Behavior of Concrete Beams Strengthened with Fiber-Reinforced Polymer Laminates under Impact Loading

Most of the research on application of composite materials in civil engineering during the past decade has concentrated on the behavior of structural elements under static loads. In engineering practice, there are many situations in which structures undergo impact or dynamic loading. In particular, the impact response of concrete beams strengthened with composite materials is of interest. This paper presents the results of an experimental investigation conducted to study the impact effects on concrete beams strengthened with fiber-reinforced polymer laminates. Two types of composite laminates, carbon and Kevlar, were bonded to the top and bottom faces of concrete beams with epoxy. Five beams were tested: two strengthened with Kevlar laminates, two strengthened with carbon laminates, and one unretrofitted beam as the control specimen. The impact load was applied by dropping a steel cylinder from a specified height onto the top face of the beam. The test results revealed that composite laminates significantly increased the capacity of the concrete beams to resist impact load. In addition, the laminates reduced the deflection and crack width. Comparing the test results of the beams strengthened with Kevlar and carbon laminates indicated that the gain in strength depends on the type, thickness, weight, and material properties of the composite laminate.     

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.     

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 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.     

Fatigue Behavior of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Plastic Laminates

Although there has been growing interest and field applications of poststrengthening concrete structures using carbon fiber reinforced plastic (CFRP) laminates, very little information exists regarding the flexural fatigue behavior of reinforced concrete beams strengthened with CFRP. This paper presents the results of an investigation into the fatigue behavior of reinforced concrete beams poststrengthened with CFRP laminates. The results of twenty 3 m and six 5 m beams loaded monotonically and cyclically to failure are discussed. Comparisons are made between beams without and with CFRP strengthening. The effect on fatigue life of increasing the amount of CFRP used to strengthen the beams is also examined.     

Strength and Ductility of Concrete Beams Reinforced with Carbon Fiber-Reinforced Polymer Plates and Steel

Seven concrete beams reinforced internally with varying amounts of steel and externally with precured carbon fiber-reinforced polymer (FRP) plates applied after the concrete had cracked under service loads were tested under four-point bending. Strains measured along the beam depth allowed computation of the beam curvature in the constant moment region. Results show that FRP is very effective for flexural strengthening. As the amount of steel increases, the additional strength provided by the carbon FRP plates decreases. Compared to a beam reinforced heavily with steel only, beams reinforced with both steel and carbon have adequate deformation capacity, in spite of their brittle mode of failure. Clamping or wrapping of the ends of the precured FRP plate enhances the capacity of adhesively bonded FRP anchorage. Design equations for anchorage, allowable stress, ductility, and amount of reinforcement are discussed.     

Rehabilitation of Cracked and Corroded Reinforced Concrete Beams with Fiber-Reinforced Plastic Patches

The behavior under static loading of fiber-reinforced plastic (FRP) retrofitted reinforced concrete beams, possessing a high chloride content and rebar corrosion, was studied both experimentally and analytically. The test beams were characterized as falling into three different groups according to the state of their corrosion damage: (1) natural corrosion, (2) cathodic protection, and (3) accelerated corrosion. The load carrying capacities of the beams, with or without FRP patching, were tested in the laboratory. The experimental results show that the state of corrosion of the steel, the water/cement ratio of the concrete material, and the arrangement and the number of FRP patches all affect the strength as well as the failure mechanisms of retrofitted RC beams. Some simple analytical models and a design concept for retrofitting cracked and corroded RC beams with FRP sheets are also presented and discussed.     

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.     

Flexural Response of Reinforced Concrete Beams Strengthened with End-Anchored Partially Bonded Carbon Fiber-Reinforced Polymer Strips

This paper presents the results of experimental and analytical studies carried out to investigate the flexural behavior of reinforced concrete beams strengthened with end-anchored partially bonded carbon fiber-reinforced polymer (CFRP) strips. A total of six beams, each 2400 mm long, 150 mm wide, and 250 mm deep with a tension steel reinforcement ratio of 1.18%, were tested. One beam was left unstrengthened as the control, another beam was strengthened with a fully bonded CFRP strip, and the remaining four beams were strengthened with partially bonded CFRP strips placed on the tension face of the beam and fixed at both ends using a mechanical anchor. The influence of varying the CFRP unbonded length (250 mm, 750 mm, 2×500 mm, and 1,250 mm) on the beam flexural response was studied. The experimental results revealed that end-anchored partially bonded CFRP strips significantly enhanced the ultimate capacity of the control beam and performed better than the fully bonded strip with no end-anchorage. This observation stresses the importance of end-anchorage in such strengthening schemes, especially considering that the end-anchored partially bonded CFRP strengthened beams showed similar flexural behavior trends. Finally, an inelastic section analysis procedure that takes into consideration the incompatibility of strains was developed to verify the obtained test results. The analysis produced good predictions of the experimental results in terms of the moment-curvature response and showed the effect of CFRP unbonded length on the strain of the internal tension steel.     

Long-Term Deflection and Cracking Behavior of Concrete Beams Prestressed with Carbon Fiber-Reinforced Polymer Tendons

This paper discusses the experimental result on the long-term deflection and cracking behavior of concrete beams prestressed with carbon fiber-reinforced polymer (CFRP) tendons, under sustained long-term service load, including cracked and uncracked sections. Six full-scale beams were cast and tested. The experimental parameters included the level of prestress, the level of sustained service loading, and concrete strengths. The experimental results showed that the performance of concrete beams prestressed with CFRP tendons meets the serviceability criteria in terms of deflection and cracking. The test results also showed that the long-term performance of concrete beams prestressed with CFRP tendons was comparable to those prestressed with steel tendons. Furthermore, the test results showed that with the increase of concrete strength, the serviceability performance also improved with concrete beams prestressed with CFRP tendons.     

Ductile Design of Reinforced Concrete Beams Retrofitted with Fiber Reinforced Polymer Plates

For reinforced concrete beams retrofitted with fiber-reinforced polymer (FRP) plates, an analytical method is derived for determining the allowable plate area to achieve a targeted value of ductility. Nonlinear models for concrete and reinforcement are applied, and the effects of concrete confinement and spalling and of FRP plate rupture are considered. The derivation of equilibrium and compatibility equations for a rectangular cross section is presented, and the solution to the nonlinear equation for determining the allowable plate area is demonstrated with examples. Analytical results are compared with numerical and experimental data reported in the literature. Subsequently a simplified version of the method is derived, based on regression analysis, to relate the curvature ductility to the FRP plate ratio. It is noted that additional conditions need to be checked to ensure ductile performance, such as local failure of the concrete layer between tension reinforcement and FRP plate or debonding of the plate itself.     

Upgrading Torsional Resistance of Reinforced Concrete Beams Using Fiber-Reinforced Polymer

An experimental investigation is conducted on the improvement of the torsional resistance of reinforced concrete beams using fiber-reinforced polymer (FRP) fabric. A total of 11 beams were tested. Three beams were designated as control specimens and eight beams were strengthened by FRP wrapping of different configuration and then tested. Both glass and carbon fibers were used in the torsional resistance upgrade. Different wrapping designs were evaluated. The reinforced concrete beams were subjected to pure torsional moments. The load, twist angle of the beam, and strains were recorded. Improving the torsional resistance of reinforced concrete beams using FRP was demonstrated to be viable. The effectiveness of various wrapping configurations indicated that the fully wrapped beams performed better than using strips. The 45° orientation of the fibers ensures that the material is efficiently utilized.     

Effect of Fiber-Reinforced Polymer Wraps on Corrosion Activity and Concrete Cracking in Chloride-Contaminated Concrete Cylinders

This paper presents the results of an experimental study designed to investigate the effect of fiber-reinforced polymer (FRP) wraps on corrosion activity and concrete cracking in chloride-contaminated concrete cylinders. Thirty-five concrete cylinders, each having 102?mm diameter and 204?mm height, concentrically reinforced with one steel reinforcing bar, were subjected to accelerated corrosion exposure for 80?days. Test parameters included level of applied potential, presence of FRP wraps, and bar diameter. The corresponding current and concrete expansion were continuously monitored throughout the corrosion exposure. At the end of the test, the steel bars were extracted, cleaned of rust, and weighed to determine the actual steel mass loss. The results showed that, for the same applied fixed potential, FRP wraps effectively reduced the corresponding current, the concrete expansion, and the steel mass loss. For the same applied potential, the current density increased as the bar diameter decreased. For the same corrosion depth, the circumferential expansion of the cylinder caused by corrosion decreased as the concrete cover-to-bar diameter ratio (c/d) increased.     

Carbon-Fiber-Reinforced Polymer Repair to Extend Service Life of Corroded Reinforced Concrete Beams

This paper presents the results of an experimental study designed to investigate the viability of using externally bonded carbon-fiber-reinforced polymer (CFRP) laminates to extend the service life of corroded reinforced concrete (RC) beams. A total of 14 beams, 152×254×3,200?mm each, were tested. Three beams were not corroded; two of them were strengthened by CFRP laminates, while one specimen was kept as a virgin. The remaining 11 beams were subjected to different levels of corrosion damage up to a 31% steel mass loss using an impressed current technique. Six of the corroded beams were repaired with CFRP laminates, whereas the remaining five beams were not repaired. Eventually, all specimens were tested to failure under four-point bending. Corrosion of the steel reinforcement significantly reduced the load-carrying capacity of RC beams. At all levels of corrosion damage, CFRP repair increased the ultimate strengths of the corroded beams to levels higher than the strength of the virgin beam but significantly reduced the deflection capacity.     

Energy Release in Fiber-Reinforced Plastic Reinforced Concrete Beams

A set of 30 concrete beams reinforced with carbon/epoxy FRP (fiber-reinforced plastic) and four reinforced with comparable size steel rebars were subjected to static bending tests. Adequate bond between the FRP and the concrete was obtained, due to the use of carbon fiber overwrap on the smooth pultruded FRP rods. With adequate bond, the large strain to failure (>2%) of the FRP determines the ductility and failure mode of the FRP reinforced beams. An analytical evaluation of the fracture energy in these experiments shows that there is ductility due to the large fraction of the total strain energy that is absorbed in the concrete, because of the formation of distributed cracking. Variations in overwrap configuration, addition of steel stirrups, addition of polypropylene fibers, and comparison with four beams reinforced with equivalent steel reinforcement were also analyzed.     

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.     

Corrosion of Steel Reinforcement in Carbon Fiber-Reinforced Polymer Wrapped Concrete Cylinders

The rehabilitation, repair, and strengthening of concrete structures has increased worldwide with a growing number of systems employing externally applied fiber-reinforced polymer (FRP) composites. However, the service life and effectiveness of FRP repair and strengthening techniques when applied to concrete in corrosive marine environments is still not well understood. This paper presents the results of an experimental study on the corrosion performance of embedded steel reinforcement in cylindrical reinforced concrete specimens with 13 different surface treatment options. Samples were subjected to an impressed current and a high salinity solution. Test variables included the type of epoxy, wrap fiber orientation, and the number of wrap layers. Samples were evaluated for corrosion activity by monitoring corrosion potentials and impressed current flow levels, and by examining reinforcement mass loss and concrete chloride content among samples. Test results indicated that FRP wrapped specimens had prolonged test life, decreased reinforcement mass loss, and reduced concrete chloride content. The performance of wrapped specimens was superior to that of either control samples or those coated only with epoxy. Epoxy type had a significant effect on the performance of samples regarding their resistance to corrosion. It was concluded that carbon FRP wraps were able to confine concrete, slowing deterioration from cracking and spalling and inhibiting the passage of salt water.     

Torsional Strengthening of Spandrel Beams with Fiber-Reinforced Polymer Laminates

Research has shown that fiber-reinforced polymer (FRP) composites can increase flexural, axial, and shear capacity of beams, columns, and walls. The present paper describes both experimental and analytical programs focused on the torsional strengthening of reinforced concrete spandrel beams using composite laminates. The variables considered in this study included fiber orientation, composite laminate, and effects of a laminate anchoring system. The study proved that the FRP laminates could increase the torsional capacity of concrete beams by more than 70%. The analytical procedure developed revealed a good comparison between experimental and analytical 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.     

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.