EP1966449A1

EP1966449A1 - Concrete floor device

Info

Publication number: EP1966449A1
Application number: EP06824514A
Authority: EP
Inventors: Jörgen FALK; Thomas Johansson; Markus Peterson; Peter Svenmar
Original assignee: Skanska Sverige AB
Current assignee: Skanska Sverige AB
Priority date: 2005-12-06
Filing date: 2006-12-05
Publication date: 2008-09-10
Also published as: WO2007067130A1; SE529224C2; SE0502666L; JP2009518564A; NO20082528L; EP1966449A4; US20090301009A1

Abstract

The present invention relates to a concrete floor assembly comprising a subfloor element with a floor side and a ground/ceiling side, and a top casting layer. The subfloor element comprises a concrete portion on the floor side, which concrete portion has a bottom casting surface with a surface roughness. The top casting layer comprises a casting composition, which comprises a self-compacting concrete and a shrinkage reducing admixture. The top casting layer is cast on the floor side of the subfloor element so that at least a portion of the bottom casting surface is covered by the top casting layer, the bottom casting surface having such surface roughness that the top casting layer adheres to the bottom casting surface. The invention further relates to a method of providing such a concrete floor assembly as well as a floor assembly.

Description

CONCRETE FLOOR DEVICE

Technical Field of the Invention

The present invention relates to a concrete floor device or assembly and to a method of providing such a concrete floor element as well as a method of providing a floor assembly.

Technical Background of the Invention

A floor structure is a substantially horizontal, structural element which separates different storeys of a building, or the ground floor of the house and the ground. In addition to a structural element, the so- called structural floor, a floor structure also includes floor and ceiling. The structural floor is part of the structural framework of a building and its main function is to carry the load of the storey, such as the floor, the ceiling, interior walls and fittings, and to transfer the weight of this load to structural walls and columns.

A floor structure must satisfy a number of

requirements. It must have the capacity to carry the load of the storey and have sufficient stiffness, so that the deflections (the resilience) are minimised and do not damage, for instance, the interior walls and fittings. A floor structure should furthermore satisfy specific requirements regarding fire resistance and acoustic and thermal insulation. Sometimes it should also have the capacity to include conduits or electrical installations.

A prior-art floor structure comprises prefabricated structural concrete elements with longitudinal holes, so- called hollow core slabs. Several hollow core slabs can be joined to form a hollow core floor. These hollow core slabs have increased stiffness and strength by being provided with pretensioned reinforcement which gives the hollow core slabs a curved shape. Due to this curvature, it is usually not possible to lay a floor directly on top of them.

There are several systems on the market for

providing a flat floor on top of a curved hollow core floor. According to one system, a plurality of pins are first attached at equal distance over the hollow core floor. Then carefully levelled joists are attached to the pins, so that the joists form a horizontal base for a parquet floor or a subfloor in the form of, for instance, chipboard. The mounting height of the joists can be selected so as to leave enough room for electric or water conduits under the floor. One disadvantage of this system is that it requires much work and is time-consuming. In addition, it is difficult to achieve high precision, which may result in a resilient or inclined floor.

Another system for providing a flat base for floor laying is application of so-called screed to the hollow core floor. One disadvantage of the use of screed is that it is expensive. Thus, it is not desirable to apply screed in layers that are thick enough for the embedment of electric or water conduits. In addition, it is

difficult to predict how well the screed will dry out and how long it will take for the screed to dry when applied in such thick layers. If the base is not dry enough when the floor is laid (for instance a parquet or a plastic carpet) , the floor can be damaged by the moisture or mildew problems can arise.

Experimental studies have also been carried out using ordinary concrete instead of screed. They have shown that if ordinary concrete is cast on a

prefabricated hollow core slab in layers thinner than 100 mm, problems arise in the adhering of the concrete to the base both centrally and along the edges due to loss of bond and edge lifting, respectively. This is above all due to the fact that thin layers have a high moisture gradient, which causes the surface layer to bend. In most cases, it is not possible to use concrete layers with sufficient thickness to avoid these problems, since this requires such large volumes that also the use of ordinary concrete would be too expensive. It is also a problem that thick concrete layers need a long time to dry out, which delays the construction process, thus also making it more expensive.

Summary of the Invention

Therefore object of the present invention is to provide a concrete floor assembly, which makes it

possible to lay a flat floor in a cost-effective manner and at a desired vertical level, and to provide a method of making such a concrete floor assembly and a method of making a floor assembly.

According to the invention, these objects are achieved by a concrete floor assembly and methods

according to claims 1, 20 and 34.

The concrete floor assembly according to the

invention comprises a subfloor element with a floor side and a ground/ceiling side, and a top casting layer. The subfloor element comprises a concrete portion on the floor side and the concrete portion has a bottom casting surface with a surface roughness. The top casting layer comprises a casting composition, which comprises a self- compacting concrete and a shrinkage reducing admixture. The top casting layer is cast on the floor side of the subfloor element so that at least a portion of the bottom casting surface is covered by the top casting layer, the bottom casting surface having such surface roughness that the top casting layer adheres to the bottom casting surface .

Owing to a sufficient surface roughness of the bottom casting surface of the subfloor element, good adhesion is obtained. Thereby, the invention allows the use of a casting composition based on a self-compacting concrete provided it contains a shrinkage reducing admixture. Such a casting composition is much more economical than prior-art screed. Due to the good

adhesion to the subfloor element and the shrinkage reducing admixture, it is possible, according to the invention, to cast a layer that is sufficiently thick to satisfy the acoustic insulation requirements and the requirements regarding embedment of conduits, but not so thick that unnecessary amounts of material are consumed. Furthermore, the casting composition, which according to the invention is based on a self-compacting concrete, advantageously requires little labour input since it does not, as ordinary concrete, have to be vibrated, but spreads itself substantially horizontally. According to the invention, a flat upper surface is thus provided, also without the use of screed. Owing to a cost-effective casting composition and a rational application method, the concrete floor assembly according to the invention is thus much more cost-effective than conventional

alternatives .

The concrete floor assembly according to the

invention is intended to be part of a floor structure for construction works, for instance buildings, such as dwelling houses, office or industrial premises, or structures such as sports centres, underground cavities or silo buildings.

The concrete floor assembly according to the

invention can be part of or constitute the entire

structural floor of the floor structure. It can also be a non-structural part of the floor structure.

According to the invention, a concrete floor

assembly is thus a portion of a floor structure, which portion forms a base which is suitable for floor laying or which itself can function as a floor.

The term subfloor element here means an element cast on the spot or a prefabricated element. The subfloor element can extend over the entire floor of a room or over the entire floor of a storey, but it can also be a slab with limited extension. The subfloor element has a floor side and a ground/ceiling side. The floor side is the side which in normal use is part of the base of a floor. The ground/ceiling side is the side which in normal use is facing the ceiling of a lower storey or the ground if the subfloor element belongs to the ground floor.

According to an embodiment of the invention, the subfloor element is a structural prefabricated hollow core slab. Several such hollow core slabs can be joined together so as to extend over an entire storey.

According to an embodiment of the invention, the subfloor element is a hollow core slab with pretensioned reinforcement. Due to the prestress, such hollow core slabs are usually curved, that is they have an arched longitudinal section. By casting the top casting layer according to the invention on the arched floor side of the hollow core slab, the concrete floor assembly can be given a flat floor side that is suitable as a base for floor laying.

If the subfloor element has a curvature, or in other words an arched cross-section, and the top casting layer is such that the concrete floor assembly has a flat floor side, the thickness of the top casting layer will vary over the cross-section. Its smallest thickness is where the curve of the arch is at its maximum. Usually, such a subfloor element has its smallest thickness approximately at the centre and its greatest thickness at two opposite edge sides.

The subfloor element according to the invention can be made of concrete or reinforced concrete, but it can also comprise other materials. According to the

invention, at least a portion of the floor side of the subfloor element has a concrete surface of which at least a portion constitutes a bottom casting surface. The bottom casting surface is intended to interact with the casting composition. According to the invention, the bottom casting surface has such surface roughness that the top casting layer adheres to the bottom casting surface. This means that the casting composition forms a top casting layer with desired thickness over at least part of the bottom casting surface of the subfloor element, the top casting layer adhering to the bottom casting surface so that the requirements of the building sector are met, for

instance, those referring to loss of bond and edge lifting and those referring to the strength of the adhesion area between the subfloor element and the top casting layer.

According to an embodiment of the invention, the bottom casting surface has a surface roughness which, when measured according to the Swedish Standard for determination of surface roughness SIS812005, is

preferably S > 1.5. Reference is made to this Standard, for instance, in the Swedish concrete code (BBK04).

According to the invention, this surface roughness can be obtained by any suitable method. One example is to seed the bottom casting surface with granular material, such as crushed stone material. Examples of particle sizes are 2-4 mm or 4-8 mm.

Another example of providing a bottom casting surface with a rough surface structure is, according to the invention, to treat the bottom casting surface by a surface roughing method. By this is meant any optional method that removes, transfers and/or compacts material unevenly over the surface so as to make the surface rugged or, in other words, rough.

According to an embodiment of the invention, the bottom casting surface is brushed or combed. This can suitably be performed in connection with the

manufacturing of the subfloor elements, by a brush or comb being attached to a casting machine. In such a method, it may sometimes be desirable to remove possible loose material originating from the brushing/combing from the bottom casting surface before the casting of the top casting layer. By this means, the adhesion properties are further improved.

According to an embodiment of the invention, the bottom casting surface is embossed. This can, for

instance, be achieved by a rotating cylinder or several rotating wheels which are attached to a casting machine in connection with the manufacturing of the subfloor elements .

According to an embodiment of the invention, at least a portion of the floor side of the subfloor element has a concrete surface with a bottom casting surface which has exposed aggregate.

In this application, "exposed aggregate" means that surface skin, surface silt and fine aggregate particles have been removed from the concrete surface so that coarse aggregate protrudes.

Such exposed aggregate surfaces are known to provide aesthetical effects. However, according to the embodiment of the invention, the exposed aggregate is used to increase the surface roughness of the surface of the subfloor element. Increased roughness of the surface of the subfloor element results in improved adhesion between the subfloor element and the top casting layer, which reduces the risk of loss of bond and edge lifting.

Another positive effect is that the upper surface is also strenghened since loose mortar is removed. Thus, the exposure of aggregate according to this embodiment provides two properties which are beneficial for the adhesion of the top casting layer to the subfloor

element, that is increased surface roughness and strong concrete in the joint.

According to the invention, substantially the entire floor side of the subfloor element can have exposed aggregate or only a portion thereof. The top casting layer can cover substantially the entire floor side of the subfloor element or a portion thereof, in which case at least a portion of the exposed aggregate surface, i.e. the bottom casting surface, is covered. In some

embodiments, the adhesion of the top casting layer can be sufficient even if the surface of the subfloor element covered by the top casting layer does not in its entirety have an exposed aggregate finish.

According to an embodiment of the invention, the concrete portion of the subfloor element has a surface with a deep exposure of aggregate, i.e. aggregate in the concrete surface is exposed to such depth that the coarse aggregate grains appear clearly. The depth can be greater than 1/3 of the maximum size of the surface aggregate.

According to an embodiment of the invention, a membrane curing compound is applied to the top casting layer so that the after-curing proceeds under the effect of the membrane curing compound. Membrane curing

compounds or (liquid membrane curing compounds) are compounds that delay the drying out of the concrete during curing. The membrane curing compound forms a thin film over the newly cast surface so that usually it does not have to be moistened. Membrane curing compounds with different chemical compositions, such as wax dispersions, acrylate polymers or latex emulsions, are commercially available. Preferred membrane curing compounds comply with the specifications laid down in ASTM C 309, Liquid Membrane-Forming Compounds for Curing Concrete, for instance the specifications in this document concerning maximum loss of moisture. Thanks to the application of the membrane curing compound to the top casting layer, the top casting layer does not dry out too quickly during the first hours after casting, whereby plastic shrinkage cracking in the surface layer is advantageously avoided.

According to an embodiment of the invention, the subfloor element is pre-moistened before the casting of the casting composition on the subfloor element. This advantageously prevents water from the casting

composition from being absorbed into the subfloor element, which would reduce the strength of the layer of the top casting layer that is closest to the subfloor element .

According to an embodiment of the invention, a plurality of subfloor elements are joined together to form a subfloor unit with or without the aid of

interconnecting means. The subfloor elements can, for instance, be joined by grouting but the subfloor elements can also be loosely placed next to each other or be attached to each other or to a base. Subsequently, the casting composition is cast so that a top casting layer forms which extends over several subfloor elements, whereby a floor assembly according to the invention is provided. According to an embodiment, substantially the entire floor side of the subfloor elements has exposed aggregate and the top casting layer extends over

substantially the entire subfloor unit. Owing to the self-levelling properties of the casting composition, it is thus possible to provide a flat base for floor laying by one single casting of a layer. In addition, electric and/or water conduits can be embedded in the top casting layer.

In some construction projects, use is made of prefabricated wall elements in which windows are already mounted. Furthermore, the construction process is

sometimes such that also the ceiling is applied before a method of providing a flat base is initiated. In such constructions, it is possible, according to an embodiment of the invention, to seal door openings and bushings, if any, in the wall elements, for instance, by means of plastics so that a relatively airtight room is obtained. Only thereafter, the top casting layer is applied.

Hereby, the moisture evaporated from the top casting layer can be maintained in the sealed room, which delays the drying out of the top casting layer. Owing to the slower drying-out process, plastic shrinkage cracking in the top casting layer can be avoided. It is often

sufficient to keep the room airtight for about 24 hours.

The inventive casting composition comprises a self- compacting concrete and a shrinkage reducer. SeIf- compacting concrete is compacted homogenously under its own weight and fills formwork without using the vibratory unit required for ordinary concrete. This makes it possible to obtain a rational casting process without vibrating, which is less time-consuming and requires less manpower. The concrete will also be self-levelling, which means that it does not have to be coated with screed before the floor is laid, which is a great saving in costs. Self-compacting concrete and various types thereof are described in The European Guidelines for SeIf- Compacting Concrete - Specification, Production and Use, May 2005, issued by the Self-Compacting Concrete European Project Group, with representatives from BIBM, CEMBUREAU, ERMCO, EFCA and EFNARC.

Concrete mainly consists of mineral particles

(aggregate) of different grain sizes which are bonded by cement paste consisting of cement and water. A common mixing ratio based on parts by volume is one part water, one part cement, two parts sand and three parts coarse aggregate. Admixtures modifying the properties of the concrete in various ways can also be included. In

general, in the case of self-compacting concrete, the ratio of fine aggregate (e.g. sand and gravel) is

greater, and the ratio of coarse aggregate (e.g. stone material) is smaller, than in traditional concrete. It is also common to use fine-particle material in self- compacting concrete, such as lime or glass filler. A common admixture in self-compacting concrete is

commercially available water reducing agents, so-called superplasticisers, which can make the self-compacting concrete plastic and workable.

According to an embodiment of the invention, the self-compacting concrete has a slump-flow of > 520 mm, preferably > 650 mm. The measurement of slump-flow is a test to assess the flowability of self-compacting

concrete in the absence of obstructions. It is based on the slump test described in EN 12350-2. The result is an indication of the filling ability of the self-compacting concrete. Slump-flow is thus the mean diameter of the spread of fresh concrete using a conventional slump cone. The measuring of slump-flow is defined in Annex B.I of The European Guidelines for Self-Compacting Concrete - Specification, Production and Use and is summarised below.

- Prepare the cone and baseplate as described in EN

12350-2.

- Place the cone coincident with the 200 mm circle on the baseplate and hold in position by standing on the foot pieces (or use the weighted collar) , ensuring that no concrete can leak from under the cone.

- Fill the cone without any agitation or rodding, and strike off surplus from top of the cone. Allow the filled cone to stand for not more than 30 s. During this time remove any spilled concrete from the baseplate. Ensure the baseplate is damp all over but without any

surplus water.

- Lift the cone vertically in one movement without interfering with the flow of concrete. Without disturbing the baseplate or the concrete, measure the largest diameter of the concrete flow and measure the diameter of the flow spread at right angles to the largest diameter.

- The slump-flow is the mean of the two measured

diameters .

Self-compacting concrete can have a very high tendency to shrink due to a small amount of coarse aggregate and a large amount of fine material and/or lime filler, which can be used to make the concrete self- compacting. To reduce shrinkage and thus the risk of loss of bond and edge lifting, shrinkage reducing admixtures are used. Such shrinkage reducing admixtures comprise, for instance, various types of alcohols. They are supposed to primarily reduce the surface tension of the pore water, whereby the shrinkage tensions in the concrete are reduced. Shrinkage reducing admixtures are commercially available and a person skilled in the art can choose a suitable agent based on the composition of the concrete and its shrinkage tendency, etc. A suitable shrinkage reducing admixture can comprise a polyalcohol, preferably 2, 2-dimethylpropane-l, 3-diol.

Lime fillers contribute to the self-compacting properties of the concrete. Lime fillers have also been found to accelerate the drying-out process due to their effect on the structure of the pore system. The

accelerated drying-out allows concrete comprising lime filler to comply with the relative humidity (RH)

requirements on concrete before carpeting. Lime fillers are commercially available in a number of qualities. For the inventive use, it has been found that a lime filler, for instance a crystalline limestone filler, with a mean particle size of < 0.05 mm, preferably 0.025-0.05 mm, provides the desired combination of contributions to the self-compacting properties of the concrete and to the accelerated drying-out.

The high amount of fine material in self-compacting concrete increases the water requirement, which further increases the shrinkage tendency. Self-compacting

concrete for use according to the invention has a water- cement ratio of > 0.5, preferably > 0.55.

By exploiting the positive effect of lime fillers on the drying-out process, quick drying-out can be achieved according to the invention without reducing the water- cement ratio, which could have increased the risk of loss of bond and edge lifting. Owing to the quick drying-out of the casting composition, the floor laying is not delayed and a quick and efficient construction process can be maintained. Thus, an embodiment of a concrete floor assembly according to the invention comprises a subfloor element with a floor side and a ground/ceiling side, and a top casting layer. The subfloor element comprises a concrete portion on the floor side and the concrete portion has an exposed aggregate surface, and the top casting layer comprises a casting composition which comprises a self- compacting concrete and a shrinkage reducing admixture. The top casting layer is cast on the floor side of the subfloor element so that at least a portion of the exposed aggregate surface is covered by the top casting layer .

Due to the exposed aggregate surface of the subfloor element, which allows good adhesion, it has been found possible, according to this embodiment of the invention, to use a casting composition based on a self-compacting concrete if it contains an admixture of a shrinkage reducer. Such a casting composition is considerably more economical than prior-art screed. According to the invention, the good adhesion to the subfloor element and the shrinkage reducing admixture make it possible to cast a layer that is sufficiently thick to comply with the requirements on acoustic insulation and the requirements on the embedment of conduits, but not so thick that an unnecessary amount of material is consumed.

Advantageously, the casting composition, which according to the invention is based on a self-compacting concrete, requires little labour input since it does not, as ordinary concrete, have to be vibrated but spreads itself substantially horizontally. Thus, according to the invention, a flat upper surface is provided, also without the use of screed. Thanks to an economical casting composition and a rational application method, the concrete floor assembly according to the invention is thus considerably more cost-effective than the

conventional alternatives. Furthermore, an embodiment of a method of making a concrete floor assembly according to the invention comprises the steps of providing a subfloor element having a floor side and a ground/ceiling side, the subfloor element comprising a concrete portion on the floor side, which concrete portion has an exposed

aggregate surface, providing a casting composition which comprises a self-compacting concrete and a shrinkage reducing admixture, casting the casting composition on the floor side of the subfloor element so that a top casting layer forms which covers at least a portion of the exposed aggregate surface.

Finally, an embodiment of a method of making a floor assembly for a floor structure in construction works comprises according to the invention the steps of providing subfloor elements which have a floor side and a ground/ceiling side and comprise a concrete portion on the floor side, which concrete portion has an exposed aggregate surface, providing a casting composition comprising a self-compacting concrete and a shrinkage reducing admixture, joining the subfloor elements to form a subfloor unit, casting the casting composition on the subfloor unit by covering at least portions of the exposed aggregate surfaces, thereby forming a top casting layer with a flat upper side which has a desired

inclination, preferably substantially horizontal.

Brief Description of the Drawings

The invention will now be described in more detail by way of examples and with reference to the accompanying drawings .

Fig. 1 is a schematic longitudinal section view of a concrete floor assembly according to the invention.

Fig. 2 is a schematic cross-section view of a concrete floor assembly according to the invention.

Fig. 3 is a schematic top plan view of a subfloor element according to the invention. Fig. 4 is a schematic top plan view of a floor assembly according to the invention, in which part of the Figure shows joined subfloor elements under the top casting layer.

Description of Preferred Embodiments

With reference to Figs 1 and 2, a concrete floor assembly according to the invention is schematically shown. The concrete floor assembly comprises a subfloor element in the form of a prefabricated hollow core slab 1. The hollow core slab 1 comprises longitudinal

reinforcement wires 2 that prestress the hollow core slab 1, whereby it obtains a curved longitudinal section. The prestress and the curved shape advantageously provide a good ability to absorb tensions and thus also a good load-carrying ability. The hollow core slab 1 is

furthermore formed with weight-reducing recesses in the form of longitudinal holes 3. The hollow core slab 1 according to the example is 10 m x 1.20 m x 0.265 m.

The hollow core slab 1 has a floor side 9 which in use is an upper side and a ground/ceiling side 10 which in use is an underside. The hollow core slab 1 has an arched shape, the curvature being directed upwards in use. If the hollow core slab 1 is placed on a flat base, the difference in height of the upper side 9 of the hollow core slab 1 above the base measured at half the length of the hollow core slab 1 and at its short ends is 15 mm.

The hollow core slab 1 is made of a prior-art concrete mix also comprising coarse aggregate grains 5 with a diameter of about 12 mm. The hollow core slab 1 has a concrete surface that covers the entire floor side 9, and in the shown example the entire concrete surface of the floor side 9 forms a bottom casting surface which has been provided with a desired roughness by aggregate exposure, cf. Fig. 3. The concrete surface has deep aggregate exposure, down to a depth corresponding to about half the diameter of the coarse aggregate. In this example, the depth of aggregate exposure is 6 mm.

A top casting layer 6 is cast on the arched floor side 9 and covers the floor side 9 over its entire exposed aggregate surface 4. The top casting layer 6 has a substantially flat upper side 7 facing away from the exposed aggregate surface. When using the hollow core slab 1 according to the shown example in a floor

structure, the upper side 7 of the top casting layer 6 is horizontal. Furthermore, conduits 8 for electric cables are embedded in the top casting layer 6. In the shown example, the top casting layer 6 has a thickness of 55 mm at the smallest cross-section where the curve formed by the hollow core slab 1 has its maximum and a thickness of 70 mm at the short ends of the hollow core slab 1.

According to the invention, the top casting layer comprises a casting composition based on a self- compacting concrete. The casting composition can, for instance, comprise the following materials:

A preferred casting composition has the following composition:

The invention also relates to a floor structure comprising at least one concrete floor assembly according to the invention. With reference to Fig. 4, such a floor structure can, according to the invention, comprise a floor assembly according to the invention, which can be obtained in the following manner.

First, a subfloor element is manufactured in the form of a hollow core slab 1 with an exposed aggregate surface 4. The hollow core slab 1 is cast in prior-art concrete and comprises a prestressed reinforcement 2. To obtain the exposed aggregate surface 4 of the hollow core slab 1, a retarding admixture (in liquid form) is applied before curing of the hollow core slab. This means that curing is delayed down to a depth of 6 mm, which has been selected in consideration of the fact that the aggregate grains have a maximum diameter of 12 mm. The hollow core slabs are then high-pressure washed, which means that the surface layer is "cleaned" of cement paste, i.e. the aggregate is exposed and a rough and clean surface 4 appears. In this example, the entire floor side 9 of the hollow core slab 1 has exposed aggregate.

A plurality of such hollow core slabs are

subsequently transported to the desired building site.

The hollow core slabs 1 are joined together on site to form a subfloor unit, which in this example extends over a storey and in which all the floor sides 1 of the hollow core slabs are upwardly directed. The hollow core slabs 1 are joined by grouting. In other embodiments of the invention, it may be sufficient to place the hollow core slabs adjacent to each other or to attach them to each other or to a base in some other convenient manner. The curved, arched longitudinal section of the joined hollow core slabs 1 gives the upper side of the subfloor unit a waveform. Furthermore, owing to the exposed aggregate floor side 9 of the hollow core slabs, the entire floor side of the subfloor unit has exposed aggregate .

Subsequently, conduits 8 are placed over the

subfloor unit in a desired pattern.

Furthermore, a casting composition according to the above formula is mixed and supplied to the building site.

The ready-mixed casting composition is pumped out over the subfloor unit so that the conduits 8 are covered and the desired thickness, for instance with regard to acoustic insulation class, is obtained. Before that, the subfloor unit has been slightly pre-moistened. The casting composition forms a top casting layer 6 which covers the entire upper side of the subfloor unit and the conduits 8. The casting composition is self-levelling, which means that it compacts under its own weight, which makes vibration unnecessary. Moreover, the casting composition spreads so that the top casting layer obtains a substantially horizontal surface.

When spreading, use is made of a laser, to obtain the desired vertical level, and by bull floating, to obtain plane and smooth surfaces. Bull floating means that the concrete surface is treated by a so-called bull float. Usually, a bull float consists of a board or a plastic tube provided with a handle. The surface is easy to work manually so that a plane and smooth surface is obtained. The top casting layer finally has a thickness of 55 mm at its thinnest point and a thickness of 70 mm at the short ends of the hollow core slab 1.

In other embodiments, it is, of course, possible to perform levelling and smoothing by other suitable

methods . Before the top casting layer has cured, a membrane curing compound is injected over the surface of the top casting layer. This is done immediately after the free water has disappeared from the concrete surface, usually within an hour after casting. Owing to this, the after- curing of the top casting layer proceeds under the action of the membrane curing compound, whereby plastic

shrinkage cracking in the surface layer is advantageously avoided.

Due to the lime fillers of the casting composition, it is possible, according to the invention, to obtain quick drying out and the relative humidity (RH)

requirements on concrete before carpeting can be complied with and long waiting times in the construction process are avoided.

According to the invention, a separate concrete floor assembly can also be provided by the above- described method.

Claims

1. A concrete floor assembly comprising a subfloor element with a floor side and a ground/ceiling side, and a top casting layer, wherein

the subfloor element comprises a concrete portion on the floor side, which concrete portion has a bottom casting surface with a surface roughness,

the top casting layer comprises a casting

composition, which comprises a self-compacting concrete and a shrinkage reducing admixture,

the top casting layer is cast on the floor side of the subfloor element so that at least a portion of the bottom casting surface is covered by the top casting layer, and wherein

the bottom casting surface has such surface

roughness that the top casting layer adheres to the bottom casting surface.

2. A concrete floor assembly as claimed in claim 1, wherein the surface roughness of the bottom casting surface, when measured according to the Swedish Standard for determination of surface roughness SIS812005, is S > 1.5.

3. A concrete floor assembly as claimed in claim 1 or 2, wherein the bottom casting surface is seeded with granular material.

4. A concrete floor assembly as claimed in claim 1 or 2, wherein the bottom casting surface is treated by a surface roughening method.

5. A concrete floor assembly as claimed in claim 4, wherein the bottom casting surface is brushed and/or combed.

6. A concrete floor assembly as claimed in claim 4, wherein the bottom casting surface is embossed.

7. A concrete floor assembly as claimed in claim 4, wherein the bottom casting surface has exposed aggregate.

8. A concrete floor assembly as claimed in claim 7, wherein coarse aggregate grains in the exposed aggregate surface of the subfloor element are exposed at a depth of > 1/3 of the maximum size of the surface aggregate.

9. A concrete floor assembly as claimed in claim 7 or 8, wherein coarse aggregate grains in the exposed aggregate surface of the subfloor element are exposed at a depth of < 1/2 of the maximum size of the surface aggregate, preferably about 6 mm.

10. A concrete floor assembly as claimed in any one of claims 1-9, wherein conduits or electrical

installations are embedded in the top casting layer.

11. A concrete floor assembly as claimed in any one of claims 1-10, wherein the bottom casting surface covers substantially the entire floor side of the subfloor element .

12. A concrete floor assembly as claimed in any one of claims 1-11, wherein the top casting layer is cast on substantially the entire floor side of the subfloor element.

13. A concrete floor assembly as claimed in any one of claims 1-12, wherein the maximum thickness of the top casting layer is < 100 mm, preferably < 70 mm.

14. A concrete floor assembly as claimed in any one of claims 1-13, wherein the minimum thickness of the top casting layer is > 40 mm, preferably > 55 mm.

15. A concrete floor assembly as claimed in any one of claims 1-14, wherein the casting composition further comprises a lime filler.

16. A concrete floor assembly as claimed in any one of claims 1-15, wherein the subfloor element is a

prefabricated element.

17. A concrete floor assembly as claimed in claim

16, wherein the subfloor element is a hollow core slab, preferably a reinforced, prestressed, curved hollow core slab.

18. A concrete floor assembly as claimed in any one of claims 1-17, wherein the self-compacting concrete has a slump-flow of > 520 mm, preferably > 650 mm.

19. A floor structure for construction works comprising a concrete floor assembly as claimed in any one of claims 1-18.

20. A method of making a concrete floor assembly comprising the steps of

providing a subfloor element having a floor side and a ground/ceiling side, the subfloor element comprising a concrete portion on the floor side, which concrete portion has a bottom casting surface with a surface roughness,

providing a casting composition which comprises a self-compacting concrete and a shrinkage reducing admixture,

casting the casting composition on the floor side of the subfloor element so that a top casting layer forms which covers at least a portion of the bottom casting surface, the bottom casting surface having such surface roughness that the top casting layer adheres to the bottom casting surface.

21. A method as claimed in claim 20, which further comprises the step of applying a membrane curing compound to the top casting layer.

22. A method as claimed in claim 20 or 21, which further comprises the step of pre-moistening the subfloor element before the step of casting the casting

composition.

23. A method as claimed in any one of claims 20-22, which further comprises the step of bull floating the top casting layer.

24. A method as claimed in any one of claims 20-23, wherein the step of providing a subfloor element

comprises the step of seeding the bottom casting surface with granular material.

25. A method as claimed in any one of claims 20-23, wherein the step of providing a subfloor element

comprises the step of treating the bottom casting surface by a surface roughening method.

26. A method as claimed in claim 25, wherein the step of treating the bottom casting surface by a surface roughening method comprises the step of brushing and/or combing the bottom casting surface.

27. A method as claimed in claim 25, wherein the step of treating the bottom casting surface by a surface roughening method comprises the step of embossing the bottom casting surface.

28. A method as claimed in claim 25, wherein the step of treating the bottom casting surface by a surface roughening method comprises the step of exposing the aggregate of the bottom casting surface.

29. A method as claimed in claim 28, wherein the step of providing a subfloor element comprises the step of casting the subfloor element in concrete, applying a retarding admixture to a surface of the floor side before curing of the subfloor element, and cleaning the surface of mortar after curing of the subfloor element, thus exposing the aggregate of the surface.

30. A method as claimed in any one of claims 20-29, which further comprises the step of arranging conduits or electrical installations on the subfloor element before the step of casting the casting composition, and in which the step of casting the casting composition comprises embedding of conduits or electrical installations in the top casting layer.

31. A method as claimed in any one of claims 20-30, wherein the casting composition is cast with a thickness of < 100 mm, preferably < 70 mm, at its thickest point.

32. A method as claimed in any one of claims 20-31, wherein the casting composition is cast with a thickness of > 40 mm, preferably > 55 mm, at its thinnest point.

33. A method as claimed in any one of claims 20-32, wherein the self-compacting concrete has a slump-flow of > 520 mm, preferably > 650 mm.

34. A method of making a floor assembly for a floor structure in construction works comprising the steps of providing subfloor elements which have a floor side and a ground/ceiling side and comprise a concrete portion on the floor side, which concrete portion has a bottom casting surface with a surface roughness,

providing a casting composition comprising a self- compacting concrete and a shrinkage reducing admixture, joining the subfloor elements to form a subfloor unit,

casting the casting composition on the subfloor unit by covering at least portions of the bottom casting surfaces, the bottom casting surfaces having such surface roughness that the top casting layer adheres to the bottom casting surface, thereby forming a top casting layer with a flat upper side which has a desired

inclination, preferably substantially horizontal.

35. A method as claimed in claim 34, wherein the step of providing a subfloor element comprises the step of exposing the aggregate of the bottom casting surface.

36. A method as claimed in claim 34 or 35, which further comprises the step of arranging conduits or electrical installations on the subfloor unit before the step of casting the casting composition, and in which the step of casting the casting composition comprises

embedding of conduits or electrical installations in the top casting layer.

37. A method as claimed in any one of claims 34-36, wherein the casting composition is cast over

substantially the entire subfloor unit.

38. A method as claimed in any one of claims 34-37, wherein the step of casting the casting composition comprises the step of pumping out the casting composition over the subfloor unit.

39. A method as claimed in any one of claims 34-38, wherein the step of casting the casting composition comprises levelling of the top casting layer.