US3055073A

US3055073A - Method and apparatus for the continuous production of prestressed concrete members

Info

Publication number: US3055073A
Application number: US851464A
Authority: US
Inventors: Jr Ben C Gerwick
Original assignee: Ben C Gerwick Inc
Current assignee: Ben C Gerwick Inc
Priority date: 1959-11-06
Filing date: 1959-11-06
Publication date: 1962-09-25
Anticipated expiration: 1979-09-25

Description

Sept- 25, 1962 B. c. GERWICK, JR 3,

METHOD AND APPARATUS FOR THE CONTINUOUS PRODUCTION OF PRESTRESSED CONCRETE MEMBERS- Filed Nov. 6, 1959 j w I.

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BY 2Xmbu ATTORNEY United States Patent 3,055,073 METHOD AND APPARATUS FOR THE CONTINU- OUS PRODUCTION OF PRESTRESSED CON- CRETE MEMBERS Ben C. Gerwick, Jr., Oakland, Calif, assignor to Ben C. Gerwick, Inc., San Francisco, Calif., a corporation of California Filed Nov. 6, 1959, Ser. No. 851,464 6 Claims. (Cl. 25-2) This invention relates to the production of reinforced, prestressed concrete by a continuous process.

Prestressed concrete members are widely used throughout the world, and are available in a great variety of shapes for many purposes (e.g., bridge girders, pilings, building columns and girders, floor and roof slabs, railroad ties, power poles, etc.). As generally employed in the United States, the pretensioned process involves the use of a long bed in which wires or strands are stretched. Abutments four hundred feet or more apart may be employed. Forms are placed about the Wires, and concrete poured inside the forms to embed the wires in the concrete. Upon curing or hardening of the concrete, the forms are removed and the wires released, the wires gripping the concrete by bond to shorten and prestress the concrete.

Where concrete members of predetermined lengths are desired, the general practice is to employ a plurality of forms or molds positioned end to end about the reinforcement. Upon cutting the wires between the units, the separate units may be lifted free and transported to the site for erection.

Some attempts have been made heretofore to produce prestressed concrete members by a continuous process. In every case however, it has been necessary to employ a plurality of mold forms or boxes which are coupled together to form a train. Such system has been used extensively, for example, in the U.S.S.R. In this country, the mold forms are considered expensive and hard to use (requiring continuous attention if proper alignment is to be achieved) and consequently have not found great favor.

In general, it is an object of the present invention to provide a method and means by which a truly continuous production of pretensioned concrete products can be obtained.

Another object of the invention is to provide a procedure of this character in which all materials are supplied at approximately the same point, work is done on the concrete as it moves, and completed concrete products are continuously removed from the process for shipment, storage or use.

Another object of the invention is to provide a pro-- cedure of this character which eliminates the requirement for separate mold forms.

Additional objects and advantages of the present invention will be apparent from the following description of a preferred embodiment, and from the drawings in which:

FIGURE 1 is a view in plan of a system for continuously producing prestressed concrete members in accordance with the invention;

FIGURE 2 is a view in side elevation thereof;

FIGURE 3 is an enlarged view in section along the line 33 of FIGURE 1; and

FIGURE 4 is a like view along the line 4-4 of FIG- URE 1.

Generally stated, my invention is directed to a procedure for the continuous manufacture of prestressed concrete wherein reinforcement strands are passed through the length of a continuously moving mold form, simultaneously with filling of the form with a fresh mix of concrete. The concrete and embedded reinforcement are 3,055,073 Patented Sept. 25, 1 962 supported for movement within the mold form until such time as the concrete is hardened sufficiently to be self-supporting. The hardened concrete is then grasped and pulled out of the mold in the direction of movement to thereby maintain the reinforcing strands under a predetermined desired tension. After it has set, the continuous length of reinforced concrete can be cut to desired lengths, with release of the tension on the reinforcement shortening and prestressing the finished concrete products.

Referring to the drawings, 10 represents a continuously moving mold form which in a typical instance may consist of a bottom endless belt 12 and a pair of endless side belts 14. In the preferred embodiment illustrated, the inner reach of each endless belt is supported within a frame 16 of desired cross sectional configuration. The

belts

12, 14, and the frame 16 together form an elongated mold form, the inner walls of which are continuously moving (i.e., to the left in FIGURES 1 and 2).

Adjacent the inlet end of the mold form, a plurality of reels 18 of reinforcement strands or wire are supported on a common unwinding axle 20. Preferably, brake means 22 are associated with the axle so that the strands 24 being pulled off the reels may be maintained under a predetermined uniform tension. The various strands are fed through a pattern or template 26, through the mold form 10, and thence to the far or discharge end of the production line.

In accordance with the invention, a freshmix of con-' crete is continuously supplied to the moving belts forming the mold 10, filling the same and embedding the strands of reinforcement in moving concrete. As illustrated, the concrete is supplied from a mixing station 30 which may include a batching substation 27 adapted to receive aggregate and cement from the

stockpiles

29 and 31. From the adjacent mixing substation 32, mixed concrete can be moved on a conveyer 33 to a hopper 34 for discharge into the mold form 10. The concrete is preferably a dense, dry mix which is sufliciently self-supporting to fill the mold form approximately to the level of the endless side belts 14. An internal vibrator 36 may be positioned immediately beyond the discharge point to compact the concrete and to eliminate moisture and air pockets. One or more vibratory screeds 38 may be employed to further compact and finish the concrete.

In operation, the

endless belts

12 and 14 forming the mold form travel at a very slow rate to permit hardening or partial setting of the concrete about the reinforcement strands. By Way of illustration, the rate of travel of the belts may be such that concrete delivered from the hopper 34 remains within the form for a period of two or more hours prior to discharge from the far end of the mold. Preferably the individual belts are fabricated of rubber or an elastomer, such as neoprene, to which the concrete material is non-adherent. From the end of the moving mold, the belts return through a cleaning bath (not shown) located underneath the mold.

The partially hardened concrete delivered from the mold 10 is immediately received on a conveyer 40 which may be an endless steel belt, as illustrated in FIGURE 4, or a plurality of transverse roller conveyers, or other suitable means. The conveyer 40 functions to movingly support the concrete as it passes into a curing tunnel 44, where it is subjected to the curing eifect of moist steam. This steam is preferably within the temperature range from about F. to 170 F. and is supplied from a boiler or other suitable source 46 through the manifold 48. Preferably the rate and conditions in the tunnel 44 are such that the concrete is initially exposed to moist air of little or no sensible heat fora period of about two hours. Thereafter the temperature of the concrete is progressively raised to the order of F. in about one hour, with the total curing time in the tunnel ranging from six to twenty hours. During the last hour or so in the tunnel, the concrete may be allowed to cool to atmospheric temperature. The concrete emerging from the curing tunnel 44 is of suflicient strength and hardness to firmly grip the reinforcement strands in bond.

I have found that the concrete leaving the curing tunnel can be positively grasped and advanced against the pull of the braking means 22 to place the reinforcement under desired prestressing tension. As illustrated at the station 50 (FIGURE 4), preferred means for this purpose include a pair of resilient roller elements 51 which frictionally engage the sides of the concrete to exert a constant and continuous longitudinal pull upon both the wires 24 and the concrete. These rollers can be two extremely large pneumatic tires that squeeze the product with suflicient force to provide the necessary frictional engagement. Any suitable means such as the motor 52 and gear train 54, 56 may be employed to rotate the rollers, provided it exerts a longitudinal pull of sufficient magnitude. By way of illustration, a pull of 80 to 200 psi. (corresponding in a typical instance to a total pull of 40 to 100 tons or more) has been found to be satisfactory. The advantage of the resilient roller elements 51 is that they not only exert sufficient longitudinal pull but also permit easy passage of the moving concrete.

It will be understood that other devices to grasp and move the concrete may be employed, for example, continuous or practically continuous jacking devices (inflatable or of other types) which similarly employ friction to advance the concrete. However, the resilient roller means have been found to possess a number of advantages, and are preferred.

"As soon as the continuous concrete body 8 has been engaged by the friction device at 50, it can be subjected to operations designed to sever desired lengths of reinforced concrete from the moving mass. Illustrative of such apparatus is the traveling saw 60, which may be of conventional design. \As will be understood, apparatus of this type travels a short distance with the product during the cutting operation, to insure a square cut.

An alternative to the method just described (e.g. manufacturing a continuous member and then cutting it to length) is to feed dividers into the mold form 10 at proper intervals in the manufacturing process. This could be accomplished .by mounting a stack of the dividers suitably apertured to receive the reinforcement adjacent the template 26, and prior to commencement of operations. The dividers could then be released and fed into the mold form as desired.

The overall operation of the apparatus just described can now be summarized as follows: To initiate operations, reinforcement strands 24 are unwound by a mechanical means from the reels 18 and against the resistance of the braking means 22. In general, it is desirable to carry the strands at least as far as thecuring tunnel 44. Once production is under way, the force for pulling the reinforcement oli the reels is derived from movement of the hardened concrete itself, as effected by the gripping means 51 at the station 40. Cement and aggregate from the stockpiles at station 30 are now batched and mixed and delivered through the conveyor 32 to the hopper 34. From the hopper, the freshly mixed concrete flows into the continuously moving mold form 10 wherein the concrete is confined by the

endless belts

12 and 14 during the preliminary compacting, screeding and finishing operations performed by the

devices

36 and 38. The concrete and embedded reinforcement strands are moved slowly within the mold so that a certain amount of setting or hardening of the concrete occurs before discharge into the curing tunnel 44. In the tunnel 44, the concrete is gradually heated, preferably by moist steam, to a temperature of the order of 160 F. and is maintained at this temperature for a period of time sufiicient to harden and cure the concrete. The concrete emerging from the curing tunnel is immediately grasped by the friction devices 51 cage).

which exerts a constant and continuous longitudinal pull. Thereafter, prestressed sections of reinforced concrete 62 are severed at the station 60, in desired lengths, and can be loaded for shipment or storage as meets construction requirements.

A number of additional variations are possible in the procedure just described without departing from the scope of the invention. For example, if spiral reinforcement is desired, it may be supplied immediately prior to entry of the reinforcement into the concrete, as at the station 64. Apparatus for this purpose may be of conventional design (e.g., a steel cage rotating transversely about the strands 24 to unwind reinforcement from the inside of the Special inserts (for example, as needed in the manufacture of railroad ties) can also be mechanically placed either before or after pouring. A further modification would be to provide blisters or humps on the inside of the

endless belts

12 or 14 so as to provide variations in the cross section of the concrete at regular intervals. External vibration may also be applied, for example, by operation of mechanical vibrators on an external surface of the frame 16. These and other variations are clearly within the skill of one versed in this art.

I claim:

1. A method for the continuous manufacture of prestressed concrete comprising the simultaneous and progressive steps of continuously discharging freshly mixed concrete into an elongated pouring zone, continuously feeding reinforcement strands lengthwise into said pouring zone to embed the same in said concrete, continuously conveying the freshly discharged concrete and embedded reinforcement strands away from said pouring zone with support of said concrete along its bottom and sides until the concrete has set sufficiently to hold said reinforcement strands embedded therein, curing said partially set concrete to form an elongated mass of hardened concrete, continuously frictionally engaging surface portions of said hardened concrete to pull and convey the same as an elongated hardened mass away from said pouring zone, said frictional engagement and resulting conveyance serving to place said reinforcement strands within the freshly mixed concrete under prestressing tension, and severing desired lengths of the elongated hardened reinforced concrete to thereby release the reinforcement strands to shorten and prestress the severed lengths of hardened concrete.

2. In apparatus for the continuous manufacture of elongated p'restressed concrete units, means forming a continuously moving mold form, said means comprising an elongated frame of desired cross sectional configuration, said frame including a bottom and sides, endless belt means reeved through said frame to provide continuously moving bottom and side surfaces of said mold form, means to continuously feed reinforcement strands lengthwise into an inlet end of said continuously moving mold form, means adjacent said inlet end of the mold form to continuously charge a fresh mix of concrete to said inlet end and about said reinforcement strands, means advancing said endless belts at a very slow rate to permit time within the mold for the concrete to partially harden, additional elongated conveyer means adjacent to a discharge end of said mold form to receive and support the partially hardened concrete, means advancing said additional conveyor means at a very slow rate to permit time for further hardening and curing of said concrete, resilient means adjacent to said additional conveyor and spaced a substantial distance from an inlet end thereof to frictionally grasp surface portions of the cured concrete carried by said additional conveyer means, said resilient means being aligned with said additional conveyer means and cooperating with the same to pull the concrete from said discharge end of said mold form, and means to sever desired lengths of concrete and reinforcement strands whereby the latter function to shorten and prestress the severed lengths.

3. Apparatus as in claim 2 wherein said reinforcement strands are supplied through braking means positioned adjacent said inlet end of the mold form and adapted to maintain said strands under predetermined tension.

4. Apparatus as in claim 2 wherein tunnel means are provided about said additional conveyor means and between said moving mold form and resilient friction means, said tunnel means including means to raise the temperature of the moving concrete to a temperature of the order of 160 F. to effect curing of the same.

5. Apparatus as in claim 2 wherein said resilient means comprises resilient roller elements rotatable about vertical axes and adapted to frictionally engage the sides of the hardened, cured moving concrete.

6. Apparatus as in claim 2 including winding means between the inlet end of said mold form and said means supplying reinforcement strands, said winding means wrapping spiral reinforcement about said strands.

References Cited in the file of this patent UNITED STATES PATENTS 1,367,227 Baumgartl -2 Feb. 1, 1921 1,956,967 Upson May 1, 1934 2,607,099 Schroder et al Aug. 19, 1952 2,757,415 Mathues et al. Aug. 7, 1956 2,912,738 Bergling et a1 Nov. 17, 1959 FOREIGN PATENTS 723.009 Great Britain Feb. 2, 1955