GB1566977A - Non-stressed high strength cement-containing pipe and its production - Google Patents

Description

(54) NON-STRESSED, HIGH STRENGTH, CEMENT-CONTAINING PIPE AND ITS PRODUCTION (71) We, CEMENT ASBESTOS PRODUCTS COMPANY, a Corporation organised and existing under the laws of the State of Alabama, United States of America, of 2144 Highland Avenue South, Birmingham, State of Alabama, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to non-stressed, reinforced cement-containing pipe and more particularly to non-stressed, reinforced, high strength, cement-containing pipe, especially asbestos-cement pipe characterized by high bursting strength and high flexural strength, and a method for producing such pipe.

U.S. Patents 3,631,897 and 3,813,098 disclose stressed concrete or asbestos-cement pipe and other tubular articles wherein tensioning strands of nylon polyester or polypropylene are wrapped or wound around the surface of the pipe and tubular article and are stretched to a length at least 5% greater than its stable, relaxed length. The material of the strands is fixed in its stretched or elongated condition around the pipe and tubular article, and the material of the strands fixed around the pipe and tubular article is in tension in an amount of 15% or more of its ultimate breaking strength. U.S. Patents 2,251,205; 2,375,971 and 2,711,291 disclose concrete pipe having metal wire, such as steel wire, wound around the pipe to reinforce it. U.S. Patent 2,791,241 relates to fibrous reinforced resin pipe comprising a first series of long reinforcing fibers of glass or asbestos helically wound at a helix angle of from 2" to 10 , and a second series of the aforementioned long reinforcing fibers superposed on the first series of helical winding and helically wound with a helix angle of from 80" to 88 . A cured resin secures the fibers together to form an integral pipe.

An important application of asbestos-cement pipe of the prior art consisting of asbestos fiber, cement and silica has been for conveying or conducting water under pressure. Cast iron pipe is also utilized in the prior art for conducting water under pressure. The cast iron pipe may require the installation of a sacrificial or impressed current cathodic protection system, not required by asbestos-cement pipe, to obviate corrosion of the iron pipe. The cathodic protection requirement adds materially to the cost of the cast iron pipe. Further, there is room for improvement of the bursting strength, as well as the flexural strength, of the aforementioned prior art asbestos-cement pipe.

In accordance with the present invention, there is provided a non-stressed, reinforced, rigid, substantially inflexible, cement-containing pipe comprising a tubular body of a composition comprising a hydraulic cement, a plurality of windings of continuous filament of an aromatic polyamide fiber having a tensile strength of at least 390,000 p.s.i. and an elongation capability not above 7% about at least one layer of the cement-containing composition within the interior of the pipe, the continuous filament of said windings also being characterized by substantially not being in tension and having its normal length.

The invention also provides a method for producing non-stressed, reinforced, rigid, substantially inflexible, cement-containing pipe which comprises forming a tubular body comprising a plurality of superposed layers of a composition comprising a hydraulic cement and water, winding a continuous filament of an aromatic polyamide fiber about at least one of said cement-containing layers within the interior of the pipe during or after the formation of the cement-containing layer, and before the formation of the next complete cement-containing layer thereon, without stretching the continuous filament and substan tially without placing the continuous filament in tension, the aromatic polyamide fiber of the continuous filament being characterized by having a tensile strength of at least 390,000 p.s.i. and an elongation capability not above 7%, and curing the thus-obtained pipe.

The continuous filament of aromatic polyamide fiber is usually wound and maintained taut about the one or more layers of cement-containing composition within the interior of the pipe. By "taut" as used herein in referring to the windings of continuous filament of the aromatic polyamide fiber about one or more layers of the cement-containing composition within the interior of the pipe is meant that the continuous filament of aromatic polyamide fiber is tightly or substantially tightly wound about the cement-containing layer or layers without any slack or substantially any slack in such filament.

Non-stressed asbestos-cement pipe of the present invention is characterized by providing a striking improvement in hydrostatic bursting strength and a considerably higher hydrostatic bursting strength than the hydrostatic bursting strength of the prior art asbestos-cement pipe which is not reinforced with a continuous filament or strand.

Non-stressed asbestos-cement pipe of this invention is also characterized by exhibiting a considerable improvement in flexural strength. In hydrostatic bursting strength tests, non-stressed asbestos-cement pipe of this invention withstood hydrostatic pressure as great as 1100 psi, which was about 110% above the hydrostatic bursting strength required by the A.W.W.A. to be withstood by the pipe. In flexing strength tests, non-stressed asbestos-cement pipe of this invention attained a flexing strength which was as much as 37% above the flexing strength required by the A.W.W.A. to be withstood by the pipe.

Strength tests conducted on the non-stressed high strength asbestos-cement pipe of this invention indicate that such pipe is well adapted for use in conveying or conducting water under considerably higher pressures than the prior art asbestos-cement pipe is capable of.

Further, an additional margin of safety is achieved in the pipe of the present invention destined for use in conveying water under roads or streets due to the continuous filament windings of the aromatic polyamide fiber, the pipe under roads or streets tending to be subjected to certain extreme conditions of heavy vibration, heavy weight, or beam conditions.

The aromatic polyamide fiber of the continuous filament utilized in the present invention has high tensile strength of at least about 390,000 psi and a low elongation capability of no more than 7%. This aromatic polyamide fiber usually has a high tensile strength in the range of about 400,000 psi to about 525,000 psi and preferably a very low elongation capability in the range of about 2% to about 4%. A suitable fiber is that obtainable from E.I. duPont de Nemours & Co. under the tradename KEVLAR spun from an aromatic polyamide which is reported to be prepared by reacting an aromatic dicarboxylic acid, such as terephthalic acid or anhydride, and an aromatic diamine such as p-phenylene diamine.

Alternatively, the aromatic polyamide may be an aromatic aminocarboxylic acid such as, for example, 4-aminobenzoic acid. The aromatic polyamides have been assigned the generic classification "aramid" by the United States Federal Trade Commission.

The preferred KEVLAR 49 aromatic polyamide fiber for use herein has a very high tensile strength in the range of about 520,000 psi to able 525,000 psi and a very low elongation capability within the elongation capability range of about 2% to about 4%. By the term "elongation capability" as utilized herein and in the appended claims is meant the maximum or substantially maximum length to which the fiber can be elongated from its stable relaxed length. Thus, for example, an elongation capability of 7% means that the fiber can be elongated to a maximum length which is 7% greater than its stable, relaxed length; and an elongation capability of about 2% means that the fiber can be elongated to a maximum length which is about 2% greater than its stable relaxed length.

The continuous filament utilized herein can be, for example, yarn, roving, or woven fabric usually of narrow width or in strip form with the strip being of width, for example, of typically about 1/4' - 1'. Conceivably the continuous filament can also be rope, or non-woven mat or fabric of narrow width or in the form of strip as disclosed immediately above. The yarn is utilizable in a wide range of deniers of, typically, 195 through 1420. By "continuous filament" as utilized herein and in the appended claims is meant a filament of considerably length. ordinarily of a plurality of feet in length, and usually of many feet or numerous feet in length, and usually of many feet or numerous feet in length as contrasted with short, discrete, fibers of asbestos of typical length of 1 - 1/4' or other short, chopped filler of similar short length.

The fibers utilized in this invention have a high strength-to-weight ratio and are unequalled in the category of high-tenacity or high tensile strength organic fibers; the preferred KEVLAR fiber having very high tensile strength which is more than twice as great as that of a representative nylon yarn of the comparable weight and considerably greater than that of Dacron polyester and polypropylene fiber. "DACRON" is a Registered Trade Mark. The striking superiority in tensile strength of KEVLAR 49 fiber over that of typical nylon, Dacron and polypropylene fibers is set forth hereafter: Tensile Strength KEVLAR 49 aramid fiber .. 500,000 psi Nylon type 728 . 140,000 psi Dacron T-68 . .. 160,000 psi Polypropylene 94,000 psi The nylon type 728 was an aliphatic polyamide.

A representative high strength aliphatic polyamide nylon yarn has a tenacity of approximately 10 grams per denier; kevlar yarn in a comparable weight has a tenacity of over 22 grams per denier and other kevlar yarns have tenacities in excess of 25 grams per denier.

Further, KEVLAR fiber has good to excellent dimensional stability due to its high or intermediate modulus and low elongation, light weight, good temperature resistance and showing good heat and fire resistance. the preferred kevlar 49 yarn for use herein has a density of 1.45 g/cc, which is 40% lower than glass; fiber elongation of 2.8% which is significantly lower than the fiber elongation of other organic fibers; tensile strength of 525,000 psi, which is considerably above the tensile strength of conventional organic fibers; modulus of 19 x 106 psi, which is twice that of E-glass; good chemical resistance, -being highly resistant to organic solvents, fuels and lubricants; and excellent temperature resistance with no degradation of the yarn properties in short exposures up to temperature of 500"F. The KEVLAR 29 yarn, also utilizable herein, has a density of 1.44 g/cc, break elongation of 3-4%-, tensile strength of 400,000 psi, modulus of 9 x 106 psi, and excellent temperature resistance having useful properties over a wide range of temperatures from subzero to high temperatures, specifically from -420 F. to 500"F.

It is critical that the continuous filament of aromatic polyamide fiber utilized in the present invention not be placed or maintained in tension during the production of the pipe.

The reason for this is that the asbestos-cement is of plastic consistency and relatively soft, and the concrete may be only partially cured and also relatively soft at the time such continuous filament is wound about the asbestos-cement layer or layers, or concrete layer or layers, and if the continuous filament was placed and maintained under significant tension at the time of winding, the continuous filament would cut into the soft asbestos-cement or concrete and could cut all the way through the soft asbestos-cement or concrete to the mandrel. In either case, the cut asbestos-cement pipe or cut concrete pipe would be damaged and undoubtedly damaged to such an extent as to render it unmarketable. The continuous filament of aromatic polyamide fiber of the high strength cementitious pipe of this invention may be placed in a certain amount of tension during use of the pipe when the pipe is subjected to certain stresses, for example, flexural stress when buried under roads or streets due to the weight of automobiles.

It is important that the aromatic polyamide fiber utilized herein have a low elongation capability not above 7%, for the reason that if such fiber had an elongation capability significantly above 7%, which is the situation with highly elastic fibers such as aliphatic polyamide nylons, "Dacron" polyester and polypropylene fibers, the reinforced pipe would undoubtedly not have the strength that the pipe has when reinforced with the continuous filament of the aromatic polyamide having the lower elongation below 7% or at least not above 7%. The reinforcing filament of the fiber having the higher elongation capability significantly above 7% would tend to stretch when the pipe reinforced therewith is subjected to material stress, and for this reason the reinforced pipe would not be as strong as when reinforced with the continuous filament of aromatic polyamide fiber having the lower elongation capability below 7% or at least not above 7%.

The plurality of windings of the continuous filament within the pipe interior can be wound manually or by conventional winding apparatus over the entire length or substantially the entire length of the pipe, over a major portion, i.e. 50% or more, of the length of the pipe, over a minor portion, i.e. less than 50%, but an appreciable portion of the length of the pipe, over a central portion, for instance an intermediate one-third, of the length of the pipe, or over another portion of the length of the pipe. The windings are ordinarily helical windings and can be a single layer of windings of typical helix angle of 50 or two or more superposed layers of windings around one or more of the asbestos-cement or concrete layers, with the helix angle of one of the layers of such windings usually being different than the helix angle or another layer of helical windings of such continuous filament superposed directly thereon, for example, a helical winding in one direction having an acute helix angle of typically 50 and a helical winding in the opposite direction having an obtuse helix angle of typically 1200.

The method of this invention for producing the non-stressed, reinforced cementitious or cement-containing pipe comprises forming a tubular article body comprising a plurality of superposed layers of a composition comprising a hydraulic cement and water, and winding a continuous filament of the aromatic polyamide fiber previously disclosed herein, and obtainable as KEVLAR aramid fiber, about one or more of such cement-containing layers usually while wet and uncured or only partially cured within the interior of the pipe during or after the formation of the cement-containing layer and before the formation of the next succeeding, complete, cement-containing layer thereon. The continuous filament of the aromatic polyamide fiber is characterized by having the high tensile strength and low elongation as well as the other properties previously disclosed herein. The thus-obtained pipe is cured. The method herein is eminently well adapted for producing asbestos-cement pipe for conveying water under high pressure. This method is also utilizable for producing concrete pipe for draining purposes or "so called" culvert pipe.

More specifically, in the method of the present invention for producing the non-stressed, high strength asbestos-cement pipe, a water-laid, wet asbestos-cement sheet is formed from an aqueous asbestos fiber-cement slurry, and water is removed from the wet asbestoscement sheet. The partially dewatered, still wet sheet is wound up on a pipe-forming mandrel as a plurality of spirally wound, wet asbestos-cement layers thereon while applying pressure to the wet asbestos-cement sheet on the mandrel. A continuous filament, e.g.

yarn, of the aromatic polyamide fiber previously disclosed herein, and obtained as KEVLAR aramid fiber, is wound, usually by helical winding and usually taut, about one or more of the spirally wound, wet asbestos-cement layers within the interior of the ultimate pipe on the mandrel during the winding of the wet asbestos-cement layers on the mandrel without any or substantially any slack in the continuous filament, without any or substantially any stretching of the continuous filament and without placing the continuous filament in tension. The continuous filament of the aromatic polyamide fiber is characterized by having the high tensile strength, low elongation, and the other properties previously disclosed herein. The mandrel is separated from the inner surface of the pipe, the pipe is ordinarily precured, the mandrel extracted from within the pipe, and the precured pipe is subsequently cured to a fully-cured or substantially fully-cured state. One spirally wound asbestos-cement layer of the pipe is one complete revolution of an asbestos-cement layer of the pipe.

The asbestos-cement pipe is ordinarily precured subsequent to separating the mandrel from the pipe inner surface, and prior to extracting the mandrel from within the pipe. The precuring is carried out for a period sufficient to render the asbestos-cement pipe sufficiently rigid or self-sustaining to enable the mandrel to be extracted from the pipe without disintegration of the pipe.

The precuring and curing of the asbestos-cement pipe is ordinarily and preferably carried out by subjecting the pipe to heating at elevated temperature in an atmosphere having a high moisture content or high relative humidity, such precuring and curing being known and conventional in the art. The curing can be also accomplished by immersing the asbestos-cement pipe in water for 28 days, but this is not preferred.

In producing the high strength asbestos-cement pipe herein, the wet asbestos-cement sheet is ordinarily formed by one or more, typically two, rotating sieve cylinders partially immersed in the aqueous asbestos-cement slurry, the sieve cylinder picking up the slurry as a water-laid, thin, wet asbestos-cement sheet or web on its sieve surface, with water being removed from the wet asbestos-cement sheet on the sieve cylinder surface by being forced through the sieve openings of the sieve cylinder. The still wet asbestos-cement sheet is transferred from the rotating sieve cylinder onto a moving, continuous, water-permeable member, usually a felt, and the asbestos-cement sheet is transported on the moving, permeable member to the pipe-forming mandrel for winding thereon while additional water is removed from the asbestos-cement sheet during its transport on the mandrel.

The aromatic polyamide-reinforced concrete pipe of this invention can be produced, in accordance with one method, by casting the concrete in a steel pipe mold having a steel cylindrical mandrel disposed and secured therein and with the mold cavity having a width equivalent to about 1/2 the radial thickness desired in the wall of the ultimate pipe when two concrete layers are to be cast in forming the pipe. When more than two concrete layers, for example three or four concrete layers, are desired in the wall of the ultimate pipe the mold cavity has a width equivalent to about 1/3rd or 1/4th respectively of the radial thickness desired in the wall of the ultimate pipe. After a concrete layer is cast, the cast concrete in the mold is precured or allowed to partially harden in conventional and known manner, for instance in ambient air, for a period sufficient to render the cast concrete self-sustaining when the outer mold casing is removed and thereafter the outer mold casing is removed to expose and render accessible the cast concrete layer, which still may be relatively soft concrete due to being only partially cured and hence only partially hardened, on the mandrel. The continuous filament of the KEVLAR aramid fiber having the characteristics of high tensile strength and low elongation as well as the other properties previously disclosed herein is then would manually or by conventional winding apparatus, ordinarly by helical winding, and usually taut, around the concrete layer without any or substantially any stretching of the filament and without any or substantially any tensioning of the filament.

The continuous filament windings of KEVLAR aramid fiber are maintained about each concrete layer by tying the filament to itself on the concrete layer by a suitable, strong knot which is the means maintaining the continuous filament windings about the one or more cement-containing or concrete layers within the pipe interior. An outer mold casing of larger diameter than the first-mentioned mold casing and of sufficient or corresponding diameter to provide a second concrete layer of the desired thickness is then disposed in proper position about the mandrel and another concrete layer cast over the continuous filament windings of KEVLAR aramid fiber and the first concrete layer. If only two layers of concrete are desired in the pipe, after removal of the mandrel, the pipe is cured in conventional manner in an autoclave under steam and pressure or, if desired, in ambient air. When more than two concrete layers are desired in the pipe, the continuous filament of KEVLAR aramid fiber is again wound, ordinarily by helical winding and usually taut, around the second layer of concrete. The continuous filament windings of the KEVLAR ararnid fiber are maintained or secured about each concrete layer by tying the filament to itself on the concrete layer by a suitable strong knot. This sequence of casting concrete layers and winding the continuous filament of KEVLAR aramid fiber about the cast concrete layer is repeated until the desired number of concrete layers are cast about the mandrel, provided however that the continuous filament of KEVLAR aramid fiber is ordinarily not wound round the last concrete layer, and hence there ordinarily are no exposed continuous filament fibers on the outermost exposed main surface of the product pipe.

The concrete can also be produced by centrifugal casting the concrete layers on the mandrel in conventional manner, followed by winding the continuous filament of KEVLAR fiber about each concrete layer in the manner disclosed previously herein.

The present invention can be utilized with any method for producing a plural layer asbestos-cement pipe or concrete pipe so long as the method enables the winding of the continuous filament of KEVLAR aromatic polyamide fiber about the asbestos-cement or concrete layer or layers as desired.

The continuous filament of KEVLAR aromatic polyamide fibre is usually untreated filament when wound about the asbestos-cement or concrete layer or layers herein in producing the high strength pipe. However, such filament could be treated, if desired, prior to such winding, by being, for example, precoated with a silica gel or asbestos-cement slurry.

Figure 1 is an isometric view partly broken away showing a length of non-stressed, high strength asbestos-cement pipe of the present invention; Figure 2 is a side elevational view partly broken away showing a length or non-stressed, high strength asbestos-cement pipe of the present invention; Figure 3 is a longitudinal view partly broken away showing a length of non-stressed, high strength concrete pipe of this invention; Figure 4 is a flow diagram illustrating apparatus utilized in producing non-stressed, high strength asbestos-cement pipe of the invention Figure 5 is a schematic, fragmentary view showing apparatus for winding the continuous filament of aromatic polyamide fiber within the asbestos-cement pipe during formation of sucn pipe on the pipe machine; and Figure 6 is a schematic, fragmentary view showing moveable bobbins of the filament winding apparatus for winding the continuous filament of aromatic polyamide within the asbestos-cement pipe during formation of such pipe on the pipe machine mandrel.

With reference to Figures 1 and 2, asbestos-cement pipe 10 comprises a plurality of convolutely wound up and compressed asbestos-cement layers 11, and a plurality of windings 12 of a continuous filament yarn of an aromatic polyamide fiber having the characteristics previously described. The continuous filament yarn 12 is compressed flat or substantially flat, or oblong or substantially oblong, between the asbestos-cement layers 11 as shown in Figure 2, during the formation of the pipe on the pipe-forming mandrel, with the cement being forced or dispersed between the fibers of yarn 12. The cement hardens between the yarn fibers with the result the yarn 12 and its fibers are bonded within the pipe by the hardened cement. The outermost exposed main surface of pipe 10 is free of the continuous filament.

As shown in Figure 1, the windings 12 of the continuous filament yarn are a helical winding around each of the asbestos-cement layers exclusive of the outer-most asbestoscement layer 13, with the windings being at different helix angles, and in different directions in succeeding layers 11.

Refering to Figure 3, a concrete pipe 15 comprises a plurality of separate, concentric layers 16 of concrete, and a plurality of windings 17 of a continuous filament yarn of aromatic polyamide fiber having the properties previously set forth. The windings 17 of the continuous filament yarn are a helical winding around each concrete layer excluding the outermost layer, with the helical windings being at different helical angles and in different directions in succeeding layers.

Referring now to Figure 4, asbestos fibers, cement and silica flour are supplied from hoppers 19, 20 and 21 respectively into vat 22, and alkaline water is fed through line 23 into vat 22. The asbestos fibers, cement, silica flour and water are mixed together in vat 22 to form an asbestos cement slurry. As exemplary, a cement of a certain type well known in the art for use in producing asbestos-cement pipe and silica flour are supplied into vat 22 in the proportions by weight, of 60% of the cement and 40% of the silica. The asbestos fiber, which is typically a mixture of long chrysotile fiber and long crocidolite fiber, is supplied into vat 22 in amount, by weight, of about 15% of the total weight of the cement plus the silica. Water, which has been previously made alkaline, is fed into vat 22 in amount sufficient to make a suitable slurry of consistency well known in the art of asbestos-cement pipe manufacture. The temperature of the thus-formed asbestos-cement slurry in vat 22 is typically about 105"F. Sieve cylinders 24 and 25 of typically 40 mesh are partially immersed in and rotate counter-clockwise in the asbestos-cement slurry in vats 26 and 27 to pick up the slurry as a thin wet sheet of asbestos-cement on the sieve cylinders, the water being forced out of the wet asbestos-cement sheets through the sieve openings of the cylinders.

Felt 28 moving on felt-carrying rolls 9 picks up the wet sheet from cylinders 25 and 24 and carries the asbestos-cement sheet to pipe-forming mandrel 29. Additional water is removed from the wet asbestos-cement sheet on porous felt 28 by a partial vacuum applied thereto by a vacuum box (not shown). The west asbestos-cement continuous sheet of reduced water content is wound up on mandrel 29 while under considerable pressure applied by press rolls 30 and supported by breast roll 31. Mandrel 29, press rolls 30, and breast roll 31 are steel rolls. Felt 32, which is pressed directly against the wet asbestos-cement pipe being formed on mandrel 29 by the press rolls, functions to further dewater the asbestos-cement pipe being formed on mandrel 29. Felt 32 runs over felt-carrying rolls 33 and vacuum boxes which remove water from felt 32.

While the wet asbestos-cement sheet is being wound up on pipe-forming mandrel 29, a continuous filament 35 of aromatic polyamide fibers as previously described is helically wound taut, but without stretching, from bobbin 36, also shown in Figure 5, around each of the plurality of asbestos-cement layers being wound up on mandrel 29 and within the interior of the thus-forming pipe. As shown in Figure 5, the continuous filament 35 passes from bobbin 36 through guide 37, power feed rolls 38, and between the blades of automatic shear 39 prior to being helically wound on the asbestos-cement layers on mandrel 29. Shear 39 cuts the continuous filament 35 to the desired length of such filament for helically winding on the asbestos-cement layers on mandrel 29. The windings of such continuous filament are maintained taut or substantially taut about the asbestos-cement layers within the interior of the pipe substantially without being stretched and substantially without being in tension by means of the asbestos-cement layers superposed directly on such windings.

With reference to Figure 6, two or more bobbins 36 are employed, with each bobbin oscillating back and forth and applying a helical winding of the continuous filament 35 over more than 50% of the length of the asbestos-cement layers, exclusive of the outermost asbestos-cement layer, conveyed to mandrel extractor 46 by chain conveyor 44. The mandrel 29 is manually removed from the asbestos-cement pipe 40 at extractor 46, and pipe 40 is then manually transferred to another chain conveyor 47. Pipe 40 is conveyed by conveyor 47 into another moist heating oven 48, and pipe 40 is held in oven 48 at temperature of typically about 120"F. and relative humidity of typically about 60So-705to for about 1 1/4 hours. The removed mandrel 29 is carried by elevator 49 to another chain conveyor 50 which returns mandrel 29 and usually also other mandrels that have been removed at mandrel extractor 46 to the pipe forming machine.

Pipe 40 is removed from oven 48 after typically about 1 1/4 hours and is loaded usually together with other asbestos-cement pipes removed from oven 48 onto a tray. The trays having the thus-formed asbestos-cement pipe 40 are then transferred by transporting vehicle 51 to autoclaves 52 and inserted into the autoclaves. The doors of autoclaves 52 are then closed and the asbestos-cement pipes are fully cured in autoclaves 52 under the following conditions: steam pressure is built up gradually to 125 psi over about 1-1 1/2 hours and such steam pressure maintained an additional 12-16 hours; temperature is about 350"F.

at such steam pressure. The asbestos-cement pipe 40 are then removed from autoclave 52 as fully cured asbestos-cement pipe. The ends of the fully cured pipe are then machined at machining apparatus to the desired configuration, usually as a spigot end onto which a coupling is installed. Prior to shipping such pipe, it is subjected to flexural testing and internal pressure hydrostatic testing.

Tests were conducted wherein measured lengths of continuous filament of KEVLAR 49 aramid fiber of three different deniers were intermittently wound into the asbestos-cement wall of asbestos-cement pipes during formation of the pipe on the mandrel of the pipe-forming machine during normal pipe production cycles. The continues filament of KEVLAR 49 aramid fiber was fed by hand with only sufficient tension on the filament to prevent any slackness. Inasmuch as 16" Class 150 asbestos-cement pipe was being produced at the time of the tests and as the weakest point in the pipe of this size was in the hydrostatic pressure tests with the primary fracture consistently being in the center or approximate center of each 13' length, the winding of the continuous filament of KEVLAR 49 aramid fiber was concentrated in the center or intermediate area of the pipe. The hydrostatic pressure tests were conducted in accordance with ASTM test method C-500, and the flexural or flex tests were conducted in accordance with ASTM test method C-500. In the tests and test results hereinafter set forth, the language "required" pressure or pressure "required" means the hydrostatic pressure or flexural pressure required by the A.W.W.A., i.e. the American Water Works Association, to be withstood by the asbestos-cement pipe.

The language "close weave" referred to in the tests and test results means the continuous filament was helically wound into the pipe with the continuous filament of the helical windings being adjacent and/or touching with other and without any large gaps between the helical windings. The language "open spiral weave" referred to in the tests and test results means the continuous filament was helically wound into the pipe with the continuous filament of the helical windings being spaced an appreciable distance apart and not being adjacent or contacting each other.

The considerable improvement in bursting strength provided by the pipe of the present invention containing the internally wound continuous filament of KEVLAR fiber over the conventional pipe not containing the continuous filament of KEVLAR fiber is shown hereinafter by the test results of Test Nos. 1, 2, 3, 4, 5, 6 and 7 as compared with the hydrostatic bursting strength results for such conventional pipe not containing the continuous filament reported hereinafter in the "Results" portion of Test No. 6. The excellent flexural strength provided by the pipe of this invention containing the internally wound continuous filament of KEVLAR fiber is shown hereinafter by the test results of Test Nos. 8, 9, 10, 11 and 12. Striking increases in hydrostatic bursting strength representing over 200% of the required hydrostatic bursting strength were achieved by pipe of the present invention containing the internally wound continuous filament of KEVLAR aromatic polyamide fiber, as shown hereinafter by the test results of Test No. 4.

Considerable increases in flexural strength of about 35% or more above the required flexural strength or "load" were attained by pipe of the present invention containing the internally wound continuous filament of KEVLAR fiber, as is shown hereinafter by the test results of Test Nos. 9 and 11.

Test No. I Lengths of each of denier 195, 390 and 1424 of continuous filament of KEVLAR 49 aramid fiber were wound into the center of different asbestos-cement pipe beginning with the first or second asbestos-cement layer of each pipe and continuing until the full length was used. 184' of 195 denier, 92' of 380 denier and 24' of 1424 denier continuous filament were wound into the different asbestos-cement pipes. A simple spiral wrap was used with no attempt at weaving.

Results: Pipe #1 which contained 184' of 195 denier withstood the hydrostatic pressure test up to 540 psi and failed at 540 psi.

Pipe #2 which contained 92' of 380 denier withstood the hydrostatic pressure test up to 500 psi and failed at 500 psi.

Pipe #3 which contained 48' of 1424 denier withstood the hydrostatic pressure test up to 490 psi and failed at 490 psi.

Test No. 2 Variations of spiral weaving of unmeasured amounts of each of denier 195, 380 and 1424 of continuous filament of KEVLAR 49 aramid fiber were used. Beginning in the center of each 16" Class 150 pipe, the continuous filament was fed by hand moving toward one end and continuing toward the opposite end and returning to the center.

Results: Pipe #1 contained 1424 denier. The continuous filament was added from the center to the drive side end and back to the operator's side but the continuous filament had to be cut before returning to the center. This resulted in the pipe being double wound in one end and only singularly wound in the other. The pipe did not burst in the hydrostatic pressure test until 540 psi and it was noted that the pipe split only from the center back to the side that contained only one strand.

Pipe #2 contained 380 denier continuous filament of KEVLAR 49 aramid fiber and did not burst in the hydrostatic pressure test until 550 psi.

Pipe #3 contained 195 denier continuous filament of KEVLAR 49 aramid fiber and did not burst in the hydrostatic pressure test until 525 psi.

Pipe #4 contained 195 denier continuous filament of KEVLAR 49 aramid fiber and did not burst in the hydrostatic pressure test until 540 psi.

Three of these four pipes passed the required 525 psi. Also worth noting is that none of the pipe split the full length of the pipe when they burst, which is normal type break in large diameter pipe.

Test No. 3 Again concentrating on the center section of the asbestos-cement pipe, all three diameters or deniers of continuous filament of KEVLAR 49 aramid fiber were woven over varying lengths of the center of the pipe.

Results: Pipe #1 had 195 denier continuous filament of KEVLAR 49 aramid fiber woven over the middle three feet moving from center left approximately 18" and wrapping at this point for 10 layers before moving to the right 18" past center, hesitating for 10 layers and then back to center for 10 layers. This was continued for the full 110 layer count. This pipe was tested in the hydrostatic pressure tests first at 500 psi, then at 545, 565, 650, 650, 650 psi and finally 685 psi before bursting. Again, this pipe did not split from end to end but had a u-shaped crack from the edge of the application area back.

Pipe #2 was woven in the manner of #1 with the 195 denier continuous filament of KEVLAR 49 aramid fiber being confined to the center two feet. This pipe was tested in the hydrostatic pressure tests at 500 psi, 650 psi and 760 psi before bursting.

Pipe #3 was woven the same as #2 but using 380 denier continuous filament of KEVLAR 49 aramid fiber. This pipe was tested twice at 650 psi and did not burst until 690 psi.

Pipe #4 was woven in the manner of above but using 1424 denier continuous filament of KEVLAR 49 aramid fiber and confining it to the middle 18' of the pipe. This pipe was tested in the hydrostatic pressure tests at 530, 590, 600 and 650 psi and did not burst until 700 psi.

None of these pipe split from end to end upon failure in the hydrostatic tests. As 80% of all 16' Class 150 asbestos-cement pipe produced on the day prior to the tests and 20% of all 16" Class 150 asbestos-cement pipe produced on the same day without the incorporation therein of the continuous filament of KEVLAR 49 aramid fiber by winding into the pipe did not have sufficient strength to withstand hydrostatic pressure testing of even 525 psi, the test results show the considerable improvement in strength by winding the KEVLAR 49 aramid fiber into the pipe. The test results indicated that a close weave in the center area of the pipe is superior to just an open spiral weave over the full length of the pipe.

Test No. 4 This was a multi-purpose test on 4" Class 150 asbestos-cement pipe since all pipe 4" through 8" in diameter must withstand both a hydrostatic pressure test as well as a flexural test. Thirteen asbestos-cement test pipes were made and stripped on the mandrel of the pipe machine.

Results: Pipes #1, #2 and #3 were woven with an open spiral weave in the middle three feet of each pipe for the duration of roll-up using 195 denier continuous filament of KEVLAR 49 aramid fiber. All three passed the required 134 psi gauge pressure or 1100 lb applied load of the flexural test. Pipe #1 was subjected to 140 psi in the flexural test and broke 7'-8" from the D/S end which was just at the edge of the application area. Pipe #2 and #3 were then subjected to hydrostatic pressures beginning at the required 525 psi and continuing in 50 ib increments until pipe #2 broke at 1000 psi, breaking on the D/S end only. Pipe #3 burst at hydrostatic pressure of 990 psi on the O/S end. Both broke outside the application area.

Pipes #4 and #5 were woven as close as possible to the previous three but using 380 denier continues filament of KEVLAR 49 aramid fiber. Both pipes passed the required 134 psi gauge pressure of the flexural test with pipe #5 withstanding an additional flexural test at 140 psi. Again both pipes were hydrostatically tested beginning at 525 psi and continuing until pipe #4 burst at 880 psi hydrostatic pressure and pipe #5 burst at 600 psi. Both broke on the O/S end and outside the application area.

Pipes #6 and #7. The continuous filament was applied as in pipe above but using 1424 denier continuous filament of KEVLAR 49 aramid fiber. Both pipes passed the required 134 psi gauge pressure of the flexural test. Pipe #6 was subjected to an additional flexural test of 140 psi and broke at 7'-9" from the D/S end -again in the edge of the application area. Pipe #7 was hydrostatically tested beginning at 525 psi and continuing to 880 psi. This pipe did not burst and was held for further tests.

Pipes #8 through #13 were woven beginning in the center for three layers, moving 1'-6" to the left and pausing for three layers, moving to the right for 3'-0" and pausing for three layers and returning to the center for the duration. Pipe #8 contained 1424 denier continuous filament of KEVLAR 49 aramid fiber and passed the required 134 psi gauge pressure flexural test, and broke during an additional flexural test at 150 psi 5'-10" from D/S end.

Pipes #9 and #10 contained 380 denier continuous filament of KEVLAR 49 aramid fiber with both passing the required 134 psi gauge pressure flexural test with #9 failing the additional flexural test at 140 psi and breaking 7'-7" from the D/S end. Pipe #10 was hydrostatically tested at 525 psi and continued at 50 psi increments until failing at a hydrostatic pressure test of 1000 psi, breaking on O/S end.

Pipes #11, 12 and 13 all contained 195 denier material and all passed the 134 psi gauge pressure flexural test, with #11 failing additional flexural tests at 150 psi.

Both pipes #12 and #13 were hydrostatically tested from 525 psi until pipe #12 failed in the center at hydrostatic pressure of 990 psi and pipe #13 failed in the center at hydrostatic pressure of 1100 psi. (1100 psi represents 210% of required hydrostatic test pressure.) It should be noted that a 4" Class 150 pipe is required to pass 525 psi in hydrostatic pressure testing. The majority of the pipe of the test runs of this Test No. 4 withstood hydrostatic pressure up to 900-1000 psi, which is a striking increase in hydrostatic bursting strength in the pipe, which represents about 190% (for 1000 psi) of the required hydrostatic test pressure.

Test No. 5 A continuation of the testing on 16" Class 150 asbestos-cement pipe was carried out.

Again, continuous filament of KEVLAR 49 aramid fiber of each denier was woven within the center three feet of each test pipe beginning at the center and pausing for 10 layers before moving to the left in helix approximately 18" and pausing again for 10 layers; then, again in helix, returned to center and paused for 10 layers before moving right approximately 18" and pausing for 10 layers, and again returning to the center. This rhythm was continued throughout the complete roll-up cycle of approximately 110 asbestos-cement layers on the pipe forming mandrel.

Results: Pipes #1 and #2 contained 195 denier continuous filament of KEVLAR 49 aramid fiber with #1 withstanding hydrostatic pressures up to but bursting at 585 psi. Pipe #2 folloed the same test pattern and burst at 580 psi.

Pipe #3 contained 380 denier continuous filament of KEVLAR 49 aramid fiber and burst at 530 psi of hydrostatic pressure.

Pipes #4 and #5 contained 1424 denier continuous filament of KEVLAR 49 aramid fiber with pipe #4 withstanding hydrostatic pressures up to but bursting at 600 psi. Pipe #5 also followed the same test pattern and burst at hydrostatic pressure of 615 psi.

Pipes made during the period of these test runs without the addition of this continuous filament of KEVLAR fiber would only withstand hydrostatic pressures in the 400-425 psi range.

Test No. 6 195 denier continuous filament of KEVLAR 49 aramid fiber was added to two asbestos-cement pipes using different methods than any tried in prior tests. In pipe #7 the filament was applied at points dividing the pipe in three equal sections but without any weaving. In pipe #8 the filament was applied at points dividing the pipe in four equal sections again without weaving. In both pipes the filament was added for the duration of the roll-up cycle.

Results: Pipe #7 burst at hydrostatic pressure of 460 psi.

Pipe #8 burst at hydrostatic pressure of 480 psi.

Test No. 7 195 denier continuous filament of KEVLAR 49 aramid fiber was wound into the asbestos-cement pipe with a very open spiral from end to end and back again for the duration of the roll-up cycle in Pipe #9.

Results: Pipe #9 burst at hydrostatic pressure of 480 psi.

Test No. 8 A follow-up series of tests on 4" Class 150 asbestos-cement pipe and Class 200 O.D.

asbestos-cement pipe similar to previous tests. Only the results of the flexural tests are available to date. Pipe #1 was wound with 195 denier continuous filament of KEVLAR 49 aramid fiber with an open spiral beginning one foot on each side of center and continuing past center toward opposite end. Pipe #2 was wound with 195 denier continuous filament of KEVLAR 49 aramid fiber with an open spiral beginning 6" from one end and continuing to within 6" of the opposite end. Pipe #3 was wound with 195 denier continuous filament of KEVLAR 49 aramid fiber in the manner of #1 but beginning 2 feet on either side of center (195 denier).

Results: Pipe #1 with a 5.36 O.D. was flexurally tested at the required 168 psi gauge pressure for Class 200 pipe, and on each subsequent test pressure was increased until the pipe broke at 210 psi gauge pressure (25% above required pressure for the flexural test).

Pipe #2 with a 5.16 O.D. was flexurally tested at the required 134 psi gauge pressure for Class 150 pipe and flexurally retested at 150 psi gauge pressure (12% above required pressure for the flexural test) and did not break.

Pipe #3 with a 5.36 O.D. was flexurally tested at the required 168 psi gauge pressure for Class 200 pipe and flexurally retested at 182 psi gauge pressure (8% above required pressure for the flexural test) and did not break.

Test No. 9 Asbestos-cement pipes #4, #5, #6 and #7 were wound with continuous filament of KEVLAR 49 aramid fiber with a close spiral weave over the middle 2-3 feet of each pipe using a single strand of the 195 denier filament. The product pipes were subjected to flexural tests.

Results: Maximum Pressure Gauge Resisted Pressure Pressure in Flex- at Class O.D. Required ural Test Failure #4 200 5.45 168 psi 218 psi 225 psi* #5 200 5.42 168 psi 212 psi #6 200 5.40 168 psi 192 psi #7 150 5.13 134 psi 150 psi 160 psi** *34% above required load **19% above required load Test No. 10 Asbestos-cement pipes #8 and #9 were wound with a close spiral weave over the middle 6 feet with two strands of 195 denier continuous filament of KEVLAR 49 aramid fiber simultaneously. The product pipes were subjected to flexural tests.

Results: Maximum Pressure Gauge Resisted Pressure Pressure in Flex- at Class O.D. Required ural Test Failure #8 150 5.08 134 psi 134 psi #9 150 5.11 134 psi 152 psi 168 psi* *25% above required load Test No. 11 Asbestos-cement pipes #10 and #11 were wound with a close spiral weave over the middle 6 feet with two strands of 380 denier continuous filament of KEVLAR 49 aramid fiber simultaneously.

Results: Maximum Pressure Gauge Resisted Pressure Pressure in Flex- at Class O.D. Required ural Test Failure #10 200 5.45 168 psi 185 psi 230 psi* #11 200 5.40 168 psi 185 psi *37% above required load Test No. 12 Pipes #12 through 17 were wound with a close spiral weave over the middle 4 feet with one strand of 195 denier continuous filament of KEVLAR 49 aramid fiber and one strand of 380 denier continuous filament of KEVLAR 49 aramid fiber simulataneously.

Results: Maximum Pressure Gauge Resisted Pressure Pressure in Flex- at Class O.D. Required ural Test Failure #12 200 5.41 168 psi 168 psi #13 200 5.41 168 psi 168 psi #14 200 5.44 168 psi 180 psi 210 psi* #15 200 5.41 168 psi 185 psi #16 200 5.40 168 psi 188 psi 210 psi* #17 200 5.40 168 psi 190 psi *25% above the required load The reason pressure values are not set forth for pipe #12, 13, 15 and 17 in the foregoing table in the "Pressure At Failure" column is that such pipe were not tested to destruction but held for other testing. In the flexural test each psi gauge pressure represents 8.209 Ibs.

actual applied load. "O.D." in all tests means outer diameter of the pipe. The "D/S" end of the pipe in the tests is that end of the pipe on the drive side of the mandrel; the "O/S" end of the pipe in the tests is that end of the pipe on the operator's side of the mandrel, which is the end opposite the drive side end.

Several of these pipes that were tested to destruction withstood pressures in the flexural tests approaching 25%-35% above the required load but it should be pointed out that all were tested at pressures equivalent to the highest breaking load of any compatible pipe produced without the addition of this material. The average breaking load of the test pipe in the flexural tests is approximately 20% higher than the average breaking load of a like number of standard pipe with highest strength.

WHAT WE CLAIM IS: 1. A non-stressed, reinforced, rigid, substantially inflexible, cement-containing pipe comprising a tubular body of a composition a hydraulic cement, a plurality of windings of continuous filament of an aromatic polyamide fiber having a tensile strength of at least 390,000 p.s.i. and an elongation capability not above 7%, about at least one layer of the cement-containing composition within the interior of the pipe. the continuous filament of the said windings also characterized by substantially not being in tension and having its normal length.

2. A pipe according to claim 1, in which the pipe body composition comprises asbestos fiber and cement.

3. A pipe according to claim 2, in which the pipe body composition also contains silica.

4. A pipe according to any one of the preceding claims, in which the windings of continuous filament are taut about the layer of cement-containing composition.

5. A pipe according to any one of the preceding claims, in which the fiber has tensile strength of 400,000 psi to 525,000 psi preferably 520,000 to 525,000 psi, and an elongation capability of 2% to 4%.

6. A pipe according to any one of the preceding claims, in which the continuous filament is yarn.

7. A pipe according to any one of the preceding claims, in which the pipe body comprises a plurality of spirally wound up layers.

8. A pipe according to any one of the preceding claims, in which the pipe body

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (26)

**WARNING** start of CLMS field may overlap end of DESC **. Test No. 12 Pipes #12 through 17 were wound with a close spiral weave over the middle 4 feet with one strand of 195 denier continuous filament of KEVLAR 49 aramid fiber and one strand of 380 denier continuous filament of KEVLAR 49 aramid fiber simulataneously. Results: Maximum Pressure Gauge Resisted Pressure Pressure in Flex- at Class O.D. Required ural Test Failure #12 200 5.41 168 psi 168 psi #13 200 5.41 168 psi 168 psi #14 200 5.44 168 psi 180 psi 210 psi* #15 200 5.41 168 psi 185 psi #16 200 5.40 168 psi 188 psi 210 psi* #17 200 5.40 168 psi 190 psi *25% above the required load The reason pressure values are not set forth for pipe #12, 13, 15 and 17 in the foregoing table in the "Pressure At Failure" column is that such pipe were not tested to destruction but held for other testing. In the flexural test each psi gauge pressure represents 8.209 Ibs. actual applied load. "O.D." in all tests means outer diameter of the pipe. The "D/S" end of the pipe in the tests is that end of the pipe on the drive side of the mandrel; the "O/S" end of the pipe in the tests is that end of the pipe on the operator's side of the mandrel, which is the end opposite the drive side end. Several of these pipes that were tested to destruction withstood pressures in the flexural tests approaching 25%-35% above the required load but it should be pointed out that all were tested at pressures equivalent to the highest breaking load of any compatible pipe produced without the addition of this material. The average breaking load of the test pipe in the flexural tests is approximately 20% higher than the average breaking load of a like number of standard pipe with highest strength. WHAT WE CLAIM IS:

1. A non-stressed, reinforced, rigid, substantially inflexible, cement-containing pipe comprising a tubular body of a composition a hydraulic cement, a plurality of windings of continuous filament of an aromatic polyamide fiber having a tensile strength of at least 390,000 p.s.i. and an elongation capability not above 7%, about at least one layer of the cement-containing composition within the interior of the pipe. the continuous filament of the said windings also characterized by substantially not being in tension and having its normal length.

2. A pipe according to claim 1, in which the pipe body composition comprises asbestos fiber and cement.

3. A pipe according to claim 2, in which the pipe body composition also contains silica.

4. A pipe according to any one of the preceding claims, in which the windings of continuous filament are taut about the layer of cement-containing composition.

5. A pipe according to any one of the preceding claims, in which the fiber has tensile strength of 400,000 psi to 525,000 psi preferably 520,000 to 525,000 psi, and an elongation capability of 2% to 4%.

6. A pipe according to any one of the preceding claims, in which the continuous filament is yarn.

7. A pipe according to any one of the preceding claims, in which the pipe body comprises a plurality of spirally wound up layers.

8. A pipe according to any one of the preceding claims, in which the pipe body

comprises a plurality of concentric layers.

9. A pipe according to claim 7 or 8, in which the continuous filament is helically wound about a central portion of one or more of the layers within the pipe interior.

10. A pipe according to claim 7 or 8, in which the continuous filament is helically wound to provide at least two superposed helical windings of the continuous filament about at least one of the layers within the interior of the pipe, the helix angle of one of the helical windings being different than the helix angle of another layer of the helical windings superposed directly thereon.

11. A pipe according to claim 10, in which two superposed helical windings of the continuous filament are provided about each cement layer within the interior of the pipe.

12. A heat cured, high strength, substantially inflexible asbestos-cement pipe having circumferentially incorporated intermediate the wall thickness thereof an unstretched, substantially non-tensioned continuous filament of an aromatic polyamide fiber, said fiber being characterized by having a tensile strength of at least 390,000 psi, and an elongation capability not above 7%.

13. A method for producing non-stressed, reinforced, rigid, substantially inflexible, cement-containing pipe which comprises forming a tubular body comprising a plurality of superposed layers of a composition comprising a hydraulic cement and water, winding a continuous filament of an aromatic polyamide fiber about at least one of said cement-containing layers within the interior of the pipe during or after the formation of the cement-containing layer, and before the formation of the next complete cement containing layer thereon, without stretching the continuous filament and substantially without placing the continuous filament in tension, the aromatic polyamide fiber of the continuous filament being characterized by having a tensile strength of at least 390,00 psi, and an elongation capability not above 7%, and curing the thus-obtained pipe.

14. A method according to claim 13, in which the tubular body is formed by first forming a wet asbestos-cement sheet frm an aqueous asbestos fiber-cement slurry, removing water from the wet asbestos-cement sheet, and applying the de-watered, still wet sheet on a pipe-forming mandrel; and the continuous filament of the aromatic polyamide fiber is wound about the wet asbestos-cement and within the interior of the ultimate pipe on the mandrel during the application of the wet asbestos-cement layers on the mandrel.

15. A method according to claim 14, in which the filament of aromatic polyamide fiber is wound taut about the cement-containing layer.

16. A method according to claim 14 or 15, in which after the winding of the continuous filament on the layer, the mandrel is separated from the inner surface of the thus-formed pipe and extracted from within the pipe, and the pipe is then cured.

17. A method according to claim 15 or 16, in which the pipe is precured subsequent to separating the mandrel from the pipe inner surface and prior to extracting the mandrel from within the pipe.

18. A method according to any one of claims 15 to 17, in which the wet asbestos-cement sheet is formed by at least one rotating sieve cylinder partially immersed in the slurry, the sieve cylinder picking up the slurry as a water-laid, thin, wet asbestos-cement sheet on its sieve surface, water being removed from the wet asbestos-cement sheet on the sieve cylinder surface by being forced through the sieve openings of said sieve cylinder, the still wet asbestos-cement sheet being transferred from the rotating sieve cylinder onto a moving, continuous, water-permeable member and said asbestos-cement sheet being transported on the moving, permeable member to the pipe-forming mandrel for winding thereon while additional water is removed frm the asbestos-cement sheet during its transport on the permeable member.

19. A method according to any one of claims 15 to 18, in which the asbestos fiber-cement slurry also contains silica.

20. A method according to any one of claims 15 to 19, in which the continuous filament is helically wound about each of a plurality of the asbestos-cement layers within the pipe interior.

21. A method according to any one of claims 15 to 19, in which the continuous filament is helically wound about a central portion of one or more of the asbestos-cement layers within the pipe interior.

22. A method according to any one of claims 15 to 19, in which the continuous filament is helically wound to provide at least two superposed helical windings of said continuous filament about at least one of the asbestos-cement layers within the interior of the pipe, the helix angle of one of the helical windings being different than the helix angle of another layer of helical windings superposed thereon.

23. A method according to any one of claims 13 to 22, in which the aromatic polyamide fiber has tensile strength of 400,000 to 525,000 psi and an elongation capability of 2% to 4%.

24. A method according to any of claims 13 to 23, in which the continuous filament is yarn.

25. A non-stressed, reinforced, rigid, substantially inflexible, cement-containing pipe substantially as hereinbefore described and as illustrated in Figures 1 to 3 of the accompanying drawings.

26. A method for producing non-stressed, reinforced, rigid, substantially inflexible, cement-containing pipe substantially as hereinbefore described with reference to Figures 4, 5 and 6 of the accompanying drawings.