BACKGROUND TO THE INVENTION In the specification of my PCT application PCT/US) 7/00362 (published as W097/26395) there is described a yarn which includes a core and a multitude of staple fibres forming a layer which envelopes the core and provides an extended surface area and interstical spaces for infiltration by cement fine. and hydrates, the staple fibres being spun around the core and having sufficient freedom of radial movement to provide said spaces and permit ingress of cement fines and the formation of its hydrates in said spaces. Such a yarn is known as a friction spun yarn.

A specific feature of the friction spun yarn of PCT/lJS97/00362 is that the fibres are attached to the core at intervals along the length cf the core. This inhibits delamination, that is, sliding of the core with respect to the fibres. The specification of my PCT application is hereby incorporated into the present specification.

As is explained in the PCT specification, a cement tious mix bonds to a

fabric woven or knitted using yarn of the type described above. The resultant product is becoming known in the art as textile concrete which is a cementitious material with one or more textile layers incorporated in it.

Layers of fabric woven or knitted using friction spun yarn can be laminated to one or both faces of a water impermeable synthetic plastics material film, or alternatively a single layer of such fabric can be laminated to one face of the film. The film is preferably of polypropylene as this is unaffected by the alkalinity of any concrete with which it comes into contact. If the fibres are of polypropylene then the film must be of the same material otherwise hot melt lamination is impractical. If acrylic fibres are used in the yarn then hot melt adhesives can be employed to secure the fibres to an acrylic film. Acrylic is inert to cement an offers a polar surface to which cementitious mixes bond. The lamination procedure assists in preventing the core and fibres from delaminating as discussed above.

A composite material comprising a layer of air impermeable synthetic plastics film laminated to at least one layer of fabric woven or knitted using friction spun yarn will be referred to hereinafter as a composite material of the kind defined.

BRIEF DESCRIPTION OF THE INVENTION According to one aspect of the present invention there is provided a method of producing a hollow article which comprises fabricating a former using a composite material of the kind defined, the former being hollow and closed except for an inflation inlet and the woven or knitted layer being on the outside of the former, inflating the former, and applying a cementitious mix to the outside of the former thereby to create an outer cementitious layer.

The method can include the additional steps of applying a further layer 5 of said fabric to said outer cementitious layer before it has set, and then applying further cementitious material to produce a wall having said further layer of said fabric embedded in it between said outer cementitious layer and said further cementitious material. These additional steps can be repeated a plurality of times to produce a wall having layers of fabric embedded in cementitious mix.

10 Said cementitious mix is preferably sprayed on and can have chopped reinforcing fibres in it.

For certain products the method can include cutting an opening in said wall to provide access to the interior of the article. In this form said film can have a woven or knitted fabric layer on both sides thereof, and the method can include the 15 further step of applying cementitious mix to the layer of fabric on the inside of the former after cutting an access opening in the former.

The method can include the further step of providing an outer mould, and expanding the former against a patterned surface of the mo. jtd by increasing the pressure in the former before the outermost cementitious layer applied to the former

has set thereby to emboss onto said outermost cementitious layer the surface pattern of the mould.

The air pressure in the former is preferably increased to expand the former by heating the air in the former.

According to a further aspect of the present invention there is provided a method of making a hollow article which comprises inflating a bladder, laying up on the outside surface of the bladder fabric which has been knitted or woven from a friction spun yarn thereby to form a fabric covering over the bladder, and applying a cementitious mix to the fabric covering.

This method can include the step of applying further fabric layers and further cementitious mix to build up a wall comprising layers of fabric embedded in cementitious mix.

An outer mould can be provided and the bladder can be expanded by increasing the pressure in the bladder before the outermost cementitious layer applied to the bladder has set thereby to emboss onto said outermost layer a surface pattern of the mould. An increase in the air pressure in the bladder can be achieved by heating the air in the bladder.

DETAILED DESCRIPTION OF THE INVENTION

For a better understanding of the present invention the manufacture of a pair of boat hulls, a water pipe and a building structure will be described by way of example. The manufacture of a water tank will be described with reference to the single Figure of the drawing. However, it will be understood that hollow articles of any shape and for a variety of purposes can be manufactured by the techniques described hereinafter.

WATER TANK FABRICATION A composite fabric comprising a single layer of woven or knitted fabric laminated to a layer of synthetic plastics material is cut into panels. There are a number of panels 10 (see the drawing) each having the form of a truncated triangle, plus a smaller disc 12 and a larger disc 14. The narrower ends of the panels 10 are sewn or preferably ultrasonically welded to the periphery of the smaller disc 12. The larger ends of the panels 10 are sewn or otherwise secured to the periphery of the larger disc 14. The panels 10 have their edges sewn or ultrasonically welded together. A hole 16 is left, preferably in the larger disc 14, through which an inflation pipe 18 can be inserted and sealed in place. Once inflated the former has the shape of a truncated cone with a flat bottom surface of larger diameter and a flat top surface of smaller diameter. The woven or knitted fabric is outermost, but there can be fabric layers on the inside and the outside of the tank.

The entire outer surface of the former is then coated with a cementitious mix. The mix can have chopped reinforcing fibres in it and can be

applied by hand or sprayed on. The interaction between the cementitious mix and the sheath of the yarn is described in the specification of my PCT application and will not be repeated here. After one or more layers of cementitious mix, and one or more further layers of fabric, have been applied to achieve a desired wall thickness, reinforcing hoops 20 of metal or plastics tape are dropped over the partly finished tank and slide downwardly to their correct positions. Hoops 20 of different diameters are used so that they engage the outer surface of the truncated cone at different levels. They are temporarily fixed in place. Thereafter a final layer of fabric and a final layer of cementitious mix is applied to conceal the hoops. The final layer of fabric is, of course, applied before the previous cementitious layer has set. This final layer thus integrates the hoops into the tank's wall.

If the hoops are of synthetic plastics, then they are preferably of ultrasonically welded polypropylene strapping tape, or polyester or nylon webbing.

Instead of inflating the tank using air it is possible to use water, in which event the hoops must be in place before filling takes place to prevent the outwardly directed forces bursting the former.

An opening 22 can be cut in the smaller diameter disc using, for example, an angle grinder and a pre-cast manhole dropped into the opening. Holes 24,26 can be drilled in the tank to permit water pipes to enter the tank.

BOAT HULLS The construction of a pair of boat hulls is similar to the procedure just described. The shapes are, however, more complex and construction is facilitated by the use of a computer program to enable composite material of the kind defined to be cut into panels of the correct shape with an appropriate allowance for seam width. The composite material used has woven or knitted fabric on both sides of the synthetic plastics layer. In producing the boat hull, ultrasonic welding is preferred as the method for joining the panels. If the panels are sewn together then sealing tapes are required between adjacent panels. Two hull shapes are formed and joined together at the gunnels. One hull is thus upside down with respect to the other. A stiff diaphragm is provided at this level to prevent the hulls bulging when they are inflated. The diaphragm can be sewn or pop rivetted to the fabric.

Inflation can be achieved by inserting a steel pipe from bow to stern into the double hull structure. The pipe preferably lies horizontally and includes a rotary joint at one end so that the two hulls can be rotated about the axis of the pipe during the fabrication procedure. The other end of the pipe is closed and it has a hole in the walling thereof to permit air to be pumped into the two hulls. If the panels forming the hulls are ultrasonically welded it should only be necessary to inflate the hulls once as there should be no leakage.

The inflated hulls are preferably coated with a mixture of neat cement and bonding liquid, mixed into a smooth paste. A thixotropic additive, for example,

carboxy-methyl-cellulose, can be used to help keep the cement in suspension. The paste may be applied by brush or spray as only small quantities are required. A pressure pot sprayer with mixing agitator can also be used.

Once the hull surfaces are coated, one or more layers of material of the kind described are laid up and bonded to the hulls by painting or spraying on a cementitious mix. The fabric need not be pre-sewn or welded, merely over lapped at the joint lines. The surface should be carefully smoothed and if a conventional cement and a bonding liquid is used, should be allowed to air dry for 24 hours. If time is available the hulls should be sprayed with water, wrapped in plastics film and allowed to cure for four to six days.

The two hulls are separated at the diaphragm and the steel pipe removed. The wall thickness of the textile concrete should be about 3mm, and at this thickness it can be cut with a sharp knife, a garden scissors, metal shears, or by using a cutting disk in an angle grinder.

The inside surface of the two hulls has a textile bondable surface. A hollow keel and seats can be shaped and fitted into place, and bonded to the textile surface by neat cement bonding liquid such as was used on the outside of the hulls.

Gunnel rails either of wood or fabricated from strips of textile cor, crete lapped over the trimmed gunnel edges can be fitted. The insides of the hulls are desirably painted with the cement bonding liquid mix.

The finished hulls are water tight due to the laminated layers of polypropylene in the fabric. The keel and seats can be hollow and injected with polyurethane foam to provide buoyancy. The hulls can be decorated with outdoor quality PVA/acrylic paint or a water based paint.

WATER PIPE A concrete water pipe can be produced by overlapping the longer edges of an elongate strip of material of the kind defined and securing the edges together to form a seam. The strip thus takes on the form of a tube. If the composite material inclues a single layer of fabric then the fabric is outermost.

Caps are used to close the ends of the tube, one of the end caps including an air inlet. The tube is then inflated and subsequently treated with layers of cementitious mix and further layers of fabric until the desired wall thickness has been achieved.

Inflation of the pipe can continue to pre-stress the fabric layers.

BUILDING STRUCTURE A former of the requisite shape, preferably much like an igloo or a tent, is formed by ultrasonically welding panels of the kind defined together. In this form the synthetic plastics material is preferably sandwiched betweer two fabric layers so that the building structure can be treated with cementitious mix and further layers of material both on the inside and on the outside. The former is entirely closed except for an air inflation pipe entrance. Once the former has been inflated cementitious mix and further layers of fabric are applied externally until the desired wall thickness has been achieved.

Windows and doors can be cut using, for example, an angle grinder and thereafter the inner surface of the former can be treated with cementitious mix and layers of fabric to obtain the desired surface finish.

5 A floor can be cast on the laminated sheet constituting the base of the former. A frame is erected inside the former to provide additional support for the walls and roof.

Hot air under pressure can be applied to the inside of the former. This causes the film to stretch and hence the former increases in size. If there is an outer 10 mould against which the former is inflated, fine detail can be embossed into the outer surface of the former.

It is also possible to omit the film and use an inflatable bladder of the requisite shape. Fabric made of the yarn described is then laid up on the outside of the bladder and a cementitious mix applied. Further layers of fabric and mix can be 15 applied to achieve the desired wall thickness.

In this latter form it is also possible, whilst the wall ; s still flexible, to further inflate the bladder thus pressing the wall outwardly against the inner surface of a mould. This enables fine detail to be impressed into the outrer surface of the wall. If the wall thickness has to be increased further, this can be done from the inside when the bladder has been deflated and removed.

It is also possible to produce an article which has a hollow in one face thereof. In one form of the method a sheet of composite material of the kind defined 5 is placed on a horizontal platen surface. The surface has an inlet in it through which air can be supplied to the space between said surface and said composite material.

The face of the sheet which is uppermost has the fabric layer on it.

Cementitious mix is applied to this layer so that the layer is embedded. Further layers and further mix can be applied to achieve the desired thickness.

10 A mould, which can be a rubber mould, is then placed over the sheet.

Edges of the mould contact said sheet and press it against said platen surface. The mould is then clamped. When air is supplied through said inlet into said space, the space expands and presses the mix against a patterned surface of the mould. This is done before the mix sets so that the configuration of the patterned surface is 15 impressed into the mix. The mould has at least one recess into which the sheet is pressed thereby to leave a hollow in the rear face.

If desired a deflated bladder can be used in place of the composite material. The knitted or woven fabric is placed on the bladder a 1d the mix then applied to embed the fabric. Several layers of fabric and mix can be applied before the mould is placed in position and clamped. Thereafter the bladder is inflated. This has the effect, as previously described, of expanding the space behind the composite material.