US3422586A

US3422586A - System for post-stressing concrete slabs,beams or other structures

Info

Publication number: US3422586A
Application number: US549637A
Authority: US
Inventors: Domenico Parma
Original assignee: Individual
Current assignee: Individual
Priority date: 1966-05-12
Filing date: 1966-05-12
Publication date: 1969-01-21
Anticipated expiration: 1986-01-21

Description

D. PARMA SYSTEM FOR PosT-sTREssING CONCRETE Jan. 21, 1969 SLABS, BEAMS OR OTHER STRUCTURES Sheet of 5 Filed May 12, 1966 NW Nm.

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3,422,586 CRETE UREs Jan. 21,v 1969 D. ARMA v SYSTEM FOR Po sTREssING coN sLABs BEAMs oa OTHER sTRucT Filed May 12, 1966 Jan. 21, 1969 D. PARMA SYSTEM FOR PosT-sTEEssING CONCRETE SLABS, BEAMS OR OTHER STRUCTURES sheet Filed May l2, 1966 fire. 7

United States Patent O 6 Claims ABSTRACT OF THE DISCLOSURE A post-stressing system embedded lin a concrete structure comprising a doubled tension member, a sheath received around said tension member, a looped end p-ortion on said tension member and projecting from one end of said sheath, a first grout tube connected to the same end of said sheath and extending beyond said looped end portion, an end fitting connected to the opposite end of said sheath, a mass of concrete embedding said sheath and said looped end portion, as well as a portion of said end fitting and a .portion of said grout tube, said looped end portion effectively dead anchoring said tension member in said concrete mass, said tension member having a pair of free end portions extending out of said sheath through said end fitting and disposed outside said concrete mass for use in post-stressing said tension member, wedging means for anchoring said free end portions to said end fitting, and a second grout tube connected to said end fitting, said first grout tube affording preliminary support for said sheath and said tension member during the casting of said concrete mass, said grout tubes being usable for injecting grout into said sheath and said end fitting to maintain the tension in said tension member.

This invention relates to a new and improved system or apparatus for developing compressive stresses in reinforced concrete structures, such as slabs, beams or the like, after the structures have been poured and allowed to set, but before the normal working loads are imposed upon the structures.

It is highly advantageous to impose an initial compressive stress upon a concrete structure, because such initial compressive stress makes it possible for the concrete structure to resist greater bending moments without cracking. Moreover, the concrete structure is better able to resist other types of loads which tend to produce tensile stresses in the concrete. In general, ythe initial compressive stress is imposed upon the concrete structure by tensioning the reinforcing rods, cables, or the like, which extend through the structure.

One object of the present invention is to provide a new and improved post-stressing system which is constructed and arranged so that the reinforcing cables or the like may `be post-tensioned from only one end or edge of the concrete structure, without any necessity for access to the opposite end.

A further object is to provide a new and improved poststressing system in which each of the post-stressing cables. or the like is doubled back upon itself so that the free ends of the cable may bebrought out of the concrete structure, while the loop formed by doubling the cable is embedded in the concrete structure, whereby the cable may be post tensioned by applying tractive forces to the free ends ofthe cable.

It is a further object to provide a post-stressing system of the foregoing character, in which a sheath or conduit is provided to cover each of the doubled cables, except for the looped portion of each cable, so that the sheath and the looped portion will become embedded in the concrete, while the portions of the cable within the sheath are free to be tensioned uniformly by the ICC external application of tractive forces to the free ends of the cable.

Another object is to provide a post-stressing system of the foregoing character, in which a wedging arrangement is provided to anchor the free ends of each cable after it has Ibeen tensioned.

A further object is to provide a post-stressing system of the foregoing character, in which an overflow conduit or tube is connected to the end of the sheath, adjacent the cable loop, so as to provide an outlet or vent for cement grout or the like, which is forced into the sheath after the cable has been tensioned, to form a permanent bond between the cable and the sheath, the outlet tube affording positive assurance that the sheath has been filled with grout.

Another object is to provide a post-stressing system which is highly effective and easy to use, yet is highly economical.

Further objects and advantages of the present invention will appear from the following description, taken with the accompanying drawings, in which:

FIG. l is a vertical section showing a post-stressing device to be described as an illustrative embodiment of the present invention, the device being shown in place in a set of forms which are to be used in pouring a concrete beam, slab or other similar structure.

FIG. 2 is a view similar to FIG. l, but showing the device after the concrete has been poured into the forms so as to embed the device.

FlG. 3 is an enlarged longitudinal section taken in a horizontal direction through the post-stressing device of FIGS. l and 2.

FIG. 4 is a fragmentary horizontal section showing the manner in which jacks are employed to tension the cable yof the post-stressing device.

FIG. 5 is a fragmentary horizontal section showing the manner in which a wedge is driven into place between the free ends of the cable to anchor the ends after the cable has been tensioned.

FIG. 6 is a horizontal section showing which concrete grout is forced into the post-stressing device.

FIGS. 7, 8, and 9 are cross-sectional views taken generally along the lines 7 7, 8 8, and 9-9 in FIG. 3.

FIG. l0 is a fragmentary longitudinal section taken generally along the line 1tl10 in FIG. 3.

FIG. ll is a cross-sectional view taken generally along the line 11-11 in FIG. 3.

Considered in greater detail, the drawings illustrate a post-stressing device or system 20 for applying an initial compressive stress to a concrete beam, slab or other structure 22. Only one device is illustrated, but it will be understood that a plurality of such devices may 4be employed in the concrete structure to provide the desired amount and distribution of initial stresses. The location of the post-stressing devices may be similar to the location of reinforcing cables or rods in a conventional concrete structure.

The illustrated post-stressing device 20 comprises a tension element or member 24, which is shown in the form of a iiexible cable or wire rope, but may also take the form of one or more wires or rods. The use of a cable is lpreferred fo its high flexibility and strength.

It will be seen that the cable 24 is doubled back upon itself. The doubling of the cable 24 produces a loop 26 in the cable. The loop 26 is adapted to be embedded in the concrete structure 22 so as to be permanently anchored therein. Anchor rods 27 maybe connected across the loop 26, if desired. Due to the doubling of the cable, the free end portions 28 of the cable are adapted to be brought out at one end or edge of the concrete structure 22.

A sheath or conduit 30 is provided to cover the doubled the manner in sheath of the cable 24, except for the looped portion 26. The sheath 30 is adapted to be embedded in the concrete structure 22. It will be appreciated that the sheath 30 prevents the embedding concrete from coming into contact with the cable 24, except for the looped portion 26 thereof, so that it is readily possible to develop a uniform tensile stress in the portions of the cable within the sheath, after the concrete has been allowed to set. It is preferred that the sheath 3f) be flexible, so that it will be easy to place and manipulate, but the sheath may also be relatively rigid. The sheath is normally made of metal but may also be made of plastics or other suitable materials. The illustrated sheath is of the flexible type which is made by the spiral winding or wrapping of a sheet metal strip. 'I'his type of sheath has sufficient strength so that it will not `be crushed by the weight of the concrete. It will be understood that other types of flexible metal conduit may be employed in the sheath.

Adjacent the free ends 28 of the cable 24, the sheath 30 is provided with an end fitting or housing 32 having a portion 34 which flares outwardly from the sheath to receive a wedge 35. Grooves 39 are formed in the edges of the wedge 35 to receive the free ends 28 of the cable 24. As will be described in greater detail shortly, the wedge 35 is adapted to be driven into the flaring or tapering housing 34 and between the free ends 28 of the cable 24, so as to clamp the free ends of the cable against the housing 34.

The concrete structure 22 is formed by pouring wet concrete into a set of forms 36, which include opposite end walls 3S and 40, as well as side walls 42 and a `bottom Wall 44. The end fitting 32 is adapted to be mounted in a suitable opening 46 formed in one of the end walls 38. The end fitting 32 is formed with a flange 48 for securing the fitting to the end wall 38. The end fitting 32 may be welded or otherwise secured to the sheath 30.

At the looped end of the cable 24, the sheath 30 is provided with an end fitting 50 through which the loop 26 is brought out. The end fitting 50 has a flaring wall 52 which is Welded or otherwise secured to the sheath 30. The larger end of the flaring wall 52 is closed by an end plate or wall 54. The cable 24 passes outwardly through openings S6 in the end wall 54. The flaring of the wall 52 corresponds to the flaring of the cable as it passes into the loop 26.

An overflow or outlet tube or conduit 58 is connected to the end fitting 50 and is adapted to be brought out of the concrete structure 22 at the end thereof opposite from the end through which the free ends of the cable 24 are brought out. As shown, the outlet tube 58 is mounted in an opening 60 formed in the end wall 54 of the end fitting 50.

The tube 58 provides on outlet for cement grout, as will be described in greater detail shortly. In addition, the tube 58 may be employed to support the looped end of the cable 24 in the forms 36, while the concrete is being cast. Thus, as shown in FIG. 1, the tube 58 is secured to the end Wall 40 of the concrete forms 36 and is arranged to extend through a suitable opening 62 in the wall 40, the tube 58 thereby affording preliminary structural support for the sheath 30 and the tension member 24.

The post-stressing device is suspended in the forms 36 in accordance with the position which it is to occupy in the finished concrete structure. In the illustrated construction, the post-stressing device is positioned so as to reinforce the lower portion of the concrete structure, where tensile stresses are most likely to be produced due to bending moments.

With the post-stressing device 20 in place, as shown in FIG. 1, wet concrete is poured into the forms 36 to the desired depth. The concrete flows around the loop 26 and the sheath 3f), but not into the interior of the sheath. Thus, when the concrete is allowed to set and cure, the loop 26 and the sheath 30 are permanently embedded in the cast concrete structure. The

end fittings

32 and 50,

embedded in the cast concrete. The forms 36 may be removed frorn the concrete structure 22, shortly after it has taken its initial set. When the concrete has cured sufficiently to be loaded safely, the cable 24 is tensioned so as to develop an initial compressive stress in the concrete structure. As shown in FIG. 4, the cable may be tensioned by applying tractive forces to the free ends 28 of the cable. The forces may be provided by suitable jacks 66, preferably of the hydraulic type. While one jack may be employed, it is preferred to utilize a pair of jacks, as shown, with a yoke or crossbar 68 between the plungers 70 of the jacks. A clamping device 72 may be employed to connect the free ends 28 of the cable 24 to the yoke `68. The jacks 66 react against the concrete structure 22.

When the jacks 466 are actuated, tractive forces are applied to the free ends 28 of the cable 24 so that both portions of the cable within the sheath 30 are uniformly tensioned. The cable 24 is movable within the sheath 30 so that the tension in the cable will be uniform. The forces exerted by the cables are transferred to the concrete structure 22 by the loop 26, which is embedded in the concrete structure. Thus, the tensile forces in the cable 24 produce a compressive stress in the concrete structure.

To maintain the tension in the cable 24, the wedge 35 is driven between the free end portions 28 of the cable so as to clamp the end portions against the flaring wall 34 of the end fitting 32, as illustrated in FIG. 5. A suitable hammer may be employed to drive the wedge into place.

It is preferred to form a permanent bond between the cable 24 and the sheath 30 by forcing cement grout or the like into the sheath and around the cable. For this purpose, the end fitting 32 is provided with an inlet tube or pipe 76. It will be seen from FIGS. 1 and 2 that the end of the pipe 76 is brought out of the concrete structure.

Thus, after the cable 24 has been tensioned and anchored with the wedge 35, a pump 78, or some other source of cement grout under pressure, may be connected to the inlet pipe 76, as shown in FIG. 6. Cement grout is forced into the pipe 26 until the grout comes out of the outlet pipe 58 at the opposite end of the concrete structure. This gives a positive indication that the cement grout has filled the entire sheath 30 as well as the

end fittings

32 and 50, The provision of the outlet pipe 58 makes it possible to vent all of the air out of the sheath 3f) and the

end fittings

32 and 50. Thus, no air spaces or voids will be left within the sheath or the end fittings. When the cement grout is allowed to set and cure, it forms a solid bond between the cable 24 and the sheath 30, as well as the

end fittings

32 and 50. The grout insures that the tension in the cable will be maintained, even if the wedge 35 should become loosened, due to vibration or other factors.

The operation of the post-stressing system has already been described, but a brief summary may be helpful, As shown in FIG. l, the post-stressing device 20 is mounted in the forms 36 for the concrete product. The end fitting 32 is mounted on one end wall 38 of the forms, while the outlet tube 58 is brought out through the other end wall 40. The post-stressing device 20 is positioned so as to provide the desired reinforcement of the finished con- Crete structure.

Wet concrete is then poured into the forms 36, as shown in FIG. 2. The concrete fiows around the looped portion 26 of the cable, and also around the sheath 30 and the

end fittings

32 and 50. The concrete is allowed to set, and then the forms may be removed. After the concrete has cured sutciently, the cable 24 is tensioned by exerting tractive forces on the free ends 28 of the cable, preferably with the use of hydraulic jacks 66. Due to the provision of the sheath 30, the portions of the cable 24 within the sheath are not bonded to the concrete and are free to move within the sheath, so that the cables may be uniformly tensioned. The forces exerted by the cable 24 are applied to the concrete through as well as a portion of the outlet conduit 58, are also the loop 26, which is embedded in the concrete. The wedge 35 is driven into place within the end fittings 32 so as to clamp the cable ends 28 against the tapering wall 34. Thus, the free ends of the cable are anchored so as to maintain the tension in the cable.

'Cement grout is then forced into the sheath 30 and the

end fittings

32 and 50 through the inlet pipe 76, as shown in FIG. 6. The air displaced by the cement grout is vented through the outlet pipe 58. The introduction of the grout is continued until it iiows freely from the outlet pipe 58. Positive assurance is thus given that the sheath and the fittings have been filled with grout, After the grout has set and cured, it forms a solid, permanent bond between the cable 24 and the sheath 30, so that the tension in the cable will be maintained even if the wedge 35 becomes loosened.

It will be recognized that the post-stressing system of the present invention is highly effective and dependable. Nevertheless, it is easy to use and low in cost.

It will be understood that various modifications, alternative constructions and equivalents may be employed, without departing from the true spirit and scope of the invention, as exemplified in the foregoing description and defined in the following claims.

I claim:

1. A post-stressing device for a concrete structure, comprising the combination of a hollow tubular sheath,

an elongated tension member extending through said sheath and slidable therein,

a hollow end fitting connected to one end of said sheath,

said tension member having a dead anchoring end portion projecting out of the opposite end of said sheath and adapted to be embedded in the concrete structure,

said opposite end of said sheath being remote from said hollow end fitting,

said tension member having a free end portion extending out of said sheath through said hollow end tting for post-stressing the tension member,

an anchoring device for selectively anchoring said free end portion of said tension member to said endA fitting,

and a grout tube connected to said sheath and communicating therewith at said opposite end thereof adjacent said dead anchoring end portion of said tension member,

said grout tube having a structural connection to said sheath,

said grout tube extending along and beyond said dead anchoring end portion to afford preliminary structural support for said sheath and said tension member during the casting of the concrete structure,

said grout tube affording means for grouting the interior of said sheath subsequent to the post-stressing of said tension member,

a hollow end fitting connected to one end of said sheath,

said opposite end of said sheath being remote from said hollow end fitting,

said tension member having a free end portion extending out of said sheath through said hollow end fitting for post-stressing the tension member,

an anchoring device for selectively anchoring said free end portion of said tension member to said end fitting,

said grout tube having a structural connection to said sheath,

said grout tube extending along and beyond said dead anchoring end portion and affording preliminary structural support for said sheath and said tension member during the casting of the concrete structure,

said grout tube affording means for grouting the interior of said sheath subsequent to the post-stressing of said tension member.

2. A device according to claim 1,

in which said dead anchoring end portion comprises a looped end portion of said tension member,

said tension member being doubled back upon itself to form said looped end portion.

3. A device according to claim 1,

including a second hollow end fitting connected between said sheath and said grout tube,

said dead anchoring end portion of said tension member extending out of said sheath through said second end fitting.

4. A device according to claim 1,

including a second grout tube connected to said first mentioned end fitting.

S. A device according to claim 1,

in which said anchoring device comprises wedging means for clamping engagement with said free end portion of said tension member.

6. A device according to claim 1,

said dead anchoring end portion comprising a looped end portion of said tension member extending out of said sheath through said second end fitting,

References Cited UNITED STATES PATENTS 3,293,811 12/1966 Rice 52--223 FOREIGN PATENTS 225,759 12/ 1956 Australia. 774,546 5 1957 Great Britain. 939,124 4/ 1948 France. 192,006 10/ 1964 Sweden.

FRANK L. ABBOTT, Primary Examiner.

JAMES L. RIDGILL, I R., Assistant Examiner.

U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,422,586 January 2l, 1969 Domenico Parma It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, after line 75 insert as well as a portion of the outlet conduit 58, are also Column 5, line l, cancel "as well as a portion of the outlet conduit 58, are also 7 line 58, the comma should be a period;

9, beginning with "a hollow end" cancel all to and including "said line 5 in column 6, line 2l.

tension member.

Signed and sealed this 17th day of March l970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, IR.

Edward M. Fletcher, Ir.

Commissioner of Patents Attesting Officer