Journal of Composites for Construction

Despite the fact that fiber reinforced polymer (FRP) composites are a reliable structural material with reasonable durability performance, the environment to which the strengthened structure is exposed can make the strengthening system vulnerable. In this study, the effectiveness of externally bonded reinforcement (EBR) systems when external compressive stresses are applied to glass fiber reinforced polymers (GFRP)-to-concrete interfaces in several aggressive environments is analyzed. The compressive stress imposed on the GFRP-to-concrete interface intends to simulate, for instance, the effect produced by a mechanical anchorage system applied to the EBR system. The design and the region to set those mechanical anchorage systems are not yet well understood and are mostly applied without really knowing how they will behave. This work shows an exhaustive experimental program based on several double shear tests subjected to salt fog cycles, dry/wet cycles and two distinct temperature cycles: from $-10°\mathrm{C}$ to $+30°\mathrm{C}$ and $+7.5°\mathrm{C}$ to $+47.5°\mathrm{C}$. The Mohr-Coulomb failure criterion was found to provide a good representation of the performance of the GFRP-to-concrete interface, and changes of cohesion and the internal friction angle of those interfaces during the hours of exposure to the aggressive environments are reported.

Presented in this paper is a combined experimental and analytical modeling study of the strength of pultruded fiber-reinforced polymer (FRP) single-bolted double-lap joints subjected to tensile loading and elevated temperatures. Dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) are conducted on the polymeric composite material to determine the glass transition temperature and decomposition temperature, respectively. Based on the DMA and TGA results, and to cover glass transition without any material decomposition, the six temperatures selected for the test program are $+23$, $+60$, $+100$, $+140$, $+180$, and $+220°\mathrm{C}$. Three nominally identical joints are tensioned to failure at each temperature. A total of 36 double-lap joints are tested, comprising 18 joints fabricated with ordinary steel bolting and the other 18 with novel blind bolting. A comparison is made based on load-displacement curves, failure modes, and maximum (ultimate) loads. It is found that both methods of mechanical fastening experience a reduction of 85% in maximum load as the test temperature increases from $+23$ to $+220°\mathrm{C}$. Three proposed empirical or mechanism-based models for characterizing strength under elevated temperatures are shown to provide good predictions for the maximum loads obtained in the test program.

Reinforced concrete (RC) beams and slabs are frequently strengthened or stiffened in flexure by adhesively bonding fiber-reinforced polymer (FRP) plates to their surfaces using a strain-based moment-curvature design technique. This design technique is generally based on the intermediate crack (IC) debonding strain of the FRP reinforcement, that is, on the start of IC debonding; from this analysis it is often deduced that FRP plating is ineffective at the ultimate limit state because FRP debonding occurs before yield of the steel reinforcement. In this paper, it is shown that the strain-based approach is generally a lower bound at the ultimate limit state. Instead, a displacement-based approach is described that shows that FRP plated beams can be designed to achieve a higher strength than that of the RC beam by itself no matter when IC debonding first occurs. The mechanics of the analysis approach developed here treat the FRP debonded plate as a FRP prestressing tendon with a force equal to the IC debonding force. Consequently all FRP plated beams have the potential to achieve strengths greater than that of the unplated beam specifically when designed for ductility, which makes the system much more effective at the ultimate limit state. This paper describes, in a form suitable for the development of numerical solutions, the fundamental mechanics that control local IC debonding at a section or segment as well as global IC debonding along a member. It is shown how FRP plates and their extent of plating can be chosen through mechanics to increase the strength and if necessary the ductility of a member as well as allowing for both stable and unstable debonding.

The demand for sustainable concrete structures with longer service life and lower maintenance cost has been driving the use of glass fiber-reinforced polymer (GFRP) reinforcement in new concrete structures. The behavior of GFRP bars under seismic loading in reinforced concrete (RC) frame structures has not been widely investigated. Furthermore, the ability of FRP-RC frame structures to dissipate energy in seismic events is still questionable due to the elastic-linear behavior of the FRP reinforcement. Very few studies have been conducted on the seismic behavior of GFRP-RC exterior beam-column joints, and none studied the shear capacity of joints. Therefore, this study attempts to partially fill this gap by investigating the shear capacity of beam-column joints reinforced with GFRP bars and stirrups. Six full-scale exterior beam-column joint prototypes (T-shaped) were constructed and tested under simulated seismic load conditions. Test parameters in this study included the concrete strength and the shear stress level in the joint. Diagonal shear failure in the joint exhibited in some specimens showed the significance of evaluating the shear capacity in the joint.

This paper presents the results of an investigation of reinforced concrete (RC) T-beams retrofitted in shear with near-surface mounted (NSM) fiber-reinforced polymer (FRP) rods. Six full-scale 4,520-mm-long RC T-beams were tested to study the effects of important parameters such as the presence of NSM FRP rods, the presence of steel stirrups, and the steel stirrup ratio. This paper provides an insightful and comprehensive description of the behavior of strengthened T-beams under increasing load, from the formation of the first crack to ultimate failure. The results of this study and those gathered in the presented database show that existing steel stirrups and strengthening NSM FRP did not diminish each other’s effect when failure modes unrelated to shear resistance of RC beams were prevented. The experimental results of this study and those in the database were used to verify a newly proposed model to predict the shear contribution of NSM FRP rods and laminates in RC beams strengthened in shear. The proposed model showed an improved accuracy compared with the existing models in the literature.

In this paper a reinforcement system that utilizes unique geometries in order to improve reinforced concrete beam stiffness, shear strength, and reinforcement bond properties, as compared with conventional glass-fiber reinforced polymer (GFRP) bar-style reinforcement is introduced. The proposed GFRP reinforcement is formed into modular units intended for use in place of traditional flexural reinforcement. Prototype reinforcement units were manufactured and cast into eight concrete beam specimens. These specimens were tested and compared to similar specimens with conventional straight GFRP bars in order to document their behavior and assess the effect on beam performance of the new geometry under typical loading. Bond failures that occurred in the control specimens were not present in the prototype specimens. Ultimate shear capacity in one of the prototype variations was increased by 26% over the bar-reinforced control specimens. Flexural performance for one of the prototypes was also found to be comparable to the bar-style control reinforcements.

In this research, the flexural resistance factors in the American Association of State Highway and Transportation Officials’ (AASHTO) load and resistance factor design (LRFD) format were calibrated for bridge girders that are prestressed with carbon fiber–reinforced polymers (CFRP). The underlying principle of LRFD is to achieve a uniform probability of failure (target reliability) for all possible design scenarios, which is realized through application of load and resistance factors. Calibration of resistance factors requires an extensive design space to be applicable to different design scenarios. For this purpose, 12 design cases with various girder span lengths, girder positions, girder spacing, roadway widths, and failure modes were considered. The load and resistance model random variables, their statistics, and flexural resistance model accuracy were obtained from material testing and literature. Existing load factors were used, and flexural resistance factors were derived using Monte Carlo simulation and a comparative reliability method for different target probabilities of failure for interior and exterior girders failing in tension and for interior girders failing in compression.

To study the fatigue performance of a reinforced-concrete (RC) beam strengthened with textile-reinforced concrete (TRC), a four-point bending fatigue test was conducted, in which the strengthening forms, reinforcement ratio, textile ratio, static load damage, and sustained load corrosion were considered. The results of this test show that TRC can modify the fatigue failure mode and significantly improve the fatigue life of RC beams. The strengthened beams exhibited better performance than the unstrengthened beams with regard to controlling cracks. TRC can increase the number of cracks and decrease the crack width in RC beams, and the strengthened beams exhibit an increase in midspan deflection that rapidly progresses, while the unstrengthened beams do not exhibit this progression. In terms of improving the fatigue life of beams, the single-sided strengthening form is shown to perform better than the three-sided form, and a beam with a high reinforcement ratio is superior to that with a low ratio. The textile ratio is also shown to influence the fatigue life of the RC beam: when the textile ratio is low, the strengthened and unstrengthened beams have nearly identical fatigue lives; however, when the ratio reaches a given value, the fatigue life of the strengthened beams increases with an increasing textile ratio. Also, TRC performs better when used to strengthen damaged beams, and the damage degree influences the fatigue life of a beam strengthened with TRC. However, the fatigue life of strengthened beams will be reduced if the static-damage degree is high. In addition, in a chlorate environment, corrosion or a sustained load acting alone reduces the fatigue life of strengthened beams, and the initial deformation generated by a sustained load also affects the fatigue life. The effect becomes more evident when corrosion and a sustained load are present concurrently. It was also found that corrosion weakens the bond between the TRC and the original concrete, resulting in damage to the interface between the TRC and the strengthened beams. The ratio of the realistic stiffness to the computed stiffness was proposed to judge the safety of the strengthened beams under a fatigue load.

Roofing is one of the most vulnerable parts of the building envelope in a residential structure, often damaged during extreme wind events such as hurricanes or tornadoes. Conventional roofing systems in residential buildings mostly consist of separate the main wind force resisting system (MWFRS) and components and cladding (C&C). High wind uplift forces often cause partial or full destruction of one or more of these elements, leading to water intrusion and losses of interior contents. This study focuses on the design, development, and testing of a new composite roofing system that integrates the functionalities of the MWFRS and C&C. The new system is composed of lightweight concrete panels reinforced with fiber-reinforced polymer mesh and rods. The architectural shape of a conventional residential roof with high profile tiles was adopted for the composite roof panels to help provide the requisite structural stiffness and strength of the MWFRS, while maintaining similar C&C aesthetics and avoiding failure of individual tiles that often become wind-borne debris. The panel system was subjected to a battery of tests under equivalent wind loads. In addition, three connections for the system were designed and validated through testing. The panel-to-wall connection provided ample continuity of the vertical load path; the panel to panel connection was found adequate for shear transfer; and the ridge connection allowed for load transfer between the windward and leeward sides of the roof. Tests demonstrated the structural viability of the new system as an alternative to conventional roofs, making it ideal for residential buildings in hurricane zones and tornado alleys. The purpose of the paper is to present details of the system and their structural reliability. Durability and constructibility issues, including cracking, are the object of future work.

Concrete, when exposed to elevated temperatures, undergoes deterioration and loses much of its strength. The strengthening of heat-damaged concrete is possible with the help of external reinforcing materials such as fiber-reinforced polymers (FRPs). The strength of FRP-strengthened flexural concrete members depends on the bond between the FRP and the concrete. The purpose of this experimental study was to investigate the behavior of the bond between the glass fiber-reinforced polymer (GFRP) laminate and the heated concrete using a single-shear test (SST). The specimens were initially heated to temperatures of 200, 400, 600, and 800°C. The heat-damaged specimens were subsequently bonded with FRP sheets with various bond lengths (100, 150, and 200 mm). The test variables were bond length and elevated temperatures. The results show that bond strength increased with increasing bond length and noticeably decreased as temperatures exceeded 400°C. The thickness of the delaminated concrete layer with GFRP composite was less than 4 mm for the specimens subjected to temperatures less than 400°C. However, the thickness of the delaminated concrete ranged 4–25 mm in 600 and 800°C heat-damaged specimens. The ultimate bond stress was influenced by the bond length and increased as the bond length decreased. A model is proposed to estimate the bond strength between GFRP laminate and heated concrete.

One-way reinforced concrete joist floor with wide shallow beams (WSBs) are used widely in building construction throughout the Middle East. Situations arise when it is desirable to strengthen a specific member or a group of members in a floor to reduce their deflections. Extensive work has been made on the strengthening of isolated members using fiber-reinforced polymers (FRP) for providing much needed data on the member performance and characteristics where the ultimate goal is to apply these findings to actual complex existing structures. Judging by studies on individual isolated members strengthened with FRP, a substantial improvement in deflection control is expected. However, this cannot be directly applied to members in redundant structural systems before confirming that experimentally. This paper presents the results of an investigation on the effectiveness of carbon FRP (CFRP) external strengthening for the purpose of reducing deflection of WSBs as part of joist floor of a prototype reinforced concrete building with highly redundant structural system. The results indicated that using CFRP strengthening did not show much improvement in controlling the deflection of the beams. The experimental work was also complemented with theoretical explanation of the expected effectiveness in terms of parallel spring analogy. Further, a nonlinear finite element analysis for the floor system was conducted. The results of that analysis were in good agreement with the experimental findings.

Application of carbon fiber-reinforced polymer (CFRP) composites has been promoted successfully for strengthening or repairing concrete structural members. However, if a column member is strengthened with longitudinal FRP sheets in which fibers are laid parallel to the column axis, the FRP composite will soon buckle under compression to ultimately debond off the concrete surface. It is the objective of the current research to investigate the effect of the recently developed grooving method (GM) on cylindrical specimens strengthened with FRP sheets under compressive loading, and to determine whether GM postpones the buckling of FRP sheets and their debonding off the concrete substrate. For the purposes of this study, 35 cylindrical reinforced concrete specimens, 150 mm in diameter and 500 mm in height, were tested under compressive loading with eccentricities of zero, 30, 60, and 90 mm. The specimens were additionally subjected to the four-point flexural bending test to represent infinity eccentricity. Fiber-reinforced polymer (FRP) sheets with longitudinal fibers parallel to the column axis were installed using the conventional externally bonded reinforcement (EBR) and grooving method, taking advantage of the special technique of externally bonded reinforcement on grooves (EBROG). Experimental results revealed that the grooving method was much more effective than the EBR in increasing the load-carrying capacity of the specimens. Moreover, it was found that longitudinal fibers exhibited an enhanced effect with increasing load eccentricity. The results showed that in the columns strengthened with longitudinal FRP strips using the EBROG method, the load-carrying capacity enhancements of 8.3, 12.2, 25.8, 36.0, and 53.3% relative to those of the controls were achieved for 0, 30, 60, 90 mm, and infinity eccentricities, respectively.

Continuous unbonded posttensioned concrete beams might require strengthening to cope with higher demands. Strengthening with carbon-fiber-reinforced plastics (CFRP) is a viable option; however, because of the limited number of studies in this area and lack of experimental data, the guidelines covered only bonded prestressed concrete members, in particular when these are made from high-strength concrete (HSC). This paper reports on the results of an experimental investigation on the flexural behavior of continuous unbonded posttensioned HSC beams strengthened using CFRP. In this application, the CFRP is either externally bonded reinforcement (EBR) or internally near-surface mounted (NSM) in both hogging and sagging regions. The results show that both service and ultimate states of continuous unbonded posttensioned concrete beams were considerably improved by CFRP strengthening, and also that the efficiency of the NSM method is greater than that of the EBR method, especially in crack propagation and ultimate load.

With the widespread use of fiber-reinforced polymer (FRP) composites in the construction sector as a strengthening technique, the development of design guidelines for the field application of externally bonded FRP systems is ongoing in Europe, Japan, Canada, and the United States. The main goal of this study is to evaluate the current seven international design guidelines and four other design models for the prediction of confined concrete compressive strength of FRP-wrapped plain concrete cylinders against the experimental results of a large database of 812 specimens reported in the literature. The results clearly show that the reliability of predictions of confined concrete compressive strength of FRP-wrapped plain concrete cylinders by the design guidelines significantly varies for different ranges of unconfined concrete compressive strength. For example, the gain in confined concrete compressive strength of FRP-wrapped low- and medium-strength concrete cylinders is larger than that of high- and ultrahigh-strength concrete cylinders. Furthermore, a simplified model for the prediction of design/characteristic–confined concrete compressive strength is developed based on the design-assisted-by-testing approach. The developed simplified model accounts for the variation in confinement effectiveness for different ranges of unconfined concrete strengths.

Fiber-reinforced polymer (FRP) started to find its way as an economical alternative material in civil engineering in the early 1970s. The behavior and failure modes for FRP composite structures were studied through extensive experimental and analytical investigations. Although research related to the flexural behavior of FRP-strengthened elements has reached a mature phase, studies related to FRP shear strengthening are less advanced. In all proposed models to predict shear capacity, the constitutive behaviors of concrete and FRP are described independently. The true behavior, however, should account for the high level of interaction between the two materials. Constitutive relations for FRP-strengthened reinforced concrete (RC) elements should provide a better understanding of the shear behavior of the composite structure. To generate these relations, large-scale tests of a series of FRP-strengthened RC panel elements subjected to pure shear were conducted. This paper presents the results of the test program and the calibration of the parameters of the constitutive model. These constitutive laws could easily be implemented in finite-element models to predict the behavior of externally bonded FRP-strengthened beams. The focus in this work is on elements failing because of concrete crushing and not because of FRP debonding. The newly developed model provides a good level of accuracy when compared with experimental results.

This study presents an experimental study on responses of eight constrained masonry walls with tie columns and ring beams, subjected to in-plane cyclic loading before and after being retrofitted with basalt-fiber-reinforced polymer (BFRP). Among these eight walls, two without any strengthening measures served as reference specimens; two were directly strengthened with horizontal and diagonal BFRP strips after they were constructed; and the rest were tested to defined damage levels and then repaired for retesting. The opening in the wall, the layer of the BFRP strip, and the vertical compression level were considered in the test design. Experimental results showed different failure modes of the specimens. Details of testing results were discussed regarding strength and drift, hysteretic characteristics, stiffness degradation, energy dissipation, and BFRP strip strains. A performance comparison analysis was also conducted between the constrained masonry walls and the unreinforced ones. Finally, a calculation method was proposed to estimate the ultimate lateral strength of BFRP retrofitted masonry walls.