Journal of Composites for Construction

In the last decade, noncorrodible fiber-reinforced polymer (FRP) reinforcing bars have been increasingly used as the main reinforcement for concrete structures in harsh environments. Also, owing to their lower cost compared with other types of FRP bars, glass-FRP (GFRP) bars are more attractive to the construction industry, especially for implementation in bridge deck slabs. In North America, bridge deck slabs are exposed to severe environmental conditions, such as freeze-thaw action, in addition to traffic fatigue loads. Although the bond strength of GFRP bars has been proved to be satisfactory, their durability performance under the dual effects of fatigue-type loading and freeze-thaw action is still not well understood. Few experimental test data are available on the bond characteristics of FRP bars in concrete elements under different loading and environmental conditions. This research investigates the individual and combined effects of freeze-thaw cycles along with sustained axial load and fatigue loading on the bond characteristics of GFRP bars embedded in concrete. An FRP-reinforced concrete specimen was developed to apply axial-tension fatigue or sustained loads to GFRP bars within a concrete environment. A total of thirty-six test specimens was constructed and tested. The test parameters included bar diameter, concrete cover thickness, loading scheme, and environmental conditioning. After conditioning, each specimen was sectioned into two halves for pullout testing. Test results showed that fatigue load cycles resulted in approximately 50% loss in the bond strength of sand-coated GFRP bars to concrete, while freeze-thaw cycles enhanced their bond to concrete by approximately 40%. Larger concrete covers were found more important in cases of larger bar sizes simultaneously subjected to fatigue load and freeze-thaw cycles.

This paper evaluates the benefits of computing deflection with an equivalent moment of inertia based on integration of curvature to account for changes in member stiffness along the span. Results are evaluated for steel and fiber-reinforced polymer reinforced (FRP-reinforced) concrete flexural members with different loading arrangements and support conditions. Closed-form solutions of integrated expressions for deflection are expressed in terms of an equivalent moment of inertia $I_e^{\prime }$ and compared to deflection computed with an effective moment of inertia $I_e$ based on the stiffness at the critical section. Results from this comparison are validated with measured deflections from an experimental database for FRP-reinforced concrete. Current code-related approaches are also compared to the experimental database. It is shown herein that the use of an integration-based expression for the moment of inertia can lead to improved prediction of deflection, though the use of an effective moment of inertia based on member stiffness at the critical section gives a reasonably conservative estimate of deflection in many cases. The benefits of taking account of changes in stiffness along the member span are more evident when low reinforcing ratios are used in combination with FRP reinforcement, and use of the integration-based expression $I_e^{\prime }$ may be warranted when deflection control is critical in such cases.

The study presented in this article concentrated on investigating the ductility and characterization of damage in concrete beams post tensioned with hybrid carbon-glass fiber-reinforced polymer (HFRP) composites. The investigation included an approach for design of flexural members with HFRP tendons and characterization of damage, load deformation response, ultimate strength, and failure modes. Direct tensile tests of hybrid FRP rods in a previous study had indicated elastoplastic response, enhanced ductility, and increased strain capacity. In this context, the current study focused on design and fabrication of post tensioned beams using glass or steel rebars for partial prestressing. All the beams were tested in flexure under four-point bending configuration. Results of the study are presented in terms of ductility index and enhanced load-deflection response in comparison with the conventional FRP materials. Damage characterization involved evaluating the specific features of the acoustic emissions for detecting the elastoplastic transition in the hybrid tendons. The method involved use of a high-resolution fiber-optic interferometer for detection and separation of acoustic emissions. By using the time domain response, it was possible to spatially localize the damage at various stages of the loading. Spectral energy of the acoustic emissions facilitated separation of carbon and glass fiber fractures.

The tensioning of carbon-fiber-reinforced polymer (CFRP) rods for prestressed concrete applications or posttensioning repair and strengthening has been met with mixed success. This is primarily because of limitations inherent in the use of traditional wedge anchors typically used for steel tendons. Recently, an integrated sleeve-wedge anchorage has been successfully developed specifically for CFRP rods. This paper presents a numerical simulation of the newly developed anchorage by using ABAQUS. The three-dimensional (3D) finite-element (FE) model, which considers material nonlinearity, uses hexagonal elements for the barrel, CFRP rod, and tetrahedral elements for the integrated sleeve wedge. The simulated barrel surface strains are shown to compare well with optically measured strains; however, the numerical results are shown to be sensitive to the mechanical properties of the anchorage and CFRP rod and especially the transverse elastic modulus of the CFRP rod. Finally, the simulated strain distributions throughout the anchorage as well as the distribution of CFRP rod confining pressure are presented. Such strain and pressure distributions enable insights into the inner workings of the anchorage to be achieved.

In general, steel-reinforced concrete involves a ductile steel material and a very strong and ductile bond between the steel reinforcement and concrete, so that debonding rarely governs the design. In contrast, fiber-reinforced polymer (FRP) reinforcement is a brittle material with a weak and brittle bond, making debonding a major issue. Consequently, there has been an extensive amount of research on FRP debonding and in particular intermediate crack (IC) debonding. This paper shows that the very good research by the FRP research community on the mechanics of IC debonding can be applied to a wide range of apparently disparate reinforced concrete behaviors to produce a unified approach. Hence, a single mechanism, or unified approach, based on IC debonding is proposed in this paper for dealing with moment rotation, tension stiffening and deflections, member ductility and moment redistribution, shear capacity, confinement, and fiber concrete for FRP RC beams.

The flexural behavior of fiber-reinforced polymer (FRP)–strengthened beams after exposure to elevated temperatures in an electrical furnace was investigated. Twenty-five specimens making up unstrengthened beams and FRP-strengthened beams were fabricated. Glass and basalt FRP systems were used with and without protective systems, which included a cement mortar overlay and two types of commercially available intumescent coatings. Typical temperature-time histories at the surface of FRP laminates, FRP-concrete interface, internal steel bars, and center of beams were monitored by using two specimens. The other specimens were tested to failure under three-point bending after subjecting them to elevated temperatures. Test results indicated a general decrease in the initial stiffness and ultimate strength of the specimens with an increase in the exposed temperature. The protective systems appeared to preserve the structural integrity of glass FRP systems when the elevated temperature was less than approximately 700°C. Basalt FRP-strengthened beams exhibited smaller deterioration in ultimate strength than glass FRP-strengthened beams.

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.

To prevent the casualties that can result from the collapse of earthquake-damaged structures, it is important that structures be rehabilitated as soon as possible. This paper proposes a rapid rehabilitation scheme for repairing moderately damaged reinforced concrete (RC) beam-wide column joints. Four nonseismically detailed interior beam-wide column joints were used as control specimens. All four subassemblages were subjected to similar cyclic lateral displacement to provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks with epoxy and externally bonding them with carbon-fiber-reinforced polymer (CFRP) sheets and glass-fiber-reinforced polymer (GFRP) sheets. These repaired specimens were then retested and their performance compared with that of the control specimens. This paper demonstrates that the repair of damaged RC beam-wide column joints by using FRP can restore the performance of damaged RC joints with relative ease, suggesting that the repair of beam-column joints is a cost-effective alternative to complete demolition and replacement

Intermediate crack-induced debonding (IC debonding) is a common failure mode of RC beams strengthened with externally bonded fiber-reinforced polymer (FRP) reinforcement. Although extensive research has been carried out on IC debonding, much work is still needed to develop a better understanding of the failure mode and a more reliable strength model. This paper presents an advanced finite-element (FE) model on the basis of the smeared-crack approach for predicting IC debonding failure. Existing FE models of the same type are generally deficient in capturing localized cracks (both their pattern and widths). This deficiency is overcome in the proposed FE model through the accurate modeling of interfaces between the concrete and both the internal steel and the external FRP reinforcements. The capability and accuracy of the proposed model are demonstrated through comparisons of its predictions with selected test results. The importance of accurate modeling of localized cracking is also explained using numerical results obtained from the FE model.

The aim of the present paper is the development of a simple procedure for the analysis of the bond behavior of fiber-reinforced polymer (FRP) sheets or plates externally applied to masonry supports for the strengthening or repair of masonry constructions. The procedure allows evaluation of the bond strength and the fracture energy developed during the debonding process through simple formulas based on a few parameters, evaluated either by standard tests performed on the materials making up the support and the strengthening system or by theoretical considerations. A brief discussion on the main experimental evidence and the theoretical models provided by the literature is also reported in this paper. The comparison between the theoretical results obtained by applying the proposed procedure and the experimental data deduced from literature is carried out.

This paper presents a statistically designed experimental program to present and analyze an effective and practical strengthening system for deteriorated crossarms of the Gulfport timber structures using glass-fiber-reinforced polymer (GFRP) wrap. In this research, a total of 14 strengthened and six control specimens with a length of approximately 3.1 m were tested to failure. The experimental program was conducted in two phases: feasibility study and parameter optimization designed by using the Taguchi method and analysis of variation (ANOVA). The feasibility study concluded that the proposed strengthening system was sufficient for increasing the strength of the deteriorated crossarms of the Gulfport structures well above the critical end-of-life (EOL) design threshold value of the crossarms. The parametric optimization of the strengthening system identified the critical variables to be the crack-repair material, the width of the wrap, and sanding the surface. Using statistical advantages of the Taguchi method of experimental design, the mean strength of the strengthened specimens was estimated, and a 95% confidence interval for that mean was calculated. This estimated mean was 70% higher than the EOL threshold of the crossarms.

Embedded through-section (ETS) technique is a recently developed method to increase the shear capacity of reinforced concrete (RC) using fiber-reinforced polymer (FRP) rods. The ETS method presents many advantages over existing methods, such as externally bonded FRP sheets (EB FRP) and near-surface mounted FRP rods (NSM FRP). Unlike EB and NSM methods where the FRP relies on the concrete cover of RC beams, in the ETS method, the FRP relies on the concrete core of the RC beam, which offers a greater confinement and hence improves bonding performance. Additionally, the ETS method requires less concrete preparation compared with EB and NSM methods. The objective of this paper is to present results of an experimental investigation that studies the effectiveness of the ETS method and compares the performance of the ETS method with both EB and NSM methods. In total, 12 tests are performed on 4,520-mm-long T-beams. The parameters investigated are as follows: (1) the effectiveness of the ETS method, compared with EB FRP sheet and NSM FRP rod methods; (2) the presence of the internal steel; and (3) the internal transverse steel reinforcement ratio (i.e., spacing). The test results confirm the feasibility of the ETS method and reveal that the performance of the beams strengthened in shear using this method is significantly superior compared with that of the beams strengthened with EB and NSM methods.

The study presented in this paper proposes a new theoretical framework to interpret and capture the mechanics of the fiber-reinforced polymer (FRP) confinement of square reinforced concrete (RC) columns subjected to pure compressive loads. The geometrical and mechanical parameters governing the problem are analyzed and discussed. A single-parameter methodology for predicting the axial stress–axial strain curve for FRP-confined square RC columns is described. Fundamentals, basic assumptions, and limitations are discussed. A simple design example is also presented.

The onset and evolution of bar buckling at the plastic hinge of old-type reinforced concrete (RC) columns confined with composite material [fiber-reinforced polymer (FRP) and textile-reinforced mortar (TRM)] jackets was investigated experimentally and analytically in this study. The interaction between composite jacket (or concrete cover, for unconfined concrete) and embedded longitudinal compression reinforcement at the onset and evolution of bar buckling was achieved through strain measurements of the compression reinforcement. Moreover, the implementation of a recent stress-strain confinement model, which relates lateral with axial strains, allowed the description and monitoring of the axial-lateral strain relationship at the base of the columns throughout the evolution of bar buckling. Based on the aforementioned model and the experimental measurements, the postbuckling behavior of columns was related to the jacket stiffness. Finally, a semiempirical expression that gives the drift ratio of RC columns at the onset of bar buckling was modified herein for the case of FRP- or TRM-jacketed columns.

Structural fire endurance experiments were conducted on full-scale cellular glass fiber-reinforced (GFRP) columns under axial compression and subjected to ISO834 fire exposure from one side. Unprotected columns can resist fire for more than 30 min, which is sufficient for occupants to be evacuated from smaller buildings. The closed cellular cross-section prevented the rapid heating of the webs, which could therefore continue stabilizing the face sheet on the cold side against buckling. Water cooling was proven to offer an effective active fire-protection system. The structural function of the column could be maintained for two hours. Previously developed models were capable of accurately predicting the time-dependent temperature responses, modulus degradation, and time to failure.

The postfire mechanical performances of water-cooled full-scale GFRP cellular columns subjected to axial compression were evaluated after exposure to an ISO-834 fire from one side for periods of 60 and 120 mins. The elastic modulus and postfire buckling loads exhibited significant recovery after cooling because of the partial reversibility of glass transition. Existing methods to predict ultimate loads based on experimental buckling curves have proved inaccurate. Ultimate loads were accurately predicted, however, based on a shear failure criterion and by taking large initial imperfections caused by fire damage into account. Recently developed models to predict time- and temperature-dependent material and postfire properties showed good agreement with the experimental results and were thus further validated.

This paper investigates large-scale sandwich panels ($9,145\times 2,440\times 78\mathrm{mm}$) subjected to out-of-plane loading. The panels comprise glass-fiber-reinforced polymer (GFRP) skins connected by orthogonal GFRP ribs and a polyurethane foam core. The lightweight insulated panel is supported at three levels along its 9,145-mm height and is proposed for cladding of buildings, where the main loading is caused by wind. A full-scale panel was tested under uniform air pressure by using a specially designed setup equipped with pressure load actuators. The panel failed at 7.5 kPa, 2.6 times the factored design pressure for the windiest region in Canada. Failure occurred by outward wrinkling and crushing of the GFRP compression skin near the middle supports. The deflection under the maximum design service wind pressure did not exceed $\mathrm{span}/360$. Another test was carried out on a different specimen by using conventional mechanical loading. It confirmed the ultimate strength of the panel and also revealed a successive failure response, associated with residual capacity beyond first failure. After skin wrinkling in the negative moment region, the load remained stable, within a 3% variation, until shear failure of the core followed near the middle supports. Because the system is statically indeterminate, it did not suffer global collapse, and the load dropped only 24%. By further loading, the strength increased up to 85% of the first peak when failure occurred by outward wrinkling and crushing of the compression skin in the positive moment region.

Thin webs of glass-fiber-reinforced polymer (GFRP) girders are sensitive to shear buckling, which can be considered an in-plane biaxial compression-tension buckling problem, according to the rotated stress field theory. An extensive experimental study was performed, which shows that an increasing transverse tension load significantly increases the buckling and ultimate loads caused by a decrease in the initial imperfections and additional stabilizing effects. The stacking sequence also greatly influenced the buckling behavior. Higher bending stiffness in the compression direction increased the buckling and ultimate loads, while higher bending stiffness in the tension direction changed the buckling mode shape. The general solution obtained using the Fok model accurately modeled the experimental results, while the simplified solution (modified Southwell method) provided accurate results only at higher tension loads.

This paper reports on research investigating a nongrouted sleeve-type connection used to attach fiber-reinforced polymer (FRP) decks to steel girders. The connection system was investigated for stiffness, strength, fatigue resistance, and degree of composite action. Static and fatigue tests were conducted first at the component level on push-out specimens to obtain $P-\Delta$ (load-displacement) and $S-N$ (stress range–fatigue life) relationships, from which design formulas were developed. Then tests were conducted at the system level on a $1:3$ scaled-bridge model by using this sleeve-type connection, and the results showed this shear connection can satisfy requirements from AASHTO specifications for fatigue, strength, and function. Further, three-point bending tests were conducted on a T-section model cut from the scaled bridge, and approximately 25% composite action was achieved for two different connection spacings. The structural efficiency of this shear connection is shown, and this connection design is applied in practice.

In recent years, fiber-reinforced polymer (FRP) composites have found wide applications in new construction. A popular type of hybrid member incorporating FRP is the concrete-filled FRP tube (CFFT), consisting of an FRP tube filled with plain or steel-reinforced concrete. CFFTs have several advantages over traditional column forms, including their excellent corrosion resistance and ductility. Much research has been conducted on CFFTs over recent years, but no systematic procedure for designing such members has been developed, which has been one of the factors preventing CFFTs from becoming more widely accepted in practice. This paper presents a summary of the design provisions for CFFTs given in the Chinese Technical Code for Infrastructure Application of FRP Composites and explains the rationale behind these provisions. These provisions have been developed on the basis of available research and work undertaken at The Hong Kong Polytechnic University.

This paper presents the results of experimental and theoretical investigations that study the flexural behavior of reinforced concrete-filled fiber-reinforced polymer (FRP) tubes (RCFFTs) beams. The experimental program consists of 10 circular beams [6 RCFFT and 4 control reinforced concrete (RC) beams] with a total length of 2,000 mm, tested under four-point bending load. The experimental results were used to review and verify the applicability of various North American code provisions and some available equations in the literature to predict deflection of RCFFT beams. The measured deflections and the experimental values of the effective moment of inertia were analyzed and compared with those predicted using available models. The results of the analysis indicated that the behavior of steel and FRP-RCFFT beams under the flexural load was significantly different than that of steel and FRP-RC members. This is attributed to the confining effect of the FRP tubes and their axial contribution. This confining behavior in turn enhanced the overall flexural behavior and improved the tension stiffening of RCFFT beams. For that, the predicted tension stiffening of steel and FRP-RCFFT beams using the conventional equations (steel or FRP-RC member) underestimates the flexural response; therefore, the predicted deflections are overestimated. Based on the analysis of the test results, the Branson’s equation for the effective moment of inertia of RC structures is modified, and new equations are developed to accurately predict the deflection of concrete-filled FRP tube (CFFT) beams reinforced with steel or FRP bars.