CH622073A5 - Reinforced asbestos/cement pipe and its method of manufacture - Google Patents

Abstract

The pipe (10) is made of asbestos/cement reinforced with a continuous unprestressed filament (12) made of an aromatic polyamide of tensile strength greater than 27,300 kg/cm<2>. The pipe is formed of several concentric layers (11) of asbestos/cement, the filament being wound without elongation in helicoid turns, of alternate pitch, around one or several internal layers of the pipe. <IMAGE>

Description

       

  
 

** ATTENTION ** start of the DESC field may contain end of CLMS **. 

 



   CLAIMS
 1.  Reinforced asbestos-cement pipe, characterized by a body composed of several layers made of a composition comprising hydraulic cement, and comprising several turns of a continuous filament of an aromatic polyamide fiber having a tensile strength of at least minus 27,300 kg / cm2 and an elongation capacity not exceeding 7%, wound at least around an inner layer of the pipe, the filament being relaxed and having substantially the same length as at rest. 



   2.  Pipe according to claim 1, characterized in that the body material also comprises silica. 



   3.  Pipe according to one of the preceding claims, characterized in that the turns of continuous filament are at least partially embedded in the layer situated inside them. 



   4.  Pipe according to one of the preceding claims, characterized by a tubular body comprising several helical turns of filament. 



   5.  Pipe according to one of claims 1 to 3, characterized by a tubular body comprising several concentric layers. 



   6.  Pipe according to one of the preceding claims, characterized in that the filament is wound in a helix on the central part of one or more internal layers of the pipe. 



   7.  Pipe according to one of Claims 1 to 5, characterized in that the continuous filament is wound in a helix so as to form at least two helical turns of superimposed filament around at least one of the inner layers of the pipe, two directly overlapped turns having no different. 



   8.  Method for manufacturing a pipe according to claim 1, characterized in that a tubular body is formed comprising several layers of a material comprising hydraulic cement and water, which is wound without drawing a filament continuous of an aromatic polyamide fiber around at least one of the inner layers during or after its formation, and before the complete formation, around it, of the next layer containing cement, and in that it is hardened the pipe thus obtained. 



   9.  Method according to claim 8, characterized in that the tubular body is produced by first forming an aqueous asbestos-cement sheet from a slurry based on water, cement and asbestos fibers, then by removing water from this sheet, then applying the still wet sheet to a forming mandrel, and winding the continuous filament of aromatic polyamide fiber around one or more wet sheets of asbestos-cement intended to form one or more inner layers of the body. 



   10.  Method according to one of claims 8 or 9, characterized in that the filament is kept taut during the winding around the asbestos-cement sheet. 



     He.     Method according to one of claims 8 to 10, characterized in that the filament is wound in helical coils around each of several of the inner layers of the pipe. 



   The present invention relates to asbestos-cement reinforced pipes or conduits, and more particularly to non-prestressed asbestos-cement pipes or conduits which are characterized by high resistance to bursting and to bending, as well as to a process for manufacturing these pipes or conduits. 



   U.S. Patents Nos. 3631897 and 3813098 describe reinforced asbestos-cement pipes, as well
 than other tubular parts, in which tensioning strands
 Nylon, polyester or polypropylene, are wrapped around
 the surface of the pipe or tubular part and are stretched to a
 length exceeding at least 5% their stable and relaxed length.  The material of these arms is fixed, in its stretched or elongated state, around the pipe or the tubular part, and the material of the strands which is fixed around the pipe or the tubular part is subjected to a tension representing at least 15% of its breaking strength.  United States patents Nos. 2251205, 2375971 and 2711291 describe concrete pipes which are wrapped with a metallic wire, for example a steel wire, to reinforce them. 

  U.S. Patent No. 2,791,241 relates to fiber reinforced resin pipes comprising a first series of long reinforcing fibers, glass or asbestos, which are helically wound with a helix angle of 2 to 10 #, and a second series of long reinforcing fibers mentioned above which are superimposed on the first series forming the helical winding, and which are wound in a helix with a helix angle of 80 to 880.     A hardened resin joins the fibers together to form a one-piece pipe. 



   An important application of asbestos-cement pipes of the prior art, composed of asbestos fibers, cement and silica, is the transport or routing of water under pressure.  Cast iron pipes are also used in the prior art for conveying water under pressure.  These cast iron pipes may require the installation of a consumable cathodic protection device which is not required by asbestos-cement pipes, to avoid corrosion of the cast iron pipe.  The cathodic protection requirement materially adds to the cost price of cast iron pipes.  Furthermore, it is still possible to improve the burst strength, as well as the flexural strength, of asbestos cement pipes of the aforementioned prior art. 



   The present invention provides a reinforced asbestos cement pipe, characterized by a body composed of several layers made of a composition comprising hydraulic cement, and comprising several turns of a continuous filament of an aromatic polyamide fiber having a tensile strength at least 27,300 kg / cm2 and an elongation capacity not exceeding 7%, wound at least around an inner layer of the pipe, the filament being relaxed and having substantially the same length as at rest. 



   The invention also relates to a method of manufacturing this pipe, characterized in that a tubular body is formed comprising several layers of a material comprising hydraulic cement and water, which is wound without drawing a filament continuous of an aromatic polyamide fiber around at least one of the inner layers during or after its formation, and before the complete formation, around it, of the next layer containing cement, and in that it hardens the pipe thus obtained. 



   The continuous filament of aromatic polyamide fiber will preferably be wound and kept stretched around the composition layer or layers containing the cement inside the pipe.  The term stretched as it is used here to describe the turns of continuous filament of the aromatic polyamide fiber around the
 or layers of the composition containing the cement inside
 of pipe means that the continuous filament of polyamide fiber
 aromatic is wrapped tightly or relatively tightly
 around the layer (s) containing the cement, without slack
 or practically without slack in the same filament. 



   The asbestos-cement pipe thus obtained is not prestressed
 and provides a remarkable improvement in resistance to
 bursting under hydrostatic pressure, which is considerable
 larger than that of the asbestos cement pipes of the
 prior art, which are not reinforced with a filament or
 continuous strand.  Asbestos cement pipes according to this
 invention are also characterized by resistance to bending
 considerably better.  In resistance tests to
 bursting under hydrostatic pressure, asbestos pipes
 cement according to the invention withstood hydrostatic pressure
 reaching up to 77 kg / cm2, about 110% more than the



  bursting strength under hydrostatic pressure which is required by the American Water Works Association (AWWA).  In the flexural strength tests, the asbestos-cement pipes according to the invention achieved a flexural strength which was not less than 37% higher than that required by AWWA. 



   The resistance tests which have been subjected to the asbestos-cement pipes according to the invention reveal that these pipes can perfectly be used to convey water under considerably higher pressures than the pipes made of them are capable of. asbestos-cement of the prior art.  Furthermore, the present invention provides an additional safety margin for the pipes which are intended to convey water under the roads or under the streets, thanks to the coils of continuous filament of the aromatic polyamide fiber, the pipes which are under roads or under streets that tend to be subject to certain extreme conditions of vibration or load, for example. 



   The aromatic polyamide fiber of the continuous filament which is used in the present invention has a high tensile strength, at least equal to approximately 27,300 kg / cm 2, and a low elongation capacity, of not more than 7%.  This aromatic polyamide fiber usually has a high tensile strength, between approximately 28,000 kg / cm 2 and approximately 36,750 kg / cm 2, and preferably a very low elongation capacity, between approximately 2% and approximately 4%.  A suitable fiber is that which is marketed by the company E. I.  DuPont of
Nemours & Co. , under the trademark Kevlar, spun from an aromatic polyamide which is prepared by reacting an aromatic dicarboxylic acid, for example terephthalic acid or anhydride, with an aromatic diamine such as pphenylenediamine. 

  Or the aromatic polyamide can be an aromatic aminocarboxylic acid such as 4-aminoben zoic acid, for example.  These aromatic polyamides will be designated below by the generic term aramid. 



   The aromatic polyamide Kevlar 49 fiber which is preferred to be used here has a very high tensile strength, between approximately 36,400 kg / cm2 and approximately 36,750 kg / cm2, and a very low elongation capacity, between 2% about and about 4%.  By the term elongation capacity as used herein and in the claims is meant the maximum or practically maximum length that the fiber can take from its stable relaxed length.  For example, an elongation capacity of 7% means that the fiber can be elongated to a maximum length which is 7% greater than its stable relaxed length, and that a capacity of elongation of approximately 2% means that the fiber can be elongated to a maximum length which is approximately 2% greater than its stable relaxed length. 



   The continuous filament used here can be, for example, a thread, a roving, or a woven fabric usually of small width or in the form of a strip, this strip having for example a typical width of approximately 6 mm to 2, 5 cm.  Optionally, this continuous filament can also be a cord, or a nonwoven web of a fabric of small width or in the form of a strip, as we have just said.  The wire can be used within wide denier limits, typically between 195 denier and 1420 denier. 



  By the term continuous filament as used herein and in the claims is meant a filament of considerable length, usually of the order of 1 m in length, and usually of several meters in length, by as opposed to short, independent asbestos fibers with a typical length of 3.2 cm, or other short cut fibers of similar length. 



   The fibers which are used in the present invention have a high strength / weight ratio and are unmatched in the category of organic fibers of high toughness or high tensile strength; the preferred Kevlar fiber has a very high tensile strength, which is more than twice that of a representative nylon yarn of comparable weight, and considerably greater than that of a polyester fiber Dacron or d '' a polypropylene fiber.  The remarkable superiority of the tensile strength of Kevlar 49 fiber over that of typical nylon, Dacron and polypropylene fibers is demonstrated below:
 Resistance
 tensile
 (kg / cm2)
Aramid fiber Kevlar 49 35000
Nylon type 728 9800
Dacron T-68 11200
Polypropylene 6580
Nylon type 728 is an aliphatic polyamide. 



   A nylon thread, which is a representative highly resistant aliphatic polyamide, has a toughness of about 10 g / denier; Kevlar yarn, with a comparable weight, has a toughness of more than 22 g / denier, and the other Kevlar yarns have tenacity of more than 25 g / denier. 



   Furthermore, Kevlar fiber has a dimensional stability which is good and even excellent, thanks to its high or medium modulus of elasticity, and its low elongation, its lightness and its good temperature resistance, because it has good resistance to heat and cold.  The Kevlar 49 yarn that we prefer to use here has:

   a density of 1.45 g / cm3, 40% less than glass; 2.8% fiber elongation, significantly less than the fiber elongation of other organic fibers; a tensile strength of 36750 kg / cm2, which is considerably more than the tensile strength of conventional organic fibers; a modulus of elasticity of 1.33.     106 kg / cm2, twice as much as glass E;

   good chemical resistance, being very resistant to organic solvents, fuels and lubricants, and excellent temperature resistance, without any degradation of the properties of the wire by brief exposure to temperatures of 260O C.     Kevlar 29 yarn, which can also be used in the present invention, has a density of 1.44 g / cm3, an elongation at break of 3 to 4%, a tensile strength of 28000 kg / cm2, a modulus of elasticity of 6.3.     105 kg / cm2 and excellent temperature resistance, having useful properties within wide temperature limits from temperatures below OC C to high temperatures, especially from - 250 C to + 260 C. 



   It is crucial that the continuous filament of aromatic polyamide fiber which is used in the present invention is not energized or maintained during the manufacture of the pipe.  In fact, asbestos cement has a plastic consistency and is relatively soft, and the concrete risks being only partially hardened and also being relatively soft when the continuous filament is wrapped around the layers of asbestos-cement, or of the layer (s) of concrete, and, if the continuous filament is placed and maintained under significant tension at the time of winding, it will sink into the asbestos-cement or the concrete soft and may go through it all the way to the mandrel. 

   In either case, the asbestos-cement or concrete pipe thus cut would be damaged, and probably enough damaged to no longer be salable.  The continuous filament of aromatic polyamide fiber of the highly resistant cementitious pipe according to the invention risks in fact being subjected to a certain degree of tension during its use when it is exposed to certain stresses, for example a bending stress when 'it is buried under roads or streets due to the weight of cars.   



   It is important that the aromatic polyamide fiber which is used here has a low elongation capacity, of not more than 7%, because, if this fiber had a clearly greater elongation capacity than 7%, which is the case for very elastic fibers such as aliphatic nylon nylon fibers, polyesters
Dracon and polypropylene fibers, the reinforced pipe would certainly not have the resistance it has when it is reinforced by the continuous filament of aromatic polyamide having the elongation capacity less than 7% or at most equal to 7 %. 

  The fiber reinforcing filament having an elongation capacity greater than or clearly greater than 7% would tend to stretch when the pipe which it reinforces is subjected to a material stress, and this is why the reinforced pipe would not be not as strong as when reinforced by the continuous filament of aromatic polyamide fiber having the lower elongation capacity, that is to say less than 7% or at most equal to 7%. 



   It is possible to wind manually, or using a conventional winding device, the different turns of the continuous filament inside the pipe, over the entire length or almost the entire length of the pipe, over the major part, i.e. at least 50%, of the length of the pipe, on a small part, i.e. less than 50%, but an appreciable part of the length of the pipe, on a central part , for example the middle third, of the length of the pipe, or even on another part of the length of the pipe. 

  The turns are usually helical turns and can form a single layer of turns with a typical helix angle of 50, or at least two superimposed layers of turns around one or more of the layers of asbestos-cement or of concrete, the helix angle of one of the layers of these turns is usually different from the helix angle of another layer of helical turns of the same continuous filament which is directly superimposed on the first layer, for example a helical turn having, in one direction, an acute helix angle of 50 and a helical turn having, in the other direction, an obtuse helix angle of 120.    



   The proposed method for manufacturing the non-prestressed cement-based reinforced pipe consists in forming a tubular body comprising a number of superimposed layers of a composition comprising a hydraulic cement and water, and in winding a continuous filament of fiber of aromatic polyamide which has already been mentioned, and which may be an aramid fiber
Kevlar, around one or more of these same layers containing cement, usually while they are still wet and not hardened or only partially hardened inside the pipes, during or after the formation of the layer containing cement and before the formation of the complete and immediately following cement-containing layer. 

  The continuous filament of the aromatic polyamide fiber is characterized by the high tensile strength and the low elongation, as well as by the other properties which have been mentioned.  The pipe thus obtained is hardened.  The method of the invention is eminently suitable for the manufacture of asbestos-cement pipes for conveying water under high pressure.  This process can also be used to make concrete pipes for evacuation or sewer pipes. 



   More specifically, a wet asbestos cement sedimentation sheet is formed from an aqueous slurry of cement and asbestos fibers, and water is removed from this wet asbestos cement sheet.  This partially dehydrated and still wet sheet is wound on a pipe-forming mandrel in the form of a number of wet layers of asbestos-cement wound in a spiral, while applying pressure to the wet sheet of asbestos-cement which is on the chuck. 

  A continuous filament, for example a thread, is wound from the aromatic polyamide fiber which has already been mentioned, and which has been obtained in the form of an aramid Kevlar fiber, usually in helical turns and usually in stretched state, around one or more of the wet layers of asbestos-cement wound in a spiral, inside the last pipe which is on the mandrel, during the winding of the wet layers of asbestos-cement on the mandrel, without leaving or practically leaving slack in the continuous filament, but without stretching or practically without stretching the continuous filament and without subjecting tension to the continuous filament.  The continuous filament of the aromatic polyamide fiber is characterized by the high tensile strength, the low elongation, and the other properties which have already been mentioned. 

  The mandrel is separated from the interior surface of the pipe, the pipe is usually subjected to prior hardening, the mandrel is removed from the interior of the pipe, and the previously hardened pipe is then subjected to total or almost total hardening.  One of the layers of asbestos-cement spirally wound in the pipe constitutes a complete revolution of a layer of asbestos-cement in the pipe. 



   Typically, the asbestos cement pipe is pre-cured after separating the mandrel from the interior surface of the pipe, and before extracting the mandrel from inside the pipe.  The pre-hardening is carried out for a sufficient period of time to make the asbestos-cement pipe rigid or self-supporting enough to be able to extract the mandrel from the pipe without disintegrating the latter. 



   Usually and preferably, the pre-hardening and the final hardening of the asbestos-cement pipe are carried out by subjecting the pipe to high temperature heating in a very humid atmosphere, this prior hardening and this final hardening being known and conventional. in the technique.  The final hardening can also be carried out by immersing the asbestos-cement pipe in water for 28 days, but this is not preferable. 



   To make a very resistant asbestos-cement pipe, the wet sheet of asbestos-cement is usually formed using one or more rotating perforated cylinders, typically two in number, which are partially immersed in the aqueous slurry. asbestos cement, the perforated cylinders collecting the slurry in the form of a thin, wet sheet or strip of sediment on its perforated surface, the water being removed from the wet sheet of asbestos cement on the perforated surface of the cylinder being expelled by the perforations in the cylinder. 

  The still wet asbestos-cement sheet is transferred from the rotating perforated cylinder to a movable and continuous water permeable part, usually a felt, and the asbestos-cement sheet is conveyed to this movable and permeable part. water to the pipe forming mandrel to be wound up there, while water is still removed from the asbestos-cement sheet during transport on the mandrel. 



   The concrete pipe reinforced with an aromatic polyamide can be manufactured, according to a first process, by pouring the concrete into a steel pipe mold inside which is arranged and fixed a cylindrical steel mandrel and the cavity of which has a width equivalent to about half the radial thickness that is desired for the wall of the final pipe, in the case where two layers of concrete must be poured to form the pipe. 



  When it is desired that the wall of the final pipe is formed by more than two layers of concrete, for example by three or four layers of concrete, the mold cavity has a width equivalent respectively to approximately 1/3 and 1/4 of the radial thickness you want for the wall of the final pipe. 

   After having poured a layer of concrete, the poured concrete which is in the mold is subjected to a prior hardening, or else it is allowed to partially harden in a conventional and known manner, for example in ambient air, for a lapse of time. sufficient time to make the poured concrete self-supporting when the outer casing is removed from the mold, and then the outer casing is removed from the mold to expose and make accessible the layer of poured concrete, which may still be relatively soft because that it is only partially hardened, on the mandrel. 

  Is then wound manually, or using a conventional winding device, usually by helical winding, and usually in the stretched state, the continuous filament of the aramid fiber Kevlar having the properties of high tensile strength and low elongation, as well as the other properties which have already been mentioned, around the concrete layer, without drawing or practically without drawing of the filament and without tension or practically without tension of the filament.  The turns of continuous filament of aramid Kevlar fiber are maintained around each layer of concrete by binding the filament to itself on the layer of concrete using a suitable resistant knot, which constitutes the means for maintaining the turns of continuous filament around the layer or layers containing cement or the layer or layers of concrete inside the pipe. 



  Then, in the correct position around the mandrel, an outer mold envelope of larger diameter than the first mentioned mold envelope and of sufficient diameter corresponding to form a second layer of concrete of the desired thickness is placed, and poured. another layer of concrete over the turns of continuous filament of aramid Kevlar fiber and the first layer of concrete.  If two layers of concrete are sufficient for the pipe, after removing the mandrel, the pipe is hardened in a conventional manner in an autoclave under steam pressure or possibly in ambient air.  When the pipe is to have more than two layers of concrete, the continuous filament of aramid Kevlar fiber is rewound, usually by helical winding and usually in the stretched state, around the second layer of concrete. 

  The turns of continuous filament of the aramid Kevlar fiber are maintained or fixed around each layer of concrete by bonding the filament to itself on the concrete layer using a suitable resistant knot. 



  These two operations are repeated for casting the layers of concrete and for winding the continuous filament of aramid fiber Kevlar around the layer of poured concrete until the desired number of layers of concrete has been poured around the mandrel, it being understood however that the continuous filament of aramid fiber
Kevlar is usually not wrapped around the last layer of concrete, and therefore, there is ordinarily no continuous filament fibers that are exposed on the outermost main exposed surface of the pipe product. 



   The concrete pipe can also be made by centrifugal casting of the concrete layers on the mandrel in a conventional manner, then by winding the continuous filament of fibers
Kevlar around each layer of concrete, in the manner that has already been described here. 



   Any method of manufacturing an asbestos cement pipe or a multi-layer concrete pipe can be used in the present invention, as long as this process allows the winding of the continuous filament of fibers of aromatic polyamide Kevlar around the asbestos-cement or concrete layer (s), as appropriate. 



   The continuous filament of aromatic polyamide Kevlar fibers is usually an untreated filament when it is wrapped around the layer (s) of asbestos-cement or concrete to form the very resistant pipe.  However, this filament could possibly be treated, before this winding, for example by coating it beforehand with silica gel or an asbestos-cement slurry. 



   On the attached drawing boards:
 fig.  1 is an isometric view, partially broken away, showing a section of asbestos-cement asbestos-cement pipe which is very resistant according to the present invention;
 fig.  2 is a side elevational view, partially broken away, showing a section of asbestos-cement pipe unconstrained and very resistant according to the present invention;
 fig.  3 is a longitudinal view, partially broken away, representing a section of unconstrained and very resistant concrete pipe according to the present invention:

  :
 fig.  4 is a technological diagram representing an apparatus used to manufacture a very resistant and unconstrained asbestos-cement pipe according to the invention;
 fig.  5 is a partial schematic view showing an apparatus which makes it possible to wind the continuous filament of aromatic polyamide fiber inside the asbestos-cement pipe during the formation of this same pipe in the machine for manufacturing the pipe, and
 fig.  6 is a schematic partial view showing the moving coils of the filament winding device which makes it possible to wind the continuous filament of aromatic polyamide inside the asbestos cement pipe during the formation of this same pipe on the chuck of the pipe-making machine. 



   We see in fig.  1 and 2 that the asbestos-cement pipe 10 comprises a number of layers 11 of asbestos-cement which are wound in a spiral and compressed, and a number of turns 12 of a continuous wire or filament of a fiber of aromatic polyamide having the properties already indicated.  The continuous wire or filament 12 is compressed flat or practically flat, or else lengthwise or practically lengthwise, between the layers 11 of asbestos-cement which are shown in FIG.  2, during the formation of the pipe on the pipe-forming mandrel, the cement being forced or dispersed between the fibers of the wire 12.  The cement hardens between the fibers of the wire, with the result that the wire 12 and its fibers are bonded inside the pipe by the hardened cement.  The outermost exposed main surface of the pipe 10 does not contain the continuous filament. 



   As shown in fig.  1, the turns 12 of the continuous wire or filament form a helical winding around each of the layers of asbestos-cement, except the layer 13 of asbestos-cement which is the outermost, these turns forming angles different helix, and in different directions in successive layers 11. 



   We see in fig.  3 a concrete pipe 15 which comprises a number of separate concentric layers 16 of concrete and a number of turns 17 of a continuous wire or filament of aromatic polyamide fiber having the properties already indicated.  The turns 17 of this continuous wire or filament form a helical winding around each layer of concrete, except the outermost layer, these helical turns forming different helix angles and in different directions in the successive layers. 



   We see in fig.  4 that asbestos fibers, cement, and ordinarily also silica flour are poured through respective hoppers 19, 20 and 21 into a tank 22, and that alkaline water is poured into the tank 22 through a pipeline 23.  The asbestos fibers, cement, silica flour and water mix with each other in the tank 22 so as to form a slurry of asbestos and cement.  By way of example, a cement of a certain type well known in the art for manufacturing asbestos-cement pipes, and silica flour, are poured into the tank 22 in the weight proportions of 60% of cement for 40% silica. 

  The asbestos fibers, which are usually a mixture of long chrysotile fibers and long crocidolite fibers, are poured into tank 22 in the proportion by weight of about 15% of the total weight of the cement, plus silica. 



  Water, which has previously been made alkaline, is poured into the tank 22 in sufficient quantity to form a suitable slurry of consistency well known in the field of manufacturing asbestos-cement pipes.  The temperature of the asbestos-cement slurry thus formed in the tank 22 is typically around 40 ° C.  Perforated cylinders 24 and 25, whose meshes typically have 420 11, are partially submerged in, and rotate counterclockwise in the asbestos-cement slurry in tanks 26 and 27 so as to collect this slurry in the form of a thin wet sheet of asbestos cement on the perforated cylinders, the water being expelled from the wet sheets of asbestos-cement by the perforations of the cylinders. 

   A felt 28 which passes over conveyor rollers 9 collects the wet sheet from the cylinders 25 and 24 and transports the asbestos-cement sheet to a mandrel 29 for forming a pipe. 



  Water is also removed from the wet sheet of asbestos-cement which is on the porous felt 28, by a vacuum which is applied to this felt by a suction box (not shown). 



  The continuous wet sheet of asbestos-cement, the water content of which has been reduced, is wound on the mandrel 29, while considerable pressure is applied to it by pressure rollers 30 and it is supported by a roller 'support 31.  The mandrel 29, the pressure rollers 30 and the support roller 31 are steel rollers.  The felt 32, which is applied directly against the wet asbestos-cement pipe during its formation on the mandrel 29, by the pressure rollers, has the role of further dehydrating the asbestos-cement pipe during its formation on the mandrel 29 .  The felt 32 passes over the transport rollers 33 and suction boxes which remove the water from the felt 32. 



   While the wet asbestos-cement sheet is wound on the pipe-forming mandrel 29, a continuous filament 35 of aromatic polyamide fibers already described is wound in a helical state in the stretched state, but without stretching, from a coil 36, also shown in FIG.  5, around each of the different layers of asbestos-cement which are wound on the mandrel 29 and inside the pipe thus formed.  As shown in fig.  5, the continuous filament 35 passes from the reel 36 into a guide 37, between rollers 38 for supplying energy, and between the blades of an automatic shear 39 before being wound in a helix on the layers of asbestos. cement on the mandrel 29. 

  The shears 39 cut the continuous filament 35 to the desired length of this same filament with a view to its helical winding on the asbestos-cement layers which are on the mandrel 29.  The turns of this continuous filament are kept stretched or relatively stretched around the layers of asbestos-cement which are inside the pipe, practically without being stretched and practically without being subjected to tension, by means of the layers of asbestos -cement which directly cover these same turns. 

  We see in fig.  6 that at least two coils 36 are used, each coil oscillating back and forth and applying a helical turn of the continuous filament over almost 50% of the length of the asbestos-cement layers, with the exception of the outermost asbestos-cement layer, from their opposite ends on the mandrel 29, with overlap on the central part of the asbestos-cement layers. 



   Let's go back to fig.  4.  The mandrel 29 and the asbestos cement pipe 40 thereon are transported together by the chain conveyor 41 to a calender 42 comprising two steel rollers 43 on which the asbestos cement pipe 40 is separated from the mandrel 29.  An operator ordinarily initiates the separation of the asbestos-cement pipe from the mandrel with the aid of a tool, at the end part of the pipe, and the action of the rollers 43 of the shell results in that the pipe asbestos cement is separated from the steel of the mandrel along the entire length of the asbestos cement pipe.  The mandrel 29, on which the separate pipe 40 is still located, is transported by a chain conveyor 44 in an oven 45 with moist heat for its prior hardening. 

  The temperature prevailing in the oven 45 is typically about 1200C, and the hydrometric degree prevailing there is typically 95 to 100%, and the mandrel, on which the separate asbestos-cement pipe is located, is maintained typically in the oven 45 for 1 Y4 h.  The temperature and the hydrometric degree of the oven 45 are supposed to accelerate certain reactions which occur in the cement of the asbestos-cement pipe 40 so that, at the outlet of the mandrel 29 and the pipe 40 of the oven 45, the asbestos-cement pipe 40 has sufficient rigidity and mechanical strength to enable the mandrel 29 to be removed from the pipe 40. 



   At their exit from the oven 45, the mandrel 29 and the pipe 40 are transported to the extractor 46 of the mandrel by the conveyor
 chain 44.  Manually remove the mandrel 29 from the pipe 40
 asbestos cement at the extraction station 46, then the hose 40 is transferred manually to another chain conveyor 47. 



  The pipe 40 is transported by the conveyor 47 in another humid heat oven 48, and the pipe 40 is maintained in the oven 48 at a typical SOC C temperature and at a typical hydrometric degree of about 60 to 70% for approximately 1 Y4 h. 



  The withdrawn mandrel 29 is transported by a freight elevator 49 on another chain conveyor 50 which returns the mandrel 29, and usually also other mandrels which have been withdrawn at the extraction station, to the pipe forming machine.
 mandrels 46. 



   We remove the pipe 40 from the oven 48 generally after
   1 Y4 h approximately, and it is loaded on a tray, usually in the company of other asbestos-cement pipes removed from the oven 48. 



  The trays carrying the asbestos-cement pipes 40 thus formed are then transferred by a transport vehicle 51 into an autoclave 52, into which they are introduced.  The doors of the autoclaves 52 are then closed and the asbestos-cement pipes are subjected to a total hardening in the autoclaves 52 under the following conditions: the water vapor pressure is gradually raised to 8.75 kg / cm2 in I about 1/2 hour, and this water vapor pressure is maintained for another 12 to 16 hours; the
 temperature is about 1800 C for this water vapor pressure.  The asbestos-cement pipe 40 is then removed from the autoclave 52, in the form of a fully hardened asbestos-cement pipe. 

  The ends of this completely hardened pipe are then machined using a machining device, to give them the desired shape, usually that of a male end on which a fitting is installed.  Before shipping this pipe, it is subjected to bending tests and internal hydrostatic pressure resistance tests. 



   Tests were carried out in which intermittently wound measured lengths of continuous filament of aramid fiber Kevlar 49 of three different denier in the asbestos-cement wall during the formation of these pipes on the chuck of the machine pipe forming and during normal pipe making cycles.  The continuous filament of aramid fiber Kevlar 49 was unwound by hand with barely sufficient tension on the filament to prevent any slack. 

  Since it was 40 cm class 150 asbestos cement pipes that were manufactured at the time of the tests, and the weakest point of the pipe of this size was in the hydrostatic pressure tests, the main fracture appearing constantly in the center or approximately in the center of each 4 m section, the winding of the continuous filament of aramid fiber Kevlar 49 has been concentrated in the center or in the middle part of the pipe.  Hydrostatic pressure tests were performed according to test method ASTM C-500, and bending tests were performed according to test method
ASTM C-500. 

  In the tests and test results indicated below, the expression pressure required means the hydrostatic pressure or the bending pressure at which FAWWA requires that the asbestos-cement pipes resist.  The expression tight weave which is used in the tests and the results of the tests means that the continuous filament was wound in a helix in the pipe, the helical turns of the continuous filament being contiguous and / or contiguous and not being separated by large free spaces. 



   The expression open spiral weave used in the tests and the results of the tests means that the continuous filament was wound in a helix in the pipe, the helical turns of the continuous filament being separated from each other by an appreciable distance and being neither contiguous neither contiguous. 



   The considerable improvement in the burst strength of the pipes according to the present invention containing the continuous filament with internal winding of Kevlar fiber, compared to conventional pipes not containing a continuous filament of Kevlar fiber, is shown below. -after by the results of tests N s 1, 2, 3, 4, 5, 6 and 7, by comparison with the results of tests of resistance to bursting under hydrostatic pressure for these same conventional pipes not containing the continuous filament , recorded below in the results section of test No 6. 

  The excellent flexural strength of the pipes according to the present invention, containing the continuous filament with internal winding of Kevlar fiber, is demonstrated below by the results of tests N s 8, 9, 10, 11 and 12.  The remarkable improvements in burst strength under hydrostatic pressure, representing more than 200% of the burst strength under hydrostatic pressure which is required, have been achieved by the pipes according to the present invention containing the continuous internally wound filament aromatic polyamide Kevlar fiber, as shown below in the results of test No 4. 

  Considerable improvements in flexural strength, of at least about 35% over the required flexural strength or load, have been achieved by the hoses according to the present invention containing the continuous internally wound filament of Kevlar fiber, as demonstrated below by the results of tests N05 9 and 11. 



  Test N0 I
 We have wound in the center of different asbestos cement pipes lengths of continuous filament of aramid fiber Kevlar 49, of deniers 195, 390 and 1424, starting with the first or the second layer of asbestos cement each pipe and continuing until the full length is occupied.  56.1 m of 195 denier continuous filament, 28 m of 380 denier continuous filament and 7.3 m of 1424 denier continuous filament were wound in the various asbestos cement pipes.  We used a simple spiral winding without trying to form an armor. 



  Results: Pipe N0 1 which contained 56.1 m of 195 denier filament
 withstood the hydrostatic pressure test up to
 37.8 kg / cm2 and yielded to this pressure. 



     ¯The pipe N0 2, which contained 28 m of 380 denier filament,
 withstood the hydrostatic pressure test up to 35 kg / cm2
 and gave in to this pressure. 



     Pipe N0 3, which contained 7.3 m of 1424 filament of
 niers, withstood the hydrostatic pressure test up to
 34.3 kg / cm2 and yielded to this pressure. 



     Test No. 2
 Variations in spiral weave of unmeasured lengths, of deniers 195, 380 and 1424, of continuous filament of aramid fiber Kevlar 49 were used.  Starting at the center of each 40 cm class 150 pipe, the continuous filament was unwound by hand, moving to one end and continuing to the other end, then returning to the center. 



  Results: Pipe N0 1 contained the 1424 denier filament.  We hoar
 milk the continuous filament from the center to the driving side
 then back to the operator side but there
 had to cut the continuous filament before returning to the
 center.  This made the pipe carry a double winding
 at one end and a single winding at the other end. 



   Pipe did not burst in hydrostatic pressure test
 before the pressure of 37.8 kg / cm2 is reached, and it has been noted
 that the pipe only split from the center to the side that
 contained only one winding. 



  - Pipe N0 2 contained a continuous filament of 380 deniers of
 aramid fiber Kevlar 49 and did not burst in the test
 hydrostatic pressure before the pressure reaches
 38.5 kg / cm2. 



     Pipe N0 3 contained a 195-denier continuous filament of
 aramid fiber Kevlar 49 and did not burst in the test
 hydrostatic pressure before the pressure reaches
 36.8 kg / cm2. 



     Pipe No. 4 contained a 195-denier continuous filament of
 aramid fiber Kevlar 49 and did not burst in the test
 hydrostatic pressure before the pressure reaches
 37.8 kg / cm2. 



   Thus, three of these four pipes withstood the required pressure of 36.8 kg / cm2.  It should also be noted that none of these pipes cracked over the entire length when it burst, as is normally the case with large diameter pipes. 



     Test ND 3
 Concentrating again on the central part of the asbestos-cement pipe, continuous filaments of aramid fiber Kevlar 49, having the previous three deniers respectively, were wound up over different lengths of the center of the pipe. 



  Results: - The pipe NO 1 carried a continuous filament of 195 denier
 aramid fiber Kevlar 49 on 90 cm from its central part, in
 starting from the center left over a length of about 45 cm and
 by wrapping at this point on ten layers before going 45 cm
 to the right beyond the center, hesitating over ten layers,
 then back to the center over ten layers.  We continued
 so for all 110 layers.  We tried this hose
 in hydrostatic pressure tests first at 35 kg / cm2,
 then at 38.2, 39.6, 45.5, 45.5, 45.5 kg / cm2 then finally
 48.0 kg / cm2 before bursting.  Here again, the pipe did not
 burst from end to end, but presented a crack in the form
 of U relative to the area of application of the filament. 



     Pipe N0 2 carried an armored winding like the
 pipe N0 1, the 195-denier continuous filament of macaw fiber
 mide Kevlar 49 being wound only on the central 60 cm.  We
 tried this hose in hydrostatic pressure testing at
 35, 45.5 and 53.2 kg / cm2, before it bursts. 



  - Pipe N0 3 wore the same armor as pipe N0 2, but
 with a continuous filament of 380 denier of aramid fiber
 Kevlar 49.  We tried this hose twice at 45.5 kg / cm2 and it
   did not burst before the pressure of 48.3 kg / cm2 was
 reached. 



  - Pipe NO 4 wore the same armor as the previous ones,
 but with a continuous filament of 1424 deniers of macaw fiber
 mide Kevlar 49 limited to the central 45 cm of the pipe.  We have
 tested this hose in the hydrostatic pressure test at 37.1,
 41.3, 42.0 and 45.5 kg / cm2, and it did not erupt until the
 pressure of 49.0 kg / cm2 is reached. 



   None of these pipes cracked from end to end when they broke in the hydrostatic tests.  Like 80% of all 40 cm class 150 asbestos cement pipes that were manufactured the day before the tests, and 20% of all 40 cm class 150 asbestos cement pipes that were manufactured the same day without incorporation of the continuous filament of aramid fiber Kevlar 49, by winding in the pipe, did not have sufficient mechanical strength to withstand the required hydrostatic test pressure of 36.8 kg / cm2, the test results put in evidence the considerable improvement in mechanical resistance obtained by winding the aramid fiber Kevlar 49 in the pipe.  The test results revealed that a narrow weave in the central area of the pipe is better than a simple spiral weave open over the entire length of the pipe. 



      Test No. 4
 This is a combined test on 10 cm asbestos cement pipes class 150, because all pipes from 10 cm to 20 cm in diameter must withstand a hydrostatic pressure test as well as a test bending.  Thirteen asbestos-cement test tubes were made and removed from the mandrel of the pipe forming machine. 



  Results: - On NOS pipes 1, 2 and 3, we formed a spiral armor
 open within 90 cm of each pipe during the
 winding time using a continuous filament of
 195 deniers of aramid fiber Kevlar 49.  These pipes all have
 three withstood the required gauge pressure of 9.4 kg / cm2,
 or applied load of 500 kg, from the bending test.  The pipe
   N0 1 was subjected to 10 kg / cm2 in the bending test and
 broken 2.34 m from the drive end, which was
 right at the edge of the application area.  We then submitted the
 pipes Nos 2 and 3 at hydrostatic pressures starting
 at the required 36.8 kg / cm2 and continuing in increments of
 3.5 kg / cm2 until the NO 2 pipe breaks at 70 kg / cm2,
 only on the drive side. 

  Pipe N0 3 burst on
 hydrostatic pressure of 69.3 kg / cm2, on the operator side.  The
 two pipes broke outside the application area. 



  - We formed on the pipes NOS 4 and 5 an armor as sem
 as possible to the first three, but using a
 380-denier continuous filament of Kevlar 49 aramid fiber. 



   Both pipes withstood gauge pressure
 required 9.4 kg / cm2 of bending test, pipe N0 5
 resistant to another bending test at 10 kg / cm2.  Here again, we
 subjected both pipes to hydrosta pressure tests
 tick starting at 36.8 kg / cm2 and continuing until
 pipe N0 4 bursts at 61.6 kg / cm2 and pipe N0 5 at
 42.0 kg / cm2.  Both pipes broke on the operator side
 and outside the filament application area. 



     On pipes N05 6 and 7, we applied the continuous filament
 as on the previous pipes, but using a filament
 continuous 1424 denier of aramid fiber Kevlar 49.  The
 two pipes withstood gauge pressure test
 required at 9.4 kg / cm2 of the bending test.  We subjected the
 pipe N0 6 another bending test at 10 kg / cm2 and it was
 broken 2.36 cm from the drive end - still at
 edge of the filament application area.  We subjected the
 pipe No. 7 of the hydrostatic pressure tests starting at
 36.8 kg / cm2 and continuing up to 61.6 kg / cm2.  This pipe
 did not explode and was saved for further testing. 



  - On pipes NOS 8 to 13, we formed an armor starting
 through the center on three layers, moving 45.5 cm towards
 the left and pausing on three layers,
 moving right 91.5 cm and pausing
 for three layers and then coming back to the center for the
 duration of the winding.  Pipe N0 8 contained a filament
 continuous 1424 denier of aramid fiber Kevlar 49 and a
 withstood bending test under gauge pressure
 9.4 kg / cm2 required, and broke during another test of
 bending at 10.5 kg / cm2, 1.78 m from the drive end. 



  - Hoses N05 9 and 10 contained a continuous filament of
 380 deniers of aramid fiber Kevlar 49 and both have
 withstood the required bending pressure of
 9.4 kg / cm2, hose N0 9 yielding in the additional bending test
 mental at 10 kg / cm2 and breaking at 2.31 cm from the tip
 drive.  NO 10 pipe was tested
 hydrostatic at 36.8 kg / cm2 and continued in increments
 3.5 kg / cm2 until it yields under hydrosta pressure
 70 kg / cm2 tick, breaking on the operator's side. 



  - Hoses N05 11, 12 and 13 all contained a filament
 195 denier continuous and all withstood the bending test
 under gauge pressure of 9.4 kg / cm2, pipe NO 11
 yielding in the additional bending test at 10.5 kg / cm2. 



   Tubes N05 12 and 13 were pressure tested
 hydrostatic starting from 36.8 kg / cm2, until the
 pipe N0 12 gives way in its middle under hydrostatic pressure
 of 69.3 kg / cm2, and that the pipe N0 13 gives way in its part
 central under hydrostatic pressure of 77 kg / cm2. 



   (77 kg / cm2 representing 210% of the hydrosta test pressure
 tick required. )
 It should be noted that a 10 cm class 150 pipe must withstand 36.8 kg / cm2 in hydrostatic pressure tests. 



  The majority of the pipes tested in this NO 4 test withstood a hydrostatic pressure of up to 6,370 kg / cm2, which represents a remarkable improvement in burst strength under hydrostatic pressure, i.e. around 190% (for 70 kg / cm2) of the required hydrostatic test pressure. 



     Test No. 5
 A continuation of the tests was carried out on the 40 cm class 150 asbestos cement pipes.  Here again, a weave of continuous filament of aramid fiber Kevlar 49 having the three preceding deniers respectively, was formed in the 90 cm cen. traux of each test pipe, starting from the center and pausing in ten layers before moving to the left in a helix about 45 cm and making a new break in ten layers, then, still in a helix, going back to the center and pausing over ten layers before moving to the left for about 45 cm and pausing over ten layers, and coming back again to the center.  This continued over the entire winding cycle of about 110 layers of asbestos cement on the pipe forming mandrel. 



  Results: - NOÇ pipes 1 and 2 contained a continuous filament of
 195 deniers of aramid fiber Kevlar 49 and have withstood
 hydrostatic pressures until bursting at 40.95 kg / cm2. 



   Pipe NO 2 has been subjected to the same cross pressure test
 health and burst at 40.6 kg / cm2. 



  - Pipe N0 3 contained a continuous filament of 380 deniers of
 aramid fiber Kevlar 49 and burst at hydrosta pressure
 tick of 37.1 kg / cm2. 



  - N05 pipes 4 and 5 contained a continuous filament of
 1424 deniers of aramid fiber Kevlar 49, hose N0 4
 resistant to hydrostatic pressures up to its pressure
 bursting density of 42.0 kg / cm2.  Pipe NO 5 was also
 tested for increasing hydrostatic pressure and has
 burst at hydrostatic pressure of 43.05 kg / cm2. 



   The pipes which were manufactured during the period when these tests were carried out, without the addition of this continuous filament of Kevlar fiber, only withstood hydrostatic pressures of the order of 28 to 29.8 kg / cm2. 



  Test N0 6
 A 195-denier continuous filament of Kevlar 49 aramid fiber was added to two asbestos-cement pipes, using methods different from those tried in the previous tests. 



   The filament was applied to pipe N0 7 at points dividing this pipe into three equal parts, but without forming any armor. 



  The filament was applied to the NO 8 pipe at points dividing the pipe into four equal parts, still without forming armor. 



  The filament was added to both pipes during the winding cycle. 



  Results: - The NO 7 pipe burst at a hydrostatic pressure of
 32.2 kg / cm2. 



  - Pipe N0 8 burst at a hydrostatic pressure of
 33.6 kg / cm2.    



  Test N0 7
 A 195-denier continuous filament of Kevlar 49 aramid fiber was wound in the asbestos-cement pipe with a very open spiral from one end to the other and in the other direction during the cycle of winding, on pipe NO 9. 



  Results: ¯Hose N0 9 burst at a hydrostatic pressure of
 33.6 kg / cm2. 



     Test No. 8
 A complementary series of tests was carried out on 10 cm asbestos-cement pipes class 150 and on asbestos-cement pipes class 200, similar to those of the previous tests.  Only the results of the bending tests are currently available.  On hose N0 1, we wound the continuous filament of 195 denier aramid fiber Kevlar 49 with an open spiral, starting at 30 cm on either side of the center and continuing beyond the center towards the 'other end.  On the NO 2 pipe, a 195-denier continuous filament of Kevlar 49 aramid fiber was wound with an open spiral starting at 15 cm from one end and continuing up to 15 cm from the other end. 

  On the pipe N0 3, we wound a continuous filament of 195 deniers of aramid fiber Kevlar 49, as for the pipe NO 1, but starting at 60 cm on either side of the center (195 deniers). 



  Results: ¯We subjected the NO I pipe, 13.4 cm in outside diameter
 laughing, bending tests at the required gauge pressure
 11.8 kg / cm2 for pipes of class 200, and each
 next test we increased the pressure until the hose
 breaks at a gauge pressure of 14.7 kg / cm2 (25% of
 more than the pressure required for the bending test). 



     ¯We subjected pipe N0 2, 12.9 cm in outside diameter
 laughing, bending tests at the required gauge pressure
 9.4 kg / cm2 for class 150 pipes, and it was made
 undergo further bending tests at manomated pressure
 10.5 kg / cm2 stick (12% more than the required pressure)
 for the bending test), and it did not break. 



     ¯We subjected pipe N0 3, 13.4 cm in outside diameter
 laughing, bending tests at the required gauge pressure
 11.8 kg / cm2 for class 200 pipes, and it was made
 undergo further bending tests at manomated pressure
 12.7 kg / cm2 stick (8% more than the pressure required for
 bending test), and it did not break. 



  Test N0 9
 On the asbestos cement pipes N "4, 5, 6 and 7, a continuous filament of aramid fiber Kevlar 49, with a narrow spiral weave, was wound on the 40 to 60 cm from the middle of each pipe, using a single strand of 195 denier filament.  The resulting pipes were bent. 



  Results:
 Class Diameter Pressure Pressure Pressure at
 mano-exterior breaking
 (cm) metric reached in (kg / cm2)
 required testing
 (kg / cm2) bending
 (kg / cm2) N04 200 13.84 11.8 15.3 15.8 'N05 200 13.77 11.8 14.8 N06 200 13.72 14.8 13.4 N07 150 13.03 9.4 10.5 11.22 134% more than the required load. 



     219% more than the required load. 



  Test N0 10
 On the asbestos cement pipes N (> s 8 and 9, we formed a narrow spiral armor on the middle 180 cm, with two strands of continuous filament of 195 denier of aramid fiber Kevlar 49, at the same time .  The pipes obtained were subjected to bending tests. 



  Results: N0 Class Diameter Pressure Pressure Pressure at
 mano-exterior breaking
 (cm) metric reached in (kg / cm2)
 required testing
 (kg / cm2) bending
 (kg / cm2) 8 150 12.90 9.4 9.4 9 150 12.98 9.4 9.4 11.81 125% more than the required load. 



  Test N0 il
 On the asbestos cement pipes Nos 10 and 11, a narrow spiral armor was formed on the middle 180 cm with two strands at the same time of continuous filament of 380 denier of aramid fiber Kevlar 49. 



  Results: N0 Class Diameter Pressure Pressure Pressure at
 mano-exterior breaking
 (cm) metric reached in (kg / cm2)
 required
 (kg / cm2) bending
 (kg / cm2) 10,200 13.84 11.8 12.95 16.1 11,200 13.72 11.8 12.95 137% more than the required load. 



  Test N0 12
 On the pipes NOS 12 to 17, a narrow spiral weave was formed on the middle 120 cm with a strand of continuous filament of 195 denier of aramid fiber Kevlar 49, at the same time. 



  Results: N0 Class Diameter Pressure Pressure Pressure at
 mano-exterior breaking
 (cm) metric reached in (kg / cm2)
 required test
 (kg / cm2) of bending
 (kg / cm2) 12,200 13.74 11.8 11.8 13,200 13.74 11.8 11.8 14,200 13.82 11.8 12.6 14.71 15,200 13.74 11.8 12 , 95 16 200 13.72 11.8 13.2 14.71 17 200 13.72 11.8 13.3 125% more than the required load. 



   The reason why the pressures are not indicated for pipes N0 12, 13, 15 and 17 in the previous table in
 the rupture pressure column is that we haven't tried these
 pipes until destroyed, but have been kept for
 other tests.  In the bending test, each pressure
 size in kg / cm2 represents an actual applied load of
 3724 kg. 



   Several of these hoses which were tested to destruction withstood bending tests in bending tests.
 pressures 25 to 35% higher than the required load, but
 note that all of them were tested at pressures equivalent to the maximum breaking load of any compatible pipe manufactured without the addition of the filament.  The average breaking load of the test pipe in bending tests is approximately 20% higher than the average breaking load of the same number of normal pipes having the maximum resistance.  


    

Claims (11)

 CLAIMS  1. Reinforced asbestos-cement pipe, characterized by a body composed of several layers made of a composition comprising hydraulic cement, and comprising several turns of a continuous filament of an aromatic polyamide fiber having a tensile strength d '' at least 27,300 kg / cm2 and an elongation capacity not exceeding 7%, wound at least around an inner layer of the pipe, the filament being relaxed and having substantially the same length as at rest.  2. Pipe according to claim 1, characterized in that the material of the body also comprises silica.  3. Pipe according to one of the preceding claims, characterized in that the turns of continuous filament are at least partially embedded in the layer located inside them.  4. Pipe according to one of the preceding claims, characterized by a tubular body comprising several helical turns of filament.  5. Pipe according to one of claims 1 to 3, characterized by a tubular body comprising several concentric layers.  6. Pipe according to one of the preceding claims, characterized in that the filament is wound in a helix on the central part of one or more internal layers of the pipe.  7. Pipe according to one of claims 1 to 5, characterized in that the continuous filament is wound in a helix so as to form at least two helical turns superimposed of filament around at least one of the inner layers of the pipe, two turns directly superimposed having different steps.  8. A method for manufacturing a pipe according to claim 1, characterized in that a tubular body is formed comprising several layers of a material comprising hydraulic cement and water, which is wound without stretching it. a continuous filament of an aromatic polyamide fiber around at least one of the inner layers during or after its formation, and before the complete formation, around it, of the next layer containing cement, and in that hardens the pipe thus obtained.  9. Method according to claim 8, characterized in that the tubular body is produced by first forming an aqueous asbestos-cement sheet from a slurry based on water, cement and asbestos fibers , then removing water from this sheet, then applying the still wet sheet to a forming mandrel, and winding the continuous filament of aromatic polyamide fiber around one or more wet sheets of asbestos-cement intended to form one or more inner layers of the body.  10. Method according to one of claims 8 or 9, characterized in that the filament is kept taut during the winding around the asbestos-cement sheet. He. Method according to one of claims 8 to 10, characterized in that the filament is wound in helical coils around each of several of the inner layers of the pipe.  The present invention relates to asbestos-cement reinforced pipes or conduits, and more particularly to non-prestressed asbestos-cement pipes or conduits which are characterized by high resistance to bursting and to bending, as well as to a method for manufacturing these pipes or conduits.  U.S. Patents Nos. 3631897 and 3813098 describe reinforced asbestos-cement pipes, as well  than other tubular parts, in which tensioning strands  Nylon, polyester or polypropylene, are wrapped around  the surface of the pipe or tubular part and are stretched to a  length exceeding at least 5% their stable and relaxed length. The material of these arms is fixed, in its stretched or elongated state, around the pipe or the tubular part, and the material of the strands which is fixed around the pipe or the tubular part is subjected to a tension representing at least 15% of its breaking strength. United States patents Nos. 2251205, 2375971 and 2711291 describe concrete pipes which are wrapped with metallic wire, for example steel wire, to reinforce them. U.S. Patent No. 2,791,241 relates to fiber reinforced resin pipes comprising a first series of long reinforcing fibers, glass or asbestos, which are helically wound with a helix angle of 2 to 10 #, and a second series of long reinforcing fibers mentioned above which are superimposed on the first series forming the helical winding, and which are wound in a helix with a helix angle of 80 to 880. A hardened resin assembles the fibers l 'to each other so as to form a one-piece pipe.  An important application of asbestos-cement pipes of the prior art, composed of asbestos fibers, cement and silica, is the transport or routing of water under pressure. Cast iron pipes are also used in the prior art for conveying water under pressure. These cast iron pipes may require the installation of a consumable cathodic protection device which is not required by asbestos-cement pipes, to avoid corrosion of the cast iron pipe. The cathodic protection requirement materially adds to the cost price of cast iron pipes. Furthermore, it is still possible to improve the burst strength, as well as the flexural strength, of asbestos cement pipes of the aforementioned prior art.  The present invention provides a reinforced asbestos cement pipe, characterized by a body composed of several layers made of a composition comprising hydraulic cement, and comprising several turns of a continuous filament of an aromatic polyamide fiber having a tensile strength at least 27,300 kg / cm2 and an elongation capacity not exceeding 7%, wound at least around an inner layer of the pipe, the filament being relaxed and having substantially the same length as at rest.  The invention also relates to a method of manufacturing this pipe, characterized in that a tubular body is formed comprising several layers of a material comprising hydraulic cement and water, which is wound without drawing a filament continuous of an aromatic polyamide fiber around at least one of the inner layers during or after its formation, and before the complete formation, around it, of the next layer containing cement, and in that it hardens the pipe thus obtained.  The continuous filament of aromatic polyamide fiber will preferably be wound and kept stretched around the composition layer or layers containing the cement inside the pipe. The term stretched as it is used here to describe the turns of continuous filament of the aromatic polyamide fiber around the  or layers of the composition containing the cement inside  of pipe means that the continuous filament of polyamide fiber  aromatic is wrapped tightly or relatively tightly  around the layer (s) containing the cement, without slack  or practically without slack in the same filament.  The asbestos-cement pipe thus obtained is not prestressed  and provides a remarkable improvement in resistance to  bursting under hydrostatic pressure, which is considerable  larger than that of the asbestos cement pipes of the  prior art, which are not reinforced with a filament or  continuous strand. Asbestos cement pipes according to this  invention are also characterized by resistance to bending  considerably better. In resistance tests to  bursting under hydrostatic pressure, asbestos pipes  cement according to the invention withstood hydrostatic pressure  reaching up to 77 kg / cm2, about 110% more than the ** ATTENTION ** end of the CLMS field may contain start of DESC **.