A pre-stressing sheath for preventing wet concrete from penetrating between the sheath and strands being debonded, and for covering sections of reinforcing strands in pre-stressed concrete beams such that any potential strand tension transmitted to a beam is nullified by allowing the strands to slide freely within the pre-stressing sheath. The pre-stressing sheath is made up of a first hollow cylindrical sheath section having a longitudinal slit that allows the first sheath section to be opened and placed about the concrete reinforcing element, and a second hollow cylindrical sheath section having a longitudinal slit that allows the sheath section to be opened and placed about both the concrete reinforcing element and the first sheath section. The first sheath section further includes an integral outwardly projecting ridge for interlocking with the longitudinal slit of the second hollow cylindrical sheath section.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to a pre-stressing sheath for preventing wet concrete from penetrating between the sheath and strands being debonded. In particular, the present invention relates to plastic clip-on type sleeves used to encircle and debond concrete reinforcing members such as cables.

(2) Description of the Prior Art

Pre-cast concrete construction elements such as bridge girders, etc., are stressed by placing tension on an elongated cylindrical rod or cable bundle having a generally circular cross-sectional area normally formed of twisted wires generally referred to as strands. Strands that are placed in a straight configuration near the bottom of a pre-stressed girder tend to overstress near the end of the girder at the moment of de-tensioning. In order to avoid this overstress, several strands are usually draped upwardly near each girder end to reduce the eccentricity of the force they cause about the centroid of the girder. The effort required to position and stress the draped strands is expensive, time consuming, difficult and dangerous.

Recently, a less expensive, faster, easier and safer technique for eliminating overstress has been developed and used commercially to prevent the undesirable overstress. The improved procedure, places and stresses the strands necessary for flexure at mid-span of a girder in a straight configuration extending over the entire length of the girder. The overstress near the ends of the girder at the moment of de-tensioning is avoided by debonding several strands from the end of the girder towards mid-span over a length which would otherwise be overstressed. This eliminates the eccentric forces that the debonded strands would otherwise generate about the centroid of the girder, and thus reduces stresses.

Generally, the debonding of one or more strands is accomplished by placing a debonding sheath about the strands to be debonded. Once the sheath is installed on the strands, the ends of the sheath are sealed to help prevent the strands from coming into contact with a mortar that is used to bond unprotected strands to the girder's concrete mass. At present, the preferred debonding sheath is a plastic tube having a longitudinal slit that allows the tube to be clipped over the strands to be debonded. However, because of the longitudinal slit, this type of sheath is not as protective in preventing wet mortar from coming into contact with the strands to be debonded. Therefore, while the split debonding sheath is currently favored, it by itself does not offer an adequate strand debonding solution.

Another attempt to prevent mortar from coming into contact with strands uses a hollow cylinder having a continuous wall that slides over the strands to be debonded. While a hollow cylinder having a continuous wall offers the best protection for preventing wet mortar from coming into contact with the strands, it is difficult and sometimes impossible to apply. What is needed is a pre-stressing sheath that offers the debonding protection of a hollow cylinder having a continuous wall along with the ease of use of a split debonding sheath.

SUMMARY OF THE INVENTION

Generally, the pre-stressing sheath of the present invention is comprised of a pair of hollow cylindrical sheath sections used in combination to protect and debond strands. Each hollow cylindrical sheath section has a central longitudinal axis and a longitudinal slit parallel to its longitudinal axis extending along its length so that it can be opened along the slit and readily placed around more than half the circumference of a cable bundle or strands to be debonded.

Each sheath section is a single sheet of material, in which case the sheath section must be sufficiently flexible to permit it to be opened along the longitudinal slit and placed around a cable bundle or strands to be debonded. The circumference of each sheath section is greater than 180 degrees. Moreover, each sheath section is made of a relatively rigid and resilient material, preferably from a relatively rigid plastic such as a high-density polyethylene, polypropylene, polyvinyl chloride or the like. Moreover, it is preferred that the inside diameter of each sheath section be substantially equal to the given effective diameter of a generally cylindrical bundle of cables or strands to be debonded whenever the sheath section is in an unflexed state.

In use, a first sheath section is opened along its longitudinal slit just enough to be placed over a strand to be debonded. A second sheath section is opened along its longitudinal slit and is placed over both the strand and the first sheath section. The first and second sheath sections should be rotated about the enclosed strand relative to each other such that the longitudinal slit of the first sheath section does not coincide with the longitudinal slit of the second sheath section. Preferably, the longitudinal slits are positioned opposite to each other to maximize the seal protecting the strands from wet mortar.

In a preferred embodiment of the invention, the sheath sections can be interlocked in order to prevent the inadvertent and coincidental alignment of the respective longitudinal slits during mortar pouring operations. An interlocking structure consists of an outwardly projecting ridge integral with the outer surface of the longitudinally slit hollow cylinder serving as the first sheath section and is sized to fit within the slit of the second sheath section whenever the second sheath section is clipped about the first. Preferably, this outwardly projecting ridge extends the length of the first sheath section and is opposite the longitudinal slit of the first sheath section. An added benefit is that the outwardly projecting ridge also functions as a reinforcing spine.

In a preferred embodiment, the first and second sheath sections are identical with one advantage being minimized manufacturing costs. Another advantage is that identical first and second sheath sections allow interchangeability. Therefore only one type of sheath section needs to be in inventory to complete any given sheathing task.

The present invention also includes a method for preventing wet mortar from bonding to strands used to provide tension in pre-stressed concrete construction. The method is comprised of steps of providing a first longitudinally slit hollow cylindrical sheath section made of a flexible and resilient material, further including an outwardly projecting ridge, then opening the first sheath section along the first sheath sections longitudinal slit and placing it about the strands. A second longitudinally slit hollow cylindrical sheath section of a flexible and resilient material is then provided and is opened along its longitudinal slit and placed about the first sheath section. An additional step can be rotating the second sheath section relative to the first sheath section, such that the first sheath section's outwardly projecting ridge engages the longitudinal slit of the second sheath section. In this configuration, the sheath of the present invention will prevent wet mortar from contacting the protected strand through either longitudinal slit of either sheath section. Mortar can be prevented from entering the ends of the sheath by traditional methods such as taping or plugging up the ends.

These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, terms such as horizontal, upright, vertical, above, below, beneath, and the like, are used solely for the purpose of clarity in illustrating the invention, and should not be taken as words of limitation. The drawings are for the purpose of illustrating the invention and are not intended to be to scale.

Referring to the drawings and first toFIG. 1, a sheath section10comprises a first hollow cylinder12having a longitudinal slit14bounded by longitudinal edges16and18. Sheath section10has an outer surface20that includes an outwardly projecting ridge22for aligning with the longitudinal slit of a second sheath section. Outwardly projecting ridge22, also serves as a reinforcing spine extending the length of sheath section10. Preferably, ridge20is located directly opposite longitudinal slit14. Sheath section10has an inside surface24that has an inside diameter or circumference sized to envelop over half the circumference of a concrete reinforcing member to be debonded.

FIG. 2shows an end view of a pair of sheath sections combined to complete a sheath26according to the present invention. Sheath26, is made up of first sheath section10shown encircling a plurality of strands28. A second sheath section30is clipped over first sheath section10such that it seals longitudinal slit14. Second sheath section30has longitudinal edges32and34that capture ridge22of first sheath section10whenever ridge22engages slit36of sheath section30. A reinforcing spine38is also included for sheath section30. However, this spine can be eliminated if sheath section30is deemed strong enough not to need reinforcement.

Sheath26is used by opening sheath section10by urging longitudinal edges16and18away from each other until longitudinal slit14is open enough to span the effective circumference of strands28. Next, sheath section10is placed about strands28and longitudinal edges16and18are released. Once released, edges16and18snap back towards each other due to the resiliency of the material from which sheath section10is made. Next, sheath section30is opened by urging longitudinal edges32and34away from each other until longitudinal slit36is open enough to span the effective circumference of strands28and outer surface20. Then sheath section30is placed about both sheath section10and strands28. Once released, edges32and34snap back towards each other due to the resiliency of the material from which sheath section30is made. Next, sheath section30is rotated axially relative to sheath section10until ridge22of sheath section10engages longitudinal slit36of sheath section30. If necessary, sheath section30can be slid longitudinally to a position that maximizes the sealing of longitudinal slit14. At this point with the exception of plugging the ends of sheath sections10and30, sheath26is properly configured to seal strands28from mortar intrusion.