Journal of Composites for Construction

During earthquakes, diagonal members of concentrically braced frames are meant to dissipate hysteretic energy through their postbuckling performance. However, crack formation at the location of plastic hinges obstructs the brace operation. This paper proposes the use of fiber-reinforced polymer (FRP)—both glass fiber–reinforced polymer (GFRP) and carbon fiber–reinforced polymer (CFRP)—to improve the behavior of square hollow section (SHS) braces, and this retrofitting strategy is selected in a way to mitigate the local buckling which plays a key role in the failure of braces. The effectiveness of the proposed approach is validated using an experimental program, and the strengthening specimens are tested under axial compressive loading. The influence of parameters—including the slenderness ratio, the number of FRP layers, and the FRP length—is evaluated. Results indicate that the externally overwrapped FRP fabrics in the transverse direction are highly capable of postponing the local buckling, and the strengthening does not noticeably increase the axial strength of braces. Furthermore, a numerical model using ABAQUS software is developed to extend the experimental results for further studies after verification.

Existing RC moment-resisting frames (MRF) built with inadequate detailing or before the introduction of detailed seismic design codes (pre-1970s) are highly vulnerable to brittle failure mechanisms under earthquake loading. To prevent potentially catastrophic failures and consequent human and economic losses in future earthquakes, efficient and practical retrofit solutions are required for these buildings. This paper presents an experimental study focused on the development of retrofit solutions that adopt carbon fiber–reinforced polymers (CFRP) to improve the seismic performance of existing RC MRF at their beam–column connections. It is highlighted that to date, most experimental studies in this field used simplified test specimens that have ignored the presence of slabs and secondary beams at beam–column connections. This may lead to an unrealistic assessment of FRP retrofit schemes. Hence, in this study, results from six full-scale cyclic tests on typical pre-1970s interior beam–column joints with slab and transverse beams are presented. The tests are used to assess three proposed CFRP schemes composed of a combination of FRP strengthening methods and selective slab weakening. Each scheme is designed to meet a distinct retrofit objective: (1) enhancement of the lateral-load capacity, (2) enhancement of ductility, and (3) enhancement of the lateral-load capacity and ductility, as well as changing the dominant failure mode of the joint from a column hinging mechanism to one where the plasticity is mainly concentrated in the beams. A comparison of the retrofitted specimens with the behavior of a deficient specimen and a specimen designed to modern guidelines highlights the successful achievement of the respective retrofit objectives and the necessity to weaken the slab to achieve a favorable failure mechanism that will allow compliance to be achieved with current retrofit codes.

This study evaluates the use of a composite material composed of multiaxial hybrid glass-fibers and polypropylene fabric coated in a hydraulic lime-based mortar (referred to as textile-reinforced mortar) to improve the in-plane shear strength and displacement capacity of solid clay-brick and hollow clay-block masonry walls with the aim of defining a comprehensive test protocol. Design parameters of the adopted strengthening system are investigated, and both single-sided and double-sided configurations for application on wall surfaces are considered. The influence of the presence of an external thermal insulation layer on walls’ in-plane performance is also considered. The first stage characterizes the materials, their bonding properties, and the initial shear strength of the masonry bed-joints. The second stage investigates the in-plane behavior of strengthened masonry wall panels using diagonal compression testing. No premature debonding is observed, even with no mechanical anchorage between the masonry substrate and the strengthening layer. Out-of-plane deformation due to structural asymmetry is observed in the single-sided strengthening configuration.

Textile reinforced mortars (TRM) and thermal insulating materials are integrated in a new system for structural (including seismic) and energy retrofitting of masonry. The proposed system is investigated experimentally by performing medium-scale tests on three series of masonry walls subjected to in-plane cyclic loading. Key parameters under investigation included the use of one-sided versus two-sided insulation and/or TRM jacketing, and placement of the TRM outside the insulation or between the insulation and the masonry. On the basis of the results obtained in this study the authors believe that TRM jacketing may be combined effectively with thermal insulation to substantially increase the capacity of masonry walls subjected to in-plane cyclic loading. The experimental results prove that the exact positioning of the TRM and the insulating material does not play an important role in the in-plane response of retrofitted walls, as long as proper bonding between the different layers is achieved.

Intermediate crack (IC) debonding is one of the most dominant failure modes associated with reinforced concrete beams strengthened with fiber-reinforced polymer (FRP) plates. To obtain a better understanding of the mechanism of IC debonding, a comprehensive finite-element (FE) model based on the smeared-crack method is proposed to predict IC debonding failure. This model adopts a crack band model for concrete and incorporates a bond–slip relationship between the steel bars and surrounding concrete, as well as between the FRP plates and concrete substrate. The proposed model is found to be capable of accurately predicting the appearance, development and distribution of cracks in concrete. Furthermore, the concentration of interfacial slip and abrupt changes in the interfacial shear stress and FRP plate strain due to cracking can be predicted accurately, and IC debonding is successfully captured. Comparisons between the numerical and experimental results of 35 beams confirm that the proposed FE model is appropriate for predicting the IC debonding of the strengthened beams. Finally, a simple and accurate model of the FRP debonding strain is proposed based on the FE interfacial shear stress distribution. The accuracy of the model is verified by comparing the strength model with the results of 116 beam tests.

The Friendship Trail Bridge linking St. Petersburg to Tampa, Florida, was demolished in 2016. This was the site of 13 fiber-reinforced polymers (FRP) repairs of corroding reinforced concrete piles undertaken in three separate demonstration studies completed in 2003–2004, 2006, and 2008. The repairs used carbon or glass fiber, wet layup or prepreg, and epoxy or polyurethane resins. Installation was by shrink wrap in the initial series and by pressure bagging in the next two. Residual FRP–concrete bond was evaluated in 2015–2016 through 120 pull-off tests conducted on 10 representative repaired piles. Results showed wide variation in the measured pull-off strength depending on the resin type, the number of FRP layers, the prevailing conditions at the time the epoxy was mixed, and how it was placed. Ambient conditions at installation influenced bond in epoxy-based systems. The highest residual bond was recorded in epoxy-based repair on piles that were installed by pressure bagging in 2008.

The structural performance of carbon fiber–reinforced polymer (CFRP) pretensioned structures is controlled by the bond between the CFRP tendons and concrete. The bond strength of CFRP sand-coated tendons can be affected by humid environments due to the porous epoxy matrix structure or by defects in the external sand coating layer of CFRP tendons, e.g., due to storage conditions. Pullout tests were carried out to assess the bond strength performance of sand-coated tendons embedded in high-strength concrete and immersed in water at either 23 or 40°C. Sand-coated CFRP tendons with two different core diameters of either 4.2 or 5.4 mm were studied. To assess the effect of the sand coating coverage on the bond, half sand-coated and uncoated tendons were also tested. An image processing technique was developed to help correlate bond strength variations with variations in the sand coating. An average difference of 24% between the bond strengths of the half sand-coated and full sand-coated tendons was recorded. A large scatter in the pullout results for the sand-coated tendons of diameter 5.4 mm was observed and this was attributed to the manufacturing process. There was no clear trend of bond strength degradation in the sand-coated tendons even after roughly 1.5 years of full immersion in water irrespective of the exposure temperature. However, an increase in the bond strength of the uncoated tendons and in the bond stiffness of all CFRP tendons was observed. This was felt to be the result of concrete autogenous shrinkage in high-strength concrete and the potential swelling effects of the tendons in a humid concrete environment. Analytical models are used to describe the bond stress-slip behavior, and their suitability for sand-coated CFRP tendons is studied.

Fabric-reinforced cementitious matrix (FRCM) composites represent a new technique for strengthening concrete and masonry structures. Various studies confirm the efficiency of FRCM systems in terms of ultimate capacity gain, compatibility with the parent material, and viability in corrosive environments and under elevated temperatures. An appealing use of FRCM composites is to strengthen concrete bridges. Vehicular traffic imposes cycles of loading and unloading on the structure that can lead to fatigue failure, but the fatigue life of FRCM materials is yet to be investigated. This study examined fatigue performance of FRCM-strengthened reinforced concrete beams. Twelve concrete beams reinforced with carbon fabric were tested, including four benchmark beams subject to monotonic loading. The effects of fiber architecture and reinforcement ratio were accounted for by using two types of fabrics. Inspection of the fractured sections indicated that fatigue failure in the steel reinforcement was the predominant cause of failure. Analysis of applied stress range versus number of cycles to failure suggested that FRCM systems enhance fatigue life by controlling crack propagation in concrete.

The bending fatigue test results on cracked rectangular hollow section (RHS) steel beams repaired with prestressed carbon fiber-reinforced polymer (CFRP) plates are presented. The steel beams were artificially cut on the bottom of the midspan at various depths (3, 10, and 30 mm) before the test. Six prestressed CFRP plate-repaired steel beams were prepared with two prestressing levels. Six non-prestressed CFRP plate-repaired steel beams were repaired with two types of CFRP. Three more specimens were arranged as a control group. All specimens were subjected to fatigue loading. Fatigue crack propagation, failure modes, deflections, and strains were recorded and analyzed. The objective of this investigation was to determine the effect of the prestressing level in CFRP plates, repairing the material’s modulus and initial crack depth on the fatigue behavior of RHS beams. The test results indicated that a higher modulus material or a larger prestressing level could improve the fatigue performance. Repaired beams with prestressed CFRP plates exhibited a slight stiffness degradation, whereas repaired beams with non-prestressed CFRP plates exhibited significant stiffness degradation during the fatigue loading.

Because stirrups corrode earlier and faster than longitudinal steel bars, reinforced concrete (RC) structures may fail in shear instead of bending. Since shear failure is brittle and catastrophic, it is of significance to strengthen RC beams with corroded stirrups in shear. However, the shear behavior in strengthened RC beams has not yet been fully documented and explained. This paper presents a numerical investigation of the shear behavior of corroded RC beams strengthened in shear with fiber-reinforced polymer (FRP) sheets. The model described in this paper was based on modified compression field theory and can simulate the whole process from loading to shear failure. The proposed model was validated by collected experimental results. Influencing parameters that have prominent effects on the shear behavior were studied, including the corrosion degree, concrete strength, stirrup ratio, FRP sheet strengthening ratio, and strengthening methods. The numerical results proved that FRP strengthening, whether included as the wrapping or U-shaped bonding of FRP sheets, was effective to improve the shear strength of RC beams. It was found that the cracking load barely changed with the increasing corrosion degree of the stirrups, whereas the shear strength decreased gradually. With the increase in the stirrup ratio or the decrease in the concrete strength and FRP sheet strengthening ratio, stirrups provided a larger contribution to the shear behavior, and the shear strength decreased more significantly, as expected with corroded stirrups.

Hybrid fiber-reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs) are a new form of composite columns that consist of an outer FRP tube and an inner steel tube, with the space between them filled with concrete. Although many studies have been conducted on the hybrid DSTCs, no studies have been conducted on their behavior under blast loading. This study presents the results of a numerical study on the behavior of hybrid DSTCs under close-in blast loading. Numerical models of hybrid DSTCs are developed using finite-element code LS-DYNA, and the reliability of the developed models are validated with available testing results. With the validated models, numerical simulations are carried out to investigate the structural responses of hybrid DSTCs under blast loading. The simulation results indicate that the hybrid DSTCs behave in a ductile manner under blast loading. The outer FRP tube can effectively provide confinement to the infilled concrete, and the inner steel tube plays a key role in resisting the blast loading. Detailed parametric analyses are conducted to investigate the influences of different parameters on the blast behavior of hybrid DSTCs. The blast resistance capacities of the hybrid DSTCs, concrete-filled steel tubes (CFSTs), and concrete-filled double-skin steel tubes (CFDSTs) are compared and discussed based on the simulation results.

Beam-column joints are fundamental structural components to provide stability to a concrete frame. Compared to other types of joint, corner joints are more susceptible to failure due to their smaller size and inferior confinement; therefore, it is necessary to develop efficient techniques for strengthening three-dimensional (3D) corner deficient joints of reinforced concrete (RC) structures. However, most research has focused on two-dimensional (2D) joints due to the complexity of casting and testing 3D ones. Moreover, the effect of lateral beams on the confinement of the joint zone has been neglected in most cases. These shortcomings undermine the applicability of some of the retrofitting configurations proposed so far to real 3D structures. This study investigates the shear strengthening of nonseismic 3D corner joints, i.e., those lacking adequate transverse reinforcement in the joint zone, using carbon fiber reinforced polymer (CFRP) sheets. To delay the debonding of CFRP sheets off the concrete substrate, the externally bonded reinforcement on groove (EBROG) technique is used, and the hysteretic response of 3D corner joints strengthened via the EBROG method is analyzed. Five 3D joint specimens in half-scale size were manufactured and tested. The specimens consisted of one control specimen, one standard specimen conforming to code requirements, and three FRP-retrofitted specimens with L-shaped strengthening configurations. Results indicated that the adopted strengthening configurations prevented the shear failure of the joint. Furthermore, the EBROG method enhanced the load-carrying capacity and ductility of the RC joints by up to 38 and 69% respectively, compared with the control specimen. Comparison of the maximum loads achieved at a drift ratio of 4.5% revealed that the EBROG technique combined with CFRP fans prevented any load reduction up to the test termination.

This paper aims to investigate the behavior of short-span concrete members with a shear span–to–effective depth ratio ($a/d$) less than 2, internally reinforced with glass fiber–reinforced polymer (GFRP) bars. The study comprised testing of 12 specimens. Test parameters included the $a/d$ ratio, GFRP reinforcement ratio, and concrete grade. The study was supplemented by an analytical investigation to predict the strength of the tested specimens. The experimental study highlighted that increasing the longitudinal GFRP reinforcement ratio together with the concrete strength in the absence of web reinforcement might be detrimental to the strength of GFRP-reinforced short-span concrete members due to a shift in the mode of failure from crushing of a diagonal strut to diagonal splitting. Experimental results indicate also that increasing the $a/d$ does not necessarily reduce the strength of short-span GFRP-reinforced concrete specimens with a low concrete strength due to a greater angle of inclination of the critical shear crack developed during testing relative to that considered in the analysis. The strut-and-tie modeling (STM) provided conservative predictions for the nominal strength of the tested specimens except for those failed by diagonal splitting where it overestimated the strength by a maximum of 12%. The accuracy and validity of two published analytical approaches were examined. These approaches tended to provide less conservative, yet reasonable, nominal strength predictions but overestimated the strength of the specimens with the diagonal splitting mode of failure.

The structural performance of corrosion-damaged reinforced concrete (RC) beams repaired with fabric-reinforced cementitious matrix (FRCM) was investigated. Eleven RC beams were constructed and tested in flexure under four-point load configuration. Nine beams were subjected to an accelerated corrosion process for 70 days to obtain an average mass loss of 13% in the tensile steel reinforcing bars while two other beams were tested as controls. One corroded beam was repaired with carbon fiber-reinforced polymer (CFRP) before testing for comparison. The test parameters included the number of fabric plies (1–4), the FRCM repair scheme (end-anchored and continuous U-wrapped strips), and FRCM materials [carbon and polyparaphenylene benzobisoxazole (PBO)]. Test results showed that corrosion slightly reduced the yield and ultimate strengths of the beams. The use of FRCM increased the ultimate capacity of corroded beams between 5 and 52% and their yield strength between 6 and 22% of those of the uncorroded virgin beam. Beams repaired with U-wrapped FRCM strips showed higher capacity and higher ductility than those repaired with the end-anchored bottom strips having a similar number of layers. A high gain in the flexural capacity and a low ductility index were reported for specimens with a high amount of FRCM layers. A new factor was incorporated in the design equations of the ACI 549.4R-13 to account for the FRCM scheme.

This paper provides a comprehensive account of using fabric-reinforced cementitious matrix (FRCM) composites to strengthen corrosion-damaged reinforced concrete (RC) structures subjected to monotonic loading and fatigue. Twelve beams were constructed and tested to failure under four-point loading configuration. Prior to testing, 10 beams were subjected to accelerated corrosion for 140 days, leading to an average mass loss in the steel reinforcement of 19%. Eight corrosion-damaged beams were strengthened and tested while the other two beams remained unstrengthened. Two other virgin beams that were not subjected to corrosion were used as benchmarks. The test parameters included the fabric material (polyparaphenylene benzobisoxazole and carbon), the number of FRCM plies, the strengthening configuration, and the type of loading (monotonic and fatigue). Test results showed that the corrosion of steel bars dramatically decreased the fatigue life of the beams. After strengthening, the corrosion-damaged beams fully restored the load-carrying capacity of the virgin beam. The FRCM-strengthened beams endured more load cycles than those endured by their unstrengthened benchmarks but could not restore the original fatigue life of the virgin beam. The effect of FRCM configuration was more pronounced in the beams subjected to fatigue than those tested monotonically. PBO-FRCM composites were more effective than the carbon counterparts in enhancing the fatigue performance of the corrosion-damaged beams.

Shear testing of glass fiber–reinforced polymer (GFRP) bars can be considered for material specifications, quality control, quality assurance, and structural design purposes. ASTM and the Canadian Standards Association (CSA) specify the testing apparatus and the procedure to test fiber-reinforced polymer (FRP) matrix composites, in which the principal variables are the size (diameter) and type of the FRP bars. This study develops a device to test GFRP bars in shear based on both ASTM and CSA standards. The paper provides information about the significance of shear testing of GFRP bars and the potential outcomes of this type of test. For that purpose, different types of GFRP bars (straight and bent) provided by two different suppliers are studied. Two test series are considered. The first examines straight and bent bars from two different companies, and tests two different diameters of the GFRP bars. The straight bars are tested with and without their outer coating (provided by the companies) and the bent bars are tested in their straight portion. The same bars are tested in shear after being in an alkaline solution (pH 13) for 5 months (150 days) at 60°C. The second test series considers GFRP bars of various diameters (original GFRP bars are shaved). The main objective of this work is to propose a shear testing protocol for GFRP bars of various diameters by reducing the number of cutting blades of the shear testing device. Finally, the failure modes and the significance of stiffness are discussed.

This paper presents an experimental study on the bond durability of basalt fiber–reinforced polymer (BFRP) bars, steel–fiber reinforced polymer (FRP) composite bars (SFCB), and steel bars embedded in seawater sea-sand concrete (SWSSC) and subject to a simulated ocean environment. The effects of the environment type, exposure period (i.e., 3, 6, and 9 months), and reinforcement type on bond durability were investigated. The degradation mechanism was analyzed using scanning electron microscopy (SEM). The test results showed that after being conditioned, the failure mode changed from shear failure at the BFRP rib-concrete occlusal interface to the peeling of the BFRP surface. For the BFRP bars, after 9 months of aging, the bond stress retention was 92% in the wet-dry cycling environment and 78% in the immersion environment. For the SFCBs, after 9 months of aging, the retention in the wet-dry cycling environment was 100% and that in the immersion environment was 90%. For the steel bars, although there was no reduction in bond strength, pitting corrosion was observed at the bar surface. The predicted bond stress retention of BFRP bars embedded in SWSSC after 50 years of service life in various environmental conditions ranged from 47% to 83%.

This paper describes experimental and analytical investigations on a highly durable composite structural system comprising polyvinyl alcohol (PVA) fiber-reinforced concrete (PVA-FRC) reinforced with glass fiber-reinforced polymer (GFRP) bars. To aid in the development of a design guideline for this novel composite beam system, a generic analytical approach that is equally applicable to both conventional concrete and FRC beams reinforced with steel or FRP rebar is applied. Predicted load-deflection and load-crack width responses are compared to the experimental results, showing very good correlation for the flexural behavior of PVA fiber–reinforced concrete members.

Two models, Model Fiber and Model Rotational Spring (RS), simulating the seismic performance of repaired column-to-cap beam/footing connections using a carbon fiber-reinforced polymer (CFRP) donut are presented in this paper. In Model Fiber, distributed plasticity was assumed over a plastic hinge length of the nonlinear beam-column element. In Model RS, concentrated plasticity was considered using a nonlinear moment rotational spring located at the repaired cross section. Previous concrete damage and low-cycle fatigue of longitudinal steel bars as well as bond-slip between the damaged steel bars and surrounding concrete were included in the proposed numerical models. Numerical simulations show that the results are in good agreement with the experiments in terms of structural response, cumulative hysteretic energy, and moment-rotation capacity. Model Fiber can also predict the local response and low-cycle fatigue of the longitudinal steel bars. Using appropriate pinching parameters, Model RS can model accurate pinching behavior for both the repaired cast-in-place (CIP) and precast concrete specimens.