US3086273A

US3086273A - Method for pre-stressing concrete

Info

Publication number: US3086273A
Application number: US468A
Authority: US
Inventors: Welborn Jack Vernon
Original assignee: Super Concrete Emulsions Ltd
Current assignee: Super Concrete Emulsions Ltd
Priority date: 1959-12-28
Filing date: 1959-12-28
Publication date: 1963-04-23
Anticipated expiration: 1980-04-23

Description

9 0 m6 moss REFERENCE EX MQ April 1963 J. v. WELBORN 7 3,086,273

METHOD FOR PRE-STRESSING CONCRETE Filed Dec. 28, 1959 I INVENTOR.

J. VERNON Waaoau BY Ma 5004 A 7'7'ORIVEYS- United States Patent 3,086,273 METHOD FOR PRE-STRESSING CONCRETE Jack Vernon Welborn, Santa Monica, Calif., assignor to Super Concrete Emulsions, Ltd., Los Angeles, Calif, a corporation of Nevada Filed Dec. 28, 1959, Ser. No. 468 2 Claims. (Cl. 25-154) This invention relates generally to construction operations and more particularly to an improved method for prestressing concrete constructions. This application is a continuation-in-part of my copending application Serial No. 789,199, filed January 26, 1959, now abandoned. The term prestressed concrete as used herein and generally in the art is intended to mean reinforced concrete structures wherein the concrete is under compression and the reinforcing steel is under tension, without regard to the manner in which this is accomplished, i.e., by pretensioning, wherein the steel is stretched or put under tension before the concrete is poured, or by post-tensioning, wherein the steel is stretched or tensioned after the concrete has hardened. The present invention is primarily concerned with the post-tensioning process, although it comprehends within its scope pretensioning operations wherein it is desired to prevent or temporarily prevent a bond between the concrete and a portion or portions of the stressing tendon or tendons, such as where it is desired to achieve maximum efiiciency of the prestressing force in respect to the external forces applied to various areas of the member.

For many construction operations, post-tensioning of concrete is preferable to the use of pretensioned concrete. For example, post-tensioned concrete does not require the use of special molds since the concrete itself is sufficiently cured at the time of tensioning to retain its shape at full stress. Further, the stressing or tensioning tendons can assume desired curves to provide compressive forces and force components in the concrete where de sired. Pretensioning tendons are substantially straight at the time the concrete is poured. Another advantage flowing from the use of post-tensioning and the elimination of special molds is that the tensioning operation may be carried out directly on the job. Thus, considerably larger slabs or other structural forms of concrete can be created as compared to constructions formed from pre formed, pretensioned concrete.

In post-tensioning of concrete, a plurality of stressing tendons such as wires, rods, or the like are stretched between points in a desired pattern and concrete material in its plastic state is molded into the pattern to cover the tendons. The tendons will form catenaries under their own weight and by varying the stretching force the curves assumed by the tendons can be adjusted so that ultimate compressive stresses and stress components in the concrete are maximum where desired with respect to other externally applied loads. After the concrete has cured to an extent that it can withstand stresses established by tensioning of the tendons, the actual post-tensioning is carried out. The tensioning operation itself results in relative movement between the tendons and the concrete and unless some means are provided for eliminating or considerably reducing the friction between the tendons and the concrete proper post-tensioning cannot be accomplished.

It has generally been the practice, therefore, to employ a guide tube or metal tube material through which the tensioning tendon is passed, which tube is cast with the concrete. The tube then permits slippage to occur during the post-tensioning step. After proper tensioning of the tendon has been effected, the tube is filled with grout or the like and becomes a permanent part of the final 3,086,273 Patented Apr. 23, 1963 post-tensioned concrete structure. However, there is no assurance of bonding between the concrete and the tendon. In other words, it is difficult to determine whether the tube has been completely filled with grout.

In other instances, it has been proposed to grease and wrap paper or similar material about the tendon to enable ultimate slippage after partial curing of the concrete during the tensioning operation. While in certain structures, the provision of grease and paper wrapping is adequate because complete adhesion bond between the concrete and the tendon is not necessary, there is no way of assuring that the grease or paper is not removed accidentally during placing, and it is generally more desirable from a structural standpoint to provide an ultimate bond or adequate friction between the cable and the concrete. With the grease and paper wrapping methods, the tensioning load is permanently concentrated at the opposite end portions of the concrete pattern between which tensile stress is provided.

In the above examples of prior art methods of posttensioning employing grout, there is danger of water from the grout becoming entrapped between the tendon and concrete surrounding the tendon. As a result, serious fractures and even destruction of the concrete can occur under high temperatures or freezing conditions. Further, the presence of water can result in rusting and corrosion of the stressing tendon thereby seriously weakening the structure.

Bearing the foregoing in mind, it is a primary object of the present invention to provide an improved method for post-tensioning concrete in which the foregoing difficulties are substantially overcome.

More specifically, it is an object to provide a method of post-tensioning concrete which avoids the use of guide tubes, grouting, wrappings and the like so that no auxthe post-tensioning has been completed.

Another important object is to provide a post-tension- I ing method in which the possibility of the presence of water and resulting possible rusting or destruction of concrete under freezing or high temperatures is minimized. I

Still another object is to provide a method of post-tensioning which does not require the use of any separate grouting and yet which is capable of resulting in a direct mechanical friction or adhesion bond between the tendon and the concrete.

Briefly, the generally preferred method of the invention contemplates the steps of providing a tendon in the form of a wound cable in which the exterior surfaces are formed with spiraled crevices or other inundations and coated with a concrete setting retarder or retardant. The use of smooth wires or rods is within the scope of the present invention, but for some applications the wound cable type of tendon is superior in that with wound cable the concrete retardant will penetrate the crevices in the exterior of the wound cable. The treated cable is suspended between suitable end fastenings covered by molding concrete in a desired pattern. The ends of the cable are accessible at different portions of the pattern between which tensile stress is to be provided. After curing of the concrete to an extent in which it is sufliciently strong to retain its shape under full stress, pulling forces are applied at opposite ends of the wound cable to tension the same. This tensioning force may be maintained by suitable end fastenings. However, since foreign material such as guide tubes, wrappings, and the like are not present, the concrete itself may form a direct mechanical friction or adhesive bond to the wound cable. In the case of pretensioning operations, only a portion of the tendon, such as the ends or a single end portion, is coat- 3 ed with the retarder, thereby permitting slipping of the tendon in the coated portions and full bond with stress transfer to the concrete in the uncoated portions.

A better understanding of the overall invention will be had by now referring to the accompanying drawings, in which:

FIGURE 1 is a schematic perspective view of a concrete slab post-tensioned in accordance with the method of the present invention;

FIGURE 2 is a cross sectional view showing one of the post-tensioning tendons;

FIGURE 3 is an enlarged perspective view of a portion of one of the tendons employed in the method of the invention; and

FIGURE 4 is a cross section of the tendon of FIG- URE 3 taken in the direction of the arrows 44 after a coating has been applied thereto.

Referring first to FIGURE 1, there is shown, by way of example, a concrete slab including a plurality of groups of post-tensioning tendons running transversely of the slab as indicated at 11, 12, 13 and 14. The spacing and number of tendons in each group is a matter of design depending upon the particular strength requirements for the proposed construction in question. Also shown in FIGURE 1 are longitudinally extending tensioning tendons 15 to provide tensile strength in the slab in a longitudinal direction. In some instances the tendons may extend at angles between the transverse and longitudinal directions to provide components of tension force in the transverse and longitudinal directions.

Referring now to FIGURE 2, there is shown one stressing tendon 16 having exposed

ends

17 and 18 accessible at difierent portions of the pattern defined by the concrete slab 10. The tendon is supported by end fasten ings 19 and 20 and will, under its own Weight, or by mechanical positioning means, 'define a catenary. While only one tendon is shown, it will be understood that multiple catenaries of any number or direction may be formed by supporting the tendons in the desired positions.

With one or more stressing tendons such as illustrated at 16, properly stretched across the desired pattern, concrete is molded to cover the tendons such as the tendon shown in FIGURE 2. Curing of the concrete takes place in a conventional manner with attendant shrinkage thereof so that the cable 16 is normally tightly engaged by the concrete. As a consequence of such contraction, it has been necessary as described to provide a guide tube or otherwise protect the cable 16 from bonding to the concrete so that after the concrete has cured, proper tensioning of the tendon can take place. Such tensioning is effected by pulling on the

ends

17 and 18 of the tendon as indicated by the arrows thereby establishing stresses within the slab 10. In the particular example chosen in FIGURE 2, these stresses have an upward component as a consequence of the catenary defined by the tendon which upward component of force is desirable when the slab 10 is used as a beam to support weight on its upper surface.

The tendon 16 is preferably provided 'with a special exterior having a non-cylindrical surface. Thus, referring to FIGURE 3 and FIGURE 4 a preferred type of tendon is shown as comprising a plurality of rod-

like members

21, 22, 23, 24 preferably spiraled together to form a wound cable. -By this construction, the exterior surfaces of the cable include spiraled crevices 25. The wound cable is coated with a concrete retardant 26 which penetrates between the non-engaging peripheral portions of the rod-like members. These spiraled crevices or cavities constitute an important feature of the present invention as they provide room for the shrinking concrete to move during the curing process and enable effective action of the concrete retardant to take place.

Thus, when the coated cable as described is employed in a post-tensioning operation as shown in FIGURE 2,

contraction or shrinkage of the concrete about the cable during initial curing will result in cement paste from the shrinking concrete being forced into the cavities. The concrete retardant includes a resinous emulsion which will react with alkaline water liberated during the hydration of the cement present in the concrete within these crevices. The coating itself will be destroyed releasing the actual retardant which forms a molecular film about the cement particles. In other words, the coating will initially penetrate into the cavities and mix with the cement paste forced into the cavities by shrinkage. The retardant then reacts with the cement paste by coating particles thereof to delay their effective bonding properties and form a cylindrical coating of retarded cement paste about the tendon in turn delaying any direct bonding of the concrete to the cable. Therefore, relative movement between the cable and the concrete surrounding the cable can take place during post-tensioning.

The preferred type of concrete retardant employed with the method of the present invention includes one or more salts of tmmrmlpi diformed in a resinous emulsion. A suitable t 1c enmg, quick drying agent, and preferably a dye to facilitate visual inspection during coating may be added. The resinous emulsion is characterized in that it is soluble in or destroyed by alkaline water and the retardant is thereby liberated as described during hydration of the cement.

Generally speaking, any type of cement inhibitor or retarder can be used in carrying out the process of the present invention. As indicated, the preferred class of retarder is the salts of tetra-h ic acid, such as is disclosed in Winkler, Patent No. 2,174,05115, a?

pa mer-smrssie inarem's'fih as the general class of hydroxylated organic compounds containing aliphatic radicals, such as tartaric acids and tartar, citric acid, saccharic acid and other hydroxylated carboxylic acids. Other well kn n retarders useful in the present invention include the sulfonated lignin materials such as lignin sulfonic acid, calcium and other metal salts of lignin su onic aci form or in the form of waste sulfite liquor; cellulosic matermluding-etarek amr-watefiofnble gums such as gum arabic, gum gamboge and the like; polyatomic monobasic phenolic acids such as gallic acid and gallitinic acid; the diatomic phenols, such as pyrocatechin, resorcin, hydroquinone and the higher phenols such as pyrogallic acid; the glucosides, including tanning agents extracted from various vegetable materials such as cutch and quebracho; the carbohydrates such as sugars; cyanides, cyanates, cyanamides and the like; borate salts; zinc salts such as magnesium-zinc silicofluorides; and many other organic and inorganic compounds.

In its broadest aspect, the present invention comprehends within its range the application to the tendons of the retardant as such or merely in aqueous solution. Unless the retarder itself provides adequate body when used in aqueous solution, a suitable thickening agent such as carboxy methyl cellulose, agar or other organic Water soluble thickening agent should be added to prevent uncontrollable running or dripping of the coating during application and to permit a coating of proper thickness. Another method comprises the application to the tendons of a retarded concrete or mortar mix or slurry of desired thickness. If desired, the concrete or mortar may be encased in a shell or tube, suitably perforated to permit bonding with the main concrete mass after the tendons have been placed and the main concrete mass has been poured.

However, as indicated, the superior method is to apply the retarder in a resinous emulsion or dispersion or in the form of an alcohol solution of the retarder and a film-forming agent. In either case the resinous component is water insoluble but soluble or destroyed by the alkaline water in the concrete. A thickener and a dye may be added, if needed, to facilitate inspection. Suitable combined thickeners and dyes are the inert pigments such as iron oxide, titanium dioxide and the like. Examples of suitable water insoluble alkali-reactive resins are casein; gums such as shellac, Manila gums and rosin, dissolved in alcohol or alkaline solution; drying oils; synthetic saponifiable resinous compositions such as partially unsaturated modified alkyds, rosin modified alkyds, preferably dissolved in aliphatic or aromatic solvents; and the like.

In actual practice, the concrete retardant resinous emulsion or other retardant coating is coated onto the wound cable prior to any initial positioning of the cable on the job. The resinous emulsion is impervious to normal water and therefore will protect the cable for many days against rust and corrosion from the elements. On the other hand, the emulsion will disintegrate in the presence of alkaline water from hydrated cement and, therefore, will not inhibit the action of the concrete retardant when the cables are in use. The dye employed facilitates visual inspection during coating so that all surfaces of the cable will be properly covered and protected. If desired, a conventional corrosion resistant coating can be applied to the cable or rod prior to application of the retardant.

After the cables are positioned and the concrete poured, initial curing of the concrete for fourteen days or so, for example, is sufficient to enable it to withstand the contemplated stress. About ninety days after the tension ing step, the concrete will cure suificiently and the retardant action of the coating will have been completed to the extent that a direct mechanical, frictional or adhesive bond may be elfected between the concrete and the cable. This bond, of course, will improve with age. It is to be understood, however, that the eventual production of a bond is not essential. Indeed, in some cases, depending upon the amount of coating, size of wire, type of concrete mix and the like, true adhesive bond will never be effected, but this will not result in any significant adverse properties.

A specific example of a retardant coating composition in the form of an aqueous solution and dispersion is as follows: Ten grains of tannic acid are added to one gallon of water and to this is added 100 grams of cornstarch and the whole boiled until the starch grains have exploded to form a viscous colloidal paste which may be conveniently applied with a brush or otherwise, as desired.

A specific example of a resinous emulsion or dispersion is as follows: Two pounds of orange shellac gum and two and one-quarter pounds of Manila gum are dissolved in about three and one-half quarts of methyl alcohol and to this is added with agitation a mixture of one-fourth ounce aluminum chloride, six ounces magnesium saccharate and two ounces magnesium chloride in onehalf pint of water. This mix produces about a gallon of product suitable for brush or other application to the tendons in thicknesses of up to three-eighths of an inch.

A further specific example comprises a composition the same as the one in the preceding paragraph, but substituting the sodium salt of tetra-hydroxy adipic acid for the aluminum and magnesium salts and including six ounces of titanium dioxide.

From the foregoing description, it will be evident that by the method of this invention, post-tensioning can be effected without auxiliary material or any foreign matter being left within the concrete. Moreover, the necessity for any grouting is completely avoided and thus the possibilities of entrapped water which could rust or actually destroy concrete in high temperatures is avoided. The entire process is extremely simple and a considerable savings in time and labor can be efiected because of the elimination of various steps heretofore thought necessary in post-tensioning operations.

Having fully described my invention, it is to be understood that I do not wish to be limited to the details set forth, but my invention is of the full scope of the appended claims.

I claim:

A method of pre-stressing concrete comprising the steps of: coating at least one tendon with a thin coat of concentrated retardant including a salt of tetra-hydroxy adipic acid in a resinous emulsion soluble in alkaline water and free of any concrete mix prior to the positioning of said tendon in any concrete mold; transporting the coated tendon to and positioning the coated tendon in a concrete mold; pouring concrete in a desired pattern to cover said tendon, the ends of said tendon being accessible from portions of said pattern between'which tensile stress is to be provided; curing said concrete sufiiciently,

to enable said pattern to retain its shape against compressive forces resulting from tension of said tendon; and thereafter tensioning said tendon by applying pulling forces to said ends.

2. The method of claim 1, including the step of forming said tendon from a plurality of rod-like members to provide a single cable having crevices between the nonengaging peripheral portions of said members, said concentrated retardant penetrating said crevices during said coating step.

References Cited in the file of this patent UNITED STATES PATENTS 1,684,663 Dill Sept. 18, 1928 2,174,051 Winkler Sept. 26, 1939 2,315,895 Cram Apr. 6, 1943 2,582,751 Fitzpatrick Jan. 15, 1952 2,702,424 Bakker Feb. 22, 1955 FOREIGN PATENTS I 777,318 Great Britain June 19, 1957

Claims

1. A METHOD OF PRE-STRESSING CONCRETE COMPRISING STEPS OF: COATING AT LEAST ONE TENDON WITH A THIN COAT OF CONCENTRATED RETARDANT INCLUDING A SALT OF TETRA-HYDROXY ADIPIC ACID IN A RESINOUS EMULSION SOLUBLE IN ALKALINE WATER AND FREE OF ANY CONCRETE MIX PRIOR TO THE POSITIONING OF SAID TENDON IN ANY CONCRETE MOLD; TRANSPORTING THE COATED TENDON TO AND POSITIONING THE COATED TENDON IN A CONCRETE MOLD; POURING CONCRETE IN A DESIRED PATTERN TO COVER SAID TENDON, THE ENDS OF SAID TENDON BEING ACCESSIBLE FROM PORTIONS OF SAID PATTERN BETWEEN WHICH TENSILE STRESS IS TO BE PROVIDED; CURING SAID CONCRETE SUFFICIENTLY TO ENABLE SAID PATTERN TO RETAIN ITS SHAPE AGAINST COMPRESSIVE FORCES RESULTING FROM TENSION OF SAID TENDON AND THEREAFTER TENSIONING SAID TENDON BY APPLYING PULLING FORCES TO SAID ENDS.