GB1596805A

GB1596805A - Method and apparatus for producing cast-in-place pipe employing permanent pipe mould

Info

Publication number: GB1596805A
Application number: GB628578A
Authority: GB
Original assignee: Keller G M
Current assignee: Keller G M
Priority date: 1978-02-16
Filing date: 1978-02-16
Publication date: 1981-09-03

Description

(54) METHOD AND APPARATUS FOR PRODUCING CAST-IN-PLACE PIPE EMPLOYING PERMANENT PIPE MOULD (71) I, GEORGE MONTEITH KELLER, a citizen of the United States of America, formerly of 162 Wolf Grade, Kentfield, and now of 290 Via Casitas, No. 302, Green brine both in the State of California 94904, United States of America do hereby declare the invention, for which I pray that a Patent may be granted to me and the method by which it is to be performed, to be particularly described in and by the following statement: The invention concerns a method of and an apparatus for producing cast-in-place pipe of concrete or like cementitions material.

The in situ production of continuous, jointless concrete pipe in open trenches in now widely accepted because it yields high quality pipe at modest costs. Under one method pipe is produced by employing the bottom of an open trench as the supporting medium for the lower pipe half while special moulds are provided to support the upper pipe half until the concrete has hardened.

The moulds are relatively short lengths of metal sheets having a curved shape complementary to the interior curvature of the pipe.

After the concrete has hardened, the moulds must be removed by a workman who enters the pipe, releases the moulds and then removes them by dragging them to the open end of the pipe. Methods and apparatus for constructing such pipe are described, for example, in U.S. Patents 2,731,698; 3,106,760; 3,113,364; and 3,534,449. U.S. Patent 3,785,759 proposes to delete interior pipe moulds for supporting the upper pipe half while the concrete hardens. As far as is known to applicant, the method proposed in the last mentioned patent is technically and/or economically not feasible and has not been employed on a commercial scale.

Although the pipe-forming methods ari: machines proposed in the patents have resulted in substantial improvements for laying large diameter pipes, they have short cnmincr Thp npprl fnr rirmrlrmPn to an+n* fo duction of such pipe to a minimum pipe diameter of at least about 24 inches.

Moreover, the removal of the moulds in and of itself requires substantial manual labour and is, therefore, expensive.

In addition, it is necessary to store large numbers of pipe moulds at the construction site and each mould must be individually handled and inserted into the pipe-forming machine. Again, this involves substantial labour and complicates the construction of the machine. The initial costs, as well as the subsequent maintenance of the machine, are thereby significantly increased.

The prior art concrete pipe forming methods and machines proposed in the patents have the further drawback of requiring an intermittent cessation of the pipe forming method to permit the removal of pipe moulds from the interior of already laid pipe. This significantly reduces the produc- tion rate attainable with such prior art pipe forming machines. Additionally, once the moulds are removed from the pipe they leave ring-shaped depressions on the interior of the pipe. These depressions adversely affect the hydraulic characteristics of the pipe since they cause turbulence in the medium flowing through the pipe.

Lastly, jointless concrete pipe produced in accordance with the prior art expose the raw concrete to potentially corrosive fumes, which in time, could damage or destroy the pipe. The problem is particularly acute for sewage drain pipe which releases concrete corroding fumes. In the past, such corrosion was counteracted by coating the finished pipe with a suitable protective agent. This, however, is an expensive task since low cost coating techniques such as dip coating of short pipe sections cannot be employed in seamless in situ formed pipe.

The present invention seeks to overcome disadvantages when producing in situ formed pipes in accordance with the prior art by eliminating the multiple handling of the concrete support moulds and by automatically providing a protective layer on the The present invention provides a method for the in situ formation of a long, continuous length of cementitious pipe in an open trench comprising the steps of; providing a sled for movement along the trench, the sled having an intake for receiving a deformable cementitious material, means for forming the cementitious material into a lower pipe portion resting in the trench, and an upwardly convex, rigid mandrel; drawing the sled through the trench, pouring a plastics material over the mandrel and hardening the plastics material whilst supported by the mandrel to form a substantially rigid, downwardly open plastics mould, retaining the mould stationary relative to the trench, pouring the cementitious material into the intake and over the mould to thereby cast the pipe; withdrawing the mandrel whilst the cementitious material is in its deformable state. supporting an upper section of the deformable cementitious material with the mould only until such cementious material hardens and becomes self-supporting; and leaving the mould permanently in the hardened pipe.

Thus, the mould is formed in situ, much like the pipe itself and it becomes a permanent part of the pipe. The mould is centered relative to the upper pipe portion and extends over at least a major portion thereof; preferably over an arc of 1800 or more and normally over an arc of less than 360".

The invention also provides a method for constructing a continuous length of pipe from a deformable and hardenable material including the steps of providing an exterior form for supporting a lower portion of the pipe; providing an interior mould for supporting an upper portion of the pipe to prevent a collapse of the pipe while the material is deformable; casting the deformable material over the exterior form and the interior mould to construct homogeneous, initially deformable pipe; and thereafter permitting the material to harden into a rigid continuous length of pipe; the improvement to the step of providing the interior mould comprising the steps of providing a continuous length of a relatively thin plastics material; shaping the plastics matenal into a configuration to conform it to an interior surface of the upper pipe portion and so that it extends over less than the full circumferential extent of the interior pipe surface; providing a support for the plastics material in said configuration and supporting the plastics material with the support until it is substantially rigid, rendering the plastics material substantially rigid to thereby form a continuous, longitudinally extending mould; casting the deformable material about the mould; withdrawing the support from the plastics material before the deformable material has hardened; and supporting the deformable material while it is in its deformable state and until it is in its hardened state with the mould.

Furthermore, the invention also provides apparatus for the formation of long lengths of continuous pipe in an open trench comprising a sled for movement through the trench; first means connected with the sled for forming a lower pipe portion in the trench; second means connected with the sled for forming an upper pipe portion homogeneously with the lower portion; and intake means for feeding cementitious material to the first and second pipe-forming means while the sled moves through the trench; the second means including a mandrel connected with the sled and having an upwardly convex surface disposed forward of the intake means and having a shape closely approximating the shape of an interior surface of the upper pipe portion; means for placing a relatively thin layer of a deformable and hardenable plastics material in its deformable state over the convex mandrel surface to conform the material to the mandrel and release means providing for the separation of the plastics layer from the mandrel whereby the mandrel is movable relative to the plastics layer while the plastics layer remains stationary relative to the trench.

The hardenable plastics material is applied over the convex mandrel surface at a point positioned sufficiently forward of the intake means so that the applied plastics material has time to harden before it passes beneath the intake means and comes into contact with the fresh concrete.

Any suitable thermoplastics, thermosetting or chemically curable plastics material may be used, several examples of which are hereinafter described.

Various ways of forming the plastics mould can be employed. For example, a liquid polymerisable resin can be sprayed via a suitable spray nozzle onto the convex mandrel surface. The spray nozzle can be a mixing nozzle which mixes the resin with a suitable catalyst and promoter. Alternatively, separate nozzles for the resin and the catalyst/promoter can be employed.

A wide variety of resins can be used. They include styrene polyester copolymer such as SELECTRON Registered Trade Mark 50081, manufactured by PPG Industries, Inc.; LAMINAC Registered Trade Mark 4116, manufactured by the American Cyanamid Company, Inc.; or PARAPLEX Registered Trade Mark P43, manufactured by the Rohm & Haas Company, Inc. Generally speaking, these resins comprise a solid or liquid polyester dissolved in a reactive monomer such as styrene monomer. The reactive polyester copolymerizes with the styrene by virtue of the unsaturated molecules within its composition, such as maleic anhydride, fumeric acid, and the like.

Other available resin materials include vinyl ester, such as DERAKANE Registered Trade Mark, manufactured by the Dow Chemical Company, Inc.; acrylic syrups; and condensation polymerization resins such as phenol formaldehyde resins, urea formaldehyde resins, resorcinal formaldehyde resins and the like.

Typical catalyst and promotor combinations for the polyester resins are a combination of methyl ethyl ketone peroxide and cobalt naphthanate; dimethyl aniline and benzoyl peroxide, or ascorbic acid and methyl iso-butyl ketone peroxide and a number of other activating systems.

To increase the strength of the cured plastics material, it is preferably reinforced by incorporating therein fibers such as glass fibers, celluloid fibers, synthetic polymeric fibers, such as Nylon or Dacron (Registered Trade Mark), carbon fibers and the like.

Lastly, to reduce material costs, low cost fillers can be incorporated. The fillers include such products as calcium carbonate, talc, china clay, sand, glass microspheres and the like.

The incorporation of both the fiber reinforcements and the filler follows conventional practice. Thus, the fibers may be supplied from roving or fiber spools or they may be incorporated in the form of woven cloth, woven roving or a random fiber mat.

When roving or fiber spools are employed they are introduced into the spray streams either upstream of the spray nozzles or by spraying or blowing them into the resin spray as it is applied to the mandrel.

An alternative method of forming the plastics mold is to provide a flat sheet of a thermoplastic material which may be reinforced. The flat sheet is positioned over the convex mandrel and heated so that it drapes over the mandrel where it cools, hardens and takes on the shape of the mandrel.

Fresh concrete can thereafter be poured over it. Since the mandrel supports the mold when fresh concrete is poured thereover, the cooling and, thereby, the hardening of the thermoplastic sheet is enhanced by its contact with the cold concrete. Faster operating times can thereby be achieved.

Materials for the thermo-plastic sheet include polystyrene, polyethylene, polyvinyl chloride, acrylic, polyester and the like.

They are heated to a temperature which is normally between 200-300 F, sufficient to drape the sheet over the mandrel.

Another alternative for the construction of the plastics mold is to extrude plastics material of the desired shape from an extrusion die. In this alternative, powdered or granulated thermoplastic material is fed from a hopper to an extruding screw. The plastics material is heated to soften it and extruded or protruded from an opening in the die which corresponds to the desired shape of the mold. This alternative has the advantage that the finsh formed shape of the mold is achieved without requiring the production of an intermediate flat sheet such as is required in the preceding alternative.

Other methods for constructing the mold will, of course, come to mind. For example, hot melt resins such as asphalt crystalline waxes, crystalline polymers and the like, which can be reinforced, can be sprayed or poured over the mandrel. Also, rigid foams, such as polyurethane foam can be employed for forming the mold over the mandrel.

In each instance (except in the extrusion alternative) the mold is formed over the mandrel and the mandrel supports the mold in its desired orientation and shape until the pipe has been finish tormed. further, the mold is continuously formed, that is, without joints or interruptions and once the mold has been formed on the mandrel, it remains stationary relative to the trench and the pipe being cast about it while relative movement takes place between the mold and the supporting mandrel.

To prevent the mold from adhering to the mandrel, suitable mold release means are employed. The release means can comprise a highly polished convex mandrel surface, a coating of tetrafluorethylene such as TEF LON (Registered Trade Mark) applied to the mandrel, or the application of a thin separating film such as a plastic or cellophane film which is placed between the mandrel and the mold being formed. The film adheres to the mold and slides relative to the mandrel.

From the above it will be apparent that the present invention eliminates the heretofore troublesome and costly mold forms. By eliminating the reuse of molds the wear and tear inflicted on them from the continuous handling is avoided. Moreover, the need for a workman to enter the pipe to release and remove the mold forms has been eliminated.

This not only reduces labor costs for producing the pipe but enables the production of virtually any pipe size well below the heretofore minimum pipe size of about 24 inches.

The present invention further enables the continuous, uninterrupted 24-hour a day production of pipe since the need for intermittently terminating production to allow for the removal of mold forms is eliminated.

Also, surface irregularities on the interior of the pipe caused by the mold forms are eliminated. This in turn eliminates a source of turbulence for the medium flowing through the pipe. Thus, the present invention permits a more rapid production of pipe, reduces production costs and enhances the flow characteristics of the pipe. Thus, the present invention both facilitates the construction of the pipe and reduces production costs.

In addition, since the plastics mold becomes a permanent part of the pipe which spans at least the upper half thereof the mold provides an ideal protection for the concrete against concrete attacking corrosives, such as are frequently found in sewage fumes. This protection is achieved without any additional labor. Thus, the present invention substantially enhances the durability of concrete pipe.

The invention will now be described further, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a fragmentary, schematic side elevational view of a seamless concrete pipe forming machine constructed in accordance with the present invention; Fig. 2 is a front elevational view, in section, and is taken on line 2-2 of Fig. 1; Fig. 3 is a fragmentary, side elevational view of the arrangement shown in Fig. 2; Fig. 4 is a front elevational view, in section, similar to Fig. 2 but illustrates another embodiment of the present invention; Fig. 5 is a side elevational view, with parts broken away, of the arrangement shown in Fig.4; Fig. 6 is a fragmentary side elevational view of another embodiment of the present invention;; Fig. 7, 8 and 9 are cross-sectional figures taken on lines 7-7. 8-8 and 9-9, respectively of Fig. 6; and Fig. 10 is a cross-sectional view of a finished jointless concrete pipe produced in accordance with the present invention.

Referring to Figure 1, a pipe-forming machine 2 constructed in accordance with the present invention is generally illustrated.

Principally, it comprises a sled or carriage 4 which can be drawn along an open trench 6 via a cable 8 connecting the carriage with a pulling mechanism 10, for example. The carriage rests on trench bottom 14 and mounts a pipe-forming mechanism 16 which is broadly defined by a lower trough form 18 which compresses fresh (deformable) concrete 20 into a lower pipe half 22 having an interior, concave pipe surface 24. As is best seen in Figure 10, the lower pipe half is fully supported by trench bottom 14.

The pipe-forming mechanism 16 further includes an upper forming section 26 which casts the fresh concrete into an upper pipe half 28 about an interior pipe form 30 the exact construction of which is described hereinafter. The pipe-forming mechanism 16 communicates with an upright concrete intake hopper 32 into which fresh concrete is continuously poured by conventional means while the sled 4 is pulled in a forward direction, that is, to the right as viewed in Figure 1.

For the purposes of the present invention the pipe-forming mechanism 16 is only schematically illustrated. The abovereferenced issued U.S. Patents describe its construction in greater detail. Furthermore, for the purposes of this specification the term "concrete" is intended to include and includes any initially readily deformable and subsequently hardenable cementitious material having such binders as cement or gypsum, and fillers such as sand aggregate and the like.The "plastics material" used is one of those falling within the earlier described class of materials, which have the general characteristic of being liquid, soft or pliable and hardenable by adding suitable chemical agents thereto, by subjecting the material to heat, radiation or other physical hardening agents or materials which are supplied in a rigid or semi-rigid form but which can be temporarily softened as by subjecting them to heat.

Referring now to Figures 1-3, the pipeforming machine includes a mandrel 34 which extends from adjacent a forward end 36 of the sled rearwardly past concrete intake 32 and, preferably, past or to closely adjacent the aft end of the concrete forming mechanism 16. In the illustrated embodiment the mandrel has a generally cylindrical configuration and it includes an upwardly facing convex surface 38. The mandrel has a diameter which equals the diameter of the concave pipe surface 24 less the thickness of a mold 40 formed over the convex surface for supporting the freshly formed pipe as is more fully described hereinafter. A lower portion 42 of the mandrel may be flattened and is supported on the sled.

One or more, e.g., three spray nozzles 44 are secured to sled 4 and mounted about the convex mandrel surface 38 so their respective spray cones 46 fully cover the convex surface. The spray nozzles are in fluid communication with a liquid resin supply 48 defined by a tank 49 which may be carried by sled 4. The nozzles may be of any conventional construction and they mix the resin with a suitable catalyst/promoter as above discussed. The detailed construction and operation of the nozzles does not form part of this invention and is therefore not described herein. Also provided are means 50 for adding to the nozzle spray suitable fiber reinforcement and/filler material is supplied from a fiber/filler source 52 schematically illustrated in Figure 2.

In operation, nozzles 44 spray a plastics layer 54 onto the convex mandrel surface 38 over the desired arc. Since the resin is initially of a relatively low viscosity, it is pre ferred to include an enlarged diameter lower portion 56 which defines the ends of the convex mandrel surface and which prevents resin from flowing in an uncontrolled manner down the mandrel before it has sufficiently hardened.

While plastics layer 54 is formed the sled is drawn in a forward direction (to the right as seen in Figure 3). The layer remains stationary relative to trench 6 which while it slides relative to sled 4 and mandrel 34. The distance between spray nozzles 44 and concrete intake 32 is chosen so that plastics layer 54 has cured or hardened into a substantially rigid, convex, continuous and homogeneous interior pipe mold before it comes into contact with fresh concrete. Curing of the plastics can be enhanced by including a heater 60 between the nozzles and the hopper. The heater may be an infrared heater, a hot air heater and must, of course, be combined with the use of a heat sensitive catalyst such as benzoyl peroxide, in which case the temperature of the curing plastics can be raised to above 180 F.

Alternatively, other physical means for enhancing the curing of the plastics can be employed. For example, in most addition polymerization resins, polyester resins, acrylic resins, vinylester resins, and the like, polymerization can be enhanced by using actinic light. In general, ultraviolet light of wave lengths between 2,850 and 3,600 angstroms and a proper light sensitive catalyst will cause rapid polymerization of the resin at room temperature. Furthermore, the addition of heat increases the reaction rate.

Typical ultarviolet light sensitive catalysts are benzoin, acetoin, alpha chloro methy naphthalene and the like; they can be employed in concentrations of between one-half to three percent.

As the sled is drawn forward, concrete intake 32 is drawn over the freshly formed and hardened mold and the pipe is formed thereabout. It is preferred that the pipeforming mechanism 16 be constructed so that the mold is ultimately embedded in concrete as is illustrated in Figure 10. In this manner, the wet concrete helps to support the mold in its arcuate position; after the concrete hardens a mechanical lock for the mold is formed to prevent its accidental release.

To increase the bond between the plastics mold 40 and the concrete, it is presently preferred that the exterior surface of the mold be roughened. This can be done with a suitable roll or other equipment (not shown in the drawings) which forms grooves, depressions and/or protrusions in the exterior mold surface so that the concrete, when it is poured over the mold, can enter and surround the grooves, depressions and protrusions. In this manner, the mold is mechanically locked to the pipe over the full exterior surface of the mold. Alternatively, barbs, hooks and the like of a sufficent size can be embedded in the freshly formed plastics before the plastics hardens so that they protrude beyond the plastics mold 40 and extend into the concrete poured thereabout.

This also yields a mechanical interlock which enhances the bond between the plastics mold and the concrete pipe.

To facilitate the separation of the plastics layer 54 - and the finished plastic mold from the convex mandrel surface 38, the surface is either highly polished or coated with a release agent such as tetra fluoroethylene, commonly sold under the trademark TEF LON. Alternatively, a continuous separating film 62 is rolled off a supply roll 64 carried by sled 4 and nozzles 44 spray the resin onto the film so that the film separates the resin from the mandrel. Suitable materials for the separating film include cellophane (Registered Trade Mark) and polyethylene.

Furthermore, the film and resin materials may be chosen so that the film adheres permanently to the resin. The film furnishes additional corrosion resistance and can be chosen to help protect the concrete or the plastics mold against a particularly corrosive substance.

Referring to Figures 1,4 and 5, in another embodiment of the invention the spray nozzles 44 shown in Figures 1-3 are replaced with a horsehoe-shape resin curtain coating device 66 which is placed over the convex mandrel surface 38. The curtain-forming device has a plurality of closely adjacent holes 68 through which the resin is discharged onto mandrel 34. As before, a source 48 supplies the resin to the device 66 and fiber reinforcements and/or suitable fillers may be incorporated in a conventional manner for the purposes discussed above. In all other respects the embodiment shown in Figures 4 and 5 are operational similar or identical to the previously discussed embodiment of the invention.

The reference numerals illustrated in Figure 4 not specifically discussed refer to like elements shown in and discussed in connection with the description of the embodiment of the invention shown in Figures 1-3.

Referring to Figures 1 and 6-9, in yet another embodiment of the invention, mold 40 is formed from an initially flat sheet 70 constructed of a thermo-plastic material such as polyethylene, PVC and the like. The flat sheet is drawn off a supply roll 72 mounted to sled 4 and passed beneath a heater 74 also carried by the sled and positioned forward of the concrete intake hopper 32. The heater may, for example, be an infrared rod heater which heats the flat sheet above the softening point for the material to allow it to sag and drape over the convex mandrel surface 38. The crosssectional Figures 7-9 illustrate the progress of the initially flat sheet (Figure 7) to the finish formed mold 40 (Figure 9). It is apparent that the flat sheet can be supplied in forms other than on rolls.

Additional alternative methods for forming mold 40 will come to mind to those skilled in the art. As already briefly mentioned, these alternatives include the extrusion of the mold from a die with an appropriately shaped die opening (in which case thermosetting or thermo-curing plastics can be employed), the use of hot melt resins or the use of foamed materials. Since the construction and use of the underlying equipment is conventional, it is not further described herein.

The thickness of the finish formed mold depends upon such factors as the rigidity of the plastics. the extent to which it has cured when the underlying support furnished by the mandrel is withdrawn (as the sled moves forward through the trench), the weight of the concrete and the wall thickness of the pipe, the maximum permissible deflection of the mold, the span of the mold arc as determined by the pipe diameter, and of course, the strength of the material of which the mold is constructed. A thickness of the mold between 0.010 and 0.100 inch would normally be sufficient for the most commonly encountered pipe diameters, wall thicknesses and mold materials.

WHAT I CLAIM IS: 1. A method for the hz situ formation of a long. continuous length of cementitious pipe in an open trench comprising the steps of; providing a sled for movement along the trench, the sled having an intake for receiving deformable cementitious material, means for forming the cementitious material into a lower pipe portion resting in the trench, and an upwardly convex, rigid mandrel; drawing the sled through the trench, pouring a plastics material over the mandrel and hardening the plastics material whilst supported by the mandrel to form a substantially rigid, downwardly open plastics mould. retaining the mould stationary relative to the trench, pouring the cementitious material into the intake and over the mould to thereby cast the pipe; withdrawing the mandrel whilst the cementitious material is in its deformable state; supporting an upper section of the deformable cementitious material with the mould only until such cementitious material hardens and becomes selfsupporting; and leaving the mould permanently in the hardened pipe.

2. In a method for constructing a continuous length of pipe from a deformable and hardenable material including the steps of providing an exterior form for supporting a lower portion of the pipe; providing an interior mould for supporting an upper portion of the pipe to prevent a collapse of the pipe while the material is deformable; casting the deformable material over the exterior form and the interior mould to construct homogeneous, initially deformable pipe; and thereafter permitting the material to harden into a rigid, continuous length of pipe; the improvement to the step of providing the interior mould comprising the steps of providing a continuous length of a relatively thin plastics material; shaping the plastics material into a configuration to conform it to an interior surface of the upper pipe portion and so that it extends over less than the full circumferential extent of the interior pipe surface; providing a support for the plastics material in said configuration and supporting the plastics material with the support until it is substantially rigid rendering the plastics material substantially rigid to thereby form a continuous, longitudinally extending mould; casting the deformable material about the mould; withdrawing the support from the plastics material before the deformable material has hardened; and supporting the deformable material while it is in its deformable state and until it is in its hardened state with the mould.

3. A method according to claim 2, wherein the support comprises a support mandrel, and wherein the step of supporting the plastics material whilst forming the mould comprises the step of positioning the support mandrel beneath the plastics material and including the step of withdrawing the mandrel from beneath the mould after the deformable material has been finished cast.

4. A method according to claim 2, wherein the plastics material comprises a liquid polymerisable resin, and including the step of applying the resin over a mandrel, and curing the resin before bringing into contact with the deformable material.

5. A method according to claim 4, including the step of adding fibre reinforcements to the resin before the resin hardens.

6. A method according to claim 4, including the step of applying heat to the resin after the resin has been deposited on the mandrel to facilitate the curing step.

7. A method according to claim 2, wherein the support comprises a support mandrel, including the step of supporting the plastics material during the rendering step with the mandrel, and placing a thin plastics film between the mandrel and the plastics material to prevent the plastics material from adhering to the mandrel.

8. A method according to claim 7, including the step of forming a continuous length of film and leaving the film attached to the plastics material in the hardened pipe.

9. A method according to claim 2, wherein the support comprises a support

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. convex mandrel surface 38. The crosssectional Figures 7-9 illustrate the progress of the initially flat sheet (Figure 7) to the finish formed mold 40 (Figure 9). It is apparent that the flat sheet can be supplied in forms other than on rolls. Additional alternative methods for forming mold 40 will come to mind to those skilled in the art. As already briefly mentioned, these alternatives include the extrusion of the mold from a die with an appropriately shaped die opening (in which case thermosetting or thermo-curing plastics can be employed), the use of hot melt resins or the use of foamed materials. Since the construction and use of the underlying equipment is conventional, it is not further described herein. The thickness of the finish formed mold depends upon such factors as the rigidity of the plastics. the extent to which it has cured when the underlying support furnished by the mandrel is withdrawn (as the sled moves forward through the trench), the weight of the concrete and the wall thickness of the pipe, the maximum permissible deflection of the mold, the span of the mold arc as determined by the pipe diameter, and of course, the strength of the material of which the mold is constructed. A thickness of the mold between 0.010 and 0.100 inch would normally be sufficient for the most commonly encountered pipe diameters, wall thicknesses and mold materials. WHAT I CLAIM IS:

1. A method for the hz situ formation of a long. continuous length of cementitious pipe in an open trench comprising the steps of; providing a sled for movement along the trench, the sled having an intake for receiving deformable cementitious material, means for forming the cementitious material into a lower pipe portion resting in the trench, and an upwardly convex, rigid mandrel; drawing the sled through the trench, pouring a plastics material over the mandrel and hardening the plastics material whilst supported by the mandrel to form a substantially rigid, downwardly open plastics mould. retaining the mould stationary relative to the trench, pouring the cementitious material into the intake and over the mould to thereby cast the pipe; withdrawing the mandrel whilst the cementitious material is in its deformable state; supporting an upper section of the deformable cementitious material with the mould only until such cementitious material hardens and becomes selfsupporting; and leaving the mould permanently in the hardened pipe.

9. A method according to claim 2, wherein the support comprises a support

mandrel, including the step of supporting the plastics material during the rendering step with the mandrel and wherein the steps of shaping and rendering the plastics material comprises the step of providing a flat, thermoplastic sheet, placing the sheet over the mandrel, heating the sheet to permit it to sag and assume the shape of the mandrel, and thereafter cooling the sheet before casting the deformable material over the sheet.

10. A method according to claim 2, wherein the support comprises a support mandrel, wherein the steps of shaping and rendering the plastics material comprises the steps of extruding the plastics material onto the mandrel, and thereafter hardening the material to form a mould before casting the deformable material over the mould.

11. Apparatus for the formation of long lengths of continuous pipe in an open trench comprising a sled for movement through the trench; first means connected with the sled for forming a lower pipe portion in the trench; second means connected with the sled for forming an upper pipe portion homogeneously with the lower portion; and intake means for feeding cementitious material to the first and second pipe-forming means while the sled moves through the trench: the second means including a mandrel connected with the sled and having an upwardly convex surface disposed forward of the intake means and having a shape closely approximating the shape of an interior surface of the upper pipe portion; means for placing a relatively thin layer of a deformable and hard enable plastics material in its deformable state over the convex mandrel surface to conform the material to the mandrel and release means providing for the separation of the plastics layer from the mandrel whereby the mandrel is movable relative to the plastics layer while the plastics layer remains stationary relative to the trench.

12. Apparatus according to claim 11, wherein the release means comprises a separating film bewteen the plastics layer and the mandrel.

13. Apparatus according to claim 12 wherein the separating film comprises a layer of tetra-fluorethylene applied to the portion of the mandrel adjacent the plastics layer.

14. Apparatus according to claim 12, including means for placing said film between the mandrel and the plastics layer, whereby the plastics film is stationary relative to the layer and the mandrel is movable relative to the film.

15. Apparatus according to claim 11, wherein the intake means for the cementitious material comprises an upwardly opening intake chute for receiving the cementitious material and wherein the mandrel extends in a rearward direction relative to the sled travel through the trench past the intake chute to provide support for the hardened plastics layer while the cementitious material is applied thereover.

16. Apparatus according to claim 11, wherein the plastics layer placing means comprises at least one spray nozzle for applying a plastics resin to the convex mandrel surface.

17. Apparatus according to claim 16, including means for applying fibre reinforcements to the plastics layer before the latter hardens.

18. Apparatus according to claim 17, including means for applying a filler material to the plastics layer before the lattter hardens.

19. Apparatus according to claim 11 wherein the plastics layer placing means comprises means for continuously feeding an elongate plastics sheet to the mandrel surface forward of the intake means, and means for heating the sheet to soften it and drape it over the mandrel surface.

20. Apparatus according to claim 11, wherein the plastics layer placing means comprises means for extruding a layer of plastics material and placing the extruded material onto the mandrel surface.

21. The method for the in situ formation of a long continuous length of cementitious pipe in an open trench substantially as herembetore described with reference to and as illustrated in the various figures of the accompanying drawings.

22. In the method for constructing a continuous length of pipe from a deformable and hardenable material, the improvement to the step of forming the interior mould substantially as hereinbefore described with reference to and as illustrated in the various figures of the accompanying drawings.

23. A pipe constructed of a homogeneous, continuous and uninterrupted length of cementitious- material defining an interior passage substantially as hereinbefore described with reference to and as illustrated in the various figures of the accompanying drawings