Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1995-06-06
2001-03-13
Bueker, Richard (Department: 1763)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S372000, C428S377000, C428S379000, C428S380000, C427S461000, C427S185000, C427S195000, C427S201000, C427S202000, C427S204000, C427S205000, C427S385500, C427S386000, C427S409000, C427S410000, C264S228000, C052S233000
Reexamination Certificate
active
06200678
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to steel members for strengthening concrete, particularly prestressing tendons of wire and strand types for prestressing by pretensioning or posttensioning, but applicable in some respects to reinforcing bars, wires, or the like. The invention more particularly relates to improvements in corrosion resistance and/or bond control of such members.
BACKGROUND
Corrosion of steel strengthening members in concrete has long been a problem in the art, and has received a great deal of attention. For instance, it is known to coat reinforcing bars with an epoxy coating applied by electrostatic spray guns, and the American Society for Testing and Materials has issued standard specifications for epoxy-coated reinforcing bars and steel, under ASTM designations A 775-81 and D 3963-81, covering deformed and plain steel reinforcing bars with protective epoxy coating applied by the electrostatic spray method. This approach is not without its problems in that the coating thickness is specified as 5 to 12 mils, apparently in order to avoid bond problems encountered at greater thicknesses, and the lesser thicknesses involve problems of integrity or permeability of the coating, exemplified by the ASTM specifications permitting two holidays (pin holes not discernible to the unaided eye) per linear foot of the coated bar. Epoxy coating materials are available on the market for use specifically in coating reinforcing bars. A problem remains, however, in assuring an adequate corrosion-protective coating while maintaining good bonding qualities with the concrete.
Corresponding problems, but of greater magnitude and importance, exist in the case of high strength steel wire and strand used for prestressing concrete (hereafter referred to as PC wire or strand). Strand, of course, is formed by spinning a number of wires (typically six) together around a central core. The magnitude of the problem is exemplified by the fact that use of PC strand or wire is discouraged or prohibited in certain areas where it advantageously could be used. Thus, in a Memorandum dated Feb. 10, 1981, of the Federal Highway Administration, U.S. Department of Transportation, captioned “Corrosion Protection of Reinforcement in Bridge Decks,” and dealing with criteria to be applied to all reinforcement in bridge decks, prestressed or otherwise, where deicing salts or a salt water environment present the potential for corrosion, it is suggested that all conventional reinforcement be epoxy coated, but that “Pretensioning should not be permitted in bridge decks since there is no known way of eliminating the potential for corrosion,” and that “Polyethylene ducts should be provided for protection of posttensioned tendons in addition to grouting.” In a follow-up Memorandum dated Apr. 14, 1981, indicating that epoxy coating of rebars was not intended to be the only method of corrosion protection of bridge decks, it was stated that “In pretensioned work, there are currently no known methods for epoxy coating the strands, and the potential for corrosion exists in a salt water environment as well as in areas where deicing chemicals are used.”
There have been efforts to develop corrosion resistant PC tendons, and some are in use because nothing more efficient and/or more economical was available. Thus, the use of galvanized strand is often suggested by designers concerned about corrosion and not familiar with the properties of galvanized strands. Galvanized strands are not as strong as stress-relieved strands of the same size, and they cannot be fabricated so that they possess all of the desirable properties that are obtained by stress-relieving uncoated strands. They are appreciably more expensive per unit of strength, their bond properties are not consistent, and there can be a chemical reaction between the zinc coating and the cement paste in concrete. Although galvanized strands have been available since before the development of prestressed concrete, they are seldom used. Single unbonded posttensioned strands are used in the construction of flat slab floors for garages, apartment and office buildings, etc., and tendons made of several parallel wires are used in a similar manner. These tendons are typically coated with a corrosion resisting grease, encased in tubing, fastened in place and the concrete slab is cast around them. When the concrete has cured the tendon is tensioned and then permanently held under tension by an anchor at each end. At present, tendons of this type are being coated with grease and encased in plastic tubing. This is an improvement on the former paper wrap, but they are still subject to corrosion in the anchorage area because typically the tubing must be removed to permit the anchor to grip the strand. Additionally, the relatively thin plastic is sometimes damaged during handling.
Posttensioned grouted tendons have been in use as long as prestressed concrete itself. The tendon is threaded through a cavity that has been cast in the concrete, or the tendon is encased in an oversized flexible metal or other type of tube before concrete is cast. After the concrete is cured, the tendon is tensioned, and the cavity around the tendon is pumped full of liquid cement grout. The cavity can be filled if the tendon is properly detailed and fabricated, and if the grout is properly injected. In actual practice, this is frequently not the case, and areas susceptible to corrosion are left in the cavity.
In precast pretensioned members, the tendons typically are seven-wire strands which are tensioned and anchored in the forms. Concrete is cast around the strands. When the concrete has cured, the strands are released from their external anchors, and their prestressing force is transferred to the concrete by bond between the steel and the concrete. Thus, in such pretensioned PC tendons, there is a problem not only of corrosion protection, but also one of bond transfer between the pretensioned PC tendon and the concrete.
The patented technology is replete with various approaches to the problems of corrosion protection and/or bonding characteristics, including some incidental disclosures. For instance, Billner U.S. Pat. Nos. 2,319,105 and 2,414,011 mention thermoplastic or thermosetting coverings which will harden and effect a bond between the concrete body and its reinforcement. Simonsson U.S. Pat. Nos. 2,591,625 and 2,611,945 involve coatings including siliceous material. Wijard U.S. Pat. No. 3,030,664 discloses reinforcing elements provided with a coating comprising a suspension of a hydraulic cement and rubber in suitable proportions as to be converted by steam curing of the concrete into a hard strong layer having good adhesion to the reinforcing elements and the concrete, and supposedly serving also as a rust-protective film. Rice U.S. Pat. No. 3,293,811 discloses an epoxy resin coating on PC strand to protect against notching by serrated teeth carried by anchor wedges. Mager U.S. Pat. No. 3,377,757 relates to steel storage tanks prestressed by plastic coated tendons extending about the tank, the plastic coating being for the purpose of protecting the tendons from corrosion forces. Lang U.S. Pat. No. 3,513,609 relates to posttensioned type tendons, including one embodiment in which the tendon incorporates a curable plastic material such as an epoxy resin between the wire or strand and an outer plastic coating, the curable resin being cured while the wire is held under tension so as to anchor the wire to the outer plastic coating and thus to the concrete structure along the length of the wire. The curable resin initially provides a lubricating effect and, after curing, provides a bonding effect. Curing of the resin is by passing electric current through the core wire. Lang U.S. Pat. No. 3,579,931 is of the same substance. Scott U.S. Pat. No. 3,596,330 discloses a structural tensile member made of steel wire or like material provided with a sheath or coating of polypropylene or other impermeable corrosion resistant material. Lang U.S. Pat. No. 3,646,748 discloses a PC strand encased in a
Gillette Donald J.
Hunt Frederick F.
Bueker Richard
Florida Wire & Cable, Inc.
Holt William H.
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