改造现有的结构.doc

外文翻译：
RETROFIT OF EXISTING STRUCTURES
2.1 Introduction
Many structures have historically been constructed using reinforced concrete. Typical ordinary concrete consists of four constituents: gravel, sand, water, and cement. Reinforced concrete has some type of reinforcement, typically steel combined with concrete to produce a stronger system than plain concrete. Concrete is strong in compression but weak in tension. Tensile forces cause concrete to crack and eventually fail in a brittle manner at stresses significantly lower than the compression strength of concrete. Steel, or another type of reinforcement material, can be used to compensate for the 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 include spalling, flaking, or cracking of the concrete, and subsequent corrosion of the reinforcing steel. These occurrences can significantly affect the strength of structural members. Members displaying these adverse affects may be rehabilitated using an appropriate retrofit method.
Retrofitting is typically done for two reasons: rehabilitation or strengthening. As previously discussed, rehabilitation is fixing the structural deficiencies of a damaged structure or structural member. This may be necessary for aging members that no longer display the strength of the original design. Strengthening increases the load-carrying capacity of a structural member (Ersoy et al. 1993). This may be necessary if the supported load is altered through the life of the structural member, or if current design standards have more stringent reinforcement requirements. Additionally, structural members 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 concrete jacketing, steel jacketing, and FRP strengthening (Wipf et al. 1997).
Reinforcement for concrete 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.
2.2 Prefabricated Cage System (PCS) Reinforcement
PCS reinforcement, as shown in Figure 2.1, is essentially a continuous steel section with holes applied as a secondary process. The PCS reinforcement provides longitudinal and transverse reinforcement simultaneously and can be used in circular or rectangular sections. This monolithic connection reduces the need for separate longitudinal and transverse reinforcement. PCS can be used as reinforcement in columns, shear walls, foundations or beams. PCS reinforcement can be concentrically layered if large reinforcement ratios are required (Figure 2.2). For a column, vertical steel strips provide longitudinal reinforcement while the horizontal steel strips provide transverse reinforcement. Tests have shown that PCS provides higher confinement capacities than traditional rebar reinforcement systems (Shamsai and Sezen 2005).
2.2.1 PCS Production
PCS reinforcement is produced from steel tubes or plates. Standard steel tubes or plates are punched or cut to provide holes with the desired dimensions. In lieu of tubes, steel plates can be bent into the required shape and then welded. Casting of PCS reinforcement has also been suggested as a method of production.
Cutting of holes in the PCS can be performed with a variety of methods including laser (as shown in Figure 2.3), plasma, flame, and abrasive water jet cutting. Additional suggested methods include milling, electrochemical machining, electrical discharge machining, and chemical etching. Precision, time, cost, availability, and thickness limitations vary among the different cutting methods (Shamsai 2006).
Punching holes to form the PCS reinforcement is suggested for mass production. Computer numerical control (CNC) machines can program the punching spaces and dimensions for PCS reinforcement. Three suggested CNC punching machines include a single station press, a mechanical turret punch press, and a
hydraulic turret punch press. Punching also has disadvantages including steel thickness restrictions and possible steel deformations (Shamsai 2006).
2.2.2 PCS Benefits
PCS reinforcement has some unique properties. The longitudinal and lateral reinforcement for PCS are located the same distance from the center of the member cross-section. This provides increased flexural capacity, using the same amount of steel as a traditional rebar system, and results in more efficient use of the reinforcing steel. The monolithic action of PCS eliminates separation of longitudinal and transverse reinforcement. Additionally, PCS reinforcement is spread in a planar configuration which 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 reinforcement are determined by the designer to produce any desired amount of transverse and longitudinal reinforcement. This allows a great deal of flexibility and efficiency in the design 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 of reinforcement, which prolong construction time. Off-site fabrication also provides an increased level of quality control for the reinforcement. In PCS production, dimensions and spacing are far more accurate which minimizes opportunities for human error and eliminates acceptance of sub-par detailing or inadequate construction. This, in turn, results in an increased factor of safety for construction projects.
2.3 Retrofit Methods
As previously mentioned, common retrofit techniques include concrete, FRP, and steel jackets. Concrete jackets are constructed by enlarging the existing cross section with a new layer of concrete and reinforcement (Ersoy et al. 1993). This reinforcement is traditionally provided by hoop or spiral rebar, or welded wire fabric. FRP reinforcement is typically applied two ways: prefabricated jackets or wraps.
Both methods have been experimentally researched (Morshed and Kazemi 2005). Steel jackets are constructed by placing 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).
2.3.1 Concrete Jacketing
Addition of a concrete jacket is used to enhance flexural strength, ductility, and shear strength of columns. This technique is more commonly used for building columns but has been applied to some bridge members in Japan. The enhanced confinement is achieved with the use of ties or spirals at a small pitch, or transverse reinforcement spacing (Priestley et al. 1996). Concrete jackets can be used to retrofit beams as well as columns (Cheong and MacAlevey 2000). Additional materials can be used to reinforce the retrofit, as long as confinement is enhanced.
2.3.1.1 Rebar Reinforcement
Concrete jacketed columns with hoop and spiral reinforcement effectively enhance the structural capacity of retrofitted members. Ersoy et al. (1993) ran two series of tests to study the behavior of strengthened and repaired concrete jacketed columns. The first series compares the behavior of jacketed columns with a monolithic reference specimen under monotonic axial loading. All the concrete for the monolithic specimen was cast with the base column and retrofit reinforcement in place, to provide a specimen with perfect interaction and bond between the base column and retrofit material. Hoop reinforcement is used in the base column and retrofit reinforcement. The jackets are applied under two conditions: after the compression loading was applied and removed, as well as while the axial load is still applied. It is determined that columns jacketed after unloading performed well, reaching 80 to 90 percent of the strength of the monolithic reference specimen. Repair jackets applied while the column is still under load did not perform as well and only reached 50 percent of the axial load carried by the monolithic specimen. The second series of tests study the effectiveness of concrete jackets with columns tested under combined axial load and bending. Both repair and strengthened jackets behave adequately under monotonic and reversed cyclic loading.

M represents monolithically cast specimens. L and U represent specimens retrofitted under load or after unloading, respectively. S and R represent strengthening or repair retrofits, respectively. From the results it is obvious that the strengthened specimens perform adequately and carry axial loads comparable to the monolithic reference specimen.
Rodriguez and Park (1994) conducted further testing on rectangular columns repaired and strengthened by concrete jackets under compressive axial loading as well as lateral loading. Rebar hoops are provided as the retrofit reinforcement for the concrete jackets. Concrete jackets increase the strength and stiffness of the as-built (unretrofitted or base) columns by up to three times. It is also shown that damage before the retrofit has no significant influence on the performance of the jacketed columns. Overall, concrete jackets with rebar reinforcement significantly improve stiffness, strength, and ductility of typical reinforced concrete columns, but construction is very labor-intensive.
Similar results are reported by Bett et al. (1988) for similarly retrofitted columns. Three square column test specimens are constructed, retrofitted with a concrete jacket, and then tested. One of the specimens is tested, repaired, and then retested. The other two
specimens are strengthened before testing. Specimens are tested under axial and lateral loads simultaneously to simulate earthquake loading. Again, the experiment determines that a damaged retrofitted column has nearly the same strength and stiffness as an undamaged retrofitted column with a similar concrete jacket.
Lehman, et al. (2001) use concrete jackets to repair severely damaged columns. Three repair methods are considered and implemented for the damaged columns, which were built to modern seismic specifications. Initial damage to the columns includes crushing of concrete, buckling and fracture of longitudinal reinforcement, and fracture of the spiral reinforcement, which was the result of axial and lateral loading. The concrete jacket retrofitted column displays increased stiffness and strength, comparable to the original column before damage.
2.3.1.2 Additional Reinforcement Options

Additional reinforcement options for concrete jacketing have been introduced. Welded wire fabric is another common material that could be used to reinforce a concrete jacket retrofit. Morshed and Kazemi (2005) propose a similar system produced from expanded steel mesh, shown in Figure 2.9. The research determines that retrofitting members with the expanded steel mesh and mortar significantly increases the shear strength and ductility of the retrofitted members.
2.3.2 Steel Jacketing
Steel jackets prevent concrete from expanding laterally as a result of high axial compression strains. The steel jacket is equivalent to continuous hoop reinforcement and can be used for circular columns or rectangular columns with slight modifications, as shown in Figure 2.10. Steel jacketing of rectangular columns is not recommended because while shear strength is enhanced, flexural ductility is only provided at the corners. An elliptical steel jacket with concrete infill should be provided for rectangular members to fully confine all the concrete (Priestley et al. 1996).
A comprehensive two-part study was performed by Priestley et al. (1994 a, b) to determine the enhanced shear strength provided by steel jacket retrofitting. The first part of the research focuses on theoretical considerations and test design. It is determined that ACI design equations are overly conservative and new design equations are presented for circular or rectangular columns in need of shear enhancement. The second part of the research focuses on the actual testing of the columns designed according to the proposed equations. 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. Specimens with an A are as-built columns and an R represents retrofitted columns.
Further research performed by Shams and Saadeghvaziri (1997) determines several needs for research into concrete-filled steel tubes (CFT). It is concluded that the ultimate strength of CFT’s is not accurately predicted and the confinement benefits need to be accounted for. Additionally, the research includes axial compression tests performed on circular and rectangular CFT specimens, shown in
Figure 2.11. It is suggested that further research is needed to accurately understand the bond between the steel and concrete, ductility enhancement, and to provide a new design method.
Aboutaha et al. (1999) uses various types of rectangular steel jackets to strengthen concrete columns. Thin steel plates are also used to measure the confinement and strengthening of thin plates used with large columns. The research shows that even thin steel jackets are effective for strengthening reinforced concrete columns. The steel jackets applied include full and partial steel jackets, which do not fully encompass the entire retrofitted member. Both jackets enhance the ductility and strength of the members. Additionally, partially stiffened steel jackets are proposed by Xiao and Wu (2003a) to enhance strength and ductility for square or rectangular columns.
Zhang et al. (1999) applies steel jacket retrofit measures typically used for single columns, to multicolumn bridge applications. The retrofitted bridge structure is compared to an unretrofitted bridge structure under a simulated seismic load using a dynamic bridge analysis program.
2.3.3 FRP Jacketing
Fiber reinforced polymer (FRP) confinement can be provided using several composite materials including fiberglass, carbon fiber, and Kevlar bonded to the confined concrete surface using epoxy Priestley et al. 1996). Weight and cross-section of the retrofitted member are not significantly affected with FRP jackets. FRP jackets are most applicable for circular columns, as stress concentrations can develop in the FRP wrap around the corners of square or rectangular cross-sections. FRP j