Journal of Composites for Construction

This paper presents an innovative strengthening methodology for damaged steel beams with carbon-fiber-reinforced polymer (CFRP) sheets. Emphasis is placed on controlling premature CFRP debonding using an emerging material called silyl-modified polymer (SMP) in conjunction with its hybrid bond application with a conventional epoxy adhesive. The hypothesis proposed is that the low-modulus SMP addresses stress singularity along the CFRP-steel interface, while the high-modulus epoxy preserves interfacial capacity. CFRP anchorage assists the efficacy of such a hybrid bond configuration. A two-phase experimental program is conducted to evaluate the performance of various CFRP-strengthening schemes, including element- and structural-level investigations (i.e., interface and beam tests, respectively). Test results reveal that the SMP bond mitigates CFRP debonding that is commonly observed in epoxy bond. The strengthening systems employed significantly improve the flexural behavior of the test beams in terms of ductility and failure characteristics.

This paper presents the experimental results of a study investigating the effectiveness of using externally bonded fiber-reinforced polymer (FRP) tow sheets for in-plane bending strengthening and repair of reinforced concrete (RC) shear walls. The repair/strengthening scheme is designed to enhance the flexural load-carrying capacity of the walls and prevent brittle shear-related failures. The experimental program consisted of the performance evaluation of the FRP system in seven RC shear wall specimens tested to failure under lateral loads. The effectiveness of two different anchoring systems to transfer the loads carried by the FRP sheets to the supporting elements of the wall was also investigated. The experimental results showed that the FRP system was successful in restoring (in repair applications) and increasing (in strengthening applications) the initial stiffness and ultimate load-carrying capacity of the walls, while maintaining a ductile response behavior and avoiding brittle shear failures. A simple strength model to predict the peak strength of walls with externally bonded FRP sheets suitable for design applications is presented and its limitations discussed.

To evaluate the performance of externally bonded fiber-reinforced polymer (FRP) composites to concrete structures, the bond strength is predicted at the maximum transferable load. Most of the existing prediction models neglect the adhesive material properties. As such, a previously proposed criterion on the FRP debonding initiation considering the adhesive material properties is extended by determining the fracture depth parameter and introducing a strength factor. The fracture depth represents a certain depth of a failure in concrete substrate and the strength factor revises the concrete tensile strength to be applied as the limit bond stress. Both the fracture depth parameter and strength factor are determined as a function of concrete compressive strength through experimental measurements and regression analysis, respectively. The proposed criterion is validated through the collected 342 experimental data points. Comparison between the predicted and the experiment results shows good agreement, providing a relatively high correlation factor. In addition, the predicted results are compared with existing prediction models.

The mechanisms of shear transfer in fiber-reinforced polymer (FRP) reinforced concrete members with shear reinforcement are discussed, and it is explained how these were used to derive the shear design provisions of the Canadian standard for design and construction of building structures with FRPs. Subsequently, the accuracy of these provisions and the validity of their underlying assumptions are assessed by comparing the predicted shear strengths of over three hundred FRP-reinforced beams with their corresponding experimental values. Although the focus of the paper is mainly on beams with FRP shear reinforcement, for completeness beams with and without shear reinforcement are analyzed. It is determined that the mean and standard deviation of the ratio of the test to predicted shear strength of the beams without shear reinforcement are 1.16 and 0.24, respectively, whereas those of beams with shear reinforcement are 1.15 and 0.23. The strengths of these beams are also computed using the recommendations and it is shown that overall recently proposed Canadian method yields more accurate and consistent results.

The technique of externally bonding fiber-reinforced polymer (FRP) composites has become very popular worldwide for retrofitting existing reinforced concrete (RC) structures. Debonding of FRP from the concrete substrate is a typical failure mode in such strengthened structures. The bond behavior between FRP and concrete thus plays a crucial role in these structures. The FRP-to-concrete bond behavior has been extensively investigated experimentally, commonly using a single or double shear test of the FRP-to-concrete bonded joint. Comparatively, much less research has been concerned with numerical simulation, chiefly due to difficulties in the accurate modeling of the complex behavior of concrete. This paper presents a simple but robust finite-element (FE) model for simulating the bond behavior in the entire debonding process for the single shear test. A concrete damage plasticity model is proposed to capture the concrete-to-FRP bond behavior. Numerical results are in close agreement with test data, validating the model. In addition to accuracy, the model has two further advantages: it only requires the basic material parameters (i.e., no arbitrary user-defined parameter such as the shear retention factor is required) and it can be directly implemented in the FE software ABAQUS.

Rehabilitation and upgrading of existing civil infrastructures have become a major concern that definitely requires attention. The use of externally bonded carbon-fiber-reinforced polymer (EB-CFRP) to strengthen deficient RC beams has gained in popularity and has become a viable and cost-effective method. Fatigue behavior of RC beams strengthened with fiber-reinforced polymer (FRP) is a complex issue due to the contribution of many variables such as the load range, the applied frequency, and the number of cycles. Very few studies have been conducted in shear under cyclic loading. This study has examined the fatigue performance of RC beams strengthened in shear using EB-CFRP sheets. The investigation involved nine laboratory tests performed on full-size 4,520-mm-long T-beams. The specimens were subjected to fatigue loading up to 6 million load cycles at a rate of 3 Hz. Two retrofit (loading) options (upgrading and repairing) and three different transverse-steel reinforcement ratios (Series S0, S1, and S3 specimens) were considered. Test results were compared with the upper fatigue limits specified by codes and standards. The specimens that did not fail in fatigue were then subjected to static loading up to failure to evaluate their residual capacity. The test results confirmed the feasibility of using the EB-CFRP sheets to extend the fatigue service life of RC T-beams for both loading options. They also revealed that the presence of transverse steel enhanced the fatigue performance of strengthened RC beams. Finally, it was found that the gain contributed by EB-FRP sheets to shear resistance decreased with the addition of internal steel stirrups, confirming the existence of an interaction between transverse-steel reinforcement and EB-FRP.

This study experimentally and numerically investigates the effect of longitudinal steel ratio on the flexural performance of RC beams externally strengthened with fiber-reinforced polymer (FRP) composites. The experimental program consisted of testing 11 beams under four-point bending until failure. Each beam was duplicated to verify the repeatability of the results. Three beams were tested as control specimens; the remaining eight beams were externally strengthened in flexure with FRP composites. The primary experimentally studied parameters were longitudinal steel ratio and axial FRP stiffness. Three different steel ratios were examined. For the lowest steel ratio, four different FRP systems with six axial stiffness values were investigated. However, for the other two steel ratios, only one FRP system was studied. In addition to the experimental program, a numerical study utilizing nonlinear finite-element (FE) analysis was conducted. As a result of the numerical study, new FRP stiffness and reinforcement parameters were introduced in this research. These parameters were used in the categorization of failure modes of FRP-upgraded beams.

Concrete-filled fiber reinforced polymer (FRP) tubes (CFFTs) have received significant research attention over the last two decades, with recent experimental studies identifying significant benefits of CFFTs filled with high-strength concretes (HSC). However, many test parameters of high-strength CFFTs, such as specimen slenderness, remain experimentally limited. This paper presents an experimental investigation on the axial compressive behavior of 33 monotonically loaded circular normal-strength and high-strength CFFTs (NSCFFTs and HSCFFTs). The influence of specimen slenderness is investigated on test specimens with height-to-diameter ratios ($H/D$) of 1, 2, 3, and 5. The CFFT specimens were instrumented with numerous lateral strain gauges to examine the development of hoop strains along the specimen height and around specimen perimeter. The experimentally recorded stress-strain relationships are presented graphically and the ultimate axial stresses and strains, and FRP tube hoop strains at rupture are tabulated. The results indicate that specimens with a height-to-diameter ratio ($H/D$) of 1 outperform specimens with a $H/D$ ratio of 2 to 5, with significantly increased strength and strain enhancements. The influence of slenderness on specimens with a $H/D$ ratio between 2 and 5 was found to be significant in regards to axial strain enhancement, with a decrease observed as specimen slenderness increased. Conversely, the influence of slenderness on axial strength enhancement of specimens with a $H/D$ ratio between 2 and 5 was found to be negligible. The strain results indicate that hoop rupture strains along the height of CFFTs become more uniform for specimens with higher amounts of confinement. On the other hand, the variation of hoop strains around the perimeter of CFFTs was not observed to be significantly influenced by slenderness, concrete strength or amount of confinement.

This study investigated the punching shear behavior of two-way concrete slabs with glass-fiber-reinforced polymer (GFRP) bars as flexural reinforcement and FRP stirrups (glass or carbon) as shear reinforcement. A total of 10 full-scale interior slab-column specimens measuring $\mathrm{2,500}\times \mathrm{2,500}\mathrm{mm}$, with thicknesses of either 200 or 350 mm, and $300\times 300\text{-}\mathrm{mm}$ square column stubs were fabricated and tested under monotonic concentric loading until failure. These tests aimed at investigating the behavior of GFRP-reinforced two-way concrete slabs reinforced with FRP stirrups as shear reinforcement and evaluating their contribution to the punching shear capacity. The investigated parameters were the flexural reinforcement ratio and the shear reinforcement type (glass FRP and carbon FRP stirrups) and ratio. The test results revealed that using FRP stirrups as shear reinforcement increased the punching shear strength and deformation capacity of the test slabs. The increased punching shear strength and deformation capacity were proportional to the flexural- and shear-reinforcement ratios. In addition, the performance was enhanced by reducing the brittleness of the specimens when FRP stirrups were used as shear reinforcement.

An experimental and analytical study that includes testing to failure of fiber-reinforced polymer (FRP) strengthened medium-scaled reinforced concrete shallow arches and the application of a specially tailored high-order finite element for their analysis is presented. Three arches are tested. One is used as a control arch, whereas the other two are strengthened with externally bonded FRP strips in different patterns. The loading system includes six nonsymmetric vertical point loads equally spaced along the arch. The theoretical study is based on a multilayered finite-element approach, which accounts for the deformability of the adhesive layer and its shear and radial normal (through the thickness) stiffness. The arch is modeled as a polygon that consists of inclined specially tailored high-order finite elements. The results show that applying the FRP strips leads to an increase of about 40% in the failure load of the arch, changes to the cracking pattern, and a significant increase in deflection capacity. Edge debonding of the FRP strip is observed during the test but without causing total failure of the arch. The ability of the theoretical model to describe the overall structural response, the stress transfer mechanism in the strengthened arch, and the interfacial stresses and the local stress concentrations near irregular points are also demonstrated. Together they throw light on the behavior of the FRP-strengthened arch and on the evolution of its failure mechanism.

An international committee on shear and torsion reported that giving physical significance for the torsion design is an upcoming challenge. The purpose of this paper is to propose a reasonably accurate and relatively simple model capable of predicting the torsional strength of concrete beams reinforced with glass fiber–reinforced polymer (GFRP) stirrups. In this paper, a database for concrete beams reinforced with GFRP stirrups, tested under torsion, is compiled. The implementation of the torsion design provisions of the conventional steel-reinforced-concrete design codes is discussed. A few selected strength models were used to predict the ultimate torsional strength of the tested beams. The predicted strength was compared with that measured during testing. The comparison showed that more improvement is required in calculating the inclination of the diagonal concrete strut and the effective strain in the GFRP stirrups. Two strength models were modified and proposed. The proposed models showed better compliance and consistency with the experimental results compared to the available models and design codes. In addition, the proposed models are noniterative and simple to implement. However, further experimental testing will help refine the proposed models.

Presented in this paper are test results to determine the moment-rotation characteristics of joint details for a portal frame specific to a pultruded fiber reinforced polymer assembly for the Startlink house. Two joints having beam-to-column dowel connections, with and without extra adhesively bonding, were statically loaded in increments of moment or rotation to ultimate failure. The floor beam and stud column members are bespoke closed sections developed for Startlink Lightweight Building System. The serviceability design calculations for the demonstrator house at Bourne, England assumed the joints rigid. Clauses in an English standard are applied to classify the measured rotational stiffnesses against the rigid requirement, and an evaluation is made of the modes of failure with respect to the design moments. Only the joint with extra bonding between the mating surfaces of the members is classified as rigid. Both joints are shown to have an acceptable strength.

Given developments in the construction material industry, existing knowledge of concrete confined with fiber-reinforced polymer (FRP) must be validated for newly developed FRP-type materials. Polyethylene terephthalate (PET), which has a high ultimate tensile strain, is a relatively new type of fiber. Given the limited number of studies on PET-FRP, the goal of this study is to observe and define the mechanical behavior of PET-FRP-confined concrete specimens subjected to monotonic/cyclic compression loading. In this study, the behavior of concrete confined with PET-FRP is compared with the behavior of concrete confined with other types of FRP. The general form of the compression stress-strain relationship of PET-FRP-confined concrete consists of two parts that are connected with a transition part, which is similar to concrete confined with other FRP types. Additionally, to understand whether available models are able to predict the compressive behavior of PET-FRP-confined concrete, the predictions from four models are compared with the corresponding test results. The results indicate that the compression stress–axial strain relationship of PET-confined concrete is not predicted in a sufficient manner using the four models.

The study presents the experimental behavior of reinforced concrete columns strengthened externally by transverse glass fiber-reinforced polymer (GFRP) sheets. The columns had a square section and very low concrete strength. The internal steel reinforcement comprised sparse stirrups and longitudinal bars with two different qualities, S220 or B500C. The specimens were lightly confined by full wraps or partial GFRP straps. The straps were placed in between the existing steel stirrups. Three different widths of GFRP straps were examined. A series of plain concrete columns with identical GFRP strengthening was designed in order to assess the partial wrapping effects and compare them with reinforced concrete columns. The columns were subjected to axial compressive monotonic load up to failure. The achieved strength and ductility levels of the strengthened columns suggest that partial wrapping can efficiently upgrade the mechanical behavior of columns with a square section. Fully or partially GFRP wrapped columns with S220 slender bars require relatively higher GFRP strengthening than their counterparts with B500C columns to achieve similar ultimate strains.