Journal of Composites for Construction

Fiber-reinforced polymers (FRPs) are widely used to enhance the flexural capacity of existing concrete structures subjected to various loading actions. However, when FRPs are applied to structural elements as externally bonded flexural reinforcement, premature debonding of the FRP may occur in the form of concrete cover separation (CCS). This study explores the potential of gene expression programming (GEP) for the development of a new model for the prediction of the shear force required to induce FRP CCS failure. A comprehensive database consisting of FRP-strengthened RC beams that failed by CCS was compiled and comprised of 127 experimental data points. GEP was employed to develop an accurate prediction model for CCS failure, encapsulating all the influencing parameters into a single equation. A comparative study was also conducted to assess the performance of the new GEP model against several existing analytical models from the literature. The proposed GEP model demonstrated significantly higher correlations between predicted and actual CCS failure loads when compared with all available predictive models.

This paper focuses on the conceptual use of a fiber-reinforced polymer (FRP) wind turbine blade that is repurposed for a second life as an electrical transmission pole. Thousands of tons of fiber-reinforced polymer composite wind turbine blades are currently coming out of service globally and are being landfilled or incinerated. These are not environmentally preferable disposal methods. This paper presents a detailed structural analysis of a Clipper C96, 46.7-m-long turbine blade used as an electrical pole. The analytical procedure needed to characterize the wind turbine blade for repurposing includes determining the external and internal geometry of the blade, identifying the types of materials and laminates used throughout the blade, and calculating effective moduli and section properties for structural analysis. Code-specified load combinations are then used to analyze the transmission line BladePole to determine internal forces and deformations and stresses. Maximum stresses were compared to those obtained from theoretical models. The results indicate that wind turbine blades can safely be used as electrical transmission poles.

Reinforced concrete (RC) columns can be effectively rehabilitated or strengthened by externally bonded fiber-reinforced polymer (FRP) wraps, which can improve the axial load capacity and ductility of these columns through a confinement effect. Modern design codes require that seismically designed RC columns have significant minimum amounts of both longitudinal and transverse steel. This transverse steel can apply a considerable confining pressure into the concrete core in addition to that produced by the external FRP sheets. This simultaneous confinement can be accurately modeled using a recently developed FRP-and-steel confined concrete model for finite-element analysis. This paper presents a comprehensive parametric study to investigate the steel confinement effects and the relative importance of key modeling and design parameters on the axial strength of FRP-confined RC columns. The results show that the steel confinement effect can significantly increase the axial strength of FRP-confined RC columns, particularly for large cross sections, low concrete compressive strengths, and low amounts of confining FRP. The steel confinement effects induce two distinct behaviors depending on the ratio between the FRP lateral confinement and the unconfined concrete peak strength. These two behaviors can be described as functions of a relative confinement coefficient. The results of this study can be used to achieve more efficient and economical retrofit of RC columns through FRP confinement.

The use of externally bonded (EB) carbon fiber–reinforced polymer (CFRP) fabrics for strengthening existing structures has received worldwide acceptance since the late 1990s. The composites produced at the time were of low density and have established the experimental database used by codes and guidelines to develop their analytical design models. As for projects requiring higher densities, the design guidelines recommend the use of several plies (multilayers) of CFRP fabrics. Use of a multilayer fabric leads to higher installation costs. Therefore, the need for high-density CFRPs has been recognized over the last few years by manufacturers. However, outstanding issues remain to be investigated, such as (1) the maximum number of EB-CFRP layers that can be installed without exhibiting slippage or debonding, particularly on vertical surfaces where strengthening in shear is required; and (2) the effectiveness of shear strengthening with multilayer CFRP fabrics, taking into account premature debonding failure and the effect of CFRP thickness. This study aims to evaluate and compare experimentally the behavior of 18 full-size reinforced concrete (RC) T-beams strengthened in shear with multilayer CFRP sheets to that of beams with monolayer strengthening fabric of equivalent density. The influence of internal transverse steel reinforcement (steel stirrups) on EB-CFRP fabric performance is also examined. The test results revealed that the increase in CFRP weight enables an increase in shear resistance, but not in a proportional way, up to an optimal value (300 g/m² in this study). In addition, strains in CFRP featured lower values as the number of layers increased and were more significant in monolayers than multilayers of equivalent weight. Finally, predictions of current design guidelines were closer to experimental results for beams without steel stirrups because they do not yet consider the interaction between EB-FRP and transverse steel.

Large rupture strain (LRS) fiber-reinforced polymer (FRP) composites, typically formed from polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) fibers, generally exhibit ultimate rupture strains >5%. Such fibers are particularly suited to the confinement of concrete columns on account of their LRS and sufficient elastic modulus. There are currently a limited number of studies on LRS FRP-confined concrete, particularly with high- and ultrahigh-strength concrete, so their behavior across a range of variables is still unknown. To improve this understanding, this paper systematically investigates the influence of fiber type, fiber thickness, and concrete strength on the behavior of FRP-confined concrete. To achieve this objective, the current investigation presents the results of 66 circular FRP-confined cylinders that are loaded concentrically. Three main parameters are investigated, namely, fiber type (i.e., PEN, PET, carbon, glass, and aramid), concrete strength (i.e., normal, high, and ultrahigh strength), and fiber thickness. The results show that regardless of fiber type, the stress–strain response is bilinear when the concrete is sufficiently confined. However, when there is insufficient confinement provided to the concrete core, the stress–strain response becomes trilinear. This trilinear response is more pronounced for LRS FRP-confined specimens because the confinement stiffness of the LRS FRP jacket is lower than that of a traditional FRP-confined specimen with an equivalent confinement ratio. Increasing the confining hoop stiffness (i.e., increasing FRP layers) reduces the magnitude of strength reduction after initial concrete cracking. It is also evident that as the unconfined concrete strength increases, the minimum confinement stiffness ratio necessary to prevent strength reduction after initial concrete cracking increases.

To investigate the shear behavior of post-tensioned concrete beams with fiber-reinforced polymer (FRP) reinforcements, six large-scale post-tensioned beams without transverse reinforcement and with a shear span-to-effective depth ratio of approximately 3.0 were tested to failure. All beams were longitudinally reinforced with draped prestressed carbon FRP tendons and non-prestressed glass FRP bars. The test variables included the amount of flexural reinforcement and the prestressing level. With the aid of full-field measurement on the beam surface using digital image correlation, the kinematics of the critical shear crack of each beam were tracked. Two shear failure modes, including shear compression and shear tension, were observed in the tested beams. Generally, when the amount of flexural reinforcement increased, there was a corresponding increase in the maximum shear force. When the total prestressing force was increased from 360 to 440 kN, the shear cracking strength and the maximum shear strength increased by 6.9% and 10.0%, respectively. It was demonstrated that an arch mechanism formed in the tested FRP post-tensioned beams, although the contribution of aggregate interlock to the shear capacity was negligible. The predictions of the shear capacity calculated from various shear design models showed that the American and Japanese recommendations were highly conservative, whereas the Canadian recommendations were more consistent with the experimental results.

Although externally bonded fiber-reinforced polymer (FRP) composites are commonly employed to strengthen reinforced concrete members, the durability of these rehabilitation measures is not well understood. This work includes the examination of FRP-strengthened girders after 12 years of service exposed to brackish water, including assessments of remaining structural strength and investigations of the FRP. Structural load tests of the rehabilitated girders—compared to companion, unstrengthened girders—demonstrate that FRP strengthening maintains influence on the girder strength and failure mode at ultimate strength. The combination of glass fiber–reinforced polymer (GFRP) and carbon fiber–reinforced polymer (CFRP) reinforcement provided a modest improvement in bridge girder strength (12% beyond residual strength) while limiting ductility. Inspection of the FRP microstructure, mechanical property tests, and bond pull-off tests, however, suggest that the FRP’s contribution to structural strength may be greatly influenced by construction techniques and exposure conditions. Strength was calculated using the strain limits from relevant guidelines and was conservatively estimated for the design scenario herein evaluated.

Glass fiber–reinforced polymer (GFRP)-reinforced flat slabs offer a convenient construction option that is both simple to build and highly resistant to corrosion. The relatively low thickness of the floor plates and absence of beams results in increased susceptibility to punching shear failure, which may become the governing design factor. This paper presents an experimental investigation on the use of synthetic fiber–reinforced self-consolidating concrete (SNFRSCC) to improve punching shear capacity of GFRP-reinforced flat slabs. Synthetic fiber was used because it is inert and corrosion resistant, and SCC facilitates use of fibers in relatively high dosages without adverse effects on concrete placing quality. Six large-scale interior slabs were experimentally tested for punching shear behavior assessment. Three SNFRSCC specimens with varying longitudinal reinforcement ratios were compared with their SCC control counterparts. It was confirmed experimentally that the punching shear capacity did not display high dependency on the longitudinal reinforcement spacing. However, the punching shear capacities of the SNFRSCC specimens were marginally improved in comparison with the control specimens. In addition, the SNFRSCC specimens exhibited substantial toughness improvements (2.34 times on average). Moreover, analytical expressions were developed to estimate the punching shear capacity as well as load-rotation relationships for the GFRP-reinforced SCC and SNFRSCC slabs. The analytical expressions were based on the critical shear crack theory (CSCT). The modified CSCT predictions showed a considerable agreement with the experimentally observed load-rotation behavior and punching shear capacity for SCC and SNFRSCC specimens alike.

Strengthening of existing reinforced concrete (RC) structures by externally bonding carbon fiber–reinforced polymers (CFRP) applied to the concrete substrate has proven to be effective in enhancing the shear capacity of RC beams. However, a premature debonding of CFRP laminates does not allow a full utilization of the strength of laminates. This paper reports on a research program that aimed at studying the effect of anchoring CFRP sheets used for shear strengthening using CFRP splay anchors. The anchors were used to further improve the shear strength contribution of FRP by delaying debonding and achieving a higher strain in the external CFRP reinforcement. A total of nine shear-deficient T-beams were cast and externally strengthened with high-modulus CFRP laminates in the form of U-wraps and anchored with different configurations of high-modulus CFRP splay anchors. The variables considered for the experimental program included anchor embedment depth and anchor splay length. In addition, the effect of installing the anchors directly on the concrete substrate prior to installing the U-wraps was investigated. The performance of the anchored U-wrapped specimens in terms of failure modes, load–deflection responses, and shear strength and strain in CFRP laminates was compared with that of unstrengthened and unanchored strengthened control specimens. The test results showed that high-modulus CFRP anchors delayed the debonding of the U-wraps and significantly improved the shear strength and ductility of the beam specimens. It was also concluded that anchoring CFRP U-wrap shear reinforcement by properly designed CFRP anchors allows for a higher strain to be attained in the CFRP laminates. A comparison between the experimental capacities of the tested specimens with the theoretical values predicted using current standards provides reasonable predictions on the shear capacity of anchored specimens.

This paper proposes, for the first time, a design methodology against the web-crippling failure of pultruded glass fiber–reinforced polymer (GFRP) I-section beams, based on the direct strength method (DSM). This study took into consideration previous experimental and numerical data reported by the authors to calibrate the DSM expression. Approximate formulas were derived to estimate the web-buckling and web-crushing loads for beams under end two flange (ETF) and interior two flange (ITF) loading cases. Finite-element (FE) analyses were also performed to generate complementary data, particularly for higher levels of slenderness, providing a more robust basis for the calibration of the proposed design formulations. Both experimental and numerical results were very well approximated by unified DSM expressions, that fitted both ETF and ITF configurations simultaneously, for a significant variety of materials and section dimensions. Finally, the proposed DSM formula also provides an in-depth and important novelty by identifying the slenderness ranges for which the web-crippling failure is triggered by web crushing, web buckling, or an interaction thereof.

Few studies have investigated the eccentric compression behavior of fiber-reinforced polymer (FRP) confined reinforced concrete (RC) columns. In particular, the studies on the eccentrically loaded large-sized FRP-confined RC columns are limited. In this study, the mechanical responses and size effect behavior of FRP-confined RC square columns under eccentric loading will be investigated experimentally and numerically. Nine carbon fiber–reinforced polymer (CFRP) confined columns and three control columns with cross-sectional widths (h) from 200 to 600 mm will be tested, in which the influence of the structural size and eccentricity ratio (e/h₀) will be studied. The failure patterns, load–deflection responses, and nominal stress–strain curves of CFRP and longitudinal reinforcement will be presented. The nominal axial strength and corresponding lateral deflection of tested CFRP-confined RC columns size effect existed. Subsequently, a finite-element (FE) model was developed and verified, in which the mesoscale structure of the concrete was considered. The experimental results are supplemented by FE modeling with a wider range of structural sizes and e/h₀. In addition, the predicted equation for nominal axial strength for eccentrically loaded CFRP-confined RC columns will be established, of which the accuracy will be verified by comparison with the experimental and numerical data.

A fiber-reinforced polymer (FRP)-reinforced concrete–steel (FRCS) double-skin tube was developed for use as a new type of coupling beam, and its hysteretic shear performance was investigated. Reversed cyclic loading experiments were performed using five full-sized coupling beams. The FRCS coupling beams had high shear capacities and energy dissipation capabilities, which were attributed to the confining effects of the FRP and steel tubes. Steel stirrups contributed negligible shear capacity. Most increases in the ductility and shear capacity were contributed by the FRP and steel tubes, respectively. The double-skin tube and dual arrangement of the coupling beam decreased the shear deformation and changed the failure mode from brittle shear to ductile concrete crushing. A finite-element model was developed based on the open system for earthquake engineering simulation system. The results of the validation indicate that the model can accurately predict the hysteretic behavior of the FRCS coupling beam. Results of a subsequent parametric study indicate that by increasing the FRP and steel tube thicknesses it is possible to postpone the decrease in shear capacity effectively.

This paper will present the results from an experimental study to examine the effect of severe environmental conditions on the performance of carbon fiber–reinforced polymer (CFRP) prestressed concrete members. Six 4.8 m (16 ft) long decked bulb-T beams will be prestressed with CFRP strands and will be loaded at three different temperatures: hot, ambient, and cold. Subsequently, four of the CFRP-prestressed beams along with five unbonded stressed CFRP specimens will be subjected to 300 freeze–thaw cycles following ASTM C666. Residual strength after freeze–thaw exposure will be assessed by testing the prestressed beams to failure under a flexural loading configuration and the CFRP specimens through a uniaxial tensile test setup. The results show that due to the difference in the coefficient of thermal expansions for concrete and CFRP, the temperature change caused a fluctuation in the level of the prestressing force in the CFRP-prestressed beams. This fluctuation in the prestressing force should be estimated and implemented in the design of CFRP-prestressed beams that are exposed to temperature fluctuations during service, such as bridge beams. In addition, exposing the CFRP-prestressed beams to freeze–thaw cycles resulted in a reduction in their flexural strength of approximately 7.5%. The reduction in the strength was triggered by the deterioration in the concrete strength and a change in the mode of failure. However, the mechanical properties of CFRP strands were not adversely affected.

This paper presents the effectiveness of strengthening slab–column connections against punching shear failure with near surface–mounted (NSM) carbon fiber–reinforced polymer (CFRP) bars. The experimental program consists of preparing and testing eight samples, two control, and six strengthened samples. The main variables of the experiment are the strengthening layout and the cross-sectional area of CFRP bars. The results show that NSM strengthening increases the ultimate load by up to 44%. The strengthening delays the formation of the first crack in concrete, thus maintaining a linear behavior for load–displacement and load–strain curves for a higher level of load. The NSM strengthening increases the flexural stiffness by over 100% and maintains a strong bond with concrete throughout the loading. The flexural strength of the slab increases, which subsequently improves the punching shear capacity. The experimental results are compared with several design codes by modification and implementation of Chen and Li’s method. There is a good agreement between the calculated ultimate capacity of the strengthened samples and the obtained experimental results.

Glubam (glued-laminated bamboo) is a newly developed structural material with properties comparable to other wood and wood-based materials. Fiber-reinforced polymer (FRP) bar is an attractive reinforcement due to its lightweight, high strength, and high corrosion resistance. In order to explore the merits of combined utilization of glubam and FRP bars in structures, pull-out tests of carbon FRP (CFRP) bars glued-in glubam were conducted. The bond behavior, failure modes, strength, and stiffness of glued-in CFRP bar glubam joints were investigated. The main experimental parameters included glue-line thickness, anchorage length, and the angle between the bar and bamboo fiber. The test results illustrated an increasing tendency of peak loads with the anchorage length, glue-line thickness, and angle. The complex relationships between the bond stress and three influencing parameters were discussed. A simple analytical model was derived and shown to be capable of capturing the initial bond stress–slip behaviors of glued-in CFRP bar glubam joints. Finally, a design equation for estimating the pull-out strength of the CFRP bar in glued-in glubam joints was proposed based on the modification of an existing code equation.

A design-oriented analytical model able to evaluate the shear capacity of reinforced concrete (RC) beams strengthened with fiber-reinforced polymer (FRP) sheets or strips oriented in any direction is proposed. The formulation of the model is based on the variable-inclination stress-field approach, aiming to extend the provisions of current European standards to beams strengthened in shear using FRP. The main novelty of the model lies in taking into account the possible different inclination of steel stirrup and FRP reinforcement in determining the orientation of a compressed concrete stress field, and in shear strength evaluation, overcoming the approximation of the known models with variable inclination of the concrete strut in the assessment of concrete strut capacity, in which the value that has to be assigned to the shear reinforcement direction is not defined, that is, either that of the steel stirrup or the external FRP reinforcement. The proposed model is able to take into account different steel stirrup and external FRP shear reinforcement orientation in assessing the reduction of the steel transverse reinforcement efficiency due to the brittle failure of the composite and also as a function of the effective composite to yielding steel strain ratio. Moreover, regarding the former aspect, a simplified approximate procedure is proposed for solving the drawbacks related to verifying compressed concrete strength in the suggested method of application of code models for RC beams strengthened by means of FRP reinforcement inclined with a different slope from the pre-existing steel stirrup. Complete and U-shaped schemes are considered. The effectiveness of the proposed model adopting different relations for assessment of the FRP effective strains proposed in the literature is investigated, differentiating them by shape of the cross section and by the possible presence of fiber-anchoring devices. The shear capacity predicted by the model and those obtained using international codes and literature models are compared against the experimental results, proving that the proposed model is the most effective in predicting the shear strength when considering specimens having steel stirrups and FRP shear reinforcement arranged with different inclinations.

The Mohaka Township, New Zealand, reinforced concrete bridge was designed in 1958, measuring 239 m long and divided into 14 spans, each 17.1 m long. The piers are doubly reinforced concrete walls over a pile cap with 12 piles. Four prestressed T-beams and a singly reinforced deck form the superstructure. All individual structural members were deemed to comply with current standards, with the main issue being the connection between substructure and superstructure. The integral connection did not have enough reinforcement bars to resist lateral and rotational movement of the girders, resulting in significant cracking at the beams’ ends. A solution was devised to release those movements by physically separating the beams and the pier cap and installing an elastomeric bearing. However, the new detailing required an enlarged pier cap, which in turn increased the lever arm and the moment demand on the pier cap—from both traffic loads and seismic loads. Vertical layers of fiber-reinforced polymer (FRP) were bonded on the face of the wall piers, anchored at the top and bottom using FRP spike anchors. The capacity of the new pier was calculated using section analysis, but no method was available at the time to design the anchors, which were grossly oversized. More current research enables this design, which would result in significant savings in materials and labor. An innovative method using metallic U-tubes was used to try to minimize drilling through concrete members, but it was not satisfactory, owing to constructability.