外文翻译(英文)试验研究钢筋混凝土柱改造方法.doc

EXPERIMENTAL RESEARCH OF REINFORCED CONCRETE COLUMNRETROFIT METHODSIntroductionAs the infrastructure of our country continues to age, the need for effective retrofittreatments has increased. Many building and bridge structural components no longerprovide capacity sufficient to meet the required code standards. Seismic upgrading andreinforcement protection are two of the major issues requiring retrofits. Additionally,many aging structural members no longer provide the load capacity of the original designbecause of concrete cracking, steel corrosion, or other damage. In this research, severalretrofit methods for increasing the axial load capacity of reinforced concrete columnswere tested and analyzed.Several currently applied methods for retrofitting columns include concrete jacketing,steel jacketing, and fiber reinforced polymer (FRP) jacketing. All three methods havebeen shown to effectively in increase the axial load capacity of columns. In addition, anew reinforcement product, Prefabricated Cage System (PCS) reinforcement, wasintroduced as a possible reinforcement option for concrete jacket retrofit applications.Project ScopeThree retrofit reinforcement options, concrete jacketing, steel jacketing, and FRPwrapping, were experimentally analyzed in the research. Three different materialsprovided reinforcement for the concrete jackets: a rebar cage with spiral transversereinforcement, welded wire fabric (WWF), and the aforementioned PCS. All specimenswere tested under axial compression loading only. Applied load and displacement weremeasured for all specimens throughout testing until failure. Additionally, response of theloaded specimens was observed and documented throughout the testing. Behavior of thenewly introduced PCS reinforcement was compared with traditionally used retrofitapplications already currently utilized in the structural engineering industry. Advantagesand disadvantages of each retrofit measure were identified. Additionally, model conceptswere presented for the response of concrete jacket retrofitted columns based on the workof Mander et al. (1988) and Cai (1987).Project SummarySeventeen circular columns were constructed and tested to failure in compression. First,similar reinforced concrete base columns were constructed with spiral transversereinforcement. All base specimens had the same dimensions, transverse, and longitudinaleinforcement. The base specimens were then retrofitted with the different retrofitmethods previously discussed. One specimen, referred to as the base specimen, wastested without any retrofit applied. Three specimens were retrofitted with a spiral rebarreinforced concrete jacket. Two specimens were reinforced with a welded wire fabricreinforced concrete jacket. Three specimens were retrofitted with FRP wraps and twomore specimens were retrofitted with steel jackets. In addition, six specimens wereretrofitted with a PCS reinforced concrete jacket, including two different thicknesses ofPCS.Nine specimens had the entire cross section of the base column and retrofit area loaded.This simulated a retrofit condition when the applied load is distributed across the entirecross section. Five specimens had the cross section of only the base column loaded,which simulated a condition when the load is only applied to the original member. Threeother specimens had increased lateral reinforcement spacing with similar transversereinforcement strength per spacing. These specimens also had the entire retrofit and basecolumn cross-section loaded.RETROFIT OF EXISTING STRUCTURESMany structures have historically been constructed using reinforced concrete. Typicalordinary concrete consists of four constituents: gravel, sand, water, and cement.Reinforced concrete has some type of reinforcement, typically steel, combined withconcrete to produce a stronger system than plain concrete. Concrete is strong incompression but weak in tension. Tensile forces cause concrete to crack and eventuallyfail in a brittle manner at stresses significantly lower than the compression strength ofconcrete. Steel, or another type of reinforcement material, can be used to compensate forthe weak tensile strength of concrete. This system is referred to as reinforced concrete.As reinforced concrete ages, a variety of detrimental effects can occur. These includespalling, flaking, or cracking of the concrete, and subsequent corrosion of the reinforcingsteel. These occurrences can significantly affect the strength of structural members.Members displaying these adverse affects may be rehabilitated using an appropriateretrofit method.Retrofitting is typically done for two reasons: rehabilitation or strengthening. Aspreviously discussed, rehabilitation is fixing the structural deficiencies of a damagedstructure or structural member. This may be necessary for aging members that no longerdisplay the strength of the original design. Strengthening increases the load-carryingcapacity of a structural member (Ersoy et al. 1993). This may be necessary if thesupported load is altered through the life of the structural member, or if current designstandards have more stringent reinforcement requirements. Additionally, structuralmembers in seismic regions may need to be upgraded to current seismic requirements.Retrofitting can be applied to any structural members, including beams and columns.Several methods are traditionally utilized for retrofitting. These include concretejacketing, steel jacketing, and FRP strengthening (Wipf et al. 1997). Reinforcement forconcrete jackets can be provided by rebar reinforcement or welded wire fabric (WWF).Additionally, a relatively new product, Prefabricated Cage System reinforcement (PCS),is suggested as a possible reinforcement alternative for concrete jacket retrofits.PCS BenefitsPCS reinforcement has some unique properties. The longitudinal and lateralreinforcement for PCS are located the same distance from the center of the membercross-section. This provides increased flexural capacity, using the same amount of steelas a traditional rebar system, and results in more efficient use of the reinforcing steel.The monolithic action of PCS eliminates separation of longitudinal and transverseeinforcement. Additionally, PCS reinforcement is spread in a planar configurationwhich offers greater confinement than rebar reinforcement, as displayed in Figure 2.4.Thickness of the PCS steel determines the dimensions of the reinforcement confining the concrete.PCS reinforcement offers several additional benefits. Dimensions of the reinforcementare determined by the designer to produce any desired amount of transverse andlongitudinal reinforcement. This allows a great deal of flexibility and efficiency in thedesign process, as reinforcement choices are not limited to available stock materials.Additionally, PCS reinforcement can be fabricated off-site and immediately placed for concrete casting without additional fieldwork, such as tying, cutting, or bending ofreinforcement, which prolong construction time. Off-site fabrication also provides anincreased level of quality control for the reinforcement. In PCS production, dimensionsand spacing are far more accurate which minimizes opportunities for human error andeliminates acceptance of sub-par detailing or inadequate construction. This, in turn,results in an increased factor of safety for construction projectsRetrofit MethodsAs previously mentioned, common retrofit techniques include concrete, FRP, and steeljackets. Concrete jackets are constructed by enlarging the existing cross section with anew layer of concrete and reinforcement (Ersoy et al. 1993). This reinforcement istraditionally provided by hoop or spiral rebar, or welded wire fabric. FRP reinforcementis typically applied two ways: prefabricated jackets or wraps. Both methods have been experimentally researched (Morshed and Kazemi 2005). Steel jackets are constructed byplacing a steel tube with a slightly larger diameter around the member to be retrofitted.The area between the existing member and steel tube is typically filled with grout(Priestley et al. 1996).Concrete JacketingAddition of a concrete jacket is used to enhance flexural strength, ductility, and shearstrength of columns. This technique is more commonly used for building columns buthas been applied to some bridge members in Japan. The enhanced confinement isachieved with the use of ties or spirals at a small pitch, or transverse reinforcementspacing (Priestley et al. 1996). Concrete jackets can be used to retrofit beams as well ascolumns (Cheong and MacAlevey 2000). Additional materials can be used to reinforce the retrofit, as long as confinement is enhanced.Rebar ReinforcementConcrete jacketed columns with hoop and spiral reinforcement effectively enhance thestructural capacity of retrofitted members. Ersoy et al. (1993) ran two series of tests tostudy the behavior of strengthened and repaired concrete jacketed columns. The firstseries compares the behavior of jacketed columns with a monolithic reference specimenunder monotonic axial loading. All the concrete for the monolithic specimen was castwith the base column and retrofit reinforcement in place, to provide a specimen withperfect interaction and bond between the base column and retrofit material. Hoopreinforcement is used in the base column and retrofit reinforcement, as shown in Figure 2.5. The jackets are applied under two conditions: after the compression loading wasapplied and removed, as well as while the axial load is still applied. It is determined thatcolumns jacketed after unloading performed well, reaching 80 to 90 percent of thestrength of the monolithic reference specimen. Repair jackets applied while the columnis still under load did not perform as well and only reached 50 percent of the axial loadcarried by the monolithic specimen. The second series of tests study the effectiveness ofconcrete jackets with columns tested under combined axial load and bending. Both repairand strengthened jackets behave adequately under monotonic and reversed cyclic loading.Steel JacketingSteel jackets prevent concrete from expanding laterally as a result of high axialcompression strains. The steel jacket is equivalent to continuous hoop reinforcement andcan be used for circular columns or rectangular columns with slight modifications, asshown in Figure 2.10. Steel jacketing of rectangular columns is not recommendedbecause while shear strength is enhanced, flexural ductility is only provided at thecorners. An elliptical steel jacket with concrete infill should be provided for rectangularmembers to fully confine all the concrete (Priestley et al. 1996).A comprehensive two-part study was performed by Priestley et al. (1994 a, b) todetermine the enhanced shear strength provided by steel jacket retrofitting. The first partof the research focuses on theoretical considerations and test design. It is determined thatACI design equations are overly conservative and new design equations are presented forcircular or rectangular columns in need of shear enhancement. The second part of theresearch focuses on the actual testing of the columns designed according to the proposedequations. It is concluded that steel jackets significantly increase the shear strength and flexural ductility of shear deficient columns, which is shown in Table 2.1. Specimenswith an A are as-built columns and an R represents retrofitted columnsFRP JacketingFiber reinforced polymer (FRP) confinement can be provided using several compositematerials including fiberglass, carbon fiber, and Kevlar bonded to the confined concretesurface using epoxy (Priestley et al. 1996). Weight and cross-section of the retrofittedmember are not significantly affected with FRP jackets. FRP jackets are most applicablefor circular columns, as stress concentrations can develop in the FRP wrap around thecorners of square or rectangular cross-sections. FRP jackets can be used for rectangularor other shaped members, if shape modification is performed to prevent stress concentrations from developing. Two types of FRP retrofits, wraps and prefabricatedcomposite jackets, are typically utilized.FRP WrapsFRP wraps have several benefits including high strength, light weight, resistance tocorrosion, low cost, and versatility (Saadatmanesh et al. 1997). FRP wrapping isperformed by first cleaning the surface of the member to be retrofitted. Epoxy or resin isthen used to attach the flexible FRP fabric to the surface of the member to be retrofitted.The fabric can be applied dry, but the preferred method is to soak the fabric in epoxybefore application to allow for better cohesion with the surface of the member to beretrofitted. The fabric is then smoothed out to ensure no air pockets exist and extra epoxyis squeezed out the sides. A ne