WO2000070167A1

WO2000070167A1 - Bonded prestressing system

Info

Publication number: WO2000070167A1
Application number: PCT/US2000/013340
Authority: WO
Inventors: Bradford J. Raffensperger
Original assignee: Raffensperger, Patricia
Priority date: 1999-05-14
Filing date: 2000-05-15
Publication date: 2000-11-23
Also published as: AU4851500A

Abstract

A prestressing apparatus (100) and a method for producing such an apparatus are disclosed, the apparatus including an elongated sheath (120), at least one elongated high tensile strength tendon (130) disposed within and longitudinally enclosed by the elongated sheath (120), and a quantity of grout (140), disposed within each elongated sheath and capable of curing to bond the tendon (130) to the sheath (120) along its length. The prestressing apparatus (100) may be provided within a body of concrete (110), tensioned and cured to provide a reinforced, bonded prestressed concrete structure. The prestressing apparatus (100) may also be utilized in numerous other applications, such as the fields of mechanically stabilized earth (MSE) walls, ground anchors, rock and soil anchors and tiebacks, cable stays, tension roof members and offshore oil platform tethers.

Description

BONDED PRESTRESSING SYSTEM

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The invention relates to a bonded prestressing apparatus for use in a variety of

applications, most commonly for reinforcing concrete structures, as well as methods for

producing such an apparatus. Additionally, the prestressing apparatus of the invention

may be utilized in a plurality of other applications, including the fields of mechanically

stabilized earth (MSE) walls, ground anchors, rock and soil anchors and tiebacks, cable

stay systems, tension roof members and tethers for offshore oil platforms.

DESCRIPTION OF RELATED ART

Conventional prestressed concrete structures include a body of concrete and a

reinforcing element which is imbedded in the concrete and tensioned to provide a pre¬

existing compressive stress within the body of concrete. Two types of concrete

prestressing systems are currently in widespread use: unbonded post-tensioned systems

and bonded post-tensioned systems. In an unbonded system, a reinforcing element is

supplied within a passage through the body of concrete. The passage is ordinarily

provided by a conduit or duct constructed of metal or plastic which encloses the

reinforcing element and lends some measure of protection from corrosive forces. After the reinforcing element is positioned within the concrete, tension is applied across the

reinforcing element and anchorages at the distal ends of the body of concrete are

utilized to maintain tension in the reinforcing element.

Bonded systems differ from unbonded systems in that the reinforcing element is

bonded to the body of concrete along its length after tensioning of the reinforcing

element. Bonding has been accomplished in the prior art by providing a coating or

grout around the reinforcing element which bonds either to the interior of the conduit

which envelops the reinforcing element and is in turn bonded to the interior of the body

of concrete, or to the concrete itself if no conduit is utilized.

A bonded system generally provides several advantages over unbonded

systems. First, corrosion may be significantly reduced along the strand if a bonding

material is utilized which is capable of preventing water or other fluids from coming in

contact with the strand. Second, continuous bonding of the tensioned strand throughout

the body of concrete provides safety advantages. If corrosion weakens the reinforcing

element to the point of failure, or if the element is ruptured by other means, the entire

prestressing force in an unbonded system is lost along the full length of the element.

This failure may be instantaneous and violent, potentially causing damage to other parts

of the structure or causing injury to bystanders. Further, retrofit and renovation of a

structure having a bonded prestressing system can be performed more safely and easily

than an unbonded system. If the use of the structure is changed such that plumbing,

mechanical, electrical or other modifications are required, coring through the existing slab is often attempted. In unbonded systems, damage to one prestressing element

might ruin the structural integrity of the entire slab, where a bonded system would

suffer only a localized loss of prestress. Finally, building codes often require a

minimum percentage of bonded reinforcement for slabs and beams within building

structures. Use of bonded prestressing systems reduces the amount of supplemental

reinforcing material required to satisfy code requirements.

Disadvantages of bonded systems generally include an increased cost of

manufacture, due in large part to increased material cost and an increased labor and

skill demand. Bonded systems require skill and labor in providing and properly curing

the bonding material, often a type of cementitious grout. Finally, bonded systems are

often bulkier than unbonded systems, creating greater difficulty in handling and in

installation.

Thus, it can be seen that needs exist for improved methods and apparatus for

fabricating reinforced concrete structures. It is to the provision of a prestressing

apparatus capable of such use, and methods and apparatus for producing such systems,

that the present invention is primarily directed. It should be noted that, as described in

greater detail below, the prestressing apparatus disclosed herein may be used

advantageously in a multitude of other applications in which a flexible tension bearing

member is desired. Nothing in this specification or in the claims should be read to limit

the scope of the invention to use of the apparatus and methods disclosed herein to use

in the field of reinforced concrete structures. BRIEF SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the present invention which,

in one aspect, is a prestressing apparatus. Each prestressing apparatus includes an

elongated sheath fixedly enclosed along its length within the body of concrete, an

elongated high tensile strength tendon disposed within and longitudinally enclosed by

the sheath, and grout disposed within the sheath which may be cured to bond the

tendon to the sheath along its length.

In another aspect, the invention includes a prestressing apparatus as described

above having an epoxy-resin grout which, prior to curing, is soft and pliable in ambient

atmospheric conditions. Such grout may be supplied within the sheath during

manufacture of the prestressing apparatus, well in advance of installation of the

prestressing apparatus in the selected installation environment.

In another aspect, the invention includes a prestressed and reinforced concrete

structure, in which a body of concrete is reinforced by the operation of at least one

prestressing apparatus as described above. After placement of the prestressing

apparatus within the body of concrete, grout may be cured by application of heat to the

grout material.

In yet another aspect, the invention includes a prestressing apparatus as

described above, wherein the elongated sheath is fabricated from a generally flat elongated ribbon. The ribbon has a first longitudinal edge and an opposite second

longitudinal edge, and is rolled to form an elongated tube. The sheath is formed by

sealing the first and second longitudinal edges to each other to form a watertight seam.

The seam may be formed by welding the first and second longitudinal edges to one

another, or by the application of adhesive to bond the first and second edges together.

The seam may form a generally spiral pattern around an axis along the lengthwise

direction of the sheath, or may be substantially parallel to the axis of the sheath.

In yet another aspect, the invention includes a prestressing apparatus as

described above, wherein the elongated sheath includes corrugations formed

transversely of the axis along its lengthwise direction.

These and other aspects of the invention will become apparent from the

following description of the preferred embodiments taken in conjunction with the

following drawings. As would be obvious to one skilled in the art, many variations and

modifications of the invention may be effected without departing from the spirit and

scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

Fig. 1 is a schematic cross-sectional side view of one embodiment of the prestressing

apparatus of the present invention, disposed within a body of concrete to form a

prestressed and reinforced concrete structure. Fig. 2 depicts opposed wheels used to produce corrugations within a sheath according

to one embodiment of the present invention.

Fig. 3 is a perspective view of a corrugated sheath according to one embodiment of the

present invention.

Fig. 4A is a perspective view of a sheath according to another embodiment of the

present invention.

Fig. 4B is a perspective view of a sheath according to still another embodiment of the

present invention.

Fig. 5 is a schematic cross-sectional view of an embodiment of the prestressing

apparatus of the present invention, depicting corrugations in the sheath.

DETAILED DESCRIPTION OF THE INVENTION

Representative embodiments of the invention are now described in detail.

Referring to the drawings, like numbers indicate like parts throughout the views. As

used in the description herein and throughout the claims, the following terms take the

meanings explicitly associated herein, unless the context clearly dictates otherwise: the

meaning of "a," "an," and "the" includes plural reference, the meaning of "in" includes

"in" and "on." Referring now to Fig. 1, in one embodiment, the invention comprises a

prestressing apparatus 105. Each prestressing apparatus 105 includes an elongated

sheath 120 which encloses a high tensile strength tendon 130 surrounded by a quantity

of grout 140.

As shown in Figs. 4A and 4B, in one embodiment, the sheath 120 is constructed

of a non-metallic material to minimize corrosion due to oxidation or water damage. In

another embodiment, a metallic material may be used to form the sheath 120. A variety

of corrosion resistant plastic materials are suitable for use in fabricating the sheath 120,

including but not limited to low, medium or high density polyethylene, polypropylene,

teflon, nylon, and high molecular weight plastic. Plastic sheaths may be manufactured

by a variety of methods, including for example, extruding directly in tubular form, or

extruding as a flat elongated ribbon 124 which is subsequently formed into a sheath

120. In the latter instance, plastic material may be extruded as a substantially flat

elongated ribbon 124, having a first longitudinal edge 170 and an opposite second

longitudinal edge 172. The ribbon 124 may then be rolled to form an elongated tube

127, wherein the first and second longitudinal edges 170, 172 of the ribbon 124 are

sealed to form a watertight seam 126.

In one embodiment, the watertight seam 126 may be formed by welding the

first and second longitudinal edges 170, 172 of the ribbon 124 at the area where the

longitudinal edges 170, 172 overlap. The weld may be accomplished by the application

of heat sufficient to melt the sheath material, localized at the area of overlap of the longitudinal edges 170, 172. A torch may be used to effect this seal, or an extrusion

welder may be utilized in association with the extrusion process. In other

embodiments, the seam 126 may be formed by utilizing Radio Frequency ("RF")

welding, wherein pressure and mechanical vibrations at radio frequencies are applied to

the overlapping longitudinal edges 170, 172 of sheath material to bond the longitudinal

edges 170, 172 to each other. The seam 126 may also be formed by other welding

methods, such as laser or sonic heat welding methods, by application of a solvent or

other chemical sealing methods, by the creation of a mechanical lock by crimping or

otherwise applying pressure to the overlapped or abutting longitudinal edges 170, 172,

or by application of a water- impermeable adhesive to effect the watertight seal between

the longitudinal edges 170, 172 of the elongated ribbon 124.

Referring now to Fig. 4 A, the sheath 120 may be formed such that the seam 126

is oriented transverse of the lengthwise axis of the sheath 120, so that the seam 126

forms a substantially "spiral" pattern with respect to the lengthwise axis of the sheath

120. Alternatively, as shown in Fig. 4B, the seam 126 may be oriented longitudinally

along the sheath 120, parallel to the lengthwise axis of the sheath 120.

In embodiments wherein the prestressing apparatus 105 is utilized to form a

prestressed and reinforced concrete structure 100, relative movement between the

prestressing apparatus 105 and the body of concrete 110 is undesirable. If the sheath

120 is not fixed within the body of concrete 110 in such an application, stressing of the concrete 110 will not occur optimally, because slippage of the tensioned prestressing

apparatus 105 with respect to the body of concrete 110 will occur.

Referring now to Fig. 2, relative movement between the sheath 120 and the

concrete 110 is prevented, in one embodiment of the present invention, by imparting

corrugations 150 to the sheath 120 which provide enhanced engagement between the

body of concrete 1 10 and the outer surface 128 of the sheath 120. In one embodiment,

the corrugations 150 are 0.375" in width, and are distributed over substantially the

entirety of the outer surface 128 of the sheath 120. In another embodiment,

corrugations 150 can be provided to selected portions of the outer surface 128, or the

outer surface 128 can be generally smooth and without corrugations 150. It should be

understood, however, that while corrugations 150 of a generally ribbed configuration

are demonstrated in Figs. 2 and 3, corrugations 150 of any configuration capable of

increasing the frictional and engagement forces between the sheath 120 and the body of

concrete 110 are intended to be included within the definition of "corrugation" in this

specification. Such alternate corrugations (not shown) may take the form of cross

hatched ribs, diamonds, squares, rectangles, dimples, protrusions, waveforms, or other

configurations. Similarly, while corrugations 150 forming substantially axial rings

around the lengthwise axis of the sheath 120 are depicted in Fig. 3, other corrugation

configurations, such as a generally spiraled pattern with respect to the axis of the sheath

120, or even a random pattern are contemplated within the scope of the invention and

the claims set forth below. Referring to Fig. 5, in embodiments in which corrugations 150 are imparted to

the sheath 120, the sheath 120 includes inwardly protruding areas 152 and outwardly

protruding areas 154. In one embodiment of the invention, the total inward facing

surface area created by the inwardly protruding areas 152 is greater than the total

outward facing surface area created by the outwardly protruding areas 154. For

example, in an embodiment depicted in Fig. 5, the width of the inwardly projecting area

152 is approximately twice the width of the outwardly projecting area 154. The result

is that the tendency for snagging of the sheath 120 by the tendon 130 (in embodiments

where the tendon 130 is loaded into an already formed sheath 120) is reduced by

providing a more continuous inner surface along the inner diameter of the sheath 120.

Additionally, the more continuous inner surface provides for reduced localized wear of

the sheath 120 due to sliding contact between the sheath 120 and the tendon 130 during

tensioning. Reduction of localized wear between the sheath 120 and tendon 130

reduces the possibility of rupturing the sheath 120, thereby improving the resistance of

the apparatus to corrosion and preserving the structural integrity of the sheath 120.

Corrugations 150 may be imparted to the sheath 120 in several ways, depending

on the selected material and method of manufacture of the sheath 120. According to

one embodiment of the method of the present invention, the corrugations 150 are

imparted to the elongated ribbon 124 after it is created by extruding or other means, but

prior to sealing of the ribbon 124 into a sheath 120. Referring to Fig. 2, the elongated

ribbon 124 may be passed through a pair of opposed drums or wheels 160, 162, the

wheels 160, 162 having complementarily textured surfaces for producing corrugations 150. By the action of the opposed wheels 160, 162, corrugations 150 are imparted to

the ribbon 124. The ribbon 124 is then rolled into an elongated tube 127 and the

overlapping longitudinal edges 170, 172 sealed to form the sheath 120.

The opposed wheels 160, 162 may be maintained at a temperature which is

adequate to raise the sheath material to a temperature at which plastic deformation will

be imparted to and maintained within the sheath material. Similarly, the sheath

material can be heated by separate heating means prior to introduction to the opposed

wheels 160, 162. Alternatively, the corrugations 150 may be imparted to the sheath

material in proximity to the extruding process, where the temperature of the sheath

material has not been given the opportunity to cool below a temperature at which

deformations are easily received by the sheath material. In still another alternative, the

corrugations 150 may be imparted to the sheath material at room temperature without

heat application.

Referring again to Fig. 1, within each sheath 120 is a high tensile strength

tendon 130. In one embodiment, the tendon 130 is constructed of seven-strand wound

steel strand, of 0.5" or 0.6" diameter. Alternately, strands of other diameters may be

utilized, typically ranging from .5" to .75" in diameter. In other embodiments, the

tendon 130 may be constructed of other materials capable of withstanding high tensile

stresses, such as high strength prestressing bars of carbon fiber, aramid or fiberglass,

reinforcing steel bars, prestressing wire or wires, or bundles of more than one prestressing steel bars or strands (not shown). Other tendon 130 diameters as

appropriate for a particular application may also be used.

Still refeπing to Fig. 1, the interstitial space between the tendon 130 and the

sheath 120 is filled with a grout 140, which coats the tendon 130 and acts as a bonding

material to capture the tendon 130 and inhibit movement of the tendon 130 with respect

to the sheath 120 and concrete 110. Although there is no specific restriction on the

composition of the grout 140, some embodiments may contain as a principal ingredient

an epoxy resin, a polyurethane resin, a polyester resin, a polyepoxide resin or a cyanate

resin which, when cured, exhibits sufficient strength to bond the tendon 130 securely

within the sheath 120. In an uncured state, the grout 140 is preferably of a consistency

which is capable of injection into the space between the sheath 120 and tendon 130

through substantially the entire length of the sheath 120. The grout 140 may be of a

type that cures without further intervention with passage of time, or may cure in

response to the application of heat (referred to herein as "heat-cured") or the release of a

catalyst.

In one example embodiment, the grout 140 is composed of a heat-cured epoxy

resin that, prior to curing, is pliable in ambient atmospheric conditions. One such grout

according to the present invention remains pliable between 0°C and 30 °C and cures as

follows: at 50°C, cure time is 10 hours; at 150°C, cure time is 10 minutes. Thus, after

placement of the reinforcing apparatus within the body of concrete 110 and after full

hardening of the concrete 110 at temperatures below 30 °C, the grout 140 remains pliable, and the tendon 130 may be easily elongated and tensioned within the sheath

120 without excessive binding or seizure of the tendon 130 within the sheath 120.

In one embodiment, curing of the epoxy resin grout 140 is activated by the

selective application of heat to the grout 140, which may be accomplished in a variety

of ways. For example, heat may be applied by conducting an electric current through

the tendon 130 (if a steel or other conductive tendon is utilized), or through small

conductive wires (not shown) which can be placed within the sheath 120 or within the

interstitial area between the sheath 120 and tendon 130 during the manufacturing

process. Alternately, a non-corroding dielectric material (not shown) can be added to

the plastic sheathing material prior to its extrusion or other mode of manufacture to

establish conductivity within the sheath 120, a conductive material can be placed

outside the sheath 120 within the body of concrete 1 10, or a radiation/susceptor

material may be added to the grout 140. This list of curing mechanisms is not intended

to be exhaustive, and those skilled in the art may utilize other curing methods without

departing from the scope of the invention.

The grout 140 is preferably comprised of a mixture of epoxy resin and silica to

produce an effective bond between the tendon 130 and the sheath 120 in a cost effective

manner. In one embodiment, a 50/50 mixture of epoxy resin to silica has been found to

exhibit sufficient tensile strength for use as a reduced cost grout 140. Such a mixture

exhibited tensile strength exceeding 7,500 psi. Alternately, other inert fillers such as cellulose may be utilized in mixture with epoxy resin to formulate a suitable grout 140

at costs below the cost of a pure epoxy resin grout 140.

Several benefits are derived from utilization of a heat-cured grout 140 which is

pliable in its uncured state, as described above in a preferred embodiment. First, such a

grout 140 enables shop fabrication of the prestressing apparatus, including sheath 120,

tendon 130 and grout 140, which may be coiled and stored indefinitely in a

prefabricated state. Such an apparatus has an extended shelf life over a product with a

grout 140 which cures over time, because the apparatus is incapable of curing without

application of the requisite heat energy. Further, such an apparatus is less expensive

and requires less skilled field labor to install, because tendon 130 and grout 140 need

not be placed within the sheath 120 at the job site.

The prestressing apparatus described herein may be utilized in any of a

multitude of applications. Referring again to Fig. 1 , at least one prestressing apparatus

105 may be provided within a body of concrete 110 to fonri a prestressed and

reinforced concrete structure 100. A nearly unlimited variety of bodies of concrete 110

may be reinforced using the prestressing apparatus 105 of the present invention,

including slabs, beams, arch ribs, girders, retaining walls, pavings, columns and other

bodies (not shown).

Additionally, the prestressing apparatus 105 may be utilized in a variety of other

applications, providing improved corrosion and wear resistance properties over current systems. The following description of possible applications is illustrative only, and is

not intended to be an exhaustive list.

Mechanically stabilized earth (MSE) wall systems, which are generally

stabilized using passive tension elements, may be enhanced by incorporation of the

prestressing apparatus 105 of the present invention. MSE walls may be constructed and

anchored using at least one prestressing apparatus 105, then tensioned as described

herein and the grout 140 set to provide a bonded prestress which would tie the earth and

wall together as a compression block.

Similarly, ground, rock and soil anchor systems may be enhanced by

incorporation of the prestressing apparatus 105 of the present invention. Such systems

may include at least one prestressing apparatus 105 which is fed into an anchor hole

drilled into the soil, ground or rock. After the prestressing apparatus 105 is in place,

the anchor hole may be filled with a cementitious or other grout material which is cured

to capture the prestressing apparatus 105 within the anchor hole. The prestressing

apparatus 105 may then be tensioned and fixed to an anchor plate to retain the soil,

ground or rock. Various alterations to this general description may be made, such as

the initial curing of grout 140 within the prestressing apparatus 105 to create a "bond

zone," prior to insertion of the prestressing apparatus 105 into the anchor hole.

Creation of a bond zone in the prestressing apparatus 105 increases the anchoring

surface which is grasped and pulled in tension by the remaining "free length" of the

prestressing apparatus 105 after the free length is stressed and bonded. Cable stay systems, such as those used in bridge systems and dome structures,

as well as tension roofs and tethers for offshore platforms may also be improved by

utilization of the prestressing apparatus 105 of the present invention.

Also according to the present invention, a method of forming prestressing

elements 105 is provided. The method includes the steps of providing a high tensile

strength tendon 130, enclosing the tendon 130 in an elongated watertight sheath 120,

providing a heat-curable grout 140 within the sheath 120 and coating the tendon 130.

The tendon 130 may be enclosed within the sheath in several ways, depending

upon the method by which the sheath 120 is formed. In embodiments where the sheath

120 is extruded directly as a seamless tube 127, the tendon 130 may be enclosed within

the sheath 120 as the extrusion is formed. If the sheath 120 is formed by sealing the

longitudinal edges 170, 172 of an elongated ribbon 124, the tendon 130 may be

enclosed by the sheath 120 as part of the sealing step, or may be inserted into the sheath

120 after the seal has been completed.

Grout 140 may be provided within the sheath 120 in a variety of ways,

depending upon the method of manufacture of the sheath 120 and the method of

enclosing the tendon 130 within the sheath 120. If the tendon 130 is enclosed within a

seamless extruded sheath 120 as the extrusion is formed, the grout 140 may be

provided at the time that the tendon 130 is enclosed within the sheath 120. If the

tendon 130 is enclosed within a sheath 120 comprising a sealed elongated ribbon 124, the grout 140 may also be provided within the sheath 120 at the time the seal is formed.

Alternately, in either embodiment of the sheath 120, grout 140 may be injected into the

sheath 120 after the tendon 130 is enclosed by the sheath 120.

The tensioning step as referred to above may be carried out through the use of

jacks, as known to those skilled in the art. Finally, the step of curing the selected grout

140 may be accomplished in a variety of ways, including those described hereinabove

in the description of the grouts 140 themselves.

The invention also includes a method of forming a prestressed and reinforced

concrete structure 100, utilizing at least one prestressing apparatus 105. After the

prestressing apparatus 105 is formed as described above, the prestressing apparatus 105

may be introduced to the concrete 110 such that the apparatus 105 traverses a selected

section of concrete 110. The prestressing apparatus 105 may be introduced by

placement of the prestressing apparatus 105 within molds or forms prior to pouring the

concrete 110, or by placement within a freshly poured body of concrete 110 before the

concrete 110 has set. All that is required is that the prestressing apparatus 105 should

be introduced to the concrete 110 before setting of the concrete 110 makes movement

of the prestressing apparatus 105 into place impossible. As the concrete 110 hardens,

the sheath 120, which is in direct contact with and enveloped by the concrete 110, is

captured by the body of concrete 110. It is preferable that movement of the sheath 120

with respect to the cured body of concrete 110 be prevented, as such movement would tend to relieve the compressive forces which are beneficially maintained within the

reinforced concrete structure 100.

After the concrete 110 has set, the tendon 130 may be tensioned, and the grout

140 cured so that and the tensioned tendon 130 is bonded to the concrete 110 through

the engagement between the tendon 130, the grout 140, the sheath 120 and the concrete

110.

The above described embodiments are given as illustrative examples only. It

will be readily appreciated that many deviations may be made from the specific

embodiments disclosed in this specification without departing from the invention.

Accordingly, the scope of the invention is to be determined by the claims below rather

than being limited to the specifically described embodiments above.

Claims

What I claim is:

1. A prestressing apparatus, comprising:

a. an elongated sheath;

b. an elongated high tensile strength tendon, at least partially disposed

within the elongated sheath; and

c. heat-curable grout which is initially pliable in ambient atmospheric

conditions, disposed within the elongated sheath.

2. The apparatus of Claim 1, wherein the grout is cured by application of heat to

the grout material.

3. The apparatus of Claim 2, wherein the grout further comprises a material which

is pliable prior to curing in ambient atmospheric conditions.

4. The apparatus of Claim 3, wherein the grout comprises an epoxy resin material.

5. The apparatus of Claim 1, wherein the elongated sheath is formed from a

generally flat elongated ribbon, having a first longitudinal edge and an opposite

second longitudinal edge, wherein said ribbon is rolled to form an elongated

tube, and wherein said first and second longitudinal edges of said ribbon are

sealed to form a watertight seam.

6. The apparatus of Claim 5, wherein the watertight seam comprises a weld

between said first and second longitudinal edges of said ribbon.

7. The apparatus of Claim 5, wherein the watertight seam comprises an adhesive

bond between said first and second longitudinal edges of said ribbon.

8. The apparatus of Claim 5, wherein the elongated sheath further comprises an

axis along its lengthwise direction, and wherein the watertight seam is

substantially parallel to the axis of the sheath.

9. The apparatus of Claim 5, wherein the elongated sheath further comprises an

axis along its lengthwise direction, and wherein the watertight seam is formed

transverse of the axis of the sheath.

10. The apparatus of Claim 1, wherein the elongated sheath further comprises an

axis along its lengthwise direction and includes corrugations formed transverse

of said axis.

11. The apparatus of Claim 10, wherein the corrugations are formed generally

perpendicularly to the axis of the sheath.

12. The apparatus of Claim 10, wherein the elongated sheath further comprises a

generally smooth inner surface and a corrugated outer surface.

13. The apparatus of Claim 1, wherein the elongated sheath comprises a seamless

tube.

14. The apparatus of Claim 1, wherein the elongated sheath is fabricated from a

coπosion resistant water impermeable plastic.

15. A prestressed and reinforced concrete structure, comprising:

a. a body of concrete;

b. at least one elongated sheath, each fixedly enclosed along its length

within the body of concrete;

c. at least one elongated high tensile strength tendon, each disposed at least

partially within an associated elongated sheath; and

d. grout, disposed within each elongated sheath and cured to bond each

tendon to an associated sheath along its length.

16. A sheath for use in combination with a tendon for providing a prestressing

apparatus, said sheath comprising a generally tubular element defining an

internal passage extending lengthwise therethrough, and having corrugations

which provide inwardly protruding areas and adjacent outwardly protruding

areas, the inwardly protruding areas providing a greater surface area than the

outwardly protruding areas.

7. A method of forming prestressed and reinforced concrete structures, comprising

the steps of providing a high tensile strength tendon: at least partially enclosing

the tendon in an elongated sheath; providing a heat-curable grout within the

sheath and coating the tendon; introducing the tendon and sheath to an unset

body of concrete; tensioning the tendon after the concrete has set; and heat-

curing the grout so that and the tensioned tendon is bonded to the concrete

through the grout and the sheath.

18. A method of forming prestressed and reinforced concrete structures, comprising

the steps of providing a high tensile strength tendon; forming an elongated

sheath from a substantially flat elongated ribbon by rolling the ribbon into an

elongated tube and sealing the overlapping longitudinal edges of the ribbon to

form a seam; at least partially enclosing the tendon in the sheath; providing an

uncured grout within the sheath and coating the tendon; introducing the tendon

and sheath to an unset body of concrete; tensioning the tendon after the concrete

has set; and curing the grout so that the tensioned tendon is bonded to the

concrete through the grout and the sheath.

19. The method of Claim 18, wherein the sealing step comprises heat sealing the

longitudinal edges of the ribbon to one another.

20. The method of Claim 18, wherein the sealing step comprises applying adhesive

to a selected one of the longitudinal edges of the elongated ribbon and bonding

the selected longitudinal edge to the other of the longitudinal edges.

21. A method of forming a corrugated sheath, comprising forming an elongated

ribbon of material; passing the ribbon through a pair of opposed wheels, the

first wheel having an iπegular surface for producing corrugations, and the

second wheel having a substantially smooth surface; rolling the ribbon into an

elongated tube; and sealing the overlapping longitudinal edges of the ribbon to

form a watertight seam.

22. A method of fabricating a prestressing apparatus, comprising providing a high

tensile strength tendon; at least partially enclosing the tendon in an elongated

sheath; and providing a heat-curable grout within the sheath and coating the

tendon.