Journal of Composites for Construction

Fiber-reinforced cementitious matrix (FRCM) composites can be preferred to other techniques to strengthen reinforced concrete (RC) members. Due to the limited cross-sectional area of textiles in FRCM, multilayer composites are often needed to provide adequate strength increase. Although multilayer FRCM composites have been already employed, quite limited research is available regarding the bond behavior of multilayer FRCM–concrete joints. Direct shear tests of FRCM–concrete joints and bending tests of FRCM-strengthened RC beams showed that when one or two textile layers are employed, debonding at the matrix–fiber interface generally occurs. When more than two layers are employed, detachment of the FRCM strip was observed in FRCM-strengthened RC beams, while limited studies of corresponding FRCM bond behavior were performed. The bond behavior of a multilayer bidirectional polyparaphenylene benzobisoxazole (PBO) FRCM composite applied onto a concrete substrate is investigated in this paper. Single-lap direct shear tests of PBO FRCM–concrete joints comprising two or four textile layers are performed, along with tensile tests of bare textile strips and single-layer FRCM coupons. Tests were conducted in both textile directions. The results obtained provide fundamental information on the effect of the textile direction and number of layers to be considered when designing the reinforcing system.

Sulfate ions in saltwater can considerably deteriorate the strength and durability of reinforced concrete (RC). To date, there has been no study that systematically investigates the effects of a sulfate attack on the capacity of mid-/large-scale RC columns confined with carbon fiber–reinforced polymer (CFRP). The capacity of CFRP-confined concrete columns subjected to sulfate attacks with various schemes and durations (up to 210 days) was examined in this study. The primary objectives of this study were to (1) examine the effect of a sulfate attack on the structural properties of midscale RC columns confined with CFRP; (2) quantify the contribution of the sulfate-damaged concrete core and CFRP confinement to the load-carrying capacity of the columns; and (3) propose a new formula to estimate the compressive strength of FRP-confined sulfate-damaged concrete considering the exposing time, number of FRP layers, and concrete strength. The experimental results showed that the sulfate attack significantly reduced the initial axial stiffness of the column up to 36% and increased its ultimate axial displacement up to about 40%. These influences in the CFRP-strengthened columns were slightly lower than those of unstrengthened ones. The sulfate attack considerably reduced the strength of the unstrengthened columns (up to 29.4%), but it only slightly affected the capacity of the strengthened columns with a 3.7% reduction. The strength reduction increased with the number of wet–dry cycles. CFRP confinement effectively mitigated the penetration of sulfate ions into the concrete core and, thus, slowed the degradation of the strength up to 90%. An empirical formula was proposed to estimate the strength of the CFRP-confined concrete subjected to a sulfate attack with close predictions regarding the experimental results.

The growing interest in fiber-reinforced polymer (FRP) composites in recent years for use in strengthening concrete structures bears witness to their efficiency and ease of use. A major issue in FRP strengthening designs, however, is the determination of the bond strength (ultimate load capacity) of the FRP strips, which is quite dependent on their debonding off the concrete substrate. Identification of the major parameters involved in the FRP–concrete bond capacity forms an important step toward its prediction. The parameters commonly considered in previous models include the geometric and mechanical properties of FRP sheets as well as the properties of both the concrete and the adhesive material used as determinants of bond strength. The present study, however, explores the additional parameter of cement-to-aggregate weight ratio for its effect on the failure plane. In the experimental phase of the study, a wide range of FRP strips is used in three different series of concrete specimens to investigate the failure plane in each. Based on the post-test conditions of the specimens, a novel method is adopted for the first time to measure the failure depth of the externally bonded reinforcement (EBR) strengthening technique. Accordingly, the depth of the failure plane is expressed as a function of the cement-to-aggregates weight ratio. In the second phase, an analytical model is developed that draws upon interfacial fracture energy and in which failure depth is introduced as one of the parameters involved in interfacial fracture energy. The unknown parameters of the new model are then derived based on 196 specimens (85% of the data set) from the experimental results of the present study and the extracted data from previous studies. Finally, the model is validated against three existing well-known ones using 35 random specimens (15% of the data set) of the database to show the satisfactory performance of the proposed model.

Fiber-reinforced polymer (FRP) reinforced concrete-steel double-skin tubular columns (DSTCs) consist of an exterior glass FRP tube, an interior steel tube, and reinforced concrete in between. They have attracted the attention of researchers owing to their high ductility, lightweightedness, resistance to corrosion, and ease of construction. However, studies regarding FRP-reinforced concrete-steel DSTCs under eccentric loading are fewer compared with those regarding unreinforced DSTCs. To investigate the behavior of composite columns under eccentric compression, 12 circular cross-section columns with a height of 700 mm and an outer diameter of 210 mm were tested. They included two axial compression columns and 10 eccentric compression columns. The main parameters considered were the eccentricity, void ratio, longitudinal reinforcement ratio, and concrete strength. The results showed that the arrangement of steel bars inside the hybrid DSTCs could improve the eccentric carrying capacity and ductility of the composite columns. Furthermore, this study showed that the eccentric carrying capacity and initial stiffness decreased as the eccentricity and void ratio increased, whereas they increased with the longitudinal reinforcement ratio and concrete strength. The growth rate of the carrying capacity decreased when the longitudinal reinforcement ratio increased to 3.66%. When the void ratio exceeded 0.5 and the eccentricity ratio was 0.4, the ductility and carrying capacity decreased. Finally, a simplified model was proposed to predict the axial load–moment curves of reinforced DSTCs. The theoretical predictions were in good agreement with the experimental results of this study.

This study discusses the behavior of ambient-cured circular geopolymer concrete specimens tested under different loading conditions. Twelve specimens were cast and tested to investigate the influence of the type of the reinforcement, that is, steel and glass fiber-reinforced polymer (GFRP), the pitch of the transverse reinforcement, and loading conditions (concentric axial load, 15 and 35 mm eccentric load, and four-point bending). The axial load carrying capacity, confinement efficiency, and deformability of the specimens decreased for the replacement of the steel reinforcement with the same amount of GFRP reinforcement under concentric and eccentric loads. However, the deformability of the GFRP reinforced concrete (RC) specimen was higher than the deformability of its steel counterpart specimen under four-point bending. Overall, the reduction in the pitch of the GFRP helices in the GFRP-RC specimens resulted in a significant improvement in the confinement efficiency, deformability, postpeak behavior, and the axial load–bending moment capacity of the specimens under different loading conditions. Moreover, theoretical equations based on the confinement pressure of transverse reinforcement provide accurate predictions of the theoretical load-carrying capacity of the columns reinforced with GFRP bars and helices.

The durability of steel-reinforced concrete (RC) members reduces significantly under aggressive environmental conditions due to the corrosion of steel rebars. The use of fiber-reinforced polymer (FRP) rebars is an attractive alternative for steel rebars in RC members. However, the brittle failure of FRP RC members is a significant concern. The addition of discrete fibers in FRP RC members can improve their postcracking behavior and provide pseudoductility. The objective of this study is to understand the effects of macrosynthetic polyolefin (PO) and a hybrid combination of steel and macrosynthetic PO fibers on the compression behavior of glass FRP (GFRP) RC columns. Fourteen square GFRP-RC columns were cast with various fiber dosages. The test matrix included (1) GFRP RC control specimen with no fibers; and (2) GFRP RC columns with six different fiber dosages which include: (a) 0.35% of PO; (b) hybrid dosage of 0.175% of steel and 0.175% of PO; (c) 0.70% of PO; (d) hybrid dosage of 0.35% of steel and 0.35% of PO; (e) 1.0% of PO; and (f) hybrid dosage of 0.50% of PO and 0.50% of steel fibers. Test results revealed that the fiber addition improved peak load, postpeak behavior, and pseudoductility under pure compression. Specimens with hybrid fibers had higher energy absorption and pseudoductility than those with macrosynthetic fibers.

This paper discusses the flexural response of thin marble panels strengthened with carbon and glass fiber–reinforced polymer (FRP) laminates when subjected to out-of-plane loading. Two types of panels were investigated, including 20 mm-thick single panels used for facades exposed to load from one direction, thus reinforced from the tension side, and 40 mm-thick composite panels used for balcony decking consisting of two 20 mm-thick single panels sandwiching the FRP laminate. The composite panels resist incident blast pressures from above and underneath and protect the laminate from fire damage. Load-deflection curves were developed experimentally from three-point bending tests. The results for ultimate loads and deflections for either reinforced panel type were in the range of 9 kN and 12 mm compared with typical values of 1.37 kN and 1 mm or 5 kN and 0.6 mm for 20 mm- or 40 mm-thick unreinforced marble panels, respectively. Analytical models for the load-deflection curves were obtained from moment-curvature diagrams using equilibrium and strain compatibility based on a compressive stress–strain model of marble that best fits the experimental data. The analytical model incorporated the effect of crack width opening and crack depth of the marble. The benefits of the reinforcement were to preserve durability, bowing, warping, and structural integrity of the stone panels. In addition, ductility will be improved, thus preventing brittle failures that cause personal injury due to fragmentation when subjected to blast pressures or thermal stresses.

Glass fiber-reinforced polymer (GFRP) bars are a common alternative to steel reinforcement, offering economical and safety benefits of avoiding/limiting corrosion in traditionally reinforced concrete structures (e.g., parking garages, buildings, and bridges). However, estimating the life of GFRP structures represents a challenge for the construction industry. The experimental program described in this paper is designed to evaluate progressive damage of GFRP bars due to a highly alkaline environment using ultrasonic nondestructive testing (NDT). The evaluation of damage in GFRP bars is performed using a new methodology based on the analysis of relative changes not only in wave velocity and but also in wave amplitude. The actual displacement time histories (nm) generated by the ultrasonic transducers are directly measured with a high-frequency laser Doppler vibrometer. The measurement of the actual ultrasonic displacements as a function of frequency is a required step for the new amplitude-based approach used in this study. Finally, NDT results (low-strain measurements) are correlated with the reduction of shear strength obtained from destructive shear tests (large-strain measurements). The results show that ultrasonic parameters (e.g., wave velocity and amplitude) correctly capture deterioration in the GFRP bars, and for the samples studied, ultrasonic parameters can be used to estimate the shear strength reduction with the induced damage. The maximum error in the estimation of shear strength using ultrasonic measurements is less than 7%.

Standard pullout tests (e.g., 150 mm cubes) with a short embedment length have shown a significant reduction (80% loss at 200°C) in the bond strength of glass fiber-reinforced polymer (GFRP) bars at high temperatures but little information is known about how this information can be extended to longer embedment lengths. To address this knowledge gap, a more detailed bond behavior of GFRP reinforcing bars has been examined in this study with a modified pullout test method in which pullout specimens consisting of GFRP bars embedded in 350 mm (22d_b) long concrete cylinders were tested at high temperatures. The tests examined the bond degradation under different temperature distributions (i.e., uniform and gradient). The results were compared with the previous pullout tests with 68 mm (4d_b) embedment length and showed that the pullout capacity of GFRP bars in this test was 3.3, 4.0, and 2.0 times higher at 115°C, 135°C, and 300°C, respectively. The results were the first to prove the effectiveness of longer embedment length particularly at temperatures below 200°C. Additionally, pullout tests with temperature gradients simulated the bond behavior of GFRP bars at the unexposed end zones of GFRP reinforced concrete beams or slabs. The results showed that the tensile force in GFRP bars gradually transferred to the lower temperature zones as the heated parts of the bar lost bond with concrete. The results showed that the concentrated forces over a short length of GFRP bars can trigger concrete splitting and loss of bond even if the GFRP bars still maintain the bond with concrete. Providing thicker concrete cover at the end zones of GFRP reinforced members is recommended to enhance fire resistance. The findings of this study are being used in developing forthcoming FRP reinforced concrete design codes.

In fabric-reinforced cementitious matrix (FRCM) systems, one technique to increase the strengthening efficiency is to utilize the deformability of dry fiber bundles and anchor them at their ends. In this study, the effectiveness of anchored carbon fiber bundles was thoroughly investigated through pull-out tests of 220 specimens and corresponding microstructural analyses. The results show that the peak pull-out load can be increased by up to 165% after anchoring the fiber bundles, and the dominant failure mode shifts from fiber pull-out to fiber rupture. Parametric analyses show that a larger anchorage angle and a larger total embedded length help to increase the pull-out strength, while a proper combination of the bonded length and the anchorage length is essential for high strengthening efficiency. Further microstructure analyses suggest that these improvements are primarily attributed to the additional constraint at the anchorage point, and the anchorage length is critical for the pull-out behavior.

Good bond and water impermeability in fiber-reinforced polymer (FRP) bonded/coated systems are essential if the durability of FRP-rehabilitated concrete structures is to be ensured. In addition, water impermeability is required in some special FRP applications, such as the strengthening of underground water pipes. So far, there has been no method in the literature guaranteeing water impermeability in FRP strengthening works. This paper studies the feasibility of using a waterproof coating as the initial primer on cementitious materials before applying externally bonded FRP. In this preliminary investigation, by examining the two most important indicators (i.e., pull-off bond tests and water penetration tests), it was found that the use of an initial waterproof layer in the proposed FRP bonding system did not influence the pull-off bond strength but significantly improved the system's water impermeability. It is, therefore, suggested that an initial waterproof layer can be included in method statements for externally bonded FRP systems to upgrade the effectiveness and durability of FRP systems.