US3863891A

US3863891A - Concrete prestressing device

Info

Publication number: US3863891A
Application number: US430874A
Authority: US
Inventors: Constantin Leonte
Original assignee: Institutul Politehnic Iasi
Current assignee: Institutul Politehnic Iasi; INSTITUTUE POLITEHNIC IASI
Priority date: 1971-12-15
Filing date: 1974-01-04
Publication date: 1975-02-04
Anticipated expiration: 1992-02-04

Abstract

A device for the prestressing of concrete and assembly of prestressed concrete structures composed of precast concrete units. It allows the post-tensioned cables to follow paths identical with or similar to those of monolithic structures without needing any scaffolding. The units are assembled at ground level by prestressing the members with which the structure is assembled thereafter. The structure prestressing continuity is realized by connecting the poststressed cables of the adjacent members, by coupling the anchorages and extracting from under the coupled anchorages the devices that have been located there before having prestressed the individual members. In order that tension stresses may not appear at the joints, when assembling, the joint spaces are partially filled with mortar or concrete.

Description

Leonte CONCRETE PRESTRESSING DEVICE [75] Inventor: Constantin Leonte, Iasi, Romania [73] Assignee: Institutue Politehnic Iasi, August,

Romania [22] Filed: Jan. 4, 1974 [21] Appl. N0.: 430,874

Related US. Application Data [62] Division of Ser, No. 208,293, Dec. 15, 1971, Pat. No.

[52] US. Cl 254/29 A [51] Int. Cl ..E21b 19/00 [58] Field of Search 254/29 A, 104

[56] References Cited UNITED STATES PATENTS 2,l70,690 8/1939 Mafera 254/104 3,285,568 11/1966 Biach 254/29 A 3,285,569 11/1966 Marr et al. 254/29 A 3,597,830 8/1970 Yegge 254/29 A X Feb. 4, 1975 Primary Examiner-Othell M. Simpson Attorney, Agent, or.Firm-Karl F. Ross; Herbert Dubno [57] ABSTRACT A device for the prestressing of concrete and assembly ofprestressed concrete structures composed of precast concrete units lt allows the post-tensioned cables to follow paths identical with or similar to those of monolithic structures without needing any scaffolding.

3 Claims, 17 Drawing Figures PATENTEU FEB 4|975 SHEET 10F 5 CONCRETE PRESTRESSING DEVICE CROSS-REFERENCE TO RELATED APPLICATION This application is a division of my copending application Serv No. 208,293 filed l5 Dec. 1971 now US. Pat. No. 3.790,657 issued 5 Feb. I974.

FIELD OF THE INVENTION The invention relates to means for assembling and prestressing resisting structures composed of concrete units.

BACKGROUND OF THE INVENTION It is known to assemble prefabricated concrete units, on scaffolding so that the continuity of channels for post-tensioned reinforcement is assured; the joints between units being filled with mortar,

These procedures present many disadvantages among which the more important are the following: considerable consumption of material, of manual labor and of implements for setting up and taking down the scaffolding; the limitation of transport capacity and immobilization of raising equipment; long assembly times; considerable consumption of high-strength steel because ofthe friction inherent in the use ofpretensioned reinforcement of large length and polygonal or curvilinear configuration; and the difficult execution of reinforcement pretensioning of the whole structure.

There are also known some procedures consisting of assembling by prestressing in two stages of the structures composed of concrete prefabricated units. During the first stage the units are assembled at ground level by prestressing the members (beams, columns etc.) of the resisting structure. During the second stage these members are mounted in a resisting structure, and the continuity of prestressing is achieved by means of cap cables.

These procedures eliminate part of the disadvantages of the first procedures by: eliminating the scaffoldings bearing the resisting structure before its prestressing; the execution at the ground level ofa part of the prestressing operations; and the reduction of tension losses due to friction by decreasing the layout length of pretensioned reinforcements. These systems have, how ever, other disadvantages including the increased consumption of high-strength steel because of a less rational positioning of the pretensioned reinforcements, in order to satisfy the more severe requirements of preventing cracking at transfer ofjoints between concrete units and between the members made of units during the first stage; besides, these have the disadvantage of using the cap cables contributing to an additional consumption of high strength steel, of anchorages, of manual labor, and pretensioning and injection implements etc. Also, the danger of accidents when pretensioning is carried out at high levels is greater by comparison with pretensioning at ground level.

SUMMARY OF THE INVENTION The present invention eliminates the mentioned disadvantages by the fact that, in order to form resisting structures made up of prefabricated concrete units assembled by prestressing with post-tensioned reinforcements, in a first stage the concrete units are assembled at the ground level in prestressed concrete members, by arranging the concrete units end so that the continu- LII ity for reinforcement is assured, allowing the necessary joint spaces to remain between them so as to be filled up with cement mortar subsequently. The cables are pretensioned and blocked by means of steel anchorages of the ring and cone type, after having previously been provided with transfer devices. The latter comprise metallic wedges, cylindrical members with interior hollows for temporary support of the anchorages and cylindrical nuts of the devices that join together the reinforcements.

After prestressing and blocking ofthe reinforcements the spaces between the concrete units are completely filled with mortar.

During the second stage the prestressed members with post-tensioned reinforcements are mounted in a structure by putting them end to end so that the anchorages are juxtaposed with each other and on the same axes, and coupling the reinforcements of the anchorages with connection devices after hardening of the mortar in. the joints. Next the stressing of the connection devices of the reinforcements is carried out by transferring the prestressing forces developed by each cable to the concrete members which have been assembled, from the anchorage levels to connection devices and to the joint spaces between the members by the extraction of the transfer devices from below the anchorages, by means ofa conventional jack. Then follows the injection of mortar and the filling with concrete of the groove in which are found the connection devices after completely filling up with mortar the spaces between members, after the hardening of mortar and after the obtained structure has been loaded with permanent loads.

The devices comprise a cylindrical piece made of steel pipe having an interior diameter of 2 to 3 mm greater than the external diameter of the anchorages, threaded insides at the ends, and having a hole in the lateral wall for checking upon the injection, and two nuts in the shape of cylindrical washers, one threaded on the external lateral surface and one of the frontal surfaces being provided with cylindrical holes or radial notches of rectangular section. A fixed wrench is used for driving the washers, each wrench being equipped with pegs or rectangular ribs on one of their faces, the nuts being placed below the anchorages before pretensioning each reinforcement on cylindrical pieces having an inner hole whose exterior diameter is 2 to 3 mm less than of the inner hole diameter of the nuts.

The coupling of reinforcements in the prestressed concrete members which are to be assembled into the prestressed concrete resistance-structure is made by threadedly interconnecting the cylindrical pieces with the nuts existing behind the anchorages from one of each of the prestressed concrete members, the successive arrangement of prestressed concrete members being carried out so that, during the setting up, the anchorages placed at the end located opposite the connection of the member that is to be mounted go into the cylindrical pieces of patch devices assembled on anchorages at the end opposite the connection of the prestressed concrete member previously mounted. The tightening of the nuts at the back of those anchorages to the cylindrical pieces of the patch devices after mounting all of the prefabricated members, is performed with a strong pressing of patch devices on anchorages by a simultaneous threading of the two nuts of the patch devices by means of two fixed wrenches.

the transfer devices in one construction variant are of parallelepipedic shape with a cylindrical hole having a diameter a bit greater than the diameters of reinforcement pipes, composed of a U-shaped wedge with two slopes placed between two lateral wedges, also of U shape, with one slope only, and of two steel pieces which fill in the spaces between the arms of the U wedges beveled only on one side and which are assembled by means ofa rib system and some flanges as well as by means of two screws which are to be tightened at their thinner frontal part of the middle wedge.

In another construction variant, the transfer devices are made of two wedges which are beveled on two faces and are placed between two steel pieces, each of them being cut by a plane passing through the axes of the reinforcements and beveled only on the inner faces, with cylindrical holes having their axes within the sectioning plane and assembled by means of ribs on the edges of exterior pieces at their inner side and flanges laterally of the middle wedges at their upper and lower parts, as well as by means of screws tightened in one of the central wedges, to pass through the cylindrical holes provided in the other wedge, and are fixed by nuts. The drawing out of the transfer devices, disposed below anchorages, is performed by means of conventional presses. The pistons of the latter have a central longitu dinal cavity. By threading a steel rod into the middle wedge and placing a U-shaped support, with a central cavity of somewhat long shape in its plane of symmetry, on the marginal wedges of the transfer device so that the rod could pass through, the wedge may be withdrawn. Thus the rod passes through and is fixed by means of one nut which is introduced at the free end of the rod and is pressed until it becomes tight against the press piston and then, by unscrewing the assembling screws of the device operating the press by beams of a pump until the middle wedge begins to slide between the marginal wedges of the transfer device. In the second variant, instead of the U-shaped support, two rods are used that pass through the holes of that wedge in which the traction-rod has been threaded, and is supported on the other wedge.

BRIEF DESCRIPTION OF THE DRAWING FIGS. la, lb and represent a schematic presentation of the performing stages of a continuous beam with a polygonal upper sole;

FIGS. 2a, 2b and represent the same for a continuous beam with a linear upper sole;

FIG. 3 represents a view of a connecting device;

FIG. 4 represents a view of a fixed key type necessary to stress the connecting devices on anchorages;

FIGS. 5a and 5b represent two perspective views of a transfer device;

FIG. 6 represents a perspective view of another kind of transfer device; and

FIG. 7 represents six stages (steps) a, b, c, d, e andf of the formation of a connection between two prestressed concrete precast members.

SPECIFIC DESCRIPTION According to the invention, the assembling by means of prestressing with post-tensioned reinforcement l of a structure from units 2,.made from precast concrete, can be carried out in two stages.

In the first stage, the units 2 are to be assembled on the ground over the table 3 in prestressed concrete members, in this instance, in boundary beams (FIGS. la and 2a) and central beams (FIGS. lb and 2b).

In the second stage, the prestressed concrete elements made in the first stage are to be assembled into structures of prestressed concrete. This is done when applying the invention for prestressed concrete continuous girders, by disposing these elements (boundary and central girders) on supports end to end (FIGS. I0 and 20) so that some space should remain between them by the connection of reinforcement l. by the filling up of the space-joints among the beams with mortar or concrete and the achievement of the continuity of the prestressing force in the connected reinforcement I and the connecting zone prestressing.

For assembling, connecting devices of reinforcement and some devices for the transfer of the prestressed forces developed by post-tensioned reinforcement on the prestressed concrete member tops to the connecting devices are necessary.

A connecting device of two post-tensioned cables (FIG. 3) comprises a cylindrical piece 4 of steel pipe, internally threaded at both ends, provided with a hole c in the lateral wall in order to check the injection, and two screw nuts Sin the form of cylindrical washers also formed of steel pipe d, threaded on the outer lateral sides; on one of the front sides the nuts are provided with cylindrical holes e or radial notches having rectangular section. In order to to stress the screw nuts 5, a key or wrench 6 is used (FIG. 4), provided on one side with cylindrical pegsfor with ribs having a rectangular section.

A transfer device (FIGS. 5a and 5b) is formed with a central wedge 7 of U-shape, having two slopes, disposed between two lateral wedges 8 (also U-shaped.) having only one slope. They form a rectangular parallelipiped with a central cylindrical hole g by addition of two metallic pieces 9 at the lateral wedge arms 8.

The assembling of the device is facilitated by ribs h disposed on the elements 8 and of some flanges i with channels j corresponding to the ribs h, disposed on the upper part and on the lower part of the wedge 7 as well as by means of two screws 10 which are threaded to the thinner front part in the arms of the wedge 7.

In another constructional form (FIG. 6), the transfer device comprises two U-shaped wedges 11 having two slopes, arranged between the steel elements 12 having slopes only to the wedges 11, and forming together a rectangular parallelepiped having a central cylindrical hole k; the elements 12 are sectioned along a plane passing through the transfer device along the axis of the cylindrical hole k. Assembling is made possible by means of ribs 1 disposed at the inner borders of the elements l2 and flanges m provided by the channels n cor.- responding to the ribs I disposed at the upper and lower parts of the wedges 11. The screws 13 are stressed in one of the wedges 11, pass by cylindrical holes formed in the other wedges and are fixed by means of screw nuts 14.

The assembling in the first stage of the units 2 of prestressed concrete elements is carried out as follows.

The concrete precast units making up a prestressed concrete member are set end to end on a platform 3, so that the continuity of each longitudinal channel p for post-tensioned reinforcements is assured and the spaces r remain in the range of I to 2 mm.

The spaces between units are partially filled with cement mortar (FIGS. la, lb and FIGS. 2a, 2b) to a depth such that each of the cross-sections of the partially filled up spaces should have one of the central nucleus limits, the nearest to the post-tensioning reinforcement, at the level of the application point of the prestressed force at transfer (taking into account the loading effect from member eigenweight, when required).

After the mortar hardens in the spaces, the cables 1 are introduced into channels for prestressing.

At the ends that emerge from the channels (FIG. 7a) the cables are coated with pieces of thin pipe 15, with their external diameter equal to the inner diameter of the channels p which are fixed in the concrete by inserting them into the channels p to an adequate depth.

On each ofthe fittings is placed a transfer device, the central cylindrical hole g of which has a diameter equal to the washer channel diameter p and, also, a thick washer 16 made of thick wall steel pipe, that may be made up from two semicylindrical pieces; over each washer 16 there must be introduced a nut 5 ofthe cable connecting devices.

Then there are applied the steel anchorages of ring 17 and a double cone 19. It is desirable that the anchorage rings 17 be located by means of a template fixed at the ends of the beam, and maintained as long as the pretensioning operation of all the cables continues.

Then, the cable prestressing and anchoring with metallic ring-cone anchorages l7 and 19 is carried out.

After prestressing and anchoring, the wire ends are bent and cut so that they do not exceed the outward edge of the rings 17 of the anchorages.

Next the spaces between the units are completely filled up with cement-mortar.

With the prestressed concrete members made up in the first stage, having post-tensioning reinforcements without any bond, some resistive structures are assembled in a second stage into continuous prestressed beams, as follows.

One of the border beams is transported and is placed on supports 20. At the time of the preparation for assembling, or after assembling on supports, the anchorages of the next span of the continuous beams are covered with cylindrical pieces 4 of the connection devices which are threaded onto the nuts 5 from the back side of the anchorages (FIG. 7b). Considering the possible operating errors, the inner diameter of the cylindrical pieces 4, the external diameter of nuts 5 respectively must be 2 to 3 mm bigger than the external diameter of the rings 17 of the anchorages.

Then the beam for the next span is brought up and placed on supports so that its anchorages enter the cylindrical pieces of the connection devices from the end of the previously assembled beam (FIG. 76); between the two beams a space of l 2 cm remains.

Next the nuts 5 from the back side of the anchorages of the second member mounted into the structure are threaded onto the cylindrical pieces of the connection devices and by means of two wrenches 6, the nuts 5 of the connection devices (FIG. 7d) are tightened as much as is practically possible on the anchorages.

In the same way, successively the other elements of the continuous beam are assembled.

Thereafter the joint spaces s between the beams are partially filled up with cement mortar (with high initial strength, preferably expansive) such that each of the cross-sections of the partially filled up spaces has a central nucleus limit nearest to the post-tensioned cable connection device at the gravity center level of the connection device cross-section.

After the mortar has hardened in the spaces, the extension cable connection devices, and simultaneously the assembling zones between the precast prestressed concrete members, are prestressed by extraction of the transfer devices from under the anchorages, to provide the continuity of the prestressing force in the connected cables at the joining levels.

The drawing out of the transfer devices as shown in FIG. 5 is carried out as follows.

A steel rod 21 is threaded into wedge 7.

After that, on the wedge ends 8 is places a support 22 of U shape, with a hole I lengthened along its symmetry axis so that the rod 21, which is supporting it. passes through this hole t.

Then, a hydraulic jack 23 with a piston 24, with a central hole u, is mounted on support 22 so that rod 21 passes through the axis of the jack and the nuts 25 are tightened onto the free end of the rod 21.

The screws 10 of the transfer device are unscrewed and the hydraulic jack 23 is operated until the wedge 7 starts to slip and then is drawn out. At that moment the prestressing force in the cable beneath which the anchorage transfer device has been pulled out, is transmitted to the cable with which it is coupled by means of the connection device, while the prestressing force developed by the two connected cables is transfered from the anchorages to the whole span of the prestressed concrete members that are to be assembled. After drawing out the wedges 7, the wedges 8 and 9 of the transfer devices (FIG. 7f) are also taken out together with the washers 16 when they are made up from two pieces.

After drawing out of the transfer device, the joint spaces s between elements are filled up completely with initial high strength cement mortar, preferably mixed with an expanding cement.

After the mortar hardening and after loading with permanent loads the prestressed concrete structure that is so made up the channel is tightened at the joining zones; then the channels are injected with cement mortar and the inner spaces of the connection devices are cast-in-place with concrete at the grooves in which the connection devices are provided; the following operations have been previously achieved: the wrapping with networks of steel bars of the connection devices, the joining by superposition or by welding of the joining bars of mild steel in the grooves, and the assembly of stirrups on their lengths.

I claim:

1. A device for stressing a beam having a longitudinally extending throughgoing cable projecting from said beam at least at one end thereof, said device comprising:

a wedge assembly surrounding said end of said cable and laterally removable therefrom, said assembly including a pair of oppositely converging wedge members and means relatively displaceable transversely to said cable to spread said members, one of said members bearing against said beam;

a cable clamp fixed to said end of said cable beyond said wedge assembly;

a sleeve interposed between said cable clamp and the other of said members and in axial forcetransmitting relationship therewith;

an externally threaded ring mounted rotatably on said sleeve and having an external diameter in excess of that of said cable clamp and an internal diameter less than that of said cable clamp; and

an internally threaded pipe for joining said beam to a similar beam having a similar ring by threading said rings into opposite ends of said internally threaded pipe.

2. The device defined in claim 1 wherein said wedge assembly comprises a pair of U-shaped blocks having mutually parallel outer faces and shanks straddling said cable, the mutually confronting flanks of said shanks converging toward the bights of said blocks, said blocks forming said members; a U-shaped wedge received between said flanks and having legs straddling said cable, said legs having outer flanks converging away from the bight of the wedge and slidingly engaging said flanks, said blocks and said wedge together having a rectangular parallelepipedal configuration with a throughgoing bore defined between said shanks and said legs; and means for displacing said wedge relative to said blocks.

3. The device defined in claim I wherein said members each include a pair of blocks having confronting semicircular recesses, said pair of blocks having mutually parallel planar outer faces and mutually confronting inner faces, said wedge assembly further comprising a pair of wedges received between said inner faces and provided with means preventing separation of the blocks of each pair. and means for drawing said wedges together, said wedges and said blocks together having a generally rectangular parallelepipedal configuration. i

Claims

1. A device for stressing a beam having a longitudinally extending throughgoing cable projecting from said beam at least at one end thereof, said device comprising: a wedge assembly surrounding said end of said cable and laterally removable therefrom, said assembly including a pair of oppositely converging wedge members and means relatively displaceable transversely to said cable to spread said members, one of said members bearing against said beam; a cable clamp fixed to said end of said cable beyond said wedge assembly; a sleeve interposed between said cable clamp and the other of said members and in axial force-transmitting relationship therewith; an externally threaded ring mounted rotatably on said sleeve and having an external diameter in excess of that of said cable clamp and an internal diameter less than that of said cable clamp; and an internally threaded pipe for joining said beam to a similar beam having a similar ring by threading said rings into opposite ends of said internally threaded pipe.

3. The device defined in claim 1 wherein said members each include a pair of blocks having confronting semicircular recesses, said pair of blocks having mutually parallel planar outer faces and mutually confronting inner faces, said wedge assembly further comprising a pair of wedges received between said inner faces and provided with means preventing separation of the blocks of each pair, and means for drawing said wedges together, said wedges and said blocks together having a generally rectangular parallelepipedal configuration.