Journal of Composites for Construction

The results of an experimental study of the flexural performance of full-scale, fire-damaged, two-way composite-topped precast concrete slabs with different postfire exposure-strengthening methods are presented. Four slabs were subjected to the standard ISO 834 fire conditions for varying exposure times (30, 60, 90 min and failure at 133 min). A fifth control slab was tested without having been exposed to fire conditions. Different strengthening methods, selected based upon the extent of fire damage, included externally bonded carbon fiber–reinforced polymer (CFRP) sheets, steel strips, and combining CFRP sheets and midspan I-beam support were demonstrated to restore fire-damaged slab load-bearing capacity, although postfire exposure stiffness was degraded in every case. Load-carrying capacity of the strengthened fire-damaged slabs was increased at least 23% over the capacity of the control specimen without strengthening. Finite-element modeling combining an initial thermal analysis followed by a mechanical analysis which accounted for temperature-degraded material properties was conducted to capture the mechanical behavior of the strengthened slabs after fire. The results were shown to correlate well with experimentally determined results, exhibiting conservative errors of less than 20%.

The favorable mechanical properties of basalt fiber–reinforced polymer (BFRP) bars, such as their excellent strength-to-weight ratio, resistance to corrosion, lower environmental impact, and electromagnetic neutrality, make them attractive for internal reinforcement of concrete elements with specific service requirements. Prestressing has emerged as a possible method for limiting the deflections and cracking of BFRP-reinforced flexural RC elements. However, long-term behavior of such structural members has not yet been investigated extensively. Therefore, this paper aims to give information on long-term behavior and prestress losses of pretensioned BFRP-reinforced concrete beams based on the collected experimental data. The testing program includes long-term analysis of six BFRP concrete beams pretensioned to different prestress levels—namely, 20%, 30%, and 40% of the ultimate tensile capacity of the bars. The monitored testing stages included initial tensioning of the bars, casting and curing of concrete, transfer of prestressing force to the concrete, long-term unloaded phase after transfer, sustained-loading application, long-term sustained loading phase, unloading, and final destructive testing, conducted in a controlled indoor environment. During all phases of the testing, strain levels in the BFRP bars and deflections were continuously monitored. The results of continuous strain monitoring show that the average loss of strain over the period of initial 90 days of unloaded monitoring was 7% of the initial strain. During the following 6 months of monitoring under sustained loading, an additional 0.3% reduction was recorded on average. The loss was dependent on the initial strain.

The out-of-plane (OP) strength of masonry infills is reduced when damaged under the action of the in-plane (IP) forces. A strengthening approach using a fabric-reinforced cementitious matrix (FRCM) was proposed for improving the OP strength of damaged infill walls. A finite-element (FE) model of a reinforced concrete (RC) frame, infilled with FRCM-strengthened masonry, was developed and validated using experimental results and analytical predictions. Interaction curves representing the interplay between in-plane damage and out-of-plane response were generated for unstrengthened (ordinary) and FRCM-strengthened infill walls using FE analysis. Results of the FE analysis showed that FRCM-strengthened infills had greater OP strength in the undamaged state and suffered less strength degradation at high-damage states compared to unstrengthened infills. Anchoring of the fabric proved effective in utilizing the strength and ductility of the FRCM reinforcement. The fabric with ±45° orientation contributed more in resisting the diagonal tension under in-plane shear but was not as effective as 0°–90° orientation in resisting the OP flexural stresses corresponding to in-plane damage at 2.2% drift level. A novel definition of damage state was introduced for fragility analysis that can consider the effect of in-plane damage and out-of-plane strength using interaction curves. A four-bay, five-story masonry infilled RC frame was analyzed using the capacity spectrum method, and fragility curves were developed for unstrengthened and FRCM-strengthened infills. The effect of variation in out-of-plane demand and in-plane damage on each floor was included in the analysis. The RC frame using FRCM strengthened infills exhibited a smaller probability of exceedance of the performance limit than the unstrengthened infills. The proposed methodology helped identify critical infills that require strengthening.

Using locally available raw materials, such as seawater, sea sand, and coral reefs, for construction on remote islands is conducive to saving costs and shortening the construction time. These raw materials contain high amounts of corrosive substances such as chloride ions, which increase the corrosion risk of steel reinforcement. This study investigated the flexural behavior of seawater sea-sand coral aggregate concrete (CAC) beams reinforced with the noncorrosion fiber-reinforced polymer (FRP) bars. Nine beam specimens were loaded under four-point bending to evaluate the influence of concrete type and reinforcement ratio on their flexural behavior. A model of equivalent rectangular stress block parameters was derived, and the accuracy of the model was validated by the test results. It is found that the ultimate moment capacity and deformability of FRP-reinforced CAC beams were lower than those of the corresponding FRP-reinforced natural aggregate concrete beams. The proposed model reduces the prediction error of the flexural strength and is also safer and more conservative than the existing design provisions, which is critical for nonductile FRP-reinforced concrete beams.

In this study, a new composite shear wall is proposed, which consists of fiber-reinforced polymer (FRP)-confined concrete cores embedded in the concrete-filled steel tubular (CFST) columns as boundary elements of concrete-filled multicellular steel tube shear walls (MCSTWs). In order to study the seismic performance of this new composite shear wall, the following five different shear-wall specimens were designed and tested under constant axial load and cyclic lateral loading: (1) two reinforced concrete (RC) shear walls with glass FRP (GFRP) tube-enhanced CFST boundary elements; (2) an MCSTW with only CFST columns as boundary elements; and (3) two MCSTWs with FRP-confined concrete cores embedded in CFST columns as boundary elements. The failure modes, hysteretic performance, skeleton curves, strength and stiffness degradation, energy dissipation capacity, and deformation characteristics of the specimens were compared and discussed based on the test results. The results show that the proposed shear wall has a better seismic performance than the RC shear wall with GFRP tube-enhanced CFST boundary elements, such as a higher energy dissipation capacity and a more gradual strength degradation. For MCSTWs, incorporating a GFRP tube in the CFST boundary leads to a higher energy dissipation capacity, a higher load-carrying capacity, more gradual degradation of strength and stiffness. It can be concluded that the newly proposed shear walls have excellent seismic performance and are well-suited for application in high-rise buildings and other structures where the demand for seismic resistance is high.

The combined torsion and bending response of concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) was investigated in this study. The CFFT samples were produced from the same 166-mm-diameter nearly cross-ply filament wound glass-FRP (GFRP) tubes with a 30 MPa concrete infill. The samples were tested at torque-to-bending moment ratios (T/M) of 2, 1, and 0.67, as well as under pure bending and pure torsion, to cover a wide range of loading cases. The study showed that ultimate torque and bending moment were minimally affected when combined with normalized bending moments (M_u/M_uo) and normalized torques (T_u/T_uo) ≤ 0.5, respectively. A circular normalized ultimate strength interaction could reasonably predict the trend of the experimental data. The postcracking torsional stiffness was not greatly affected by flexure; however, late-stage stiffness was improved at T/M = 2, while it was reduced at T/M = 0.67. The flexural stiffness remained largely unchanged with T/M. Reducing T/M from 2 to 0.67 reduced the twist capacity from 52% to 27% of that at pure torsion but increased the deflection from 41% to 75% of that at pure flexure. Reducing T/M from pure torsion reduces the hoop and shear strains and produces a longitudinal strain gradient over the section height. The strain gradient limits the formation of diagonal cracks in the flexural compression region, reduces the number of cracks, and produces a crack angle variation through the section depth. Failure initiates from rupture of the FRP tube under a complex state of in-plane normal and shear stresses at the flexural tension side of the tube, followed by failure of the concrete core, which experiences partial confinement.

This investigation addresses the durability of the adhesive bond between near-surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips and concrete, under real-time field conditioning and laboratory-accelerated conditioning. Four natural outdoor environments were considered in the experimental program to induce aging mainly by carbonation, freeze-thaw cycles, elevated temperatures, and airborne chlorides from seawater. Additionally, a control environment (20°C and 55% RH) and a water immersion environment under controlled temperature (20°C) were considered. The durability was studied mainly throughout the mechanical properties obtained from (1) the involved materials (concrete, epoxy adhesive, and CFRP strips); and (2) the bond specimens, with a period of exposure up to 2 years. The bond performance of NSM-CFRP strips to concrete was slightly affected by environments under investigation, being water immersion and freeze–thaw cycles the most deleterious ones. A maximum average bond strength decrease of approximately 12% was registered for the specimens immersed in water, while in the case of the outdoor environments, the maximum bond degradation (approximately 8%) occurred for the specimens of freeze–thaw cycles after 2-year exposure.

This study develops a new form of composite beam theory to study the connections of fiber reinforced polymers (FRPs), reinforced concrete beams, and closed-form solutions of four typical connections are derived and discussed. Comparisons with experimental data also show agreement. The maximum strain incompatibility of the mechanically anchored FRP system at two ends appears at the position where the first-order derivative of the applied bending moment $M_{ex}^{\mathrm{\prime }}$ is zero or at the boundary positions. Compared with strain incompatibility, the slip is more critical for determining the composite behavior of the FRP-reinforced structure. The first-order derivative function of the slip is strain incompatibility. It is demonstrated that the structural response of this FRP system can be improved theoretically by introducing additional mechanical anchors at the critical locations from the slip perspective.

Carbon fiber–reinforced polymer (CFRP) systems have been widely used to strengthen concrete structures. To further study the flexural strengthening of concrete beams with prestressed CFRP plates, flexural tests of large-scale reinforced concrete beams were performed, and a tensioning and anchorage system suitable for engineering applications was used in this study. The effects of the prestressing level, strengthening amount, type, and bonding position of the prestressed CFRP plate on the flexural behavior were investigated. The test results showed that the cracking load, yielding load, and ultimate load of all prestressed strengthened beams could reach 2.25–3.38 times, 1.27–1.53 times, and 1.24–1.61 times those of the unstrengthened beam, respectively. The postcracking stiffness, cracking load, and yielding load could be increased by increasing the prestressing level and strengthening the amount of the prestressed CFRP plate. However, a high prestressing level of 50% could lead to a premature intermediate debonding of the CFRP plate before yielding of the steel reinforcements and a reduction in the ultimate load. The use of a low-strength prestressed CFRP plate could achieve a strengthening efficacy similar to that of a high-strength CFRP plate before yielding of the steel reinforcements, whereas the low-strength CFRP plate reduced the ultimate load by 25 kN compared with the high-strength one. The prestressed CFRP plate bonded on the two sides provided a lower flexural strengthening efficacy than that bonded on the bottom of the beam owing to the shorter force arm and the nonuniform stress of the side-bonded CFRP plate.

Fiber-reinforced polymers (FRPs) have enabled the construction of lightweight footbridges, whose structural design is often governed by a serviceability limit state. A suitable approach to avoid overdimensioning an FRP footbridge may be to adopt a motion-based design strategy, where excessive human-induced vibrations are mitigated through the installation of tuned vibration absorbers (TVAs). In this sense, human–structure interaction (HSI) phenomena should be considered to estimate accurately the acceleration response of lightweight footbridges and size TVAs properly. Thus, this paper presents the design, installation, and performance assessment of a passive inertial controller for completing the construction of a full-scale FRP pedestrian structure. First, a general frequency-domain procedure to design TVAs for structures susceptible to HSI is proposed. The methodology considers a multiobjective optimization problem that minimizes simultaneously the structural response and the controller inertial mass. Second, the HSI load model of a bouncing pedestrian is identified experimentally to be used within the proposal to design TVAs. Third, a TVA of 25 kg is designed, assembled, and installed in the lightweight FRP structure, employing the proposed procedure. Then, the enhancement of the dynamic response due to the controller is assessed considering a person bouncing and two streams of walking pedestrians. For the different load scenarios, the TVA exhibits an adequate behavior to mitigate the vertical acceleration, demonstrating the feasibility to deliver an ultralightweight FRP footbridge with an inertial controller to meet requirements at different limit states.

The elliptical FRP–concrete–steel double-skin tubular column (DSTC) is a promising compression member due to its favorable mechanical performance and constructability. To provide insights into the interaction mechanism of the individual parts (i.e., elliptical FRP tube, elliptical steel tube, and sandwiched concrete), the compressive behavior of elliptical DSTC stub columns was systematically studied and compared with two types of reference specimens, including FRP-confined solid concrete (FCSC) and FRP-confined hollow concrete (FCHC) specimens. Based on a comparative investigation on the compressive performance and dilation properties of concrete cores in different column forms, a three-stage interaction mechanism in elliptical DSTC was clarified: unconfined stage, transition stage, and hardening stage. The confined concrete strength in elliptical DSTC increased by 20%–55% compared with the corresponding elliptical FCSC with the same column dimension and FRP confinement level. The possible reason is that the less-effective confinement zone in elliptical FCSC was replaced by a hollow steel tube. The existing model could not be directly adopted to predict the behavior of elliptical DSTC using high-strength concrete.

Impressed current cathodic protection (ICCP) has been proven to be an effective technique to inhibit steel corrosion. Based on the principle of ICCP, a hybrid carbon fiber–reinforced polymer (CFRP)/steel reinforced concrete (RC) structural system with cathodic protection has been proposed considering the dual function of CFRP, namely, the anode material of ICCP and reinforcement of concrete structures at the same time. For this system, the larger structural deflection and brittle failure caused by the low elastic modulus and linear elastic behavior of the CFRP bar can be avoided, and ICCP is activated only when the steel bar is at risk for corrosion. In this paper, the flexural performance of the proposed structural system with different reinforcement configurations was investigated. First, an accelerated precorrosion test was conducted for the steel bar with a theoretical corrosion ratio of 8%. Then, wet–dry cycle corrosion was performed by companies with ICCP treatment for 150 and 180 days. Finally, four-point bending tests were carried out, followed by a comparison of the experimental bending capacities and predicted values. The results showed that for the proposed structural system, the further corrosion of steel bars was inhibited significantly with a protection current density of 10 mA/m², leading to the maintenance of satisfactory structural performance without a marked deterioration of the CFRP anode performance or the CFRP-concrete bond behavior. The reinforcement configuration with CFRP bars in the lower row and steel bars in the upper row was the most effective to maintain a satisfying bending capacity and inhibit steel corrosion for the proposed structural system in a corrosive environment.

Results are presented from a comprehensive experimental study to assess the occurrence of heat-induced longitudinal splitting cracks in concrete specimens reinforced with carbon fiber–reinforced polymer (CFRP) or steel when exposed to severe heating from one side, as would likely occur during a fire in a building. Tests were performed on large- and medium-scale precast CFRP-reinforced or prestressed specimens. Large-scale specimens were tested in a standard fire-resistance test, while medium-scale specimens were tested using a novel Heat-Transfer Rate Inducing System (H-TRIS) that controls thermal exposure by imposing a time-history of incident heat flux at a specimen’s exposed surface. The formation of thermally-induced longitudinal splitting cracks and failure of the concrete cover to provide sufficient confining action, and thus sufficient bond strength, is shown to be more likely for FRP-reinforced or prestressed concrete elements than for those reinforced or prestressed with steel. This appears to be at least partly due to thermomechanical incompatibility between CFRP reinforcement and concrete; the formation of heat-induced longitudinal splitting cracks is related to rapid thermal expansion of CFRP tendons relative to the surrounding concrete. Many aspects of bond performance at elevated temperature remain poorly understood, and these require additional investigation before FRP-reinforced or prestressed elements can be used in fire-rated applications with confidence.

Strengthening-by-stiffening (SBS) is a technique for enhancing the shear performance of buckling-prone webs of steel beams by bonding glass fiber–reinforced polymer (GFRP) stiffeners. This paper focuses on changes in load capacity, ductility, and mode of failure of SBS under cyclic loading and after exposure to weathering conditions for 1 year. First, weathering effects on mechanical properties of the adhesive used in bonding the GFRP stiffeners are presented. The ductility of the epoxy was significantly reduced; however, the ultimate stress increased by about 68% over the ultimate stress of the nonweathered coupons. Furthermore, weathering led to the formation of a thin film of rust that propagated around the edges of the bonding adhesive for the full-scale beams. Consequently, the ultimate load capacity of the weathered beam dropped about 11% after 1 year of environmental exposure. Cyclic loading, however, resulted in negligible change in the ultimate load capacity. For all tested beams, the ductile adhesive used in bonding the GFRP stiffeners prevented a sudden drop in load capacity due to debonding of the GFRP stiffener in all tests.

This study introduces a novel test apparatus that can be used to test the adhesive bond between fiber-reinforced polymer (FRP) laminate and concrete in both double shear and mixed mode (shear/peeling). It was deployed to evaluate the behavior of carbon FRP (CFRP)–concrete interfaces at six peel angles. The apparatus is usable with any servohydraulic load test frame and requires only one-half of the traditional concrete block specimen used in both double shear and mixed-mode tests. Experimental results revealed a decrease in bond capacity as peel angle increased. The reduction is quantified using a power-law relationship with respect to the tangent of the peel angle. A three-dimensional (3D) finite-element model utilizing a cohesive zone model to characterize the FRP–concrete interface was developed to corroborate experimental results and carry out parametric studies. The percentage increase in CFRP–concrete bond capacity resulting from an increase in the FRP modulus or its thickness declined as the peel angle increased.