FR2795111A1 - REINFORCED CONSTRUCTION MATERIAL, COATING PRODUCT AND PLATE OR SLAB OF MOLDED MATERIAL COMPRISING SAID MATERIAL AND THEIR PREPARATION METHOD - Google Patents

Abstract

The subject of the invention is a reinforced construction material consisting of a reinforcement embedded in the thickness of a composite material, said material being obtainable by mixing a curing agent with a filler comprising: (iii) a binder of hydraulic or chemical setting cement type which hardens in contact with the hardening agent, and (iv) sand, gravel or possibly crushed aggregates, characterized in that said reinforcement consists of a mesh network of wires chosen from among yarns of poly (vinyl alcohol), yarns of a copolymer of vinyl alcohol and mixtures thereof.

Description

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  The invention relates to a reinforced construction material which can be used for the renovation or consolidation of civil engineering works such as galleries, tunnels and sewerage collectors. This reinforced material can also be used for the preparation of prefabricated parts intended for construction such as thin slabs or floating docks. The reinforced material of the invention is based on concrete or mortar. In the field of construction, materials with high mechanical strength, in particular in tensile bending and crack resistance, 1o are necessary. This is the reason why reinforced concrete and mortar have appeared in the art. EP 51 101 proposes in particular the manufacture of concrete plates having in the thickness of the plate a reinforcement constituted by a mesh fabric of glass fibers. Likewise, DE 2,854,228

  describes the reinforcement of concrete slabs with glass fiber mats.

  However, these techniques are not fully satisfactory since the manufacture of concrete and mortar is carried out in a highly alkaline medium, it requires the use of materials which are very resistant to bases. In fact, in the case of composites reinforced by glass fibers with an unmodified Portland cement matrix, there is, during aging, a loss of ductility of the material following an excessive increase in the adhesion between

  fibers and the unmodified lime-rich matrix (Ca (OH) 2).

  Metal reinforcements, for example steel, are known in the art. Their development is however not desirable being

  given their tendency to oxidize and their poor maneuverability on site.

  - Reinforcements based on carbon fibers have also been proposed. The materials reinforced by these reinforcements combine high mechanical resistance with excellent durability. In addition, the maneuverability of

frames is optimal.

  The major drawback of these materials is their exaggerated cost price. The high cost of carbon fibers thus limits the use of these materials.

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  The present invention therefore aims to provide a reinforced material based on mortar or concrete which is both inexpensive, durable and which has

  high mechanical resistance.

  According to the invention, the material used to reinforce the mortar, respectively the concrete, is very easy to handle. The invention relates more specifically to a reinforced material for renovation or construction, consisting of a frame embedded in the thickness of a composite material, said composite material being obtainable by mixing a curing agent with a filler comprising : (i) a cement-type binder with a hydraulic or chemical setting which hardens on contact with the hardening agent, and (ii) sand, gravel or possibly crushed aggregates. The reinforced material of the invention is characterized in that the frame consists of a mesh network of threads chosen from poly (vinyl alcohol) threads, threads of a vinyl alcohol copolymer and their mixtures. Particularly suitable vinyl alcohol copolymers

  are those resistant to bases, whether sequenced, alternate or statistical.

  Advantageously, said copolymer comprises more than 40% by weight, preferably more than 50%, better still more than 80% by weight

  of motifs derived from vinyl alcohol.

  By unit derived from vinyl alcohol, is meant the unit -CH2-CH-,

- OH

  which corresponds to the formula of vinyl alcohol except that the double bond

  is engaged in binding to the other monomers.

  The copolymers which can be used according to the invention are obtained for example by copolymerization of a precursor of the unit -CH2-CH- (such as I OH vinyl acetate) with various comonomers such as the α-olefins, for example ethylene or propylene, or acrylic or methacrylic monomers, for example acrylic and methacrylic acids and their lower alkyl esters. By lower alkyl is understood according to the invention C1-Cs alkyl. Then, the pattern -CH2-CH- is generated by simple transformation I

OH

  chemical. For example, when the copolymerization was carried out from vinyl acetate, then the transformation takes place by hydrolysis of the acetate function or saponification. As vinyl alcohol copolymers, mention may thus be made of ethylene / vinyl alcohol copolymers and alcohol copolymers

  vinyl / methyl methacrylate.

  According to a particularly preferred embodiment,

  selects for the manufacture of the reinforcement of poly (vinyl alcohol) wires.

  Poly (vinyl alcohol) or copolymer yarns are monofilament or multifilament yarns. The multifilament yarns are prepared in a conventional manner, and for example by twisting, from monofilament yarns of

  poly (vinyl alcohol) or a copolymer of vinyl alcohol.

  The wire used generally comprises up to 2000, preferably

  up to 2500 filaments of (co) polymer.

  The yarns of polymer or copolymer of vinyl alcohol are

  commonly available commercially.

  It is preferable to use yarns having a grammage varying between 20 and 1000 tex (better still between 100 and 300 tex), and a density varying

  between 1.1 and 1.4 (better still between 1.25 and 1.35).

  Generally, we will opt for wires with a module

  Young's range between 1900 and 3250 kg / mm2 or between 15N / tex and 25 N / tex.

  More preferably, the wires have a tensile strength (as measured according to French, European and

  international: NF 5079, EN 5079 and ISO 5079) varying between 80 and 180 kg / mm2.

  Similarly, it is preferred that the wires used have an elongation at break (as measured according to French standards,

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  European and international: NF 5079, EN 5079 and ISO 5079) from 1 to 20%,

  better still from 3 to 13%, for example from 4 to 7%.

  Another preferred characteristic of the threads used according to the invention is a toughness of between 65 kg / mm2 and 260 kg / mm2 (or between 0.5 N / tex and 2 N / tex), preferably between 100 kg / mm2 and 195 kg / mm2 (or between 0.8 N / tex and 1.5 N / tex), for example between 120 kg / mm2 and 195 kg / mm2 (or

between 0.9 N / tex and 1.5 N / tex).

  Note that an N / tex unit corresponds to pGPa o p is the

  density expressed in g / cm3 of the wire concerned.

  The frame used in the context of the invention is presented under

  the shape of a mesh network of weft and warp threads.

  The weft threads and the warp threads respectively consist of one or more threads chosen from poly (vinyl alcohol) threads and

  vinyl alcohol copolymer yarns.

  Preferably, each warp yarn consists of 2 to 10 yarns chosen from polyvinyl alcohol yarns and vinyl alcohol copolymer yarns. Similarly, it is preferred that each weft thread consists of 2 threads chosen from poly (vinyl alcohol) threads and copolymer threads

vinyl alcohol.

  According to a preferred embodiment of the invention, the weft threads used are all identical to each other and the warp threads used are all identical to each other. However, the invention is not intended to be limited to a

such an embodiment.

  - Thus, the invention intends to cover materials reinforced with reinforcements made up: - exclusively of poly (vinyl alcohol) yarn, - exclusively of vinyl alcohol copolymer yarn, and - of a mixture of poly (alcohol alcohol vinyl) and yarn

vinyl alcohol copolymers.

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  The meshes formed by the woven network have any shape, for example square, rectangular or the shape of a rhombus. For simplicity we

  prefers square and rectangular shapes.

  The opening of the meshes delimited by the woven network preferably varies between 1 mm2 and 2500 mm2, preferably between 10 and 200 mm2. Generally, the grammage of the woven network varies between 20 and

  500 g / m2, preferably between 50 and 300 g / m2.

  The mesh network is obtained conventionally by

knitting, weaving or sizing.

  To do this, the (co) polymer wires are previously sized, in order to improve their sliding properties. Sizing is a classic technique in the textile field, which consists in treating the yarn on the surface so as to facilitate its handling in knitting,

weaving and sizing.

  According to a preferred embodiment of the invention, the mesh network is manufactured by gluing. This technique involves coating the wires with a sizing material and positioning the wires in the weft and warp directions so as to constitute the desired mesh network, then a processing step ensuring the curing of the sizing material. , whereby the weft and warp threads are securely held within the mesh network. The curing of the sizing material can be obtained by heating the mesh network to an appropriate temperature or by drying at room temperature. How to do this depends on the exact nature of the sizing material. The hardening can be caused by crosslinking or other chemical reaction, or by evaporation of the water contained in the

  sizing material and coalescence.

  Examples of sizing materials are latexes and adhesive resins. Mention may be made, as latex, of the aqueous dispersions of any of the following polymers: styrenebutadiene polymers, polyurethane polymers, poly (vinyl alcohol), polyvinyl chloride

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  and their mixtures. As preferred sizing material, the latexes will be chosen

  of styrene-butadiene polymer.

  A better solidity of the mesh network is obtained by gluing.

  The reinforcement based on (co) polymer of vinyl alcohol is embedded in the thickness of the reinforced construction material according to the invention. The positioning of the reinforcement in the thickness of the composite material takes place during the manufacture of said composite material, this positioning possibly being carried out in situ on site. The exact location of the reinforcement depends on the end use of the reinforced building material prepared and the type of stresses to which this material will be subjected. The frame must be placed

  so as to reinforce the most stressed area of the material.

  The composite material is concrete or mortar. More generally, the composite material can be obtained by mixing a hardening agent with a filler comprising (i) a cement-type binder hardening in contact with the hardening agent and (ii) sand, gravel or

possibly crushed aggregates.

  The nature of the curing agent depends on the nature of the cement used, as a binder. There are hydraulic binders

chemical setting binders.

  Examples of cements with hydraulic setting are Portland Artificial Cement (CPA), sulphate resistant CPA cements or CPA PMES cements (with selenitic seawater cements), slag cements (CHF), slag cements with ash (CLC), Clinker slag cements (CLK), or

aluminous cements.

  The designation of these cements conforms to the standard

  European prEN 197.1 (1998) or to French standard NF P 15-301.

  By way of cement with chemical setting, mention may also be made of cements of the aluminosilicate type and cements based on silica fume and

fly ash.

  A typical example of a suitable chemical setting cement comprises about 10% by weight of silica smoke and about 90% by weight of fly ash.

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  In the case of cements with hydraulic setting, the

hardening is water.

  According to a preferred embodiment of the invention, the material

  composite is prepared from water and Portland hydraulic cement.

  In the case of chemically set cements, the curing agent is a basic aqueous solution (the pH of which is generally around 14)

  of alkali metal silicate, for example potassium or sodium silicate.

  The respective proportions of binder (i), sand, gravel and / or aggregates (ii) and curing agent are those generally used.

  works in the technique for the preparation of concrete and mortar.

  For example, in the case of a cement with hydraulic setting, the quantity of cement varies from 60 kg to 400 kg per tonne of load, preferably from 100 to 200 kg / t. In this case, the amount of water used as curing agent varies between 8 and 35% by weight relative to the total weight of the

  filler, preferably from 10 to 25% by weight.

  By way of indication, in the case of a cement with chemical setting, the curing agent represents from 7 to 20% by weight, generally from 9 to 16% by weight relative to the total weight of the mixture of the curing agent. , of component (ii) and of said cement. In this case, the binder is generally present at

  from 200 to 300 kg per tonne of load.

  According to the invention, the charge can also comprise various adjuvants. The adjuvants which can be used according to the invention are those generally

implemented in the art.

  Among these are water retention agents, water reducers, superfluidifiers, plasticizers and more generally agents

improving rheology.

  Examples of plasticizing additives are fine silica (in particular of particle size less than 100 μm), tripolyphosphates and neopentylglycol. Preferably, the adjuvants constitute from 4 to 10% by

  weight of the total mass of the load.

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  Advantageously, the filler comprises, in addition to the constituents (i) and (ii) above and any additives, reinforcing fibers which are fibers from 0.1 to 500 mm long, preferably from 1 to 50 mm long, chosen from glass fibers, metallic fibers or organic fibers. Examples of organic fibers are polyvinyl alcohol fibers, polyethylene fibers, polypropylene fibers, polyacrylonitrile fibers, polyamide fibers, polyterephthalate fibers

  ethylene, cellulose acetate fibers or cellulose fibers.

  i0 Examples of glass fibers are glass wool fibers

  and base-resistant glass textile fibers.

  Examples of metallic fibers are bare steel fibers or

  galvanized, cast iron fibers and brass fibers.

  Resistant alkali glass fibers and organic fibers chosen from polyvinyl alcohol fibers, polyethylene fibers, polypropylene fibers, polyacrylonitrile fibers,

  polyamide fibers, or polyethylene terephthalate fibers.

  In the context of the invention, it is possible to add to the charge

  one, two or more types of reinforcing fibers.

  The quantity and nature of the fibers to be added to the filler depends on the end use of the reinforced material. They depend in particular on the type of tensile and bending stresses to which said material will be subjected. When the building material has to undergo significant tensile and bending stresses, it is desirable to use at least two types of additional short fibers in order to obtain the multiple cracking of said material on macroscopic, mesoscopic and microscopic scales and thus to guarantee minimum opening of all cracks. The amount of fiber added generally varies from I to 50 kg per tonne of composite material, better still from 2 to 40 kg / t, for example from 2 to

kg / t.

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  Sands, gravel and aggregates, possibly crushed, which can be used

  according to the invention are those usually used in the art.

  The sands are generally characterized by a grain size

from about 100 to 400 pm.

  s Aggregates are characterized by a particle size of approximately

350 pm to 4 mm.

  Gravel is characterized by a grain size greater than 4 mm. As crushed aggregates, we know the fillers whose

  particle size is less than 100 µm.

  The nature of the filler and that of the binder depend on the intended application. When it comes to preparing a material for renovation (or repair) and comfort, we will choose for example a cement type binder with hydraulic setting and a load consisting of silica smoke, limestone fillers, sands silicon, various additives and fibers in the following proportions: - silica smoke: 10 to 30% o; - limestone fillers: 20 to 40% o; - silica sands: 500 to 700% o; - adjuvants + fibers: 10 to 50% o; and -binder: 100 to 350% o, these quantities being calculated relative to the total weight of all of these constituents. The binder hardening is obtained in this case by contact with water, the amount of water preferably being between 11 and 18% by weight.

  the total weight of the filler and the binder.

  In order to prepare a material for prefabrication, we will choose a cement type binder with hydraulic setting and a load consisting of limestone fillers, sands, aggregates, various additives and pigments in the following proportions: - limestone or siliceous fillers: 30 to 100% o; - silica sands, aggregates or gravel

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  or silica-limestone (with a particle size of 1 to 50 mm): 400 to 600% o; various additives: 10 to 30% o; -pigments: 5 to 15% o; and - binder: 150 to 450% o, these quantities being calculated relative to the total weight of all of these constituents. The binder hardening is obtained in this case by contact with water, the amount of water preferably being between 10 to 20% by weight.

  lo of the total weight of the filler and the binder.

  The process used for the preparation of the reinforced material

  construction of the invention depends on the envisaged application.

  By way of example, the following is illustrated: - the preparation of coating products for the renovation or consolidation of building elements; and - the preparation of molded plates and slabs;

  comprising the reinforced materials of the invention.

  According to one of its aspects, the invention therefore relates to a coating product for the renovation or consolidation of a construction element

  comprising the reinforced material of the invention.

  According to another of its aspects, the invention relates to a plate or

  molded material slab comprising the reinforced material of the invention.

  The processes for preparing these products form another

subject of the invention.

  The coating products of the invention can be prepared by carrying out the steps consisting in: a) preparing a paste by adding a curing agent to a filler comprising: (i) a cement-type binder with hydraulic setting or chemical curing on contact with the curing agent, and (ii) sand, gravel or possibly crushed aggregates; b) depositing a first layer of said paste on the surface to be renovated or consolidated; c) placing on the surface of said first layer of dough an armature consisting of a mesh network of threads chosen from poly (vinyl alcohol) threads, threads of a vinyl alcohol copolymer and their mixtures; d) impregnating the frame with said paste; e) depositing a second layer of the paste obtained in step a) on said frame; f) let stand for the time necessary for setting and

hardening of the dough.

  The slabs or plates of molded material can be obtained by implementing the following steps: a) preparing a paste by adding a curing agent to a filler comprising: (i) a cement type binder with hydraulic setting or chemical curing on contact with the curing agent, and (ii) sand, gravel or possibly crushed aggregates; b) depositing a first layer of said paste in a suitable mold; c) placing on the surface of said first layer of dough an armature consisting of a mesh network of threads chosen from poly (vinyl alcohol) threads, threads of a vinyl alcohol copolymer and their mixtures; d) impregnating the frame of said paste e) depositing a second layer of the paste obtained in step a) on said frame; f) let stand for the time necessary for setting and

hardening of the dough.

  A preferred embodiment of these two methods consists in depositing the cement paste by spraying using a spraying means

  suitable, for example a P tzmeister type machine.

  The impregnation of the reinforcements with the paste can be carried out mechanically or manually, for example by masking. The precise positioning of the reinforcements can be facilitated by

  use of specific spacers or spacers.

  The location of the reinforcement essentially depends on the type of

  stresses that the final work will have to bear.

  The curing agent is mixed with the filler by kneading according to the usual techniques. When the load comprises reinforcing fibers, these are incorporated dry or in a pasty medium. When the fibers added are mineral fibers (such as glass fibers), these are preferably incorporated into the cement paste at the end of

mixing.

  The reinforced materials of the invention can be used in the construction, building and civil engineering industry. Some possible applications are: (1) repair and consolidation of civil engineering works, such as galleries, sewerage collectors (sewers) and tunnels, (2) the preparation of thin sails in prefabricated mortar or concrete for the new building or renovation market (load-bearing or decorative elements), and (3) the design of new parts allowing for lightening

  manufactured concrete structures (thin slabs and floating docks).

  The following example further illustrates the invention.

EXAMPLE 1

  A reinforcement (mesh network) is prepared by gluing from a 200 tex (200 g / 1000 m) poly (vinyl alcohol) wire and a density of 1.3 g / cm 3 having a toughness 0.98 N / tex (- 127 kg / mm2),

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  an elongation at break of 6.6 to 6.7%, a tensile strength of 132 kg / mm2, and a Young's modulus of 20.3 N / tex (- 2630 kg / mm2). Matter

  size is a styrene / butadiene polymer latex.

  The mesh network of this frame is characterized by a mesh opening of 8 x 8 mm2, a grammage of 40 g / m2 in warp and 40

g / m2 in weft.

  A mortar paste is then prepared by mixing in a kneader a dry mortar ready for use with water, in a ratio

0.18 water / mortar weight.

  At the bottom of a 20 mm x 100 mm x 300 mm mold, there is a first layer of this paste with a thickness of approximately 5 mm. On this dough, deposit 0 (reference example "without grid"), 1 (example of the invention "with 1 grid") or 2 (example of the invention "with 2 grids") grids of

the frame prepared above.

  If necessary, the paste (s) is then impregnated with the paste (s), by masking, then the mold is filled with a second layer of paste, the same quantity of paste being used in each case (reference example and

examples with 1 or 2 grids).

Let the whole harden.

  The mechanical performance of the mortar test pieces obtained is then tested, under the following conditions: (1) after curing for 28 days, at 20 ° C. and 98% relative humidity; (2) after an accelerated aging cycle of 84 days in a bairr of hot water at 50 C (which corresponds to natural aging

about 20 years).

  The test used is the 4-point bending test in accordance with the

European standard for EN 1170-5.

  After aging and just before the bending test, the test pieces tested (20 mm x 100 mm x 300 mm) are immersed in water

for 24 hours.

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  According to this test, the breaking stress (MOR), the

  elastic limit (LOP) and the strain at break (EPS).

  The results obtained are collected in the following tables 1 to 4:

Table 1

  Plate ripening: 28 days, 20 C, 98% relative humidity MOR LOP EPS test tube Elastic limit stress Deformation at break rupture Without grid 4.8 MPa 4.8 MPa 0.03% with 1 grid 4.2 MPa 3.5 MPa 0.2% with 2 grids 6.8 MPa 4.3 MPa 3.5% Table 2 Plate cure: 28 days, 200C, 98% relative humidity + 28 days in water at 50 C MOR LOP EPS specimen Stress at elastic limit Deformation at break rupture Without grid 7.0 MPa 6.5 MPa 0.03% with I grid 6.0 MPa 5.7 MPa 0.3% with 2 grids 6.3 MPa 3.6 MPa 3.5%

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Table 3

  Plate ripening: 28 days, 20 C, 98% relative humidity + 56 days in water at 500C MOR LOP EPS test tube Stress at elastic limit Deformation at break rupture Without grid 6.5 MPa 5.4 MPa 0 , 03% with 1 grid 5.5 MPa 2.7 MPa 1.1% with 2 grids 6.7 MPa 4.3 MPa 3.0%

Table 4

  Plate ripening: 28 days, 20 C, 98% relative humidity + 84 days in water at 50 C MOR LOP EPS test tube Stress at elastic limit Deformation at break rupture Without grid 6.6 MPa 5.3 MPa 0.04% with 1 grid 5.7 MPa 3.3 MPa 0.4% with 2 grids 6.5 MPa 3.4 MPa 3.2% The results obtained show that the presence of reinforcements of poly (vinyl alcohol) allows obtaining superior mechanical characteristics in bending: we notice in particular deformations at break

times higher.

  These characteristics are retained after aging.

EXAMPLE 2

  The mesh networks having the characteristics indicated in the following tables 5 and 6 are prepared by gluing, using the

  same polyvinyl alcohol wire as in Example 1.

  The sizing material used is a latex of styrene butadiene polymers.

Table 5

  N of the network Number of wires of Number of wires of Number of wires of Number of mesh wires PVA per PVA wire per warp thread per weft unit per unit warp weft length in length in length in direction orthogonal direction orthogonal 2.1 6 4 0.875 thread / cm 0.875 thread / cm 2.2 4 2 0.875 thread / cm 0.625 thread / cm 2.3 6 4 0.75 thread / cm 0.625 thread / cm

Table 6

  N of the mesh network Weight of the mesh Mesh opening 2.1 260 g / m2 7 x 5 mm2 2.2 140 g / m2 8 x 12 mm2 2.3 190 g / m2 9 x 10 mm2

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the 17

Claims (16)

  1. Reinforced construction material consisting of a reinforcement embedded in the thickness of a composite material, said composite material being obtainable by mixing a curing agent with a filler comprising: (i) a cement type binder to hydraulic or chemical setting hardening in contact with the hardening agent and (ii) sand, gravel or possibly crushed aggregates, characterized in that said reinforcement consists of a mesh network of wires chosen from poly ( vinyl alcohol), yarn of an alcohol copolymer vinyl and their mixtures.   2. Reinforced material according to claim 1, characterized in that said frame consists of a mesh network of poly (vinyl alcohol) threads. 3. Reinforced material according to any one of   Claims 1 or 2, characterized in that the yarns with a grammage of 20 to   1000 tex, preferably from 100 to 300 tex are of the multifilament type.   4. Reinforced material according to any one of   Claims 1 to 3, characterized in that the mesh network of wires has a   grammage from 20 to 500 g / m2, preferably from 50 to 150 g / m2.   5. Reinforced material according to any one of   Claims 1 to 4, characterized in that the mesh network is bonded with   coating of an adhesive resin, of an aqueous dispersion of a polymer chosen from a styrene-butadiene polymer, a polyurethane polymer,   poly (vinyl alcohol), polyvinyl chloride or a mixture thereof.   6. Reinforced material according to any one of   Claims 1 to 5, characterized in that the opening of the delimited meshes   by the mesh network varies between I mm2 and 2,500 mm2. 279511 1   "" "-" has 18 7. Reinforced material according to any one of   Claims I to 6, characterized in that the binder is a hydraulic cement   Portland and that the curing agent is water.   8. Reinforced material according to any one of   Claims 1 to 6, characterized in that the binder comprises smoke from   silica and fly ash and in that the curing agent is a   basic aqueous solution of alkali metal silicate.   9. Reinforced material according to any one of   Claims I to 8, characterized in that the solid filler further comprises one or more adjuvants.   10. Reinforced material according to any one of   previous claims, characterized in that the solid filler comprises   in addition fibers from 0.1 mm to 500 mm long chosen from fibers of   resistant alkali glass, metallic fibers and organic fibers.   11. Reinforced material according to claim 10, characterized in that the fibers are resistant alkali glass fibers, poly (vinyl alcohol) fibers, polyethylene fibers, polypropylene fibers, polyacrylonitrile fibers, fiber polyamide, or fibers of polyethylene terephthalate.   12. Coating product for the renovation or consolidation of a building element comprising the reinforced material   according to any one of claims 1 to 11.   13. Plate or slab of molded material comprising the material   reinforced according to any one of claims 1 to 11.   14. A method for renovating or consolidating a building element, characterized in that it comprises the steps consisting in: a) preparing a paste by adding a curing agent to a filler comprising: (i) a cement type binder with hydraulic or chemical setting hardening in contact with the hardening agent, and (ii) sand, gravel or possibly crushed aggregates; 279511 1   b) depositing a first layer of said paste on the surface to be renovated or consolidated; c) placing on the surface of said first layer of dough an armature consisting of a mesh network of threads chosen from poly (vinyl alcohol) threads, threads of a vinyl alcohol copolymer and their mixtures; d) impregnating the frame with said paste; e) depositing a second layer of the paste obtained in step a) on said frame; f) let stand for the time necessary for setting and hardening of the cement paste.   15. A process for the preparation of a plate or slab of molded material comprising the steps consisting in: a) preparing a paste by adding a curing agent to a filler comprising: (i) a cement-type binder with hydraulic setting or chemical curing on contact with the curing agent, and (ii) sand, gravel or possibly crushed aggregates; b) depositing a first layer of said paste in a suitable mold; c) placing on the surface of said first layer of paste an armature consisting of a mesh network of wires chosen from poly (vinyl alcohol) wires, son of a copolymer - vinyl alcohol and of their mixtures; d) impregnating the frame of said paste e) depositing a second layer of the paste obtained in step a) on said frame; f) let stand for the time necessary for setting and hardening of the dough.   16. Method according to any one of claims 14 or   , characterized in that the dough is deposited by projection.