State of Research on Seismic Retrofit of RC Beam-Column Joints with Externally Bonded FRP

A considerable amount of research has been directed recently toward understanding and promoting the use of externally applied fiber-reinforced polymer (FRP) for the seismic retrofit of reinforced concrete (RC) structures. In this paper, a comprehensive review and synthesis of published experimental studies on the seismic rehabilitation of RC frame beam-column joints with FRP is presented, and the issues that need to be addressed for further research are discussed. In addition, the paper presents a simple design model for predicting the contribution of the FRP to the shear strength of retrofitted joints. The key element in the model is the derivation of an expression for the effective FRP strain, based on the calibration of test data reported in the literature. A total of 54 tests carried out worldwide were considered in the review, and a database of the published studies, encompassing all relevant design parameters, was assembled. The reported test results confirm the structural effectiveness of the FRP strengthening technique for the seismic retrofit of RC joints. However, there are some gaps which need to be addressed. For instance, there is a lack of a rationale explanation of the resistance mechanisms involved in the beam-column joints retrofitted with FRP. Such a rational explanation is a prerequisite for the development of more comprehensive and rigorous design procedure.     

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.     

Performance-Based Seismic Retrofit Strategy for Existing Reinforced Concrete Frame Systems Using Fiber-Reinforced Polymer Composites

The feasibility and efficiency of a seismic retrofit intervention using externally bonded fiber-reinforced polymer composites on existing reinforced concrete frame systems, designed prior to the introduction of modern standard seismic design code provisions in the mid-1970s, are herein presented, based on analytical and experimental investigations on beam-column joint subassemblies and frame systems. A multilevel retrofit strategy, following hierarchy of strength considerations, is adopted to achieve the desired performance. The expected sequence of events is visualized through capacity-demand curves within M-N performance domains. An analytical procedure able to predict the enhanced nonlinear behavior of the panel zone region, due to the application of CFRP laminates, in terms of shear strength (principal stresses) versus shear deformation, has been developed and is herein proposed as a fundamental step for the definition of a proper retrofit solution. The experimental results from quasi-static tests on beam-column subassemblies, either interior and exterior, and on three-storey three-bay frame systems in their as-built and CFRP retrofitted configurations, provided very satisfactory confirmation of the viability and reliability of the adopted retrofit solution as well as of the proposed analytical procedure to predict the actual sequence of events.     

Residual Performance of FRP-Retrofitted RC Columns after Being Subjected to Cyclic Loading Damage

Numerous recent research findings evidenced the success of retrofitting existing RC columns using fiber-reinforced plastic (FRP) jacketing. However, little is known about the residual performance of FRP-retrofitted RC columns following limited seismic damage. In this paper, the residual performance of FRP-retrofitted columns damaged after simulated seismic loading is studied. Eight model columns with a shear aspect ratio of 5.0 were tested first under cyclic lateral force and a constant axial load equal to 20% of the column gross axial load capacity. The main parameters considered were the type of FRP jacket and peak drift ratio where the lateral loading was interrupted. Glass fiber-reinforced plastic (GFRP) and carbon fiber-reinforced plastic (CFRP) were both used for retrofitting. Five of the model columns were subjected to long-term axial loading after being subjected to limited damage by lateral cyclic loading. From the results of long-term loading test, it was found that FRP-retrofitted columns had much smaller creep deformation than the counterpart as-built model. The deformation of retrofitted columns under long-term axial loading depended on the previous damage intensity and the modulus of elasticity of FRP. The effective creep Poisson’s ratios of the retrofitted columns were much smaller than the as-built column but identical for GFRP and CFRP retrofitted columns. Under the testing conditions of this study, the long-term axial deformation of retrofitted columns tends to be sufficiently stable, despite the simulated earthquake damage.     

Seismic Behavior and Retrofit of Outrigger Knee Joints

Experimental tests were conducted on six 1/3-scale specimens to define the vulnerabilities of existing outrigger bents under in-plane and out-of-plane seismic loading and to develop retrofit measures that address the identified vulnerabilities. The specimens represented knee joints in the SR 99 Spokane Street overcrossing in western Washington State but included deficiencies present in a number of older bridges. The as-built specimens failed at low ductility levels due to shear distress, low torsional strength of the outrigger beam, and reinforcement bond failures within the joint. Threshold principal tension stress values describing the expected condition of the joints were established and compared to values obtained by other researchers. Circular steel jackets were used to retrofit the as-built specimens. The retrofitted specimens developed plastic hinging in the column, with enhanced strength, energy, and ductility capacities. Design and detailing guidelines for retrofitting outrigger bents were proposed.     

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.     

Comparison of CFRP and Alternative Seismic Retrofitting Techniques for Bare and Infilled RC Frames

The opportunities provided by the use of modern repair schemes for the seismic retrofit of existing RC structures were assessed on a comparative experimental study of carbon fiber-reinforced polymer (CFRP) and more-conventional seismic retrofitting techniques for the repair of reinforced concrete members and masonry walls of bare and infilled RC frames, respectively, damaged because of cyclic loading. Four 1-story, one-bay, one-third-scale frame specimens are tested under cyclic horizontal loading up to a drift level of 4%—two bare frames with spirals or stirrups as shear reinforcement, respectively, and two infilled frames with weak infills and spirals or stirrups as shear reinforcement, respectively. The applied repair techniques are mainly based on the use of thin epoxy resin infused under pressure into the crack system of the damaged RC joint bodies or on the additional use of CFRP plates to the surfaces of the damaged structural RC members as external reinforcement and the use of a polymer modified cement mortar or two-sided diagonal CFRP fabrics for the damaged infill masonry walls. After repair, specimens were retested in the same way. Conclusions concerning the comparison of the effectiveness between conventional and CFRP seismic retrofitting applied techniques on the basis of maximum cycles load, loading stiffness, and hysteretic energy absorption capabilities of the tested specimens are drawn.     

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 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     

Tests on Seismically Damaged Reinforced Concrete Structural Walls Repaired Using Fiber-Reinforced Polymers

This paper presents the results of an experimental study on the seismic performance of axially loaded reinforced concrete (RC) walls with boundary elements confined by limited transverse reinforcement. These specimens were initially subjected to axial compression loading and cyclic lateral loading to failure, and subsequently repaired and subjected to loading again. The test specimens include two low-rise walls of aspect ratio 1.125 and two medium-rise walls of aspect ratio 1.625. Results show that significant drift capacities were achieved from the strengthened walls. The performance of the repaired walls was similar to the original walls before repair in terms of the flexural behavior, shear strength, and ductility capacities. While the fiber-reinforced polymer (FRP) anchorage may undergo premature failure, it however failed only after the peak lateral strength of the repaired wall was attained. This paper demonstrates that repair of damaged RC walls using FRP is able to restore the performance of damaged RC walls while also serving as repair method of relative ease.     

Strengthening of Reinforced Concrete Bridge Decks Using Carbon Fiber-Reinforced Polymer Composite Materials

This paper presents the results of an investigation of the monotonic and fatigue behavior of one-way and two-way reinforced concrete slabs strengthened with carbon fiber-reinforced polymer (CFRP) materials. The five one-way slab specimens were removed from a decommissioned bridge in South Carolina. Three of the slabs were retrofitted with CFRP strips bonded to their soffits and the other two served as unretrofit, control specimens. Of the five one-way slab specimens, one unretrofit and two retrofit slabs were tested monotonically until failure. The remaining two specimens, one unretrofit and one retrofit, were tested under cyclic (fatigue) loading until failure. In addition, six half-scale, two-way slab specimens were constructed to represent a full-scale prototype of a highway bridge deck designed using the empirical requirements of the AASHTO LRFD Bridge Design Manual. Of the six square slabs, two were unretrofitted and served as the control specimens, two were retrofitted using CFRP strips bonded to their soffits making a grid pattern, and two were retrofitted with a preformed CFRP grid material bonded to their soffit. Three slabs, one unretrofit, one CFRP strip, and one CFRP grid retrofitted, were tested monotonically until failure and the remaining three slabs were tested under cyclic (fatigue) loading until failure.     

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.     

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.     

Comparison of Three Flexural Retrofit Systems under Monotonic and Fatigue Loads

The majority of experimental work involving the flexural retrofit of concrete bridge girders has been conducted on beam specimens with adhesive-applied, soffit-mounted, fiber-reinforced polymer (FRP) composite systems, referred to in this study as conventional adhesive application (CAA). It has been observed that the performance of such girders is often controlled by the quality of the bond between the FRP and the concrete substrate and the substrate’s ability to transfer stress from the steel to the FRP. With the goal of improving the performance of bonded FRP in mind, two additional soffit-mounted retrofit schemes are investigated: near-surface mounted (NSM), where the FRP strips are embedded in adhesive within slots cut into the substrate concrete, and, powder-actuated fastener-applied (PAF) FRP, which uses a powder-actuated nail gun to install mechanical fasteners through predrilled holes in the FRP into the concrete substrate, “nailing” the FRP in place. The PAF application is a recent development, and little work has been done on it other than by the proprietors of the system. This study reports on a comparative study of the static and fatigue performance of reinforced concrete beams retrofitted with CAA, NSM, and PAF FRP retrofit systems. Ten medium-scale beams were tested: six strengthened specimens, two per retrofit method, were tested under cyclic loading conditions, and four specimens, one per retrofit method and one control specimen, were tested monotonically to failure. The results of this study indicate that although all three methods of FRP application result in significant strength increases over the control specimen under monotonic loading conditions, the CAA method is outperformed by the other methods under cyclic conditions. A number of other relevant detailed conclusions with respect to performance and practical application issues are presented for each of the methods of retrofit examined in this study. Significantly, clear evidence of FRP debonding in the midspan region prior to specimen failure is presented.     

外加强环式H型钢梁-方钢管角柱节点抗震性能

通过对5个试件进行拟静力加载试验，研究了加载方式对角柱和边柱节点抗震性能的影响.试验通过对加载方式（单向加载、双向轴对称加载和双向中心对称加载）和钢管柱宽厚比（D/t=22和33）主要参数的变化分析，着重研究了试件的滞回性能、刚度退化和耗能性能等特性.试验结果表明:加载方式对试件刚度及承载力影响十分明显.在双向中心对称荷载作用下，试件的承载力比在单向荷载作用下试件的承载力降低约20%；而在双向轴对称荷载作用下，试件的承载力与在单向荷载作用下试件的承载力基本相同.方钢管柱宽厚比是影响试件承载力的主要因素之一，随着宽厚比的增加，试件承载力逐步减小.所有试件的滞回曲线均呈饱满的纺锤体状，等效黏滞阻尼系数在0.2左右，具有稳定的耗能能力.      

Seismic Performance Assessment of Damage-Controlled FRP-Retrofitted RC Bridge Columns Using Residual Deformations

Despite the improved performance of fiber-reinforced plastic (FRP)-retrofitted bridges, residual deformations in the event of an earthquake are inevitable. Little consideration is currently given to these deformations when assessing seismic performance. Moreover, important structures are currently required not only to have high strength and high ductility but also to be usable and repairable after high intensity earthquakes. This paper presents a definition of an FRP-RC damage-controllable structure. An intensive study of 109 bridge columns, extracted from recent research literature on the inelastic performance of FRP retrofitted columns with lap-splice deficiencies, flexural deficiencies, or shear deficiencies, is used to evaluate the recoverability of such retrofitted columns. The residual deformation, as a seismic performance measure, is used to evaluate the performance of 39 FRP-retrofitted RC columns from the available database. Based on this evaluation, a requirement for the recoverable and irrecoverable states of FRP-RC bridges is specified. Finally, the Seismic Design Specifications of Highway Bridges for RC piers is adapted to predict the residual deformations of FRP-RC columns.     

Behavior of FRP-Retrofitted Joints Built with Plain Bars and Low-Strength Concrete

Two series of tests on eight full-scale exterior beam-column joint subassemblages built with plain bars and low-strength concrete were conducted. No transverse reinforcement was present in the joint cores. In the first series of tests, which included three specimens, the behavior of joints before fiber-reinforced polymer (FRP) retrofitting was investigated. In the second series, which included five specimens, the behavior of the FRP-retrofitted joints was investigated. The six specimens consisted of a column, an in-plane beam, a transverse beam, and a slab part, and two specimens were plane members without transverse beams and slabs. The utilized retrofitting scheme is easily applicable for actual exterior beam-column joints, even in the presence of a transverse beam and a slab. Two types of strength limitation were observed for specimens in the first series. The strength of the specimen with beam longitudinal bars sufficiently anchored to the joint core was limited by the shear strength of the joint. The strengths of the other two specimens were limited by the slip of the beams’ longitudinal bars at their anchorages. In the second series of tests, significantly better performance was obtained both in terms of shear strength and ductility, provided that the slip of the beam bars was prevented. Furthermore, by using a simple theoretical algorithm based on truss analogy, the strength and deformability characteristics of the tested reference and FRP-retrofitted joints are predicted with reasonable accuracy. The same algorithm is used for predicting the joint shear strength of specimens tested by other researchers, and satisfactory agreement is obtained between the predictions and test results.     

Active Confinement of Reinforced Concrete Bridge Columns Using Shape Memory Alloys

This paper presents experimental and analytical work conducted to explore the feasibility of using an innovative technique for seismic retrofitting of RC bridge columns using shape memory alloys (SMAs) spirals. The high recovery stress associated with the shape recovery of SMAs is being sought in this study as an easy and reliable method to apply external active confining pressure on RC bridge columns to improve their ductility. Uniaxial compression tests of concrete cylinders confined with SMA spirals show a significant improvement in the concrete strength and ductility even under small confining pressure. The experimental results are used to calibrate the concrete constitutive model used in the analytical study. Analytical models of bridge columns retrofitted with SMA spirals and carbon fiber-reinforced polymer (CFRP) sheets are studied under displacement-controlled cyclic loading and a suite of strong earthquake records. The analytical results proves the superiority of the proposed technique using SMA spirals to CFRP sheets in terms of enhancing the strength and effective stiffness and reducing the concrete damage and residual drifts of retrofitted columns.     

Seismic Rehabilitation of Deficient Exterior Concrete Frame Joints

The performance of rehabilitated reinforced concrete beam-column joints was compared with the response of existing joints designed to preseismic codes to assess proposed rehabilitation techniques. Six beam-column subassemblies with nonductile reinforcement detailing were tested. The joints were subjected to quasi-static load that simulates seismic forces. The first three specimens had inadequate anchorage length of the bottom beam bars. Two of them were strengthened by using carbon-fiber-reinforced polymer sheets attached to the bottom beam face and then tested. The other three specimens had no steel ties installed in the joint zone, in addition to inadequate anchorage length of the beam bars. Two of the beam-column joints were strengthened by glass-fiber-reinforced polymer jackets of the joint zone and steel rods or plates. The rehabilitation techniques were found effective in eliminating the brittle joint shear and steel bar bond-slip failure modes, and ductile beam hinging instead occurred.