Journal of Composites for Construction

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been studied extensively, but a lingering question remains regarding the vulnerability of the exposed tube to accidental damage or vandalism. In this paper, the effect of damage induced in the tubes on the flexural strength of CFFTs is studied. A total of 18 specimens with tubes having nearly orthogonal fiber arrangements were tested in four-point bending, including one undamaged control specimen to establish its full nominal strength ($M_{no}$). A total of 11 specimens with artificially induced damage in the form of through-thickness cuts on the tension side, and six with cuts on the compression side at midspan were also tested. Longitudinal, circumferential, and square cuts were introduced. Cut length and proximity to neutral axis were varied. It is shown that the most critical cut is the circumferential one on extreme tension. A sharp decline occurred initially in the nominal residual strength ratio ($M_n/M_{no}$), from 1.0 to 0.55 at a cut length-to-perimeter ratio of $0.02\pi D$, followed by a much less steep trend up to $M_n/M_{no}$ of 0.25 at a $0.2\pi D$ cut. As the location of a $0.03\pi D$ circumferential cut shifted from extreme tension to an 80° from the bottom, $M_n/M_{no}$ increased from 0.52 to 1.0. A longitudinal cut of $0.1\pi D$ on the tension side resulted in $M_n/M_{no}$ of 0.69 as opposed to 0.38 for the circumferential cut. Compression side longitudinal and circumferential cuts were less critical with $M_n/M_{no}$ of 0.8 to 0.9. For practical design, a case study based on ultimate static strength showed that the CFFT can tolerate a circumferential tension cut of up to $0.29\pi D$ before it has to be taken out of service.

Studies on unbonded post-tensioned concrete (UPC) beams strengthened with fiber-reinforced polymer (FRP) sheets are limited and existing studies focus on monotonic loading; in particular, there are no studies on flexural behavioral characteristics of UPC beams strengthened with carbon FRP (CFRP) sheets subjected to repeated loads. This study deals with the flexural behavior of CFRP-strengthened UPC T-beams under repeated and post-repeated monotonic loading in which influences of varying load amplitudes, CFRP sheet ratios, and different CFRP U-wrap anchors were investigated. Experimental results showed that the high-amplitude repeated load considerably affected the response of the beams including creep strain in concrete, residual displacement, residual load-carrying capacity, crack width, CFRP sheet strain, and stress range in prestressing steel. During repeated loading, the CFRP sheets considerably decreased the residual displacement, energy dissipation, stress range in prestressing steel, maximum displacement, and crack width of the strengthened beams; these reductions were proportional to the number of CFRP layers and load cycles. The repeated loads tremendously affected the beams’ behavior under post-repeated monotonic loading particularly at the serviceability state but the influence became insignificant at the ultimate state. The U-wrap anchors considerably affected beam failure, increased the maximum strain of CFRP sheets and tendons, and enhanced the ductility of the beams under the post-repeated monotonic loads. The debonding strain of the CFRP sheets estimated from guidelines or codes was much smaller than the actual values for UPC beams.

Strengthening concrete bridges using fiber-reinforced polymers (FRPs) in flexure, shear, and torsion is limited by the occurrence of premature FRP debonding which often controls the degree of strengthening possible using this approach. To address this issue, anchorage systems have been developed in recent years to mitigate FRP debonding failure modes with the objective of achieving higher levels of strengthening using less material. Of the many types of anchorage systems researched to date, joint level tests have revealed that bidirectional fiber patches placed at the ends of the FRP laminates can significantly delay the occurrence of FRP debonding. This study presents the first experimental application where patch anchors are evaluated in post-tensioned concrete beams strengthened in shear using FRP laminates. Three specimens were tested including an unstrengthened control beam and strengthened beams with and without anchors. The anchors were proven to significantly improve the shear capacity of the member and increase the FRP laminate strain utilization prior to failure. The experimental results were verified against predictions provided by the modified compression field theory (MCFT) and the ACI 318. A semiempirical model (SE model) was proposed to predict the shear contributions provided by the anchored FRP laminates.

Eccentric corner beam-column joints of existing reinforced concrete (RC) frames, in which the beam axes are eccentric to the column axis, can be highly vulnerable to brittle joint shear failure under earthquake loading. Externally bonded fiber-reinforced polymer (FRP) composites provide a strengthening option, although research on this method is limited. This paper addresses such a knowledge gap by considering the influence of key parameters of joint transverse reinforcement ratio, eccentricity, and carbon FRP (CFRP) strengthening on the seismic performance of large-scale eccentric corner beam-column-slab joints containing a transverse beam and slab. Seven specimens, including four control specimens and three strengthened specimens, were designed and tested. Three CFRP strengthening schemes, aimed at improving joint shear capacity and overall seismic behavior, were proposed. Upon the comparison and discussion of test results, the influence of joint transverse reinforcement ratio, eccentricity, and the proposed CFRP strengthening schemes on the seismic behavior of such eccentric joints was established. Irrespective of the joint region and with or without adequate transverse reinforcement, all control specimens experienced joint shear failure due to the adverse effects of eccentricity. The CFRP intervention changed the failure mode from brittle joint shear failure to ductile beam-hinge failure. The mechanism and effectiveness of the proposed CFRP strengthening schemes were finally validated and several suggestions on CFRP strengthening are provided for eccentric joints.

In coastal regions, chloride penetration causes steel reinforcing bar (rebar) corrosion in reinforced concrete structures, leading to durability problems in existing structures. A new intervention method, impressed current cathodic protection and structural strengthening (ICCP-SS), was adopted to rehabilitate sea–sand concrete columns. A carbon fiber-reinforced cementitious matrix (C-FRCM) was used as a dual-functional material in the ICCP-SS system, wherein the C-FRCM served as both an anode and a strengthening material. This study aimed to consider the effects of the total charge density on the confinement effect of C-FRCM jackets and the compressive strength of columns under ICCP-SS intervention to demonstrate the long-term effectiveness of the ICCP-SS intervention method for sea–sand RC columns and to investigate the appropriateness of existing strength models for RC columns strengthened by C-FRCM jackets under impressed current cathodic protection (ICCP). The experimental program included a total of nine reinforced concrete stub columns. Prior to the compression tests, the columns were subjected to 270 days of accelerated corrosion and 250 days of cathodic protection under protective cathodic current densities of $20$ and $60\mathrm{mA}/{\mathrm{m}}^2$. This paper presented an experimental program, a comparison between short-term and long-term test results of ICCP-SS, a comparison of existing strength models, and a discussion on the appropriateness of the existing models for C-FRCM jackets subjected to an applied current.

The method provided in ACI 440.2R for predicting the shear strength of reinforced concrete (RC) beams strengthened with externally bonded reinforcement (EBR) gives conservative estimates of shear capacity due to its inability to capture the variation of $\beta$ and $\theta$. This paper presents three iterative approaches as well as a noniterative approach to predict the shear capacity of RC beams strengthened with EBR with greater accuracy and improved appropriate safety. The first iterative approach is based on the combination of a slightly revised version of the modified compression field theory and another model proposed in the literature. The second iterative approach is based on the combination of the simplified modified compression field theory and the aforementioned proposed model, and the third iterative approach is based on the combination of a revised version of AASHTO’s model with the aforementioned proposed approach. The noniterative approach is developed using Monte Carlo analyses using the first iterative model as the target function and includes two simple closed-form equations to capture the variation in $\beta$ and $\theta$. All proposed approaches are compared with the ACI 440.2R method in terms of accuracy and appropriate safety. A global sensitivity analysis is performed to assess the influence of various parameters on the shear capacity of RC beams strengthened with EBR using various approaches. A database of 172 RC beams strengthened with EBR is compiled to appraise the predictive capability of the proposed models and to demonstrate the improvement that they offer regarding accuracy and appropriate safety.

In regions of high seismic activity, flat plate systems can be used as gravity force resisting systems, where special moment frames are provided as seismic force resisting systems. Nevertheless, all elements of the reinforced concrete (RC) structure must have sufficient deformability to be capable of deforming into the inelastic range. An efficient method of enhancing the deformability of flat plate systems is providing slab shear reinforcement. This paper presents the results of an experimental program evaluating the efficiency of two new types of glass fiber-reinforced polymer (GFRP) shear reinforcement in enhancing the deformability of GFRP-RC slab-column edge connections. Three full-scale connections were tested under gravity and uniaxial reversed-cyclic lateral loading, one connection was reinforced with GFRP shear studs, one with GFRP corrugated bars, and one had no shear reinforcement. The use of GFRP shear studs and corrugated bars increased the lateral load capacity of the connections by 47% and 44%, respectively. In addition, both types of GFRP shear reinforcement were able to enhance the deformability of the connections significantly, whereas the connection with shear studs was able to sustain deformations associated with 3.50% drift ratio without jeopardizing its gravity load capacity.

Existing studies have shown that fiber-reinforced polymer (FRP) jacketing/wrapping is highly effective in strengthening circular concrete columns but is much less effective for rectangular/square concrete columns due to the existence of flat sides and sharp corners in the latter. Rounding the corners can enhance the effectiveness of FRP confinement, but its benefit is limited, particularly for large-scale columns, because the corner radius is limited by the presence of internal steel reinforcement. An alternative strengthening technique therefore has been proposed by some researchers, in which the flat sides of a rectangular/square section are modified into slightly curved sides (i.e., curvilinearization) before FRP jacketing/wrapping. This section curvilinearization (SC) method increases the section size by only a limited extent but can substantially enhance the effectiveness of FRP confinement for rectangular/square columns. However, only very limited research has been conducted on curvilinearized rectangular/square columns (CRCs or CSCs), and the few existing studies are limited in scope and were conducted using small specimens. This paper presents the results of a systematic experimental program on the behavior of CSCs, in which 16 small-scale and 10 large-scale FRP-confined square concrete columns with or without section curvilinearization were tested under axial compression. A comparison of the results for columns of the two different sizes indicates that the size effect is very limited in these FRP-confined CSCs. The accuracy of two existing stress–strain models for FRP-confined concrete in CRCs was evaluated using the test results, revealing the need for the development of an improved stress–strain model for use in design.

This study presents an innovative hybrid profiled steel- and fiber-reinforced polymer (FRP) reinforced concrete (HPSFRC) structural system that primarily consists of thin semiclosed T-shaped cold-formed steel sheeting that is externally enclosed with an FRP sheet and filled with concrete. The experimental study was conducted in two interdependent parts: a development study and a validation study. In the development study, six specimens were tested to determine the best interlocking technique between the concrete flange and the other components of the cross section. In addition, two specimens were examined to define the shear strength of the steel-concrete composite profiled system. The validation study presented the flexural behaviors of HPSFRC T-beams with different reinforcement configurations. The test results of the HPSFRC beams were assessed in terms of the behavior of a conventional reinforced concrete T-beam and a composite profiled T-beam. The HPSFRC T-beams achieved a ductility comparable to that of a composite profiled beam but exhibited a higher flexural strength. The flexural behaviors of the HPSFRC beams can be controlled using additional longitudinal reinforcement at the beam tension side. The beam with additional steel bars exhibited ductile behavior with a stable increase in the beam resistance to the applied load; however, the addition of FRP layers enhanced the flexural capacity of the beam and greatly controlled the deformability of the beam after steel yielding, resulting in the lowest measured residual deflection.

A practical design optimization of fiber-reinforced polymer (FRP) patches for repairing fatigue cracks in metallic structures is presented. The design procedure combines finite-element (FE), genetic programming (GP), and genetic algorithm (GA) approaches. An optimum patch design is defined as the combination of design parameters that simultaneously minimizes the patch volume and reduces the stress intensity factor (SIF) range below the fatigue threshold range. A patching correction factor, which accounts for the positive effects of material and geometric properties of the patch and adhesive layer on the SIF solution, is proposed. The correction factor is developed by performing symbolic regression via GP analyses on the SIF values obtained from the three-dimensional FE models. The closed-form SIF solution facilitates the visualization of the effects of design parameters, simplifies the calculation of fatigue life, and reduces the computation effort for design optimization. An example of the center-cracked steel plate subjected to constant amplitude fatigue loading and then repaired with double-sided adhesive-bonded FRP patches is used to illustrate the optimization procedure.

This study investigated the flexural behavior of a new one-way concrete slab system reinforced longitudinally with basalt fiber–reinforced polymer (BFRP) bars and cast with basalt fiber–reinforced concrete (BFRC). The study included experimental testing and three-dimensional finite-element (FE) modeling of eight slab strips, $500\times 175\times \mathrm{2,500}\mathrm{mm}$ each. The investigated parameters included the volume fraction of the basalt fibers added to the concrete mix (0%, 0.5%, 1%, and 2%) and the BFRP reinforcement ratios (1.4 and 2.8 times the balanced reinforcement ratio). The effect of varying the fiber volume fraction on the mechanical properties of concrete was first assessed. The test results showed that increasing the fiber volume fraction increased the compressive strength and the modulus of rupture of the concrete. Slab strips with higher dosages of fibers showed an increased number of cracks and a considerable enhancement in their cracking and ultimate capacity. A volume fraction of 0.5% of basalt fibers had an insignificant effect on the flexural performance of the specimens, and therefore 1% of basalt fibers were recommended as a minimum dosage. Increasing the fiber volume fraction led to a noticeable increase in the ductility of the slab strips at all stages of loading. The FE models provided reasonable prediction of the nonlinear structural behavior of the slab strips. The Variable Engagement Model, initially developed for steel fiber–reinforced concrete, was assessed to describe the BFRC mixes. Good correlation between the numerical and experimental results in terms of cracking loads, load-carrying capacities, deflections, and crack pattern was obtained.

Civil works construction is now facing a transformational change with the growing implementation of fiber-reinforced polymer (FRP) materials as rebars and tendons for concrete structures. To avoid surprises and decrease risk, a contractor must be cognizant of FRP’s differences with traditional steel reinforcement in terms of procurement, quality control, and installation. A bridge replacement project under construction in Florida is presented in this paper as a case study to address the technology’s constructability and adaptability in a number of structural elements for both substructure and superstructure. The paper also provides some productivity considerations for the case of FRP reinforcing cage assembly in terms of work hours, including a comparison with the traditional steel solution. The case study points out critical issues such as procurement, acceptance, and workforce experience that can provide guidance for both implementation and standardization of the technology.