In the provision of cementitious structures, such as the building of tunnels by boring or excavating and then shotcreting. there is often a need to provide waterproofing. This is typically achieved either by providing drainage or by firstly injecting fissures in the rock with materials such as cement. microcement and reactive resin. this being followed by a layer of shotcrete. To this layer is applied a waterproof membrane. and to this membrane is applied a further layer of shotcrete or in situ concrete. The waterproof membrane is typically a prefabricated sheet of waterproof polymeric material which is anchored in place by, for example. anchoring bolts. The result is thus a "sandwich" construction.

Sometimes there may be applied by in situ placing or spraying a further layer of concrete with reinforcing (fibres or mesh), or this reinforcing may be incorporated into the final concrete coating.

The labour intensity of such a method, plus the fact that the existing membranes have not been wholly satisfactory, particularly with respect to leaking. has led to the search for alternatives. Recently. attempts have been made to spray on a suitable membrane. These have used a two-pack polyurethane material. but these have suffered from drawbacks such as complicated application. toxicity problems (from the isocyanate curing agents), bonding problems to wet shotcrete and inhibition of polyurethane formation because of high tunnel humidity has prevented the obvious potential benefits of such a method being realised.

It has now been found that it is possible to provide a suitable membrane by means of a method which avoids most or even all of the drawbacks hereinabove mentioned. There is therefore provided, according to the present invention, a process of providing a waterproof concrete structure comprising the steps of (a) providing a primary layer of a concrete composition on a support; (b) spraying on to this concrete composition a waterproofing layer; and (c) applying to the waterproofing layer a layer of a concrete composition;

characterised in that the waterproofing layer is provided by a composition which comprises as a binder an aqueous dispersion of coalescable particles of thermoplastic polymer.

The invention further provides a waterproof sandwich concrete structure comprising two layers of concrete separated by a coherent intermediate polymeric layer whose binder is coalesced particles of thermoplastic polymer.

By "support" is meant a untreated, water-permeable surface to which the waterproof concrete structure of this invention is applied. This will usually be a rough rock surface, but it may equally well be a man-made surface. such as a ceiling. The concrete compositions of steps (a) and (c) hereinabove described may be the same or different. In the case ofthe concrete composition ofthe primary layer (a), the scope ofthis term is considered to encompass not only simple concrete compositions (cold sprayed or prefabricated) but also such known variants as concrete followed by a levelling or smoothing layer of a sprav mortar (up to 4mm).

By "binder" is meant a material which on drying forms a coherent, continuous film and by thermoplastic" is meant a material which does not react to form a crosslinked structure. By "coalescable particles" is meant particles of a film-forming suspension or emulsion which on the removal ofthe continuous medium in which they are dispersed, at least partially merge to form a continuous. coherent film. The polymer dispersions which are useful in this invention are thus similar in nature to the film-forming aqueous dispersions which are used, for example. as film-forming media in aqueous "emulsion" household paints. Included in this definition of"binder" are compositions which include thermoplastic coalescable particles and cement.

The aqueous dispersion of coalescable thermoplastic polymer particles may be chosen from a wide range of materials known to the art, the nature of the polymer itself being unimportant. Thus, for example, suitable polymers include poiyurethanes. polyesters, vinyls and acrylics. Particularly suitable polymers are addition polymers derived from ethylenically-unsaturated monomers by addition polymerisation. Particularly suitable materials within this group are those which have a glass transition temperature (Tg) such

that they remain in the "rubbery" condition under conditions of use, those having a degree of elastomeric properties being particularly desirable. It is preferred that the Tg be below l50C, more preferably below -15"C. The weight solids contents of such dispersions typically lie within the range of 30-60%. Specific examples of suitable materials include polyurethanes, styrene-butadiene copolymers, ABS (acrylonitrile- butadiene-styrene) polymers, acrylonitrile-butadiene copolymers, styrene-acrylic copolymers, polsulphide dispersions, polyurethane-acrylic dispersions, polyisoprene and PVC latexes and copolymers of vinyl chloride and/or vinyl acetate with acrylic monomers such as (meth) acrylic acid and esters thereof. Materials such as bitumen emulsions may be used in conjunction with these materials, but as such materials do not coalesce, they should not comprise more than 50% by weight solids ofthe binder. This list is not exhaustive. and the skilled person equipped within the concept of this invention will readily be able to identify other suitable materials. Many such materials are available commercially and examples of suitable commercial materials include those sold by BASF AG. under the trade mark "Acronal" and those sold by Synthomer under the trade mark "Synthomer".

In addition to the aqueous dispersion, the composition may include other ingredients.

One especially useful ingredient is filler. This not only "extends" the composition, but also roughens the surface, thus providing a "key" for a subsequently applied cementitious composition. Its presence is preferred. Typical examples of suitable fillers include quartz sand and quartz flour of average diameters in the range of from 0.04-1.5mm, as well as dolomite, talc, mica, barytes, iron oxide, titanium dioxide, rubber and plastics granules, lightweighted aggregates and glassy fumace residues such as "holospheres". Fibres of steel, glass or polymeric material can also be used, preferred examples of polymeric fibre being those ofthermoplastic material, especially polyethylene and polyacrylonitrile, preferably with length of from 0.2-12mm and surface area of from 6-8M2/g.

There may also be added to the compositions of this invention standard ingredients in art-recognised quantities. Typical examples of additives whose presence may be beneficial include flow-enhancing agents, defoamers, dispersants, colouring materials, wetting agents, rheology modifiers and catalysts.

When there is no filler present, a composition for use in this invention may comprise 100% of dispersion. When desired, additives may comprise up to 5%, preferably 0. 1-5% by weight (calculated on active ingredients of additive per weight of liquid dispersion).

When filler is present (the preferred case), the composition preferably comprises 20-80% (more preferably 30-50%) by weight dispersion, 79.9-15% (more preferably 69.9-15%) by weight filler, and 0.1-5% by weight additive(s).

The weight of dispersion is the weight of the liquid dispersion and the weight of the additive is the weight of active ingredient.

The composition may be applied by spraying to a primary layer concrete. This is preferably shotcrete, but it may be other suitable cementitious compositions. such as polymer concrete. The membrane layer applied may vary in thickness. depending on the material used, the conditions of application and the properties (such as crack bridging ability) desired, but as a general rule it should have a thickness of 2-15mm, preferably 2- 6mm. The application may be done in a single spraying pass or in several passes. To this sprayed composition is applied a further layer of cementitious composition, preferably shotcrete applied by spraying. The result is a "sandwich" construction. The preferred ways in which such a sandwich construction may be created are shown below: Variation A B C 1 st. layer shotcrete (typically prefabricated shotcrete 50-250mm thick) concrete elements (200-500mm) 2nd. layer 2-10mm thick 2-5mm thick membrane (5-10mm) membrane (Example membrane (Example 2 composition) 4 composition) 3rd. layer shotcrete (typically shotcrete or concrete poured concrete or 50-250mm thick) placed in situ concrete placed in situ

The preferred variation is the A variation. Included in the first layer is the possibility that there may be applied thereto a levelling mortar, this being from 2-1 5mm thick. The membrane can be sprayed on to the concrete 1-90 days after the application of the concrete. If spraying is consistent within 3 days after concrete application, there is the additional advantage that the water in the dispersion can help with the cement hydration.

When the concrete of the first layer has been in place for a longer time (3-60 days), it may be necessary to wash the surface with water. This removes only residues and prewastes the surface, giving better membrane adhesion. As previously mentioned, it can be applied in a single pass or in a number of passes with suitable pauses to allow drying and/or hardening. In the multi-layer approach, one or more of the sprayed layers may include reinforcing fibres of glass, steel, ceramic or polymer; alternatively, reinforcing in the form of mesh or roving can be applied to still-wet membrane and the reinforcing then covered by a further layer of membrane.

The structure thus formed has excellent waterproofing properties, coupled with excellent water vapour permeability, thus allowing any trapped water to escape through the membrane and avoiding any interfacial failure. In addition, it is durable, safe and easy to apply and relatively inexpensive.

The invention is further described with reference to the following non-limiting examples in which all parts are expressed by weight.

A number of compositions are prepared by blending together the following combinations of ingredients: Example 1 polymer dispersion' 30 parts barytes 27 parts calcium carbonate 42.5 parts titanium dioxide 0.5 parts 1. styrene-acrylic ester copolymer emulsion 50% solids by weight ("Acronal" (trade mark) S361 (ex BASF))

Example 2 polymer dispersion2 40 parts calcium carbonate 55 parts defoamer3 2 parts dispersant4 1 parts pigment5 2 parts 2. "Acronal" S361 3. "BYK-035" ex Byk Chemie, a mixture of hydrophobic components in a paraffin- based mineral oil 4. "Pigment dispersant A" ex BASF, a 20% aqueous solution of ammonium polyacrylates 5. black iron oxide "Bayferrox" (trade mark) 316 ex Bayer Examnle 3 polymer dispersion6 50 parts barytes 48 parts defoamer7 1 parts dispersant8 1 parts 6. "Acronal" 6210 (styrene-acrylic ester copolymer) 7. "Agitan" (trade mark) 731 ex Münzing Chemie GmbH. a modified organopo lysiloxane 8. "Pigment dispersant A".

Example 4 polymer dispersion9 40 parts calcium carbonate 32 parts defoamer'0 2 parts pigment dispersant" 0.5 parts pigment2 0.5 parts barytes 22 parts acrylic fibres 0.5mm long 1 parts

9. "Acronal" S361 10. BYK-035 11. "Pigment dispersant A" 12. red iron oxide "Bayferrox" 105M Example 5 polymer dispersion13 60 parts calcium carbonate 40 parts 13. styrene-butadiene-acrylic ester copolymer emulsion 53% solids ("Synthomer" 33y 20 (ex Synthomer)) Example 6 An example of a mixed system (polymer dispersion + cement) polymer dispersion'4 55 parts portland cement 14.8 parts quartz sand 0.1-0.4mm 8 parts quartz flour 22 parts polyethylene fibres, average length 0.2mm 0.2 parts 14. acrylic ester-acrylonitrile copolymer emulsion 55% solids by weight ("Acronal" DS 6137X) ExamPle 7 polymer dispersion15 35 parts polyethylene/polypropylene fibres 1.5 parts barytes 48 parts hollow glass spheres 10 parts water 5.5 parts

Testing of compositions The compositions according to Examples 2 and 3 are sprayed on to concrete plates of dimensions 500x500x40mm at a film build of 2mm and tested. The results are shown in Table 1.

Table 1 Property measured Result Example 2 Example 3 bonding strength (N/mm2)' 0.6 0.6 strain at rupture (%)2 - at room temperature 250 140 - at -20°C 63 40 stress (N/mm2)3 at rupture - at room temperature 0.8 1.7 - at -20°C 8.5 7.7 tear propagation (N/mm)4 11 12 (at room temperature) water absorption (%)' 5 10 (3d at room temperature) 1. measured according to DIN/ISO 4624 2. measured according to DIN 53504 3. measured according to DIN 53504 4. measured according to DIN 53515 5. measured according to DIN 62617 The crack-bridging ability of the compositions of the invention depends on the thickness of the membrane deposited. The excellent crack-bridging ability of the compositions according to the invention is demonstrated by the testing of the composition of Example 2 on a universal testing machine. In this method, a cylindrical specimen of concrete (diameter 50mm, thickness 30mm) is stretched on the machine at a rate such that there is generated in the concrete a continuous transverse crack at a rate of 0. lmm/min. The

crack width in the concrete is measured at the point when there occurs in the membrane a crack extending completely through it. The results are shown in Table 2.

Table 2 membrane test temperature crack width in concrete thickness (mm) (°C) (mm) at through cracking point in membrane 0.7 20 1.5 -20 1.0 1.7 20 2.7 -20 2.1 2.3 20 4.1 -20 3.6