Near Surface Mounted Fiber Reinforced Polymer Research Articles

Bond behaviour between NSM CFRP laminate and concrete using modified cement-based adhesive

Near-surface mounted (NSM) techniques have been successfully utilized for the strengthening of concrete structures with CFRP composites. Although the adhesive most commonly used for strengthening of structures is epoxy, this adhesive is suitable only for low temperature environments due to the deterioration of its mechanical properties at elevated temperatures, in addition to its other disadvantages such as flammability and emission of toxic fumes. Substituting the epoxy with cementitious adhesive is therefore of great interest. However, few investigations on the strengthening of concrete structures using cementitious bonding materials have been reported, particularly for NSM CFRP strengthening systems. Since the bond between NSM FRP reinforcement and the concrete substrate plays a major role in the efficiency of NSM strengthening systems, in this paper, the bond properties of NSM CFRP laminate using modified cement-based adhesive are investigated. 15 concrete specimens were studied, and parameters including bond length, strain distribution, and bond-slip curve were considered. Finite element simulation was conducted to verify the experimental results using ATENA Software from Cervenka. The results showed excellent bond properties with reasonable correlation between experimental and finite element analysis results.

Using data mining algorithms to predict the bond strength of NSM FRP systems in concrete

This paper presents the effectiveness of soft computing algorithms in analyzing the bond behavior of fiber reinforced polymer (FRP) systems inserted in the cover of concrete elements, commonly known as the near-surface mounted (NSM) technique. It focuses on the use of Data Mining (DM) algorithms as an alternative to the existing guidelines’ models to predict the bond strength of NSM FRP systems. To ease and spread the use of DM algorithms, a web-based tool is presented. This tool was developed to allow an easy use of the DM prediction models presented in this work, where the user simply provides the values of the input variables, the same as those used by the guidelines, in order to get the predictions. The results presented herein show that the DM based models are robust and more accurate than the guidelines’ models and can be considered as a relevant alternative to those analytical methods.

Experimental investigation into the use of NSM FRP to increase the torsional resistance of RC beams using epoxy resins and cement-based adhesives

Carbon fibre-reinforced polymers (CFRPs) have been used to increase the strength of reinforced concrete (RC) members using both externally bonded reinforcement (EBR) and near surface mounted (NSM) techniques with a great deal of success. However, currently no studies have been conducted on torsional strengthening using NSM methods as current research on torsional strengthening has been conducted using EBR techniques only. It is well documented that flexure and shear strength of RC beams has been improved using NSM strengthening technology applied with epoxy adhesives. However, due to epoxy adhesive problems associated with toxic fumes and poor performance in high-temperature environments, a new cement-based adhesive has been developed and used as an alternative to epoxy in several studies. This paper reports on experimental results of ten RC beams tested in torsion using a specialized torsional rig system. Two control beams were considered and eight beams strengthened with CFRP laminates applied to all four faces of the beam and embedded into pre-cut grooves using the NSM technique were prepared. An epoxy adhesive was used in four beams and a new cement-based adhesive was used as a substitute for epoxy in the remaining four beams. For each type of adhesive, two different spacings between the grooves were examined. The performances and the ultimate strengths of the beams are presented and the results expressed in terms of the maximum torsional capacity with respect to the peak angle of twist. According to the results, the torsional behaviour of the beams was improved in the strengthened beams to varying degrees when using epoxy resin as the bonding agent. On the other hand, the NSM technique with the cement-based adhesive was also an effective method in increasing the torsional strength.

Fatigue performance of NSM CFRP strips embedded in concrete using epoxy adhesive

Strengthening of reinforced concrete (RC) structures using the near-surface mounted (NSM) fibre reinforced polymer (FRP) technique is becoming an attractive system for upgrading these structures. The efficiency of this method depends on the bond between the concrete and the NSM FRP. Although different experimental and analytical studies on the bond of NSM FRP to concrete substrate under monotonic loading have been carried out, very few studies have been conducted under fatigue loading conditions. This paper presents the results of an experimental study of NSM carbon FRP bonded to concrete with epoxy adhesive under fatigue loading using single-lap shear tests. The test variables were: CFRP strip dimensions (1.4×10mm, 1.4×20mm), and CFRP strip surface conditions (smooth, rough). 52 specimens were tested under different fatigue load ranges to develop the load range (LR)-fatigue life (N) relationships. Finally, equations were developed to predict fatigue lives based on the experimental results. It was found that the rough surface CFRP strips 1.4×20mm in dimensions showed the best fatigue life under different fatigue load levels. Moreover, the proposed equations can predict fatigue life reasonably well and can be used in the design of NSM CFRP-strengthened RC members.

Damaged RC beams strengthened with NSM CFRP rectangular rods under vibration in different constrain conditions

The strengthening of damaged reinforced concrete (RC) beams through near surface mounted (NSM) fiber reinforced polymers (FRPs) has been shown to be a suitable method in practice. Availability of this technique is based on maintaining the adhesion between FRP rods and the concrete surface of grooves, both undamaged and damaged by cracking of beams. Controlling the status of RC beams strengthened with NSM FRP rods may be carried out through non destructive experimental tests measuring dynamic parameters such as frequency values.This paper analyses experimental vibration of four RC beam models without strengthening and strengthened with NSM carbon-FRP rectangular rods after damage due to bending cracking of concrete by static tests. The investigation of vibration was carried out considering RC beams at different static loading damages in different constrain conditions: two beam models were studied assuming free-free end condition and the other two with hinge-hinge end condition. Envelope of Frequency Response Functions (FRFs) obtained through dynamic testing are shown in the paper and changes of frequency values are compared and discussed.Finally, a comparison between experimental and theoretical dynamic values obtained by finite element analysis is developed to verify the control of RC beams strengthened with NSM CFRP rods.

Preliminary Study to Enhance Ductility of CFRP-Strengthened RC Beam

AbstractFiber-reinforced polymer (FRP) strengthening systems are widely accepted in engineering practice as means to enhance the flexural strength of RC members, but resulting in decreased ductility. Brittle failure, i.e., FRP rupture, FRP delamination, or concrete cover separation, impairs the reliability of FRP strengthening system. A near surface–mounted (NSM) FRP strengthening system has similar drawbacks. As a possible solution, a design philosophy is proposed to enhance the ductility of the NSM-FRP system by triggering a global slip at the FRP–adhesive interface. The corresponding design method is developed to induce the ductile failure and predict the ultimate strength of NSM FRP–strengthened RC member in flexure. A series of experimental tests were conducted to verify the effectiveness of the proposed method. The designed RC beams strengthened with NSM–carbon FRP (CFRP) outperformed the externally bonded CFRP systems both in ductility and flexural strength. Additionally, the proposed design method...

EXPERIMENTAL STUDY OF THE INFLUENCE OF ADHESIVE PROPERTIES AND BOND LENGTH ON THE BOND BEHAVIOUR OF NSM FRP BARS IN CONCRETE

The near-surface mounted (NSM) fibre reinforced polymer (FRP) technique is a relatively recent system for strengthening concrete structures. Bond is a key factor in its behaviour, and is affected by many factors whose influence can only be tested through experimental studies. In this study, the modified pull-out test was used to study the effect of epoxy properties and bond length on the behaviour of NSM FRP bars. Three epoxy types, two FRP materials (carbon and glass) and four bond lengths (6db, 12db, 24db and 30db) are used. The load capacity, slips at the loaded end and free end and average bond stress are all analysed. The test results indicate that the role of epoxy properties appear to be a key factor in bond performance in the NSM FRP strengthening technique, and that their effect varies depending on bond length and FRP properties.

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.

Flexural behavior of RC beams strengthened by NSM GFRP Bars having different end conditions

Near surface mounted (NSM) fiber reinforced polymer (FRP) bars became more effective in strengthening reinforced concrete (RC) beams. This is because it increases the bond capacity and makes a protection against external damage. Most of previous related researches stated that the failure of the tested RC strengthened beams with NSM FRP is due to debonding or concrete cover separation. In this research the ends of the NSM glass fiber reinforced polymer (GFRP) bars were bent to delay or prevent NSM FRP debonding and concrete cover separation and thus increasing the load carrying capacity of the strengthened beams. The inclination angles of GFRP bars with bent ends were 90° and 45°. Straight GFRP bars with variable lengths were also used for comparison. The test results demonstrated that the GFRP bars with bent ends prevented the concrete cover separation and increased the load carrying capacity of the strengthened beams. The load carrying capacity of the strengthened beams by straight NSM bars and those having 45° and 90° inclined ends were 177%, 201%, and 185% of that of their control beam, respectively.

Flexural Strength of RC Beam Strengthened by Partially De-bonded Near Surface-Mounted FRP Strip

This paper presents an experimental work to study the flexural strength of reinforced concrete (RC) beams strengthened by partially de-bonded near surface-mounted (NSM) fiber reinforced polymer (FRP) strip with various de-bonded length. Especially, considering high anchorage capacity at end of a FRP strip, the effect of de-bonded region at a central part was investigated. In order to check the improvement of strength or deformation capacity when the bonded surface area only increased without changing the FRP area, single and triple lines of FRP were planned. In addition, the flexural strength of the RC member strengthened by a partially de-bonded NSM FRP strip was evaluated by using the existing researchers’ strength equation to predict the flexural strength after retrofit. From the study, it was found that where de-bonded region exists in the central part of a flexural member, the deformation capacity of the member is expected to be improved, because FRP strain is not to be concentrated on the center but to be extended uniformly in the de-bonded region. Where NSM FRP strips are distributed in triple lines, a relatively high strength can be exerted due to the increase of bond strength in the anchorage.

Flexural performance of RC beams strengthened with near surface mounted CFRP strips

The near surface mounted (NSM) fiber reinforced polymer (FRP) reinforcement is emerging as a promising alternative strengthening technique to externally bonded reinforcement (EBR) for increasing the load carrying capacity of reinforced concrete (RC) members. NSM FRP technique has several advantages, in comparison with the EBR method, such as reducing the risk of debonding, and a better protection from the external sources of damage. In this research, the performance and effectiveness of the NSM and EBR techniques for the flexural strengthening of RC beams are compared. In order to achieve this objective, six full-scale, RC beams were strengthened with different carbon FRP (CFRP) schemes and tested. Such beams were designed to fail in a flexural mode. Test results indicated that if the same amount of CFRP is used, beams strengthened with NSM strips achieved higher ultimate load than those strengthened with EBR. Such increase in the ultimate load ratio ranged between 12% and 18%. Furthermore, a design approach for computing the moment capacity of RC flexural members strengthened with NSM CFRP strips is developed and presented in this paper.

Flexural performance of reinforced concrete beams strengthened with prestressed near-surface-mounted FRP reinforcements

Abstract A numerical method for estimating the curvature, deflection and moment capacity of reinforced concrete beams strengthened with prestressed near-surface-mounted (NSM) FRP bars/strips is presented. A sectional analysis is carried out to predict the moment–curvature relationship from which beam deflections and moment capacity are then calculated. Based on the amount of FRP bars, different failure modes were identified, namely tensile rupture of prestressed FRP bars and concrete crushing before or after yielding of steel reinforcement. Comparisons between experimental results available in the literature and predicted curvature, moment capacity and deflection of reinforced concrete beams with prestressed NSM FRP reinforcements show good agreement. A parametric study concluded that higher prestressing levels improved the cracking and yielding loads, but decreased the beam ductility compared with beams strengthened with nonprestressed NSM FRP bars/strips.

Fiber-Element Modeling for Seismic Performance of Square RC Bridge Columns Retrofitted with NSM BFRP Bars and/or BFRP Sheet Confinement

This paper presents an analysis of the seismic performance of square RC bridge columns retrofitted with near-surface-mounted (NSM) basalt fiber-reinforced polymer (BFRP) bars and/or BFRP sheet confinement based on fiber element modeling. An axial stress versus loaded-end displacement model of NSM FRP bars, including both elastic elongation and bond-slip effects, is proposed to account for the significant slippage of FRP bars in the plastic hinge region of strengthened columns. The simplified FRP bar model is then converted into the equivalent stress-strain relationship for easy implementation in fiber-based analysis. Moment-curvature analysis is performed based on the proposed FRP bar model, and the analytical moment versus fixed-end rotation relationship of a strengthened column is calculated and assigned to the rotational spring in the fiber element model. The proposed method avoids the nested iterations in sectional analysis that require force equilibrium and deformation compatibility. Comparisons between the numerical simulations and experimental results indicate that the proposed method is appropriate for predicting pushover curves and the hysteretic response of strengthened columns. Furthermore, the mechanism of the hybrid effect of combining NSM FRP bars with externally bonded FRP sheets can be interpreted after the analytical study; that is, confining the column with FRP sheets improves the bond conditions for NSM FRP bars, while the bond-slip effect mitigates the premature fracturing of the FRP bars, leading to more effective utilization of the NSM reinforcement and a more ductile column behavior.

콘크리트에 표면매입 보강된 FRP판의 전단키 및 연단거리 효과

This paper presents a bond test to find the effect of shear key and edge length from the bonded FRP in near surface-mounted(NSM) retrofit using FRP plate. Main parameters in the test are the location and size of shear key and the edge length. For the test, 10 specimens were made by embedding FRP plate of into concrete block and fixing it by using epoxy. Tensile load was applied to the FRP of the specimens until failure and was recorded at each load increase. In addition, the bond slip and elongation of FRP were measured during the test. From the test, it was found that the further the shear key located from the loading, the higher strength we could get. The bond strength inversely depended on the size of shear key. Especially, when the size of shear key was to be lagger than certain size, the bond strength decreased to very low value; even less than that of the case without shear key. The bond strength somewhat increased corresponding to the increase of edge length from the bonded end of FRP to loading in spite of same bond length. The bond-slip between FRP and concrete governed overall deformation in the bond test of NSM FRP so that the effect of excessive slip is necessary to be considered in the design.

Analytical solution for interaction forces in beams strengthened with near-surface mounted round bars

The use of near-surface mounted (NSM) FRP composites for strengthening RC beams in flexure has become increasingly popular in the last decade. Compared to the externally bonded (EB) FRP strengthening method, the bond between FRP and concrete is much stronger when the NSM FRP strengthening method is used. However, debonding is still a likely failure mode in RC members strengthened with NSM FRP bars. The debonding in an NSM FRP-strengthened beam may initiate from either of the two ends of an NSM bar (i.e. end debonding) in the form of interfacial debonding or concrete cover separation, both of which are closely related to the existence of large localized interaction forces between the NSM bar and concrete near the bar ends in such beams. This paper presents an analytical solution for the interaction forces in RC beams strengthened with NSM FRP round bars, which are one of the most popular types of FRP bars used for NSM strengthening. The key elements of the proposed analytical solution are two interfacial stiffness parameters (i.e. tangential interfacial stiffness and normal interfacial stiffness). The validity of the proposed analytical solution is also verified using a sophisticated 3D FE model of a RC beam strengthened with a NSM round bar.

Strength model for end cover separation failure in RC beams strengthened with near-surface mounted (NSM) FRP strips

As an alternative to externally bonded FRP reinforcement, near-surface mounted (NSM) FRP reinforcement can be used to effectively improve the flexural performance of RC beams. In such FRP-strengthened RC beams, end cover separation failure is one of the common failure modes. This failure mode involves the detachment of the NSM FRP reinforcement together with the concrete cover along the level of the tension steel reinforcement. This paper presents a new strength model for end cover separation failure in RC beams strengthened in flexure with NSM FRP strips (i.e. rectangular FRP bars with a sectional height-to-thickness ratio not less than 5), which was formulated on the basis of extensive numerical results from a parametric study undertaken using an efficient finite element approach. The proposed strength model consists of an approximate equation for the debonding strain of the FRP reinforcement at the critical cracked section and a conventional section analysis to relate this debonding strain to the moment acting on the same section (i.e. the debonding strain). Once the debonding strain is known, the load level at end cover separation of an FRP-strengthened RC beam can be easily determined for a given load distribution. Predictions from the proposed strength model are compared with those of two existing strength models of the same type and available test results, which shows that the proposed strength model is in close agreement with test results and is far more accurate than the existing strength models.

Investigation of Near Surface–Mounted Method for Shear Rehabilitation of Reinforced Concrete Beams Using Fiber Reinforced–Polymer Composites

This paper presents the results of an investigation of reinforced concrete (RC) T-beams retrofitted in shear with near-surface mounted (NSM) fiber-reinforced polymer (FRP) rods. Six full-scale 4,520-mm-long RC T-beams were tested to study the effects of important parameters such as the presence of NSM FRP rods, the presence of steel stirrups, and the steel stirrup ratio. This paper provides an insightful and comprehensive description of the behavior of strengthened T-beams under increasing load, from the formation of the first crack to ultimate failure. The results of this study and those gathered in the presented database show that existing steel stirrups and strengthening NSM FRP did not diminish each other’s effect when failure modes unrelated to shear resistance of RC beams were prevented. The experimental results of this study and those in the database were used to verify a newly proposed model to predict the shear contribution of NSM FRP rods and laminates in RC beams strengthened in shear. The proposed model showed an improved accuracy compared with the existing models in the literature.

Experimental investigation of the bond behavior of the interface between near-surface-mounted CFRP strips and concrete

The technique of strengthening reinforced concrete structures with near-surface mounted (NSM) fiber reinforced polymers (FRPs) has been widely used in structural retrofit, and the load-carrying behavior of the strengthened structures is greatly influenced by the bond between the NSM FRP and concrete. In the present study, a total of 28 specimens, which consisted of concrete prism and NSM CFRP strip, were manufactured and the pull-out test was conducted on them. The failure mode, bond capacity, and CFRP strain distribution of the specimens were investigated. Meanwhile, the influences of concrete strength, bond length, and the distance between the groove and concrete edge on the bond behavior of the NSM FRP–concrete interface were analyzed. The test results revealed that the bond performance of the interface between the NSM CFRP and concrete is improved with the increase in concrete strength. Moreover, increasing the bond length of CFRP strips increases the bond capacity, and as such, the failure mode of specimens transforms from interface debonding into the rupture of CFRP. There exists residual friction stress at the interface between the CFRP and concrete after the interface debonding, which is independent of the bond length. An insufficient distance between the groove and the specimen edge reduces the bond capacity and could result in a splitting failure of the concrete corner. When the edge distance exceeds 60mm, the bond behavior of the specimens tends to be stable.

Dynamic response of RC beams strengthened with near surface mounted Carbon-FRP rods subjected to damage

Near surface mounted (NSM) technique with fiber reinforced polymer (FRP) is becoming a common method in the strengthening of concrete beams. The availability of NSM FRP technique depends on many factors linked to materials and geometry - dimensions of the rods used, type of FRP material employed, rods’ surface configuration, groove size - and to adhesion between concrete and FRP rods. In this paper detection of damage is investigated measuring the natural frequency values of beam in the case of free-free ends. Damage was due both to reduction of adhesion between concrete and carbon-FRP rectangular and circular rods and cracking of concrete under static bending tests on beams. Comparison between experimental and theoretical frequency values evaluating frequency changes due to damage permits to monitor actual behaviour of RC beams strengthened by NSM CFRP rods.

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.