Journal of Composites for Construction

Green concrete made with ground granulated blast-furnace slag (GGBS) has presented itself as a sustainable alternative to traditional concrete. Similarly, the noncorroding fiber-reinforced polymer (FRP) reinforcing bars have demonstrated reliable structural performance at a competitive price. The combination of these materials substantially improves the service lifespan for reinforced concrete (RC) structures. Earlier research investigations and existing standards do not account for FRP reinforcement in the design for compressive forces. This study revisits this design approach while experimentally investigating the axial compressive behavior of slender circular GGBS-based green concrete columns reinforced with double layers of steel and glass fiber reinforced polymer (GFRP) bars and spirals. The experimental program consists of nine large-scale specimens loaded concentrically. All specimens had a diameter of 260 mm and a length of 2,500 mm, yielding a slenderness ratio of 38.5. Two control specimens were reinforced with a single steel layer having 1.2% and 2.5% longitudinal reinforcement ratios. The remaining seven specimens had an outer layer of GFRP reinforcement and an inner layer of steel reinforcement. The investigated parameters were: (1) longitudinal reinforcement ratio; (2) spiral reinforcement ratio; and (3) spiral reinforcement configuration (pitch and diameter). The experimental findings revealed that all tested columns failed in a material-type failure. However, the slenderness effects reduced the strength of the hybrid-reinforced columns compared with the control counterparts. The steel-reinforced control specimens demonstrated load-carrying capacities that agreed well with nominal capacity predictions under existing code. Conversely, the nominal capacity predictions in current codes and standards underestimated the strength of the hybrid specimens. A proposed equation from the literature that considers the compression contribution of GFRP bars was also evaluated, and a proposed correction factor was introduced to count for second-order effects. Due to the limited number of specimens in this study, future research is warranted to refine this proposed correction factor further.

The utilization of geopolymeric composites can considerably promote sustainability in the construction sector due to their low carbon footprint. In this work, an experimental study was performed to investigate the quasi-static cyclic behavior of geopolymeric composites confined by carbon fiber-reinforced polymer (CFRP). In total, 40 CFRP-confined geopolymeric composite columns were fabricated and tested. The primary variables included the matrix composition (i.e., slag contents of 0% and 10%), type of confined material (i.e., mortar and concrete with recycled aggregate replacement ratios of 0%, 50%, and 100%), and the load pattern (i.e., cyclic loading with a single cycle and cyclic loading with repeated cycles). Careful examination and analysis of the experiment results led to a series of conclusions regarding failure patterns, stress–strain behaviors, and characteristics under ultimate conditions. In addition, discussions were provided concerning the effects of the primary variables on the key characteristics of the stress–strain curves, which included plastic strain, residual and reloading moduli, and stress deterioration for the envelope and internal cycles. In addition, the prediction results by the existing models for fiber-reinforced polymer (FRP)-confined conventional concrete were compared with the test results to assess their applicability for CFRP-confined geopolymeric composites.

Fiber-reinforced polymer (FRP) reinforced concret flexural members are typically designed as an overreinforced concrete section and to satisfy the design requirements, the overlap of bars is often ubiquitous. This paper presents innovative FRP bar connectors made of woven E-glass fiber cloth and vinyl-ester resin with various shapes and configurations. FRP bar coaxial and off-axis connections were investigated. Connector configurations, including the connection system number, length, and diameter as well as the bar type, were considered as the experiment’s variables. Connection system lengths of 25 and 50 mm, glass fiber tape lengths of 2 and 4 m (for wrapping the splice length), and glass fiber–reinforced polymer (GFRP) bar full and reduced cross sections (50% and 25% cross section reductions) were selected. To study the impact of bar type, sand-coated and helically wrapped GFRP bars were used. Based on the obtained results, increasing the connection system length, diameter, and number all have a significant effect on improving the GFRP bar’s developed tensile stress. Among all variables, using two connection systems with 50-mm connection length and 4-m tape length showed the most promising result in reaching the maximum developed tensile strength (bar rupture failure). The research contributes to reducing GFRP bar splicing length and efficiently achieving the required tensile stress.

Determination of fiber-reinforced polymer (FRP) confinement-induced improvements in the mechanical properties of concrete columns under compression is a current concern, particularly if partial confinement applied on a noncircular cross-sectional shape is to be considered. Although several design-oriented predictive formulations have been proposed for the calculation of axial strength and axial strain ductility of FRP-confined concrete, their applications are, in general, limited to a specific cross-sectional shape (circular, square, or rectangular cross section) and a certain confinement arrangement (fully or partially confining system). Accordingly, the aim in this study is to establish new unified strength and ductility models for concrete columns of circular or noncircular cross sections with fully or partially confining FRP systems. To achieve the highest level of predictive performance through a nonlinear regression technique, two datasets, consisting of 2,117 test data of peak strength and 2,050 test data of strain ductility, available in the literature, were collected. The dominance degrees of size effect, sectional noncircularity (corner radius ratio), cross-sectional aspect ratio, and confinement configuration type on confinement effectiveness were evaluated and reflected in the development of these regression-based models. Through predictions of test data compiled in the datasets and a comparison with the performances of available predictive models, the proposed unified formulations demonstrated a high level of reliability and were found to be proper for design purposes.

The effect of in-plane damage on the out-of-plane response of retrofitted and thermally insulated masonry infills was examined in this paper through a set of experiments performed on a medium-scale reinforced concrete frame. Structural reinforcement was realized through the use of textile reinforced mortar (TRM), while expanded polystyrene boards were used for thermal insulation. Various specimen configurations were tested both in- and then out-of-plane sequentially for each infill specimen. Experimental results have shown that the TRM-based strengthening scheme can improve the out-of-plane response of masonry infills both in terms of strength and stiffness, especially in the case of predamaged walls, where strength increases of above 80% were achieved. The addition of insulation arranged in a sandwich configuration resulted in a slight out-of-plane improvement but was not as effective in the case of predamaged infills due to prior in-plane loading, which caused partial debonding of the board. An analytical model is also proposed and validated against the experimental data, which can predict the out-of-plane behavior of a masonry infill while also accounting for the existence of reinforcement and prior damage. Finally, using the same model in a number of case studies, generalized response diagrams are produced and a set of simplified empirical equations is suggested.

An experimental and analytical study on the bond durability of near-surface-mounted fiber-reinforced polymer bars is reported in this paper. C-shaped pullout specimens reinforced with either glass fiber–reinforced polymer (GFRP) or basalt fiber–reinforced polymer (BFRP) bars were tested after being exposed to aggressive environments, namely (1) immersion in salted water (SW) at 50°C for 30, 60, and 120 days; (2) exposure to 120 wet–dry cycles (WD) at 50°C during the wet duration and at 30°C during the dry duration; and (3) exposure to 120 freeze–thaw cycles (FT). Beside the environmental exposures, the examined parameters comprised the bar texture (either sand-coated or deformed) and the epoxy adhesive used (two epoxy–based adhesives). The effect of each parameter on the bond–slip curves, the adhesion to the surrounding epoxy adhesive, the bond strength of the bars, and the mode of failure are reported and discussed. The test results revealed that the unconditioned NSM-BFRP and GFRP specimens exhibited almost identical average bond strengths. The unconditioned deformed BFRP and GFRP bars exhibited higher adhesion stress than their sand-coated counterparts. The unconditioned specimens with SD (Sikadur-30) epoxy adhesive exhibited less bond strength than those with NG (NSM-Gel) epoxy adhesive whereas those with SD epoxy demonstrated excellent bond durability after being immersed in SW at 50°C for 120 days. Specimens exposed to wet–dry cycles showed slight variation in their bond strength after conditioning. However, specimens immersed in SW solutions for 30 and 60 days and those subjected to FT cycles showed significant losses in their bond strengths. The obtained scanning electron microscope data confirmed the modes of failure observed during testing. The Eligehausen, Popov, and Bertero analytical model was calibrated to model the bond–slip relationships of the NSM-BFRP bars.

This paper presents an experimental study that investigated the physical properties, longitudinal tensile properties, and bond strength of a new generation of glass fiber–reinforced polymer (GFRP) bars. Five commercially available types of GFRP bars with different surface treatments (deformed/ribbed, helically deformed, helically grooved, double helical wrap/sand-coated, and sand-coated) were selected for this investigation. Two bar sizes (No. 5 and No. 8)—with 15.9 and 25.4 mm nominal diameters representing the range of GFRP-reinforcing bars typically used in practice as longitudinal reinforcement in concrete flexural members—were selected from each of the manufacturers. The test results reveal that these new higher modulus GFRP bars have physical properties, tensile strength, and moduli of elasticity greatly exceeding the requirements of ASTM and Canadian Standards Association (CSA) standards. The GFRP bars had measured cross-sectional areas near or slightly over the maximum cross-sectional area allowed by ASTM standards. The mechanical properties reported in this study were based on nominal cross-sectional areas. The longitudinal tensile properties and bond strength of the GFRP bars satisfied the limits in ASTM and CSA standards. GFRP bar manufacturers have developed and are producing GFRP bars with guaranteed strengths that significantly exceed the ASTM minimum specifications. However, ASTM bases qualification methods for tensile strength on the minimum ASTM specified values, which could differ from the values implemented in the design. Until ASTM bases rejection on the strength used in design, the designer should add rejection criteria to the project specifications to ensure that GFRP bar acceptance is based on the strength assumed for design, which could be between the minimum value specified in ASTM and the manufacturer’s guaranteed value.

The new American Concrete Institute (ACI) design code for glass fiber–reinforced polymer (GFRP)–reinforced concrete (RC) members specifies a single value of 1.20 for the bond-dependent coefficient (k_b) of all types of GFRP bars. This value was chosen based on test data from this project, as well as a compilation of data available in the literature. This paper reports on an experimental study that assessed the k_b for the new generation of GFRP bars from five different manufacturers with different surface types: deformed/ribbed, helically deformed, helically grooved, double-helical wrap/sand-coated, and sand-coated. Two bar sizes (No. 5 and No. 8)—with 15.9 and 25.4 mm nominal diameters representing the typical range of GFRP-reinforcing bars used in practice as longitudinal reinforcement in concrete members subjected to bending—were selected from each of the manufacturers. Five RC beam replicates were used to increase experimental accuracy. Therefore, a total of 60 beams, including 50 beams reinforced with GFRP bars and 10 control beams reinforced with conventional steel bars for comparison purposes, were constructed and tested to failure according to a predetermined test method. Based on the analysis, the study confirms using the bond-dependent coefficient value of 1.20 as adopted in the new ACI design code for GFRP–RC members. This value is recommended for GFRP bars complying with (or exceeding) the material specification listed in the ASTM International standard specification for solid round GFRP bars for concrete reinforcement.

Bond-type anchorages with plain carbon fiber–reinforced polymer (CFRP) tendon, ribbed CFRP tendon, and CFRP tendon with metallic ribs were fabricated and tested. The contributions of chemical adhesion force, friction force, and bearing force of the surface ribs to the maximum pullout load of a bond-type anchorage with ribbed CFRP tendon were discussed. The influences of the metallic rib type, metallic rib number, and preloading force on the anchoring performance of a bond-type anchorage with CFRP tendon were also investigated. Finite-element simulation was also conducted to help study the working mechanism of wedge plug type ribs. Results show that bilinear and trilinear bond-slip models can characterize the bond performance of a bond-type anchorage with plain and ribbed CFRP tendon, respectively. In addition, bearing forces of surface ribs are limited due to the low shear strength of the CFRP material. However, metallic ribs can address this problem and promote the anchoring performance if the connection between the CFRP tendon and the metallic rib is reliable. A wedge plug type rib exerts a larger contact pressure on the surface of the CFRP tendon during the plugging and loading procedure compared with chip-type rib, making it more reliable than the chip-type rib. Moreover, preloading the wedge plug type rib after plugging eliminates the initial slip between the CFRP tendon and the metallic rib. This allows the wedge plug type rib to function sensitively under pullout load. Consequently, the bond between the CFRP tendon and the grout can work together with the friction between the CFRP tendon and the wedge plug type rib, and thus can avoid the “snap back” phenomenon in the pullout test.

Precast fiber-reinforced concrete (FRC) tunnel lining segments are designed using fundamental principles recommended in various international design provisions and guidelines. Current design provisions, however, do not apply to designing precast concrete tunnel (PCTL) segments reinforced internally with fiber-reinforced polymer (FRP) bars. Moreover, the behavior of PCTL segments reinforced internally with glass-FRP (GFRP) bars under quasi-static cyclic flexural loading is a field in which no experimental research results are available in practice. This paper reports on investigating the cyclic behavior of GFRP-reinforced precast FRC tunnel lining segments. Four full-scale GFRP-reinforced PCTL segments were fabricated and tested under quasi-static cyclic flexural loading. The segments had a total length, width, and thickness of 3,100, 1,500, and 250 mm, respectively. The investigated test parameters were the concrete type, the longitudinal reinforcement ratio, and the transverse reinforcement configuration. The hysteresis response, cracking pattern, ductility index, deformability, unloading stiffness degradation, and secant stiffness damage index of the test segments were identified and evaluated. The experimental results from this study showed that the hysteretic response of the GFRP-reinforced precast FRC tunnel lining segments exhibited stable cyclic behavior with no or limited strength degradation until failure. Moreover, the test results showed that the segments demonstrated adequate ductility index and deformability limits. An analytical model to predict the hysteresis behavior of the test segments was produced, and its results were compared to the experimental results. The results of this study showed the effectiveness of using FRC for GFRP-reinforced PCTL segment applications under quasi-static cyclic flexural conditions.

A fiber-reinforced polymer grid-reinforced engineered cementitious composite (FRPECC) is a promising strengthening composite that addresses the drawbacks of FRP and textile-reinforced mortar (TRM). The present study focused on the axial compressive behavior and constitutive models of predamaged concrete repaired with carbon FRP grid-reinforced ECC (CFRPECC). The test parameters included predamage level, grid number, and concrete strength. The test results indicated that compared with plain concrete, the peak stress (strength), strain at peak stress (peak strain), and energy dissipation capacities of CFRPECC-retrofitted predamaged concrete increased by 9%–82%, 28%–96%, and 60%–322%, respectively. The improvement increased with the number of CFRP grids and ECC thickness but decreased with concrete strength. On the other hand, the strength and elastic modulus of concrete after retrofitting degraded with an increase in the predamage level, especially when the concrete was severely damaged, while the peak strain and lateral rupture strain were barely influenced. When the predamage level increased, the enhancement of strength decreased from 41%–106% to 9%–60%. CFRPECC directly carried 11.0%–22.7% of the strength of the retrofitted concrete columns. The enhancements of strength and elastic modulus after retrofitting were mainly attributed to the confinement and axial stiffness of CFRPECC, respectively. Further, models for the strength, peak strain, and stress–strain relationship were developed for CFRPECC-retrofitted predamaged concrete. The applicability of the proposed models was verified by test results of the existing studies.

Externally bonded composite systems are commonly used for the structural strengthening of existing arched masonry structures. Textile-reinforced mortar systems applied to masonry arch intrados allow the load carrying capacity of the structure to be increased and the infrastructure to be kept in service during application of strengthening systems. The structural performance of mortar-based materials [i.e., fabric-reinforced cementitious matrix (FRCM), textile-reinforced mortar (TRM), and steel-reinforced grout (SRG)] depends on the bond properties between the matrix and the substrate and between the matrix and the fibers. Also, when a strengthening system is applied to a concave masonry substrate, the additional stress component (normal to the substrate) at the matrix–substrate and matrix–fiber interface, due to the substrate curvature, negatively effects the stress-transfer mechanism between the strengthening system and the masonry substrate. In current engineering practice, spike anchors are generally used to improve bonding properties between the composite material and the substrate in order to reduce the negative effects of a strengthening system applied to a concave masonry/concrete substrate. In this paper, we report and discuss the results obtained from direct shear tests performed on an FRCM with a pozzolanic reaction on strengthened curved masonry specimens. In particular, the experimental campaign revealed the effects of spike anchors in the stress-transfer mechanism between the FRCM and the masonry substrate. The presence of spike anchors, as we determined, caused local effects that altered the stress-transfer mechanism. However, the maximum force obtained from single shear-lap tests on specimens with spike anchors was, on average, higher than that obtained from specimens without spike anchors.

This study aims to investigate the flexural performance and design methods of flexural concrete beams reinforced with basalt fiber–reinforced polymer (BFRP) and steel bars. Eight concrete beams, which included six reinforced with a combination of BFRP and steel bars, one reinforced with only steel bars, and one reinforced with only BFRP bars, were constructed and then tested under four-point loading. For hybrid-reinforced concrete (RC) beams, two types of BFRP bars (ribbed and sand-coated) and different reinforcement ratios were considered. The test results are discussed in terms of the flexural capacity, reinforcement strain, deflection responses, and cracking behavior. Experimental results showed that the crack width of the hybrid-RC beam, obtained by adding BFRP bars in the concrete cover of the steel-RC beam, reduced by approximately 40% compared with that of the steel-RC beam, and the strength utilization of BFRP bars in hybrid-RC beams was higher than that in the BFRP-RC beam. Additionally, the cracking moment, nominal flexural strength, and midspan deflection of the hybrid-RC beams were predicted by using existing design models. The bond-dependent coefficients (k_b) of the ribbed and sand-coated BFRP bars were assessed by using test data.