and ductility are discussed based on measured load, deflection, and strain data. Test results show measurable increases in yield and ultimate strengths in all beams strengthened with CFRP as well as predictable nominal strengths and failure modes. Force transfer between the CFRP, epoxy grout, and surrounding concrete was able to develop the full tensile strength of the CFRP strips. Energy and deflection ductilities were reduced for CFRP strengthened beams. Future research needs are addressed.

Fiber Reinforced Polymer (FRP) rods are considered to be the most effective in retrofitting to increase the strength of reinforced concrete (RC) structures through Near-Surface Mounted (NSM) technique. There are, however, frequent cases encountered by engineers where the embedment depth mandated by ACI 440.2 R-08 code is not achievable in the field implementation. It has also been discovered that it is more challenging to strengthen the negative moment region of concrete members in comparison with the positive region. This research was conducted to determine the behavior of RC T-beams strengthened in the negative moment region using half-embedded NSM FRP rods. The findings were associated with the Modified Compression Field Theory (MCFT) which was applied in the analytical models. The model proposed was validated through a comparison with previous experimental study that showed half-embedded NSM FRP was effective as another method in comparison with the conventional soffit strengthening systems in retrofitting RC T-beams in the negative moment region, and good agreement was obtained. After that, a parametric analysis was initiated to determine the influence of FRP rod diameters, the compressive strength of concretes, the ratio of steel reinforcement as well as the materials used for the FRP on the flexural behavior of strengthened beams.

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

<p>Strengthening concrete structures with FRP(fiber reinforced polymer) have grown to be a widely used method over most parts of the world today, which FRP was developed in 1960's. A method to apply prestressing force to FRP is developed newly in these days, which can use the maximum performance of FRP materials. This study investigated the flexural behavior of simply supported Reinforced Concrete(R/C) beams strengthened with Prestressed Near Surface Mounted (NSM) CFRP(carbon fiber reinforced polymer) . CFRP plate and rod were used for flexural strengthening. Prestressing level changed from 0 % of CFRP tensile strength to 50 %. Any mechanical device has not been used to maintain the prestress during testing. Static four point loading tests are conducted for eleven R/C beams strengthened with Prestressed Near Surface Mounted (NSM) CFRP and Non-prestressed Near Surface Mounted (NSM) CFRP. The test shows that the beams with prestressed NSM CFRP exhibited a higher yielding load and a higher ultimate load, compared to the beams with non-prestressed NSM CFRP and the control beams. Beams strengthened by CFRP rod failed due to fiber rupture of the FRP in the groove, but beams strengthened by CFRP plate failed due to concrete cover separation.</p>

Effect of maximum coarse aggregate size upon shear strengthening of RC beams using NSM-CFRP strips

Prestressed concrete structures are facing serviceability challenges due to rising live loads, material degradation, and seismic demands. Retrofitting with carbon fiber-reinforced polymer (CFRP) offers a cost-effective alternative to full replacement. This study presents a finite element (FE) modeling framework to simulate the seismic performance of prestressed concrete T-beams retrofitted in the negative moment region using near-surface-mounted (NSM) CFRP rods and sheets. The model incorporates nonlinear material behavior and cohesive interaction at the CFRP–concrete interface and is validated against experimental benchmarks, with ultimate load prediction errors of 4.41% for RC T-beams, 0.49% for prestressed I-beams, and 1.30% for prestressed slabs. A parametric investigation was conducted to examine the influence of CFRP embedment depth and initial prestressing level under three seismic conditions. The results showed that fully embedded CFRP rods consistently improved the beams’ ultimate load capacity, with gains of up to 10.84%, 16.84%, and 14.91% under cyclic loading, near-fault ground motion, and far-field ground motion, respectively. Half-embedded CFRP rods also prove effective and offer comparable improvements where full-depth installation is impractical. The cyclic load–displacement histories, the time–load histories under near-fault and far-field excitations, stiffness degradation, and damage contour analysis further confirm that the synergy between full-depth CFRP retrofitting and optimized prestressing enhances structural resilience and energy dissipation under seismic excitation.