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.     

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.     

Textile-Reinforced Mortar versus FRP Jacketing in Seismic Retrofitting of RC Columns with Continuous or Lap-Spliced Deformed Bars

The effectiveness of a new structural material, namely, textile-reinforced mortar (TRM), was investigated experimentally in this study as a means of confining oldtype reinforced concrete (RC) columns with limited capacity due to bar buckling or due to bond failure at lap splice regions. Comparisons with equal stiffness and strength fiber-reinforced polymer (FRP) jackets allow for the evaluation of the effectiveness of TRM versus FRP. Tests were carried out on nearly full scale nonseismically detailed RC columns subjected to cyclic uniaxial flexure under constant axial load. Ten cantilevertype specimens with either continuous or lap-spliced deformed longitudinal reinforcement at the floor level were constructed and tested. Experimental results indicated that TRM jacketing is quite effective as a means of increasing the cyclic deformation capacity of oldtype RC columns with poor detailing, by delaying bar buckling and by preventing splitting bond failures in columns with lap-spliced bars. Compared with their FRP counterparts, the TRM jackets used in this study were found to be equally effective in terms of increasing both the strength and deformation capacity of the retrofitted columns. From the response of specimens tested in this study, it can be concluded that TRM jacketing is an extremely promising solution for the confinement of reinforced concrete columns, including poorly detailed ones with or without lap splices in seismic regions.     

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.     

Hysteretic Behavior of Bridge Columns with FRP-Jacketed Lap Splices Designed for Moderate Ductility Enhancement

This paper presents test results of six specimens representing older bridge columns with inadequate reinforcement detailing consisting of short lap splices at the base and widely spaced transverse reinforcement. Four of these specimens were rehabilitated using fiber-reinforced polymer (FRP) jackets of two different composite materials (carbon and aramid) to avoid premature failure of the lapped bars after a limited number of postyield cycles. The test results indicate that thin FRP jackets can be used to avoid failure of short lap splices at moderate displacement ductilities. Displacement capacities consistent with expected demands in regions of moderate or low seismicity were achieved after jacket retrofitting. The hysteretic behavior of rehabilitated columns was assessed with emphasizing issues related to variation of stiffness and damping ratio as a function of ductility demand for this class of columns. Equations that account for the effect of axial load level on estimates of effective stiffness and damping as a function of displacement ductility are proposed for this class of columns.     

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.     

Experimental research on the seismic behavior of CSPSWs connected to frame beams

The seismic performance of composite steel plate shear walls (CSPSWs) that consist of a steel plate shear wall (SPSW) with reinforced concrete (RC) panels attached to one or both sides by means of bolts or connectors is experimentally studied. The shear wall is connected to the frame beams but not to the columns. This arrangement restrains the possible outof-plane buckling of the thin-walled steel plate, thus significantly increasing the bearing capacity and ductility of the overall wall, and prevents the premature overall or local buckling failure of the frame columns. From a practical viewpoint, these solutions can provide open space in a floor as this type of composite shear walls with a relatively small aspect ratio can be placed parallel along a bay. In this study, four CSPSWs and one SPSW were tested and the results showed that both CSPSWs and SPSW possessed good ductility. For SPSW alone, the buckling appeared and resulted in a decrease of bearing capacity and energy dissipation capacity. In addition, welding stiffeners at comers were shown to be an effective way to increase the energy dissipation capacity of CSPSWs.     

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.     

Posttensioned FRP Composite Shells for Concrete Confinement

Experiments have shown that externally bonded fiber-reinforced polymer (FRP) jackets for square and rectangular columns are not as effective as they are for circular columns. The results of experiments on shape-modified concrete columns using posttensioned FRP shells are presented. Posttensioning was achieved by radially straining the precured FRP shell outwards to a substantial strain level, using expansive cement concrete, over a period of 60?days. The prefabricated FRP shell was also used as a stay-in-place formwork. The effectiveness of shape modification using posttensioned FRP shells is compared to FRP-confined original square and rectangular columns, as well as shape-modified columns with nonshrink grout and externally bonded FRP jackets. It is shown that shape modification with posttensioning of FRP shells, using expansive cement concrete, can change the confinement from passive to active and improve significantly the axial strength and ultimate compressive axial strain capacity of square and rectangular 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.     

Numerical Modeling of Concrete Confined by Fiber-Reinforced Composites

Lateral confinement of reinforced concrete columns can significantly increase their lateral deflection capability and load-carrying capacity. While such retrofits were initially completed using steel jackets, fiber-reinforced polymer (FRP) composites have been used successfully and extensively for seismic and blast upgrades. Numerical modeling of such structures requires the use of a concrete material model that can accurately represent the volumetric behavior of concrete under triaxial stress states, to capture the interaction between concrete expansion and the resulting stress increase in the confining jacket. Test data by Suter and Pinzelli, Karbhari and Gao, and Mirmiran and Shahawy on concrete cylinders and prisms confined by aramid, carbon, and glass FRP sheets are analyzed numerically. The concrete material model used was developed for the study of the effect of blast loading on reinforced concrete structures and was verified and validated for a variety of triaxial stress paths. The numerical analyses closely reproduce the strength enhancements observed in the test specimens for various levels of confinement. The model also confirms the observed inefficiency of low levels of lateral confinement and the superior enhancement provided by circular cross sections as compared to rectangular ones.     

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.     

Effect of Sand Columns on the Undrained Load Response of Soft Clays

When sand columns are used as vertical drains in soil improvement schemes, the possible reinforcing role that these columns can play in regards to improving the bearing capacity is usually neglected in design. The objective of this paper is to evaluate the degree of improvement in the mechanical properties of soft clays in practical applications involving the use of sand drains or sand columns in clayey soils. For this purpose, 32 isotropically consolidated undrained triaxial tests were performed on normally consolidated kaolin specimens. The parameters that were varied were the diameter of the sand columns, the height of the columns, the type of columns (geotextile encased versus nonencased), and the effective confining pressure. Test results indicated that sand columns improved the undrained strength significantly even for area replacement ratios that were less than 18%. The increase in undrained strength was accompanied by a decrease in pore pressure generation during shear and an increase in Young’s modulus. The drained shear strength parameters were found to be relatively unaffected by the sand column reinforcement, except for fully penetrating columns with high area replacement ratios.     

Acoustic Emission Monitoring of Hybrid FRP-Concrete Columns

Fiber-reinforced plastic tubes can be used as permanent formwork and external reinforcement for concrete columns. Failure of such columns is often accompanied by audible noise from cracking of concrete and shifting and settling of aggregates, snapping of the inner layers of the jacket, and ultimately fiber fracture. Since the state of the concrete core is not readily visible from the outside, nondestructive evaluation tools such as acoustic emission (AE) can help assess its structural integrity. The objective of this research was to investigate the applicability of AE technique to hybrid columns, and to correlate the AE parameters to the state of stress in concrete. The AE signature of hybrid columns was studied on over 40 specimens with different lengths, cross sections, jacket types, and jacket thicknesses. The study showed that (1) rate of change of cumulative AE counts with respect to the applied load correlates well with the degree of damage sustained by the concrete core; (2) longer specimens, thicker jackets, and unbonded tubes display higher AE activities; and (3) the Kaiser effect is not present in hybrid columns.     

Analysis of RC Hollow Columns Strengthened with GFRP

Reinforced concrete (RC) hollow piers in bridges withstand high moment and shear demands ensured with reduced mass and lower stress on foundations compared with solid piers. Failure of hollow columns is typically affected by premature buckling of reinforcing bars and concrete cover spalling. At present, no guidelines are available for the design of their upgrade, and few research investigations can be found on hollow columns strengthened by using fiber-reinforced polymer (FRP) materials. This paper discusses an experimental program carried out on purely compressed RC hollow columns externally wrapped with glass-fiber-reinforced polymer (GFRP). Three specimens were tested: one specimen was unstrengthened and used as the benchmark; the other two specimens were GFRP-wrapped with different confining reinforcement ratios. Each specimen was designed according to dated codes (i.e., prior to 1970) accounting only for gravity loads. In particular, steel longitudinal bars cross section and steel tie-spacing were designed with the minimum amount of longitudinal reinforcement and minimum tie area at maximum spacing. Tests results highlight that the GFRP-jacket mainly provided ductility increases before low strength increments could be obtained. Refined and simplified numerical models for hollow square RC columns, previously proposed by the authors, herein extend to hollow rectangular members. Comparisons of experimental results and theoretical predictions on the basis of both refined and simplified confinement models were performed and showed good agreement. In the case of the simplified model, a value for the effective ultimate FRP strain was suggested.     

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.     

Debonding in RC Beams Shear Strengthened with Complete FRP Wraps

Substantial research has been conducted on the shear strengthening of reinforced concrete (RC) beams with bonded fiber reinforced polymer (FRP) strips. The beams may be strengthened in various ways: complete FRP wraps covering the whole cross section (i.e., complete wrapping), FRP U jackets covering the two sides and the tension face (i.e., U jacketing), and FRP strips bonded to the sides only (i.e., side bonding). Shear failure of such strengthened beams is generally in one of two modes: FRP rupture and debonding. The former mode governs in almost all beams with complete FRP wraps and some beams with U jackets, while the latter mode governs in all beams with side strips and U jackets. In RC beams strengthened with complete wraps, referred to as FRP wrapped beams, the shear failure process usually starts with the debonding of FRP from the sides of the beam near the critical shear crack, but ultimate failure is by rupture of the FRP. Most previous research has been concerned with the ultimate failure of FRP wrapped beams when FRP ruptures. However, debonding of FRP from the sides is at least a serviceability limit state and may also be taken as the ultimate limit state. This paper presents an experimental study on this debonding failure state in which a total of 18 beams were tested. The paper focuses on the distribution of strains in the FRP strips intersected by the critical shear crack, and the shear capacity at debonding. A simple model is proposed to predict the contribution of FRP to the shear capacity of the beam at the complete debonding of the critical FRP strip.     

Size Effect of Concrete Short Columns Confined with Aramid FRP Jackets

Most previous studies on concrete short columns confined with fiber-reinforced polymer (FRP) composites were based on small-scale testing, and size effect of the columns still has not been studied thoroughly. In this study, 99 confined concrete short columns wrapped with aramid FRP (AFRP) jackets and 36 unconfined concrete short columns with circular and square cross sections were tested under axial compressive loading. The circular specimens were divided into six groups, and the square specimens were divided into five groups, with each group containing different levels of the AFRP’s confinement. In each group, the specimens were geometrically similar to one another and had three different scaling dimensions. Statistical analyses were used to evaluate the size and interaction effects between the specimen size and the AFRP’s confinement, and a size-dependent model for predicting the strength of the columns was developed by modifying Baz?nt’s size-effect law. The experimental results showed that the size of a specimen had a significant effect on the strength of AFRP-confined concrete short columns, lesser effect on the axial stress-strain curves, and slight effect on the failure modes. The modified Baz?nt model was in good agreement with the experimental data.     

An Experimental Study on the Behavior of Square Concrete-Filled High Strength Steel Tube Columns

The paper presents an experimental study on the behavior of square concrete-filled high strength steel tube columns until failure.Four specimens were tested to investigate the effect of high strength steel and plain strength concrete on the load carrying capacity of the square concrete-filled steel tube columns.The effect of the concrete which filled in the steel tube was also investigated.Also,the effect of high strength steel and the presence of concrete were also investigated.The main parameters in the tests were:(1) the ultimate strength of steel (800 Mpa),(2) the presence of concrete in steel tube (hollow steel tube and concrete-filled steel tube),and (3) slender ratio.The main objectives of these tests were three-step:(1) to describe a series of tests on composite columns,(2) to analyze the influence of several parameters,and (3) to compare the accuracy of the predictions by using the specifications in the code (AISC-LRFD,EC4 and AIJ etc.) for the design of high-strength composite columns.Experimental results indicate that the ultimate strength of steel and presence of concrete had significant influence on both the axial compressive load capacity and the ductile of the square steel tube.     

Shear Strengthening of Interior Slab–Column Connections Using Carbon Fiber-Reinforced Polymer Sheets

This paper presents the results of an experimental investigation undertaken to evaluate the punching shear capacity of interior slab–column connections, strengthened using flexible carbon fiber-reinforced polymer (CFRP) sheets. Sixteen square (670×670?mm) slab–column connections with different slab thicknesses (55 and 75 mm) and reinforcement ratios (1 and 1.5%) were tested. Twelve specimens were strengthened using CFRP sheets and the remaining four specimens were kept as controls. Without strengthening, all specimens were designed to experience punching shear failure. The CFRP sheets were bonded to the tension face of the specimens in two perpendicular directions parallel to the internal ordinary steel reinforcement. The test results clearly demonstrate that using CFRP leads to significant improvements in the flexural stiffness, flexural strength, and shear capacity of beam–column connections. Depending on the content of the ordinary reinforcement, thickness of the slab, and area of CFRP sheet, the flexural strength increased between 26 and 73% and the shear capacity increased between 17 and 45%. The measured stress in the CFRP sheets at nominal strength varied between 22 and 69% of the ultimate tensile strength of the fibers. Comparison with available prediction equations showed that the punching shear capacity can be predicted with reasonable accuracy if the contribution of CFRP reinforcement to the increase in flexural strength is accounted for. On the other hand, the code design expressions were conservative in predicting the capacity observed in the tests.