Fiber Reinforced Polymer Strips Research Articles

Study on Flexural Performance of Damage RC Beams Strengthened with near Surface Mounted FRP Strips

The contrast tests of 1 reinforced concrete(RC) beams and 4 RC beams strengthened with near-surface mounted carbon fiber reinforced polymer (FRP) strips have been carried out, under conditions of different damage degree of RC beams. It is mainly conducted to investigate the effects of FRP strips reinforcement of flexural strength. The experimental results indicate that significant strength on the ultimate load and rigidity of reinforced concrete beams can be realized by mounting FRP strips to the beams, and propagation of cracks were reduced obviously.

The effectiveness of FRP strips in repairing moderately and severely damaged RC beam–column connections

An effective means to increase the shear resistance of shear deficient reinforced-concrete (RC) beam–column connections is by bonding fibre-reinforced polymer (FRP) composites. The majority of research to date has focused on the strengthening of two-dimensional reinforced-concrete connections with fibre-reinforced polymer; that is, the strengthening of virgin (undamaged) connections. This paper reports the results of tests on the effectiveness of fibre-reinforced polymer strips in repairing two-dimensional connections with different degrees of damage. The results of tests assessing the effectiveness of fibre-reinforced polymer strips in strengthening virgin connections are also reported. All test specimens are extensively instrumented and their behaviour and failure comprehensively documented. In the case of the repaired specimens, instrumentation is concentrated on the fibre-reinforced polymer around the damage region, thus enabling the strength contribution and behaviour of the fibre-reinforced polymer repair to be closely monitored. The results of this experimental programme enable a better understanding of the strengthening and repair effect of fibre-reinforced polymer strips in reinforced-concrete beam–column connections, and the results will also facilitate the future development and calibration of analytical and numerical models.

Strengthening of concrete slab-column connections using CFRP strips

This paper presents experimental data and results on the effect of externally bonded carbon fiber-reinforced polymer (CFRP) strips on the punching shear of interior slab-column connections. A total of six square slabs with a concentric column were constructed with overall dimensions of 1220 mm by 1220 mm and 100 mm thick slab and 150 × 150 mm column. The test variables were the configuration and amount of CFRP strips externally bonded to the tension face of the slab. The specimens were simply supported along their edges and tested in punching with vertical load applied through the central column. The test results clearly showed that CFRP strengthening leads to significant improvements in the structural behaviour of slab-column connections. The increase in punching capacity of strengthened slabs was up to 29%, while the increase in stiffness was up to 80% compared to the unstrengthened slab. Punching capacities of the test specimens were evaluated using a recently developed model that accounts for both configuration and amount of CFRP strips. The predicted punching capacities using the analytical model were compared with the experimental results, and good agreement was found.

Fatigue Retrofitting of Welded Steel Cover Plates using Pre-Stressed Carbon Fibre Reinforced Polymer Strips

Past research on the use of carbon fibre reinforced polymer (CFRP) strips for the fatigue retrofitting of steel structures has shown that these materials have considerable potential for this application. Several investigations have found that this approach can be significantly improved by first pre-stressing the strips. To further explore this possibility, a study was recently undertaken with the objectives of: (a) fatigue testing steel beams with welded cover plates strengthened using pre-stressed CFRP strips and (b) employing analytical models to predict the resulting fatigue life increase. This paper summarises the main findings of this study. Specifically, it is shown that a significant fatigue life increase can be achieved with the application of pre-stressed CFRP strips to welded details, such as cover plates. This increase will be modest, however, if the introduced stress is less than the residual stress already present as a result of the welding process. The test results are predicted by a fracture mechanics model, wherein the stresses in the weld are determined by finite element analysis.

Effect of curing conditions on strength development in an epoxy resin for structural strengthening

Civil structures such as bridges and buildings can be strengthened with prestressed fibre reinforced polymer (FRP) strips to enhance both their stiffness and load-bearing capacity. End anchorage is a crucial issue for prestressed FRP strips. An innovative anchorage procedure, called the “gradient anchorage method” and based on the possible accelerated curing of the epoxy-resin in the end region of the FRP strip, has recently been conceived with the aim of avoiding more invasive mechanical fastening systems. An in-depth knowledge of the actual development of the key mechanical properties of resins under different curing conditions (i.e., in terms of curing temperature) is of paramount importance for employing the above mentioned gradient method in practical applications. This paper presents experimental results and analytical investigations aimed at developing a better understanding of the strength development of a commercial adhesive under different curing times and temperatures. Firstly, direct tensile tests on epoxy specimens were performed at different curing temperatures. It was shown that the necessary curing time to reach the maximum tensile strength can be significantly reduced from several hours at room temperature to approximately 30min at 90°C. Furthermore, higher curing temperatures reduced the activation time after which strength starts to increase. The experimental observations are shown graphically with both the activation time and reaction duration at different curing temperatures. Secondly, pull-off bond tests were conducted on 100mm wide and 1.2mm thick FRP strips bonded to concrete using epoxy adhesives cured either at 90°C for different durations or at room temperature. An optical image correlation system (ICS) allowed the load transfer behaviour of the inhomogeneous cured adhesive between the FRP strip(s) and concrete to be studied. Finally, using the experimental measurements, the bond shear stress–slip interface relationships for the different test specimens were identified in order to present the effect of elevated curing temperatures and curing durations.

The Influence of Concrete Surface Preparation when Fiber Reinforced Polymers with Different Anchoring Devices are Being Applied for Strengthening R/C Structural Members

The aim of this study is to investigate the influence of concrete surface preparation when either steel or carbon fiber reinforced polymers (FRP) are applied for strengthening applications of R/C members. The present study also investigates the anchorage or not of the applied FRP strips on the volume of concrete. For this purpose special concrete specimens were fabricated and were used to attach CFRP or SRP strips with or without anchoring and with or without contact surface treatment. The experimental results indicate that the concrete surface preparation is important and results in an increase of the load bearing capacity when the FRP strip is not anchored. When an anchoring device is employed, the concrete surface preparation is of no significance. With a properly designed anchoring device, a significant increase in the bearing capacity was observed and the failure was that of the fracture of the FRP strips for all such specimens. The highest FRP material exploitation was achieved in the specimen that utilises the patented anchoring device together with two layers of SRP strips. Debonding of the FRP strips, or failure of the anchoring device, results as was to be expected, in relatively unsatisfactory FRP material exploitation.

Experimental analysis of historic masonry walls reinforced by CFRP under in-plane cyclic loading

External bonding of Fiber Reinforced Polymers (FRPs) has become a popular technique for strengthening historic masonry wall buildings in seismic area. Although FRPs seem to improve response of historic masonry walls under in-plane shear loading, further investigations must specify a number of aspects for both researchers and practitioners. This paper presents an experimental analysis pertaining to the response of unreinforced and reinforced historic masonry walls built using full clay bricks in scale 1/3rd. On the basis of previous experimental research carried out by shear tests on triplets and unreinforced walls, a shear criterion for historic unreinforced masonry (HURM) has been assumed. In this experimental analysis two HURM walls, characterized by double T shape, were subjected to in-plane cyclic loading to shear cracking. The damaged walls were reinforced using horizontal–vertical and diagonal Carbon FRP strips. An anchorage system was also put into place to improve the adhesion of the strips used; the historic reinforced masonry (HRM) walls were tested by cyclic loading until failure. The experimental results are illustrated and discussed taking into account the delamination failure of the CFRP strips. Finally, the increase obtained in walls’ shear capacity is analysed considering theoretical bilinear diagrams and the coefficient of ductility.

Seismic retrofit of plain concrete piers of railroad bridges using composite of FRP-steel plates

This study is planed to solve the overturning problem and manifestation of tensile cracking of plain concrete piers of railroad bridges. For the overturning problem, earth anchors are used to fix the bottom of a pier to a rock-foundation using prestressing cables. Composite of Fiber Reinforced Polymer (FRP) and steel plates (FSP) are attached longitudinally on the surface of the pier to prevent cracking. Then, FRP band strips are wrapped onto the composite of FRP and steel plate to provide lateral confinement. Push-over tests in field show that the earth anchors are effective in preventing the overturning of the pier, and that the FSPs and the FRP strips prevent the cracking of concrete and increase the strength in bending.

Shear Strengthening of RC Beams with Externally Bonded FRP Composites: Effect of Strip-Width-to-Strip-Spacing Ratio

The results of an experimental and analytical investigation of shear strengthening of reinforced concrete (RC) beams with externally bonded (EB) fiber-reinforced polymer (FRP) strips and sheets are presented, with emphasis on the effect of the strip-width-to-strip-spacing ratio on the contribution of FRP (Vf). In all, 14 tests were performed on 4,520-mm-long T-beams. RC beams strengthened in shear using carbon FRP (CFRP) strips with different width-to-spacing ratios were considered, and their performance was investigated. In addition, these results are compared with those obtained for RC beams strengthened with various numbers of layers of continuous CFRP sheet. Moreover, various existing equations that express the effect of FRP strip width and concrete-member width and that have been proposed based on single or double FRP-to-concrete direct pullout tests are checked for RC beams strengthened in shear with CFRP strips. The objectives of this study are to investigate the following: (1) the effectiveness of EB discontinuous FRP sheets (FRP strips) compared with that of EB continuous FRP sheets; (2) the optimum strip-width-to-strip-spacing ratio for FRP (i.e., the optimum FRP rigidity); (3) the effect of FRP strip location with respect to internal transverse-steel location; (4) the effect of FRP strip width; and (5) the effect of internal transverse-steel reinforcement on the CFRP shear contribution.

Cyclic testing of steel I-beams reinforced with GFRP

Flange and web local buckling in beam plastic hinge regions of steel moment frames can prevent beam-column connections from achieving adequate plastic rotations under earthquake-induced forces. This threat is especially valid for existing steel moment frame buildings with beams that lack adequate flange/web slenderness ratios. As the use of fiber reinforced polymers (FRP) have increased in strengthening and repair of steel members in recent years, using FRPs in stabilizing local instabilities have also attracted attention. Previous computational studies have shown that longitudinally oriented glass FRP (GFRP) strips may serve to moderately brace beam flanges against the occurrence of local buckling during plastic hinging. An experimental study was conducted at Izmir Institute of Technology investigating the effects of GFRP reinforcement on local buckling behavior of existing steel I-beams with flange slenderness ratios (FSR) exceeding the slenderness limits set forth in current seismic design specifications and modified by a bottom flange triangular welded haunch. Four European HE400AA steel beams with a depth/width ratio of 1.26 and FSR of 11.4 were cyclically loaded up to 4% rotation in a cantilever beam test set-up. Both bare beams and beams with GFRP sheets were tested in order to investigate the contribution of GFRP sheets in mitigating local flange buckling. Different configurations of GFRP sheets were considered. The tests have shown that GFRP reinforcement can moderately mitigate inelastic flange local buckling.

Modeling of flexural behavior of RC beams strengthened with mechanically fastened FRP strips

An alternative to fiber reinforced polymer (FRP) materials adhesively bonded to the concrete substrate is the implementation of mechanically fastened FRP (MF-FRP) systems using steel anchors to secure the laminate to the substrate. The benefit of MF-FRP, compared to adhesive bonding for FRP flexural strengthening, is the speed of installation with unskilled labor, minimal or absent surface preparation under any meteorological condition and immediate use of the strengthened structures. Some of the potential shortcomings are: possible concrete damage during anchoring and limited opportunity of installation in the presence of congested internal reinforcement in the members to be strengthened. Laboratory testing and a number of field applications have shown the effectiveness of the MF-FRP method. In this paper, an analytical model is discussed for reinforced concrete (RC) members strengthened with MF-FRP strips. The model accounts for equilibrium, compatibility and constitutive relationships of the constituent materials; in particular, it accounts explicitly for the slip between the substrate surface and the FRP strip due to the behavior of the fasteners. The proposed flexural model, coupled with the computation algorithm, is able to predict the fundamentals of the behavior of RC flexural members strengthened with MF-FRP strips, in terms of both ultimate and serviceability limit states. Comparisons between the analytical predictions and the experimental results have been successfully performed.

Application of improved hybrid bonded FRP technique to FRP debonding prevention

External bonding of fiber reinforced polymer (FRP) is one of the most efficient techniques for retrofitting and strengthening concrete structures. However, the bond interface is the weakest link in such retrofitted or strengthened concrete members, resulting in premature FRP debonding. Therefore, a newly improved hybrid bonded FRP (IHB-FRP) technique is developed in this paper for strengthening concrete members. Unlike the conventional mechanical fastening method, the mechanical fasteners in the technique do not penetrate the FRP strips and the capping plates serve to apply vertical pressure to the FRP strips. An experimental study on the flexural strength of IHB-FRP technique strengthened beams is conducted. It is found that all the beams strengthened with the IHB-FRP technique fail due to tensile rupture of FRP strips even for seven plies of FRP strips. Based on the experimental results, the effects of the steel reinforcement ratio, the number of FRP plies, and the fastener spacing on the ultimate load are evaluated. Finally, a simplified method is proposed for estimating the ultimate bending moment of the IHB-FRP strengthened beam. The validity of the method is verified with the experimental results.

Hybrid Externally Bonded/Mechanically Fastened Fiber-Reinforced Polymer for RC Beam Strengthening

Externally bonded (EB) fiber-reinforced polymer (FRP) systems and mechanically fastened (MF) FRP strips can both be used to strengthen reinforced concrete (RC) structures, but each has some drawbacks. This paper proposes a new technique for the flexural strengthening of RC beams that combines the EB and MF FRP systems. The technique uses nylon anchors inserted inside the concrete prior to installing the fasteners. These anchors provide a better grasp of the fasteners with the concrete. The hybrid EB/MF-FRP-strengthened specimens showed higher load capacity and post-cracking stiffness than those of the corresponding MF-FRP counterparts. Extending the FRP strips for the entire beam span is necessary for achieving the noticeable enhancement of the load capacity and stiffness with this hybrid EB/MF-FRP system. The failure of the strengthened beams in the system is associated with diagonal cracks at the strip end locations due to fastener rotation and bearing damage.

Behavior and Characteristics of Innovative Composite Plate Shear Walls

Fiber reinforced polymer (FRP) bonding has been investigated as an innovative method for enhancing behavior of thin steel plates subjected to in-plane shear. The steel plate undergoes post-buckling and inclined tension field forms in the plate at a certain angle. Carbon fibers can be oriented at the angle of tension field in the plate and significant results can be obtained by strategic fiber orientation and clever placement of FRP strips. A finite element analysis research program was conducted and a steel plate shear wall model was designed. A global design rule has been taken into consideration in every model which dictates that complete yielding of the infill steel plate must precede any failure or hinging in the boundary frame and that hinging in the column must be the last step. Modeling and analysis of the steel shear wall was carefully validated with some of the most renowned laboratory tests available in the literature. Four-node shell elements with material and geometric nonlinearity were used and damage initiation and propagation in the FRP layer is considered in the model. The optimum angle of fibers is found to be in the direction of tension field in the plate and increases of over 20% have been observed in strength and enclosed area under the load-displacement curve and 10% in stiffness of the shear wall. The FRP layer is found to participate in load carrying mainly after the steel plate has completely yielded and fiber damage and stress distribution are observed along the diagonal and corners of the plate.

Theoretical Model and Computational Procedure to Evaluate the NSM FRP Strips Shear Strength Contribution to a RC Beam

This paper presents a computational procedure to evaluate the shear strength contribution provided to a reinforced concrete (RC) beam by a system of near-surface mounted (NSM) fiber reinforced polymer (FRP) strips. This procedure is based on the evaluation of (1) the constitutive law of the average-available bond-length NSM FRP strip effectively crossing the shear crack, and (2) the maximum effective capacity it can attain during the loading process of the strengthened beam. Because of complex phenomena such as (1) interaction between forces transferred through bond to the surrounding concrete and the concrete fracture, and (2) interaction among adjacent strips, the NSM FRP strip constitutive law is largely different than the linear elastic one characterizing the FRP behavior in tension. Once the constitutive law of the average-available bond-length NSM strip is reliably known, its maximum effective capacity can be determined by imposing a coherent kinematic mechanism. The self-contained and ready-to-impl...

Recent Developments in Long-Term Performance of FRP Composites and FRP-Concrete Interface

This paper presents recent developments in long-term behavior of fiber reinforced polymer (FRP) composites and FRP-strengthened concrete beams with special emphasis on the FRP-concrete interface subjected to creep and fatigue loads. Although short-term behavior of FRP-strengthened structures is intensively reported thus far, their long-term performance has not been thoroughly elucidated yet. This state-of-the-art paper provides a synthesis of recent findings on the long-term performance of FRP composites and their time-dependent bond behavior for strengthening concrete structures, including externally-bonded FRP sheets and near-surface-mounted FRP strips. The review examines the bond-stress slip response, damage accumulation associated with progressive crack propagation, and failure modes, including descriptions of the predictive models. The identified research needs to further advance FRP-strengthening technologies are addressed.

Moving-Wheel Fatigue for Bridge Decks Strengthened with CFRP Strips Subject to Negative Bending

This paper presents the negative bending of reinforced concrete slabs strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. Six slab specimens, three of which are strengthened with CFRP strips, are tested in static and fatigue loads. A wheel-running fatigue test machine is used to simulate vehicular loads on a bridge deck. The effectiveness of CFRP strengthening for bridge decks in cantilever and pseudonegative bending is examined based on moment-carrying capacity and cyclic behavior under the wheel-running fatigue loads, including crack patterns and damage accumulation. The moment-carrying capacity (static) of the cantilever slab strengthened with the NSM CFRP strips is improved by 68.4% when compared to that of an unstrengthened slab. The damage accumulation rate of the strengthened cantilever slab owing to the fatigue load is significantly lower than that of the unstrengthened slab. The damage accumulation of the strengthened slab gradually increases and is irrever...

In-Plane Shear Behavior of Masonry Panels Strengthened with NSM CFRP Strips. I: Experimental Investigation

An experimental investigation was conducted to study the in-plane shear behavior of masonry panels strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer strips (CFRP). As part of the study four unreinforced masonry panels and seven strengthened panels were tested in diagonal tension/shear. Different reinforcement orientations were used including vertical, horizontal, and a combination of both. The effect of nonsymmetric reinforcement was also studied. The results of these tests are presented in this paper, and include the load-displacement behaviors, crack patterns, failure modes, and FRP strains. The results showed that the vertically aligned reinforcement was the most effective, with significant increases in strength and ductility observed. The dowel strength of the vertical reinforcement did not likely contribute significantly to the shear resistance of the masonry. Instead, it was likely that the vertical reinforcement acted in tension to restrain shear induced dilation and restrain sliding. In some panels cracking adjacent to the FRP strip, through the panel thickness was observed. This type of cracking reduced the bond between one side of the FRP strip and the masonry, and led to premature debonding. A comparison of the test results with the results of other tests from the literature is also presented in this paper.

Assessment of the Predictive Performance of Existing Analytical Models for Debonding of Near-Surface Mounted FRP Strips

The near-surface mounted (NSM) strengthening technique, in which pultruded fiber reinforced polymer (FRP) plates or bars are adhesively bonded into grooves cut in the concrete cover, has proven to overcome several of the drawbacks of externally bonded FRP plates on reinforced concrete flexural members. However, as research on the NSM technique is relatively recent, the theoretical foundation for its application is not yet as complete, and some of the debonding mechanisms are not yet fully understood. This paper presents the main findings of a literature review aimed at identifying the existing analytical models to predict the debonding strength of reinforced concrete flexural elements retrofitted with NSM FRP strips, and of a thorough assessment of the adequacy of such models by comparison with all relevant published experimental data. It is concluded that for debonding of NSM FRP strips induced by flexural (or flexural-shear) cracking or the critical diagonal shear crack of the reinforced concrete member, the currently available models capture the behavior of the debonding mechanism and provide reasonably conservative estimations of the debonding strength. However, for debonding induced by stress concentrations at the strip ends, existing models do not fully capture the debonding behavior and as such, are shown to be excessively conservative.

Numerical Investigation of Creep Effects on FRP-Strengthened RC Beams

Numerical analysis using a finite-element model was performed to simulate and investigate the long-term behavior of two RC beams with similar steel reinforcement, cast from the same batch of concrete. One beam was a plain RC beam and the other beam was strengthened using carbon fiber-reinforced polymer (FRP) strips. The deflections of both beams have been monitored for 5 years after loading. The finite-element model included both creep of concrete and viscoelasticity of the epoxy adhesive at the concrete-carbon FRP (CFRP) interface. The results of the finite-element analysis are compared to experimental observations of the two beams. The finite-element analysis was found to be able to simulate the long-term behavior of the CFRP-strengthened beam and help us understand the complex changes in the stress state that occur over time.