1. In a system for stressing steel tendons in a concrete structure formed on one side by a bulkhead, the combination of a jacket loosely fitted on a tendon for defining an annular space therebetween, an anchor member inboard of said bulkhead and defining an aperture for receiving said tendon, said aperture including a tapered seating surface opening toward said bulkhead and a threaded portion inboard of said seating surface, and a rigid hollow sleeve of closed section fitting over said tendon and provided with intermediate exterior threads for mating with the threads of said anchor member and a cylindrical extension for telescopically fitting within said annular space and snuggly engaging the interior of said jacket to form a grout-tight seal therewith.
2. The system of claim 1 wherein said jacket of said tendon is a continuous plastic tube sealing said cable from said cement.
3. The system of claim 1 further comprising a solid frusto-conical grommet of flexible resilient material interposed between said bulkhead and said anchor member, said grommet defining an axial aperture for receiving said sleeve and adapted to seal said conical seating aperture against said anchor member.
4. An anchorage system for stressing steel cable in concrete partially defined by a form comprising stranded steel cable, a tubular jacket loosely fitted about said cable to seal against direct contact between said cable and said concrete, an anchor member defining a conical seating surface opening toward said form and receiving said cable, a tubular sleeve adapted to fit through an aperture in said form and over said cable and including means sealingly engaging said anchor member inboard of said conical seating aperture to seal the same against grout and further including an inboard cylindrical section sealingly engaging the interior of said tubular jacket to protect said cable against grout said sleeve including an outer end carrying means for abutting a form.
5. The system of claim 4 wherein said anchor member defines internal screw threads between said conical seating aperture and the inboard surface of said anchor member and wherein said tubular sleeve member defines shoulder means for engaging the outboard surface of said form and exterior screw threads for mating with the interior threads of said anchor member, said exterior threads of said tubular member being located intermediate said cylindrical section and said shoulder member.
6. The system of claim 5 further comprising a solid, frusto-conical grommet defining a central axial aperture adapted to receive said tubular sleeve and interposed between said bulkhead and said anchor member to seal said conical seating aperture of said anchor member against grout when said tubular sleeve is threaded into said anchor member to draw said anchor member toward said bulkhead and compress said grommet whereby after the concrete is poured and set, said tubular sleeve may be removed from its threaded connection with said anchor member and said form removed, and said compressed grommet will become unseated from its engagement with said anchor member.
7. A system for stressing concrete comprising a steel tendon having one end extending through an aperture in a bulkhead form, a plastic jacket surrounding said tendon and terminating short of said one end thereof, an anchor member defining a seating surface for receiving a permanent grip in securing said cable to said anchor member after tensioning and receiving said tendon, said aperture defining internal threads along a portion thereof, a flexible, resilient grommet defining an aperture for receiving said cable in alignment with the aperture of said anchor member and adapted for placement between said anchor member and said bulkhead form, and a sleeve defining an interior conduit for receiving said tendon, said sleeve passing through said apertures of said bulkhead, said grommet and threadably engaging said aperture of said anchor member and providing a cylindrical inboard portion received in the annular space between said tendon and said jacket and snuggly fitting within said jacket, the outboard end of said sleeve defining a shoulder for bearing against said bulkhead form whereby when said sleeve is rotated, said anchor member will be drawn to compress said grommet against said bulkhead thereby firmly securing said anchor member in place before pouring said concrete and sealing said seating surface thereof and the outboard end of said tendon as well as the aperture of said anchor member against grout.
8. A tensioning assembly for concrete slabs and beams, including an elongated cable and means for holding one end of the cable within a concrete mass, an anchorage positioned around the other end of said cable, said anchorage being defined by a generally flat bearing plate extending transversely to said cable and a hub projecting toward the other end of said cable, said tension applying anchorage including reinforcing gussets extending between said hub and said plate and facing outwardly towards the other end of the cable, said anchorage being in screw-threaded relation to a protective tube surrounding said cable and projecting inwardly and outwardly from said anchorage, an elastomeric hole former surrounding said cable and bearing against the outer end face of said hub, a protective sheath surrounding said cable and fitted over the inwardly projecting end of said tube, and means on said other end of said tube for holding said tension applying anchorage against said elastomeric former and against a form for a concrete slab or beam.
9. Stressing anchorage apparatus for stressing steel tendons in a concrete structure including a plate having a hub formed thereon, said hub having an aperture formed therethrough, said aperture being screw threaded through a portion of its length, said aperture having a conical seating between one side of said hub and said screw threaded aperture, said conical seating having its large diameter positioned outwardly, and a cable receiving tube positioned within said hub, said tube having a portion in screw threaded relation with the screw threaded portion of said hub, said tube having one end portion extended beyond said hub from said screw threaded portion, said tube having another portion extended outwardly from said conical seating, said outer portion including abutment means thereon for engagement with a form board.
BACKGROUND AND SUMMARY
The present invention relates to the prestressing of poured concrete members such as slabs or beams. More particularly, the present invention relates to an anchorage system for prestressing concrete wherein the tendons are stranded steel cable, typically having seven separate strands formed into a solid continuous tendon.
The tendons for prestressing systems are laid in place prior to pouring the concrete; and after the concrete has been poured and sufficiently cured to about 2,500 psi., a hydraulic jack pulls the tendon (usually out of a side of the slab) to induce a predetermined stress in the tendon which is anchored to the slab at its inboard end. After the desired stress has been induced in the tendon, the tendon is anchored to the concrete at its outboard edge so that the tendon remains permanently stressed. This takes place, of course, prior to loading the concrete structure with its intended load.
The present invention, then, relates to the anchorage system for permitting the stressing of the tendon and for securing the tendon to an anchor member after it has been properly stressed. One commercially-available anchorage system provides a paper overlay spirally wrapped about the cable; and a lubricant covers the steel strand beneath the paper overlay so that during stressing the cable may move relative to the concrete. Alternatively, the cable may be obtained with a plastic tubular sleeve. In this system, a plastic grout excluder in the form of a flexible, split bushing is first placed over the cable after the paper overlay (or tube) has been stripped back for a short distance. Next, a precast anchor plate having side nail holes and a central conical seating aperture is placed over the cable with the cable extending through the seating aperture and the wider portion of the seating aperture facing the bulkhead form which defines the side edge of the slab. Next, a plastic bayonet fitting is temporarily secured to the anchor plate by means of a bayonet lock. The bayonet fitting is hollow for receiving the cable and has a longitudinal slot for fitting over the cable at an intermediate location. The bayonet fitting extends through the bulkhead and defines a set of exterior screw threads outboard of the bulkhead. The inboard side of this fitting connects to the anchor plate by means of a bayonet type lock. A frusto-conical pocket former, also of plastic, is interposed between the anchor plate and the inboard edge of the bulkhead. The function of the pocket former is to form a recess between the outboard edge of the slab and the conical seating surface of the anchor plate.
The anchor plate is then fixed in place by nailing it to the bulkhead through the nail holes provided in the anchor plate. This has been found to be a time-consuming and cumbersome job; and one of the principal features of the present invention is to obviate the need to nail the anchor plate to the bulkhead. When in place, the stripped cable extends through the grout excluder, the precast anchor plate, the pocket former, the slotted bayonet fitting, and the bulkhead form. A plastic nut is then secrewed onto the exterior threads of the bayonet fitting outboard of the bulkhead form, but because the bayonet fitting and nut are made of plastic sufficient torque cannot be obtained to hold the heavy cast anchor plate in place without nailing.
Aside from the disadvantage mentioned above of having to nail the anchor plate to the bulkhead form in order to keep it in place prior to pouring the concrete, there are a number of other difficulties with this system. For example, the grout excluder does not adequately seal the inboard end of the conical seating surface of the anchor plate. Thus, when the concrete is vibrated immediately after it is poured in order to eliminate air pockets from the setting concrete, grout finds its way through the grout excluder and into the conical seating aperture. This problem is further accentuated because vibration in the immediate neighborhood of the anchor member is essential in order to get uniform compaction. Any grout that enters the anchorage must be thoroughly cleared prior to stressing because the frusto-conical gripping wedges require a smooth seating. It has also been found that the grout secures the cables to the anchor plate; and during stressing, the cable must be pulled through the anchor plate. Further, the paper wrap does not exclude the concrete from contacting at least a portion of the cable which is exposed immediately adjacent the grout excluder.
It has also been found that the grout excluders come loose after the cables have been set up and the anchor plates initially secured to the bulkhead by nailing when workmen walk on the cables. That is, if a person steps on the cable adjacent the anchor plate, the grout excluder may be unseated; and if it is not replaced prior to pouring and vibrating, the entire conical seating aperture of the anchor plate will be filled with grout.
If the plastic grout excluder slips out of the anchor, the system provides no means for positioning the strand in the center of the conical seating aperture for anchoring. When the strand moves to the side of the aperture and is held there by the poured concrete, uneven seating of the parts of the multi-part stressing wedger results. This condition can cause irratic losses in stressing forces and failure of the strand or the wedges.
Further, if grout does enter the conical seating aperture of the anchor plate, it will secure the bayonet fitting in place because it is slotted so as to make its removable more difficult. As mentioned, the bayonet fitting is provided with a longitudinal slot so that it may be wedged open and secured onto the cable at any intermediate length, but this slot also causes difficulty in that if grout enters it, the fitting will be secured to the cable, and it permits movement of the cable all the axis of the conical seating aperture -- particularly when the grout excluder becomes unseated from the anchor plate.
It has also been found difficult to remove the plastic pocket former after the concrete has been poured and set and the bulkhead stripped away. Frequently, it has been found necessary to destroy the pocket former in order to remove it because grout enters around its edges and secures it in place.
The present invention provides a steel strand cable in a plastic, tubular jacket with an annular space between the cable and jacket. Prior to assembling the anchor member, the plastic jacket is cut short of the end of the cable and stripped from the cable. The anchor member includes a central conical seating aperture and an internally-threaded portion for receiving the stripped portion of the cable. A frusto-conical solid rubber grommet with an axial aperture is placed over the cable between the bulkhead form and the anchor member. The smaller flat surface of the grommet engages the anchor plate and seals the conical seating aperture; and the larger flat surface of the grommet engages the inboard surface of the bulkhead to provide a recess if it is desired.
Next, a metallic sleeve of closed section and having an intermediate threaded portion is placed over the cable and through the grommet and the anchor member for threaded connection with the internally-threaded portion of the anchor member. The outboard end of the sleeve is provided with a shoulder for engaging the outboard surface of the bulkhead; and the inboard end of the sleeve is provided with a reduced cylindrical portion which is telescopically received in the annular opening between the cable and the jacket -- the exterior surface of which snugly engages the interior surface of the jacket for sealing the same against grout. The sleeve positively positions the strand in the center of the conical aperture for anchoring. That is highly advantageous for the anchor system to develop its design strength, and to achieve uniform distribution of stress through the anchor member.
As the sleeve member is tightened, (by turning with a wrench) the anchor member is drawn toward the bulkhead to compress the grommet and hold the two in place solely by reaction with the bulkhead while positioning the cable. Thus, there is no need to nail the anchor member in place. A seal is effected by the threaded connection between the sleeve and the anchor member and by the reduced cylindrical portion of the sleeve fitting snuggly within the plastic jacket of the cable. The outboard edge of the conical seating aperture is sealed against grout by the flexible, resilient grommet. After the concrete is poured, vibrated and set, the metallic sleeve is loosened and the wooden bulkhead is removed, and the compressed grommet will "pop" from its location thus facilitating access to the recessed conical seating surface of the anchor member and obviating the difficulty in the above-mentioned prior system of having to remove the plastic pocket former. The threaded sealing engagement between the sleeve and the anchor member insures direct positive sealing against grout and easy removal by simply backing the threaded sleeve out of engagement with the anchor member and the plastic jacket of the cable.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will be used to refer to like parts in the various views.
FIG. 1 is an exploded view, taken from the side, of an end anchorage system according to the present invention;
FIG. 2 is a side view, partially in cross section, of the system of FIG. 1 in its assembled condition;
FIG. 3 is a transverse cross sectional view of the assembled anchorage system;
FIG. 4 is a cross section view taken through sight line 4--4 of FIG. 3; and
FIG. 5 is an exploded view in perspective of the inventive system.
Turning then to the drawing, reference numeral 10 generally designates conventional stranded steel cable comprised of seven separate steel strands twisted to form a single tendon. An anchor member is generally designated 11, a rubber grommet 12, and a metallic sleeve member 13. The cable 10 is provided with a tubular plastic jacket or sheath 14 having an interior diameter slightly larger than the outer diameter of the cable 10 to provide an annular aperture generally designated 15 (FIG. 4). That is, the cable 10 is loosely located within the plastic jacket 14.
Prior to assembly, the jacket 14 is cut as at 16 and the outboard (left as viewed in the drawing) portion of the sleeve 14 is stripped from the strand to leave the exposed portion 10a.
The anchor member 11 (also see FIG. 3) is a steel casting designed to accept and distribute the final force induced in the strand 10a; and it includes a transverse plate 17, a first inboard cylindrical boss 18 which is provided with a central aperture 19 which is internally threaded as at 20. A plurality of radially-extending rib members 21 brace the boss 18 against to the plate 17.
Extending from the opposite side of the transverse plate 17 (i.e., the outboard side thereof) is a second boss or hub 22 which defines a tapered or conical seating surface 23 opening in an outboard direction. The boss 22 is supported relative to the plate 17 by a plurality of radially-extending ribs or reinforcing gussets identified by reference numeral 24.
The grommet 12 is located between the anchor member 11 and the inboard surface of a bulkhead form designated by reference numeral 26. The bulkhead 26 includes a drilled hole 26a for receiving the strand; and it forms the edge of the slab, although the invention, of course, is not limited to slabs. The grommet 12 has a generally frustoconical shape; and it defines a central cylindrical aperture 27 extending entirely through its axis. The grommet 12 includes an inboard flat surface 28, an outwardly-tapered side surface of frusto-conical shape 29 and a forward or outboard flat surface 30. The grommet 12 is a solid member formed of flexible, resilient material such as rubber or neoprene. The function of the grommet 12 is to provide a recess at the outboard edge of the concrete slab being formed, free of cement, to permit access to the conical seating surface 23 of the anchor member 11, and at the same time, to temporarily seal that seating surface from grout while the concrete is being poured or vibrated.
The aperture 26a of bulkhead 26 receives the sleeve 13. The sleeve 13 is preferably formed from a hollow steel tube of closed section to provide a central closed conduit 32 for receiving the stripped portion 10a of the strand with little or no wobble. That is, the inside diameter of the sleeve 13 is preferably only slightly greater than the dimensions of the cable 10a to allow an easy fit while holding the strand in position placement during setting of the cement. The outboard end of sleeve 13 is provided with a lock nut 33 which provides a shoulder 34 for engaging the outboard surface of the bulkhead 26. At the inboard side of the sleeve 13 there is formed a smooth cylindrical portion 35 of reduced diameter for fitting into the annular space 15 (see FIG. 2) between the strand 10a and the plastic jacket 14. Intermediate the lock nut 33 and the reduced cylindrical portion 35, the sleeve 13 is provided with a set of exterior threads, generally designated 36, for threaded engagement with the interior threads 20 of the aperture 19 in boss 18 of anchor member 11 (also see FIG. 2).
The tendons or cables may be supplied in predetermined lengths; and they are provided with conventional dead-end anchorages placed at locations within the forms prior to pouring the concrete. When the tendons are first placed in their desired location, the plastic jacket 14 is cut short of the bulkhead sufficient to allow room for the grommet and anchor member. The cut portion is stripped away. Next, the anchor member 11 is placed over the exposed cable 10a with the inboard transverse surface of the boss 18 engaging the cut edge 16 of the plastic jacket 14. The grommet 12 is then fitted over the exposed cable 10a; and the outboard edge of the cable 10a is placed through the aperture 26a of the bulkhead form 26 (see FIGS. 2 and 3).
Then the sleeve 13 is placed over the exposed strand 10a with the reduced cylindrical portion 35 fitting into the annular space 15 between the jacket 14 and the strand 10a. The exterior surface of the reduced cylindrical portion 35 snuggly but removeably engages the interior surface of the jacket 14 to form a seal and prevent the flow of grout between them. The exterior intermediate threads 36 of the sleeve 13 are then screwed into the interior threads 20 of the boss 18 until the outboard surface of the boss 22 forces the grommet 12 into engagement with the inboard surface of the bulkhead 26. The lock nut 33 is further tightened with a wrench or other suitable means with the shoulder 34 bearing against the outboard surface of the bulkhead 26 to force the grommet 12 into a compressed state.
The sleeve 13, after it is sufficiently screwed into the anchor member 11, holds the anchor solidly in place by reaction with the bulkhead without the need of nailing the anchor in place or otherwise affixing it, as was required in the above-described prior system. This is considered to be an important aspect of the present invention not only because it saves the time and labor of nailing the anchor member in place, but also because the sleeve secures the cable positively in place relative to the conical seating aperture so that the concrete is set, the sleeve 13 is removed and the cable stressed, the gripper wedges may be securely placed in position. If the axis of the cable is not colinear with the axis of the conical seating aperture, the wedges (not shown) will not be properly seated and may even slip through the anchor member.
It will be appreciated that the exterior threads 36 of the sleeve 13 together with the interior thread 20 of the boss 18 form a grout-tight seal to prevent the entering of grout, and at the same time, the reduced cylindrical portion 35 of the sleeve 13 enters further into the annular space 15 of the cable 10 as the sleeve 13 is tightened to form an even better seal between these elements. It has been found that even if the cut edge 16 of the plastic jacket 14 is shorter than desired so that the cement does engage the exterior surface of the reduced cylindrical portion 35 of the sleeve 13, this contact is easily broken when the sleeve 13 is retracted after the concrete has been poured, vibrated and set.
After the concrete has sufficiently cured, the wrench is used to remove the sleeve 13, and when the bulkhead 26 is next removed, the grommet 12, being in a state of compression, forces itself outwardly to "pop" out of contact with the set concrete thereby providing a recess for facilitating access to the tapered seating surface 23.
Next, suitable means such as a hydraulic jack pulls the exposed portion of the strand 10a against the outboard surface of the hardened concrete to induce a stress in it; and when sufficient, predetermined stress has been induced in the cable, wedge-shaped gripper members (not shown) are positively seated within the seating surface 23 of the boss 22 of the anchor 11 to grip the exposed portion 10a of the cable and securely hold the same in place. The hydraulic jack may then be removed.
Thus, the sleeve 13 as well as the grommet 12 may be reused in subsequent applications, and the conical aperture of the anchor member 11 is free from grout because the compressed, resilient grommet 12 effects a very good seal. The grommet 12 also seals the aperture 26a of the bulkhead 26 from grout; and the reduced cylindrical portion 35 seals with the interior surface of the plastic jacket 14 to protect the cable from grout.
It will also be appreciated that for those applications wherein it is not necessary to provide a recess for the anchor member (as when a long slab is poured and stressed in sections) the grommet 12 may be omitted and the flat, outboard surface 22a of the anchor member will abut the inboard surface of the bulkhead 26. In this case, the sleeve is suitable shorter than shown and the anchor member is still held in place solely by tightening of the sleeve 13. The resultant firm securement of the anchor member, and the closed section of the sleeve will properly align the cable in the conical seating aperture while sealing that aperture against grout.
Persons skilled in the art will be able to substitute elements equivalent to those which have been illustrated and described and to modify the structure disclosed while continuing the practice the principle of the invention; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the invention.