Journal of Composites for Construction

Current guidelines do not cover the subject of the fiber-reinforced polymer (FRP) reinforced concrete (RC) members subjected to simultaneous flexural and axial loads. The obvious implication is that, presently, RC columns cannot be designed with FRP longitudinal bars and ties. This paper suggests the consideration of this type of application of glass FRP (GFRP) reinforcement for the next revision of guidelines by (1) showing the theoretical approach at the basis of the behavior of GFRP-RC members subject to simultaneous flexural and axial loads; (2) providing revised design and analysis provisions for GFRP-RC columns similar to those in practice for steel-RC columns; (3) showing how interaction diagrams can be developed for both rectangular and circular cross sections; (4) explaining the rationale for a new formulation of the strength reduction factor for simultaneous flexural and axial resistance that is consistent with current guidelines, and; (5) reformulating the shear strength computation. Recognizing the limited availability of experimental data on GFRP-RC column behavior, the design provisions proposed in this paper are not applicable to structures in seismic zones and may be applicable only to buildings of limited size and height. An example of the design of a GFRP-RC square column for a hospital facility concludes the paper.

The writers evaluated the confinement that was provided by glass fiber-reinforced polymer (GFRP) spirals in concrete columns under axial load. Given that GFRP spirals are resistant to chloride-induced corrosion, the option of replacing steel spirals with GFRP spirals was explored to determine whether this would reduce the corrosion of the vertical steel bars in hybrid columns. The writers investigated the axial load behavior of 10 spirally reinforced concrete columns. Six of the 254-mm diameter columns were confined with a GFRP spiral and four were confined with a steel spiral. Some of the columns that were confined with a GFRP spiral utilized steel vertical bars (hybrid columns), whereas others utilized GFRP vertical bars (all-GFRP columns). The stress-strain and load-displacement behavior of all columns was studied. Analytical expressions predicted the axial load capacity of the hybrid and all-GFRP-reinforced concrete columns. Axial compression tests of all-steel-reinforced and hybrid specimens subjected to accelerated corrosion were also carried out. The latter exhibited a smaller corrosion rate, similar axial load capacity, and equal or higher ductility relative to steel corroded columns.

In this paper, the bond strength of lap-spliced glass fiber–reinforced polymer (GFRP) bars in concrete beams is experimentally investigated. Thirteen beam specimens were manufactured for laboratory experiments. The parameter of transverse reinforcement along the splice length was selected as the main variable for the beam specimens. Other variables were surface properties of GFRP bars, bar diameter, and concrete compressive strength. Test results show that the effect of transverse reinforcement on bond strength between GFRP bars and concrete depends on the surface properties of reinforcing bars. The experimental results obtained in the present study and the results reported in the literature are compared with the current design equation provided by the American Concrete Institute (ACI) Committee 440 guidelines. The equation of ACI Committee 440 for evaluating the bond strength of GFRP bars underestimates the bond strength of spliced GFRP bars confined with large amounts of transverse reinforcement. For cases of small values of transverse reinforcement and splices without transverse reinforcement, the equation of ACI Committee 440 is unconservative and overestimates the bond strength. In this paper, an equation for predicting the bond strength of GFRP bars is also proposed on the basis of test results and the Monte Carlo simulation method.

Using externally bonded (EB) fiber-reinforced polymer (FRP) laminates for strengthening steel-concrete composite girders has recently received more attention from researchers. By prestressing the EB FRP laminates, the material is used more efficiently because a greater portion of its tensile capacity is employed and it contributes to the load-bearing capacity under both service and ultimate conditions. This is an ideal technique because it combines the advantage of using noncorrosive and lightweight advanced composite materials in the form of bonded FRP laminates with the high efficiency offered by external prestressing. An innovative mechanical anchorage system was developed to prestress the FRP laminates directly by jacking and reacting against the steel girder itself. The efficiency of the system was investigated using two types of FRP laminates for flexural strengthening of large-scale steel-concrete composite girders. The used FRP composite materials included carbon-fiber-reinforced polymer (CFRP) plate and steel-fiber-reinforced polymer (SFRP) sheets. The developed anchorage/prestressing system was easy to use/apply and proved to be a feasible and practical system for prestressing both CFRP plate and SFRP sheet. The prestressing levels in the FRP laminates were sufficiently maintained. The prestressing losses were insignificant.

This paper presents the results of a study on the mechanical behavior of concrete columns strengthened with fiber-reinforced polymer (FRP) and subjected to concentric and eccentric compression loading. A numerical analysis model was developed based on the effectively confined concrete area and the stress-strain relationship of the confined concrete established based on compression tests of circular plain concrete columns. The model was verified against the test results of square concentrically loaded plain concrete columns and square eccentrically loaded reinforced concrete columns. An analytical formula of the maximum compression load of square or rectangular fiber reinforced polymer strengthened concrete columns, with respect to the same but nonstrengthened columns, was developed based on the parametric study results by using the verified model. The analytical formula was further verified with the test results of 23 square and rectangular fiber reinforced polymer strengthened columns reported in the literature. Good agreement was achieved. It was found that the increase of the maximum compression load of FRP-strengthened concrete columns, with respect to the same but nonstrengthened columns, increases linearly with the amount of FRP sheets used and decreases linearly with the load eccentricity and exponentially with the concrete compression strength. The last observation implies that the FRP strengthening technique (through wrapping) is most suitable for low-strength concrete buildings.

This paper reports on axial compression experiments on 18 specimens that were strengthened using fiber-reinforced polymer (FRP) fabric and bamboo sticks to improve buckling resistance. The core steel members were first bundled with bamboo sticks longitudinally using steel wire and were then wrapped longitudinally and laterally with FRP fabric. The bending stiffness and load-bearing capacity of the members were increased by the strengthening system, and the bamboo sticks acted as lightweight and free-shaped filling between the steel and FRP. Three major parameters were examined: (1) the cross-sectional shape; (2) the nominal slenderness ratio of the steel members; (3) the number of FRP fabric layers. Although global buckling that is analogous to that in unstrengthened specimens was observed, the strengthened specimens exhibited significant local deformation at the end of the steel member and rupture of the FRP fabric at midheight. The load-displacement results demonstrated an increase of 25–114% for load-bearing capacity and a maximum increase of up to $6.7$ times for ductility. The load-strain developments also evidenced a reduced effective compression stiffness of the strengthened section. Based on the test results, a calculation method for the load-bearing capacity of the strengthened specimens was proposed.

This paper presents a comprehensive overview of the characteristics of reinforced concrete beams strengthened with fiber-reinforced polymer (FRP) sheets. A large database consisting of 230 test beams is constructed and analyzed using advanced statistical approaches such as multivariate analysis of variance, parameter estimation analysis, and discriminant analysis. Pearson correlation is conducted to identify linear dependency between test parameters. Parameters influencing debonding of the FRP are determined at a confidence level of 95%: concrete strength, steel reinforcement ratio, FRP thickness, and beam geometry. Transition of failure modes between FRP debonding and cover delamination is affected by FRP rupture strain and a steel reinforcement ratio. Discriminant functions are developed to infer the failure mode of FRP-strengthened beams. Existing design proposals for FRP debonding are evaluated using the test data. A new statistics-oriented equation is proposed to predict the effective strain of FRP-concrete interface on the basis of Monte-Carlo simulations associated with random sampling of contributing parameters.

The strengthening of beam-column joints in RC structures is considered an effective approach for improving their seismic resistance and overall performance. This paper presents a numerical investigation into the effectiveness of carbon fiber–reinforced polymer (CFRP) sheets in enhancing the seismic performance of RC joints under combined axial and cyclic loads. For this purpose, a case-study joint subassemblage was retrofitted using three different retrofitting configurations (L-shaped, web bonded, and flange bonded), all commonly used for external strengthening with composite materials. Following the verification of the nonlinear numerical model against the existing experimental data, the analysis outcomes of the retrofitted specimens were compared with those of the control specimen in terms of the tip beam load distribution versus tip beam displacement, energy dissipation, and plastic hinge relocation. Compared with the results of the original joint, the results of the retrofitted joints confirmed an improved load-carrying capacity for all strengthening schemes. However, some configurations led to a decrease in the ductility and dissipated energy. It was shown that the L-shaped and flange-bonded retrofitting schemes could relocate the plastic hinge from the column face toward the beam. This represents a good outcome because it can potentially eliminate the possibility of joint core brittle failure.

This paper presents the functionality of truss systems damaged and strengthened with carbon fiber reinforced polymer (CFRP) tendon. Sixteen damage scenarios are designed by disconnecting various web members of a laboratory-scale truss. A posttensioning technique is proposed using steel tubes and non-shrink mortar embedded with the CFRP. Tension tests are conducted to evaluate the feasibility of the anchorage. Experimental trusses are monotonically loaded in service and a three-dimensional finite-element model is constructed to simulate the test data. Deflection characteristics of the truss system are influenced by the presence of local damage. The proposed posttensioning method enhances the serviceability of the strengthened truss system by reducing decompression–deflection. However, the level of posttension force is not related to the flexural stiffness of the truss from a practical perspective. Stress redistribution is observed in the truss system due to the CFRP strengthening, including a load-sharing mechanism among the members. CFRP strengthening enhances structural redundancy and functional performance of the truss systems.

The writers report an efficient technique for the analysis and design of fiber-reinforced polymer (FRP) web core decks for highway culverts. This technique is based on finite-element modeling and an iterative optimization scheme for different spans, ranging from 2–5 m, that have a carriageway width of 7.5 m, in accordance with Indian loading conditions. The FRP web core deck system that is considered in this paper consists of laminated web cores with equally spaced webs, oriented only in the transverse direction, and were placed between the top and bottom laminated face plates. The culvert decks were simply supported on two opposing edges, whereas the remaining edges were assumed to be free. These were subjected to self-weight and imposed live load. The thicknesses of the webs, top and bottom face plates, number of webs in the core, and depths of the web core of the FRP culverts are considered as the basic design parameters for the structural optimization. The optimization followed different criteria based on the deflection limit, global buckling, the Tsai-Hill failure model, and the Tsai-Wu failure model. The writers used computer software for parametric modeling with modified scripts. The design parameters were varied to obtain different trial solutions in a two-stage optimization process. The optimum dimensions were finalized based on the solutions that corresponded with the minimum volume of the FRP material.

The purpose of this paper is to provide a review of and observations on progressive failure and ductility of fiber-reinforced polymer (FRP) composites of interest to civil and infrastructure construction applications. The primary reason for this is that although FRP composites have over the last 25 years successfully penetrated niche markets in civil engineering applications, one of the most frequently heard concerns from designers is their discomfort with the ductility of these composites and the structures built or reinforced with them, and that if the market for FRP applications in construction is to be expanded, the community must address this issue in greater depth. One approach is to use systemwide, structural, progressive failure behavior of the composite material itself to dissipate internal strain energy in lieu of the elastoplastic behavior of metallic materials. Specific applications of FRP composites in construction where progressive failure mechanisms have been considered are reviewed. These include FRP profiles, FRP frame connections, FRP reinforcing bars, externally bonded FRP or mechanically fastened FRP strengthening strips, and FRP column wraps.