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.     

Behavior of FRP-Retrofitted Joints Built with Plain Bars and Low-Strength Concrete

Two series of tests on eight full-scale exterior beam-column joint subassemblages built with plain bars and low-strength concrete were conducted. No transverse reinforcement was present in the joint cores. In the first series of tests, which included three specimens, the behavior of joints before fiber-reinforced polymer (FRP) retrofitting was investigated. In the second series, which included five specimens, the behavior of the FRP-retrofitted joints was investigated. The six specimens consisted of a column, an in-plane beam, a transverse beam, and a slab part, and two specimens were plane members without transverse beams and slabs. The utilized retrofitting scheme is easily applicable for actual exterior beam-column joints, even in the presence of a transverse beam and a slab. Two types of strength limitation were observed for specimens in the first series. The strength of the specimen with beam longitudinal bars sufficiently anchored to the joint core was limited by the shear strength of the joint. The strengths of the other two specimens were limited by the slip of the beams’ longitudinal bars at their anchorages. In the second series of tests, significantly better performance was obtained both in terms of shear strength and ductility, provided that the slip of the beam bars was prevented. Furthermore, by using a simple theoretical algorithm based on truss analogy, the strength and deformability characteristics of the tested reference and FRP-retrofitted joints are predicted with reasonable accuracy. The same algorithm is used for predicting the joint shear strength of specimens tested by other researchers, and satisfactory agreement is obtained between the predictions and test results.     

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.     

External Prestressed Carbon Fiber-Reinforced Polymer Straps for Shear Enhancement of Concrete

The use of fiber-reinforced polymers (FRPs) for the strengthening and repair of existing concrete structures is a field with tremendous potential. The materials are very durable and, hence, ideally suited for use as external reinforcement. Although extensive work has been carried out investigating the use of FRPs for flexural strengthening, a fairly recent development is the use of these materials for the shear strength enhancement of concrete. The current system investigates the use of posttensioned, nonlaminated, carbon fiber-reinforced polymer (CFRP) straps as external shear reinforcement for concrete. Experiments were carried out on an unstrengthened control beam and beams strengthened with external CFRP straps. It was found that the ultimate load capacity of the strengthened beams was significantly higher than that of the control specimen. Existing design codes and analysis methods were found to underestimate the ultimate resistance of the control specimen and the strengthened beams. Nevertheless, the modified compression field theory provided insight into possible failure mechanisms and the influence of the strap prestress level on the structural behavior. It is concluded that the use of these novel stressed elements could represent a viable and durable means of strengthening existing concrete infrastructure.     

Efficient CFRP Strap Configurations for the Shear Strengthening of Reinforced Concrete T-Beams

A prestressed carbon fiber-reinforced polymer (CFRP) strap retrofitting system has been found to significantly enhance the shear capacity of existing reinforced concrete beams. In previous studies, the CFRP straps were supported on metal pads placed on the top and bottom of a beam necessitating top surface access. The goal of the current work was to develop a system where the straps were installed from underneath a slab without compromising the strengthening efficiency. A series of T-beam experiments was conducted where the CFRP straps were inserted through holes that were drilled from below the flange, thereby avoiding the need for access to the top surface. The depth of penetration of the CFRP straps into the compression flange, the concrete strength, the CFRP strap spacing, the presence of holes in the compression flange, and the size of the loading pads were all found to affect the shear performance. Using the most successful installation technique, the resulting CFRP strengthened beam failed at a load that was approximately 50% higher than that of an unretrofitted control beam.     

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.     

Interaction between Steel Stirrups and Shear-Strengthening FRP Strips in RC Beams

RC beams shear strengthened with either fiber-reinforced polymer (FRP) U-jackets/U-strips or side strips commonly fail due to debonding of the bonded FRP shear reinforcement. As such debonding occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups may not have reached yielding. Consequently, the yield strength of internal steel stirrups in such a strengthened RC beam cannot be fully used. In this paper, a computational model for shear interaction between FRP strips and steel stirrups is first presented, in which a general parabolic crack shape function is employed to represent the widening process of a single major shear crack in an RC beam. In addition, appropriate bond-slip relationships are adopted to accurately depict the bond behavior of FRP strips and steel stirrups. Numerical results obtained using this computational model show that a substantial adverse effect of shear interaction generally exists between steel stirrups and FRP strips for RC beams shear strengthened with FRP side strips. For RC beams shear strengthened with FRP U-strips, shear interaction can still have a significant adverse effect when FRP strips with a high axial stiffness are used. Therefore, for accurate evaluation of the shear resistance of RC beams shear strengthened with FRP strips, this adverse effect of shear interaction should be properly considered in design.     

Time-Dependent Behavior of RC Beams Retrofitted with CFRP Straps

A retrofitting technique that uses prestressed unbonded carbon-fiber-reinforced polymer (CFRP) straps to provide additional shear capacity has previously been shown to be successful under short-term static loading conditions. The current study explores the longer-term behavior of this retrofitting technique through two experiments (a sustained load and a cyclic load experiment) and the development of a model based on the modified compression field theory. The experiments indicated that the strain in the CFRP straps changes with time due to changes in the load sharing with the concrete (caused by creep) and the steel stirrups (caused by yield of these elements). The predictive model was initially validated against static experimental results before being applied to the longer-term experiments. The model predicts the trends in behavior well although it is conservative in its estimates of strap strain. The model was then used to determine the influence of stirrup yielding, the load level before and after retrofitting, and the duration of loading on the CFRP strap strains. The initial results suggest that the largest increases in long-term strap strain will occur when the straps are installed early in the structure’s service life although further experimental validation is required.     

Fatigue Behavior of RC Beams Strengthened with GFRP Sheets

The objective of the presented study is to examine the effects of glass fiber reinforced polymer (GFRP) composite rehabilitation systems on the fatigue performance of reinforced concrete beams. Experiments were conducted on beams with and without GFRP composite sheets on their tensile surfaces. The specimens were 152 × 152 × 1,321 mm reinforced concrete beams with enough transverse reinforcement to avoid shear failure. The results of this study indicate that the fatigue life of reinforced concrete beams with the given geometry, subjected to the same cycling load, can be significantly extended through the use of externally bonded GFRP composite sheets. An interesting finding is that, although the fiber strengthening system increases the fatigue life of the beams, the failure mechanism, fatigue of the steel reinforcement, remains the same in both strengthened and nonstrengthened beams. Thus, it is possible to predict the fatigue life of a cyclically loaded beam using existing fatigue models.     

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

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

Effect of Partial Unbonding on Prestressed Near-Surface-Mounted CFRP-Strengthened Concrete T-Beams

The flexural behavior of RC T-beams strengthened with prestressed near-surface-mounted (NSM) carbon fiber-reinforced-polymer (CFRP) reinforcement was investigated. The specific objective was to study the effect of partial unbonding of the CFRP reinforcement on the beam flexural behavior to increase the deformability. A total of eight RC T-beams were tested under four-point monotonic loading. The main variables were the level of prestressing force in the CFRP bars and the unbonded length at the midspan of the beam. The test results showed that all of the prestressed strengthened beams effectively improved the ultimate load-carrying capacity and the serviceability performance compared to the unstrengthened beam. The partially bonded prestressed beams exhibited an enhancement of the deformability compared to the fully bonded beams while minimizing the reduction of the load-carrying capacity. Partial unbonding was more effective to improve the deformability at higher levels of prestressing force. The general behavior of the partially bonded beams was reasonably well predicted by an analytical model developed previously by the writers.     

Behavior of RC Beams Shear Strengthened with Bonded or Unbonded FRP Wraps

Reinforced concrete (RC) beams shear-strengthened with fiber-reinforced polymer (FRP) fully wrapped around the member usually fail due to rupture of FRP, commonly preceded by gradual debonding of the FRP from the beam sides. To gain a better understanding of the shear resistance mechanism of such beams, particularly the interaction between the FRP, concrete, and internal steel stirrups, nine beams were tested in the present study: three as control specimens, three with bonded FRP full wraps, and three with FRP full wraps left unbonded to the beam sides. The use of unbonded wraps was aimed at a reliable estimation of the FRP contribution to shear resistance of the beam and how bonding affects this contribution. The test results show that the unbonded FRP wraps have a slightly higher shear strength contribution than the bonded FRP wraps, and that for both types of FRP wraps, the strain distributions along the critical shear crack are close to parabolic at the ultimate state. FRP rupture of the strengthened beams occurred at a value of maximum FRP strain considerably lower than the rupture strain found from tensile tests of flat coupons, which may be attributed to the effects of the dynamic debonding process and deformation of the FRP wraps due to the relative movements between the two sides of the critical shear crack. Test results also suggest that while the internal steel stirrups are fully used at beam shear failure by FRP rupture, the contribution of the concrete to the shear capacity may be adversely affected at high values of tensile strain in FRP wraps.     

Plastic Deformations of Impulsively Loaded, Rigid-Plastic Beams

Rigid body dynamics is used to determine the deformation of a fixed-end, rigid-plastic beam subjected to uniformly distributed impulsive loading. The proposed solution methodology allows calculations of deformations at plastic hinges and can be used to establish rigid-plastic fracture criteria for rigid-plastic beams. Unlike previous solutions to this problem, rotary inertia and the shear deformations at the support are considered. The solution for beam deformations is described in three phases: shear, bending, and membrane. Each phase ends when the corresponding component of the strain rate vector vanishes. The initial shear phase is completed when the transverse shear velocity at the support vanishes. The beam then undergoes only rigid body rotation and axial stretching at plastic hinges in the bending phase. The bending phase ends when the angular velocity vanishes. In the membrane phase, the beam acts like a string until the transverse velocity vanishes. It has been found that beams subjected to low impulse velocity attain permanent deformation in the bending phase, while beams subjected to high impulse velocity reach permanent deformation in the membrane phase. The predictions of the beam deflections using the proposed methodology are within 15% of the experimental results.     

Mixed-Mode Fracture of Hybrid Material Bonded Interfaces under Four-Point Bending

A combined analytical and experimental approach is presented to characterize mixed-mode fracture of hybrid material bonded interfaces under four-point bending load, and closed-form solutions of compliance and energy release rate (ERR) of the mixed-mode fracture specimens are provided. The transverse shear deformations in each sublayer of bimaterial bonded beams are included by modeling the specimen as individual Timoshenko beams, and the effect of interface crack-tip deformation on the compliance and ERR are taken into account by applying the interface deformable bilayer beam theory (flexible-joint model). The higher accuracy of the present analytical solutions for both the compliance and ERR of mixed-mode fracture specimens is manifested by comparing them with the solutions predicted by the conventional beam theory (CBT) and finite-element analysis (FEA). As an application example, the fracture of wood–fiber-reinforced plastic (FRP) bonded interface is experimentally evaluated by using mixed-mode fracture specimens [i.e., four-point asymmetric end-notched flexure (4-AENF) and four-point mixed-mode bending (4-MMB)], and the corresponding values of critical ERRs are obtained. Comparisons of the compliance rate change and the resulting critical ERR based on the CBT, the present theoretical model, and FEA demonstrate that the crack-tip deformation plays an important role in accurately characterizing the mixed-mode fracture toughness of hybrid material bonded interfaces under four-point bending load.     

Concrete Beams Strengthened with Externally Bonded FRP Plates

The structural behavior of reinforced concrete beams strengthened with adhesively bonded fiber-reinforced plastics (FRP) is presented. The experimental work included flexural testing of 2.3-m-long concrete beams with bonded external reinforcements. The test variables included the amount of conventional (internal) reinforcement and also the type and amount of external reinforcement. For comparison, some of the beams were strengthened with bonded steel plates. Theoretical analyses included 2D nonlinear finite-element modeling incorporating a “damage” material model for concrete. In general there were reasonably good correlations between the experimental results and nonlinear finite-element models. It is suggested that the detachment of bonded external plates from the concrete, at ultimate loads, is governed by a limiting principal stress value at the concrete∕external plate interface.     

Full Torsional Behavior of RC Beams Wrapped with FRP: Analytical Model

Torsion failure is an undesirable brittle form of failure. Although previous experimental studies have shown that using fiber-reinforced polymer (FRP) sheets for torsion strengthening of reinforced concrete (RC) beams is an effective solution in many situations, very few analytical models are available for predicting the section capacity. None of these models predicted the full behavior of RC beams wrapped with FRP, account for the fact that the FRP is not bonded to all beam faces, or predicted the ultimate FRP strain using equations developed based on testing FRP strengthened beams in torsion. In this paper, an analytical model was developed for the case of the RC beams strengthened in torsion. The model is based on the basics of the modified compression field theory, the hollow tube analogy, and the compatibility at the corner of the cross section. Several modifications were implemented to be able to take into account the effect of various parameters including various strengthening schemes where the FRP is not bonded to all beam faces, FRP contribution, and different failure modes. The model showed good agreement with the experimental results. The model predicted the strength more accurately than a previous model, which will be discussed later. The model predicted the FRP strain and the failure mode.     

Flexural Performance of Carbon Fiber-Reinforced Polymer Prestressed Concrete Side-by-Side Box Beam Bridge

The effect of varying transverse posttensioning levels and arrangements on the load response of a one-half scale 30° skewed seven box beam bridge model was investigated. The effective span of the bridge model was 9.45?m (31?ft) with a width of 3.35?m (11?ft) and depth of 355.6?mm (14?in.). The bridge model was prestressed and reinforced with carbon fiber composite cables (CFCCs). CFCCs were also used as shear reinforcement. The bridge model was provided with five transverse diaphragms equally spaced along the length of the bridge. The experimental investigation included load and strain distribution tests and a flexural ultimate load test. The load and strain distribution tests were conducted on the bridge model with and without full-depth longitudinal cracks at the shear-key locations. The investigation showed that the application of an adequate transverse posttensioning force was successful in restoring the load distribution of the bridge model with full-depth longitudinal deck cracks to that of the case without deck cracks. The ultimate load and the associated compression-controlled failure mode of the bridge model agreed well with that predicted according to ACI 440.4R-04 and numerical analysis. The behavior of the bonded pretensioned and reinforced CFCC strands was linear elastic and remained intact throughout the collapse of the bridge model. The unbonded transverse posttensioned CFCC strand also remained intact.     

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.     

Intermediate Crack Debonding in FRP-Strengthened RC Beams: FE Analysis and Strength Model

Reinforced concrete (RC) beams strengthened in flexure with a bonded fiber-reinforced polymer (FRP) plate may fail by intermediate crack (IC) debonding, in which debonding initiates at a critical section in the high moment region and propagates to a plate end. This paper first presents a finite-element (FE) model based on the smeared crack approach for concrete for the numerical simulation of the IC debonding process. This finite-element model includes two novel features: (1) the interfacial behavior within the major flexural crack zone is differentiated from that outside this zone and (2) the effect of local slip concentrations near a flexural crack is captured using a dual local debonding criterion. The FE model is shown to be accurate through comparisons with the results of 42 beam tests. The paper also presents an accurate and simple strength model based on interfacial shear stress distributions from finite-element analyses. The new strength model is shown to be accurate through comparisons with the test results of 77 beams, including the 42 beams used in verifying the FE model, and is suitable for direct use in design.