Near-surface Mounted Fiber-reinforced Polymer Reinforcement Research Articles

Numerical Analysis of an Innovative Double-Strap Joint for the Splicing of Near-Surface Mounted Fiber-Reinforced Polymer Bars for Reinforced Concrete Beam Strengthening

The issue of the cut-off splicing of an additional fiber-reinforced polymer (FRP) bar in the near-surface mounted (NSM) technique for reinforced concrete (RC) beam strengthening exposed dominantly to bending is insufficiently investigated. A possible solution of this issue is a new proposed technique: a double-strap joint. It implies the widening of the groove at the cut-off location and the symmetrical installing of additional supplements of FRP reinforcement. In this research, beam strength has been determined for the following cases: additional NSM FRP reinforcement without a cut-off, with a cut-off, and without overlapping, and with different lengths of splice overlapping. A nonlinear analysis based on the finite element method (FEM) has been applied. The length of the cut-off splice of the additional FRP reinforcement with glass fibers (GFRP) was 20Ø, 40Ø, and 60Ø. The validation of the numerical model and a comparison of the results were conducted by using the authors’ experiments. It has been shown that, in the case of a cut-off of NSM GFRP bars, a significant loss in strengthening efficiency occurs, and that, with an increase in the overlapping length, this loss decreases. An overlapping length of 60Ø provides full strengthening. An efficiency assessment was carried out via the use of a parametric study, varying the FRP bar material type and its diameter for a constant splicing length.

Parametric study and improved capacity model for RC beams strengthened with side NSM CFRP bars

Near Surface Mounted (NSM) fiber reinforced polymer (FRP) technique with rods/strips attached at the bottom face (B-NSM) has become widely used method for retrofitting concrete structures but has several practical limitations such as accessibility problems and premature debonding. A viable alternative by attaching the NSM FRP reinforcement at the side faces (S-NSM) has recently been presented and showed excellent effectiveness in limited research studies. In this study, a robust finite element (FE) model, featuring state-of-the art modelling techniques and nonlinear properties, is developed to study the flexural behaviour of reinforced concrete (RC) beams strengthened with S-NSM FRP bars. After validating its load, strain, and failure mode predictions with 6 full-size beam experiments, the model was used in an extensive parametric study on 108 new models, evaluating the effects of FRP bar diameter (df), strengthening length (SL), groove elevation (hg), tensile steel reinforcement ratio (ρs), and concrete compressive strength (fc’). In general, the beam ultimate load (Pu) increased with SL, df, ρs, and fc’. The beam ductility also seems to be affected by the studied parameters but in general was satisfactory, averaging 2.30 to 2.73. A regression-based formula was presented for the effective FRP bar length, beyond which the brittle concrete peel-off failure is prevented, and Pu becomes constant. A sectional analysis was also performed coupled with regression, resulting in the development of a simple equation to calculate the FRP strain at beam failure and consequently an improved analytical model for predicting Pu.

Characterization and Simulation of the Bond Response of NSM FRP Reinforcement in Concrete.

The near-surface mounted (NSM) technique with fiber reinforced polymer (FRP) reinforcement as strengthening system for concrete structures has been broadly studied during the last years. The efficiency of the NSM FRP-to-concrete joint highly depends on the bond between both materials, which is characterized by a local bond–slip law. This paper studies the effect of the shape of the local bond–slip law and its parameters on the global response of the NSM FRP joint in terms of load capacity, effective bond length, slip, shear stress, and strain distribution along the bonded length, which are essential parameters on the strengthening design. A numerical procedure based on the finite difference method to solve the governing equations of the FRP-to-concrete joint is developed. Pull-out single shear specimens are tested in order to experimentally validate the numerical results. Finally, a parametric study is performed. The effect of the bond–shear strength slip at the bond strength, maximum slip, and friction branch on the parameters previously described is presented and discussed.

A Research on Modification Method for NSM FRP-Concrete Bonded Joints Strength Models

A modification method was proposed for near-surface mounted (NSM) fiber-reinforced polymer (FRP)-concrete bonded joints strength prediction models considering model uncertainty. A database consisting of 246 test records was involved. Three bonded joints strength prediction models for NSM FRP reinforcement system were selected for modification. All the three selected models have model uncertainty factors associated with input design parameters. Spearman correlation analysis was used to prove the systematic correlation of the model uncertainty factors. For each model, a regression functionfwas established to eliminate the systematic nonrandom part of the model uncertainty factor. Then, the model uncertainty factors could be described by random variables obeying logarithmic normal distribution. A reliability analysis using the JC method was carried out to validate the practical significance and value of model modification. This study improves the predictability of FRP NSM reinforcement systems and provides valuable references for model calibration in practical engineering.

End concrete cover separation in RC structures strengthened in flexure with NSM FRP: Analytical design approach

Fiber-reinforced-polymer (FRP) composite materials applied according to the near-surface-mounted (NSM) technique are very effective for the flexural strengthening of reinforced-concrete (RC) structures. However, the flexural strengthening effectiveness of this NSM technique is sometimes compromised by end concrete cover separation (CCS) failure, which is a premature failure before occurring the conventional flexural failure modes. Due to the complexity of this failure mode, no analytical approach, with a design framework for its accurate prediction, was published despite the available experimental results on this premature failure. In the present study, a novel simplified analytical approach is developed based on a closed form solution for an almost accurate prediction of CCS failure in RC structures strengthened in flexure with NSM FRP reinforcement. After demonstrating the good predictive performance of the proposed model, it was used for executing parametric studies in order to evaluate the influence of the material properties and FRP strengthening configuration on the susceptibility of occurring the CCS failure. At the end, regarding to the FRP strengthening configuration, some design recommendations were proposed to maximize the resistance of NSM FRP strengthened structures to the susceptibility of occurring the CCS failure.

Theoretical solution to fatigue bond stress distribution of NSM FRP reinforcement in concrete

Abstract This study presents an innovative theoretical model for bond stress distribution under fatigue loading of reinforced concrete structures strengthened with near-surface mounted (NSM) fiber reinforced polymer (FRP) reinforcement. The model has a unique ability in predicting the bond stress between concrete and NSM FRP reinforcement under fatigue loading by considering the bond heterogeneity due to fatigue damage. A bilinear bond-slip relation, describing the pre- and post-crack bond behavior, is introduced to model the FRP-concrete interface as a cohesive zone. Theoretical solutions on bond stress distribution are derived for regions in front of and behind the crack tip. During the fatigue loading, the propagation of crack length (i.e., debonded length) is represented by a cohesive model that is based on Paris law. One-dimensional mesh is then used to account for different levels of bond degradation along the embedded length and appropriate boundary conditions are applied to obtain the closed-form solutions. To verify the accuracy of the model, test results from an existing experiment are used, which shows satisfactory predictions. The parametric study conducted indicates that high bond strength and axial stiffness can slow down the growth of the debonded length.

End cover separation in RC beams strengthened in flexure with bonded FRP reinforcement: simplified finite element approach

The flexural performance of RC beams can be improved using externally bonded (EB) or near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. One of the commonly observed failure modes of such FRP-strengthened RC beams is the end cover separation failure mode which involves the detachment of the bonded FRP reinforcement together with the cover concrete along the level of the longitudinal steel reinforcement. This paper presents a simplified 2-D (plane stress) nonlinear finite element (FE) approach for end cover separation failure in RC beams strengthened in flexure with either EB or NSM FRP reinforcement. In this simplified FE approach, only the part of the RC beam between the two cracks near the critical end of the FRP reinforcement is considered. This approach leads to the determination of the FRP debonding strain at the cracked section in the strengthened region adjacent to the crack at the end of the FRP reinforcement, which can then be used to determine the moment acting on this cracked section at debonding failure through conventional section analysis. The validity of the proposed simplified FE approach is demonstrated through comparisons with results from the full 2-D (plane stress) nonlinear FE approach and laboratory tests.

Fire behavior of concrete T-beams strengthened with near-surface mounted FRP reinforcement

This paper presents results from experimental and numerical studies on fire behavior of reinforced concrete (RC) T-beams strengthened with near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. Four RC T-beams, strengthened with NSM FRP reinforcement, were tested by subjecting them to ASTM E119 standard fire exposure and service load conditions. The test variables included load level, restraint conditions, and fire protection applied on the beams. Data from fire resistance experiments is utilized to validate predictions from numerical model to trace the fire response of NSM FRP strengthened concrete T-beams. Results from fire tests and numerical studies indicate that NSM FRP strengthened beams can sustain service loads for three hours under standard fire conditions, even without any external fire protection. In addition, fire induced axial restraint enhances the fire response of NSM FRP strengthened RC beams, while higher load levels lead to much larger deflections and lower fire resistance in these beams.

Effect of high temperature on bond strength of near-surface mounted FRP reinforcement

This paper presents results from an experimental study on the effect of temperature on bond strength and modulus of near-surface mounted (NSM) fiber-reinforced polymer (FRP) strengthened concrete. 36 NSM FRP specimens, fabricated using different types of epoxy adhesive and FRP reinforcement, were tested to evaluate bond strength in 20–400°C temperature range. Results from these tests indicate that bond strength and modulus decrease significantly in 20–200°C temperature range, and only retain 20–30% of their original values at 200°C. NSM FRP strips and rods possess negligible bond strength beyond 400°C. Data from tests also indicates that NSM FRP system exhibits similar bond stress–slip response at both room and high temperatures. However, the peak value (bond strength) and the slope (bond modulus) are much lower at high temperatures. Data from the tests is utilized to propose empirical relations for variation of bond strength and modulus of NSM FRP strengthening system with temperature.

An experimental study of different factors affecting the bond of NSM FRP bars in concrete

Near-surface mounted (NSM) fiber reinforced polymer (FRP) reinforcement has proven to be an efficient technique for strengthening concrete members. The bond behavior of the NSM system depends on the bond in the two existing interfaces, while the response in terms of load–slip curves and load capacity may be strongly affected by many other parameters. As a consequence of the variety of factors affecting the bond, different studies into the behavior of NSM FRP reinforcement have been carried out in recent years. In this paper, an experimental study is performed in which the effect of adhesive properties, bar type, bar size, FRP properties, groove geometry and the use of mechanical interlocking on the capacity and bond behavior of a NSM joint is investigated. The failure load, average bond stress, loaded end slip, free end slip, transverse strains (on adhesive and concrete), and the mode of failure for the tested joints are reported and discussed. The results show that the adhesive properties, FRP bar size and bar surface treatment were critical factors in the capacity and mode of failure of the specimens tested.

Evaluating the Fire Response of Concrete Beams Strengthened with Near-Surface-Mounted FRP Reinforcement

This paper presents an approach for evaluating the fire response of reinforced concrete (RC) beams strengthened with near-surface-mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. The model is based on a macroscopic finite-element approach and is capable of tracing the response of NSM FRP-strengthened RC beam from the preloading stage to collapse under a specified fire exposure and loading conditions. The model accounts for high-temperature properties of constitutive materials, realistic load and boundary conditions, and temperature-induced bond degradation at the FRP-concrete interface. The validity of the model is established by comparing predictions from the model with measured data from tests at both ambient and fire conditions. The validated model was applied to carry out numerical studies to compare the fire response of concrete beams with different types of strengthening systems and insulation layouts. Results from numerical studies indicate that a concrete beam strengthened with NSM FRP reinforcement yields slightly lower fire resistance as compared to a conventional RC beam but achieves higher fire resistance than that of externally bonded FRP. It is also shown that appropriate location of a NSM FRP reinforcement and insulation scheme can increase the fire resistance of concrete beams strengthened with NSM FRP reinforcement.

Behavior of Beams Strengthened with Novel Self-Anchored Near-Surface-Mounted CFRP Bars

A commonly observed failure mode in laboratory tests involving surface bonded fiber-reinforced polymer (FRP) laminates or near-surface-mounted (NSM) bars is premature delamination, that is, the separation of the FRP from the substrate well before the FRP reaches its ultimate strain capacity. To delay the onset of delamination and to ensure that the NSM FRP reinforcement continues to contribute to member strength after partial delamination, a new self-anchored carbon fiber-reinforced polymer (CFRP) bar was developed and tested for this investigation. This bar is made with a series of monolithic spikes that can be anchored deep inside the concrete. In addition to cutting grooves into the concrete cover for the placement of the primary reinforcing bar, holes are drilled deep into the concrete to insert the spikes. To test the performance of this bar, six large, simply supported, reinforced, concrete beams were retrofitted with NSM bars and tested in four-point bending. Two beams were strengthened with NSM bars without anchors or spikes but were otherwise similar to the self-anchored bar and served as control specimens (Series B1). Two beams were strengthened in flexure with the new self-anchored NSM bars (Series B2), and the remaining two beams (Series B3) were strengthened in flexure and shear by using the self-anchored NSM bars as partial shear reinforcement. The effect of the proposed strengthening system on the beams’ strength, failure mode, deformability, and ductility are discussed on the basis of the experimental results. The anchors delayed delamination and enabled the NSM bar to experience at least a 77% higher strain at failure than the companion bar without anchors. The anchors also increased beam displacement ductility and energy ductility at a 20% strength degradation by at least 34% and 42%, respectively.

Modeling of debonding failure for RC beams strengthened in shear with NSM FRP reinforcement

The shear capacity of reinforced concrete members can be successfully increased using near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. Tests conducted thus far have shown that failure is often controlled by diagonal tension associated to debonding between the NSM reinforcement and the concrete substrate. In absence of steel stirrups and/or when the spacing of the NSM reinforcement is large, debonding involves separately each of the bars crossed by the critical shear crack. In order for shear strengthening of beams with NSM reinforcement to be safely designed, an analytical model able to encompass the failure mode mentioned above must be developed. This paper presents two possible approaches, a simplified and a more sophisticated one, to predict the FRP contribution to the shear capacity. In the first approach, suitable for immediate design use, an ideally plastic bond–slip behavior of the NSM reinforcement is assumed, which implies a complete redistribution of the bond stresses along the reinforcement at ultimate. The second approach, implemented numerically, accounts for detailed bond–slip modeling of the NSM reinforcement, considering different types of local bond–slip laws calibrated during previous experimental investigations. It also takes advantage of an approach developed by previous researchers to evaluate the interaction between the contributions of steel stirrups and FRP reinforcement to the shear capacity. The paper illustrates the two models and compares their predictions, with the ultimate goal to evaluate whether the first simple model can be used expecting the same safety in predictions of the second model.

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

A recent and promising method for shear strengthening of reinforced concrete (RC) members is the use of near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. In the NSM method, the reinforcement is embedded in grooves cut onto the surface of the member to be strengthened and filled with an appropriate binding agent such as epoxy paste or cement grout. Only a few studies have been conducted to date on the use of NSM FRP reinforcement for shear strengthening of RC beams. These studies identified some critical failure modes related to debonding between the NSM reinforcement and the concrete substrate. However, more tests need to be conducted to identify all possible failure modes of strengthened beams. Moreover, virtually no test results are available on the behavior of shear-strengthened beams containing steel shear reinforcement, and on the effect of variables such as the type of epoxy used as groove filler. This paper illustrates a research program on shear strengthening of RC beams with NSM reinforcement, aimed at gaining more test results to fill the gaps in knowledge mentioned above. A number of beams were tested to analyze the influence on the structural behavior and failure mode of selected test parameters, i.e. type of NSM reinforcement (round bars and strips), spacing and inclination of the NSM reinforcement, and mechanical properties of the groove-filling epoxy. One beam strengthened in shear with externally bonded FRP laminates was also tested for comparison purposes. All beams had a limited amount of internal steel shear reinforcement to simulate a real strengthening situation. Test results are presented and discussed in the paper.

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

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.