Journal of Composites for Construction

This paper discusses the mechanical behavior of reinforced concrete columns wrapped with fiber-reinforced polymer (FRP) sheets. A numerical routine was developed to predict the behavior of the columns using a step-by-step technique. The routine is based on an existing model and was modified to account for confinement provided by the traditional steel as well as the external FRP wraps. Several empirical equations for the confined concrete were calibrated with results from experimental tests from different published papers. The most accurate equation was incorporated into the routine to predict the stress-strain relation of the column up to failure. A different confinement to the outer concrete cover and the inner core was used to account for the FRP wraps and the transverse steel. The model was calibrated with experimental results from different experiments on FRP-wrapped reinforced concrete columns.The model was taken one step further by using it to predict the behavior of reinforced concrete columns, with a combination of steel corrosion and CFRP wraps. The columns modeled were subjected to harsh corrosive environment over 44 months. The model successfully predicted the load deformation in both axial and circumferential directions in corroded and intact columns, both wrapped and unwrapped, with good accuracy. The analysis forms a solid foundation for accurate evaluation of the effect of corrosion and wrapping on reinforced concrete columns.

This paper presents test results of six specimens representing older bridge columns with inadequate reinforcement detailing consisting of short lap splices at the base and widely spaced transverse reinforcement. Four of these specimens were rehabilitated using fiber-reinforced polymer (FRP) jackets of two different composite materials (carbon and aramid) to avoid premature failure of the lapped bars after a limited number of postyield cycles. The test results indicate that thin FRP jackets can be used to avoid failure of short lap splices at moderate displacement ductilities. Displacement capacities consistent with expected demands in regions of moderate or low seismicity were achieved after jacket retrofitting. The hysteretic behavior of rehabilitated columns was assessed with emphasizing issues related to variation of stiffness and damping ratio as a function of ductility demand for this class of columns. Equations that account for the effect of axial load level on estimates of effective stiffness and damping as a function of displacement ductility are proposed for this class of columns.

In this paper, efficiency and effectiveness of carbon fiber-reinforced polymers (CFRP) in upgrading the shear strength and ductility of seismically deficient beam-column joints have been studied. For this purpose, four reinforced concrete interior beam-column sub-assemblages were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing preseismic code design construction practice of joints and encompassing the vast majority of existing beam-column connections. Out of these four, two specimens were used as baseline specimens (control specimens) and the other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epoxy bonded to the joint, beams, and part of the column regions. In the second scheme, however, sheets were epoxy bonded to the joint region only but they were effectively prevented against any possible debonding through mechanical anchorages. All four subassemblages were subjected to cyclic lateral load histories so as to provide the equivalent of severe earthquake damage. Further, the damaged control specimens were repaired after filling the cracks through epoxy and wrapping them with CFRP sheets under the same two above-mentioned schemes. These repaired specimens were subjected to the similar cyclic lateral load history and their response histories were obtained. Hence, a total of six specimens were tested: two control; two strengthened; and two repaired. Response histories of control, repaired, and strengthened specimens were then compared. The results were compared through hysteretic loops, load-displacement envelopes, column profiles (maximum horizontal displacements of column along its height), joint shear distortion, ductility, and stiffness degradation. The comparison shows that CFRP sheets improve the shear resistance of the joint and increase its ductility. Results of two chosen schemes of strengthening were also compared and the importance of beam upgrading was highlighted.

In this paper a procedure for analytical prediction of joint shear strength of interior beam-column joints, strengthened with externally bonded fiber-reinforced polymer (FRP) sheets, has been presented. The procedure is based on the formulation available in the literature. To implement the available formulation for shear capacity prediction a computer program has been developed. Using this program shear capacity of the joint and joint shear stress variation at various stages of loading have been predicted and compared with experimental observations; presented in Part I of this study. Predictions show good agreement with experimental test results. The formulation is further extended to predict diagonal tensile stresses in the joint. The effectiveness of FRP quantity on joint shear strength and on various strains has been studied on parametric basis. It is observed that even a low quantity of FRP can enhance shear capacity of the joint significantly and its effectiveness can be further increased if debonding is suppressed (e.g., through mechanical anchorages). Effect of column axial load on shear strength of the joint has also been studied. It is observed that axial load increases the confinement of the joint core, which in turn increases the shear capacity of the joint.

Buildings are vulnerable to blast loads from accidental or terrorist explosions. Key structural components, such as columns, can be shattered and result in the collapse of the whole building and a large number of casualties. Recent retrofit procedures have shown that composites can be used to strengthen structural components so that they can survive the blast load and maintain their load carrying capacity, insuring that building integrity is not affected. This paper is a review of the use of composites for retrofitting key structural components such as columns, beams, and walls subjected to blast loading.

External bonding of fiber-reinforced plastics (FRP) to concrete members has been established as an efficient and effective method for structural strengthening and retrofitting. Direct shear test is often employed to study the crack-induced debonding failure in reinforced concrete members flexurally strengthened with FRP composites. In many existing models, the bond capacity (which defines ultimate load capacity of the specimen in the direct shear test) is considered to be strongly dependent on the compressive or tensile strength of the concrete. However, since debonding behavior is affected by interfacial friction due to aggregate interlocking within the debonded zone, the concrete composition should also play an important role in determining the bond capacity. In this study, the direct shear test is performed with 10 different compositions of concrete. The test results indicate that the bond capacity has little correlation with either the concrete compressive or splitting tensile strength. On the other hand, the bond capacity is found to have reasonable correlation with the concrete surface tensile strength but correlates very well with the aggregate content. As a geometry independent parameter corresponding to bond capacity, the interfacial fracture energy is empirically proposed to relate to these two parameters. The consideration of aggregate content leads to much better agreement between predicted bond capacity and test result. Hence, the effect of concrete composition on the FRP/concrete bond should be considered in practical design.

It has been demonstrated, through laboratory investigations and various field projects, that the external bonding of fiber- reinforced polymer (FRP) laminates is an effective technique for the structural enhancement of reinforced concrete slabs. In such applications, failure is generally governed by debonding of the FRP laminate. Nevertheless, numerical simulations to date of FRP-strengthened slabs have usually been based on the assumption of full bond between the concrete and FRP. In this study, the interfacial behavior between the FRP laminates and the concrete substrate is accounted for by introducing appropriate bond-slip models for the interface in a nonlinear finite-element analysis of FRP-strengthened two-way slabs. The numerical model is capable of simulating slabs strengthened in shear or in flexure; it can be applied to arbitrary FRP configurations, and can also accommodate both passive as well as prestressed FRP strengthening schemes. Results are presented in terms of load-deflection relationships, ultimate load capacities, failure modes, and interfacial slip and stress distributions. When compared to test results reported in the literature, the analysis is shown to lead to excellent predictions in that, for the entire set of FRP-strengthened specimens considered, the average of the numerical-to-experimental load capacity ratios is 0.966, with a standard deviation of 0.066. Furthermore, in all cases when FRP debonding was observed experimentally, the analysis correctly predicted the mode of failure.

Although there is a large amount of experimental data available on the fiber-reinforced polymer (FRP) strengthening of concrete structures, a full understanding of the various debonding phenomena is somewhat lacking. As a contribution to fill this need, two-dimensional and three-dimensional (3D) nonlinear displacement-controlled finite-element (FE) models are developed to investigate the flexural and FRP/concrete interfacial responses of FRP-strengthened reinforced concrete beams. Interface elements are used to simulate the FRP/concrete interfacial behavior before and after cracking. The analysis is carried out using two different relations for the interface; namely, nonlinear and bilinear bond–slip laws. The results predicted using these two laws are compared to those based on the full-bond assumption. The FE models are capable of simulating the various failure modes, including debonding of the FRP, either at the plate end or at intermediate cracks. The 3D model is created to accommodate cases of FRP-strengthened reinforced concrete beams utilizing FRP anchorage systems. In addition, the models successfully represent the actual interfacial behavior at the vicinities of cracks including the stress/slip concentrations and fluctuations. Results are presented in terms of the ultimate load carrying capacities, failure modes and deformational characteristics. Special emphasis is placed on the FRP/concrete interfacial behavior and cracking of the concrete. The numerical results are compared to available experimental data for 25 specimens categorized in six series, and they show a very good agreement.

An attractive technique for the shear strengthening of reinforced concrete beams is to provide additional web reinforcement in the form of externally bonded fiber-reinforced polymer (FRP) sheets. So far, theoretical studies concerning the FRP shear strengthening of reinforced concrete members have been rather limited. Moreover, the numerical analyses presented to date have not effectively simulated the interfacial behavior between the bonded FRP and concrete. The analysis presented here aims to capture the three-dimensional and nonlinear behavior of the concrete, as well as accurately model the bond–slip interfacial behavior. The finite-element model is applied to various strengthening strategies; namely, beams with vertical and inclined side-bonded FRP sheets, U-wrap FRP strengthening configurations, as well as anchored FRP sheets. The proposed numerical analysis is validated against published experimental results. Comparisons between the numerical predictions and test results show excellent agreement. The finite-element model is also shown to be a valuable tool for gaining insight into phenomena (e.g., slip profiles, debonding trends, strain distributions) that are difficult to investigate in laboratory tests.

Emerging techniques that use fiber-reinforced polymer (FRP) composites for strengthening and conservation of historic masonry are becoming increasingly accepted. In the last decades steel plates or wood frames were used for external confinement in containing the lateral dilation of masonry columns subjected to axial loads. In the last years FRP epoxy bonded strips or jackets were also employed to increase strength and ductility with encouraging results in terms of mechanical behavior and cost effectiveness. The behavior of masonry columns confined with FRP and subjected to axial compression is studied in this paper. An extended experimental investigation is presented in order to show the mechanical behavior of circular masonry columns built with calcareous blocks that may be commonly found in Italy and all over Europe in historical buildings. Different stacking schemes were used to build the columns, aiming to simulate the most common situations in existing masonry structures. Carbon FRP sheets were applied as external reinforcement; different amounts and different schemes of confining reinforcement were studied. The experiments include a new reinforcement technique made by using injected FRP bars through the columns cross section. Such a solution can be considered in place of a more traditional confinement, when external reinforcement must be avoided, or in addition to external reinforcement when an improved confinement effect is required. The structural behavior of masonry columns damaged under different levels of load and strengthened by using FRP reinforcements, was also investigated. Experimental results revealed the effectiveness of the FRP confinement for masonry columns, also for columns that were strongly predamaged before strengthening. A computation of the ultimate load was conducted using the Italian National Research Council recommendations to show an application of the design approach recently proposed in Italy. An existing analytical model, previously developed by the writers, was applied for computation of expected experimental values.

Due to corrosion and the continuous demand to increase traffic loads, there is a need for an effective system which can be used to repair and/or strengthen steel bridges and structures. This paper describes an experimental program, recently completed, to investigate the fundamental behavior of steel–concrete composite scaled bridge beams strengthened with new high modulus carbon fiber-reinforced polymer (HM CFRP) materials. The behavior of the beams under overloading conditions and fatigue loading conditions was studied as well as the possible presence of shear lag at the interface of the steel surface and the CFRP strengthening material. The test results are compared to an analytical model based on the fundamental principles of equilibrium and compatibility, to predict the behavior of the strengthened steel–concrete composite beams. Based on the findings of this research work, combined with other work in the literature, a design guideline is proposed for the use of HM CFRP for strengthening the steel flexural members typically used for bridges and structures.