US3903222A

US3903222A - Method for producing prestressed concrete

Info

Publication number: US3903222A
Application number: US460062A
Authority: US
Inventors: Jr Patrick F Brown
Original assignee: Individual
Current assignee: Individual
Priority date: 1974-04-11
Filing date: 1974-04-11
Publication date: 1975-09-02
Anticipated expiration: 1992-09-02

Abstract

The invention consists of an improved method for producing prestressed concrete products wherein the improvement comprises the utilization of the concrete itself to provide the prestressing force which must be exerted on reinforcing wire strands used in the manufacture of said prestressed concrete products. The reinforcing wire is fed into and attached to the first mold in a train of molds and then through each of the following molds in series. The molds have fluent concrete inserted therein as they are guided along an endless track to a point at which the concrete has hardened and whence said molds move by force of gravity down an incline. The hardened concrete in the initial molds descends said incline at a computercontrolled rate whereby a calculated stress is imparted to the reinforcing wire strands in the subsequent molds containing fluent concrete. The calculations are based upon the force supplied by the weight of the descending molds containing hardened concrete and the rate at which said hardening occurs. The stress upon said reinforcing wire is gradually and evenly transferred from the wire to the concrete as the molds descend the incline whereupon, when the train of molds reaches a point at which the calculated stress is nulled, said reinforcing wire strands are severed at the point of entrance into said mold. A continuous production cycle may thereby be maintained with the subject invention whereby an economic advantage is obtained over prior art techniques.

Description

United States Patent [191 Brown, Jr.

[4 1 Sept. 2, 1975 METHOD FOR PRODUCING PRESTRESSED CONCRETE [76] Inventor: Patrick F. Brown, Jr., 113

Columbia Dr., Oak Ridge, Tenn. 37830 22 Filed: Apr.1l, 1974 [21] Appl. No.: 460,062

[52] US. Cl. 264/40; 264/228', 264/297; 264/333; 425/111 [51] Int. Cl. B28B 23/04 [58] Field of Search 425/1 1 l; 264/40, 228, 229, 264/297, 333

[56] References Cited UNITED STATES PATENTS 2,394,227 2/1946 Barber 264/228 X 2,569,231 9/1951 Danhof 264/228 X 2,607,099 8/1952 Schroder et a1. 264/228 X 2,745,164 5/1956 Ros 264/228 X 2,863,206 12/1958 Kirchner 264/228 X 3,055,073 9/1962 Gerwick, .lr. 264/228 X 3,283,385 11/1966 Oakden 264/228 X 3,694,118 9/1972 Crowder 264/228 X FOREIGN PATENTS OR APPLICATIONS 6/1959 United Kingdom 264/228 Primary E.ramt'nerRobert F. White Assistant E.\-aminer-Thomas P. Pavelko Attorney, Agent, or FirmHerman L. Holsopple 5 7 ABSTRACT The invention consists of an improved method for producing prestressed concrete products wherein the improvement comprises the utilization of the concrete itself to provide the prestressing force which must be exerted on reinforcing wire strands used in the manufacture of said prestressed concrete products. The reinforcing wire is fed into and attached to the first mold in a train of molds and then through each of the following molds in series. The molds have fluent concrete inserted therein as they are guided along an endless track to a point at which the concrete has hardened and whence said molds move by force of gravity down an incline. The hardened concrete in the initial molds descends said incline at a computer-controlled rate whereby a calculated stress is imparted to the reinforcing wire strands in the subsequent molds containing fluent concrete. The calculations are based upon the force supplied by the weight of the descending molds containing hardened concrete and the rate at which said hardening occurs. The stress upon said reinforcing wire is gradually and evenly transferred from the wire to the concrete as the molds descend theincline whereupon, when the train of molds reaches a point at which the calculated stress is nulled, said reinforcing wire strands are severed at the point of entrance into said mold. A continuous production cycle may thereby be maintained with the subject invention whereby an economic advantage is obtained over prior art techniques.

9 Claims, 2 Drawing Figures METHOD FOR PRODUCING PRESTRESSED CONCRETE BACKGROUND OF TI-IE INVENTION The invention described herein relates to prestressed concrete and, in particular, to a method for prestressing reinforcing wire for the continuous production of prestressed concrete. It is an improvement of known methods for forming prestressed concrete articles such as that described in U.S. Pat. Nos. 2,863,206, 3,055,073, 3,694,118, 3,283,385 and 2,607,099.

In the production of prestressed concrete it is customary to feed pretensioned wires into a fixed mold after which fluent concrete is deposited onto and around thewires. Tension is then placed on the wires by means of jacks. After the concrete hardens the highly tensioned wires are severed at their point of entrance'to themold whereupon the stress exerted within the wires is imparted to the concrete. The individual contents of each mold are then removed and the process can then be repeated.

It is not a novel idea to produce continuously prestressed concrete as the alternative to individual production of concrete members. Numerous patents exist for such procedures. Known processes also exist for providing tension upon the reinforcing wires used-in manufacturing prestressed'concrete. These processes depend upon various gripping, grasping or pulling means for engaging the wire and exerting the necessary tension. However, no process is known wherein advantage is taken of the mass and weight of the concrete itself together with the weight of the molds to exert tension upon said reinforcing wire and to thereby contribute to the continuous production of prestressed concrete members as is herein described.

SUMMARY OF THE INVENTION It is the object of this invention to provide an improved method for the mass production of prestressed concrete articles which utilizes a moving train of mold forms on an endless track, said track having an inclined portion included therein whereby such articles as railroad ties, fence posts and small beams may be produced at low unit cost and whereby a gradual transference of stresses from the wire strands to the hardened concrete is accomplished and wherein a gradual reduction of stresses is made upon the wire between mold forms subsequent to the hardening of said concrete and prior to the removal of said articles from theirmolds.

It is another object of this invention to provide an ec onomical method for the continuous production of prestressed concrete products wherein the required prestressing force for such production is supplied by the mold train and the concrete products therein.

It is an additional object of this invention to provide an improved method for exerting tension on reinforcing wire used to continuously manufacture prestressed concrete products which does not depend upon complicated gripping, grasping or pulling means for the engagement of the wire.

It is a still further object of this invention to provide an improved method for the continuous manufacture of prestressed concrete products which will utilize the topography of hilly or mountainous terrain and which will thereby minimize the detrimental effe cts on the en vironment which would occur if said terrain was required to be altered.

Further features and objects of the invention will be apparent froman examination of the accompanying drawings which illustrate the invention.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT In general my invention relates to a procedure for the continuous manufacture of prestressed concrete products wherein reinforcing wire strands are continuously fed from wire supply reels, through a-braki'ng mecha nism, then attached within the confines-of the first mold in a train of moving mold forms; fluent concrete is poured therein as the molds are guided along a generally level portion of an endless track to a point at which the concrete becomes hard and whence the molds move by force of gravity down an incline. The hardened concrete in the initial molds descends the incline at a computer-controlled rate whereby a calculated stress is imparted to the reinforcing wire strands in the following molds containing fluent concrete. Said calculations are based upon the tension imparted by the weight of the descending molds and the rate at which the concrete hardens. The latent tension stored within each of the stressed reinforcing wire strands is gradually transferred to the hardened concrete as the molds descend the incline whereupon, when the train of mold forms reaches a point at which the calculated stress is nulled, said reinforcing wire strands are severed thereby providingsaid concrete with an effective tensile strength considerably greater than that originally present.

Conventional means are used to sever the reinforcing wire strands such as by cutting said wires with wire cutters, high speed saws or by means of gas or electric torches. Severance of the molds should take place only after the first mold in the train of molds has reached a horizontal and level portion of the endless track subsequent to reaching the end of the incline or at a point wherein retarders of the type used in railroad humping operations have offset the pulling effect of the moving mold. At this point the first mold in the train of molds is no longer exerting any significant stress on the following molds and the tension which had gradually been imparted to the reinforcing wire strands will essentially all be transferred to the hardened concrete. The concrete products may then be disengaged from their molds by conventional means.

I-Iaving reference to the drawings in detail, 10 represents a continuously moving train of

mold forms

12, 12 moving horizontally in the direction of arrow 14. Mold forms 12 are designated as those containing fluent concrete while mold forms 12 will be considered as having hardened concrete therein. The mold forms 12, 12' can comprise a plurality of shapes according to the purpose for which they may be used as for bridge girders, roof slabs, fence pos ts," railroad ties, structural columns, beams, power poles and the like.

In the preferred embodiment, mold forms 12, 12' are mounted on separate carriages having flanged wheels 16 whereby mold forms 12, 12 can be guided along endless track 18. Any conventional method may be used to initiate movement of the mold forms 12, 12' along track 18 such as by means of endless cog chain 19 or the mold forms 12, 12' may be towed or separately self-propelled by means of an electrical, gasoline, diesel or battery powered engine or track 18 may have a very slight incline leading to the position shown at A whereby friction of flanged wheels 16 on track 18 is offset. It is an important feature of this invention that it is the weight of the molds and their contents which provides the required stress upon the reinforcing wire strands. No propelling force, other than gravity, is therefore required for movement of the train once it has started down the incline.

As the moving train of molds l progresses horizontally in the: direction of arrow 14 a plurality of reels 20 havingreinforcing-wire strands 21 wound thereon feed said reinforcing wire strands 21 into a conventional wire control device 22 such as an electrical regenerative brake whereby braking action is applied to reinforcing wire strands 21 relative to the rate at which mold forms 12, 12 are moving along track 18. Said reinforcing wire strands 21 are physically attached to the first mold in the train of molds and said wire is thence fed in series through each of the following mold forms 12.

Fluent concrete is thereupon admitted to each mold form 12 from concrete hopper 23. It is important in the operation of this invention that the concrete admitted from hopper 23 be set sufficiently to accept transfer of stress from the reinforcing wire strands 21 at the time the mold forms 12 reach position A of FIG. 1. In present commercial practice most of the wires tension is transferred within 24 hours. Thus, in the preferred embodiment, a time span of approximately 1 day would occupy the movement of mold forms 12 from concrete hopper 23 toposition A.

In order to insure that mold forms 12 will proceed at a constant spacing during the concrete hardening process and to prevent longitudinal movement of wire strands 21 relative to the fluent concrete in the mold forms, at least one adjustable spacer bar 24 is secured and locked in place between mold forms 12 containing said fluent concrete. It should be noted that the mechanism for maintaining a constant spacing between mold forms relative to each other is not limited to use of spacer bar 24. It is obvious that numerous known releasable clamping devices are available for performing the function of maintaining said constant spacing between the mold forms. Spacer bar 24 or a similar appropriate constant spacing or clamping means is retained between said mold forms 12 only until the fluent concrete has hardened and the moving train of mold forms begins the descent at position A whereupon said constant spacing or clamping means is removed and the full weight of the moving train of mold forms 12' is ap plied to reinforcing wire strands 21.

While not required in the practice of this invention, it is obvious that a heated curing tunnel 26 through which mold forms 12 pass prior to their descent on incline AB would greatly decrease the time required for the concrete to harden and therefore could'be well justified on economic grounds. i

As shown in FIG. 1, when mold forms 12 reach position A said forms begin a descent covering the incline AB, said incline terminating at B where track 18 again becomes generally level. The weight of mold forms 12 and the contents thereof provides tension upon reinforcing wire strands 21 in mold forms 12 having fluent concrete therein and thereby providesthe prestressing force required for the subject process.

In the preferred embodiment, the inclined portion AB of track 18 should have a pitch of between and 60 from the horizontal and should be of such a length as to provide the calculated force and stress which is required. This force is determined primarily by the mass and weight of the mold forms employed, the concrete therein and the rate at which hardening occurs and is computer-controlled whereby retarding means 27 can be automatically applied to prevent'overstressing of the reinforcing wire strands 21 and thereby maintain a constant preset tension upon said reinforcing wire strands.

Braking retarders 27 of the type used in railroad humping operations can be conveniently used at any point on incline AB to automatically reduce the stress on the wire. It is an important feature of this invention that tension is gradually transferred to the hardened concrete. Thus, as mold forms 12' arrive at position B, track 18 is again generally level and the tension exerted by gravitational forces is correspondingly reduced. It is, therefore, seen that tension has been applied in a gradual manner by slowly increasing the load on the wire strands 21 imbedded in fluent concrete until a calculated constant tension is achieved and subsequently, when the concrete has hardened, tension is then reduced in a gradual way, either by the use of braking retarders 27 or by the arrival of the mold forms 12' to a generally level portion of track 18 at B whereby the gravitational load is lessened.

Control of tension is maintained by a conventional computer programmed to convert the tensionproduced data derived from the effect of gravitational forces on the weight and mass of the concrete and mold forms moving down the inclined zone AB into the braking action of retarders 27 whereby a programmed reduction of stresses is obtained according to the size and strength requirements of the particular article being manufactured. It is obvious that conventional concrete stress tables are of use in performing the calculations which are required prior toprogramming a computer for controlling the moving train of mold forms of the subject invention. When tension and/or the load is reduced in wire strands 21 in the aforedescribed manner of this invention said wire strands 21 may be severed by any conventional means whereby a greater percentage of prestressed force is transferred to the concrete than has heretofore been possible. I

In F IG. 2 is shown a simple and direct method to provide for continuous production of prestressed concrete articles. As the continuously moving train of mold forms 10 moves along endless track 18, tension is supplied to reinforcing wire strands 21 in the fluent concrete in mold forms 12 by means of the weight and mass of hardened concrete in mold forms 12'. When the moving train of mold forms 10 reaches concrete removal station 28 the prestressed concrete products are removed from the mold forms by conventional means suchas by the use of conventional hoist cranes or by the use of mold for ms rotatable along their horizontal axis thereby enabling the product tobe rolled onto a loading platform or loading conveyor. Empty mold forms then proceed under their own power or by means of cog chain. l9from concrete removal station 28 to the point at which wire strands 21 and fluent, concrete from concrete hopper 23 are admitted to begin the cycle anew.

It is seen from the foregoing drawings and specifications that my invention can be utilized to continuously produce prestressed concrete products with none of the complicated grasping, gripping or pulling equipment common to prior art tension producing techniques. It is further seen that my invention will make use of steep terrain such as is commonly found in the hilly Appalachian Mountains or other mountainous regions of the United States and which invention can contribute to the industrial development thereof but which invention is not limited thereto since the inclined track of this invention can be installed and erected in many and varied locations.

It will be understood that variations of the described procedure may be employed without departing from the scope of the invention. For example, instead of using an inclined plane down which the mold forms proceed, a sufficiently heavy weight can be attached to the reinforcing wire strands and suspended over a vertical shaft or cliff whereby the required tension is supplied. Such a system would employ the force of gravity to supply the stress required to stretch the reinforcing wire strands similar to that of the incline of the preferred embodiment. Said weight does not necessarily have to comprise hardened concrete or solid material but may consist, for example, of a container holding water or other liquid of the necessary weight. It is required only, in this instance, that control of tension be maintained by a computer programmed to convert the tension-produced data derived from the effect of gravitational forces on the mass of the concrete and mold forms moving downwardly into the braking action of retarders while simultaneously and automatically relating said data to control of reinforcing wire strands through a wire control device whereby a programmed reduction of stresses is obtained according to the size and strength requirements of the particular article being manufactured and that such a system be installed on an endless track as is taught by this invention.

The method of my invention is thus seen to be a procedure for producing prestressed concrete products through the utilization of gravitational forces produced when the concrete or a similarly placed weight attached to the reinforcing wire strands is caused to roll or fall under controlled conditions from a higher to a lower position with respect to the center of the earth.

What is claimed is:

1. An improved method for the continuous manufacture of prestressed concrete articles wherein the improvement comprises the simultaneous and progressive steps of:

a. continuously feeding reinforcement strands through a braking mechanism to the first mold in a train of moving mold forms;

b. discharging fluent concrete into said mold forms as the molds are guided along a substantially level portion of an endless track, said endless track having an inclined portion included therein;

c. moving said mold;forms along said substantially level-portion of said endless track to a point at which the concrete becomes hard;

d. advancing said mold forms containing said hardened concrete along said inclined portion exclusively by forceof gravity whereby said mold forms descend said inclined portion thereby loading said reinforcement strands to provide the required stress;

e. transmitting a gradual prestressing force to said hardened concrete by computer-controlled means by gradually reducing the stressing load on said reinforcement strands;

f. correlating the speed of said moving mold forms with the tension on said reinforcement strands whereby control is maintained of the prestressing force transferred to said concrete articles;

g. maintaining constant spacing between said mold forms to a point at which the concrete becomes hard; and

h. gradually reducing the prestressing forces on the strands subsequent to the hardening of said concrete.

2. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein said gradual prestressing force comprises the force supplied by said mold forms having hardened concrete therein and wherein said mold forms advance along said inclined portion exclusively by force of grav ity.

3. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein said computer-controlled means for transmitting said prestressing force to said reinforcement strands comprises a computer programmed to convert tension-producing data created by the weight of said mold cars moving down said inclined portion into the braking action required to maintain a preset tension on said reinforcement strands.

4. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein said gradual reduction of prestressing force is accomplished through the use of a series of brakes of retarders on said inclined portion of said endless track.

5. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein said endless track having an inclined portion included therein utilizes the topography of hilly or mountainous terrain.

6. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein said inclined portion of said endless track forms an angle of between 10 and from the horizontal.

7. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 including utilizing wire supply means for continuously feeding reinforcement strands into said moving mold forms, attachment means for fastening wires to the first mold form in the series and braking means for providing tension upon the mold forms in series until said fluent concrete has hardened and said mold forms have reached said inclined portion.

8. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein utilizing means for correlating the speed of said moving mold forms with the tension on said reinstrands.

9. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 8 wherein said computerized utilization of tensionproduced data and stress tables to automatically and simultaneously control said braking mechanism comprises varying the degree of slope of the incline as performed by said retarding means.

Claims

1. AN IMPROVED METHOD FOR THE CONTINUOUS MANUFACTURE OF PRESTRESSED CONCRETE ARTICLES WHEREIN THE IMPROVEMENT COMPRISES THE SIMULTANEOUS AND PROGRESSIVE STEPS OF: A. CONTINOUSLY FEEDING REINFORCEMENT STRANDS THROUGH A BARKING MECHANISM TO THE FIRST MOLD IN A TRAIN OF MOVING MOLD FORMS, B. DISCHARGING FLUENT CONCRETE INTO SAID MOLD FORMS AS THE MOLDS ARE GUIDED ALONG A SUBSTANTIALLY LEVEL PORTION OF AN ENDLESS TRACK, SAID ENDLESS TRACK HAVING AN INCLINED PORTION INCLUDED THEREIN, C. MOVING SAID MOLD FORMS ALONG SAID SUBSTANTIALLY LEVEL PORTION OF SAID ENDLESS TRACK TO A POINT AT WHICH THE CONCRETE BECOMES HARD, D. ADVANCING SAID MOLD FORM CONTAINING SAID HARDENED CONCRETE ALONG SAID INCLINED PORTION EXCLUSIVELY BY FORCE OF GRAVITY WHEREBY SAID MOLD FORMS DESCEND SAID INCLINED PORTION THEREBY LOADING SAID REINFORCEMENT STRANDS TO PROVIDE THE REQUIRED STRESS, E. TRASMITTING A GRADUAL PRESTRESSING FORCE TO SAID HARDENED CONCRETE BY COMPUTER-CONTROLLED MEANS BY GRADUALLY REDUCING THE STRESSING LOAD ON SAID REINFORCEMENT STRANDS, F. CORRELATING THE SPEED OF SAID MOVING MOLD FORMS WITH THE TENSION ON SAID REINFORCEMENT STRANDE WHEREBY CONTROL IS MAINTAINED OF THE PRESTRESSING FORCE TRANSFERRED TO SAID CONCRETE ARTICLES, G. MAINTAINING CONSTANT SPACING BETWEEN SAID MOLD FORMS TO A POINT AT WHICH THE CONCRETE BECOMES HARD, AND H. GRADUALLY REDUCING THE PRESTRESSING FORCES ON THE STRANDS SUBSEQUENT TO THE HARDENING OF SAID CONCRETE.

2. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein said graDual prestressing force comprises the force supplied by said mold forms having hardened concrete therein and wherein said mold forms advance along said inclined portion exclusively by force of gravity.

6. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein said inclined portion of said endless track forms an angle of between 10* and 60* from the horizontal.

8. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 1 wherein utilizing means for correlating the speed of said moving mold forms with the tension on said reinforcement strands comprises computerized utilization of tension-produced data and stress tables to automatically and simultaneously control braking mechanisms linked to said mold forms and said reinforcement strands.

9. An improved method for the continuous manufacture of prestressed concrete articles as recited in claim 8 wherein said computerized utilization of tension-produced data and stress tables to automatically and simultaneously control said braking mechanism comprises varying the degree of slope of the incline as performed by said retarding means.