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.     

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.     

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

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

Shear Strength of One-Way Concrete Slabs Reinforced with Fiber-Reinforced Polymer Composite Bars

This paper evaluates the shear strength of one-way concrete slabs reinforced with different types of fiber-reinforced polymer (FRP) bars. A total of eight full-size slabs were constructed and tested. The slabs were 3,100?mm?long×?1,000?mm?wide×200?mm?deep. The test parameters were the type and size of FRP reinforcing bars and the reinforcement ratio. Five slabs were reinforced with glass FRP and three were reinforced with carbon FRP bars. The slabs were tested under four-point bending over a simply supported clear span of 2,500 mm and a shear span of 1,000 mm. All the test slabs failed in shear before reaching the design flexural capacity. The experimental shear strengths were compared with some theoretical predictions, including the JSCE recommendations, the CAN/CSA-S806-02 code, and the ACI 440.1R-03 design guidelines. The results indicated that the ACI 440.1R-03 design method for predicting the concrete shear strength of FRP slabs is very conservative. Better predictions were obtained by both the CAN/CSA-S806-02 code and the JSCE design recommendations.     

Transverse Thermal Expansion of FRP Bars Embedded in Concrete

Fiber reinforced polymers (FRPs) have a thermal expansion in the transverse direction much higher than in the longitudinal direction and also higher than the thermal expansion of hardened concrete. The difference between the transverse coefficient of thermal expansion of FRP bars and concrete may cause splitting cracks within the concrete under temperature increase and, ultimately, failure of the concrete cover if the confining action of concrete is insufficient. This paper presents the results of an experimental investigation to analyze the effect of the ratio of concrete cover thickness to FRP bar diameter (c/db) on the strain distributions in concrete and FRP bars, using concrete cylindrical specimens reinforced with a glass FRP bar and subjected to thermal loading from ?30?to?+80°C. The experimental results show that the transverse coefficient of thermal expansion of the glass FRP bars tested in this study is found to be equal to 33 (×10?6?mm/mm/°C), on average and the ratio between the transverse and longitudinal coefficients of thermal expansion of these FRP bars is equal to 4. Also, the cracks induced by high temperature start to develop on the surface of concrete cylinders at a temperature varying between +50 and +60°C for specimens having a ratio of concrete cover thickness to bar diameter c/db less than or equal to 1.5. A ratio of concrete cover thickness to glass fiber reinforced polymers (GFRP) bar diameter c/db greater than or equal to 2.0 is sufficient to avoid cracking of concrete under high temperature up to +80°C. The analytical model, presented in this paper, is in good agreement with the experimental results, particularly for negative temperature variations.     

Bond Strength of Fiber Reinforced Polymer Rebars in Normal Strength Concrete

The bond behavior of reinforcing bars in concrete is a critical issue in the design of reinforced concrete structures. This study focuses on the bond strength of fiber reinforced polymer (FRP) rebars in normal strength concrete. Four different types of rebars were tested using the pullout method: aramid FRP (AFRP); carbon FRP (CFRP); glass FRP (GFRP), and steel. This involved a total of 151 specimens containing 6, 8, 10, 16, and 19?mm rebars embedded in a 203?mm concrete cube. The test embedment lengths were five, seven, and nine times the rebar diameter (db). For each rebar, the test results include the bond stress–slip response and the mode of failure. The test results showed that the bond strength of an FRP rebar is, on average, 40–100% the bond strength on a steel rebar for pullout failure mode. Based on this research, a proposal for the average bond strength of straight FRP rebars in normal strength concrete is made, which verifies an existing bond strength relationship (GFRP) and extends its application to AFRP and CFRP. It is an expression that is a function of the rebar diameter, and the concrete compressive strength.     

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.     

Postrepair Fatigue Performance of FRP-Repaired Corroded RC Beams: Experimental and Analytical Investigation

This paper presents the results of an experimental and analytical study of the fatigue performance of corroded reinforced concrete (RC) beams repaired with fiber-reinforced polymer (FRP) sheets. Ten RC beam specimens (152×254×3,200?mm) were constructed. One specimen was neither strengthened nor corroded to serve as a reference; three specimens were corroded and not repaired; another three specimens were corroded and repaired with U-shaped glass FRP sheets that wrapped the cross section of the specimen; and the remaining three specimens were corroded and repaired with U-shaped glass FRP sheets for wrapping and carbon-fiber-reinforced polymer (CFRP) sheets for flexural strengthening. The FRP sheets were applied after the main reinforcing bars were corroded to an average mass loss of 5.5%. Following FRP repair, some specimens were tested immediately to failure, while the other repaired specimens were subjected to further corrosion before being tested to failure to investigate their postrepair (long-term) performance. Reinforcement steel pitting due to corrosion reduced the fatigue life significantly. The FRP wrapping had no significant effect on the fatigue performance, while using CFRP sheets for flexural strengthening enhanced the fatigue performance significantly. The fatigue results were compared to smooth specimen fatigue data to estimate an equivalent fatigue notch factor for the main reinforcing bars of the tested specimens.     

Shear Resistance of FRP RC Beams: Experimental Study

This study investigates the shear behavior of concrete beams reinforced with fiber-reinforced polymer (FRP) reinforcement. Six beams were subjected to two successive phases of testing. Half of the beams were reinforced in flexure with conventional steel reinforcement, while the other half were reinforced with glass fiber bars. Different shear span to depth ratios, ranging from 1.1 to 3.3, were analyzed in order to study the variation in the shear behavior of beams characterized by different types of shear failure. No shear reinforcement was provided in the first phase of testing, while in the second phase, just enough glass and carbon shear reinforcement was provided to enable failure due to shear. The results of these tests are presented and compared to predictions according to the design recommendations proposed by the ACI and the Institution of Structural Engineers, U.K. The results of this study show that these approaches, which are based on modifications of equations derived for steel reinforcement, underestimate the contribution of the concrete and the shear reinforcement to the total shear capacity of FRP RC beams. It is shown that both approaches can be modified to become less conservative.     

Deflection Prediction of Steel and FRP-Reinforced Concrete-Filled FRP Tube Beams

This paper presents the results of experimental and theoretical investigations that study the flexural behavior of reinforced concrete-filled fiber-reinforced polymer (FRP) tubes (RCFFTs) beams. The experimental program consists of 10 circular beams [6 RCFFT and 4 control reinforced concrete (RC) beams] with a total length of 2,000?mm, tested under four-point bending load. The experimental results were used to review and verify the applicability of various North American code provisions and some available equations in the literature to predict deflection of RCFFT beams. The measured deflections and the experimental values of the effective moment of inertia were analyzed and compared with those predicted using available models. The results of the analysis indicated that the behavior of steel and FRP-RCFFT beams under the flexural load was significantly different than that of steel and FRP-RC members. This is attributed to the confining effect of the FRP tubes and their axial contribution. This confining behavior in turn enhanced the overall flexural behavior and improved the tension stiffening of RCFFT beams. For that, the predicted tension stiffening of steel and FRP-RCFFT beams using the conventional equations (steel or FRP-RC member) underestimates the flexural response; therefore, the predicted deflections are overestimated. Based on the analysis of the test results, the Branson’s equation for the effective moment of inertia of RC structures is modified, and new equations are developed to accurately predict the deflection of concrete-filled FRP tube (CFFT) beams reinforced with steel or FRP bars.     

Behavior of CFRP Strengthened Reinforced Concrete Beams in Corrosive Environment

This paper reports the test results of 11 reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) sheets and subjected to an aggressive environment. In this study, eight beams were cracked and repaired with CFRP sheets, while the remaining three beams were kept uncracked as a control. The beams were 150?mm wide by 250?mm deep by 2,400?mm long and lightly reinforced with a reinforcement ratio of 0.6%. Two types of carbon FRP products were considered: Sheets and strips. In terms of environmental exposure, three beams were kept at room temperature and eight beams were subjected up to 300 wetting and drying cycles with deicing chemicals (3% NaCl). Following the exposure, the beams were tested to failure in four-point bending. In addition, nondestructive tests were performed to determine the corrosion rate, as well as destructive tests to determine chloride diffusion and reinforcing bar mass loss. Based on the findings of the study, the long-term effectiveness of the CFRP strengthened reinforced concrete in aggressive corrosive environments was established.     

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.     

Tensile Lap Splicing of Bundled CFRP Reinforcing Bars in Concrete

In the case of heavily reinforced concrete structural members, bundled bars are required rather than spaced bars. The use of spliced bundled bars is necessary when available bar lengths are limited. No design recommendations regarding the use of bundled or spliced bundled FRP bars are available. The results of four-point flexural testing of nine concrete beams reinforced with spliced bundled CFRP bars are presented herein. The effects of the type of bundle and splice length on the bond strength of bundled CFRP bars are investigated. Based on the experimental results, a procedure for determining the critical splice length of FRP bars is presented and the corresponding values of bond stresses can be predicted. Moreover, the ultimate strength analysis method is used to predict the maximum stress in spliced bundled CFRP bars. Finally, comparisons with the existing recommendations regarding the use of bundled steel bars and the recommended modifications for bundled CFRP bars are presented.     

Flexural Strengthening of Reinforced Concrete Beams by Mechanically Attaching Fiber-Reinforced Polymer Strips

The current method of bonding fiber-reinforced polymer (FRP) strengthening strips to concrete structures requires extensive time and semiskilled labor. An alternative method is to use a commercial off-the-shelf powder-actuated fastening system to attach FRP strips to concrete. A series of flexural tests were conducted on 15 304.8×304.8×3,657.6?mm (12×12×144?in.) reinforced concrete beams. Two beams were tested unstrengthened, 12 were strengthened with mechanically fastened FRP strips, and one was strengthened with a bonded FRP strip. The effects of three different strip moduli, different fastener lengths and layouts, and predrilling were examined. Three of the beams strengthened with mechanically attached FRP strips showed strengthening comparable to the beam strengthened with a bonded FRP strip. The same three beams strengthened with mechanically attached FRP strips also showed a greater ductility than the beam strengthened with a bonded FRP strip.     

Realistic Bond Strength of FRP Rebars in NSC from Beam Specimens

The bond strength of reinforcing bars in concrete is a prerequisite for the evaluation of the development length in reinforced concrete structures. This study concerns these phenomena for fiber reinforced polymer (FRP) rebars in normal strength concrete (NSC). Three different types of rebars were tested using the beam specimen: Carbon, glass, and steel. This involved a total of 26 beam specimens containing 10, 16, and 19?mm rebars. The test embedment lengths were 10, 15, and 20 times the rebar diameter (db). For each rebar tested, the results concern load deflection curves, bond stress-slip responses, and the mode of failure. The results showed that the bond strength of a FRP rebar is, generally, lower than that of steel rebar. Based on this and previous research, proposals for the average bond strength and for the development length of straight FRP rebars under tension in NSC are made.     

Flexural Load Testing of Concrete-Filled FRP Tubes with Longitudinal Steel and FRP Rebar

The flexural performance of reinforced concrete-filled glass-fiber reinforced polymer (GFRP) tubes (CFFTs) has been investigated using seven specimens, 220?mm in diameter and 2.43?m long. Specimens were reinforced with either steel, GFRP, or carbon–fiber reinforced polymer (FRP) rebar of various sizes. Prefabricated GFRP tubes with most of the fibers oriented in the hoop direction were used in five specimens. One control specimen included conventional steel spirals of stiffness comparable to the GFRP tube and the other had no transverse reinforcement. Test results have shown that CFFT beams performed substantially better than beams with a steel spiral. Unlike CFFTs with FRP rebar, CFFTs with steel rebar failed in a sequential progressive manner, leading to considerable ductility. An analytical model capable of predicting the full response of reinforced CFFT beams, including the sequential progressive failure, has been developed, verified, and used in a parametric study. It is shown that laminate structure of the tube affects the behavior, only after yielding of the steel rebar. Steel reinforcement ratio significantly affects stiffness and strength, whereas concrete strength has an insignificant effect on the overall performance.     

Fatigue Life Evaluation of Concrete Bridge Deck Slabs Reinforced with Glass FRP Composite Bars

Since bridge deck slabs directly sustain repeated moving wheel loads, they are one of the most bridge elements susceptible to fatigue failure. Recently, glass fiber-reinforced polymer (FRP) composites have been widely used as internal reinforcement for concrete bridge deck slabs as they are less expensive compared to the other kinds of FRPs (carbon and aramid). However, there is still a lack of information on the performance of FRP–reinforced concrete elements subjected to cyclic fatigue loading. This research is designed to investigate the fatigue behavior and fatigue life of concrete bridge deck slabs reinforced with glass FRP bars. A total of five full-scale deck slabs were constructed and tested under concentrated cyclic loading until failure. Different reinforcement types (steel and glass FRP), ratios, and configurations were used. Different schemes of cyclic loading (accelerated variable amplitude fatigue loading) were applied. Results are presented in terms of deflections, strains in concrete and FRP bars, and crack widths at different levels of cyclic loading. The results showed the superior fatigue performance and longer fatigue life of concrete bridge deck slabs reinforced with glass FRP composite bars.     

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.     

Bond Performance of GFRP Bars in Tension: Experimental Evaluation and Assessment of ACI 440 Guidelines

The development/splice strength and the pullout local bond stress-slip response of glass fiber-reinforced polymer (GFRP) bars in tension were experimentally investigated using beam specimens and pullout specimens, respectively. Two types of 12-mm (0.47-in.)-diameter GFRP bars were evaluated, namely, thread wrapped and ribbed. The test parameters included the concrete cover, the splice length, and the area of steel confinement for the beam specimens, and the concrete compressive strength for the pullout specimens. Companion steel reinforced beams were also tested for comparison. All beam specimens reinforced with thread-wrapped GFRP bars experienced pullout mode of bond failure, while all specimens reinforced with ribbed GFRP bars or steel bars experienced splitting mode of bond failure. It was found that the bond strength of FRP bars is largely dependent on the surface conditions of the bars. The pullout local bond stress-slip response of ribbed GFRP bars is intrinsically similar to that of steel bars reported in the literature. The bond strength of both types of GFRP bars investigated was about two to three times lower than that of steel bars. Predictions of the development/splice strength of GFRP bars in accordance with the ACI Committee 440 guidelines were unconservative in comparison with the test data. Also, in contradiction with the current ACI 440 report, the use of transverse confining reinforcement increased the bond strength by a sizable 15–30%.