Development of a Specification for Bridge Seismic Retrofit with Carbon Fiber Reinforced Polymer Composites

The U.S. Interstate 80 bridge over State Street in Salt Lake City is very near the Wasatch fault, which is active and capable of producing large earthquakes. The bridge was designed and built in 1965 according to the 1961 American Association of State Highway Officials specifications, which did not consider earthquake-induced forces or displacements. The bridge consists of reinforced concrete bents supporting steel plate welded girders. The bents are supported on cast-in-place concrete piles and pile caps. A seismic retrofit design was developed using carbon fiber reinforced polymer (CFRP) composites, which was implemented in the summer of 2000 and the summer of 2001, to improve the displacement ductility of the bridge. The seismic retrofit included column jacketing, as well as wrapping of the bent cap and bent cap-column joints for confinement, flexural, and shear strength increase. This paper describes the specifications developed for the CFRP composite column jackets and composite bent wrap. The specifications included provisions for materials, constructed thickness based on strength capacity, and an environmental durability reduction factor. Surface preparation, finish coat requirements, quality assurance provisions, which included sampling and testing, and constructability issues regarding the application of fiber composite materials in the retrofit of concrete bridges are also described.     

Seismic Retrofit of State Street Bridge on Interstate 80

The State Street Bridge, in Salt Lake City, was designed and built in 1965 according to the 1961 AASHO specifications; the design did not include earthquake-induced forces or displacements since only wind loads were considered. The bridge consists of four reinforced concrete (RC) bents supporting composite welded steel girders; the bents are supported on cast-in-place concrete piles and pile caps. A vulnerability analysis of the bridge was conducted that determined deficiencies in (1) confinement of column lap splice regions, (2) anchorage of longitudinal column bars in the bent cap, (3) confinement of column plastic hinge zones, and (4) shear capacity of columns and bent cap–column joints. Seismic retrofit designs using carbon-fiber-reinforced-polymer (CFRP) composites and steel jackets were performed and compared for three design spectra, including the 10% probability of exceedance in 250 years earthquake. The CFRP composite design was selected for implementation and application of the composite was carried out in the summer of 2000 and 2001, while the bridge was in service. The paper describes the CFRP composite design, which, in addition to column jackets, implemented an “ankle wrap” for improving joint shear strength and a “U-strap” for improving anchorage of column bars in the bent cap; other retrofit measures were implemented, such as bumper brackets and a deck slab retrofit. A capacity versus demand evaluation of the as-built and retrofitted bents is presented.     

Behavior of Cyclically Loaded Squat Reinforced Concrete Bridge Columns Upgraded with Advanced Composite-Material Jackets

This paper summarizes comprehensive experimental studies on scaled models of squat bridge columns repaired and retrofitted with advanced composite-material jackets. In the experimental program, a total of 14 half-scale squat circular and rectangular reinforced concrete columns were tested under fully reversed cyclic shear in a double bending configuration. In order to provide a basis for comparison, a total of three as-built columns were tested. Another 10 column samples were tested after being retrofitted with different composite jacket systems. One circular as-built column was repaired after failure. The repair process involved both crack injection as well as addition of carbon/epoxy composite jacket. The repaired column was then retested and evaluated. Experimental results showed that all as-built columns developed an unstable behavior and failed in brittle shear mode. The common failure mode for all retrofitted samples was due to flexure with significant improvement in the column ductility. The repaired column demonstrated ductility enhancement over the as-built sample.     

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.     

Retrofitting of Rectangular Columns with Deficient Lap Splices

The cyclic behavior of eight 0.4-scale reinforced concrete column specimens is investigated. The columns incorporated deficient design details to simulate bridge columns built in Washington State prior to 1971. Two columns were tested as reference specimens, five were tested after retrofitting using carbon fiber-reinforced polymer (CFRP), and one was tested after retrofitting using a conventional steel jacket. All the specimens were tested under constant gravity load and incrementally increasing lateral loading cycles. The specimens had rectangular cross sections with aspect ratios of 1.5 and 2.0. The parameters investigated included the amount of CFRP reinforcement, different retrofitting jacket configurations, and different retrofitting materials. For the as-built specimens, two modes of failure occurred, namely low cyclic fatigue of longitudinal reinforcement and lap splice failure. For the retrofitted specimens, no lap splice failure was observed. All the retrofitted specimens failed due to low cyclic fatigue failure of the longitudinal bars. The retrofitting measures improved the displacement ductility, energy dissipation, and equivalent viscous damping. In addition, increasing the amount of CFRP reinforcement improved the performance of the test specimens.     

Evaluation of As-Built, Retrofitted, and Repaired Shear-Critical Hollow Bridge Columns under Earthquake-Type Loading

The objective of this research is to investigate the seismic performance of as-built, retrofitted, and repaired hollow bridge columns with insufficient shear strength. Two as-built full-scale columns were first tested and repaired using carbon-fiber-reinforced polymer composites (CFRP) jackets and dog-bone-shaped bars and then retested. Another two columns having the same reinforcement as the as-built columns were retrofitted with CFRP jackets. In addition to the tests, the repairability of the failed hollow columns was investigated by analytical evaluation. The test results and analysis of the retrofitted columns showed that CFRP composites can effectively strengthen shear-critical hollow bridge columns and can successfully transform the failure mode from shear to flexure. The test results of the repaired circular columns show that dog-bone-shaped bars successfully repaired the flexural damage caused by the fractured longitudinal bars.     

Full-Scale Experimental Investigation of Repair of Reinforced Concrete Interstate Bridge Using CFRP Materials

An experimental study is presented of the behavior of eight reinforced concrete bridge girders taken from a decommissioned Interstate bridge and retrofitted with three different carbon-fiber-reinforced polymer (CFRP) systems. Specimens were subjected to monotonic loading to failure with and without significant fatigue conditioning. Experimental observations indicated that intermediate crack-induced debonding was the dominant failure mode for monotonically loaded beams and that degradation of the CFRP-to-concrete interface was caused by fatigue conditioning. Conventional adhesive applied and near-surface mounted (NSM) CFRP systems behaved well under monotonic loads, with the NSM system exhibiting significantly greater ductility. Powder actuated fastener applied retrofit was observed to be less efficient, requiring a relative slip of the CFRP in order to engage the shear transfer mechanism of the fasteners. The application of current accepted design guidelines for FRP retrofit indicated that guidelines aimed at mitigating debonding failure appear to be appropriately conservative under monotonic loading conditions; however, a significant additional reduction in CFRP strain limits is required to account for even small levels of fatigue loading.     

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.     

Shear Retrofit of Hollow Bridge Piers with Carbon Fiber-Reinforced Polymer Sheets

Hollow bridge piers are currently being used in high-speed rail and highway projects in Taiwan. The flexural ductility and shear capacity of such piers with the configuration of lateral reinforcement used in Taiwan has recently been studied.?This paper reports that circular and rectangular hollow bridge piers retrofitted by carbon fiber-reinforced polymer (CFRP) sheets were tested under a constant axial load and a cyclic reversed horizontal load to investigate their seismic behavior, including flexural ductility, dissipated energy, and shear capacity. An analytical model is also developed to predict the moment-curvature relationship of sections and the lateral load-displacement relationship of piers. Based on the test results, the seismic behavior of such piers is presented. The test results are also compared with the proposed analytical model. It was found that the ductility factors of the tested piers ranged from 3.3 to 5.5 and that the proposed analytical model could predict the lateral load-displacement relationship of such piers with reasonable accuracy. All in all, CFRP sheets can effectively improve both the ductility factor and the shear capacity of hollow bridge piers.     

Seismic Retrofit of RC Columns with Continuous Carbon Fiber Jackets

The development, the validation, and the implementation of a new seismic retrofit system for reinforced concrete columns are described. The column jacketing system consists of continuous carbon fiber prepreg tows wound in an automated fashion onto existing circular or rectangular concrete columns, with variable jacket thickness along the column height based on experimentally validated design models. Jacket design criteria for various seismic column failure modes are described and detailed examples show their application to retrofits of columns with circular and rectangular column geometry, different reinforcement ratios, and detailing. The carbon jacket designs are validated through large-scale bridge column model tests and are found to be just as effective as steel shell jacketing in providing desired inelastic design deformation capacity levels. Furthermore, it is shown that the retrofit criteria and guidelines are also applicable to other advanced composite jacketing systems with appropriate considerations for differences in mechanical properties of the materials system, installation and curing technology, as well as jacket discontinuities.     

Shear Retrofit of Flared RC Bridge Columns Subjected to Earthquakes

This paper presents an evaluation of the seismic performance and retrofit of reinforced-concrete bridge columns with structural flares. Experimental and analytical studies were performed on four 40%-scale specimens. Two specimens represented the as-built columns, while the other two were retrofitted with steel jackets for shear capacity enhancement. The results indicate that some of the existing methods for evaluating the shear capacity of columns can be unconservative and could overestimate the shear capacity of the columns included in this study by 60%. It is also shown that by implementing proper detailing, steel jackets can be used to enhance the shear capacity and ductility of flared columns with no appreciable increase in the shear demand.     

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.     

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.     

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 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.     

CFRP Composite Connector for Concrete Members

Steel plate connections are frequently used in tilt-up and precast concrete building construction to tie adjacent wall panels together for shear and overturning effects, and to provide continuous diaphragm chord connections for wind and seismic loading. These welded connectors perform poorly in regions of high seismicity and are vulnerable to corrosion. Until now, retrofit and repair strategies for in-plane shear transfer strengthening were limited to attaching steel sections across panel edges. In the present paper, an experimental program is described that utilizes carbon fiber reinforced plastic (CFRP) composites to develop a viable retrofit scheme for precast concrete shear walls and diaphragms. Nine full-scale precast wall panel assemblies with CFRP composite connectors have been tested. The results show that the CFRP composite connection is an effective solution for the seismic retrofit and repair of precast concrete wall assemblies and other precast concrete elements, such as horizontal diaphragms, that require in-plane shear transfer strengthening.     

Evaluation of the Effectiveness of Strengthening Intervention by CFRP on MRWA Bridge No. 3014

Main Roads of Western Australia has a continuing program of bridge upgrading, to refurbish and strengthen bridges to allow for increasing vehicle traffic and increasing axle loads. A 40-year-old, four-span reinforced concrete slab bridge was retrofitted with application of CFRP laminate strips on the top of the deck over the piers, as well as on the deck soffit in the midspan regions, to reduce high moments in both hogging and sagging. The dynamic assessment of the bridge before and after strengthening works provided the opportunity to evaluate the effectiveness of the strengthening intervention through dynamic measurements. A performance evaluation of the repaired structure was carried out through traffic loading application on the updated numerical models of the bridge, before and after retrofit. As a main observation, the addition of CFRP laminate strips led to a significant increase of the structural capacity in flexure. The paper discusses the results obtained from the dynamic-based assessment in terms of effectiveness of the strengthening intervention as well as of efficiency in using such a methodology to evaluate the capacity increase of the retrofitted bridge.     

Analytical Models and Guidelines for the Ductility Enhancement of Circular Reinforced Concrete Single-Column Bents Using Fiber-Reinforced Polymers

The target displacement ductility requirements for circular RC single-column bridge bents are considered using a proposed multifailure mode algorithm to determine the required thickness of fiber-reinforced polymer wraps (FRPs). The procedure is developed using two in-house computer algorithms, PACCC (plastic analysis of circular concrete columns) and PACCC-FRP, to generate a moment-curvature analysis using circular segment slices and subsequent failure mode predictions in single-column bents for both FRP-wrapped and unwrapped circular RC sections. The results of the study showed good comparison to published experimental tests at the ultimate force-deflection states of RC sections and against three commercial “software test beds.” The study uses PACCC-FRP to show that single columns experiencing a brittle failure may be retrofitted with FRP wraps in order to increase the displacement ductility and satisfy target ductility values within the ductility wrap envelope, or wrap-saturation level, as established herein.     

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.