A PRESTRESSED CONCRETE MEMBER OBTAINED BY POST TENSIONING

Abstract: Steel materials for use with concrete that is prestressed by posttensioning are disclosed. in accordance with one em­ bodiment of the invention, a steel member is sheathed with a heat-shrinkable synthetic resin tube. in another embodiment, the steel member is sheathed with a foamed synthetic resin tube. Both embodiments are further applicable to the case of a stranded steel member, in which case it is preferred that the spiral grooves of the stranded member be filled with a resin be­ fore sheathing with an external tube.

Description: The present invention relates to prestressed concrete members obtained by post tensioning. Concrete has a relatively low tensile strength. In order to overcome this disadvantage, prestressed concrete has been developed. By means of high strength steel wires, bars or strands, a concrete member is precompressed. When the structure receives a load, the compression is relieved on that portion which would normally be in tension. There are two general methods of prestressing, namely, pretensioning and posttensioning. The present invention relates to steel materials for use with concrete of the type that is prestressed by posttensioning. Structural designs used to prevent direct contact between steel materials and the surrounding prestressed concrete are illustrated in Figs. 1 and 2. The design shown in Fig. 1 can be used whether the steel material is in the form of a wire, bar or strand. A steel member 1 having a grease coating 2 is sheathed with a PE (polyethylene) tube 3. When the steel member 1 with the PE tube 3 is placed within a concrete section 4, the lubricating effect of the intermediate grease coating 2 reduces the coefficient of friction between the steel member and concrete to as low as between 0.002 and 0.005 m⁻¹. Because of this low coefficient of friction, the design in Fig. 1 provides great ease in posttensioning a long steel cable in concrete. However, if the steel material is of short length, the need for preventing grease leakage from either end of the PE tube presents great difficulty in fabricating and handling the steel material. Furthermore, steel members having screws or heads at both ends are difficult to produce in a continuous fashion. The steel member 1 shown in Fig. 2, which is encapsulated in asphalt 5, has a slightly greater coefficient of friction than the structure shown in Fig. 1. This design is extensively used with relatively short steel materials since it is simple in construction, is leak-free, and provides ease in unbonding the steel material from the concrete, even if the steel member has screws or heads at end portions. One problem with the design in Fig. 2 is that the presence of the asphalt (or, alternatively, a paint) may adversely affect the working environment due to the inclusion therein of a volatile organic solvent. Moreover, the floor may be fouled by the splashing of the asphalt or paint. As another problem, great difficulty is involved in handling the coated steel material during drying or positioning within a framework, and separation of the asphalt coating can easily occur unless utmost care is taken in ensuring the desired coating thickness. From FR-A-2059452 there is known a process for an improved protection of prestressing steel elements for use with pretensioned concrete in which the surface of said steel elements is coated with a continuous coating consisting of a synthetic thermo plastic resin. From DE-A1-2018941 there are known tensioning members for the prestressing of concrete wherein said tensioning members have a double coating, namely an inner coating in contact with the tensioning member and an outer coating of which the outer surface is in contact with the concrete. Said inner coating consists of a synthetic resin and said outer coating consists of a different artifical resin. Accordingly it is the primary object of the present invention to provide a prestressed concrete member obtained by post tensioning that is free from the problems associated with the prior art techniques. The object of the present invention is achieved by a prestressed concrete member obtained by post tensioning wherein said concrete member comprises an elongate post tensioning steel material coated by a foamed synthetic resin tube, said foamed resin tube being bonded to said steel member with a wall thickness of the tube sufficient to allow slippage between the steel member and the concrete. This object of the present invention is achieved by the prestressed concrete member as defined in claim 1. Preferred embodiments and further improvements of the inventive prestressed concrete member are indicated in the sub-claims. An embodiment of the present invention is described in the following with reference to figures 3 and 4 showing a schematic presentation of a steel material for use with prestressed concrete and figure 4 shows a cross section of a steel strand sheathed with a resin tube, respectively and in which figures 1 and 2 show schematically conventional designs of steel materials for concrete prestressed by post tensioning. Hereinafter, the present invention will be described in detail with reference to figures 3 and 4, in which reference numeral 1 indicates a steel member and reference numeral 7 indicates a foamed synthetic resin tube. Embodiment ₁ According to this embodiment, the steel member is sheathed by a foamed synthetic resin tube 7 in Fig. 3. Various methods may be used to cover the steel member 1 with the resin tube. In one method, a synthetic resin powder containing a blowing agent is applied to provide a foamed coating on the surface of a preheated steel member by a fluidized dip coating or electrostatic coating technique. Alternatively, a film of synthetic resin containing a blowing agent is formed on the surface of the steel member 1, which is then passed through a heating chamber to expand the resin film into a foam. If desired, a preliminarily formed synthetic resin foam tube 6 may be slipped over the steel member 1. The resin tube 6 may or may to be bonded to the steel member 1. In order to isolate the steel material 1 sufficiently from concrete to facilitate the subsequent posttensioning, the foamed synthetic resin tube 6 must have a wall thickness of at least 300 microns. Furthermore, in order to reduce the frictional resistance and therefore the slippage between the steel member 1 and the concrete, the resin tube 6 preferably has a wall thickness of at least 500 microns. Steel bars, one example of a steel member according to the present invention, were sheathed with a foamed polyethylene tube. The tube was prepared from a blowing agent loaded polyethylene powder that was applied to preheated steel bars using a fluidized dip coating technique. The properties of these samples were as shown in Tables 1 and 2: Basic Properties of Steel Bars Bar dimensions: 17 mm⊘ X 2,830 mm L Polyethylene tube: prepared from medium-density PE powder (density: 0.925 g/cm³, m.p. 120°C) containing 1.0% heat-decomposable blowing agent Wall thickness of polyethylene tube: 1.3 - 1.5 mm Occluded cells: Open cells of a size of 0.3 - 0.5 mm distributed uniformity in a thickness of 3 - 4 microns Sample Resin coat Thickness (microns) Surface features Result Barax (unbonded) 300 - 500 unscratched No rust formed even after 2,000 hrs Barax (unbonded) 300 - 500 scratched Severe rust formed around scratches after 200 hrs Foamed polyethylene coating 300 - 500 unscrachted No rust formed even after 2,000 hrs Foamed polyethylene coating 300 - 500 scratched Rust formed only at scratches after 500 hrs The present invention is also applicable to a steel strand composed of a plurality of twisted steel wires as shown in Fig. 4. The resulting steel strand has spiral grooves as indicated by A and B in Fig. 4. Not only do these grooves render the posttensioning of the strand difficult, but they also increase the frictional resistance on the stressed concrete. In order to avoid these problems, the grooves are filled with a resin. Such filling with a resin may be accomplished by extrusion or other suitable techniques. Subsequently, the thus-treated steel strand is sheathed with the foamed synthetic resin tube as above. According to the present invention, a prestressed concrete member obtained by posttensioning, can be easily manufactured. The resulting prestressed concrete member is easy to handle during transportation and installation.

Claims: Prestressed concrete member obtained by post tensioning wherein said concrete member comprises an elongate post tensioning steel material (1) coated by a foamed synthetic resin tube (7), said foamed resin tube being bonded to said steel member with a wall thickness of the tube sufficient to allow slippage between the steel member and the concrete. The prestressed concrete member of claim 1, wherein the wall thickness of said tube (7) is at least 300 µm. The prestressed concrete member of claim 1, wherein the wall thickness of said tube (7) is at least 500 µm. The prestressed concrete member of claim 1, wherein said tube (7) is a foamed polyethylene tube. The prestressed concrete member of claim 1, wherein said synthetic resin tube (7) is formed by applying a synthetic resin powder containing a blowing agent to a surface of a preheated steel material. The prestressed concrete member of claim 1, wherein said synthetic resin tube (7) is formed by applying a film of synthetic resin containing a blowing agent to a surface of said steel material (1) and then heating said steel material to expand said resin into a foam. The prestressed concrete member of claim 1, wherein said steel material (1) comprises a steel strand having a plurality of twisted steel wires, said steel strand having a plurality of spiral grooves (A,B) formed therein; a resin filling said groove; wherein said foamed resin tube being bonded to said steel member with a wall thickness of the tube sufficient to allow slippage between the steel member and the concrete.

Inventors: INOO, EIJI C/O ITAMI WORKS OF SUMITOMO, MIZOE, MIKIO C/O ITAMI WORKS OF SUMITOMO, WATANABE, KANJI C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1984
Main classification: E04C5
IPC codes: D07B1, E04C5