Journal of Composites for Construction

This study examined the accuracy of 14 design-oriented stress-strain models of fiber-reinforced polymer (FRP)-confined concrete from the available literature to predict the cyclic responses of nine circular reinforced concrete (RC) columns. The examined columns were externally wrapped with FRP composites and subjected to both constant axial loading and cyclic lateral loading. All of the studied stress-strain models were implemented in analysis software as a new uniaxial material. The numerical test results showed that the general response of an FRP-confined RC column to cyclic loading could be predicted using design-oriented stress-strain models; however, the local stress-strain law obtained from concentric compression tests did not reflect very well the local behavior of the compression zone of members in flexure with axial force. The higher the column axial load ratio, the lower the ratio between the numerical and experimental ultimate column lateral displacements, and thus, the predicted failure mode does not match well with the experimental results.

An experimental and numerical program to investigate the behavior of intermediate crack (IC) debonding failure and the bond-slip relationship between the fiber-reinforced polymer (FRP) plates and concrete in FRP-strengthened concrete beams is presented. Simple notched-beam specimens were used to represent an existing flexural-shear crack that triggers IC debonding. In all experimental cases, it was observed that the IC debonding initiated at the tip of a diagonal crack close to the notch or a flexural crack at the beam midspan. To study the sensitivity of the debonding behavior to the location of the cracks along the beam span, the notch position was placed at different locations along the shear span. The combination of the concrete damaged plasticity model and the extended finite-element method (XFEM) based cohesive method is proposed in this paper to model the complete concrete response, and to examine the stress state of the FRP–concrete interface in the presence of IC debonding failure. The numerical results show that the diagonal crack close to the notch results from a mixed-mode stress state prior to FRP debonding. Once the diagonal crack is initiated close to the flexure-shear crack, IC debonding propagates in pure shear stress condition at the FRP–concrete interface regardless of the moment shear ratio at the location of the flexure-shear crack.

Fiber-reinforced elastomeric isolators (FREIs) are relatively lightweight, can be cut to the required size from larger pads, and can be placed unbonded (i.e., unfastened) between the supports. The two major geometric parameters that influence the vertical and lateral response of unbonded fiber-reinforced elastomeric isolators (U-FREIs) are the shape factor and the aspect ratio. The vertical response is strongly influenced by the shape factor, while the aspect ratio is the controlling parameter for the stable response of U-FREIs under lateral displacement. This paper describes an experimental study on the vertical and lateral response of U-FREIs under different loading conditions in both the lateral and vertical directions. Three U-FREIs, with different aspect ratios and shape factors, were tested using a multiload test apparatus. Results from vertical tests indicate that, in addition to the shape factor, the vertical stiffness is influenced by the vertical rate of loading. Furthermore, it was found that the static lateral offset did not significantly affect the vertical stiffness of the isolators considered. These observations are in part attributed to the composite action of the fiber reinforcement and the elastomer.

The possibility of substituting traditional steel reinforcement with glass fiber–reinforced polymer (GFRP) bars in precast concrete tunnel segmental linings is investigated. The use of GFRP reinforcement in concrete tunnel segments results in several advantages mainly related to durability enhancement or when a temporary lining is required. Full-scale bending tests were performed on concrete segments reinforced with GFRP and steel bars in order to compare their structural performance. The results show the effectiveness of the proposed GFRP solution for this type of application. Additionally, a case study is developed presenting provisions for safety checks on GFRP-reinforced tunnel segments.

The use of fiber-reinforced polymer (FRP) composites as external confinement in the manufacture of high-performance concrete-filled FRP tube (CFFT) columns has recently gained significant research attention. Accurate prediction of the lateral strain-to-axial strain relationship is of vital importance in predicting member behavior under various loading conditions, as the confinement pressure created by the FRP confining shell on the concrete core depends on the lateral expansion of concrete. A recent review of the literature revealed that while lateral-to-axial strain models exist for FRP-confined concrete, a model that is applicable to CFFT columns with prestressed FRP tubes or an interface gap caused by radial shrinkage of concrete is not yet available. To address this research gap, this study presents an accurate lateral-to-axial strain model based on the experimental observations from two test databases. Analyses of the experimental databases that consisted of 24 specimens manufactured with FRP-to-concrete interface gap and a further 23 specimens prepared with lateral prestress is presented and discussed. Based on close examination of the ultimate conditions of the column specimens and hoop strain development on the FRP confining shell, expressions to predict strain-reduction factors and lateral-to-axial strain relationships are proposed. The comparison of the proposed model predictions with the experimental test results of specimens prepared with an interface gap or prestressed FRP tubes shows good agreement, with average absolute errors (AAE) below 11% in all predictions.

Hollow structural members are highly vulnerable structural components subjected to transverse impact loads. These tubular members have been used widely in both onshore and offshore structures where lateral impact forces can be expected from moving vehicles/vessels or terrorist attacks. Thus, strengthening of steel hollow tubular members is required to safely carry both service static and imposed dynamic impact loads. This paper presents the results from a series of tests on bare and fiber-reinforced polymer (FRP) strengthened circular hollow-section (CHS) steel members subjected to transverse impact loading at midspan. A total of 14 (2 bare and 12 strengthened) medium-scale specimens were tested to investigate the effect of FRP wrapping on the global and local deformation capacities of strengthened members under drop-hammer impact. Both carbon-fiber-reinforced polymer (CFRP) and glass-fiber-reinforced polymer (GFRP) sheets were used as strengthening materials. The results indicate that the FRP strengthening of the tubes enhances their impact-resistance capacity by reducing lateral displacements up to 29% compared to bare steel CHS specimens. The influence of FRP type, CFRP orientation, CFRP thickness, and effective bond length on the structural response and failure behavior of wrapped members were investigated. The results showed that the application of FRP in the longitudinal direction was effective to control the global deformations whereas the hoop layers were effective in reducing local inward deformations of CHS members.

Strengthening of RC beams and slabs using fiber-reinforced polymers (FRPs) is known to control crack width. However, no specific provisions are provided by most international design codes for predicting crack width in FRP-strengthened members. In this paper, models for predicting crack width of conventional RC members in five international codes are extended to FRP-strengthened beams by introducing appropriate modifications to account for the effect of FRP composites. Predictions of the crack width by the resulting models are evaluated and compared with experimental results. Both analytical and experimental results showed significant reductions in crack widths due to FRP strengthening. The efficiency of controlling crack width, however, decreased with increased amount of steel reinforcement and increased with increasing the ratio of FRP plate to beam width. Comparison of the codes’ predictions of the maximum crack width against experimental results revealed that some predictions are in good agreement with the measured values while others overestimate the maximum crack width by a significant margin.

Glass fiber–reinforced polymer (GFRP) composite bar is emerging as a feasible and economical solution to eliminate the corrosion problem of steel reinforcements in concrete structures; thus, confirmation of its long-term durability is crucial. The objective of this study is to investigate the performance of GFRP bars exposed to concrete alkalinity and ambient conditions after 15 years of service. In order to monitor possible changes in GFRP and concrete, samples were extracted from a bridge for various types of analysis. Carbonation depth and pH of the concrete surrounding the GFRP bars were measured. Scanning electron microscopy (SEM) imaging and energy dispersive X-ray spectroscopy (EDS) were performed on GFRP coupons. Finally, GFRP horizontal shear strength, glass transition temperature ($T_g$), and fiber content were measured and compared with the results of similar tests performed on control samples at the time of construction. The SEM and EDS did not show any sign of GFRP microstructural deterioration or change of chemical composition, and $T_g$ and fiber content were comparable to pristine values while the results of the horizontal shear strength were inconclusive. This study adds new evidence to the validation of the long-term durability of GFRP bars as concrete reinforcement in field applications.

One of the methods for strengthening square concrete columns is shape modification. The method involves modification of the column cross section from square to circular by bonding concrete segments and then wrapping the column with fiber reinforced polymer (FRP). This paper investigates experimentally the applicability of the shape modification (circularization) for square hollow reinforced concrete (RC) specimens under different loading conditions. Five groups of four hollow RC specimens made from normal strength concrete were cast and tested. The specimens in the first group were RC hollow specimens, which served as reference specimens. The corners of the specimens in the second group were rounded to 20 mm and wrapped with two layers of carbon FRP (CFRP). The specimens in the third group were circularized and wrapped with two layers of CFRP. The specimens in the fourth group were bonded with one CFRP strap longitudinally on each side and then circularized and wrapped with two layers of CFRP. The specimens in the fifth group were wrapped with one layer of CFRP and then circularized and wrapped with two layers of CFRP. The results show that circularization increased the strength and ductility of the hollow specimen. Transverse wrapping with CFRP mainly improved the performance of the specimens under concentric axial loadings, while the longitudinal CFRP straps mainly improved the performance of the specimens under eccentric axial loading.

This paper describes the outcomes of recent research that is, for the first time, aiming to completely replace internal steel reinforcement in concrete structures with knitted prefabricated cages made of highly durable fiber-reinforced polymer (FRP) reinforcement. The proposed manufacturing technique, based on the filament winding process, allows the reinforcement to be fabricated in a precisely calculated geometry with the aim of providing tensile strength exactly where it is needed. The resulting wound FRP (W-FRP) cage designs capitalize on the extraordinary flexibility and lightness offered by FRP construction materials. This paper presents fundamental analytical and experimental studies that demonstrate the effectiveness of the wound reinforcement system and forms the basis of future efforts to develop fully automated manufacturing methods for concrete structures.

This paper presents finite-element predictions for the strength of a pultruded fiber-reinforced polymer (FRP) material subjected to pin-bearing loading with hole clearance. One of the distinct modes of failure in steel bolted connections is bearing. It is caused by the compression action from the shaft pressing into the laminate, and when there is no lateral restraint the mechanism observed at maximum load shows brooming for delamination failure. Each lamina in the glass fiber polyester matrix material is modeled as a homogeneous, anisotropic continuum and a relatively very thin resin layer is assumed to contain any delamination cracking between stacked layers. A cohesive zone model is implemented to predict the size and location of the initial delamination, as well as the load-carrying capacity in a pin-bearing specimen. Finite-element simulations (as virtual tests) are performed at the mesoscale level to validate the modeling methodology against experimental strength test results with delamination failure, and to show how pin-bearing strength varies with parameter changes. For an example of the knowledge to be gained for the design of bolted connections, the parameteric study in which the mat reinforcement is either continuous strand or triaxial ($+45°/90°/-45°/\text{chopped}$ strand) shows the latter does not provide an increase in pin-bearing strength.

A proper traction–separation law based on the physical behaviors of the concrete–epoxy interface is needed to simulate the debonding behavior of fiber-reinforced polymers–strengthened concrete structures. However, no method is available to measure the traction–separation law of the concrete–epoxy interface under mode-I loading. To address this gap, this study proposes a novel testing method to directly measure the traction–separation law of the concrete–epoxy interface. This method uses a wedge-splitting test specimen that not only requires simple testing setups but also produces stable crack propagation in the specimen. By adopting a rigid body moment assumption, the traction in the concrete–epoxy interface is expressed as a function of the applied load and the crack opening displacement. As a result, this method can directly calculate the relationship between the traction and the separation of the concrete–epoxy interface by simply recording the crack opening displacement and the applied load during the test. The success of the new method has been confirmed through experimental study. Testing results show the traction–separation law of the concrete–epoxy interface under mode-I loading can be approximated by a tri-linear law. The new method is also used to evaluate the long-term durability of concrete–epoxy interface subjected to moisture attack. It has been found that conditioning the interface specimens in water can significantly reduce the tensile strength and fracture toughness of the interface. The effectiveness of applying silane coupling agent to enhance the durability of the concrete–epoxy interface has also been confirmed by this new testing method.

Hybrid fiber-reinforced polymer (FRP)–concrete–steel double-skin tubular columns (DSTCs) consist of an outer FRP tube and an inner steel tube, with the space between them filled with concrete. Many experimental studies have been conducted on the axial compressive behavior of hybrid DSTCs, but these studies have generally been limited to small-scale specimens with a wet-layup FRP tube. In particular, there has been no systematic experimental study on the axial compressive behavior of hybrid DSTCs with a filament-wound tube and high-strength concrete (HSC), which are the two desirable features of hybrid DSTCs in practical implementation. This paper therefore presents an experimental study on hybrid DSTCs under monotonic axial compression with a particular focus on three important issues: the use of HSC, the use of filament-wound tubes, and the use of large-scale specimens. The experimental results indicate that hybrid DSTCs can possess excellent ductility even when high-strength concrete is used. The axial stress-strain curves of concrete in hybrid DSTCs feature a bilinear ascending shape if the concrete is well confined by the FRP tube, but exhibits stress fluctuations or even a sudden stress drop if this confinement is less effective. An existing stress-strain model, which was developed on the basis of test results of small-scale hybrid DSTCs with normal-strength concrete and a wet-layup FRP tube, is capable of providing reasonably accurate predictions for the present test specimens with a bilinear ascending stress-strain curve or with small fluctuations in the axial stress, but it overestimates the results of the specimens with a sudden load drop.

The truss model approach is widely used to predict the behavior of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) elements in shear. The constitutive laws of each material components, namely concrete, steel, and FRP reinforcement, are essential in the truss model theory. This paper, which is one of a series of papers emphasizing on constitutive modeling of FRP-strengthened RC in shear, focuses on experimental and analytical investigations of constitutive laws of FRP-strengthened RC under biaxial tension–compression loading. The effect of two key parameters, specifically the softening coefficient and the Hsu-Zhu ratios, were carefully investigated by testing six full-scale panels. The results indicated that the softening coefficient and the Hsu-Zhu ratios for FRP-strengthened RC vary greatly compared to RC elements without FRP reinforcement and are heavily affected by parameters such as FRP stiffness and principal tensile strains. Furthermore, analytical expressions were proposed for the softening coefficient and the Hsu-Zhu ratios based on the experimental results.

Numerous reinforced concrete frames have collapsed in major earthquakes because of severe local damage at the ends of columns. Many of these frames were designed and built according to out-of-date codes in which earthquake loads were either not taken into account or not sufficiently considered. To date, there are still a large number of such nonductile RC frames serving in the potential earthquake regions across the world. Thus, there is an urgent need for preearthquake retrofitting and strengthening of these RC frames to achieve desirable seismic performance. In this paper, a total of 11 large-scale cantilever square RC columns were constructed and nine of them were retrofitted with carbon fiber-reinforced polymers (CFRP) wraps at the column ends. These columns were then tested under combined constant axial compression and cyclic lateral displacement excursions. The variables in the tests included the axial compression ratio, the thickness of CFRP wraps, and the column size. The failure modes, hysteretic responses, energy dissipation, stiffness degradation, and equivalent viscous damping ratio of the tested columns were presented and interpreted. The test results showed that the ductility and energy dissipation capacities of nonductile RC columns can be significantly improved with the potential plastic hinge regions of the columns being retrofitted. The retrofitted columns were found to exhibit satisfactory seismic performance even when tested under an ultrahigh axial compression level (i.e., 0.75). Based on the test results, the degradation of the effective stiffness and the unloading stiffness of the columns was discussed and empirical equations were proposed through nonlinear regression analysis.

This paper presents an experimental study conducted on the static performance of bent fiber-reinforced polymer (FRP) tendons for external prestressed strengthening. First, the tension strain and compression strain at the surface of FRP tendons attributable to pure bending were experimentally tested and compared with the theoretical calculated values. Then, the load capacities of bent FRP tendons, under different bending angles and bending radiuses, were tested using a customized testing device. To improve the utilization of FRP tendons, the effectiveness of the hybrid FRP tendon, as a replacement for a pure FRP tendon, was also tested. In addition, a rolling friction deviator was developed to reduce the friction loss and lessen the damage on the FRP surface. The effectiveness of the device was also tested and verified. The test results indicated that when the ratio of the FRP tendons and bending radiuses ($r/R$) reached 0.01 during the pure bending test, the generated tension strain at the tendon surface was greater than 30% of the ultimate tension strain. The hybrid FRP tendons showed higher load capacity retentions than did the pure FRP tendons under the same conditions. The friction loss was significantly reduced when the rolling friction deviator was used.

The repair and strengthening of existing concrete structures with composite material has become more common during the last decade. The near-surface-mounted (NSM) technique was used in this research, where fiber-reinforced polymer (FRP) laminates were bonded using an epoxy or cement-based adhesive in grooves cut into the concrete surface to study the behavior of the bond between the NSM carbon fiber–reinforced polymer (CFRP) strips and concrete after exposure to heating. Twenty-four heat-damaged concrete prisms were tested using a single-lap shear test. The results were compared with 24 reference specimens without heat damage. The results demonstrate that the residual bond strength after the repair of heat-damaged concrete with CFRP using epoxy adhesive is 94, 79, and 49% for temperature exposure of 1 h at 200, 400, and 600°C, respectively. The corresponding values for 2 h of exposure are 86, 75, and 41%, respectively. However, the residual bond strength after the repair of heat-damaged concrete with CFRP using cement-based adhesive is 91, 79, and 70% for temperature exposure of 1 h at 200, 400, and 600°C, respectively. The corresponding values for 2 h of exposure are 85, 69, and 66%, respectively.

The bond behavior of carbon fiber–reinforced polymer (CFRP) sheets attached to concrete surface subject to mixed-mode I/II loading through single lap-shear tests is assessed. A 63 concrete prism specimens of $150\times 150\times 330\mathrm{mm}$ were prepared through externally bonded reinforcement (EBR) and externally bonded reinforcement on grooves (EBROG) techniques, using CFRP sheets of constant width and $100/150\text{-}\mathrm{mm}$ bond lengths. To instigate mode I loading, 31 of the specimens were tested in a manner that the loaded end of FRP sheets were angled inward $-6.0$ to $-2.3\mathrm{degrees}$; while in 24 of the specimens, the FRP strips were angled outward 2.4–4.7 degrees. By applying a noncontact digital image correlation (DIC) technique, load-slip responses of both EBR and EBROG joints were obtained and are discussed in detail. The failure criteria for CFRP sheets attached to a concrete substrate applying different bonding techniques subject to mixed-mode I/II loading are developed based on the experimental results. In general, it was observed that the EBROG technique in mixed-mode I/II loading improves the FRP-concrete bond strength by delaying or fully eliminating the inappropriate debonding failure mode.

An experimental program was conducted to investigate the shear impact behavior of RC beams without stirrups, and to evaluate the enhancement effects of carbon fiber–reinforced polymer (CFRP)–wrapping retrofit. Eleven RC beam specimens were tested under static and impact loading. Two out of the 11 specimens were control specimens without retrofit, while the rest were retrofitted with CFRP wraps. The effects of varying impact velocity and impact energy on the dynamic behavior were taken into main consideration. This paper discusses the crack patterns and failure modes, time histories of dynamic forces, midspan deflection, accelerations, and CFRP strains. The results show that the CFRP retrofit can enhance the impact resistance of RC beams, significantly reduce their deflection, and limit the damage. The effects of CFRP wrapping for RC beams are found to be different for static and impact loading. The varying impact velocity and impact energy had different effects on the dynamic response and caused changes in failure modes.

Concrete beams reinforced with a combination of steel and glass fiber-reinforced polymer (GFRP) bars exhibit promising strength, serviceability, and durability. The fatigue performance of concrete beams with hybrid reinforcement has not been studied previously. This paper designs an experimental program to investigate the fatigue flexural performance of concrete beams with longitudinal tension reinforcement of steel and GFRP bars. One static flexural specimen and three fatigue flexural specimens are tested. The influences of the fatigue load and reinforcement ratio on the fatigue behaviors are investigated. The fatigue loading level and the effective reinforcement ratio influence the fatigue life. The fatigue loading has little influence on the neutral axis depth. The curvature and midspan deflection increase significantly with the fatigue cycles before 0.01 million cycles and then increase slightly with the fatigue cycles after 0.01 million cycles. During the majority of the fatigue life, crack widths develop slowly. Different theoretical models for the midspan deflection of the concrete beams with hybrid reinforcement subjected to fatigue loading are developed and compared with the test results, and the selected model shows satisfactory accuracy.

A total of 32 fiber-reinforced polymer (FRP) confined concrete columns are manufactured and tested under axial compression including 8 FRP-confined solid columns (FCSCs), 4 FRP-confined concrete-filled steel tube columns (CCFTs), and 20 hybrid FRP-concrete-steel double-skin tubular columns (DSTCs). Key parameters include FRP layers, diameter of steel tube, and presence or absence of an expansive agent and concrete filling for the steel tube. Due to the effect of the expansive agent, prestress in concrete is detected in all specimens with expansive self-consolidating concrete (ESCC). For FCSCs and CCFTs, the normalized intercept, inflection and ultimate load for ESCC specimens are higher than those of self-consolidating concrete (SCC) specimens. For DSTCs, when the inner steel tube is filled with concrete only normalized ultimate loads are improved by in ESCC specimens compared with SCC specimens. For hollow DSTCs, however, an expansive agent does not improve the axial behavior. The number of layers of FRP show a positive influence on the normalized intercept, inflection, and ultimate load of ESCC FCSC specimens. In addition, test results for hollow DSTCs indicate that normalized ultimate loads decrease with the increase of steel tube diameter. An existing model used to predict the compressive strength and ultimate axial strain of FCSCs shows good agreement.

The standards ASTM D7205/D7205M-06 and CSA S806 contain the commonly used test methods for characterizing the tensile properties of glass fiber–reinforced polymer (GFRP) bars for use as reinforcement in concrete structures. These standards, however, use different anchor dimensions and loading rates, thereby possibly yielding different properties for the same type of fiber-reinforced polymer (FRP) bars. This paper assessed the results of a four-laboratory testing program comparing the sample preparation methods and test results according to ASTM D7205 and CSA S806. Each laboratory tested at least 10 samples prepared according to the recommendations in Annex A of the ASTM standard and Annex B of the Canadian Standards Association (CSA) standard. Each type of sample was prepared by a single laboratory to minimize variation among the test specimens. The results show a statistically significant difference between the tensile strength measured by using the CSA and ASTM provisions. Regardless of specimen preparation, the modulus of elasticity of the GFRP bars was the same with both test standards, but the ASTM standard returned a wider variation than did the CSA.

A model for the design of shear-flexure critical reinforced concrete elements strengthened with fiber-reinforced polymer (FRP) sheets and plates is presented. The model is based on the stress field approach and the equilibrium method and accounts for the different failure modes of FRP, focusing on the debonding of the FRP from the concrete surface. The efficiency of the model in the strength assessment of beams reinforced with FRP by the prediction of the shear-flexure capacity is checked by corroborating the results of several experimental tests found in the literature. Moreover, the presented model’s capacity to reproduce experimental behavior is compared with the formulations suggested by some codes. Finally, to demonstrate the model’s flexibility in evaluating the resistance domain for reinforced concrete beams strengthened with FRP and to investigate the influence of the shear-bending moment interaction, resistance domains with different arrangements of FRP reinforcement are presented.

Current standards do not consider the diameter of glass fiber-reinforced polymer (GFRP) bars used as internal reinforcement in concrete structures to be a factor influencing bar durability. This paper investigates the effects of bar diameter on the physical and mechanical properties as well as the durability of GFRP reinforcing bars conditioned for three months at 60°C in an alkaline solution simulating a concrete environment. Five diameters (nominal diameters of 9.5, 12.7, 15.9, 19.1, and 25.4 mm) were considered; bar properties were assessed before and after conditioning. Microstructural analyses and measurement of physicochemical properties were also carried out. The results show that bar size had no significant effect on bar physical properties, except for water absorption. The smaller diameter bars had higher water absorption than the larger ones because of their higher surface area-to-volume ratios. In the case of the unconditioned bars, the tensile strength and modulus were not significantly affected by bar diameter, but there was a size effect for interlaminar shear strength and flexural strength. Conversely, the conditioning in the alkaline solution had a greater negative effect on the tensile strength of the larger bars than on the smaller ones. Scanning electron microscope (SEM) observations and Fourier transform infrared spectroscopy (FTIR) analysis revealed that the degradation remained at the surface of all the conditioned specimens. Nevertheless, there were only small variations between the physical and mechanical properties of the GFRP bars of different diameters. This indicates that the current provisions in standards that do not relate strength retention limit to bar size are acceptable.

The development and advancement of cement-based composites has shown promise in recent years. It is important to examine new materials and technologies so that they can satisfy the ever-changing repair and strengthening application needs in the structural engineering field. This paper presents an experimental study on the behavior of reinforced concrete beams strengthened in shear using an externally applied fiber-reinforced cementitious matrix (FRCM). The first aim of this study is to investigate the effectiveness and the performance of the polyparaphenylene benzobisoxazole fiber-reinforced cementitious mortar (PBO-FRCM) system for shear strengthening. The second aim is to study the shear performance of the PBO-FRCM system in terms of the availability and absence of internal transverse shear reinforcements. A comparison study with previous experimental work and the relevant code is done to evaluate the shear performance of the PBO-FRCM system. The test results include the observed shear contribution of the PBO-FRCM system, the failure mode of the strengthened beams, and the influence of the internal transverse shear reinforcements on the shear performance of the PBO-FRCM system.

This study examines the axial compressive strength of sandwich panels of different lengths, with flax fiber–reinforced polymer (FFRP) skins of different thicknesses, as an alternative to glass FRP (GFRP) skins. The panels incorporate polyisocyanurate (PIR) foam cores of different densities: 32, 64, and $96\mathrm{kg}/{\mathrm{m}}^3$. A total of 78 column specimens with a $100\times 50{\mathrm{mm}}^2$ core cross section were fabricated from the three PIR foam cores. FFRP skin layers ranged from one to five, giving a core-to-skin thickness ($c:t$) ratio of $15:64$. The length of the specimens ranged from 500 to 1,500 mm. Specimens were tested under concentric axial compression using pinned-end conditions. Panels with three FFRP layers showed equivalent axial strength to those with a single GFRP layer of 85% the total FFRP skin thickness. This was the case across all lengths studied and across all core densities. Doubling core density resulted in peak load increases, across all lengths, of 76, 57, and 79% for skins with one, three, and five FFRP layers, respectively; tripling the density resulted in respective increases of 114, 131, and 176%. As panel length increased from 500 to 1,500 mm, axial strength reduced by 40% for the $96\mathrm{kg}/{\mathrm{m}}^3$ panels but only by 20% for the 32 and $64\mathrm{kg}/{\mathrm{m}}^3$ panels. Panels longer than 1,250 mm experienced global buckling at peak loads, whereas those shorter than 750 mm experienced localized failures. The intermediate range of lengths saw mixed failure modes.

Fiber-reinforced polymer (FRP) confining jackets offer an attractive solution for the seismic retrofit of RC columns. For the accurate prediction of strength and ductility of FRP-confined RC columns, it is necessary to understand interactions between the FRP jacket and the RC column at all deformation levels under seismic loading. In particular, when the transverse steel reinforcement consists of widely spaced steel stirrups or spirals, the longitudinal steel reinforcing bars (rebars) are likely to develop buckling deformations despite the lateral support provided by the FRP-confined concrete. This paper presents a theoretical study into the buckling behavior of longitudinal steel rebars embedded in FRP-confined concrete subjected to monotonic axial compression. In the theoretical model, a beam-on-elastic foundation idealization is used, where the steel rebars are laterally supported by elastic springs representing the FRP-confined cover concrete. To evaluate the stiffness of the elastic springs, a curved beam model is proposed. Predictions from the theoretical model are compared with test results, demonstrating the reliability of the theoretical model. A parametric study is then presented to examine the influence of three key factors (i.e., spring stiffness, yield strength of steel, and slenderness ratio of rebar) on the overall compressive stress–strain response of laterally supported steel rebars. Finally, an empirical compressive stress–strain model considering the effect of buckling deformation is proposed for steel rebars embedded in FRP-confined concrete subjected to monotonic axial compression and validated through comparisons with the test results.

Carbon fiber–reinforced polymer (CFRP) tendons can serve as a viable alternative to steel strands in prestressing applications in which the noncorrosive nature of the strands helps reduce maintenance costs and extend the service life of the structure. An important consideration for CFRP tendons is the prestress level, which is currently limited by prevailing standards. A series of creep stress tests were conducted on carbon fiber–composite cables with generic field-made anchorages using commercially available expansive grout. Specimens withstood 95% of their guaranteed capacity for a period of five months (3,624 h) at a room temperature of 24°C (75°F) without rupture or any sign of distress. The experiments also showed quite an acceptable long-term performance for the generic field-made anchorage. In subsequent residual strength tests, all specimens still exceeded their guaranteed capacity after the extended high creep stress test.