Flexural Strength of RC Beams with GFRP Laminates

The results of an experimental and numerical study of the flexural behavior of reinforced concrete beams strengthened with glass-fiber-reinforced-polymer (GFRP) laminates are presented in this paper. In the experimental program, ten strengthened beams and two unstrengthened beams are tested to failure under monotonic loading. A number of external GFRP laminate layers and bond length of GFRP laminates in shear span are taken as the test variables. Longitudinal GFRP strain development and interfacial shear stress distribution from the tests are examined. The experimental results generally showed that both flexural strength and stiffness of reinforced concrete beams could be increased by such a bonding technique. In the numerical study, an eight-node interface element is developed to simulate the interface behavior between the concrete and GFRP laminates. This element is implemented into the MARC software package for the finite-element analyses of GFRP laminate strengthened reinforced concrete beams. Reasonably good correlations between experimental and numerical results are achieved.     

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.     

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Using Nonmetallic Anchor Systems

This paper presents the flexural behavior of reinforced concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets using nonmetallic anchor systems. The developed nonmetallic anchor systems replace the permanent steel anchorage. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The flexural behavior of the tested beams, including detailed failure modes of each nonmetallic anchor system, is investigated. The study shows that the developed nonmetallic anchors are more effective in resisting peeling-off cracks compared to the permanent steel anchors and the beams strengthened with the nonmetallic anchors provide comparable load-carrying capacity with respect to the steel anchored control beam.     

Flexural Strengthening of Reinforced Concrete Flanged Beams with Composite Laminates

Bonding composite laminates to the tension face can effectively increase the flexural strength of the reinforced-concrete flanged beams. In comparison to rectangular concrete beams, the flange provides a larger area of concrete to resist compression stresses, and considering the role of the composite in resisting tensile stresses, its addition to flanged beams can efficiently upgrade the flexural capacity. Failure of the strengthened beam may result from crushing of concrete or rupture of the plate; however, the beam must be properly detailed to avoid local shear failure at the plate cut-off point. In this paper, equations required for strengthening of the flanged beams for gravity loads will be presented. The equations have been developed based on load and resistance factor design, and have been verified through a comparison with available experimental results. Close agreement with the experimental results indicates the accuracy of the equations. Terms, definitions, and notations compatible to ordinary reinforced-concrete design codes have been used to facilitate the application of the equations.     

Flexural Strengthening of RC Beams with Externally Bonded CFRP Systems: Test Results and 3D Nonlinear FE Analysis

This paper presents experimental results and a numerical analysis of the reinforced concrete (RC) beams strengthened in flexure with various externally bonded carbon fiber-reinforced polymer (CFRP) configurations. The aim of the experimental work was to investigate the parameters that may delay the intermediate crack debonding of the bottom CFRP laminate, and increase the load carrying capacity and CFRP strength utilization ratio. Ten rectangular RC specimens with a clear span of 4.2?m, categorized in two series, were tested to evaluate the effect of using the additional U-shaped CFRP systems on the intermediate crack debonding of the bottom laminate. Two different configurations of the additional systems were proposed, namely, continuous U-shaped wet layup sheets and spaced side-bonded CFRP L-shaped laminates. The fiber orientation effect of the side-bonded sheets was also investigated. A numerical analysis using an incremental nonlinear displacement-controlled 3D finite-element (FE) model was developed to investigate the flexural and CFRP/concrete interfacial responses of the tested beams. The finite-element model accounts for the orthotropic behavior of the CFRP laminates. An appropriate bond-slip model was adopted to characterize the behavior of the CFRP/concrete interface. Comparisons between the FE predictions and experimental results show very good agreement in terms of the load-deflection and load-strain relationships, ultimate capacities, and failure modes of the beams.     

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.     

Flexural Strengthening of Steel Bridges with High Modulus CFRP Strips

Acceptance of carbon fiber-reinforced polymer (CFRP) materials for strengthening concrete structures, together with the recent availability of higher modulus CFRP strips, has resulted in the possibility to also strengthen steel structures. Steel bridge girders and building frames may require strengthening due to corrosion induced cross-section losses or changes in use. An experimental study investigating the feasibility of different strengthening approaches was conducted. Large-scale steel-concrete composite beams, typical of bridge structures, were used to consider the effect of CFRP modulus, prestressing of the CFRP strips, and splicing finite lengths of CFRP strips. All of the techniques examined were effective in utilizing the full capacity of the CFRP material, and increasing the elastic stiffness and ultimate strength of the beams. Results of the experimental program were compared to an analytical model that requires only the beam geometry and the constitutive properties of the CFRP, steel, and concrete. This model was used to investigate the importance of several key parameters. Finally, an approach for design is proposed that considers the bilinear behavior of a typical strengthened beam to the elastic-plastic behavior of the same beam before strengthening.     

Effect of Carbon-Fiber-Reinforced Polymer Laminate Configuration on the Behavior of Strengthened Reinforced Concrete Beams

This paper presents test results of 18 small-scale reinforced concrete specimens of strengthened beams using carbon-fiber-reinforced polymer (CFRP) composites. The specimens were instrumented with strain gauges in a region where cracks in the concrete were preformed to monitor the variation of strains throughout testing. Results indicate that there can be a very large variation in the measured strains in the composites depending, not only on the location of the cracks, but also on the configuration used to bond the composites to the surface of the elements. The interface shear stresses generated at failure of the beams are compared with two existing analytical models. Additionally, the stress level in the composites was determined for all the strengthened specimens from the experimental data. The calculated stress in the composites reached between 20 and 43% of the CFRP rupture stress. The information presented in this paper provides information that can be used to validate or modify current design procedures of strengthened beams using composites.     

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.     

Flexural Strengthening of RC Beams with Cement-Based Composites

In this paper, the effectiveness of fiber-reinforced cementitious matrix (FRCM) materials for the strengthening of reinforced concrete (RC) beams is experimentally investigated. Bending tests on RC beams strengthened with different FRCM materials, made out of (1)?carbon fiber nets; and (2)?poliparafenilenbenzobisoxazole (PBO) fiber nets embedded in cement-based matrix, are performed. For case (2), different net shapes, cementitious matrices, and a number of net layers were considered. Depending on the type of fibers and matrix, different flexural debonding failure modes are identified. The fiber strain at debonding is evaluated by comparing the experimental results with those obtained with two different theoretical models. The results obtained in this study confirm the effectiveness of FRCM materials for the strengthening of RC structures and encourage further experimental and theoretical work on the topic. A better understanding of the debonding phenomenon is crucial for an optimal design of the strengthening material. The way in which the nature of fibers and matrices and the number of layers control the performance of the strengthened members is also investigated in the present paper.     

Flexural Behavior of Concrete Beams Reinforced with Hybrid (GFRP and Steel) Bars

Reinforcing concrete with a combination of steel and glass fiber-reinforced polymer (GFRP) bars promises favorable strength, serviceability, and durability. To verify its promise and to support design of concrete structures with this hybrid type of reinforcement, we have experimentally and theoretically investigated the load-deflection behavior of concrete beams reinforced with hybrid GFRP and steel bars. Eight beams, including two control beams reinforced with only steel or only GFRP bars, were tested. The amount of reinforcement and the ratio of GFRP to steel were the main parameters investigated. Hybrid GFRP/steel-reinforced concrete beams with normal effective reinforcement ratios exhibited good ductility, serviceability, and load carrying capacity. Comparisons between the experimental results and the predictions from theoretical analysis showed that the models we adopted could adequately predict the load carrying capacity, deflection, and crack width of hybrid GFRP/steel-reinforced concrete beams.     

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.     

Assessing the Strengthening Effect of Various Near-Surface-Mounted FRP Reinforcements on Concrete Bridge Slab Overhangs

The near-surface-mounted (NSM) method has proved to be a reliable alternative to the existing externally bonded (EB) method for the repair and strengthening of concrete structures using fiber-reinforced polymer (FRP) composites. This technique is especially advantageous in bridge deck upgrades for larger barriers and slab overhang strengthening. This paper presents the results of a comparison of the flexural behavior of bridge slab overhangs strengthened in negative bending moment regions with various types of NSM reinforcement that differ in surface condition (e.g., textured and sand coated), cross-sectional shape (e.g., round and square), material type (carbon and glass), and prestressing effect. Eleven full-scale overhang specimens (1.524 m long in overhang and 0.914 m wide) were tested under a cantilever condition. Test results showed that the FRP NSM reinforcements were effective in increasing both yield and ultimate strength of predamaged slab overhangs. All surface treatments were more beneficial than the smooth condition, and the square-shaped reinforcement displayed better performance than the round shape. The prestressing unit developed in this study is simple to apply and could be further explored for field applications.     

Numerical Investigation of Fiber Reinforced Polymers Poststrengthened Concrete Slabs

The present work deals with the numerical simulation of fiber reinforced polymers (FRP) poststrengthened scaled concrete slabs in order to predict their load carrying behavior. The used strengthening materials are FRPs which are of increasing interest in civil engineering applications such as textile reinforced concrete tubes, cables of cable-stayed bridges, or even entire bridges. The numerical results are compared with experiments which were conducted at the Univ. of California, San Diego, and are described in detail by H?rmann in 1997; H?rmann et al. in 1998; and later by Seim et al. in 2001. The slabs are modeled for the numerical simulation first in a two-dimensional design space, assuming a plane stress condition for the concrete and the fiber reinforced polymer, and second in a three-dimensional design space with multilayered shell elements in order to include the varying response across the depth of the slabs. The used material model for reinforced concrete was developed by Menrath et al. in 1998 and has been enhanced for multilayered shell elements by Haufe et al. in 1999. It is based on multisurface plasticity, consisting of two Drucker–Prager yield surfaces and a spherical cap, and exhibits fracture energy based evolution laws for the softening regime. The reinforcement is taken into account as additional stress contribution in a smeared manner using a one-dimensional constitutive law based on an elastoplastic isotropic hardening model.     

Structural Performance of CFRP-Strengthened RC Slabs in a Corrosive Environment

This study was undertaken to address the effect of the main steel corrosion on the structural performance of RC slabs strengthened with carbon-fiber-reinforced polymer (CFRP) strips and exposed to a corrosive environment. A total of eight specimens (500×100×1,500?mm) were constructed and tested under monotonic static loading. Three specimens were CFRP-strengthened and corroded, three specimens were CFRP-strengthened and kept at room temperature, one specimen was unstrengthened and corroded, and one specimen was neither strengthened nor corroded. Three different strengthening schemes were applied: (1) externally bonded CFRP strips; (2) externally bonded CFRP strips provided with CFRP anchors; and (3) near surface mounted (NSM) CFRP strips. During the corrosion process, the specimens were placed in a small tank filled with sodium chloride (NaCl) solution concentration (3%) which covered only the slabs’ bottom third, and corrosion was induced by means of an impressed current. The corrosion process lasted for 20 days, and the average mass loss of the main steel reinforcement due to corrosion was 9%. Following corrosion, the specimens were tested under four-point bending. The experimental results showed that the increase in flexural capacity achieved using the three strengthening schemes were significantly reduced due to corrosion of the main steel. The recorded reductions in flexural strength gains for the CFRP-strengthened corroded slabs relative to the gains for the strengthened uncorroded slabs were about 55, 38, and 41% for the externally bonded CFRP system without anchors, externally bonded CFRP with anchors, and NSM-CFRP system, respectively.     

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Development of Nonmetallic Anchor Systems

This paper presents a novel anchoring technique for strengthening reinforced concrete beams with prestressed carbon fiber- reinforced polymer (CFRP) sheets. Permanent steel anchors are commonly used for the application of prestressed CFRP sheets. The steel anchors are, however, susceptible to corrosion and may not blend into the aesthetics of the original structure. As a result, it may be preferable to remove the steel anchors after transferring the required prestress to the structure with minimal losses of sustained prestress. A technique for replacing the steel anchors with nonmetallic anchors is investigated and reported herein. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The developed nonmetallic anchorages successfully transfer the sustained prestress in the CFRP sheets with insignificant prestress losses. A closed-form solution for the transfer of prestress is developed and compared to the experimental results.     

Flexural Behavior and Deformability of Fiber Reinforced Polymer Prestressed Concrete Beams

After a brief review of the ductility and deformability indices currently used in the design of concrete beams reinforced or prestressed with steel or fiber reinforced polymer (FRP) tendons, a new definition of a deformability index (factor) for prestressed concrete beams is proposed. The new factor is defined in terms of both a deflection factor and a strength factor. The deflection factor is the ratio of the deflection at failure to the deflection at first cracking, while the strength factor is the ratio of the ultimate moment (or load) to the cracking moment (or load). The proposed deformability factor is verified not only by test results obtained by the writer, but also by other test results available in the literature and it appears to be a suitable measurement of the deformability of concrete beams prestressed with either FRP tendons or steel tendons.     

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.     

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.     

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.