BACKGROUND In traditional building constructions, two systems for the external surface coatings predominate: 1. Inorganic systems-based on mineral binders. These are thick plaster sys- tems, which are water absorbent and may become damaged by water.

2. Organic systems-based on organic materials such as latexes, and other polymers. These systems are water repellent.

Humidity and the stresses are the major factors creating various durability problems of structures. Moisture can entrain from the outside wall through vapour diffusion and also can accumulate from the inside material through condensation. Long time exposure to the moisture can lead to the fungus problem and distress during freez- ing. Stress can also develop due to the movement of the foundation and building structure.

These problems are traditionally overcome by providing an insulation layer of min- eral wool, cellular plastic (EPS), sintered clay aggregate lightweight concrete, etc.

Over this insulation layer reinforcement and plaster is placed, well anchored to the base foundation. After hardening and drying, final surface coating is applied.

The insulation layer works like a barrier for the heat transport. Thereby preventing the heat to reach the plaster, avoiding thermal deterioration. This also hinders the formation of thermal stresses caused due to the variation in the thermal properties of the structural elements, which consequently can damage the surface coating.

The known system with an insulation layer is adapted for application at the con- struction site, where the insulating layer is mounted over the wall elements, the wall elements being in exceptional cases prefabricated. The additional insulation, rein- forcement and the coating make it possible to produce a façade without joints, which is a basic property of a high quality surface coating.

There are solutions where the insulation layer is made of foam concrete plates, which meets the requirements of mineral wool and expanded polystyrene spheres (EPS). Though such solutions can produce façades without joints, they will require a large amount of man-power, which cost money. For example, supplementary isola- tion, reinforcement, and plaster may still have to be applied in situ.

Attempts made to improve the quality of prefabricated elements with insulation lay- ers have failed. The elements are quite susceptible to damages, especially during transport and mounting. Additionally, as is mentioned earlier, the insulation plates need reinforcement, and the surface has to be scratched for better adhesion, before application of the final surface coating. This requires a lot of additional man-power and is not economical.

Another adopted solution is to use prefabricated concrete elements, often used for the large suburban apartment buildings built in the 1960's and 1970's in many European cities. With this method it is not possible to provide adequate sealed joints between the elements, due to their thermal incompatibility. These tolerance prob- lems occur due to the shrinkage of concrete during hardening and deformations of

the elements caused by the changes in temperature. Reinforcement does not help re- ducing these problems to the desired level, and the temperature dependent deforma- tions continue to create problems at the joints in the finished construction. However, traditionally this is solved by the use of flexible chemical sealing substances in joints between the concrete elements, but such substances are expensive and have a disadvantage in that with age they loose their flexibility, which leads to leaking.

SUMMARY An object of the invention is to simplify the construction process of a building.

The object is reached with a method of the type mentioned initially, presenting the characterising features of claim 1.

This method provides the possibility of producing a base for a surface coating for building elements. By suitable selection of the first and the second material, it can fulfil the traditional requirements for forming of multilayer elements.

The method can be used for prefabricated building elements, such as outer walls or part thereof, or floor structures or parts thereof. The method can also be used for producing, at a building construction site, a base for a surface coating for a building element. The method is applicable for any construction, comprising a plaster form- ing a connecting layer which functions as an insulation/isolation and also as the ground plaster. The method reduces the work load and ensures interconnection be- tween the framework, foundation and the surface coating. The invention will create homes that are comfortable, more energy efficient and structurally secure than those built with the traditional construction methods.

Preferably, the first material comprises cement and the second material is a cellular plastic material. Preferably, the cellular plastic material is expanded polystyrene

spheres (EPS). Preferably, the layer is at least essentially constituted by a mixture of EPS in a cement matrix. However, some aggregates such as sand can be added to the mixture. The polystyrene spheres can fulfil the function of an outer insulating layer in traditional buildings, preventing the moisture transport to the outer surface coating. Advantageously, this is accomplished without the fragile traditional solu- tion, which creates problems for pre-fabrication, for example with transportation damages, as described above. Since the second, moisture transportation preventing material is mixed with the first material comprising cement, the first material forms a protecting ingredient for the second material, so that the risk of damages during transportation of prefabricated building elements is drastically reduced.

Additionally, the inclusion of the cellular plastic material in the cement, or concrete, will result in the layer becoming more stable, than in the case where no cellular plas- tic material is used. The reason is that the relative amount, i. e. the percentage, of cement or concrete will be considerably less, and therefore deformations and stresses typically associated with cement or concrete will be smaller. This, in turn will lead to less problems with large joints between the building elements at the construction site.

Preferably, the second material is essentially evenly distributed in the first material.

Preferably, the second material is in the form of granules with an average size of 0.5-3. 0 mm, preferably approximately 1-2 mm. Test has shown that this provides a good performance of the layer regarding toughness, strength, and isolating capabili- ties.

Preferably, the second material is at least partly removed at a surface of the layer, so that a plurality of cavities are created in said surface. Thereby, the effective surface area of the layer is drastically increased so that a surface coating applied onto the layer will bind effectively to the latter. This provides a coating for a building ele-

ment which is very durable, and which can withstand stresses, stains and wear more effectively.

Preferably, the step of removing at least partly the second material is performed by heating. This provides for a simple, low cost removal process, in which no addi- tional substances are required.

Preferably, a filler material is included in the layer. Thereby, the layer can be made stronger as well as water repellent, as will be described closer below.

Preferably, at least a part of a reinforcement is cast into the layer. Preferably, the re- inforcement is in the form of a net, and/or fibres.

Preferably, the layer is formed on a structural part comprising a planar part, which is thin in relation to the thickness of the building element. Preferably, the planar part comprises a high performance concrete as described in PCT/SE04/00148 incorpo- rated herein by reference. Preferably, the thickness of the planar part is 0. 2-10 %, of the total thickness of the structural part. Thereby, a high stiffness can be accom- plished, without deformation problems (see above) associated with thick concrete elements. Preferably, the layer and the planar part are spaced apart in the final build- ing element.

The planar part provide for a stiff and light building element. Where concrete is used, the thin planar element provides for a low shrinkage in the hardening process of the concrete. If said high performance concrete (described in said PCT/SE04/00148) is used, the shrinkage will be even lower, since this high per- formance, or high density concrete has a low shrinkage. Additionally, the structural part will be stiff enough to avoid bending caused by the layer with the first and sec- ond material. Even if shrinkage cracks appear, they will not be of significance, spe- cially when they appear on the inside wall and the material is not moisture sensitive.

Preferably, the layer is formed on a structural part comprising the planar part, and at least one beam attached to the planar part. Thereby, the layer with the first and sec- ond material can be positioned adjacent to and attached to the at least one beam, so that the layer and the planar part are spaced apart by the at least one beam. The beam can be made of any suitable material, e. g. steel or aluminium, and present any suitable cross-sectional shape, but preferably, the beam is a thin section steel beam.

Preferably, the beam has a S-or C-shaped section. The beam can be provided in any suitable number and in any suitable orientation in the building element.

The beam will provide additional stiffness to the structural part and contribute to a stiff and light building element. Thereby, said problems with damages to outer lay- ers of buildings caused by deformation due to shrinkage, etc, will be drastically re- duced.

The object is also reached with a building element according to claim 13.

BRIEF DESCRIPTION OF THE DRAWINGS Below, the invention will be described in detail with the aid of the drawings, in which - fig. 1 is a perspective schematic view of a building element according to one embodiment of the invention, - fig. 2-4 are cross-sectional views of a layer on the building element in fig. 1, and - fig. 5 is a cross-sectional view of a layer, similar to the layers in fig. 2-4, accord- ing to an alternative embodiment of the invention, - fig. 6 shows a transverse cross-section of the building element 1 shown in fig. 1, and - fig. 7 shows a perspective view of thin profile steel beams for the building ele- ment in fig. 1.

DETAILED DESCRIPTION Fig. 1 is a schematic illustration of a building element 1 according to one embodi- ment of the invention. The building element 1 is a wall element and it comprises a structural part 2 and a layer 3 applied to the structural part 2, the layer 3 being in- tended to form a base for an outer coating of the building.

Fig. 2 shows a cross-section through the layer 3, which comprises a mix of a first 10 and a second 11 material, in the form of concrete 10 and cellular plastic 11, respec- tively. Alternatively, the first material 10 can be essentially only a hardened cement and water mixture with no aggregates. The layer 3 of hardened concrete 10 and pieces 11, or spheres of cellular plastic, preferably expanded polystyrene, are pref- erably produced as described in W003018503, incorporated herein by reference.

Optionally, the layer 3 can comprise ballast material in the form of sand, etc.

In some climatic conditions, it is desirable to make the layer 3 water absorbing and in the other climates water repellent. Water absorption of the concrete is related to its pore structure. A finer pore structure results in lesser water absorption. A finer pore structure means a smaller size of the pores but larger in numbers, in compari- son to the bigger size of pores, which are smaller in number. This implies that the amount of air can be the same, but its dispersion can vary. A fine pore size can be obtained by the addition of chemical admixture.

The other possibility is by the addition of mineral admixture, which has pozzolanic effect. This will result in the admixture interacting chemically with the calcium hy- droxide produced during the Portland cement hydration. The reaction products may not be sufficient to fill up the pores, but they seal the pores. Thus the porosity may be the same but the permeability is substantially decreased.

The third possibility is by the addition of polymers. These polymers are of two types, one which makes films while drying, and the other which have film forming ability and also interact chemically, forming the calcium complexes. The polymers which only make impermeable films are not desirable as they stop vapour transport.

The concrete, like a human being must breath; if breathing stops, the human dies.

This is the case with impermeable film formation ; the concrete deteriorates.

According to one embodiment of the invention, condensed silica fume, also known as micro-silica, is added to the concrete mixture 10. This partly works as pozzolanic material, which means it interacts chemically with the calcium hydroxide produced during cement hydration, and it partly works like a filler material. In both cases it refines the pore structure. Consequently, the concrete will have small capillary pores or free water, and this will have a sealing effect on the material and will make the layer 3 essentially water repellent. However, the material will still be open to diffu- sion.

A pozzolanic reaction accelerates the hardening process. However, this can be con- trolled by diminishing the cement content in relation to the aggregates.

Preferably, the pieces 11 of cellular plastic are in sizes between 1-2 mm and evenly distributed in the layer 3. At the outer surface 21 of the layer 3, pieces 11'of cellular plastic are exposed.

In preferred step in the method according to the invention, the exposed EPS 11', or pieces 11'of cellular plastic are removed. Preferably, this is done by heating so that the exposed pieces are melted or burned away. Alternatively, the removal can be performed by applying to the surface 21 a substance which has a corrosive effect on the cellular plastic, 11', but not on the concrete 10.

As can be seen in fig. 3, the described removal step will cause the creation of a plu- rality of cavities 12 in the surface of the layer. This will drastically increase the ef- fective surface area, which gives a better adhesion of the surface coating. Prefera- bly, as can be seen in fig. 3, pieces 11 of cellular plastic located below the surface are not removed. Thereby, the risk of water accumulation in empty spaces of the layer 3 is minimised.

Preferably, all steps described so far are carried out as prefabrication steps, and the building element 1 can be transported to a construction site with or without an outer cavity structure, as shown in fig. 3.

As can be seen in fig. 4, an outer coating 15 is applied to the layer 3. Due to the cavities 12 created according to the described method, the outer coating 15 will have a large bonding surface and will bond"mechanically"in by portion 15'filling up the cavities 12 of the layer 3.

Fig. 5 shows an alternative with a reinforcement net 13 cast into the layer 3 and ex- tending essentially in the plane of the layer 3. Nails 14, bolts, or similar fastening elements are used to secure the net 13 and the layer 3 on the structure 2, (fig. 1).

Fig. 6 shows a transverse cross-section of the building element 1 shown in fig. 1. It can be seen that the structural part 2 comprises, as described in PCT/SE04/00148 incorporated herein by reference, a planar part 21 in the form of an inner wall part 21 and at least one column 22 integrated with the wall part 21. An isolating layer 23, comprising cellular plastic, mineral wool, or similar, is provided outside the inner wall part 21, and the layer 3 according to the invention is provided outside the iso- lating layer 22 and outside the columns 22.

Preferably, the wall part 21 and the columns 22 comprises a high performance con- crete as described in said PCT/SE04/00148 incorporated herein by reference.

The building element 1 comprises at least one beam 24 in the form of a thin section steel beam 24 which connects the layer 3 to the inner wall part 21. The layer 3 can be secured to the beams 24 via the above described reinforcement net 13 and fasten- ing elements 14. The layer 3 can also be secured to the structural part 2 by additional fastening elements 16,17 connecting the layer 3 to the columns 22 or the wall part 21, respectively.

The thin section steel beams 24 can be provided in any suitable number, and in any suitable position or orientation. Fig. 7 shows an exemplary arrangement of such beams 24 as positioned in a building element 1, but other arrangements of the beams are of course also possible. Alternatively, no steel beams are provided in the build- ing element. As a further alternative at least one beam, but no columns are provided.

The thin inner wall part 21, columns 22 and beams 24 provide a stiff building ele- ment at which deformations are minimized so that cracking and other damages can occur in the layer 3. Also, openings at joints between building elements 1 in the constructed building can be minimized. Additionally, the solution with a thin wall part 21 and columns 22 provides for a light building element 1, which is easy to transport, and presents less deformation due to the hardening process of the concrete in comparison to traditional concrete elements, which reduces the risk of cracking, and provides for better joints between building elements. Also, the reduced risk of cracking, and the provision of better joints between building elements is accom- plished due to the deformation caused by changes in temperature being drastically reduced in comparison to traditional building elements.

The building element described above is a wall element, but the invention is equally applicable to other building elements, on which a strong bond of an outer layer is desired, for example a floor structure.