US3838837A

US3838837A - Method and fabric for pipe reinforcement

Info

Publication number: US3838837A
Application number: US00330605A
Authority: US
Inventors: W Tolliver
Original assignee: New York Wire Mills Corp
Current assignee: New York Wire Mills Corp
Priority date: 1973-02-08
Filing date: 1973-02-08
Publication date: 1974-10-01
Anticipated expiration: 1991-10-01
Also published as: CA999537A

Abstract

Wire fabric for forming concrete pipe reinforcing cages, which cages have longitudinal and circumferential wire strands. The circumferential defining wire strands of the fabric at the bellforming end of the fabric are capable of expansion longitudinally of the strand. The longitudinal strands at the bell-forming end of the fabric are also capable of longitudinal expansion, comprising for example rigidly interconnected closed loops, or corrugations, to facilitate enlargement of the cage at the bell end of the cage.

Description

United States Patent 1191 Tolliver 1451 Oct. 1, 1974 l METHOD AND FABRIC FOR PIPE REINFORCEMENT [75] Inventor: Wilbur E. Tolliver, Holland, Mich.

[73] Assignee: New York Wire Mills C0rp.,

Tonawanda, NY.

221 Filed: Feb. 8, 1973 21 Appl. No.2 330,605

[52] U.S. C1. 245/1, 138/175 [51] Int. Cl. F161 9/08 [58] Field of Search 245/1-5, 10; 138/174, 175, 176; 52/645, 653, 669-671 [56] References Cited UNITED STATES PATENTS 860,400 7/1907 Maag 138/175 939,567 11/1909 138/175 950,264 2/1910 138/175 986,474 3/1911 52/653 2,717,615 9/1955 Peckworth 138/175 3,578,036 5/1971 Francois 140/112 FOREIGN PATENTS OR APPLICATIONS 1,241,091 8/1960 France 52/669 Primary ExaminerRichard J. Herbst Attorney, Agent, or Firm--Price, Heneveld, l-luizenga & Cooper 5 7] ABSTRACT Wire fabric for forming concrete pipe reinforcing cages, which cages have longitudinal and circumferential wire strands. The circumferential defining wire strands of the fabric at the bell-forming end of the fabric are capable of expansion longitudinally of the strand. The longitudinal strands at the bell-forming end of the fabric are also capable of longitudinal expansion, comprising for example rigidly interconnected closed loops, or corrugations, to facilitate enlargement of the cage at the bell end of the cage.

20 Claims, 9 Drawing Figures METHOD AND FABRIC FOR PIPE REINFORCEMENT BACKGROUND OF THE INVENTION The present invention relates to wire fabric for reinforcing concrete pipe. Such fabric normally is a network of wire strands. When rolled into a cylindrical cage, the wire strands extending the length of the cage are usually called longitudinals and those encircling the cage are called circumferentials. Some refer to the longitudinal wires as weft wires and the circumferential wires as warp wires.

The reinforcing cages are formed either circularly or elliptically with one enlarged end. The enlarged end is called the female" or bell end of the fabric. When the cage is formed and cast in concrete, the enlarged or female end receives the non-enlarged male or spigot end of a succeeding pipe.

One technique for forming the female end in a wire cage involves the use of non-rectilinear, for example, corrugated, circumferential strands at the female end of the fabric. After the cage is formed, the corrugated strands are expanded radially outwardly. They straighten to some extent, i.e., expand longitudinally on themselves, and thereby form the enlarged female end.

A first drawback to this method is that'the cage bells sometimes form lopsided. One side of the bell expands farther than another. When this occurs, the cage must be reoriented and the bell former again activated.

Another drawback to this method is that when stress forces are placed on the pipe, the corrugations or nonrectilinear deformations tend to straighten still further, thereby allowing the concrete to crack. When the pipe is buried in the ground, the weight of the dirt compressing the pipe tends to force the bell end to stretch outwardly to either side. The resulting forces of tension on the corrugated circumferentials tend to straighten them and thereby cause the concrete to crack.

Further, the above method is not readily applicable to forming round female or male ends on elliptical cages. Particularly, in the manufacture of larger pipe, it is necessary to form wire cages elliptically in cross section. This is true even though cages are to be used in round pipe. Even when the cage is formed elliptically, however, it is still desirable to form either or both the female and male end round. An elliptical male or female end complicates forming the actual pipe. Normally, cages are formed with no circumferentials welded on the female or male end. The longitudinal strands at the female or male end are formed to a circular radius and the circumferentials are then welded in place. In the alternative, small separate circular cages are joined to the ends of an elliptical cage to provide the female and male ends. The use of corrugated circumferentials and the radial expansion method set forth above is not readily adaptable to forming round female or male ends on elliptical cages. The circumferentials are prewelded to the female or male end, thus, inhibiting the bending of the longitudinals to a circular radius.

SUMMARY OF THE INVENTION In the present invention, fabric is employed in which bell formation can actually be done with less force than is required to form the bells on the cages referred to in the background of the invention. The fabric of the present invention not only includes at least one circumferential strand which is expandable longitudinally of itself, but also includes a plurality of longitudinal defining strands which include portions expandable longitudinally of the longitudinal defining strands, at least at the female or bell forming end thereof. These portions may comprise non-rectilinear deviations in the longitudinals. This makes it easier to bend the longitudinals outwardly at the bell forming end. For some reason, perhaps because expansion is easier, such fabric is easier to expand without the female end going askew and becoming lopsided. Secondly, because these portions of the longitudinals are expandable longitudinally, the fabric can first be formed into an ellipse and yet the bell can be expanded radially into a circle (see FIG. 4). The expandability allows some of the longitudinal defining strands at the female end to bestretched farther than others, thereby allowing the formation of a circular female end on an elliptical cage.

In a related aspect of the invention, the longitudinal defining strands may include expandable portions at the male or spigot end of the cage. This facilitates formation of a circular male end on an elliptical cage.

In another aspect of this invention, the employment of longitudinal strands having the longitudinally expandable portions makes it possible to replacen0nrectilinear circumferential wires with a sheet of nonrectilinear metal, preferably corrugated metal (see FIG. 5). This greatly increases the: strength of the bell end of the fabric and increases the resistance of the completed pipe to cracking when it is buried in the ground. If normal rectilinear longitudinals were employed, it would be extremely difficult to expand such a sheet of corrugated metal radially outwardly. By easing the resistance of bending longitudinals to radial expansion, the expandable longitudinals of the present invention make expansion of the corrugated metal much easier. The corrugated metal has much less tendency to straighten when the formed pipe is placed under pressure and, therefore, is much more resistant to cracking in the ground. Such a sheet of metal can also be employed at the male end where the longitudinals include expandable portions at the male end of the fabric.

It is preferable that the expandable portions of the longitudinals of the present invention be of a closed loop configuration. As explained in my copending patent application METHOD AND FABRIC FOR PIPE REINFORCEMENT, Ser. No. 330,510, filed on even date herewith and assigned to the same assignee as this invention, the closed loops resist expansion after they are cast in concrete. Forces tending to expand a loop further tend to work against one another because of the concrete trapped within the loop (see FIG. 7). Accord ingly, the problems of cracking after the concrete pipe has been formed and is stressed are minimized.

These and other aspects and objects of the present invention will be more fully appreciated and understood by reference to the written specification and appended drawings,

BRIEF DEBCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of fabric having both closed loop circumferentials and closed loop longitudinals at the female or bell forming end thereof;

FIG. 2 is a plan view of fabric having closed loop circumferentials and non-rectilinear longitudinals at the bell forming end;

FIG. 3 is a plan view of fabric having non-rectilinear corrugated circumferentials and longitudinals at the female forming end;

FIG. 4 is an end elevational view of the fabric of the type shown in FIG. 1, but having at least one closed loop circumferential strand at the male forming end of the fabric and having longitudinals with expandable portions at the male forming end thereof, formed into an elliptical cage with a circular female end and a circular male end;

FIG. 5 is a perspective view of fabric having nonrectilinear longitudinals and corrugated sheet metal in place of non-rectilinear circumferentials;

FIG. 6 is a perspective view of a cage formed from the fabric of FIG. 5;

FIG. 7 is a schematic stress diagram for a closed loop circumferential in concrete;

FIG. 8 is a schematic stress diagram for a corrugated circumferential in concrete; and

FIG. 9 is a fragmentary view of a closed loop longitudinal of an alternative construction.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred embodiment, the fabric 1 for forming a cage 10 having a female end 11 includes strands which will be the longitudinals and circumferentials of cage 10 (FIGS. 1 and 2). The circumferentials at the female end forming portion of the fabric, i.e., the circumferentials 40 from which female end 11 is formed, comprise a plurality of interconnected closed loops 41, expandable longitudinally with respect to the forming circumferential strands 40. The forming circumferentials 40 can be radially expanded to form female end 11, but resist any expansion once embedded in concrete (FIG. 7). The portions 50 of longitudinals 20 which are at the female end forming portion of the fabric are also comprised of closed loops 51 similar to closed loops 41 of female end circumferentials 40. The construction of longitudinal portions 50 can be identical to that of the construction of closed loop circumferentials 40. The presence of such closed loops 51 allows one to expand longitudinal end portions 50 to different lengths, thereby facilitating the formation of circular ends on an elliptical cage (FIG. 4). Yet, cracking problems are minimized by the closed loop configuration, as discussed in my copending application METHOD AND FABRIC FOR PIPE REINFORCEMENT, Ser. No. 330,5 l0, filed on even date herewith.

Longitudinal and circumferential defining

strands

20 and 30, are made of a material suitable for pipe reinforcing cages. Steel wire is the standard, but the use of equivalent materials is conceivable. Circumferential defining strands 30 are rigidly connected to longitudinal defining strands 20 by means of welding or the like. Preferably, one or the other sets of strands is merely laid on top of the other and is welded in place. However, the strands could be interwoven without departing from the spirit and broader aspects of the present invention.

The female end portions 50 of longitudinals 20 comprise a closed loop 51 at the base thereof and a straight portion extending upwardly therefrom. Portion 50 can be a strip of metal having a slit made therein and expanded to define the diamond-shaped closed loop 51. This strip of metal can then be welded to the remainder of longitudinal 20. However, a preferable way of forming longitudinals 20 is to slit a strand of wire at an appropriate location and expand it outwardly. An alternative way would be to form a deviation 51a in longitudinal 20 (see FIG. 9) and weld thereto a small piece of wire 51b formed as a mirror image of deviation 51a. The formation of the type of fabric shown in FIG. 1 into an elliptical cage with circular ends is shown in FIG. 4. Note that the longitudinals on the major axis of the ellipse are not expanded at all or only to a slight degree. The longitudinals on the minor axis of the ellipse, on the other hand, are expanded substantially so that all longitudinals at the ends are formed to the same radius.

The fabric used to make the cage of FIG. 4 includes not only closed loop circumferentials 40 at-the female forming end of the fabric, but also at the male or spigot forming end 12 of the fabric. Also, the longitudinals 20 include closed loops 52 at the male end 12 which are identical to closed loops 51. These loops 52 and the closed loop circumferential 40 at the male end facilitate compression of the male end into a circle. even though the rest of the cage is elliptical. The basic male end compression concept is disclosed in my copending patent application entitled METHOD AND FABRIC FOR PIPE REINFORCEMENT. Ser. No. 330,510, filed on even date herewith. In the broader aspects of the invention, it is important only that the longitudinals include a longitudinally expandable portion at the male or spigot forming end of the cage and that there be at least one longitudinally compressible circumferential at the male or spigot forming end of the cage.

Female end circumferentials 40 comprise a plurality of interconnected closed loops 4]. One way of forming such a strand is to slit a single strand of wire at spaced intervals and then expand it laterally. Such a strand would be a strand of expanded metal. The loops 4] would be diamond shaped, and would be interconnected at their apexes along their major axes.

Such closed loop strands could also be formed by corrugating two thin wire strands at regular intervals and welding them together in their mirror image positions at their corresponding nodes. While female or bell forming circumferentials 40 do not consist entirely of closed loops 41, it is preferable that closed loops 41 are at least regularly spaced throughout the length of female end forming circumferentials 40 since such regular spacing facilitates smoother radial expansion of female end forming circumferentials 40 into female end 1 1

Closed loops

41 and 51 must be expandable longitudinally with respect to female end forming circumferential strands 40 and longitudinal strands 20, respectively. Thus, when circumferentials 40 are expanded radially, closed loops 41 stretch out or expand longitudinally and thereby actually increase the efi'ective length of female end forming circumferentials 40. Loops 51 expand to allow end portions of longitudinals 20 to bend outwardly more readily.

The term expandable or longitudinally expandable as used herein refers to the capability of a strand to be expanded sufficiently readily and to a sufficient degree to make possible the formation of an enlarged end on the cylindrical cage. Naturally any strand of wire is to some extend longitudinally expandable in the strictest interpretation of that term. However one skilled in the art will appreciate that the term as used herein requires that the strand be capable of being expanded longitudinally on itself a sufficient degree to facilitate female end formation. Thus strands having closed loops or corrugations are capable of fairly extensive elongation and are referred to as longitudinally expandable or expandable. Normal wire strands as are conventionally used in reinforcing fabric are only slightly stretchable and are referred to as nonexpandable or non-longitudinally expandable.

Once concrete is poured around cage and female end 11, closed

loops

41 and 51 will actually resist further expansion, even though they are readily expandable before being embedded in concrete or the like. In FIGS. 7 and 8, the stress forces on a closed loop 41 or 51 (FIG. 7) are compared to those on a corrugated circumferential 70 having corrugations 71 (FIG. 8). When a pipe is placed in the ground and buried, forces of compression on the pipe are going to tend to stress corrugated circumferential 70 at the crown and invert of the pipe in an elongating direction as indicated by the arrows A. This will cause corrugated circumferential 70 to tend to expand, thereby applying forces generally in the directions indicated by arrows B. It can be clearly seen that such stressing will tend to force the concrete apart and create fissures and cracks. In contrast, the elongating or tensioning forces A on closed loop circumferentials 40 will not tend to create such fissures because the loops 41 will not tend to expand or close. Specifically, the force B on one side of the loop will be counteracted by the force C on the opposite side acting through the concrete which is trapped within the closed loop. Similarly, the force D will be counteracted by the force E. Thus, the tendency for the concrete to crack around closed loop circumferentials 40 will be greatly minimized. The same principles apply to the resistance of closed loops 51 to further closing or expan- SIOI1.

Loops All and 51 are relatively small in area, in order to maximize on the advantages of better stress distribution as illustrated in FIGS. 7 and 8. Thus, for example, the area defined by each

loop

41 or 51 should be less than the area defined by adjacent longitudinals on two sides and adjacent circumferentials 30 on the other two sides. If they are not, the advantages of stress distribution illustrated in FIGS. 7 and 8 will be minimized by the fact that forces of compression on the female end of the fabric will actually tend to compress concrete through the openings defined by loops 41. The use of loops of smaller area will minimize this effect.

Since the forces of stress placed on a pipe have less of a tendency to expand longitudinal end portions 50 than they do to expand closed loop circumferentials 40, it would be possible to use a longitudinal having a deviation or corrugation rather than closed loops 51. Thus, for example, the end portions 60 0f longitudinals 20 of the fabric shown in FIG. 2 include a plurality of deviations or corrugations 61 therein. Deformations 61 need not be regular, perhaps only one or two being sufficient to facilitate the greater expansion of some than others.

Similarly, this invention does not require that the end circumferentials be closed loop circumferentials 40. In FIG. 3, the end circumferentials are shown as corrugated circumferentlals 70 having a plurality of corrugations or deviations 71 therein. Additionally, it should be noted that the longitudinal end portions 60 shown in FIG. 3 have a greater number of deviations or corrugations 61 therein than are shown in FIG. 3. The differential expansion of the ends of longitudinals 20 can be achieved either with the corrugated longitudinal portions 60 or closed loop longitudinal portions 50.

The incorporation of deviations in the end portions of the longitudinals facilitates the incorporation of yet another innovation (FIGS. 5 and 6). The fabric in FIG. 5 eliminates the use of end circumferentials altogether. Rather, a piece of sheet steel having deformations or corrugations 81 therein is welded onto either or both the male or female end portions 60 of longitudinals 20 to provide a circumferential reinforcing member. Normally, such a corrugated sheet 80 would be extremely difficult to expand. However, the incorporation of deviations 61 in the end portions 60 of longitudinals 20 decreases the amount of force required to bend the end portions of the longitudinals upwardly or outwardly and thereby make more of the expanding force available for purposes of expanding sheet metal 80.

Sheet metal 80 additionally includes apertures 82 therein which increase the holding power of sheet metal 80 with respect to the concrete within which it is cast. The ability to use a piece of sheet steel 80 greatly increases the strength of the pipe at the bell end.

OPERATION In operation, the fabric I is first rolled into a cage 10, either circular or elliptical. The female end 11 of the fabric is then expanded longitudinally, the closed loops 41 extending longitudinally with respect to female end circumferentials 40. With the female end 11 formed, the cage 10 isplaced in a mold and concrete or other plastic is poured therearound. It is not inconceivable that the fabric of the present invention could be used to make reinforcing cages for pipe made of synthetic organic plastics as well as inorganic plastics such as concrete. Also, the male end of the cage can be compressed if made in accordance with the fabric described in conjunction with FIG. 4.

The use of closed loop portions 50 at the female or male ends of the longitudinals 20 further facilitates the job of expanding the female end Ill or compressing the male end 12. This is particularly true when a circular end must be formed on an elliptical cage as shown in FIG. 4. Those longitudinal strand. portions 50 which must deviate farthest from the minor axis of the ellipse may readily be expanded farther by the greater expansion of the individual loops 51 therein. The closed loop configuration in both the circumferentials and the longitudinals render the concrete pipe less apt to crack under stress. I

The end portions of the Iongitudlinals 20 at the ends of the fabric may be corrugated in :nature as, for example, end portions 60 (FIGS. 2 and 3). The inclusion of one or more deviations 61 facilitates the expansion of some female or male end portions of longitudinals 20 farther than others.

Indeed, the inclusionof deviations 61 makes it possible to use sheet steel corrugations 80 at the female or male ends of the fabric, rather than any circumferentials. Corrugated sheet steel 80 is expanded radially to form the female end, and more rigidly reinforces the entire female end than would corrugated circumferential strands 70.

Thus, the various aspects of the present invention greatly facilitate the formation of the female or male end of cages through radial expansion or compression. Of course. it is understood that the above are merely preferred embodiments of the invention and that many alterations can be made without departing from the spirit and broader aspects of the invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Fabric for forming concrete pipe reinforcing cages comprising: a network of strands of material suitable for forming concrete pipe reinforcing cages, said strands defining the longitudinal and circumferential strands of the reinforcing cage when said fabric is formed into said cage; said fabric including first and second end portions, one of which will define the bell portion of a reinforcing cage and the other of which will define the spigot portion of the cage; said fabric including a main body portion between said first and second ends; and at least one circumferential defining strand at said first end portion of the fabric being expandable longitudinally of said one strand, said longitudinal defining strands including a portion expandable longitudinally of said longitudinal defining strands, at least at said first end portion of said fabric, said longitudinal defining strands being generally straight and free of deviations in said main body portion of said fabric.

2. The fabric of claim 1 in which each of said longitudinal defining strands includes a corrugated portion at least at said first end thereof.

3. The fabric of claim 1 comprising: at least two of said one circumferential strands at said first end of said fabric.

4. Fabric is forming concrete pipe reinforcing cages comprising: a network of strands of material suitable for forming concrete pipe reinforcing cages, said strands defining the longitudinal and circumferential strands of the reinforcing cage when said fabric is formed into said cage; and at least one circumferential defining strand at a first end of the fabric being expandable longitudinally of said one strand, said longitudinal defining strands including a portion expandable longitudinally of said longitudinal defining strands, at least at said first end of said fabric; said longitudinal defining strands being free of direct connection to one another and including at least one closed loop therein expandable longitudinally of said longitudinal defining strands at said first end of said longitudinal defining strands.

5. The fabric of claim 4 in which said closed loop portion of said longitudinal defining strands comprises a strand having been slit and expanded.

6. The fabric of claim 4 in which said closed loop portion of said longitudinal defining strand comprises: a diamond shaped segment.

7. The fabric of claim 4 in which said longitudinal strands comprise a first strand having a deviation therein and a second strand of configuration comparable to that of said deviation joined together in their mirror image position at their respective nodes.

8. The fabric of claim 1 which includes at least one circumferential defining strand at a second end of the fabric being compressible longitudinally of said one strand, said longitudinal defining strands including a portion expandable longitudinally of said longitudinally defining strands at said second end of said fabric.

9. Fabric for forming concrete pipe reinforcing cages comprising: a network of strands of material suitable for forming concrete pipe reinforcing cages, said strands defining the longitudinal and circumferential strands on the reinforcing cage when said fabric is formed into said cage; said fabric including first and second end portions, one of which will define the bell portion of a reinforcing cage and the other of which will define the spigot portion of the cage; said fabric including a main body portion between said first and second ends; and at least one circumferential defining strand at said first end portion of the fabric being compressible longitudinally of said one strand, said longitudinal defining strands including a portion expandable longitudinally of said longitudinal defining strands, at least at said first end portion of the fabric being compressible longitudinally of said one strand, said longitudinal defining strands including a portion expandable longitudinally of said longitudinal defining strands, at least at said first end portion of said fabric, said longitudinal defining strands being generally straight and free of deviations in said main body portion of said fabric.

10. Fabric for forming concrete pipe reinforcing cages comprising: a network of strands of material suitable for forming concrete pipe reinforcing cages, said strands defining the longitudinal and circumferential strands of the reinforcing cage when said fabric is formed into said cage; a sheet of material suitable for reinforcing concrete pipe connected to said longitudinal defining strands at a first end of said fabric for providing a circumferential reinforcing means at a first end of a cage, said sheet having a non-rectilinear cross section on a plane generally parallel to one of said circumferential defining strands and said sheet being expandable along said non-rectilinear cross section; and at least some of said longitudinal defining strands including a portion expandable longitudinally of said longitudinal defining strands, at least at said first end of said fabric whereby expansion of said sheet is rendered easrer.

l l. The fabric of claim 10 in which all of said longitudinal defining strands include a non-rectilinear portion, expandable longitudinally of said longitudinal defining strands, at least at said first ends of said longitudinal strands.

12. The fabric of claim 11 in which said sheet comprises a sheet of corrugated sheet metal.

13. The fabric of claim 12 in which said sheet includes apertures therein for trapping concrete and thereby further strengthening a pipe formed with said fabric 14. The fabric of claim 12 in which each of said longitudinal defining strands includes a corrugated portion at least at said first end thereof.

15. The fabric of claim 12 in which said longitudinal defining strands include at least one closed loop therein expandable longitudinally of said longitudinal defining strands at said first end of said longitudinal defining strands.

16. The fabric of claim 15 in which said closed loop portion of said longitudinal defining strands comprises a strand having been slit and expanded.

17. The fabric ofclaim 15 in which said closed loop portion of said longitudinal defining strand comprises: a diamond shaped segment.

18. The fabric of claim in which said longitudinal strands comprise a first strand having a deviation therein and a second strand of configuration compara ble to that of said deviation joined together in their mirror image position at their respective nodes.

19. The fabric of claim 10 which includes a sheet of material suitable for reinforcing concrete pipe at a second end of said fabric for providing a circumferential reinforcing means at a second end of a cage, said sheet having a non-rectilinear cross section on a plane generally parallel to one of said circumferential defining strands and said sheet being compressible along said non-rectilinear cross section; and at least some of said longitudinal defining strands including a portion expandable longitudinally of said longitudinal defining strands at said second end of said fabric whereby compression of said sheet at said second end of said fabric is rendered easier.

20. Fabric for forming concrete pipe reinforcing cages comprising: a network of strands of material suitable for forming concrete pipe reinforcing cages, said strands defining the longitudinal and circumferential strands of the reinforcing cage when said fabric is formed into said cage; a sheet of material suitable for reinforcing concrete pipe connected to said longitudinal defining strands at a first end of said fabric for providing a circumferential reinforcing means at a first end of a cage, said sheet having a non-rectilinear cross section and a plane generally parallel to one of said circumferential defining strands and said sheet being compressible along said non-rectilinear cross section; and at least some of said longitudinal defining strands including a portion expandable longitudinally of said longitudinal defining strands, at least at said first end of said fabric whereby compression of said sheet is rendered easier.

Claims

10. Fabric for forming concrete pipe reinforcing cages comprising: a network of strands of material suitable for forming concrete pipe reinforcing cages, said strands defining the longitudinal and circumferential strands of the reinforcing cage when said fabric is formed into said cage; a sheet of material suitable for reinforcing concrete pipe connected to said longitudinal defining strands at a first end of said fabric for providing a circumferential reinforcing means at a first end of a cage, said sheet having a non-rectilinear cross section on a plane generally parallel to one of said circumferential defining strands and said sheet being expandable along said non-rectilinear cross section; and at least some of said longitudinal defining strands including a portion expandable longitudinally of said longitudinal defining strands, at least at said first end of said fabric whereby expansion of said sheet is rendered easier.

11. The fabric of claim 10 in which all of said longitudinal defining strands include a non-rectilinear portion, expandable longitudinally of said longitudinal defining strands, at least at said first ends of said longitudinal strands.

13. The fabric of claim 12 in which said sheet includes apertures therein for trapping concrete and thereby further strengthening a pipe formed with said fabric

14. The fabric of claim 12 in which each of said longitudinal defining strands includes a corrugated portion at least at said first end thereof.

17. The fabric of claim 15 in which said closed loop portion of said longitudinal defining strand comprises: a diamond shaped segment.

18. The fabric of claim 15 in which said longitudinal strands comprise a first strand having a deviation therein and a second strand of configuration comparable to that of said deviation joined together in their mirror image position at their respective nodes.