Journal of Composites for Construction

In recent years, the grooving method in the form of “externally bonded reinforcement on groove” (EBROG) has been introduced as an alternative method of the conventional externally bonded reinforcement (EBR) for strengthening concrete structures using fiber-reinforced polymer (FRP) materials. This paper is a first attempt to develop an FRP-concrete bond strength model based on nonlinear regression on the experimental results. The agreement of the model to the data is verified using different statistical tools and analysis of variance. One hundred and fifty-four single lap-shear tests are conducted on 136 specimens made through the EBROG method and 18 specimens through the EBR method. The design of experiments techniques in the form of response surface methodology and I-optimality criteria are used to design and optimize the test layouts. The effects of groove dimensions, concrete compressive strength, and FRP sheet width and stiffness on the EBROG bond strength and its efficiency are investigated and compared with those on similar EBR specimens. Although debonding failure is observed to occur in all of the specimens, the results obtained confirm the superiority of the EBROG over the EBR specimens, as evidenced by an average enhancement of 31% achieved in bond strength and the postponed debonding. A bond strength model for the EBROG method was proposed by modifying the well-known Chen and Teng model, originally used for EBR.

Nine beams reinforced with glass fiber-reinforced polymer (GFRP) bars and one reinforced with steel rebars fabricated using high-strength lightweight aggregate concrete (HSLC) were tested under four-point bending with different steel fiber contents, reinforcement ratios, and bar diameters to investigate their flexural strength and serviceability performance. The test results showed that specimens with steel fibers exhibited a lower deflection and a higher load-carrying capacity. The reinforcement ratio significantly affected serviceability performance, but bar diameter had a marginal effect. Generally, at service-load levels, the specimens exhibited acceptable midspan deflections but greater crack widths compared to the limits recommended by the codes. The experimental ultimate strengths, midspan deflections, and crack widths were used to assess the accuracy of prediction equations in standards in the US, China, and Canada. Rational deflection models for GFRP-reinforced normal weight concrete (NWC) beams were proposed based on the results from the available literature and were modified by introducing two correction factors of 0.85 and 1.35 for the GFRP-reinforced lightweight aggregate concrete (LC) and steel fiber-reinforced lightweight aggregate concrete (SFLC) beams, respectively.

The paper deals with the influence of cyclic loading on the bond between glass-fiber-reinforced polymer (GFRP) bars and concrete. The experimental study considers eccentric and centric pullout tests that vary by three different parameters: (1) concrete cover; (2) concrete strength; and (3) maximum load applied during cyclic loading. The results showed that the fatigue life of the bond between the considered GFRP rebars and concrete, with a small cover and low concrete quality, is limited by the occurrence of splitting failure. As the cover increases, the fatigue failure mode becomes a combination of splitting and bar pullout. The mechanical properties of concrete have a positive effect on fatigue life.

Syntactic foams (hollow glass macrospheres bonded with a polymer matrix) encased in glass fiber-reinforced polymer (GFRP) composite shells have the potential to enhance structural performance and energy absorption. To investigate this hypothesis, several load carrying structural columns with a GFRP composite shell encasing a syntactic foam core were designed, manufactured, and tested under axial compression, with a focus on optimal energy absorption and compressive force resistance. The structural responses of column specimens and the foam core alone were assessed via load, strain, and deflection responses. The following parameters were varied to induce different failure modes: (1) shell thickness; (2) fiber volume percent in both the longitudinal and circumferential directions; (3) syntactic foam density; and (4) cross-shaped fiber orientation of GFRP web. Based on the experimental data, a strength prediction model was developed to predict the axial load capacity, including the buckling limits of the hollow GFRP tubes and syntactic foam columns with GFRP shells. The predictions are relatively simple and provided good accuracy when compared with the experimental data.

This paper presents the results of an experimental study on the behavior of predamaged concrete prisms that were strengthened with fiber-reinforced polymer (FRP) jackets. Tests were conducted on 46 concrete columns involving variations of corner radius ratios, damage levels, and confinement pressure. This paper investigates the effect of corner radii on the main parameters of the stress–strain curve for FRP-strengthened predamaged specimens, such as compressive strength, strain capacity, and initial elastic modulus. The test results demonstrated that the efficiency of repairing damaged columns greatly depends on the corner radius ratio. The strength gains after FRP strengthening decrease as the corner radius ratio reduces. Furthermore, this paper extends a well-established stress–strain model originally proposed for FRP-confined predamaged circular concrete columns to square columns. More accurate expressions for the strength, deformation capacity, and stress–strain relationship were developed considering both the corner radius ratio and predamaged level. The proposed model was verified by test results and can predict the stress–strain behavior of a FRP-confined predamaged concrete column with good accuracy.

Fiber-reinforced-polymer (FRP) composites are widely used currently as strengthening materials for the repair and strengthening of important elements of concrete structures, especially columns. This study investigated the effects of a novel technique that involves placing a thin galvanized midlayer between the substrate concrete and the overlying polymer composite sheet to improve the confinement quality of RC columns under axial loading. The technique is meant not only to prevent the transfer of concentrated stress from the concrete cracks to FRP sheets but also to decrease the transfer of biaxial stresses to FRP sheets. For this purpose, 24 RC columns, each 150 mm in diameter and 500 mm in height, were strengthened with one or two layers of FRP strips and loaded under axial compression. The effects of confinement with intermittent strips of carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) were investigated in columns with and without the thin galvanized midlayer. The results were compared with those obtained from loading control specimens lacking any external strengthening. The midlayer was found to increase maximum load capacity and energy absorption by 16% and 53%, respectively, in specimens strengthened with one layer of carbon fiber relative to those recorded for the control. These enhancements recorded for the specimens strengthened with one layer of glass fiber were 17% and 117%, respectively.

The external jacketing method using cement-based matrices and textile reinforcement has gained popularity for the enhancement of strength and ductility characteristics of reinforced concrete members. In this study, glass fiber–reinforced mortar (GFRM) with or without basalt textile reinforcement was used for external jacketing of low-strength concrete prisms. An innovative spraying method was used during the application of the cement-based matrix to the specimen’s surface. Investigations were carried out on a large number of concrete prisms with five different cross-sectional geometries (circular, square, and rectangular, with cross-sectional aspect ratios of 1.5, 2, and 3). At the end of the tests, it was shown that the proposed retrofit methodology is effective for the enhancement of performance of low-strength concrete members. The spraying technique is practical and feasible, particularly for the cases in which access to the structural members that are destined to be retrofitted is limited. Furthermore, a simple analytical approach based on the effectively confined concrete area concept is also presented for predicting the peak strength and the ultimate strains of the retrofitted specimens.

Postearthquake reconnaissance studies show that the beam-column joints of substandard reinforced concrete buildings (particularly the corner joints) may fail to resist the demanded forces and deformations under severe seismic actions. In this study, the effectiveness of carbon fiber–reinforced polymer (FRP) composites for retrofitting beam-column joints is demonstrated through testing of four full-scale three-dimensional reinforced concrete frames that are representative of substandard reinforced concrete buildings. Additionally, analytical seismic performance predictions made using available seismic assessment and retrofit guidelines are compared with the test results.

Although the flexural behavior of fiber-reinforced-polymer (FRP) reinforced-concrete (RC) members has been the focus of many studies in recent years, no research work has examined such behavior in FRP-RC members with circular cross sections. This paper reports on a study in which the flexural strength and deformability of circular concrete members with hybrid reinforcement—carbon-FRP (CFRP) bars and glass-FRP (GFRP) spirals—were assessed experimentally and analytically. Three large-scale RC specimens with a total length of 6,000 mm and 500 mm in diameter were constructed and tested under four-point bending. Three CFRP longitudinal-reinforcement ratios (0.8%, 1.2%, and 1.8%) were considered. GFRP spirals were used to eliminate shear cracks and achieve the full flexural capacity. A steel-reinforced concrete specimen with a reinforcement ratio of 1.2% and steel spirals was fabricated as a reference. Test results indicate that the CFRP-RC specimens failed gradually at a high degree of deformability before concrete crushing. Moreover, the flexural strength of the CFRP-RC specimen with a reinforcement ratio of 1.2% was almost 3.3 times greater than the counterpart steel specimen (with a similar reinforcement ratio) at steel yielding. The ductility and deformability of the test specimens were estimated using different approaches. The estimated deformability factor was significantly higher than that required by the North American FRP-design standards. Detailed design procedures using a computer program are proposed for estimating the flexural capacity of circular CFRP-RC members. The experimental and analytical results are discussed and compared.

In this study, an analytical approach was used to predict the full torsional response of RC beams strengthened with externally bonded fiber-reinforced cementitious matrix (FRCM) composite. The analytical model was based on the softened membrane model for torsion (SMMT) modified for fiber-reinforced polymer (FRP)-strengthened beams. As a first attempt, fully wrapped beams with fiber rupture governing the mode of failure were considered in this study. The model was validated by comparing the analytical response to the experimental response of five solid, rectangular RC beams. The model was able to predict values of the cracking and ultimate torsional moment and the corresponding angles of twist per unit length with reasonable accuracy. Also, reasonable agreement was achieved between the experimental and analytical results in terms of the overall response and failure sequence. The results confirm the feasibility of the SMMT model to predict the torsional response of fully wrapped FRCM-strengthened beams with the fiber rupture failure mode. However, additional modifications are required to extend the model to U-wrapped configurations and composite debonding failure modes.

Structural retrofitting by wrapping with fiber reinforced–polymer (FRP) laminates can effectively increase the ductility of RC columns. The effect of the FRP-to-concrete interfacial bond on the behavior of FRP-confined RC columns has not yet been studied extensively. This paper reports on an experimental and numerical study of FRP-confined RC columns involving different bonding conditions between FRP and concrete. It was found that variations in the bonding condition do not have a significant effect on the global response of the FRP-confined RC columns. However, a detailed study of the plastic hinge zone indicated that slipping at the bond interface causes an adverse effect on the length of the plastic hinge zone. Mechanical anchoring to avoid interfacial slip can increase the plastic hinge length. The influence of the bond on the plastic hinge length was studied both analytically and numerically. Parametric studies based on finite-element analysis showed that the plastic hinge length first increases and then decreases as the amount of FRP increases, and a stronger interfacial bond offsets the decreasing trend of plastic hinge length at high FRP confinement.

RC walls with an opening under eccentric axial loads exhibit cracks around the opening leading to a reduction in the ultimate strength of the wall. These cracks are mainly due to changes in the load path within the wall and stress concentration around the opening. Strengthening is usually required to enhance the ultimate strength of the wall, and carbon fiber–reinforced polymer (CFRP) is a suitable option for this purpose. In this paper, 18 one-third scaled RC walls with an opening were strengthened using seven distinct externally bonded (EB) CFRP layouts and were loaded up to failure under various support conditions. These walls were subjected to a uniformly distributed axial load with an eccentricity of one-sixth of the wall’s thickness. Crack patterns, failure modes, ultimate strength, and typical strain measurement of CFRP–concrete interface are presented and discussed in detail. The experimental outcomes revealed that the CFRP layout enhances the ultimate strength of RC walls by 14.0%–59.7% and 3.0%–40.8% for walls under one-way and two-way actions, respectively. The efficiency of each CFRP layout is also investigated to prioritize the appropriate layouts for each specific support condition.

Thickening concrete cover to obtain an adequate fire resistance lowers the efficiency of glass fiber–reinforced polymer (GFRP) reinforcement and increases the cost relative to conventional steel reinforcement. This paper investigates the fire resistance of two full-scale GFRP-RC slabs with only 40 mm of clear concrete cover and 200 mm of unexposed (cool) anchor zone at the ends. Both slabs endured 3 h under the standard fire. The slabs were loaded with a sustained load, which caused a moment equal to 45% of their ultimate flexural strength. Emphasis was placed on studying the bond behavior of GFRP bars by comprehensively investigating temperature distributions particularly at the unexposed anchor zones at the ends of the slabs. The temperature reduces significantly in the unexposed zones, providing an adequate anchorage for the bars when almost the entire GFRP-to-concrete bond deteriorated in the exposed zone. The analysis of the experimental results showed that the increase in the tensile force in the GFRP bars during the standard fire is below 50% of the existing sustained force. A new model considers the bond degradation to predict the fire resistance of GFRP-reinforced slabs. The results enable an efficient, economic, and fire-safe application of GFRP reinforcement in concrete construction by reducing the concrete cover.

In this study, the bond strength of a typical FRP system subjected to long-term natural weathering in the midwest United States is experimentally investigated, and the rate of degradation is estimated. To do this, the bond strength of a fiber-reinforced polymer (FRP) system exposed to over 15 years of weathering is determined with pull-off testing, and a relationship between strength reduction and exposure time is developed using regression analysis. For unweathered specimens, it was found that the attachment strength of the FRP system was governed by the concrete substrate, while for weathered specimens, the FRP system could detach by either a failure of the substrate, at the FRP/concrete interface, or FRP failure. It was found that a logarithmic curve best matches bond deterioration.