Fiber Reinforced Polymer Strips Research Articles

Interaction between Steel Stirrups and Shear-Strengthening FRP Strips in RC Beams

RC beams shear strengthened with either fiber-reinforced polymer (FRP) U-jackets/U-strips or side strips commonly fail due to debonding of the bonded FRP shear reinforcement. As such debonding occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups may not have reached yielding. Consequently, the yield strength of internal steel stirrups in such a strengthened RC beam cannot be fully used. In this paper, a computational model for shear interaction between FRP strips and steel stirrups is first presented, in which a general parabolic crack shape function is employed to represent the widening process of a single major shear crack in an RC beam. In addition, appropriate bond-slip relationships are adopted to accurately depict the bond behavior of FRP strips and steel stirrups. Numerical results obtained using this computational model show that a substantial adverse effect of shear interaction generally exists between steel stirrups and FRP strips for RC beams shear strengthened with FRP side strips. For RC beams shear strengthened with FRP U-strips, shear interaction can still have a significant adverse effect when FRP strips with a high axial stiffness are used. Therefore, for accurate evaluation of the shear resistance of RC beams shear strengthened with FRP strips, this adverse effect of shear interaction should be properly considered in design.

Flexural and shear strengthening of RC beams with composite materials – The influence of cutting steel stirrups to install CFRP strips

Abstract Experimental, numerical and analytical investigations have revealed that Carbon Fibre Reinforced Polymer (CFRP) strips with larger cross section height improve the effectiveness of the Near Surface Mounted (NSM) technique for the flexural strengthening of existing reinforced concrete (RC) beams. However, this height is limited to the concrete cover thickness of the longitudinal steel bars, since the application of strips of cross section height larger than the cover thickness requires that the bottom arm of the steel stirrups be cut. This work aims to assess the influence, in terms of shear resistance, of cutting the bottom arm of steel stirrups to install NSM strips for the flexural strengthening of RC beams. The obtained results showed that, for monotonic loading, cutting the bottom arm of steel stirrups led to a decrease of the beam’s load carrying capacity of less than 10%. Due to the high effectiveness of the adopted NSM flexural strengthening systems, shear can be a predominant failure mode for these beams. To avoid this type of failure mode, strips of wet lay-up CFRP sheets with U configuration were used, resulting in effective strengthening solutions for RC beams. In the present paper the experimental program is described, and the obtained results are presented and discussed.

Experimental and Computational Modeling of Low Cycle Fatigue Damage of CFRP Strengthened Reinforced Concrete Beams

Improvement in the short-term behavior of deficient reinforced concrete beams is very well known through extensive testing of reinforced concrete beams strengthened by externally bonded carbon fiber reinforced polymer strips. However, long-term performance of such beams needs to be assessed before the method can gain full acceptance. Prominent modes of failure in strengthened reinforced concrete beams reported in literature are concrete crushing in compression, carbon fiber reinforced polymer plate rupturing in tension and peeling of the carbon fiber reinforced polymer plate due to high interfacial shear and peeling stresses at the plate cut-off point. Modeling of the overall system of strengthened reinforced concrete beams requires modeling of the individual components that ultimately leads to failure i.e., concrete, carbon fiber reinforced polymer and concrete - carbon fiber reinforced polymer. Component elasto-damage constitutive models are developed for concrete crushing, carbon fiber reinforced polymer rupture and concrete-carbon fiber reinforced polymer interface failure, calibrated by data from both suitably designed experiments and from experiments reported in literature, and are utilized to investigate the low cycle fatigue behavior of reinforced concrete beams strengthened by carbon fiber reinforced polymer strips. The predictive ability of the model is tested by comparison to data from two different beam strengthening schemes i.e., with end anchorage and without end anchorage.

Front and Side View Image Correlation Measurements on FRP to Concrete Pull-Off Bond Tests

Understanding the transfer of force by bond between externally bonded fiber-reinforced polymer (FRP) reinforcement and concrete is an important step in formulating good models for predicting debonding failures observed in externally bonded reinforcement strengthened systems. In this paper, a 3D optical displacement measurement system was used to capture the full-field displacements from the front and side view in pull-off bond specimens. The experiments were carried using six specimens with carbon FRP (CFRP) strips having different axial stiffnesses but a constant bond length to the concrete substrate. Using the optical measurements, it was possible to obtain the in-plane displacement or slip and the out-of-plane displacement or separation between the CFRP strip and the concrete. It was demonstrated, that the usual assumption of pure shear stresses in such pull-off tests is not true and that the bond behavior is a two-dimensional problem involving shear and peeling stresses. The bond behavior in CFRP strip to concrete pull-off tests was characterized by three stages: (1) the initiation of the first crack; (2) the initiation of debonding; and (3) failure by complete debonding. Based on the test results it was found that there was a dependency between the maximum bond shear stress, the maximum fracture energy of the FRP-concrete interface, and the stiffness of the FRP. However, the slip values after initiation of debonding (Stage 2) were independent of the FRP stiffness. The measured anchorage force and anchorage length were in good agreement with predictions from existing code equations.

New Approach for Modeling the Contribution of NSM FRP Strips for Shear Strengthening of RC Beams

This paper presents the main features of an analytical model recently developed to predict the near-surface mounted (NSM) fiber-reinforced polymer (FRP) strips shear strength contribution to a reinforced concrete (RC) beam throughout the beam's loading process. It assumes that the possible failure modes that can affect the ultimate behavior of an NSM FRP strip comprise: loss of bond (debonding); concrete semiconical tensile fracture; mixed shallow-semicone-plus-debonding; and strip tensile fracture. That model was developed by fulfilling equilibrium, kinematic compatibility, and constitutive law of both the adhered materials and the bond between them. The debonding process of an NSM FRP strip to concrete was interpreted and closed-form equations were derived after proposing a new local bond stress-slip relationship. The model proposed also addressed complex phenomena such as the interaction between the force transferred to the surrounding concrete through bond stresses and concrete fracture as well as the interaction among adjacent strips. The main features of the proposed modeling strategy are shown along with the main underlying physical-mechanical concepts and assumptions. Using recent experimental data, the predictive performance of the model is assessed. The model is also applied to single out the influence of relevant parameters on the NSM technique effectiveness for the shear strengthening of RC beams.

Creep Effects in Plain and Fiber-Reinforced Polymer-Strengthened Reinforced Concrete Beams

This study considers the potential effects of creep on reinforced concrete (RC) beams strengthened with externally applied fiber reinforced polymer (FRP) strips. The significance of creep in the epoxy adhesive and whether such creep might allow the FRP strips to unload over time is assessed. The long-term deflection behavior of two RC beams with similar dimensions and material properties was monitored. One beam was externally strengthened with fiber-reinforced polymer (FRP) strips, while the other was used as a control specimen. Both beams have been subjected to sustained loading for over 6-1/2 years. Slip movements at the ends of the FRP strips were also monitored. The experimental deflections have been compared to deflection predictions using ACI 209R-92 and CEB-FIP MC 90. The creep deformations of the FRP-strengthened beam are not as predicted from the control beam. A step-by-step in-time analysis and finite element modeling are used in the analysis, and both showed good ability to simulate long-term effects in RC beams strengthened with FRP. Results from both techniques show that creep of the adhesive layer can account for the differences observed between the predicted and actual behaviors of the beam, and that this creep should be included when assessing the long-term effects of strengthening a beam with externally applied FRP.

Out-of-plane strength of brick masonry retrofitted with horizontal NSM CFRP strips

The magnitude of damage possible in unreinforced brick masonry construction subjected to high levels of out-of-plane loading is well understood. Due to the large global building stock of masonry structures, it is essential that efficient retrofitting techniques be developed. The use of fibre-reinforced polymer (FRP) strips has been shown to improve the load-carrying and displacement capacities of masonry sections subjected to out-of-plane loading. This paper presents the results of an experimental and theoretical investigation into the behaviour of modern clay brick masonry retrofitted with horizontally oriented FRP strips subjected to horizontal bending. Experimental tests were conducted on four masonry wallettes retrofitted with horizontal near-surface mounted (NSM) carbon FRP strips, subjected to horizontal bending. The specimens were tested in a vertical orientation with variation in vertical compressive loading. The application of compressive load improved the FRP confinement efficiency, and thus increased the maximum strength of the specimens. The primary failure modes were displacement induced (DI) debonding and FRP buckling. Mathematical models were developed to predict the moment at cracking and moment capacity for the specimens. The models explicitly account for the flexural strengths of the mortar and brick units, the torsional resistance of the bed joints, the NSM FRP, as well as the contributions to bed joint shear strength from the vertical compressive stress and friction. The proposed mathematical models were validated against the results of the experimental tests.

Strengthening RC beam–column connections with FRP strips

Reinforced concrete connections, designed prior to the implementation of earthquake design standards, may be vulnerable to shear failure during a seismic attack. Addition of externally bonded fibre-reinforced polymer (FRP) composites can enhance not just the shear capacity but the deformation and energy absorption capacity of the connection. The majority of research studies to date have opted for complete or near-complete coverage of the joint region with FRP and have subjected the test specimens to cyclic (push–pull) loading. Such strengthening schemes and method of loading make it quite difficult to accurately monitor and hence understand the behaviour of the FRP and the concrete beneath. This paper presents results of a series of tests on the strengthening of shear deficient connections with FRP strips subjected to either cyclic or monotonic loading with the primary motivation being accurate description of the behaviour of the FRP. The tests also enable the failure modes to be more accurately reported and classified especially due to the use of monotonic loading. An analytical model is finally presented which accurately describes the mechanics of the FRP strengthening with the model predictions correlating reasonably well with the test data.

Debonding Phenomena in CFRP Strengthened Calcarenite Masonry Walls and Vaults

In this paper the results of an experimental investigation on full-scale reinforced masonry elements like vaults and walls are presented. The masonry type is constituted by calcarenite ashlars and bed joint mortar and the reinforcement is constituted by Carbon Fibre Reinforced Polymer (CFRP) strips. Preliminary experimental results obtained in a previous research by the authors on the local behaviour at the interface CFRP-calcarenite are summarized. Tests with controlled displacement are carried out on out-of-plane loaded walls and cylindrical vaults loaded at a quarter of the span. Results of tests carried out before and after the application of the reinforcement made it possible to evaluate the enhancement in the performance of the structural elements made of the masonry type considered in this study. Local measurements allowed interpretation of the cross-section behaviour, taking into account failure modes relating to the debonding phenomenon and the compression failure of the masonry material.

Effect of Construction Details on the Bond Performance of NSM FRP Bars in Concrete

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement has proven effective for strengthening reinforced concrete structures and is gaining increasing attention. The NSM reinforcement is installed by grooving the surface of the member to be strengthened and embedding FRP bars or strips in the grooves with an appropriate binder. This paper reports the results of an investigation designed to evaluate the influence of some construction parameters on the bond performance of NSM FRP round bars. These parameters include the groove depth and width-to-depth ratio, the groove distance from the edge of the member, the mechanical properties of the groove-filling epoxy, and the use of transverse FRP sheets for confinement of the joint. During the tests, conducted with a direct shear setup, the loaded-end and free-end slips and the strain distributions along the reinforcement and in the transverse plane were monitored. Tests were conducted on both short and long NSM bar anchorages. Results are here reported and analyzed to determine the effect of the test variables on local bond-slip behavior and development capacity. The local bond-slip curves obtained from the tests with short bond length are modelled analytically and used to predict the capacity of the specimens having long bond length and the same other test variables.

Stress Analysis of Monolithic Circular Arches Strengthened with Composite Materials

This paper presents an analytical approach for the elastic stress analysis of monolithic circular arches strengthened with externally bonded fiber-reinforced polymer (FRP) strips. Emphasis is placed on the interfacial stresses between the existing structure and the supplemental reinforcement layers. Two analytical models are presented: The first model formulates the governing equations in terms of the displacements in the arch and the FRP strip and the tangential distribution of the shear stresses in the adhesive layer as unknowns without involving any assumptions on the stress and displacement fields in the adhesive layer. The second model uses the functional form of the displacement field derived in the first model yielding a formulation in terms of displacement unknowns only. Two numerical examples that examine the capabilities of the analytical approach are discussed. The first example focuses on the stress analysis of a strengthened arch under a localized load. The second example studies the elastic ...

Seismic Performance of Strengthened Reinforced Concrete Beam-Column Joints Using FRP Composites

Three nonseismically detailed interior reinforced concrete beam-column joints, namely, one eccentric and two concentric joints, strengthened with proposed fiber-reinforced polymer (FRP) wrapping configurations using glass fiber-reinforced polymer and carbon fiber-reinforced polymer strips and sheets, were tested under constant axial compression load and reversed cyclic loading which simulated low to moderate earthquake forces. Seismic performance of the strengthened beam-column joints in terms of their hysteresis response, stiffness, and energy dissipation capacity is evaluated and compared to those of the original and unstrengthened beam-column joints. Results indicate that applying strips at 45º on a flushed eccentric joint core and as cross bracing on the beam and confinement round the column is very effective. All specimens failed with gradual strength deterioration, bond degradation, and debonding of FRP sheets was observed near the joint core. The proposed strengthening schemes were found to be efficient and economical for mass repair or upgrading of nonseismically detailed structures.

Curved FRP as concrete reinforcement

The increased use of fibre-reinforced polymers (FRPs) in reinforced concrete construction is largely driven by the requirement for improved durability, especially for those applications subjected to the most severe environmental conditions. However, the adoption of FRP as embedded reinforcement in new structures has been much slower than in repair and strengthening applications. This is owing to the limited availability of curved FRP reinforcing elements. In addition, the mechanical performance of the bent portions of FRP bars is significantly reduced under a multiaxial combination of stresses, and the tensile strength can be as low as 40% of the uniaxial tensile strength. This paper presents and discusses potential issues relating to the use of curved FRP bars as embedded reinforcement in concrete and uses as an example one of the case studies that was examined during the European funded project, CurvedNFR. A 6 m long concrete plank reinforced with thermosetting FRP bars as longitudinal reinforcement and thermoplastic FRP strips as shear reinforcement was manufactured and tested. The use of FRPs allowed the reduction in the required concrete cover without compromising durability. This study shows that current design recommendations for FRP concrete structures are effective in predicting deflections and crack widths at service load. It is also shown that in FRP concrete, serviceability limit state can control the design.

Retrofitting Precast Bridge Beams with Carbon Fiber-Reinforced Polymer Strips for Shear Capacity

Advancements in fiber-reinforced polymers (FRPs) have made this an attractive material for rehabilitation and strengthening of bridge superstructures. FRP has primarily been used with the intention of increasing the bending strength of bridge members. However, this paper investigates the use of externally placed FRP strips to increase shear capacity of short-span, 5.7 m (19 ft), precast concrete channel beam bridges. A statewide survey revealed that as many as 389 bridges in the state of Arkansas are comprised of these members. Notably, beams within these bridges were designed under provisions that did not require shear reinforcement. In this research, four sections were retrofitted using carbon fiber-reinforced polymer (CFRP) strips and load tested to failure to measure the repair effectiveness. The performance of the retrofitted sections far exceeded that of unretrofitted sections. It was concluded that the addition of the CFRP repair increased the deflection ductility at least 123%. In addition, beams retrofitted with the CFRP strips experienced at least 26% more deflection after the initiation of a shear crack; therefore reducing the risk of a catastrophic failure.

Edge debonding in FRP strengthened beams: Stress versus energy failure criteria

In the present paper we analyse the edge debonding failure of a beam strengthened by a fibre reinforced polymer. As well known from the literature, a stress concentration is found at the edge of the reinforcement which triggers the debonding of the fibre reinforced polymer strip when the load reaches a certain critical threshold. Two failure criteria are proposed to study the debonding mechanism. The former is a stress assessment criterion, i.e. failure takes place whenever the maximum shearing stress reaches a limit value (the interfacial bond strength). The latter is an energy, fracture mechanics criterion, i.e. failure takes place as the strain energy release rate reaches a critical value (the interfacial fracture energy). It is argued that the energy criterion is more effective to address the edge debonding failure mode. However, under the assumption of shear lag behaviour for the adhesive layer between the beam and the reinforcement, a general rule linking the two approaches is set, thus providing the key to bypass the rather complicated energetic analysis. The final part of the paper is devoted to the crack instabilities that may occur after the debonding initiates, i.e. snap–back and snap-through phenomena. The size effect is then investigated by means of a dimensional analysis and a simplified formula providing the critical load is proposed that could be useful in engineering practice.

Behavior of RC T-section beams strengthened with CFRP strips, subjected to cyclic load

This paper presents results of an experimental investigation on T-section reinforced concrete (RC) beams strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) strips. Specimens, one of which was the control specimen and the remaining six were the shear deficient test specimens, were tested under cyclic load to investigate the effect of CFRP strips on behavior and strength. Five shear deficient specimens were strengthened with side bonded and U-jacketed CFRP strips, and remaining one tested with its virgin condition without strengthening. The type and arrangement of CFRP strips and the anchorage used to fasten the strips to the concrete are the variables of this experimental work. The main objective was to analyze the behavior and failure modes of T-section RC beams strengthened in shear with externally bonded CFRP strips. According to test results premature debonding was the dominant failure mode of externally strengthened RC beams so the effect of anchorage usage on behavior and strength was also investigated. To verify the reliability of shear design equations and guidelines, experimental results were compared with all common guidelines and published design equations. This comparison and validation of guidelines is one of the main objectives of this work. The test results confirmed that all CFRP arrangements differ from CFRP strip width and arrangement, improved the strength and behavior of the specimens in different level significantly.

Bond Model of NSM-FRP Strips in the Context of the Shear Strengthening of RC Beams

Gluing of fiber reinforced polymer (FRP) strips by a structural adhesive into thin slits cut in the cover of reinforced concrete (RC) beams' lateral faces is a shear strengthening technique that is gaining increasing attention of the FRP community. Nonetheless, certain aspects related to the failure modes affecting the behavior at ultimate of such near surface mounted (NSM) strips need to be clarified. Recent findings have spotlighted that besides debonding and tensile rupture of the strips, another possible failure mode should be considered, which involves the fracture of concrete in the shape of semicones along the strips. Thus, a comprehensive analytical model accounting for all of those failure modes was recently developed for predicting the NSM contribution to RC beams shear capacity. Despite its consistency with experimental recordings, that model still needs improvement since some aspects related to the interaction between the force transferred by bond to the surrounding concrete and its tensile fracture still need to be clarified. The need to improve that analytical model led us to further address the issues regarding the debonding failure mode affecting the behavior, at ultimate, of a NSM FRP strip. A new local bond stress-slip relationship is proposed and closed-form equations to be implemented in that analytical model are derived and appraised on the basis of some of the most recent experimental results available in the literature.

Bond Behavior of Near-Surface Mounted FRP Strips Bonded to Modern Clay Brick Masonry Prisms: Influence of Strip Orientation and Compression Perpendicular to the Strip

In this paper the results of 18 pull tests performed on clay brick masonry prisms strengthened with near-surface mounted carbon fiber-reinforced polymer (CFRP) strips are presented. The pull tests were designed to add to the existing database and investigate variables significant to masonry construction. FRP was bonded to solid clay brick masonry; FRP aligned both perpendicular and parallel to the bed joint; and in the case of FRP reinforcement aligned parallel to the bed joint, compression applied perpendicular to the strip was used to simulate vertical compression load in masonry walls. Results including bond strength, critical bond length, and the local bond-slip relationship are presented as well as a discussion on the effect of the new variables on these results.

CFRP-Based Strengthening Technique to Increase the Flexural and Energy Dissipation Capacities of RC Columns

A strengthening technique, combining carbon fiber-reinforced polymer (CFRP) laminates and strips of wet layup CFRP sheet, is used to increase both the flexural and the energy dissipation capacities of reinforced concrete (RC) columns of square cross section of low to moderate concrete strength class, subjected to constant axial compressive load and increasing lateral cyclic loading. The laminates were applied according to the near surface mounted technique to increase the flexural resistance of the columns, while the strips of CFRP sheet were installed according to the externally bonded reinforcement technique to enhance the concrete confinement, particularly in the plastic hinge zone where they also offer resistance to the buckling and debonding of the laminates and longitudinal steel bars. The performance of this strengthening technique is assessed in undamaged RC columns and in columns that were subjected to intense damage. The influence of the concrete strength and percentage of longitudinal steel bars on the strengthening effectiveness is assessed. In the groups of RC columns of 8 MPa concrete compressive strength, this technique provided an increase of about 67% and 46% in terms of column’s load carrying capacity, when applied to undamaged and damaged columns, respectively. In terms of energy dissipation capacity, the increase ranged from 40%–87% in the undamaged columns, while a significant increase of about 39% was only observed in one of the damaged columns. In the column of moderate concrete compressive strength (29 MPa), the technique was even much more effective, since, when compared to the maximum load and energy dissipation capacity of the corresponding strengthened column of 8 MPa of average compressive strength, it provided an increase of 39% and 109%, respectively, showing its appropriateness for RC columns of buildings requiring upgrading against seismic events.

Studies on Mechanically Fastened Fiber-Reinforced Polymer Strengthening Systems

This study uses experimental tests and finite element modeling to investigate the interfacial behavior between mechanically fastened (MF)-fiber reinforced polymer (FRP) composite and concrete substrates. A total of 23 direct shear tests were conducted on FRP strips MF to concrete. Based on the experimental results of one fastener FRP/concrete connection, bearing-slip models are developed to address the MF-FRP/concrete interfacial behavior. Finite element models are then presented to address the interfacial behavior between the FRP strips and the concrete substrate for both the MF-FRP/concrete direct shear specimens, as well as for MF-FRP-strengthened reinforced concrete beams. Results are presented in terms of ultimate load capacities, load-slip relationships, and strain distributions along the FRP strip. Good agreement is obtained between the numerical predictions and experimental data in terms of ultimate carrying capacities, failure modes and load-deflection relationships.