US3207829A

US3207829A - Means and method for tensioning wire

Info

Publication number: US3207829A
Application number: US86689A
Authority: US
Inventors: Jr Allen J Nieber; Arnold S Rosner
Original assignee: Flexicore Co Inc
Current assignee: Flexicore Co Inc
Priority date: 1961-02-02
Filing date: 1961-02-02
Publication date: 1965-09-21
Anticipated expiration: 1982-09-21

Description

Sept. 21, 1965 A. J. NIEBER, JR.. ETAL 3,207,329

MEANS AND METHOD FOR TENSIONING WIRE 6 Sheets-Sheet 1 Filed Feb. 2, 1961 Sep 1965 A. J. NIEBER, JR., ETAL. 3,207,829

MEANS AND METHOD FOR TENSIONING WIRE 6 Sheets-Sheet 2 Filed Feb. 2, 1961 Se t. 21; 19% A. J. NIEBER, JR, ETAL 3,207,829

MEANS AND METHOD FOR TENSIONING WIRE Filed Feb. 2, 1961 6 Sheets-Sheet 3 m a I m bdA/vzaldd 20.57281 3 $9 mfinQ [a 12mmama Sept. 21, 1965 A. J. NIEBER, JR.. ETAL 3,207,829

MEANS AND METHOD FOR TENSIONING WIRE 6 Sheets-Sheet 4 Filed Feb. 2. 1961 fill l ll Se t. 21, 1965 A, J. NIEBER, JR. ETAL 7,

MEANS AND METHOD FOR TENSIONING WIRE Filed Feb. 2, 1961 6 Sheets-Sheet 5 ma Aiwaiddpasrzefi PM L twiiwgy Sept. 21, 1965 A. J. NIEBER, JR., ETAL 3,207,829

MEANS AND METHOD FOR TENSIONING WIRE 6 Sheets-Sheet 6 Filed Feb. 2, 1961 MN \HQW N6 M H 1 1 V United States Patent 3,207,829 MEANS AND METHOD FOR TENSIONIN G WIRE Allen J. Nicher, in, Livonia, Mich, and Arnold S. Rosner,

Chicago, 111., assignors to The Flexicore (30., Inc, Dayton, Ohio, a corporation of New York Filed Feb. 2, 1061, Ser. No. 86,639 3 Claims. Cl. 264--228) This invention relates to a means and method for tensioning wire and more particularly for tensioning wire during the manufacture of a prestressed slab of concrete or the like.

Longitudinally reinforced slabs of concrete are widely used and an example of a slab of this character is illustrated in United States Patent No. 2,299,070, issued October 20, 1942. Slabs of this general type are made in various widths, heights and thicknesses and lengths of as much as 30' or 40. A common size of such slab in wide use is 8" x 16'. Such slabs are provided with longitudinal steel reinforcement in the form of simple Wire or rod or stranded wire as desired. For convenience, these reinforcing elements will be referred to as wires. Certain of the wires, if not all, are tensioned and the tension must be maintained by external means until the cast slab is completed and cured, after which the bond of the concrete to the steel will maintain the tension for the life of the slab.

Prestressed slabs are generally disposed in side by side relation. It is important that slabs have similar deflection characteristics when laid in side by side relation. A slab which is A out of a line transverse to a number of slabs is objectionable and results in extra work and expense. An important factor in the design of a slab is camber and a major factor in camber is the tension in the prestressing wires. It is therefore very important in the manufacture of slabs to control the tension in prestressing wires at all times until the concrete is cured. In theory, the tolerance on wire tension should be very small, no more than about 1%. In practice, it is diflicult to hold this limit, without great expense, and as a result tolerances up to are common. To a large degree, this tolerance range is a result of conventional methods of manufacture.

In general, the elongation of a prestressing wire is used, during slab manufacture, as a control factor on the assumption that elongation and wire tension are related to each other in a simple relationship. This assumption does not work out in practice for a number of reasons. One is that wire diameter will vary as much as 5% and in stranded wire, such a cumulative change could result in an effective change in wire diameter of much greater than 5%. Another reason is that the modulus of elasticity may vary from one wire roll to another by 5% or more. In the case of stranded wire the tendency of the wires to alter their twist pitch makes for a much greater than 5% variation in the stress-strain relation. This untwisting tendency is particularly noticeable if high stresses are used.

To stress steel to just below its elastic limit when used in concrete results in the most eificient utilization of the steel. However, it is clear from the above considerations that the elongation control for steel wire is so inaccurate as to make it impossible to manufacture slabs with steel stressed close to the elastic limit. As a result, it has been impossible to use steel most efficiently in conventional manufacturing methods. In addition, it is not easy to compensate for slight dimensional changes in the casting forms due to Wear and tear, as well as the various lengths to which slabs are cast.

Slabs may be cast in molds or forms such as disclosed in the patent referred to above. In such case, a cast slab will have the desired length.

After a mold is set up with tensioned Wires and tubes properly inflated for providing longitudinal passages in a slab, the mold is filled with a concrete mix, tamped in suitable fashion and transported to a steam curing room. Setting up the reinforcing wires to a suitable tension has been time consuming and has constituted a rather expensive step in the entire procedure for manufacturing prestressed slabs.

Various attempts have been made to speed the tensioning steps with indifferent success. As a rule, individual wires are handled separately. Except where nuts are used on threaded end portions of tensioned wires, the tension generating means is usually separate from the tension holding means. Attempts to speed up the tensioning of such wires or rods has generally resulted in inferior products.

Another serious drawback has been due to the fact that after a desired tension in a wire has been obtained .by a tensioning means, the transfer of tension to a holding means has invariably resulted in a marked decrease in tension generally called slip loss. Thus if a wire has a yield point of 190,000 p.s.i., it would be desirable to tension this wire to 170,000 or 180,000 p.s.i. With tensioning means heretofore available, it has been practically impossible to maintain this value accurately when the tension creating means is replaced by a simple tension holding means. As a rule, the action of tension holding means has involved some wire slippage while the holding means takes a grip on the wire upon the transfer of the tension from the tension creating means to the tension holding means. Such slippage has permitted the wire to shorten enough so that in the slab lengths being handled, a serious reduction in tension has resulted. This reduction in tension is not sufiiciently constant to be a reliable factor. Even if such reduction were relied upon, it might be necessary to overstress the wire to compensate for the drop in tension or use the wire less efficiently.

In the new system disclosed herein, the accuracy of stress maintenance during the transition when the stress creating means is disconnected from the tensioned wire is very high and for all practical purposes there is no significant change in the stress of the wire during this transition. In addition, the invention makes it unnecessary to change anything to take account of different slab lengths.

For a complete disclosure of the invention, reference will now be made to the drawings wherein:

FIGURE 1 is a perspective view of the wire tensioning system applied to a Wire in a casting form.

FIGURE 2 is a perspective view on an enlarged scale of "the tensioning jack applied to one wire in a casting form, the drawing also showing two other wires which have previously been tensioned.

FIGURE 3 is a plan view illustrating various steps in connection with tensioning wires in a casting form.

FIGURE 3A is a perspective detail of a slack take-up llnllock for use in maintaining tension until the concrete as set.

FIGURE 4 is a view on broken line 4-4 of FIGURE 3, showing a wire ready for tensioning.

FIGURE 5 is a section on line 5-5 of FIGURE 3.

FIGURE 6 is an elevation partly in section showing the tension jack hooked on for operating on a wire.

FIGURE 7 is a section on broken line 7-7 of FIGURE 3, showing the wire after tensioning; FIGURE 7, however, showing the take-up block positioned on the Wire and adjustable spacer sleeve taking up excess Wire length.

FIGURE 8 shows the parts of FIGURE 7 with the jack being removed from a tensioned wire.

heads, a detailed description will not be given.

FIGURE 9 shows the jack in preparation for hooking on to a wire prior to tensioning, this view being similar to FIGURE 6 but showing the hydraulic cylinder in detail.

FIGURE 10 is a sectional detail showing the jack locating pins in recesses in the live bulkhead.

FIGURE 1'1 is a perspective detail showing the jack base and the detachable coupling yoke.

Referring first to FIGURE 1, a steel casting form, generally indicated by 19, rests upon suitable supports such as, for example, blocks 11 and 12. Casting form 10 may be constructed in a number of ways, one of which is more fully disclosed in the aforementioned patent. Inasmuch as the details of the casting form are not important for the present invention, except at the bulk Instead, a simple casting form is ilustrated having bottom 15, sides 16 and 17 and

bulkheads

18 and 19. As a rule, one of the bulkheads, such as 18, may be made adjustable insofar as location along the casting form is concerned so that slabs having any desired length up to the maximum may be cast. For convenience, bulkhead 18 will be referred to as the dead end.

Bulkhead 19, which may be considered as a live bulkhead, is generally similar to bulkhead 18 in many respects. Thus the two bulkheads have openings 21 for accommodating inflatable tubes to provide longitudinal passages in the slabs. Both bulkheads also have openings for accommodating longitudinal prestressing wires, of which three are shown by way of example. These three

wires

23, 24 and 25 are here illustrated as being disposed near the bottom of the slab. It will be understood, however, that the location of the prestressing wires is immaterial insofar as the present invention is concerned.

Sides

16 and 17 of the casting form carry guide pins 27 and 28 at opposite sides near the top to cooperate with registering openings in the bulkheads for insuring correct position of the bulkheads.

Bulkhead 19 carries adjustable spacer sleeves on the outside thereof through which the tensioning wires will pass. Referring particularly to FIGURES 2 to 8 inclusive, bulkhead 19 has rigidly attached thereto, at each opening for a wire, nut member 30. Each nut member 30 is attached to the bulkhead by bolts 31 and 32 passing through the nut and threaded into the bulkhead. Each nut member 30 is generally concentric with the wire opening through the bulkhead insofar as the threaded nut portion is concerned. Cooperating with each nut member 30 is spacer sleeve 34 having portion 35 externally threaded to extend within and cooperate with the thread in member 30. Spacer sleeve 34 has active end portion 36 presenting a smooth annular bearing surface. Sleeve member 34 may be knurled on the outside or may have a hexagon or similar shape for taking a wrench. However, as a rule, nut member 30 will be adjusted easily by an operator and the nut member can turn freely enough so that no wrench will be required. Nut member 30 and sleeve 34 together form an adjustable spacer assembly.

In addition to the adjustable spacer assemblies carried by live bulkhead 19, the live bulkhead is also provided with

recesses

38 and 39 for each adjustable assembly. Bulkhead recesses 38 and 39 accommodate locating pins 40 and 41 of a jack, to be later described.

A prestressing wire will have one free end portion 43 extending for a few inches beyond live bulkhead 19. The prestressing wire also extends beyond dead bulkhead 18 for a few inches. Thus the prestressing wire has dead end portion 44 passing through dead spacer sleeve 45 and through wire grip 46. Dead spacer sleeve 45 may consist of a short length of heavy pipe having transverse aligned

openings

48 and 49 extending through the pipe wall.

Openings

48 and 49 are large enough to permit applying a torch to the wire for burning off end portion 44 from the body of the wire between the bulkheads. This burning will be accomplished after the tensioned 4 wire has been solidly anchored to the concrete after curing of the concrete.

Wire grip 46 may consist of any one of a number of devices available on the market for tightly gripping a wire. Such devices usually have tapered jaws which are pulled toward each other to grip a wire as the wire is tensioned. Such wire grip devices are usually available as strand chucks. Since they are well known, no detailed description will be given.

Referring now to wire end portion 43 beyond the live bulkhead, this end portion is also gripped by wire grip 51, which may have the same general construction as dead end wire grip 46. However, live end wire grip 51 has rigidly attached thereto in any suitable fashion a generally T-shaped yoke anchor 53 extending away from the wire receiving end of grip 51. It will be noted that wire grip 51 has a blind chamber for receiving wire portion 43, anchor 53 terminating the chamber. In the case of dead end Wire grip 46, Wire portion 44 can pass through the wire grip and project beyond. This will permit any excess wire length to be easily accommodated, although as will be apparent later, changes in wire length will be accommodated at the live end of the bulkhead.

Yoke anchor 53 cooperates with yoke 54 having transverse passage 55 therethrough and jaws 56 accommodating the head of anchor 53. Passage 55 is large enough to permit the head of anchor 53 substantial longitudinal play. Yoke 54 is slidingly disposed upon guide plate 57 which is adjustably supported by bolts 58 on jack base 60. Jack base 60 is a flat steel plate having locating pins 40 and 41 locked in recesses within the edge of the base by set screws 61. Jack base 60 has, at the end remote from the locating pins, rigidly attached thereto end plate 63. End plate 63 carries jack top plate 64 disposed above in the normal position of the jack and generally parallel to base 60. Top plate 64 has a number of eye bolts 65 which may be used in connection with jack supporting harness 66. Supporting harness 66 also engages eye bolts 68 at the cylinder end of the jack, to be described later. Harness 66 is supported from a portable harness frame 70 which may be provided with casters for easy movement. The harness frame is of conventional construction and, as illustrated in FIGURE 1, is made from lengths of pipe and cross bars.

Jack top plate 64 also has handles '72 extending on opposite sides of manual control box 74. The control box contains some electric switches, which will be described in detail later.

Referring back to yoke 54, head portion 76 of the yoke is recessed and tapped to accommodate bolt '79. Bolt 79 is part of a differentially threaded bolt portion 81 having the exterior thereof threaded. Bolt portion 81 is larger in diameter than bolt portion 79 and has a finer thread than has portion 79.

Hexagonal portion 82 will accommodate a wrench. The two threads have slightly different pitches and may be both right handed. Thus one turn of the differential bolt will result in a slight movement between yoke 54 and a ram to be described.

Bolt portion 81 cooperates with threaded recess 84 at the end of ram 85. Rigidly supported from jack plate 63 is hydraulic cylinder 86 having

end plates

87 and 88 respectively. End plate 87 of the cylinder is rigidly attached to jack plate 63 and these two plates are provided with openings through which ram 85 may pass. It is understood that ram 85 is a piston rod and is slidable into and out of cylinder 86 through suitable glands for providing a fluid tight seal.

End plates

87 and 88 have suitable couplings to permit the introduction of fluid or liquid into the cylinder. Coupled to ram 85 is piston 90 which can operate within cylinder 86. Pressure on one side or the other side of piston 90 will result in movement of the piston and consequent movement of the ram and coupled attachments.

A brief description of the steps in tensioning a wire will now be given. Assuming that the casting form is set up as illustrated, for example, in FIGURE 1 and assuming that the prestressing wires are disposed within the casting form, the ends of each length of prestressing wire beyond the bulkheads will be prepared so that they are in the condition illustrated in FIGURE 4, for example. Dead end 44 of the wire will have dead spacer sleeve 45 and will have wire grip 46. Live end 43 of the wire will pass through the adjustable spacer sleeve assembly consisting of members 33 and 34 and will extend into and be caught by live end wire grip 51. Part 34 will be turned to its innermost position as seen in FIG- URE 5. Live end wire grip 51 will have attached thereto yoke anchor 53.

It is understood that in practice, the prestressing wire may be pulled from a reel and first passed through dead end bulkhead 18 and then successively passed through the various parts until the forward end of the wire is in wire grip 51. While the wire is still on the reel and after cutting off the length to be used, the dead end wire grip can be put on and moved up by hand snugly against dead spacer sleeve 45. Then the main wire supply from the reel can be cut off to the left of dead end wire grip 46, as seen for example in FIGURE 5, so that the various parts extend straight out as illustrated, for example, in FIGURE 5.

The jack is now moved into position. First the jack is maneuvered so that yoke anchor 53 can be fitted into space 55 between yoke jaws 56. When this has been accomplished, the jack is lined up so that pins 40 and 41 fit into the appropriate recesses in live bulkhead 19. It is understood that piston 90 in cylinder 86 will generally be in the position where it is near end plate 87.

It is obvious from FIGURE 9 that if piston 90 moves to the left, the yoke at the end of the piston rod will be moved toward the live bulkhead. If piston 90 is moved in the other direction, then the reverse will be true. The buttons on manual control box 74 are provided for the purpose of controlling the direction of movement of the piston. By thus controlling the position of piston 90 in cylinder 86, it will be possible to move the jack into position as shown in FIGURE 2, for example, so that the system is ready to begin tensioning the wire. In this position, the locating pins on the jack base engage the recesses in the live bulkhead. Slack between the yoke jaws and the yoke anchor has been taken up. The differential screw coupling between the yoke and ram can be adjusted so that when the hydraulic system is in normal starting position all lost motion will have been taken up.

The jack is now ready to tension the wire, and accordingly, the appropriate button in control box 74 is kept down. Pressure within cylinder 86 forces piston 90 to the right, as seen in FIGURE 9 for example. The hydraulic system is provided with suitable means so that the desired tension in the wire is obtained smoothly and quickly. Assuming that the desired tension has been obtained, live wire grip 51 will be pulled to the right, as seen in FIGURES 6 and 9 for example, creating a space between wire grip 51 and part 34 of the adjustable spacer assembly.

As a rule, the amount of stretch of the wire for a given length will lie between certain limits for a certain desired tension in the wire. Slack take-up block 92 shown in FIGURE 3A, suitable for the desired stretch, can be used. Take-up block 92 is slotted at 93 to permit the block to he slipped over the wire. Take-up block 92 is provided with alined windows or

apertures

95 and 96 extending transversely of the block to provide access for a torch to burn the wire after the slab has been cured.

With the wire stretched to a suitable value, slack take-up block 92 is slipped over the wire and adjustable spacer sleeve 34 is screwed out to take up excess wire length. The wire is now tensioned to the desired value with the wire grips at both ends of the wire in biting or operating position. With all excess space around the wire between the two grips fully taken up, it is now possible to relax the jack. This is accomplished by reversing the hydraulic system so that piston moves to the left toward the live bulkhead. This motion is enough to cause the yoke to move to the left as seen in FIGURE 6, for example, while the yoke anchor remains stationary.

Space 55 within the yoke will be large enough so that sufiicient clearance will be provided to permit the jack to be pulled away from the bulkhead sufficiently so that the jack may be moved laterally for disengaging the yoke and yoke anchor. The tensioned wire may be left, as illustrated for example in the bottom of FIGURE 3, with assurance that no immediate decrease in tension has occurred. After all of the wires for a casting form have been tensioned to desired values, the entire casting form can be filled with concrete and then moved into a steam room for curing. The only portions of the tensioning system which may remain with the casting form during curing are the wire grips and the dead and adjustable spacer sleeves. After the slab has been properly cured, a torch can be applied through the windows or openings in sleeves 45' and 92 to burn the wire away. Thereupon, the wire grips may be easily separated from the short wire strips and excess wire may be trimmed.

Referring to FIGURE 1, the entire hydraulic system is housed in power cabinet 100 having control panel 101. The hydraulic system within power cabinet ltlt) is connected by one electric power cable 183 running to manual control box 74 and by two hydraulic hose 1% and going to end

plates

87 and 88, respectively, of the hydraulic cylinder. It is understood that hose 104 and 105 will be of the type capable of withstanding considerable pressure.

The adjustable take-up member consisting of two telescoping sleeve members are provided with cooperating threads so that the overall length may be adjusted. It is possible to make the take-up sleeves long enough and provide them with a suificient range of adjustment to make it unnecessary to provide the slotted fixed take-up blocks as illustrated in FIGURE 3A. In such case, the adjustable take-up sleeve can be provided with transverse windows or openings to permit access of a torch to the wire. It is of course possible to make the take-up sleeve of more than two threaded members.

It is also possible to have the fixed take-up block illustrated in FIGURE 3A provided with a threaded end portion for cooperating with threads upon fixed cooperating nut member 30. In such case, take-up block 92 will function as a readily removable portion of the adjustable take-up means for covering the added length of wire between the live bulkhead and the wire gripper. It is clear that block 92, if threaded, would not have complete threads around the block due to the slot in the block. It would therefore be necessary to make the adjustable takeup more massive. It is also possible to have a removable portion fill in the slot in the block and complete the threading thus rendering this block readily removable while maintaining its strength.

It is also possible to have one length of wire from the live bulk head extend to the dead bulk head and be looped around hooks and return to the live bulk head, both ends to be tensioned.

What is claimed is:

1. A facility for manufacturing cored prestressed concrete slabs having a length no greater than about 60 feet, each slab having an integral homogeneous mass of con crete within which are individually tensioned steel strands longitudinally of the slabs within the concrete and bonded thereto throughout the length thereof, said facility ineluding a steel casting form structure having a wall portion through which strands to be tensioned extend to provide extra strand length, a sleeve-like strand chuck adapted to be disposed about a strand at the extra strand length, said strand chuck, when installed and gripping said strand, permitting the strand therein to remain straight,

means for engaging said strand chuck and tensioning said extra strand length, said strand chuck moving with strand elongation away from the casting wall portion, and means disposed between said wall portion of said casting form structure and said strand chuck and physically separate from said chuck and having passages therethrough for loosely accommodating a strand to provide incompressible spacing means about said extra strand length between said chuck and wall portion, said incompressible spacing means including sleeve-like cooperatively threaded members adapted to be turned with respect to each other for telescoping action for length adjustment, said incompressible spacing means also including means for exposing part of the extra strand length between said chuck and said wall portion to strand severing means whereby after strand tensioning and elongation and after adjustment of the length of said spacing means and transfer of the strand tensioning force to said spacing means (resulting from the removal of external strand tensioning means), said casting form and associated incompressible spacing means and strand chucks with tensioned strands may be handled for curing of the casting and thereafter permit strand severance at the exposed extra strand length portion to relieve the tension throughout the extra strand length, said threaded length adjusting means requiring adjustment only when said extra strand length is substantially free of tension and obviating the necessity for using considerable turning force at any time.

2. In the manufacture of prestressed concrete slabs having steel strands longitudinally thereof bonded to the concrete and longitudinally tensioned to a sufficiently high degree and the slabs having such a length that a reduction in strand elongation incident to the strand gripping action of a strand chuck of the type having jaws bearing on a straight length of strand results in a significant drop in strand tension, the steps which comprise: disposing a strand longitudinally of a casting form at a desired position in the casting region with said strand extending beyond said casting form through at least one end of the casting form to provide a strand gripping portion externally beyond said one casting form end, anchoring the strand portion adjacent the other casting form end to permit strand tensioning from said external strand gripping portion, applying a strand chuck of the aforementioned type to a part of the strand gripping portion, tensioning said strand to cause strand elongation and movement of said strand chuck away from said one casting form end until a desired strand tension has been obtained, adjusting the overall length between the chuck gripping part and a stationary form part of a substantially incompressible metallic spacing means loosely disposed about the tensioned strand length externally of the casting form to take up all added strand length without changing the strand tension, said adjusting including the step of relatively rotating two, telescoping, cooperatively threaded metal members disposed over tensioned strand and having an open area exposing said strand, freeing said strand chuck from external force generating means used in strand tensioning to transfer the force for maintaining strand tension to said substantially incompressible spacing means, repeating the strand tensioning procedure for desired slab strands, casting the concrete slab in the casting form, said casting including disposing the entire form and associated incompressible spacing means and strand chuck for each tensioned strand in an environment for curing to establish a bond between tensioned strands and concrete, and after curing, severing each tensioned strand through said open area at a region between the strand chuck and said one end of the casting form whereby said strand chuck is re-usable and said cooperatively threaded members may be easily turned with respect to each other for disassembly.

3. In the manufacture of prestressed concrete slabs having steel strands longitudinally thereof bonded to the concrete and longitudinally tensioned to a sufliciently high degree and the slabs having such a length that a reduction in strand elongation incident to the strand gripping action of a strand chuck of the type having jaws bearing on a straight length of strand results in a significant drop in strand tension, the steps which comprise: disposing a strand longitudinally of a casting form at a desired position in the casting region with said strand extending beyond said casting form through at least one end of the casting form to provide a strand gripping portion externally beyond said one casting form end, anchoring the strand portion adjacent the other casting form end to permit strand tensioning from said external strand gripping portion, tensioning said strand by applying force to a chuck of the aforementioned type positioned on said strand gripping portion to cause strand elongation and movement of said strand chuck away from said one casting form end until a desired strand tension has been obtained, adjusting the overall length between the chuck gripping part and a stationary form part of a substantially incompressible metallic spacing means loosely disposed about the tensioned strand length externally of the casting form to take up strand length without changing the strand tension, said adjusting including the step of relatively rotating two, telescoping cooperatively threaded metal members disposed over tensioned strand and having an open area exposing said strand, freeing said strand chuck from external force generating means used in strand tensioning to transfer the force for maintaining strand tension to said substantially incompressible spacing means, repeating the strand tensioning procedure for desired slab strands, casting the concrete slab in the casting form, said casting including disposing the entire form and associated incompressible spacing means and strand chuck for each tensioned strand in an environment for curing to establish a bond between tensioned strands and concrete, and after curing, severing each tensioned strand through said open area at a region between the strand chuck and said one end of the casting form whereby said strand chuck is re-usable and said cooperatively threaded members may be easily turned with respect to each other for disassembly.

References Cited by the Examiner UNITED STATES PATENTS 2,153,741 4/39 Cobi 25-118 2,688,175 9/54 Billner 25--118 2,728,978 1/56 Birkenmaier.

2,751,660 6/56 Nakonz 25-154 2,761,649 9/56 Woolcock 25429.5 2,799,993 7/57 Stovern et a1 6052 2,804,674 9/57 Long 25118 2,811,950 11/57 Entz 25493 XR 2,824,424 2/58 Sebenick 6052 2,826,800 3/58 Van Buren 25118 2,863,206 12/58 Kirchner 25-118 2,867,884 1/59 Brandt 25118 2,871,554 2/59 Siegfried 25--154 2,897,570 8/59 Carper 251 18 2,921,354 1/60 Pankey et a1. 25118 2,949,655 8/60 Berumen et al. 25--118 2,950,517 8/60 Brickman 25----154 2,965,356 12/60 Cheskin 25-118 3,046,631 7/62 Oliver 25-118 3,107,983 10/63 Brandestini 25-118 FOREIGN PATENTS 206,834 3/57 Australia. 623,857 7/61 Canada. 1,124,587 7/56 France.

ROBERT F. WHITE, Primary Examiner.

MICHAEL V. BRINDISI, Examiner.

Claims

1. A FACILITY FOR MANUFACTURING CORED PRESTRESSED CONCRETE SLABS HAVING A LENGTH NO GREATER THAN ABOUT 60 FEET, EACH SLAB HAVING AN INTEGRAL HOMOGENEOUS MASS OF CONCRETE WITHIN WHICH ARE INDIVIDUALLY TENSIONED STEEL STRANDS LONGITUDINALLY OF THE SLABS WITHIN THE CONCRETE AND BONDED THERETO THROUGHOUT THE LENGTH THEREOF, SAID FACILITY INCLUDING A STEEL CASTING FORM STRUCTURE HAVING A WALL PORTION THROUGH WHICH STRANDS TO BE TENSIONED EXTEND TO PROVIDE EXTRA STRAND LENGTH, A SLEEVE-LIKE STRAND CHUCK ADAPTED TO BE DISPOSED ABOUT A STRAND AT THE EXTRA STRAND LENGTH, SAID STRAND CHUCK, WHEN INSTALLED AND GRIPPING SAID STRAND, PREMITTING THE STRAND THEREIN TO REMAIN STRAIGHT, MEANS FOR ENGAGING SAID STRAND CHUCK AND TENSIONING SAID EXTRA STRAND LENGTH, SAID STRAND CHUCK MOVING WITH STRAND ELONGATION AWAY FROM THE CASTING WALL PORTION, AND MEANS DISPOSED BETWEEN SAID WALL PORTION OF SAID CASTING FORM STRUCTURE AND SAID STRAND CHUCK AND PHYSICALLY SEPARATE FROM SAID CHUCK AND HAVING PASSAGES THERETHROUGH FOR LOOSELY ACCOMMODATING A STRAND TO PROVIDE INCOMPRESSIBLE SPACING MEANS ABOUT SAID EXTRA STRAND LENGTH BETWEEN SAID CHUCK AND WALL PORTION, SAID INCOMPRESSIBLE SPACING MEANS INCLUDING SLEEVE-LIKE COOPERATIVELY THREADED MEMBERS ADAPTED TO BE TURNED WITH RESPECT TO EACH OTHER FOR TELESCOPING ACTION FOR LENGTH ADJUSTMENT, SAID INCOMPRESSIBLE SPACING MEANS ALSO INCLUDING MEANS FOR EXPOSING PART OF THE EXTRA STRAND LENGTH BETWEEN SAID CHUCK AND SAID WALL PORTION TO STRAND SEVERING MEANS WHEREBY AFTER STRAND TENSIONING AND ELONGATION AND AFTER ADJUSTMENT OF THE LENGTH OF SAID SPACING MEANS AND TRANSFER OF THE STRAND TENSIONING FORCE TO SAID SPACING MEANS (RESULTING FROM THE REMOVAL OF EXTERNAL STRAND TENSIONING MEANS), SAID CASTING FORM AND ASSOCIATED INCOMPRESSIBLE SPACING MEANS AND STRAND CHUCKS WITH TENSIONED STRANDS MAY BE HANDLED FOR CURING OF THE CASTING AND THEREAFTER PERMIT STRAND SEVERANCE AT THE EXPOSED EXTRA STRAND LENGTH PORTION TO RELIEVE THE TENSION THROUGHOUT THE EXTRA STRAND LENGTH, SAID THREADED LENGTH ADJUSTING MEANS REQUIRING ADJUSTMENT ONLY WHEN SAID EXTRA STRAND LENGTH IS SUBSTANTIALLY FREE OF TENSION AND OBVIATING THE NECESSITY FOR USING CONSIDERABLE TURNING FORCE AT ANY TIME.

2. IN THE MANUFACTURE OF PRESTRESSED CONCRETE SLABS HAVING STEEL STRANDS LONGITUDINALLY THEREOF BONDED TO THE CONCRETE AND LONGITUDINALLY TENSIONED TO A SUFFICIENTLY HIGH DEGREE AND THE SLABS HAVING SUCH A LENGTH THAT A REDUCTION IN STRAND ELONGATION INCIDENT TO THE STRAND GRIPPING ACTION OF A STRAND CHUCK OF THE TYPE HAVING JAWS BEARING ON A STRAIGHTING LENGTH OF STRAND RESULTS IN A SIGNIFICANT DROP IN STRAND TENSION, THE STEPS WHICH COMPRISE: DISPOSING A STRAND LONGITUDINALLY OF A CASTING FORM AT A DESIRED POSITION IN THE CASTING REGION WITH SAID STRAND EXTENDING BEYOND SAID CASTING FORM THROUGH AT LEAST ONE END OF THE CASTING FORM TO PROVIDE A STRAND GRIPPING PORTION EXTERNALLY BEYOND SAID ONE CASTING FORM END, ANCHORING THE STRAND PORTION ADJACENT THE OTHER CASTING FORM AND TO PERMIT STRAND TENSIONING FROM SAID EXTERNAL STRAND GRIPPING PORTION, APPLYING A STRAND CHUCK OF THE AFOREMENTIONED TYPE TO A PART OF THE STRAND GRIPPING PORTION, TENSIONING SAID STRAND TO CAUSE STRAND ELONGATION AND MOVEMENT OF SAID STRAND CHUCK AWAY FROM SAID ONE CASTING FORM END UNTIL A DESIRED STRAND TENSION HAS BEEN OBTAINED, ADJUSTING THE OVERALL LENGTH BETWEEN THE CHUCK GRIPPING PART AND A STATIONARY FORM PART OF A SUBSTANTIALLY INCOMPRESSIBLE METALLIC SPACING MEANS LOOSELY DISPOSED ABOUT THE TENSIONED STRAND LENGTH EXTERNALLY OF THE CASTING FORM TO TAKE UP ALL ADDED STRAND LENGTH WITHOUT CHANGING THE STRAND TENSION, SAID ADJUSTING INCLUDING THE STEP OF RELATIVELY ROTATING TWO, TELESCOPING, COOPERATIVELY THREADED METAL MEMBERS DISPOSED OVER TENSIONED STRAND AND HAVING AN OPEN AREA EXPOSING SAID STRAND, FREEING SAID STRAND CHUCK FROM EXTERNAL FORCE GENERATING MEANS USED IN STRAND TENSIONING TO TRANSFER THE FORCE FOR MAINTAINING STRAND TENSION TO SAID SUBSTANTIALLY INCOMPRESSIBLE SPACING MEANS, REPEATING THE STRAND TENSIONING PROCEDURE FOR DESIRED SLAB STRANDS, CASTING THE CONCRETE SLAB IN THE CASTING FORM, SAID CASTING INCLUDING DISPOSING THE ENTIRE FORM AND ASSOCIATED INCOMPRESSIBLE SPACING MEANS AND STRAND CHUCK FOR EACH TENSIONED STRAND IN AN ENVIRONMENT FOR CURING TO ESTABLISH A BOND BETWEEN TENSIONED STRANDS AND CONCRETE, AND AFTER CURING, SEVERING EACH TENSIONED STRAND THROUGH SAID OPEN AREA AT A REGION BETWEEN THE STRAND CHUCK AND SAID ONE END OF THE CASTING FROM WHEREBY SAID STRAND CHUCK IS RE-USABLE AND SAID COOPERATIVELY THREADED MEMBERS MAY BE EASILY TURNED WITH RESPECT TO EACH OTHER FOR DISASSEMBLY.