Journal of Composites for Construction

The confinement effect of concrete by fiber-reinforced polymer (FRP) jackets may significantly enhance the axial compressive performance of concrete, although the results are variable. Therefore, rational calibration of deterministic models for compressive strength and the associated strain and stress–strain (SS) curves is required based on available experimental databases and selected probabilistic models. To improve the accuracy of the probabilistic model, second branch classifications for the stress–strain curve of FRP-confined concrete are identified within a series combination of uncertainties with computerized classification algorithms before probabilistic calibrations. The Bayesian theorem and Markov chain Monte Carlo (MCMC) approaches were used to update a probabilistic model that includes the critical variables that have been established in previous research. Furthermore, eight representative deterministic strength enhancement models, three strain enhancement models, and four stress–strain models were chosen for evaluation using credible intervals (CIs) and confidence levels (CLs) at different strain levels. Different types of FRPs were also analyzed individually to ensure the validity and reliability of the findings. The suggested probabilistic models can predict the properties of ultimate axial stress and related strain, thus offering an effective method for calibrating the confidence level and computational correctness of deterministic models previously published in the literature.

The present study is focused on the retrofitting solution and its verification through experimental investigation for the two most common catastrophic modes of failure in reinforced concrete (RC) frame buildings, namely, buckling of reinforcing bars in the plastic hinge region of columns and shear failure of beam–column joints. The first mode of failure is retrofitted with interlinked coupler-box assemblies, while the second mode is repaired with external carbon fiber–reinforced polymer (CFRP) wrapping. The efficiency of the proposed retrofitting technique is evaluated by the hysteresis behavior, failure mechanism, and performance index parameters of the re-established frame, which are compared with the results of the initially tested bare frame. The recommended mechanism restricts the restored frame section against any possible movement or rotation and helps by a subsequent shift in yield location. The prime advantage of the proposed technique is that it prevents the possible slip of rebar from the sleeve, which is intercepted by the interlinking process, and eliminates the formation of inclined shear cracks at beam–column joints restricted by externally wrapped CFRP composites. The synergic effect of coupler-box and CFRP enhances the overall performance of the re-established frame by keeping the frame sections intact even after a lateral drift of 6%, which is relatively higher than the collapse prevention drift level, i.e., 4% as per FEMA guidelines. The obtained test results reveal the capability of CFRP and coupler-box assemblage as a possible futuristic approach for retrofitting outmoded RC building frames, in which buckling in longitudinal rebars of columns at their respective plastic hinge locations is inevitable.

External confinement of reinforced concrete (RC) columns with fiber-reinforced polymer (FRP) jackets and/or wraps is a technique extensively used for strengthening and retrofit of structurally deficient columns. The confinement effect produced by the externally bonded FRP acts simultaneously with the confining mechanism of the existing internal reinforcing steel, thus increasing the vertical load capacity and ductility of the member. The transverse steel confinement contribution can be significant, although it is generally ignored in existing design guidelines for FRP wrapping, potentially leading to an overconservative retrofit design. This paper proposes a modification to the design equation of FRP-confined RC circular columns subjected to axial loading that is given in the current US standards. The proposed design equation is calibrated through a structural reliability analysis approach, in which the capacity model (corresponding to the probability distribution for the axial load capacity of the columns) is generated via Monte Carlo simulation based on advanced nonlinear finite-element response analyses for multiple realistic combinations of design parameters. Under different design conditions, the newly proposed design equation provides a significantly less variable reliability index than that obtained using the current accepted design equation, which produces increasingly overconservative retrofit designs for increasing amounts of transverse steel reinforcement. A practical design procedure based on the proposed design equation is also presented.

Natural fibers have become a research hotspot in composite materials recently. This study investigated the axial compressive behavior of predamaged concrete repaired by jute fiber–reinforced polymer (JFRP) composites. Basalt FRP (BFRP) was also utilized to repair the predamaged concrete for comparison. Thirty FRP-jacketed concrete cylinders with a diameter of 150 mm and a height of 300 mm were designed and fabricated. Plain concrete cylinders were axially preloaded to three predamage levels, then repaired by JFRP and BFRP composites, and reloaded to evaluate the compressive behavior after repair. The results showed that the predamage of concrete had a noticeable adverse effect on the compressive behavior of JFRP- and BFRP-repaired concrete. Compared with the JFRP-jacketed intact concrete, the compressive strength and initial elastic modulus of the JFRP-repaired predamaged concrete decreased by 6%–18% and 16%–54%, respectively. However, the compressive strength and ultimate strain of the predamaged concrete were still improved by 5%–41% and 82%–291% after JFRP repair, respectively, when compared with those of the plain concrete. On the contrary, the ultimate axial strain and lateral strain capacities of JFRP- and BFRP-repaired concrete were insignificantly affected by the concrete predamage. Under similar confinement ratios, the compressive strengths of JFRP- and BFRP-jacketed intact concrete were close to each other, whereas the axial strain–lateral strain relationship and the variations of the Poisson’s ratios were highly dependent on the FRP types and the number of FRP layers. Based on the test results and data collected from the open literature, an ultimate strength model, an ultimate axial strain model, and a stress–strain relationship model were newly developed for JFRP-repaired predamaged concrete.

Fabric–reinforced cementitious matrix (FRCM) composites have emerged as a viable solution for the external confinement of deficient masonry members. They represent an appealing alternative to fiber–reinforced polymer (FRP) systems since the use of epoxy matrixes raises some unsolved issues, such as the poor composite–substrate compatibility, low permeability of the strengthened surface, and the difficulties in removing the FRP sheets without damaging the substrate. To investigate the efficacy of FRCM confinement, a number of experimental investigations have been published in the literature, and a wide overview will be reported in this paper. However, to date there is a lack of relevant analytical studies that propose sound models to estimate the compressive strength of FRCM confined masonry; difficulties are related to the high complexity and heterogeneity of the masonry and to the uncertainties that are derived from FRCM confinement, whose performance is affected by many variables, which include the quality of the inorganic matrix and geometry of the fabric mesh. Most of the collected studies focus on investigating the applicability of the existing formulations that are suitable for FRP systems to examples of FRCMs, a few others provide promising proposals although they were validated using a limited amount of experimental data. This paper will present an analytical study on the confinement of masonry columns with FRCM composites with two aims: (a) propose new models to estimate the compressive strength; and (b) assess the existing formulas. A wide database that includes the results of compression tests on 211 masonry members that are externally wrapped with FRCM systems will be assembled from the literature. The collected data will be organized into a systematic framework and will be analyzed based on some relevant parameters, such as the type of fiber [e.g., basalt (B), carbon (C), glass (G), steel (S), and poliparafenilenbenzobisoxazole (PBO)], the geometry of the mesh, number of layers employed, mechanical properties of the inorganic matrix, and compressive strength of the unconfined masonry (f_mc). Strength models for FRCM confined masonry will be developed by best-fit analyses, and comparisons with formulations that are available in the literature and some international guidelines will be performed.

Considering the limited experimental work carried out on fiber-reinforced polymer (FRP) bars in lightweight concrete (LWC) beams, there is a need for more investigation to understand their flexural behavior and serviceability performance. This paper reports on an investigation based upon an experimental study that evaluated the flexural capacity and serviceability performance of lightweight self-consolidating concrete (LWSCC) beams reinforced with glass-FRP (GFRP) bars. Ten reinforced concrete (RC) beam specimens (200 wide × 300 high × 3,100 mm long) were prepared and tested under four-point bending up to failure. Eight specimens were made with LWSCC, while the other two were made with normal-weight concrete (NWC) as reference specimens. The test variables were concrete density (LWSCC and NWC), reinforcement type (sand-coated GFRP, helically grooved GFRP, or steel bars), and longitudinal GFRP reinforcement ratio. The lightweight coarse aggregate (Solite 343) was used along with the two types of fine aggregate [lightweight sand (Solite 307) and natural sand (NS)] in the LWSCC mixtures, leading to a concrete density of 1,800 kg/m³. The test results indicate that the GFRP-RC beams failed as a result of concrete crushing. The normalized moment capacity of the GFRP-reinforced LWSCC beams was approximately 0.90 times that of the counterpart GFRP-reinforced NWC beams. The predicted moment capacities of the GFRP beams were estimated based on the strain-compatibility approach in the design standards, which showed good agreement between the predicted and experimental results. Moreover, the recorded deflections and crack widths of the GFRP-reinforced LWSCC beams are presented and compared to those predicted with the FRP design provisions and the literature. The comparisons reveal that the deflections and crack widths of the GFRP-reinforced LWSCC beams can be estimated with the FRP design provisions with a variable degree of conservativeness.

Conventional fiber-reinforced polymer (FRP) bar ties in square FRP-reinforced concrete (FRP-RC) columns provide low strength and ductility. This is due to the lower ultimate tensile strength at the bent sections (corner) than at the straight sections of the conventional FRP bar ties. To overcome this problem, a new system of carbon-FRP (CFRP) tube ties was developed and its applicability was investigated in high-strength concrete (HSC) square columns. Hence, this study experimentally investigated the axial and flexural behaviors of the HSC specimens reinforced longitudinally with glass-FRP (GFRP) bars and laterally with CFRP tube ties (CFRP-TTs). The effect of CFRP-TTs on the behavior of GFRP bar–reinforced HSC specimens was investigated in terms of strength development and failure pattern under different loading conditions (concentric, 25- and 50-mm eccentric axial loadings, and four-point bending). The experimental results of the GFRP bar–reinforced HSC specimens laterally reinforced with CFRP-TTs (Group GC specimens) were compared with the steel bar–reinforced HSC specimens laterally reinforced with steel ties (Group SS specimens). The test results showed that the better confinement offered by the CFRP-TTs in the Group GC specimens resulted in higher axial load and bending moment capacity than in Group SS specimens under concentric, 25-mm eccentric axial loading, and four-point bending conditions.

This study investigated the effects of different configurations of connection reinforcement and anchorages on the cyclic behavior of glass fiber–reinforced polymer (GFRP) reinforced-concrete (RC) exterior beam–column connections (T-connections). Three full-scale GFRP-RC T-connections were tested under reversed cyclic loading. One connection was detailed with 90-degree hooked anchorage and horizontal stirrups, while the other two connections were detailed with L-shaped anchorage by placing additional Z- or U-shaped bars at the connection region. The objective of this study was to investigate the influence of the anchorage type (90-degree hook or L-shaped) at the end of the longitudinal bars of the beam on the cyclic performance (strength, ductility, and energy dissipation) of GFRP-RC T-connections. Moreover, the anchorage performances in terms of anchorage resistance, bond stress–slip of bars within the joint were evaluated and compared with each other. Test results indicated that all the connections passed a 5.09% drift ratio without any loss of strength. The addition of U-bars into the L-shaped anchorage connection significantly improved the strength, ductility, and energy dissipation compared with the other two connections.

This study proposes using glass fiber–reinforced polymer (GFRP) as a stay-in-place structural formwork for casting bridge decks with ultrahigh-performance concrete (UHPC). The GFRP stay-in-place formworks completely replace the bottom layer of rebars, and the top steel reinforcement is also replaced by a GFRP mesh to mitigate the corrosion damage. The formworks were either a flat GFRP plate with square hollow section (SHS) stiffeners or a flat GFRP plate with new Y-shape stiffeners. Concentric static tests on five 1:2.75 scale decks were performed to investigate the effect of stiffener's configuration and the influence of the concrete strength on the performance of bridge decks. Rotational fixity support was used to simulate a real bridge deck connection of supporting girders. All specimens with the stay-in-place formwork exhibited punching shear failure. It was found that the use of Y-shape stiffeners significantly improved the load-carrying capacity of the proposed deck. Replacing normal concrete with UHPC further improved the loading capacity of the deck. The decks demonstrated excellent performance, with the load-carrying capacity 3.8–9.5 times higher than the established equivalent service load depending on the concrete strength and configuration of the GFRP stay-in-place formwork. Deflection at service load was less than span/1,600 for all the decks. Compared with normal-strength concrete (34 MPa), UHPC improved the maximum load-carrying capacity of the deck from 91.4 to 149 kN.

Predicting the nominal axial capacity of slender fiber-reinforced polymer (FRP) reinforced concrete (RC) columns is dependent majorly on their flexural stiffness. However, the current design provisions do not incorporate design equations to estimate the flexural stiffness of slender FRP-RC columns yet due to the limited research work on this aspect. Although limited research studies proposed flexural stiffness models for slender FRP-RC columns, these models show inaccurate results with large discrepancies. This study, therefore, compiles and analyzes a surveyed database of 53 tested slender FRP-RC columns found in the literature to construct a simplified and accurate model to predict the flexural stiffness of the slender FRP-RC columns. In this approach, the experimental-based flexural stiffness values of the tested specimens were used to build a nonlinear regression flexural stiffness model and examine the influence of the critical design parameters affecting this value. As a result, the proposed model showed a strong agreement with the experimental flexural stiffness values evidenced by having the least root mean square error (RMSE) compared to the other proposed models in the literature. Moreover, the proposed model was theoretically evaluated accounting for the second-moment order effect by which a data set of 36,000 cases were generated and compared to the results of the proposed model to increase its creditability and repeatability. Moreover, a design example was presented to quantify the difference in predicting the flexural stiffness of the slender FRP-RC columns between the proposed and available models. Accordingly, the proposed model revealed better representation of the flexural stiffness with higher accuracy compared to the available models which will help the engineers to accurately design the FRP-RC columns.

To assess experimentally the effect of flexural near surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) bars mounted on the sides of beams on the shear strength of reinforced concrete (RC) beams, 18 shear-deficient RC beams were built, strengthened with flexural NSM-CFRP bars, and tested under three-point bending until failure. The variables of the experimental program included the beam depth, the concrete compressive strength, and the flexural fiber-reinforced polymer (FRP) reinforcement ratio. It was observed that the strengthened beams exhibited up to 35% increase in shear capacity over the control beams. It was also observed that the increase in shear strength provided by concrete after strengthening was higher for beams with normal strength concrete when compared to those with higher strength concrete. The results have also revealed that the percent change in shear strength provided by concrete for the strengthened beams decreased with the increase in beam depth. Experimental data from this study showed that current standards become unconservative for beams with large depths. Five different beam shear strength models found in the literature were utilized to predict the shear strength of the tested beams. The models that exhibited the closest agreement with the experimental data were those of the University of Houston and the second order simplified modified compression field theory. It was concluded that flexural longitudinal NSM bars are a viable solution to enhance the shear strength of RC beams and that the shear strength provided by concrete can be accurately quantified using published models.

Corrugated steel-sheet culvert systems were extensively applied in the construction of hydraulic and agricultural highways underpasses from the 1980s until the beginning of the 21st century. Less than 30 years after being built, the level of corrosion in the steel sheets was found to be higher than expected, potentially compromising the structural safety and service life of these structures. In this context, it is urgent to develop durable solutions for both the rehabilitation of such underpasses and the construction of new ones. Recently, structural systems based on glass fiber–reinforced polymer (GFRP) culvert sections have been proposed as an answer to this issue and have already been used in a few rehabilitations, installed inside of existing steel culverts. However, doubts have been raised about the performance of this new solution, mainly due to the lack of consolidated knowledge about its structural behavior in this specific type of work. This paper presents an experimental and numerical investigation of the performance of a commercially available GFRP culvert system. The experimental program comprised coupon tests and full-scale flexural tests up to failure on GFRP culverts, with a 60-mm-thick wall, produced by filament winding, with a height of ∼2.15 m and a width of ∼3.40 m. Conventional finite-element (FE) models were developed with commercial FE packages to simulate the structural behavior of the GFRP culverts. Following validation, a design parametric analysis was carried out with those FE models, demonstrating that this structural solution is able to comply with serviceability and ultimate-limit states requirements. Finally, this paper presents a case study of the rehabilitation of an underpass originally built with corrugated steel sheets, using this new GFRP culvert. Overall, the results obtained in this study show the feasibility of applying GFRP culverts—both in new structures and when rehabilitating existing underpasses—and of using conventional FE tools in their design.

This paper presents an experimental and a numerical investigation of the structural behavior of precast concrete sandwich panels made with Vierendeel truss–like FRP shear connectors. Two series of tests are reported and analyzed. The first includes two concrete sandwich panels and one solid panel of the same dimensions under four-point bending, and the second includes sandwich and solid panels under eccentric axial compression. The degree of composite action and the post-cracking stiffness in these panels are discussed, along with their modes of failure and load carrying capacity. The panels subjected to bending exhibited a ductile response, with a typical flexural failure characterized by yielding of the steel reinforcement under a similar load level as their counterpart solid panel. The sandwich panels exhibited a partial composite action in the elastic range, but their post-cracking stiffness was comparable with that of the solid panel. The axially loaded sandwich panels failed due to buckling characterized by a limit-point mode, and their strength and stiffness were very close to that of a solid concrete panel with the same dimensions. A finite-element (FE) model is also developed, which describes the partial shear interaction through the elasticity of the various structural components. The predictions obtained from the model are compared with the experimental results and reveal good correlation within the elastic range. The FE model is further used to shed more light on the structural response through a parametric study. The study shows that sandwich panels built with such connectors can provide an attractive design solution given that their stiffness at the precracked stage, and their failure loads were comparable with those of solid panels with the same dimensions.

The bond between concrete and fiber-reinforced polymer (FRP) bars is severely degraded when the glass transition temperature of the FRP (set mainly by the polymeric matrix, typically a thermosetting resin) is approached, and therefore long development lengths are required to enable a proper anchorage in cooler zones of FRP–reinforced concrete (RC) members exposed to fire. In spite of the potential of bent bars to shorten such lengths, and thereby improve the fire resistance of FRP–RC members, very few studies have addressed the effects of elevated temperatures on the bond performance of bent FRP reinforcement. This paper presents experimental and numerical investigations concerning the bond behavior of straight and 90°-bent glass-FRP (GFRP) bars at elevated temperatures. Steady-state pull-out tests were first carried out on bent ribbed bars, from 20°C up to 300°C, and the results were compared with those previously obtained from straight bars. The experiments showed that the hook effect provided by the bend and tail lengths of the bars enabled bond-strength increases of between 30% and 90% compared with straight bars. Three-dimensional finite-element models were then developed to: (1) simulate the pull-out tests; and (2) perform design-oriented parametric studies, aimed at assessing the influence of elevated temperatures on the anchorage strength of straight bars with different surface finishes (sand-coated and ribbed), and of 90°-bent ribbed bars with varying tail and straight development lengths. Temperature-dependent local bond stress versus slip laws were implemented in the models in order to describe the bond interaction along the straight and bent lengths of the bars. The models provided a good agreement with the test data, in terms of load versus slip response, and a reduction in pull-out load and bond stiffness with temperature. The findings were that: (1) the adoption of 90°-bent anchorages with appropriate tail lengths is an effective and practical approach for improving the bond strength of GFRP bars at both ambient and elevated temperatures; and (2) at elevated temperatures, to mobilize the tensile strength of GFRP bars, the development lengths of straight and bent bars designed for ambient temperature must be significantly increased. Finally, optimal anchorage lengths are proposed, as a function of temperature, for beam and slab applications.

This paper theoretically analyzes the symmetrical global buckling behavior of imperfect prestressed carbon fiber-reinforced polymer (CFRP) laminate-reinforced steel columns (PCRSCs) with two hinged supports under axial and eccentric compression loadings. First, the causes of the buckling of the component are revealed; either the steel yields or the CFRP becomes slack. Therefore, four buckling cases are found, and a theoretical calculation method for the buckling capacity is established. On this basis, a theoretical calculation method for the optimal CFRP initial prestressing force and maximum buckling capacity is built from physical explanations. The results of the theoretical method fit well with the test and finite-element results. Finally, a design method of PCRSCs is proposed for engineering applications, followed by a design example, by which the optimal reinforcing efficiency is realized.

The performance of concrete structures strengthened with carbon fiber–reinforced polymer (CFRP) systems can depend heavily on the bond strength of the interface between the concrete and the reinforced polymer. Even though experimental testing can be used to derive suitable constitutive models, their interpretation and analysis is often limited by the reliability of available numerical/analytical models. The debonding in shear tests can be controlled by the highly nonlinear interaction of the bonded interface with the microcracks developing in the substrate. This process cannot be efficiently predicted by simplifying assumptions, which is why robust models accounting for those features, while relying only on material parameters that can be easily measured and interpreted, need to be developed. This paper introduces a framework for developing such models based on the discrete representation of fracture that can be easily deployed into existing finite-element codes. The substrate bond failure, in addition to the interface bond failure and any combination thereof, are automatically accounted for, and the cracks are not prespecified to the underlying finite-element mesh, which means that the results are mesh-insensitive and discretization-independent. A validation of the proposed framework was performed using modified double-shear bond tests between CFRP and concrete. An in-depth analysis was carried out to assess the influence of bond length and CFRP reinforcement area on the debonding behavior and ductility of the connection.

The current experimental investigation assessed the durability of reinforced concrete (RC) piles with glass fiber-reinforced polymer (GFRP) bars and spirals exposed to simulated marine environments. A total of 18 specimens—9 square (300 mm in cross section) and 9 circular (304 mm in diameter)—and short RC piles were prepared and tested under uniaxial compression. For each pile geometry, three specimens were reinforced with pristine GFRP bars as reference piles. The other six piles were reinforced with GFRP bars that had been exposed to marine environments at subtropical regions (22°C) or accelerated aging temperature (60°C) for 12 months, three specimens for each environment. Mechanical property and microstructural analysis tests were carried out to characterize the conditioned/aged GFRP bars. The GFRP-RC piles conditioned at 60°C exhibited similar behavior to their reference counterparts. The current environmental reduction factors for GFRP reinforcing bars stipulated in design codes and guidelines are conservative. A more accurate design equation to calculate the axial capacity of piles should consider the contribution of longitudinal GFRP bars even when exposed to marine environments.