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.     

Development of FRP Shear Bolts for Punching Shear Retrofit of Reinforced Concrete Slabs

A fiber-reinforced polymer (FRP) shear bolt system has been recently developed at the University of Waterloo in Canada. The system is used to protect previously built reinforced concrete (RC) slabs against brittle punching shear failure. The system requires drilling small holes in a RC slab around the perimeter of a column, inserting bolts into the holes, and anchoring the bolts at both external surfaces of the slab. Many existing RC slabs have been built without any shear reinforcement. Also, many of these slabs are in corrosive environments, e.g., parking garages, where the use of deicing salts accelerates reinforcement corrosion and concrete deterioration. Therefore, FRPs are ideal materials to be used for such retrofit. The challenge, however, is the development of mechanical end anchorages for FRP rods that are efficient, aesthetic, cost effective, and that can be installed on site. The research presented in this paper includes development of FRP bolts with mechanical anchorages and the results of testing done using the developed systems. A new anchorage technique for the FRP rods based on crimping the rod ends with the aluminum fittings was developed. The testing was done on isolated slab-column specimens representing interior slab-column connections in a continuous flat plate system. The specimens were subjected to simulated gravity loading. The developed FRP bolts worked very well in improving the performance of the slab-column connections and showing the benefits of using FRP in punching shear retrofit of reinforced concrete slabs in corrosive environments.     

Punching Shear Behavior of Flat Slabs Strengthened with Fiber Reinforced Polymer Laminates

The present work reports the test results of seven full-scale reinforced concrete slab-column edge connections strengthened against punching shear using different methods. In this study, three slabs contained openings in the vicinity of the column, and the other four were without openings. The dimensions of the slabs were 1,540×1,020×120 mm with square columns (250×250 mm). The openings in the specimens were square (150×150 mm) with the sides parallel to the sides of the column. The slabs were reinforced with an average reinforcement ratio of 0.75%. Except for the two reference slabs, two different strengthening techniques were considered. Technique I applies externally bonded fiber reinforced polymer (FRP) flexible sheets on the slab around the column in two schemes with one or two layers of FRP sheets glued to the tension face or both tension and compression faces of the slab. Both glass and carbon FRP sheets were considered. Technique II applies externally bonded FRP sheets using either the first or second scheme combined with installing steel bolts through holes across the slab thickness around the column. Based on the test results, it is concluded that the presence of FRP sheets and steel bolts substantially increased the punching capacity of the connections. Code design expressions were conservative in predicting the experimental results.     

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.     

Strengthening of Interior Slab–Column Connections Using a Combination of FRP Sheets and Steel Bolts

The results of an experimental investigation undertaken to evaluate a new technique for strengthening interior slab–column connection in combined flexural and shear modes are presented. The technique consists of using a combination of shear bolts inserted into holes and prestressed against the concrete surface for improving the punching shear capacity, and external [fiber-reinforced polymer (FRP)] reinforcement bonded to the tension face of the slabs in two perpendicular directions for increasing the flexural strength of the slabs. Square slab specimens of 670×670?mm dimensions were tested and the main test variables included the ratio of steel reinforcement (1.0 and 1.5%), span–depth ratio or thickness (55 and 75?mm) of the slabs, area, and configuration of steel bolts, and area of FRP reinforcement. It was found that the use of shear bolts alone improves the punching shear strength and increases the ductility of failure by changing the failure mode from punching to flexural. However, the use of a combination of shear bolts and a moderate amount of FRP reinforcement increased the flexural strength and resulted in a substantial improvement of the punching shear capacity of the slabs. The corresponding increases attained levels between 34 and 77%. A design approach is presented for evaluating the ultimate capacity of the slab–column connections when strengthened using the proposed strengthening technique. Strength results predicted by the proposed approach were in good agreement with the experimental results.     

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.     

CFRP Strengthening for Punching Shear of Interior Slab–Column Connections

This paper presents the results of an experimental investigation studying the effect of retrofitting interior slab–column connections against punching shear failure with externally bonded carbon fiber reinforced polymer (CFRP) strips. Six full-scale, 2000?mm-square×150-mm-thick slab specimens were constructed. The effect of varying the CFRP strengthening amount and configuration on the load-carrying capacity of the slab specimens was investigated. Specimens were supported along their edges and tested to failure. Strengthened slabs showed an increase in stiffness between 29 and 60% and in punching capacity between 6 and 16% with respect to the control unstrengthened slab. An analytical model was refined to predict the punching shear capacity of the specimens strengthened with CFRP strips. The model takes into account both the configuration and amount of CFRP strips. The proposed model shows good agreement with the experimental results.     

Composite T-Beams Using Reduced-Scale Rectangular FRP Tubes and Concrete Slabs

A composite system consisting of rectangular glass fiber reinforced polymer (GFRP) tubes connected to concrete slabs, using GFRP dowels has been developed. Seven beam specimens have been tested, including hollow and concrete-filled GFRP tubes with and without concrete slabs. Beam–slab specimens had two different shear span-to-depth ratios and one specimen had carbon–fiber reinforced polymer (CFRP)-laminated tension flange for enhanced flexural performance. Additionally, three double-shear GFRP tube-slab assemblies have been tested to assess the shear behavior of GFRP dowels, in both hollow and concrete-filled tubes. Three compression stubs of concrete-filled tubes were also tested by loading them parallel to the cross-section plane, to study GFRP web buckling behavior. The study showed that GFRP dowels performed well in shear and that composite action is quite feasible. While hollow tubes can act compositely with concrete slabs, more slip between the tube and slab would occur, compared to a concrete-filled tube-slab system. Simplified models are proposed to predict critical web buckling load of fiber reinforced polymer (FRP) tubes. Based on the models, a critical shear span-to-depth ratio of 4 was determined, below which web buckling may occur before flexural failure.     

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 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.     

Concrete Slabs Reinforced with FRP Grids. II: Punching Resistance

The use of fiber-reinforced polymer (FRP) grid reinforcement for concrete slabs has been investigated, considering the behavior of the slabs in one-way bending and under concentrated loading. The behavior under the latter loading type will be considered in this second part of a two-part paper. From the performed punching tests and the analysis, a fairly strong interaction between shear and flexural effects was noted for most of the tested slabs. For the FRP-reinforced slabs with an increased reinforcement ratio or an increased slab depth (needed to fulfill the serviceability criteria in bending), the punching strength was similar to or higher than the tested steel-reinforced reference slabs. For most slabs, slip of the bars occurred resulting in higher deflections at failure. The calculation of the punching failure load according to empirical-based models (from different codes), a modified mechanical model, and an analytical model is evaluated.     

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.     

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.     

Behavior of FRP-RC Slabs under Multiple Independent Air Blasts

In some terrorist attacks, it is possible that RC structures might be subjected to more than a single explosion. RC structures designed without the consideration of blast effects tend to lose their capacity after the first explosion. The use of a fiber reinforced polymer (FRP) sheet has been proven to enhance the performance and resistance of an RC member under a single explosion test. However, there appears to have been no experimental programs conducted to assess the performance of FRP-strengthened RC members subjected to multiple explosions reported in the literature. This paper, therefore, presents experimental results for the behavior of RC slabs strengthened by an FRP sheet after undergoing single, double, and triple independent explosion testing. Results from these blast tests indicate that the FRP sandwich RC slab tested was able to sustain the subsequent second explosion of greater impact. A brittle shear failure with FRP debonding was observed following the third explosion on this FRP-strengthened RC slab.     

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.     

Innovative Hybrid Reinforcement for Flexural Members

The capacity and ductility of RC slabs are affected by the use of fiber-reinforced polymer (FRP) as their main reinforcement for repair and rehabilitation. The balance between increasing the capacity and reducing the ductility of flexural members reinforced with FRP was always a matter of discussion. In this research, innovative hybrid reinforcement system (HRS) was introduced to provide the required increase in capacity while keeping the ductility within an acceptable range. Ten RC one-way slabs were tested in this investigation. They included a control slab which was reinforced with ordinary steel, while the rest of the slabs were reinforced internally with HRS with nine different profiles. The main variables considered in this study were the type of core or perimeter reinforcement and the number of perimeter reinforcing layers. It was revealed that the use of the innovative HRS resulted in remarkable increases in section ultimate load capacity as well as ductility.     

Concrete Contribution to the Shear Resistance of Fiber Reinforced Polymer Reinforced Concrete Members

Seven beams were tested in bending to determine the concrete contribution to their shear resistance. The beams had similar dimensions and concrete strength and were reinforced with carbon fiber reinforced polymer bars for flexure without transverse reinforcement. They were designed to fail in shear rather than flexure. The test variables were the shear span to depth ratio, varying from 1.82 to 4.5, and the flexural reinforcement ratio, varying from 1.1 to 3.88 times the balanced strain ratio. The test results are analyzed and compared with the corresponding predicted values using the American Concrete Institute, the Canadian Standard, and the Japan Society of Civil Engineers (JSCF) fiber reinforced polymer design recommendations. Based on these results and previous experimental data, it is shown that the ACI recommendations are extremely conservative whereas the Canadian and JSCE recommendations, albeit still conservative, are in closer agreement with the experimental data. Overall the Canadian Standard’s predictions are in better agreement with experimental data than the JSCE predictions.     

Torsional Capacity of CFRP Strengthened Reinforced Concrete Beams

Many buildings and bridge elements are subjected to significant torsional moments that affect the design, and may require strengthening. Fiber-reinforced polymer (FRP) has shown great promise as a state-of-the-art material in flexural and shear strengthening as external reinforcement, but information on its applicability in torsional strengthening is limited. Furthermore, available design tools are sparse and unproven. This paper briefly recounts the experimental work in an overall investigation of torsional strengthening of solid and box-section reinforced concrete beams with externally bonded carbon fiber-reinforced polymer (CFRP). A database of previous experimental research available in literature was compiled and compared against fib Bulletin 14. Modifications consistent with the space truss model were proposed to correct the poor accuracy in predictions of CFRP contribution to strength. Subsequently, a design tool to analyze the full torsional capacity of strengthened reinforced concrete beams was validated against the experimental database.     

Numerical Cracking and Debonding Analysis of RC Beams Reinforced with FRP Sheet

Bonding a fiber reinforced polymer (FRP) sheet to the tension-side surface of reinforced concrete (RC) structures is often performed to upgrade the flexural capacity and stiffness. Except for upper concrete crushing, FRP sheet reinforcing RC structure may fail in sheet rupture, sheet peeloff failure due to opening of a critical diagonal crack, or concrete cover delamination failure from the sheet end. Accompanying the occurrence of these failure modes, reinforcing effects of the FRP sheet will be lost and load-carrying capacity of the RC structures will be decreased suddenly. This study is devoted to developing a numerical analysis method by using a three-dimensional elasto-plastic finite element method to simulate the load-carrying capacity of RC beams failed in the FRP sheet peeloff mode. Here, the discrete crack approach was employed to consider geometrical discontinuities such as opening of cracks, slipping of rebar, and debonding of the FRP sheet. Comparisons between analytical and experimental results confirm that the proposed numerical analysis method is appropriate for estimating the load-carrying capacity and failure behavior of RC beams flexurally reinforced with a FRP sheet.     

Critical Review of Deflection Formulas for FRP-RC Members

The design of fiber-reinforced polymer reinforced concrete (FRP-RC) is typically governed by serviceability limit state requirements rather than ultimate limit state requirements as conventional reinforced concrete is. Thus, a method is needed that can predict the expected service load deflections of fiber-reinforced polymer (FRP) reinforced members with a reasonably high degree of accuracy. Nine methods of deflection calculation, including methods used in ACI 440.1R-03, and a proposed new formula in the next issue of this design guide, CSA S806-02 and ISIS M03-01, are compared to the experimental deflection of 197 beams and slabs tested by other investigators. These members are reinforced with aramid FRP, glass FRP, or carbon FRP bars, have different reinforcement ratios, geometric and material properties. All members were tested under monotonically applied load in four point bending configuration. The objective of the analysis in this paper is to determine a method of deflection calculation for FRP RC members, which is the most suitable for serviceability criteria. The analysis revealed that both the modulus of elasticity of FRP and the relative reinforcement ratio play an important role in the accuracy of the formulas.