Reinforced Concrete T-Beams Strengthened in Shear with Fiber Reinforced Polymer Sheets

This research studies the interaction of concrete, steel stirrups, and external fiber reinforced polymer (FRP) sheets in carrying shear loads in reinforced concrete beams. A total of eight tests were conducted on four laboratory-controlled concrete T-beams. The beams were subjected to a four-point loading. Each end of each beam was tested separately. Three types of FRP, uniaxial glass fiber, uniaxial carbon fiber, and triaxial glass fiber, were applied externally to strengthen the web of the T-beams, while some ends were left without FRP. The test results show that FRP reinforcement increases the maximum shear strengths between 15.4 and 42.2% over beams with no FRP. The magnitude of the increased shear capacity is dependent not only on the type of FRP but also on the amount of internal shear reinforcement. The triaxial glass fiber reinforced beam exhibited more ductile failure than the other FRP reinforced beams. This paper also presents a test model that is based on a rational mechanism and can predict the experimental results with excellent accuracy.     

Shear Strength of Large Concrete Members with FRP Reinforcement

Increasing interest in the use of fiber-reinforced polymer (FRP) reinforcement for reinforced concrete structures has made it clear that insufficient information about the shear performance of such members is currently available to practicing engineers. This paper summarizes the results of 11 large shear tests of reinforced concrete beams with glass FRP (GFRP) longitudinal reinforcement and with or without GFRP stirrups. Test variables were the member depth, the member flexural reinforcement ratio, and the amount of shear reinforcement provided. Results showed that the equations of the Canadian CSA shear provisions provide conservative estimates of the shear strength of FRP-reinforced members. Recommendations are given along with a worked example on how to apply these provisions including to members with FRP stirrups. It was found that members with multiple layers of longitudinal bars appear to perform better than those with a single layer of longitudinal reinforcing bars. Overall, it was concluded that the fundamental shear behavior of FRP-reinforced beams is similar to that of steel-reinforced beams despite the brittle nature of the reinforcement.     

Effect of Transverse Steel on the Response of RC Beams Strengthened in Shear by FRP: Experimental Study

The present paper shows and discusses some of the results obtained within an experimental investigation carried out on 15 reinforced concrete (RC) beams strengthened in shear by externally bonded fiber-reinforced plastics (FRP) sheets. The aim of the study is to analyze the influence that the geometrical percentage of transverse steel reinforcement could have on the FRP resisting action. In particular, the objectives of the experimental campaign are to explore the possible interaction between FRP and steel transverse reinforcement resisting actions, analyzing the deformation behavior of the shear resisting system (FRP, transverse steel, and concrete) and the modes of failure of the strengthened and not strengthened beams. The results of the tests in terms of shear capacity are compared to the design formulations provided by the American Concrete Institute and the National Research Council of Italy code-format recommendations.     

Experimental Observation on the Effectiveness of Fiber Sheet Strip Stirrups in Concrete Beams

Experimental observations were made for the effectiveness of fiber sheet strips (FSSs) as internal stirrups in comparison with fiber-reinforced polymer (FRP) rod stirrups and steel stirrups. A total number of 10 concrete beams were tested under three-point loading. Each beam measured 1,400 mm long, 150 mm wide, and 250 mm deep. Their shear span-depth ratios were 2.5. The beams were composed of different shear reinforcements: one without stirrups, two with steel stirrups, one with carbon FRP rod stirrups, and the rest with different types of FSS stirrups. The main variables include stirrup types, strengthening of bent portions of FSS stirrups, impregnation, and shear reinforcement ratio for FSS stirrups. Test results indicated that concrete beams reinforced with FSS stirrups had enhanced shear strength over the beam without shear reinforcements. Moreover, the FSS stirrup-reinforced beams could maintain comparable shear behavior to that of the concrete beam reinforced with steel stirrups in overall load-deflection relationships, shear strengths, crack patterns, and crack widths at maximum load.     

Fiber Reinforced Polymer Shear Strengthening of Reinforced Concrete Beams with Transverse Steel Reinforcement

The paper aims to contribute to a better understanding and modeling of the shear behavior of reinforced-concrete (RC) beams strengthened with carbon fiber reinforced polymer (FRP) sheets. The study is based on an experimental program carried out on 11 beams with and without transverse steel reinforcement, and with different amounts of FRP shear strengthening. The test results provide some new insights into the complex failure mechanisms that characterize the ultimate shear capacity of RC members with transverse steel reinforcement and FRP sheets. After the discussion of the above topics, a new upper bound of the shear strength is introduced. It should be capable of taking into account how the cracking pattern in the web failing under shear is modified by the presence of FRP sheets, and how such a modified cracking pattern actually modifies the anchorage conditions of the sheets and their effective contribution to the ultimate shear strength of the beams.     

Does the Use of FRP Reinforcement Change the One-Way Shear Behavior of Reinforced Concrete Slabs?

For members with no transverse reinforcement, numerous models have been proposed for determining shear capacity, most often based on a statistical curve fit to experimental beam test results. The shear provisions of the Canadian code (CSA) for steel-reinforced concrete, by contrast, are based on a theoretical model, the modified compression field theory. This paper demonstrates that the CSA shear provisions for steel-reinforced members can be safely applied to members with internal fiber-reinforced polymer (FRP) bars by adjusting the term EsAs in the method to ErAr. A database of 146 shear failures of specimens reinforced with carbon, glass, or aramid FRP or steel is presented and gives an average test to predicted ratio of 1.38 with a coefficient of variation (COV) of 17.2%. The CSA code equations were optimized for the typical strain range of steel-reinforced concrete and when an equation appropriate for the wider range of strains associated with FRP is used, then a better statistical result can be achieved. Application of this expression to the database resulted in an average test to predicted strength ratio of 1.15 with a COV of 14.9%. As both methods are based on a theoretical shear model that was derived for steel-reinforced concrete and since both methods work safely, it can be concluded that the use of internal FRP bars does not change the one-way shear behavior of reinforced concrete beams and slabs without stirrups.     

Behavior and Ductility of Simple and Continuous FRP Reinforced Beams

The behaviors of simply and continuously supported beams reinforced with fiber reinforced polymer (FRP) materials are presented in this paper. The experimental testing program included seven simple rectangular beams and seven continuous T-section beams. Reinforcing bars and stirrups were made of steel, carbon, or glass fiber reinforced polymer (GFRP). It was concluded that the use of GFRP stirrups increased the shear deformation, and as a result deflection increased. Also, GFRP stirrups changed the failure mode from flexural to shear or flexural-shear, depending on the type of reinforcement bars (FRP or steel). Furthermore, the use of FRP reinforcement in continuous beams increased deformation. This increase remained small and acceptable at the service load level, but significantly increased near failure. While different FRP reinforcement arrangements were found to have the same load capacity as steel reinforcements in conventional beams, failure modes and ductility differed. Failure mode was governed by both the type of reinforcing bars and the type of stirrups. Additionally, the dowel effect influences the load carrying capacity of FRP reinforced continuous beams. A method for evaluating the ductility is presented. The ratio of absorbed energy at failure to the total energy, “energy ratio,” was used as a measure of ductility. Based on this definition, a classification of ductile, semiductile, and brittle behavior is suggested. The theoretical results obtained using the suggested method were substantiated experimentally. The continuous beams experienced higher “energy ratios” than did simple beams.     

Flexural Behavior of Continuous FRP-Reinforced Concrete Beams

Continuous concrete beams are commonly used elements in structures such as parking garages and overpasses, which might be exposed to extreme weather conditions and the application of deicing salts. The use of the fiber-reinforced polymers (FRP) bars having no expansive corrosion product in these types of structures has become a viable alternative to steel bars to overcome the steel-corrosion problems. However, the ability of FRP materials to redistribute loads and moments in continuous beams is questionable due to the linear-elastic behavior of such materials up to failure. This paper presents the experimental results of four reinforced concrete beams with rectangular cross section of 200×300?mm continuous over two spans of 2,800 mm each. The material and the amount of longitudinal reinforcement were the main investigated parameters in this study. Two beams were reinforced with glass FRP (GFRP) bars in to different configurations while one beam was reinforced with carbon FRP bars. A steel-reinforced continuous concrete beam was also tested to compare the results. The experimental results showed that moment redistribution in FRP-reinforced continuous concrete beams is possible if the reinforcement configuration is chosen properly. Increasing the GFRP reinforcement at the midspan section compared to middle support section had positive effects on reducing midspan deflections and improving load capacity. The test results were compared to the available design models and FRP codes. It was concluded that the Canadian Standards Association Code (CSA/S806-02) could reasonably predict the failure load of the tested beams; however, it fails to predict the failure location.     

Punching Shear Behavior of Fiber Reinforced Polymers Reinforced Concrete Flat Slabs: Experimental Study

This paper presents the results of a two-phase experimental program investigating the punching shear behavior of fiber reinforced polymer reinforced concrete (FRP RC) flat slabs with and without carbon fiber reinforced polymer (CFRP) shear reinforcement. In the first phase, problems of bond slip and crack localization were identified. Decreasing the flexural bar spacing in the second phase successfully eliminated those problems and resulted in punching shear failure of the slabs. However, CFRP shear reinforcement was found to be inefficient in enhancing significantly the slab capacity due to its brittleness. A model, which accurately predicts the punching shear capacity of FRP RC slabs without shear reinforcement, is proposed and verified. For slabs with FRP shear reinforcement, it is proposed that the concrete shear resistance is reduced, but a strain limit of 0.0045 is recommended as maximum strain for the reinforcement. Comparisons of the slab capacities with ACI 318-95, ACI 440-98, and BS 8110 punching shear code equations, modified to incorporate FRP reinforcement, show either overestimated or conservative results.     

Mechanisms of Shear Resistance of Concrete Beams Strengthened in Shear with Externally Bonded FRP

In recent years, numerous investigations have addressed the shear strengthening of reinforced concrete (RC) beams with externally bonded fiber-reinforced polymer (FRP) composites. Despite this research effort, the mechanisms of shear resistance that are developed in such a strengthening system have not yet been fully documented and explained. This clearly inhibits the development of rational and reliable code specifications. This paper aims to contribute to the understanding of the shear resistance mechanisms involved in RC beams strengthened in shear with externally bonded FRP. It is based on results obtained from an experimental program, involving 17 tests, performed on full size T beams, and using a comprehensive and carefully optimized measuring device. The resistance mechanisms are studied by observing the evolution of the behavior of the strengthened beams as the applied loads are increased. The local behavior of the FRP and the transverse steel, in particular in the failure zones, are thoroughly examined. The operative resistance mechanisms are also studied through the load sharing among the concrete, the FRP, and the transverse steel, at increasing levels of applied load.     

Effect of Transverse Reinforcement on the Flexural Behavior of Continuous Concrete Beams Reinforced with FRP

Continuous concrete beams are structural elements commonly used in structures that might be exposed to extreme weather conditions and the application of deicing salts, such as bridge overpasses and parking garages. In such structures, reinforcing continuous concrete beams with the noncorrodible fiber-reinforced polymer (FRP) bars is beneficial to avoid steel corrosion. However, the linear-elastic behavior of FRP materials makes the ability of continuous beams to redistribute loads and moments questionable. A total of seven full-scale continuous concrete beams were tested to failure. Six beams were reinforced with glass fiber-reinforced polymer (GFRP) longitudinal bars, whereas one was reinforced with steel as control. The specimens have rectangular cross section of 200×300??mm and are continuous over two spans of 2,800?mm each. Both steel and GFRP stirrups were used as transverse reinforcement. The material, spacing, and amount of transverse reinforcement were the primary investigated parameters in this study. In addition, the experimental results were compared with the code equations to calculate the ultimate capacity. The experimental results showed that moment redistribution in FRP-reinforced continuous concrete beams is possible and is improved by increasing the amount of transverse reinforcement. Also, beams reinforced with GFRP stirrups illustrated similar performance compared with their steel-reinforced counterparts.     

Shear Strengthening of Reinforced Concrete Deep Beams Using Carbon Fiber Reinforced Polymer Laminates

For reinforced concrete beams with the same shear and flexural reinforcements, shear failure is most likely to occur in deep beams rather than in regular beams. Thus, retrofitting of deep beams with shear deficiencies is of great importance. Externally bonded reinforcement such as carbon fiber reinforced polymer (CFRP) provides an excellent solution in these situations. In order to investigate the shear behavior of deep beams with externally bonded CFRP shear reinforcement, 16 deep beams without steel shear reinforcement were cast at the concrete laboratory of New Jersey Institute of Technology. After the beams were kept in the curing room for 28 days, carbon fiber strips and fabrics were applied outside of the beams at various orientations with respect to the axis of the beam. All beams were tested on a 979?kN (220?kip) MTS testing machine. Results of test demonstrate the feasibility of using externally applied, epoxy-bonded CFRP system to restore or increase the shear capacity of deep beams. The CFRP system can significantly increase the serviceability, ductility, and ultimate shear strength of a concrete beam, thus restoring deep beam shear strength using CFRP is a highly effective technique. An analysis and design method for shear strengthening of deep beams using externally bonded CFRP has also been proposed as well.     

Role of Fiber Reinforced Plastic Sheets in Shear Response of Reinforced Concrete Beams: Experimental and Analytical Results

The paper is principally aimed at analyzing the role of externally applied fiber reinforced plastic (FRP) sheets in the shear ultimate behavior of reinforced concrete elements. A theoretical model for predicting the shear resisting contribution of FRP sheets is illustrated. The proposal is based on a complete equilibrium/compatibility approach for reinforced concrete beams failing in shear and considers the possible interactions between the composite contribution and the resisting mechanisms of an ordinary reinforced concrete beam. The proposal is discussed and tested by means of an experimental investigation carried out on beams reinforced by glass FRP composite sheets with a shear span to depth ratio equal to 3. Further comparisons are then performed that consider the predictions of other existing approaches reported in the literature.     

Shear Strengthening of Reinforced Concrete Beams Using Carbon-Fiber-Reinforced Polymer Laminates

Shear failure is catastrophic and occurs usually without advance warning; thus it is desirable that the beam fails in flexure rather than in shear. Many existing reinforced concrete (RC) members are found to be deficient in shear strength and need to be repaired. Externally bonded reinforcement such as carbon-fiber-reinforced polymer (CFRP) provides an excellent solution in these situations. To investigate the shear behavior of RC beams with externally bonded CFRP shear reinforcement, 11 RC beams without steel shear reinforcement were cast at the concrete laboratory of the New Jersey Institute of Technology. After the beams were kept in the curing room for 28?days, carbon-fiber strips and fabrics made by Sika Corp. were applied on both sides of the beams at various orientations with respect to the axis of the beam. All beams were tested on a 979?kN (220?kips) MTS testing machine. Results of the test demonstrate the feasibility of using an externally applied, epoxy-bonded CFRP system to restore or increase the shear capacity of RC beams. The CFRP system can significantly increase the serviceability, ductility, and ultimate shear strength of a concrete beam; thus, restoring beam shear strength by using CFRP is a highly effective technique. An analysis and design method for shear strengthening of externally bonded CFRP has been proposed.     

Fatigue Behavior of Carbon Fiber Reinforced Polymer-Strengthened Reinforced Concrete Bridge Girders

This study examines the effects of one-dimensional fiber-reinforced polymer (FRP) composite rehabilitation systems on the flexural fatigue performance of reinforced concrete bridge girders. Eight 508?mm deep and 5.6?m long reinforced concrete T-beams, with and without bonded FRP reinforcement on their tensile surfaces, were tested with a concentrated load at midspan under constant amplitude cyclic loading. The objective of this investigation is to establish the effect that these repair systems have on the fatigue behavior and remaining life of the girders. Results indicate that the fatigue behavior of such retrofit beams is controlled by the fatigue behavior of the reinforcing steel. The fatigue life of a reinforced concrete beam can be increased by the application of an FRP retrofit, which relieves some of the stress carried by the steel. The observed increase in fatigue life, however, is limited by the quality of the bond between the carbon FRP and concrete substrate. Debonding, initiating at midspan and progressing to a support, is common and is driven partially by the crack distribution and shear deformations of the beam.     

Influence of Reinforcement on the Behavior of Concrete Bridge Deck Slabs Reinforced with FRP Bars

This paper presents the results of an experimental study to investigate the role of each layer of reinforcement on the behavior of concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars. Four full-scale concrete deck slabs of 3,000?mm length by 2,500?mm width and 200?mm depth were constructed and tested in the laboratory. One deck slab was reinforced with top and bottom mats of glass FRP bars. Two deck slabs had only a bottom reinforcement mat with different reinforcement ratios in the longitudinal direction, while the remaining deck slab was constructed with plain concrete without any reinforcement. The deck slabs were supported on two steel girders spaced at 2,000?mm center to center and were tested to failure under a central concentrated load. The three reinforced concrete slabs had very similar behavior and failed in punching shear mode at relatively high load levels, whereas the unreinforced slab behaved differently and failed at a very low load level. The experimental punching capacities of the reinforced slabs were compared to the theoretical predictions provided by ACI 318-05, ACI 440.1R-06, and a model proposed by the writers. The tests on the four deck slabs showed that the bottom transverse reinforcement layer has the major influence on the behavior and capacity of the tested slabs. In addition, the ACI 318-05 design method slightly overestimated the punching shear strength of the tested slabs. The ACI 440.1R-06 design method yielded very conservative predictions whereas the proposed method provided reasonable yet conservative predictions.     

Numerical Modeling of FRP Shear-Strengthened Reinforced Concrete Beams

An attractive technique for the shear strengthening of reinforced concrete beams is to provide additional web reinforcement in the form of externally bonded fiber-reinforced polymer (FRP) sheets. So far, theoretical studies concerning the FRP shear strengthening of reinforced concrete members have been rather limited. Moreover, the numerical analyses presented to date have not effectively simulated the interfacial behavior between the bonded FRP and concrete. The analysis presented here aims to capture the three-dimensional and nonlinear behavior of the concrete, as well as accurately model the bond–slip interfacial behavior. The finite-element model is applied to various strengthening strategies; namely, beams with vertical and inclined side-bonded FRP sheets, U-wrap FRP strengthening configurations, as well as anchored FRP sheets. The proposed numerical analysis is validated against published experimental results. Comparisons between the numerical predictions and test results show excellent agreement. The finite-element model is also shown to be a valuable tool for gaining insight into phenomena (e.g., slip profiles, debonding trends, strain distributions) that are difficult to investigate in laboratory tests.     

Tests on Seismically Damaged Reinforced Concrete Walls Repaired and Strengthened Using Fiber-Reinforced Polymers

The behavior of six 1:2.5-scale reinforced concrete cantilever wall specimens having an aspect ratio of 1.5, tested to failure and subsequently repaired and strengthened using fiber-reinforced polymer (FRP) sheets is investigated. Specimens were first repaired by removing heavily cracked concrete, lap splicing the fractured steel bars by welding new short bars, placing new hoops and horizontal web reinforcement, and finally casting nonshrink high-strength repair mortar. The specimens were then strengthened using FRP sheets and strips, with a view to increasing flexural as well as shear strength and ductility. In addition to different arrangements of steel and FRP reinforcement in the walls, a key parameter was the way carbon-FRP strips added for flexural strengthening were anchored; steel plates and steel angles were used to this effect. Steel plates were anchored using U-shaped glass-FRP (GFRP) strips or bonded metal anchors. Test results have shown that by using FRP reinforcement, the flexural and shear strength of the specimens can be increased. From the anchorage systems tested, metal plates combined with FRP strips appear to be quite efficient. The effectiveness of the bonded metal anchors used was generally less than that of the combination of plates and GFRP strips. In all cases, final failure of the FRP anchorage is brittle, but only occurs after the peak strength is attained and typically follows the fracture of steel reinforcement in critical areas, hence the overall behavior of the strengthened walls is moderately ductile.     

Behavior of Short and Deep Beams Made of Concrete-Filled Fiber-Reinforced Polymer Tubes

Behavior of short and deep beams made of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFT) was compared experimentally to that of their slender counterparts. Ten specimens made from four types of glass FRP tubes with different fiber architecture and lamina lay-up were tested with shear span to depth ratio between 0.9 and 6.25, diameter to thickness ratio between 16 and 63, and reinforcement index between 0.11 and 2.2. The study extended the test database of CFFTs to the lowest practical limit of shear span to depth ratio. None of the CFFT beams tested, even with the lowest shear span to depth ratio of 0.9, failed in shear. Tensile bending strains at the bottom of the midspan section of the beams always remained higher than the respective diagonal tensile strain at the midpoint of the shear span. Web shear cracks were observed only in the concrete core of deep CFFT beams with high reinforcement index. Following the first flexural crack, the concrete core began to slip relative to the FRP tube. This lack of composite action made shear less critical than flexure. Finally, short and deep CFFT beams exhibited higher bending capacity than their slender counterparts, primarily due to the direct diagonal compression strut that develops in the concrete core through arching action.     

Flexural Behavior of GFRP-Reinforced Concrete Masonry Beams

An experimental and analytical study is conducted in order to investigate the flexural behavior of masonry beams that are internally reinforced using glass fiber-reinforced polymers (GFRP) rebars. Seven reinforced masonry beams with 4.0- and 2.4-m spans were tested under four-point bending setup. The beams were loaded monotonically up to failure. One had two courses of hollow concrete masonry units and the remaining six beams had three courses. Two masonry beams were reinforced using conventional steel rebars and were considered as the control specimens. The remaining five beams were internally reinforced using GFRP rods with different reinforcement ratios. Beams were detailed to have sufficient shear reinforcement such that they do not fail in shear. Flexural capacity, deformation, curvature, and strains of the tested GFRP-reinforced and steel-reinforced masonry beams were compared and discussed. Using the acquired data from the experimental and analytical studies, effectiveness of GFRP rods as internal reinforcement for concrete masonry beams is demonstrated.