Seismic Behavior of Reinforced Concrete Interior Beam-Wide Column Joints Repaired Using FRP

To prevent the casualties that can result from the collapse of earthquake-damaged structures, it is important that structures be rehabilitated as soon as possible. This paper proposes a rapid rehabilitation scheme for repairing moderately damaged reinforced concrete (RC) beam-wide column joints. Four nonseismically detailed interior beam-wide column joints were used as control specimens. All four subassemblages were subjected to similar cyclic lateral displacement to provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks with epoxy and externally bonding them with carbon-fiber-reinforced polymer (CFRP) sheets and glass-fiber-reinforced polymer (GFRP) sheets. These repaired specimens were then retested and their performance compared with that of the control specimens. This paper demonstrates that the repair of damaged RC beam-wide column joints by using FRP can restore the performance of damaged RC joints with relative ease, suggesting that the repair of beam-column joints is a cost-effective alternative to complete demolition and replacement     

CFRP Rehabilitation of Concrete Frame Joints with Inadequate Shear and Anchorage Details

The research presented in this study involves full-scale experimental evaluation of carbon fiber-reinforced polymer (CFRP) rehabilitation for existing beam-column joints designed for gravity load with common pre-1970s deficient reinforcement details when subjected to cyclic loading. Numerous studies have demonstrated effectiveness of externally bonded fiber-reinforced polymer (FRP) materials for retrofitting the deteriorating RC structures. Although these materials are widely used in bridges, their applications in buildings have been somewhat limited. In particular, the experimental investigations on external FRP retrofit of deficient beam-column joints have not thoroughly been investigated and they are mainly on scaled-down specimens. The failure of these subassemblies, which possess lack of shear reinforcement within the joint core and shortly embedded positive beam reinforcement, would possibly result in catastrophic collapse of reinforced concrete frame structure during an earthquake event. Recognizing the urgent need to upgrade these structural subassemblies, the current investigation uses CFRP retrofit techniques to enhance the performance of such deficient joints. Experimental variables studied entail the developed CFRP retrofit configurations, and magnitude of the applied column axial load. Comparative analysis of the lateral loads versus drift hysteresis loops, stiffness degradation, and total dissipated energy curves of three as-built and three corresponding CFRP-retrofitted RC joints revealed that significant improvement in the shear capacity of the upgraded joints occurred. More importantly, the slippage of short embedded beam positive reinforcement into the joint was substantially controlled due to the developed CFRP retrofit. The results demonstrate the effectiveness of CFRP retrofit configurations in enhancing the structural performance of actual size connections.     

Seismic Behavior of Concrete Columns Reinforced by Steel-FRP Composite Bars

Steel-fiber-reinforced polymer (FRP) composite bars (SFCBs) are a novel reinforcement for concrete structures. Because of the FRP’s linear elastic characteristic and high ultimate strength, they can achieve a stable postyield stiffness even after the inner steel bar has yielded, which subsequently enables a performance-based seismic design to easily be implemented. In this study, lateral cyclic loading tests of concrete columns reinforced either by SFCBs or by ordinary steel bars were conducted with axial compression ratios of 0.12. The main variable parameters were the FRP type (basalt or carbon FRP) and the steel/FRP ratio of the SFCBs. The test results showed the following: (1)?compared with ordinary RC columns, SFCB-reinforced concrete columns had a stable postyield stiffness after the SFCB’s inner steel bar yielded; (2)?because of the postyield stiffness of the SFCB, the SFCB-reinforced concrete columns exhibited less column-base curvature demand than ordinary RC columns for a given column cap lateral deformation. Thus, reduced unloading residual deformation (i.e., higher postearthquake reparability) of SFCB columns could be achieved; (3)?the outer FRP type of SFCB had a direct influence on the performance of SFCB-reinforced concrete columns, and concrete columns reinforced with steel-basalt FRP (BFRP) composite bars exhibited better ductility (i.e., a longer effective length of postyield stiffness) and a smaller unloading residual deformation under the same unloading displacement when compared with steel-carbon FRP (CFRP) composite bar columns; (4)?the degradation of the unloading stiffness by an ordinary RC column based on the Takeda (TK) model was only suitable at a certain lateral displacement. In evaluating the reparability of important structures at the small plastic deformation stage, the TK model estimated a much smaller residual displacement, which is unsafe for important structures.     

Bond Strength of Lap-Spliced Bars in Concrete Confined with Composite Jackets

The effectiveness of fiber-reinforced polymer (FRP) and textile-reinforced mortar (TRM) jackets was investigated experimentally and analytically in this study to confine old-type reinforced concrete (RC) columns with limited capacity because of bond failure at lap-splice regions. The local bond strength between lap-spliced bars and concrete was measured experimentally along the lap-splice region of six full-scale RC columns subjected to cyclic uniaxial flexure under constant axial load. The bond strength of the two column specimens tested without retrofitting was found to be in good agreement with the predictions given by two existing bond models. These models were modified to account for the contribution of composite material jacketing to the bond resistance between lap-spliced bars and concrete. The effectiveness of FRP and TRM jackets against splitting at lap splices was quantified as a function of jacket properties and geometry as well as in terms of the jacket effective strain, which was found to depend on the ratio of lap-splice length to bar diameter. Consequently, simple equations for calculating the bond strength of lap splices in members confined with composite materials (FRP or TRM) are proposed.     

Behavior of Beams Strengthened with Novel Self-Anchored Near-Surface-Mounted CFRP Bars

A commonly observed failure mode in laboratory tests involving surface bonded fiber-reinforced polymer (FRP) laminates or near-surface-mounted (NSM) bars is premature delamination, that is, the separation of the FRP from the substrate well before the FRP reaches its ultimate strain capacity. To delay the onset of delamination and to ensure that the NSM FRP reinforcement continues to contribute to member strength after partial delamination, a new self-anchored carbon fiber-reinforced polymer (CFRP) bar was developed and tested for this investigation. This bar is made with a series of monolithic spikes that can be anchored deep inside the concrete. In addition to cutting grooves into the concrete cover for the placement of the primary reinforcing bar, holes are drilled deep into the concrete to insert the spikes. To test the performance of this bar, six large, simply supported, reinforced, concrete beams were retrofitted with NSM bars and tested in four-point bending. Two beams were strengthened with NSM bars without anchors or spikes but were otherwise similar to the self-anchored bar and served as control specimens (Series?B1). Two beams were strengthened in flexure with the new self-anchored NSM bars (Series?B2), and the remaining two beams (Series?B3) were strengthened in flexure and shear by using the self-anchored NSM bars as partial shear reinforcement. The effect of the proposed strengthening system on the beams’ strength, failure mode, deformability, and ductility are discussed on the basis of the experimental results. The anchors delayed delamination and enabled the NSM bar to experience at least a 77% higher strain at failure than the companion bar without anchors. The anchors also increased beam displacement ductility and energy ductility at a 20% strength degradation by at least 34% and 42%, respectively.     

Seismic Rehabilitation of Corner RC Beam-Column Joints Using CFRP Composites

In this paper, efficiency and effectiveness of carbon fiber reinforced polymers (CFRPs) in upgrading the shear strength and ductility of seismically deficient corner or knee reinforced concrete beam-column joints have been studied. For this purpose, four as-built corner/knee joints were constructed with no transverse reinforcement, representing extreme case of preseismic code design construction practice of joints and encompassing many existing beam-column corner joints. Out of these four as-built specimens, two specimens were used as baseline specimens (control specimens) and other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epoxy bonded to joint, beams, and part of the column regions. In the second scheme, however, sheets were epoxy bonded to joint region only but they were effectively prevented against any possible debonding through mechanical anchorages. All these four subassemblages were subjected to cyclic lateral load histories to simulate loading due to earthquake and provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks through epoxy and externally bonding them with CFRP sheets under the same above two schemes. These repaired specimens were subjected to the similar cyclic lateral load history and their response histories were obtained. Response histories of control, repaired, and strengthened specimens were then compared. The results were compared through hysteretic loops, load-displacement envelopes, column profiles, ductility, and stiffness degradation. The comparison shows that CFRP sheets are very effective in improving shear resistance and deformation capacity of the corner beam-column joints and delaying their stiffness degradation. Shear capacities of control, repaired, and strengthened specimens were also predicted using writers’ published formulation. The predicted shear capacities were in a good agreement with the experimental values.     

Seismic Behavior of As-Built, ACI-Complying, and CFRP-Repaired Exterior RC Beam-Column Joints

In this paper, the efficiency and effectiveness of carbon-fiber-reinforced polymer (CFRP) sheets for upgrading the shear strength and ductility of a seismically deficient exterior beam-column joint were studied and compared with an American Concrete Institute (ACI)-based design joint specimen. One as-built joint specimen, representing the preseismic code design and construction practice for joints and one ACI-based design joint specimen, satisfying the seismic design requirements of the current code of practice were cast. The as-built specimen was used as baseline (control) specimen. These two specimens (i.e., the as-built control and the ACI-based specimens) were subjected to cyclic lateral load histories to induce damage equivalent to damage expected from a severe earthquake. The damaged control specimen was then repaired by filling its cracks with epoxy and externally bonding CFRP sheets to the joint, the beam, and part of the column regions. This specimen was identified as the repaired specimen. The repaired specimen was subjected to a similar cyclic lateral load history, and its response history was recorded. The response histories of the as-built control, the repaired, and the ACI-based design specimen were then compared. The test results demonstrated that externally bonded CFRP sheets can effectively improve both the shear strength and the deformation capacity of seismically deficient and damaged beam-column joints to a state comparable to the ACI-based design joint.     

Effect of FRP Confinement on Bond Strength of Hooked Bars: Normal-Strength Concrete Structures

To assess the viability of the external confinement of normal-strength concrete beam–column joints with carbon fiber-reinforced polymer (CFRP) sheets in increasing the bond strength of hooked bars anchored in the joints, 12 hooked bar specimens were tested. The variables were beam tensile bar size, anchorage length, mode of confinement of the beam hooked bars in the beam–column joint (whether the hooked bars were anchored within or outside the column reinforcement cage, denoted as “confined specimens” or “unconfined specimens”), and presence or absence of FRP wraps. The specimen simulated the rigid connection of a cantilever beam to a column. The tensile beam reinforcement consisted of two bars anchored in the base column using hooked-bar anchorages. Test results indicated that FRP sheets were effective in increasing the anchorage capacity and the ductility of the load–deflection history for both unconfined and confined specimens. However, FRP sheets had a more significant influence on unconfined specimens than companion confined specimens. As compared with unconfined specimens without FRP wrapping, unconfined FRP specimens had an average of a 23% increase in bond strength, confined non-FRP specimens had an average 30% increase in bond strength, and confined FRP specimens had an increase of 54%.     

Seismic Response of FRP-Upgraded Exterior RC Beam-Column Joints

Shear failure of exterior beam-column joints is identified as the principal cause of collapse of many moment-resisting frame buildings during recent earthquakes. Effective and economical strengthening techniques to upgrade joint shear resistance and ductility in existing structures are needed. In this paper, efficiency and effectiveness of carbon fiber-reinforced polymer (CFRP) sheets in upgrading the shear strength and ductility of seismically deficient exterior beam-column joints have been studied. Four as-built joints were constructed with nonoptimal design parameters (inadequate joint shear strength with no transverse reinforcement) representing preseismic code design construction practice of joints and encompassing most of existing beam-column connections. Out of these four as-built specimens, two specimens were used as baseline specimens (control specimens) and other two were strengthened with CFRP sheets under two different schemes (strengthened specimens). In the first scheme, CFRP sheets were epoxy bonded to joint, beams, and part of the column regions. In the second scheme, however, sheets were epoxy bonded to joint region only but they were effectively prevented against any possible debonding through mechanical anchorages. All of these four subassemblages were subjected to cyclic lateral load histories so as to provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks through epoxy and externally bonding them with CFRP sheets under the same above two schemes. These repaired specimens were subjected to the similar cyclic lateral load history and their response histories were obtained. Response histories of control, repaired, and strengthened specimens were then compared. The results were compared through hysteretic loops, load-displacement envelopes, column profiles, joint shear distortion, ductility, and stiffness degradation. The comparison shows that CFRP sheets are very effective in improving shear resistance and deformation capacity of the exterior beam-column joints and delaying their stiffness degradation.     

Shear Strength of One-Way Concrete Slabs Reinforced with Fiber-Reinforced Polymer Composite Bars

This paper evaluates the shear strength of one-way concrete slabs reinforced with different types of fiber-reinforced polymer (FRP) bars. A total of eight full-size slabs were constructed and tested. The slabs were 3,100?mm?long×?1,000?mm?wide×200?mm?deep. The test parameters were the type and size of FRP reinforcing bars and the reinforcement ratio. Five slabs were reinforced with glass FRP and three were reinforced with carbon FRP bars. The slabs were tested under four-point bending over a simply supported clear span of 2,500 mm and a shear span of 1,000 mm. All the test slabs failed in shear before reaching the design flexural capacity. The experimental shear strengths were compared with some theoretical predictions, including the JSCE recommendations, the CAN/CSA-S806-02 code, and the ACI 440.1R-03 design guidelines. The results indicated that the ACI 440.1R-03 design method for predicting the concrete shear strength of FRP slabs is very conservative. Better predictions were obtained by both the CAN/CSA-S806-02 code and the JSCE design recommendations.     

Effects of Variation of Axial Load and Bidirectional Loading on Seismic Performance of GFRP Retrofitted Reinforced Concrete Exterior Beam-Column Joints

Most of the experimental studies available in literature on the seismic assessment and retrofit of existing, poorly detailed, reinforced concrete (RC) beam-column joints, typical of pre-1970s construction practice, have concentrated on the two-dimensional (2D) response, using unidirectional cyclic loading testing protocol and constant axial load. Even more limited information is available on the performance of exterior (corner) three-dimensional (3D) RC beam-column joints with substandard detailing subjected to bidirectional loading regime. In this study, the results of a comprehensive experimental program is presented, aiming to show the effects of varying axial and bidirectional loading on the seismic performance of deficient exterior RC beam-column joints before and after retrofit. Ten exterior beam-column joint subassemblies are tested, including four as-built specimens and six retrofitted specimens using externally bonded glass fiber-reinforced polymer (GFRP) sheets. Test results are herein presented and conclusions are drawn on the basis of the observed global and local performance. The significance of the triaxial interaction of varying axial and bidirectional loading effects on the response of retrofitted corner joints is confirmed by the experimental findings. The proposed retrofit solution was shown to be capable of re-establishing an appropriate hierarchy of strength within the subassembly, protecting the panel zone region from shear failure while promoting the formation of a plastic hinge in the beam.     

Experimental Behavior of Carbon Fiber-Reinforced Polymer (CFRP) Sheets Attached to Concrete Surfaces Using CFRP Anchors

Fiber-reinforced polymer (FRP) composite sheets have gained popularity as a viable strengthening technique for existing reinforced concrete structures. The efficiency of the strengthening system largely depends on adequate bond between FRP sheets and the concrete substrate. In recent years, techniques to anchor FRP sheets have been proposed in applications that have limited distance to develop FRP sheet strength. One promising technique consists of fabricating and bonding FRP anchors during the FRP sheet saturation and embedding them into predrilled holes in the concrete substrate. This paper presents experimental results highlighting the complex behavior between FRP sheets and anchors. The primary failure modes that the sheet-anchor system can experience are identified. The experiments identify the main variables that influence the FRP anchor-sheet system behavior. This research contributes to the needed experimental database that will aid in future development of design recommendations of this anchorage system.     

Analysis of FRP-Strengthened RC Beam-Column Joints

Analytical models are presented in this study for the analysis of reinforced concrete joints strengthened with composite materials in the form of externally bonded reinforcement comprising unidirectional strips or flexible fabrics. The models provide equations for stresses and strains at various stages of the response (before or after yielding of the beam or column reinforcement) until the ultimate capacity is reached, defined by concrete crushing or fiber-reinforced polymer (FRP) failure due to fracture or debonding. Solutions to these equations are obtained numerically. The models provide useful information on the shear capacity of FRP-strengthened joints in terms of the quantity and configuration of the externally bonded reinforcement and may be used to design FRP patching for inadequately detailed beam-column joints. A number of case studies are examined in this article, indicating that even low quantities of FRP materials may provide significant enhancement of the shear capacity. The effectiveness of external reinforcement increases considerably if debonding is suppressed and depends heavily on the distribution of layers in the beam and column. The latter depends on the relative quantities of steel reinforcement crossing the joint panel and the level of axial load in the column. Analytical shear strength predictions were in good agreement with test results found in the literature, thus adding confidence to the validity of the proposed models.     

Experimental Investigation of FRP-Strengthened RC Beam-Column Joints

The results of a comprehensive experimental program, aimed at providing a fundamental understanding of the behavior of shear-critical exterior reinforced concrete (RC) joints strengthened with fiber reinforced polymers (FRP) under simulated seismic load, are presented in this study. The role of various parameters on the effectiveness of FRP is examined through 2/3-scale testing of 18 exterior RC joints. Conclusions are drawn on the basis of certain load versus imposed displacement response characteristics, comprising the strength (maximum lateral load), the stiffness, and the cumulative energy dissipation capacity. The results demonstrate the important role of mechanical anchorages in limiting premature debonding, and they provide important information on the role of various parameters, including: area fraction of FRP; distribution of FRP between the beam and the column; column axial load; internal joint (steel) reinforcement; initial damage; carbon versus glass fibers; sheets versus strips; and effect of transverse beams.     

Seismic Rehabilitation of Reinforced Concrete Frame Interior Beam-Column Joints with FRP Composites

An experimental research program is described regarding the use of externally applied carbon fiber-reinforced plastic (CFRP) jackets for seismic rehabilitation of reinforced concrete interior beam-column joints, which were designed for gravity loads. The joints had steel reinforcement details that are known to be inadequate by current seismic codes in terms of joint shear capacity due to the absence of transverse steel hoops and bond capacity of beam bottom steel reinforcing bars at the joint. Lap splicing of beam bottom steel reinforcement at the joint using externally applied longitudinal CFRP composite laminates is investigated. Improvement of joint shear capacity using diagonal CFRP composite laminates is another strengthening scheme employed. Concrete crack widths for the as-built specimens and the extent of CFRP delamination for the rehabilitated specimens at various drift ratios are reported. The test results indicate that CFRP jackets are an effective rehabilitation measure for improving the seismic performance of existing beam-column joints with inadequate seismic details in terms of increased joint shear strength and inelastic rotation capacity. In addition, CFRP laminates are effective rehabilitation measures for overcoming problems associated with beam bottom steel bars that have inadequate embedment into the beam-column joints.     

Residual Strength of Impact-Damaged CFRP Used to Strengthen Concrete Structures

Although previous research has demonstrated the improvement in performance of reinforced concrete structures enhanced with externally applied carbon fiber reinforced polymers (CFRP), the effect of transverse impact damage on the strength of the CFRP enhancements is unknown, and no guidelines have been provided that describe which impact events warrant CFRP repair or replacement. The impact events, such as dropped tools, collisions, and low-speed projectiles may cause critical damage to the epoxy matrix and fibers that is undetectable through visual inspection. The purpose of this research is to provide insight into the level of transverse impact needed to initiate critical damage in wet layup CFRP enhancements, which will serve as a guideline for inexpensive and immediate damage assessments. To simulate a variety of impact events, impactors (tups) of different sizes and shapes were dropped from several heights. The impacts were performed with a guided drop-weight apparatus, designed to achieve free-fall behavior. The results show that impacts that only cause indention of the FRP surface do not significantly affect the tensile strength, but impacts that cause crushing of the epoxy (seen as whitish areas) can indicate as much as a 63% residual tensile strength. Furthermore, for the test conditions considered, tests showed that impacts with a peak impact pressure greater than 21?MPa (3,000?psi) reduced the tensile strength of the CFRP.     

Shear Strength of Large Concrete Members with FRP Reinforcement

Increasing interest in the use of fiber-reinforced polymer (FRP) reinforcement for reinforced concrete structures has made it clear that insufficient information about the shear performance of such members is currently available to practicing engineers. This paper summarizes the results of 11 large shear tests of reinforced concrete beams with glass FRP (GFRP) longitudinal reinforcement and with or without GFRP stirrups. Test variables were the member depth, the member flexural reinforcement ratio, and the amount of shear reinforcement provided. Results showed that the equations of the Canadian CSA shear provisions provide conservative estimates of the shear strength of FRP-reinforced members. Recommendations are given along with a worked example on how to apply these provisions including to members with FRP stirrups. It was found that members with multiple layers of longitudinal bars appear to perform better than those with a single layer of longitudinal reinforcing bars. Overall, it was concluded that the fundamental shear behavior of FRP-reinforced beams is similar to that of steel-reinforced beams despite the brittle nature of the reinforcement.     

Experimental Evaluation of Axial Behavior of Strengthened Circular Reinforced-Concrete Columns

Insufficient or deteriorating reinforced-concrete piers in many existing bridges are required to be strengthened using economical, fast, and efficient methods. Currently, only a few methods can be used to strengthen circular columns. Steel jackets and fiber-reinforced polymer (FRP) composites are the two commonly used methods. In this study, along with these two strengthening methods, concrete jackets reinforced with spiral rebar, welded wire fabric (WWF), and a new steel reinforcement called PCS are investigated under different axial-load applications. Fifteen identical specimens were constructed, strengthened, and tested: one column with no strengthening; three columns strengthened with FRP; two with steel jacketing; and nine with concrete jacketing (two with WWF, three with spiral rebar, and four with the new reinforcement). The bare or unretrofitted specimens had a 152?mm (6?in.) diameter, while the outside diameter of concrete-jacketed specimens was 254?mm (10?in.). Effectiveness of each strengthening method in increasing the stiffness, axial capacity, and displacement ductility was investigated using the experimental data.     

Effect of Untreated Surface Disbonds on Performance of FRP-Retrofitted Concrete Beams

Retrofitting concrete structures with externally bonded fiber-reinforced polymer (FRP) laminates has become an increasingly common practice. Disbonds at the concrete-FRP interface, if left untreated, may adversely affect the performance of the retrofitted system. Disbonds in the form of surface voids and cracks are the subject of this paper. Voids may occur due to poor compaction or defective formwork, or in the form of air entrapment between resin and FRP laminate during application. Cracks, on the other hand, generally result from loading. This study is comprised of a detailed experimental program and a nonlinear finite-element analysis. In the experiments, surface cracks were simulated by cutting slits in concrete, herein termed as surface cuts. The study shows that leaving surface disbonds untreated, within the considered range of disbond variables, does not have a significant impact on the overall structural performance. A threshold of 12.7 mm for void diameter was deemed conservative for structural performance. On the other hand, 38 mm spacing was considered an appropriate threshold for epoxy injection in crack widths between 0.25 and 0.8 mm. Hence, conservative thresholds in current practice may be revised accordingly, unless considered necessary to avoid environmental effects.