US3640776A

US3640776A - Coated wire for use in prestressed concrete structures and method of producing same

Info

Publication number: US3640776A
Application number: US856836A
Authority: US
Inventors: Eugene E Haney
Original assignee: Armco Inc
Current assignee: Wire Rope Corp of America Inc
Priority date: 1969-09-10
Filing date: 1969-09-10
Publication date: 1972-02-08
Anticipated expiration: 1989-02-08

Abstract

Wire for prestressed concrete structures. The wire is initially produced by cold reduction from a high-carbon material whereby to produce a high-strength wire having a tensile strength of about 250 K s.i. (1,725MN/m.2). The surface of such wire may have a drawing compound embedded in it. This wire is first coated with a chromic acid, phosphoric acid solution which is then dried on the wire. A very thin organic coating, such as an organic phosphate, is then applied to the wire having the chromic acid, phosphoric acid coating dried thereon. The wire, now having the organic coating applied thereto, is then cured.

Description

United States Patent [151 3,640,776 Haney 1 Feb. 8, 1972 s41 COATED WIRE FOR usr: IN 3,382,081 5/1968 Cutter m1 ..117/134 x PRESTRESSED CONCRETE 2,992,131 7/1961 Bricknell et a1. ..148/6.15 Z X STRUCTURES AND METHOD OF PRODUCING SAME [72] Inventor: Eugene E. Haney, Middletown, Ohio [73] Assignee: Armco Steel Corporation, Middletown,

Ohio

[22] Filed: Sept. 10, 1969 [21] App1.No.: 856,836

[52] U.S.Cl ..148/6.l6,117/128,117/134, 148/315 [51] Int. Cl. ..C23t 7/26 [58] FieldofSearch ..148/6.16, 6.15; 117/134, 128

[56] References Cited UNITED STATES PATENTS 2,067,214 1/1937 Tanner et a1 ..148/6.16 X

2,191,435 2/1940 Ballard et a1 ..148/6.15 R

2,591,625 4/1952 Simonsson et a1. ..148/6.15

OTHER PUBLlCATlONS Churikov, Chem. Abs. Vol. 57 p. 426g July 1962 148-6.]6

Primary Examiner-Ralph S. Kendall Attorney-Melville, Strasser, Foster & Hoffman [57] ABSTRACT Wire for prestressed concrete structures. The wire is initially produced by cold reduction from a high-carbon material whereby to produce a high-strength wire having a tensile strength of about 250 K s.i. (1,725MN/m. The surface of such wire may have a drawing compound embedded in it. This wire is first coated with a chromic acid, phosphoric acid solution which is then dried on the wire. A very thin organic coating, such as an organic phosphate, is then applied to the wire having the chromic acid, phosphoric acid coating dried thereon. The wire, now having the organic coating applied thereto, is then cured.

7 Claims, No Drawings BACKGROUND OF THE INVENTION 1. Field of the Invention The wire of this invention is especially designed for use in effecting prestressed concrete structures of either the pretensioned or the posttensioned type wherein it is imperative to obtain and maintain a very thin coating which will prevent the formation of stress raisers, a localized cracking condition, in the wire when associated with the concrete. Since this wire will usually be shipped to the point of application under conditions wherein it will be necessary for the wire to be exposed to the-elements prior to the time it is incorporated with the concrete structure, it is also necessary that this wire will be highly resistant to general corrosion. Additionally, particularly when used in pretensioned concrete structures, the coating must be such as to effect a firm bond of the wire to the concrete. v

By way of additional background, the following'should be noted withrespect to the terms prestressed, pretensioned" and posttensioned. Prestressing is the art of permanently joining two or more stressed bodies in such a manner that the force'in'one member is balanced by an opposite force in the other member. By such means, compressive energy can be stored up in a brittle or cracked body of sufiicient magnitude to compensate for any tensile stresses that may be imposed by future loads.'Generally speaking,-concrete has excellent properties in compression and very poor tensile properties. Steel, on the other hand, has excellent tensile properties. Prestressed concrete combines'these two properties in a single structural member, using the concrete to handle the compressive stresses and the steel wire or strand to handle the tensile stresses. This is achieved either by pretensioning or posttensioning.

In pretensioning, long lengths of high-strength stress relieved material are tensioned by hydraulic or mechanical loading and anchored to end abutments before the concrete is poured. After the concrete has cured, the load is removed from the end abutments and transferred by physical bond to the concrete member. The high-strength, stress-relieved materials are then cut off at the ends of each member, or in some cases the concrete section may be sawed into lengths by an abrasive cutoff wheel. Pretensioning is particularly adaptable to centrally located yards and widely used for mass production of smaller members. In the cured structure the pretensioned wire or strand remains stressed by virtue of the fact that it is bonded to the concrete substantially throughout its length; a proper coating on the wire insures this result.

In posttensioning, which lends itself particularly well to onthe-site construction of larger members, the member is cast with-spaces properly positioned in the form to allow the ten- -sioning material to pass through. After the concrete has reached su'flicient compressive strength, special end anchorages are attached and the tensioning member stressed. In some instances the remaining space in the concrete member is filled with grout. The anchorage force may then react to bearing plates cast in the beams or it maybe distributed directly into the surrounding concrete by other suitable means.

2. Description of the Prior Art Although a search of the prior art has been conducted in order to ascertain the most pertinent of such art, no. assertion is made that the best'art was indeed located. The following patents, however, appear to be most representative of the art to which this invention relates. U.S. Pat. No. 3,074,827

(Hoover) discloses a process of treating a steel sheet having a substantially single-phase, alloyed zinc coating containing alu- U.S. Pat. No. 3,094,44l (Curtin) discloses providing an oxida a tion-inhibiting and bonding coat onaluminum, zinc and fer- 'rous metal surfaces by applying thereto solutions'which contain both chromic acid and phosphoric acid. U.S. Pat. No. 3,127,288 (Hines et al.) discloses a method for treating zinc and zinc-iron alloyed surfaces to protect such surfaces against corrosion or discoloration by treating the surfaces with aqueous solutions containing phosphate ions and chromate ions. U.S. Pat. No. 3,370,992 (Ilenda et al.) discloses a composition and process for conditioning metal surfaces to receive a sub sequent protective coating, the conditioning composition comprising a chromic acid, phosphoric acid bath and the subsequent protective coating being organic; the purpose of the procedure is to make the metal surface receptive to paint and the like for decorative purposes. German Pat. No. 856,545 discloses the treatment of zinc coatings with aqueous solutions of phosphoric acid and chromic acid.

One of the problems in the past has been that the wire, when shipped to the construction site, will corrode prior to the time that it is made a part of the concrete structure. Another of the problems has been to find awire which will bond properly with the concrete. The known prior art does not provide or suggest a solution for these problems. Both of these problems, however, are solved by the instant invention.

In the past the prior art has attempted to meet the problems just discussed by not treating the wire at all for it was believed just as undesirable to have a coating on the wire as it was to have rust thereon; it was believed that anything which came between the metal and the concrete was detrimental, whether this be rust, oil or any other coating. Accordingly, it was the practice in the prior art to hand wrap the wire in packages to prevent rusting (vapor phase inhibitors often being included) and to then rush the packaged wire to the customer, hopefully for immediate use so that rusting would not occur. The elimination of this expensive packaging and shipping operation has been one of the chief advantages realized by this invention. An added advantage, in pretensioned concrete and metal structures, is that an improved bond is obtained by virtue of the novel coating applied to the metal.

SUMMARY OF THE INVENTION This invention resides in providing a wire for prestressed concrete structure which, even after prolonged exposure to the elements, will not corrode (other than superficially, which can be easily wiped off) and which will permit a highly efiicient bond to the concrete when it is incorporated in pretensioned structures. And, whether used in pretensioned or in posttensioned structure, it will not fail due to stress raisers caused by stress corrosion cracking as distinguished from general corrosion. The wire is first coated with a chromic acid, phosphoric acid solution whereafter a second organic coating is applied thereto, the coating thus applied on the wire then being cured. Both coatings are necessary. The phosphoric acid acts to remove the drawing compound from the wire and present a wettable surface for reaction with the chromic acid. The chromate ions which are deposited on the surface of the wire provide some corrosion protection The extremely thin, perhaps on the nature of 1 molecular thickness, organic coating not only acts to provide the desired bond to concrete but it also cooperates with these chromate ions to make the finished wire corrosion resistant. In this manner a corrosion resistant wire which will efficiently bond to concrete when used in prestressed pretensioned structures is produced.

DESCRIPTION OF THE PREFERRED EMBODIMENT and phosphoric acid in the range of 2.0 to 10.0 percent.

Although this is a desirable and workable range for the coating solution, it is not critical. It has been determined, for example, that a coating solution which is comprised of 5 percent phosphoric acid and-2 percent chromic acid will produce satisfactory results. It is believed necessary, however, to have these acids present in at least the amount indicated; that is, the ranges set forth are to be considered as minimum. Thus, although there may be more than 10 percent phosphoric acid present, there should never be less than 2.0 percent. Similarly, there should always be at least 0.5 percent of the chromic acid, although this may go above 2.0 percent (generally, however, amounts of chromic acid above 2.0 percent are not used). Specifically, a desirable, workable chromic acid solution comprises about 2 grams of chromium trioxide (CrO to about 98 grams of water. A desirable, workable phosphoric acid solution is made up of about 3 to 7 grams of phosphorus pentoxide (P with about 97 to 93 grams of water.

Preferably the phosphoric acid and chromic acid are located in the same container or tank, simply to save time and space. These acids, however, could be located in separate containers. The wire is wetted with these acids and then promptly dried. The process does not contemplate an actual phosphating or pickling action. The primary function of the phosphoric acid is to react with any impurities which may be found on the wire and to wet the surface of the wire. In a typical operation, the wire is moved through a 2-foot-long phosphoric acid, chromic acid bath at the rate of 150 feet per minute (0.76 m./sec.). In such an operation, the wire is immediately dried by means of a 360 air wipe, thus eliminating any excess acid on the wire. In effect, a film of chromium trioxide is put on the wire and then subjected to this 360 air wipe so that as thin a film as possible is obtained without wiping all of this chromium trioxide off. Respecting the phosphoric acid, if there should be any of this left on the surface of the wire, it will form an iron phosphate which is not undesirable.

Secondly, an organic coating, extremely thin, preferably on the nature of 1 molecular thickness, is applied to the dry, acidcoated wire. In practice it has been found that an organic coating such as succinic acid (one such satisfactory coating material is known in the trade as Dacromet) will produce good results when applied according to the teachings of this invention. Other organic coatings may suffice. It is necessary, however, that these organic coatings of this second treatment must be very thin and such that they will be compatible and reactive with the chromic acid. It is also important that the organic coating be wettable with, and bondable to, concrete. It may be desirable for the organic coating to have powdered zinc entrained therein.

As indicated, the organic coating should be very thin. Accordingly, as soon after it is applied as possible, it is dried. When the coated wire is used in pretensioned concrete structures there must .be a bond, through the coating, between the wire and the concrete. If the coating is more than 1 molecule thick, even if the lcoating materials have reacted together, there could be a shearing action within the coating itself. On the other hand, if the coating is only 1 molecule thick, then the effect of this is that one side of such molecular thickness is adhered to the concrete and the other side to the steel, whereby shearing is presented for the molecule itself cannot be sheared except with atomic forces which are not encountered in these applications. These very thin coatings, therefore, prevent internal organic failure.

After the organic coating has been applied to the previously acid coated wire and dried, the wire is heated so as to insure a reaction between the organic substance and the chromate. This heating will usually be in the range of 450 to 800 F. (500 to 700 K.) It is believed that this reaction heating results in reducing, by the organic compound, the chromium trioxide, which is not reactive with iron, to chromium dioxide, which is reactive with iron, whereby to insure that the coating adheres firmly to the wire. In this manner it is believed that a chemical bond between the coating and the wire is effected.

This third step, therefore, may be considered as a curing operation in which the wire, now having the chromic acid, phosphoric acid base coat applied thereto, and on which coat there is the very thin organic coating, is heated. A curing range of 450 to 800 F. (500 to 700 K.) has earlier been indicated, and the time required may vary from 5 seconds to 30 minutes. The reaction time is very short but the total time will vary with the nature of the heat source. Actually, however, curing can vary from room temperature to 800 F. (700 K.) depending on the organic compound used and the time available for treatment.

This thin organic coating, presenting a wettable and bondable surface with concrete, applied to a previous coating having a chromate therein, gives the results desired. Not only will a wire so coated bond to concrete when used in pretensioned structures, but also stress corrosion cracking, whether the wire is used in pretensioned or in posttensioned concrete structures, is prevented. General corrosion is also prevented by the coating resulting from this invention. This insures that stress raisers will not occur and the wires, whether used in pretensioned or posttensioned structures, will be prevented from failing prematurely.

The wire itself is also important. Generally, the wire constituting a part of this invention is produced by cold reduction of a high-carbon material such that the resulting wire has a tensile strength of about 250 K s.i. (l,725MN/m.). Before treatment in accordance with the teachings of this invention, the surface of such wire will usually have a drawing compound such as calcium stearate embedded in it. In view of these factors, it is necessary that the surface protection provided by this invention must be of such a nature that, in addition to providing the corrosion resistant and bonding properties heretofore mentioned, it will not produce stress raisers in the wire surface nor will it anneal the cold-drawn structure. The coatings herein described, acting together, achieve all of these results. Any hydrogen generated anywhere along the line must be kept away from the steel because it would diffuse into the steel in the stressed condition and cause cracking. The herein described coating forms a film which prevents hydrogen diffusion.

The cold-drawn wire of this invention must also be stress relieved. This is easily accomplished by using a molten lead pot in the range of 750 to 800 F. (670 to 700 K.), as is well known in the art. The placing of this stress reliever is not critical and could even occur as a part of the reaction heau'ng step needed for the organic substance and the chromate. For example, an induction heating step could be employed to effect both a stress relief of the cold-drawn wire and the reaction between the chromate and, organic substance; thus the earlier suggested range of 450 to 800 F. (500 to 700 K.) can be used.

Wires have been referred to throughout this description. There may be, however, not only individual wires but also strands made up of a plurality of wires. Common individual wire sizes are 0.192 inch, 0.250 inch and 0.276 inch (respectively 50, 65, and 70 mm.) in diameter. The most commonly used strand sizes are one-fourth inch, five-sixteenths inch, three-eighths inch, seven-sixteenths inch, one-half inch, and 0.600 inch (respectively 65, 80, 95, 110, 130, and 150 mm.) diameter seven-wire strand. Such wires, or wires made into strands, for prestressed concrete are preferably made from heat-treated, high-carbon rods with multiple-holecold drafting to produce the required mechanical properties. It must have a high tensile strength (usually on the order of 240,000 (1 ,650MN/m. pounds per square inch minimum); it must have high yield strength (at least percent of tensile); it

must have adequate ductility for placement, anchorage and tensioning; and it must have uniform stress-strain characteristics within the elastic range. The prestressing steel must also be relatively straight to facilitate its placement in concrete, and free from excessive amounts of foreign material on its surface which might adversely affect its bonding characteristics. Individual wires and strand arepresently stress relieved by beating them in molten lead. Strands are particularly well suited for pretensioned structures in that the presence of v the valleys makes for additional mechanical bonding between the strand and the concrete.

It is believed that the phosphoric acid acts to react with the drawing compound from the wire and present a wettable surface for reaction with the chromic acid. The chromate ions which are deposited on the surface of the wire themselves provide some corrosion protection while at the same time acting with the organic coating to not only secure the efficient bond desired but also to increase the corrosion-resistant properties in the finished wire of this invention.

The wire, after the organic coating treatment, looks somewhat like a lightly pickled surface with no coating on it. In this connection, it is emphasized that the total coating thickness must be very thin so that a shear failure is prevented within the coating layer itself. This is to be distinguished from the prior art wherein an inferior pickling or cleaning preparation, along with an ordinary paint coating, will not achieve the results produced by this invention. Previous to this invention, the wires were uncoated and the resulting corrosion product which occurred seriously detracted from the bond strength which could be developed in concrete.

The wire of this invention, as produced by the method described herein, is highly resistant to corrosion and it adheres nicely in a bonding action with the concrete of the prestressed and pretensioned concrete structures with which it is used.

It is believed that the foregoing constitutes a full and complete disclosure of this invention. Numerous variations and modifications may be made without departing from the scope and spirit of this invention, and no limitations are intended except as specifically set forth in the claims which follow. It has been determined, for example, that other possible organic coatings, identified by brand name, are Dacromet 2 (believed to be basically succinic acid with suspended zinc particles) and Rustarest, an organic phosphate.

It is especially to be noted that in the claims which follow the use of the word wire is intended to include strand as earlier described in this specification. The claims, therefore, are to be considered as directed to and embracing both a single wire and a multitude of wires wound together to comprise a strand.

The embodiments of the invention in which an exclusive property of privilege is claimed are defined as follows:

1. A wire for prestressed concrete structures comprising a coated steel wire having a high tensile strength, the coating being the end reaction product of an aqueous solution of phosphoric acid in which the phosphoric acid is present in at least the range 2.0 to 10.0 percent, and an aqueous solution of chromic acid in which the chromic acid is present in at least the range of 0.5 to 2.0 percent, and an organic compound wettable with and bondable to concrete and compatible and reactive with the chromic acid on said wire, said coating being very thin, on the order of 1 molecule thick.

2. A wire for prestressed concrete structures comprising a thinly coated steel wire having a high tensile strength, said coating being the end reaction product of P and water in which the P 0 is present in at least the range of 2.0 to 10.0 percent, CrO and water in which the C10 is present in at least the range of 0.5 to 2.0 percent, and an organic compound wettable with and bondable to concrete and compatible with and reducing to the CrO whereby to form CrO said coating being very thin, on the order of 1 molecule thick.

3. A method of making wire for prestressed concrete structures which comprises providing a steel wire having a high tensile strength, passing said steel wire briefly through aqueous solutions of phosphoric acid in which the phosphoric acid is present in the range of at least 2.0 to 10,0 percent and chromic acid in which the chromic acid is present in the range of at least 0.5 to 2.0 percent, promptly drying said steel wire, passing said steel wire through an organic compound wettable with and bondable to concrete and reducing to and compatible with chromic acid (CrO drying said organic compound, and heating said steel wire to effect a reaction between said chromic acid on said wire and said organic compound, whereby to achieve a final coating on said wire which is on the order of 1 molecule thick.

4. The method of claim 3 including the step of stress relieving the steel wire.

5. A wire for prestressed concrete structures, said wire having an undercoating thereon comprised of the reaction products of an aqueous phosphoric acid, chromic acid solution applied thereto, said phosphoric acid solution having phosphoric acid in the range of at least 2.0 to 10.0 percent and said chromic acid solution having chromic acid in the range of at least 0.5 to 2.0 percent, and a very thin outercoating on said undercoating, said outer coating resulting from the application of a thin layer of a liquid organic coating wettable with and bondable to concrete and compatible and reactive with the chromic acid on the wire, whereby said wire is resistant to corrosion and to stress corrosion cracking and whereby said wire bonds to concrete with high efficiency.

5. The wire of claim 5 in which the total thickness of the under and outer coatings is very thin, said outer coating being on the order of 1 molecular thickness, whereby to prevent shear failure within the coatings.

7. A method of producing a wire for pretensioned and posttensioned concrete, which wire is resistant to corrosion and to stress corrosion cracking and which wire has a highly eificient bond to concrete, comprising the steps of applying an aqueous solution of phosphoric acid, chromic acid to the wire, said phosphoric acid solution having phosphoric acid in the range of at least 2.0 to 10.0 percent and said chromic acid solution having chromic acid in the range of at least 0.5 to 2.0 percent, drying the solution on the wire, applying a thin organic coating to the dry acid-coated wire, said organic coating being wettable with and bondable to concrete and compatible and reactive with chromic acid, and curing the thus-coated wire to dry said organic coating.

Claims

2. A wire for prestressed concrete structures comprising a thinly coated steel wire having a high tensile strength, said coating being the end reaction product of P2O5 and water in which the P2O5 is present in at least the range of 2.0 to 10.0 percent, CrO3 and water in which the CrO3 is present in at least the range of 0.5 to 2.0 percent, and an organic compound wettable with and bondable to concrete and compatible with and reducing to the CrO3 whereby to form CrO2, said coating being very thin, on the order of 1 molecule thick.
3. A method of making wire for prestressed concrete structures which comprises providing a steel wire having a high tensile strength, passing said steel wire briefly through aqueous solutions of phosphoric acid in which the phosphoric acid is present in the range of at least 2.0 to 10.0 percent and chromic acid in which the chromic acid is present in the range of at least 0.5 to 2.0 percent, promptly drying said steel wire, passing said steel wire through an organic compound wettable with and bondable to concrete and reducing to and compatible with chromic acid (CrO3), drying said organic compound, and heating said steel wire to effect a reaction between said chromic acid on said wire and said organic compound, whereby to achieve a final coating on said wire which is on the order of 1 molecule thick.
4. The method of claim 3 including the step of stress relieving the steel wire.
5. A wire for prestressed concrete structures, said wire having an undercoating thereon comprised of the reaction products of an aqueous phosphoric acid, chromic acid solution applied thereto, said phosphoric acid solution having phosphoric acid in the range of at least 2.0 to 10.0 percent and said chromic acid solution having chromic acid in the range of at least 0.5 to 2.0 percent, and a very thin outercoating on said undercoating, said outer coating resulting from the application of a thin layer of a liquid organic coating wettable with and bondable to concrete and compatible and reactive with the chromic acid on the wire, whereby said wire is resistant to corrosion and to stress corrosion cracking and whereby said wire bonds to concrete with high efficiency.
6. The wire of claim 5 in which the total thickness of the under and outer coatings is very thin, said outer coating being on the order of 1 molecular thickness, whereby to prevent shear failure within the coatings.
7. A method of producing a wire for pretensioned and posttensioned concrete, which wire is resistant to corrosion and to stress corrosion cracking and which wire has a highly efficient bond to concrete, comprising the steps of applying an aqueous solution of phosphoric acid, chromic acid to the wire, said phosphoric acid solution having phosphoric acid in the range of at least 2.0 to 10.0 percent and said chromic acid solution having chromic acid in the range of at least 0.5 to 2.0 percent, drying the solution on the wire, applying a thin organic coating to the dry acid-coated wire, said organic coating being wettable with and bondable to concrete and compatible and reactive with chromic acid, and curing the thus-coated wire to dry said organic coating.