GB2543894A

GB2543894A - Constructions and methods for casting slabs

Info

Publication number: GB2543894A
Application number: GB1614125.1A
Authority: GB
Inventors: Seaton Alastair
Original assignee: Cordek Ltd
Current assignee: Cordek Ltd
Priority date: 2015-08-18
Filing date: 2016-08-18
Publication date: 2017-05-03
Also published as: GB201614125D0; GB201514638D0

Abstract

A construction comprises a concrete slab 83 and an upper layer 92 having a grooved top face over which the slab is cast. A supporting layer 93 for supporting the upper layer comprises a plurality of passageways and/or cavities. A former 89 for defining the side edges of the slab extends beyond the upper layer and has a lower edge portion on the outside of the upper layer. An inner wall 82 of a cavity wall may be provided on top of an edge of the panel. A membrane 85 may be placed over the slab and a concrete screed may be laid over the membrane. The former may be L-shaped. The upper layer may be formed from a thermally insulating material such as polystyrene. The supporting layer may comprise at least one collapsible support formed from spaced apart walls defining hexagonal cells. The supporting layer may include one or more passageways to provide paths for fluid to the edges of the layer. The supporting layer may also include sand and hardcore layers. Also claimed is a construction having a membrane between upper and supporting layers, and methods of casting a slab.

Description

Constructions and methods for casting slabs Technical Field

This invention relates to the casting of slabs on a ground base. More particularly, the invention relates to a construction on which a slab is to be cast, to a construction including such a slab, and to a method of casting a slab on a ground base. The invention is particularly concerned with a construction and method in which the slab is cast on shuttering comprising a plurality of layers of material, for example of expanded polystyrene (EPS). Slabs of this kind may be used to form the floor of a building.

Background of the Invention

Various issues arise when seeking to design a suitable construction at the base of a building that has a slab cast over the ground. Those issues may include ease of construction in the environment of a building site, accommodating rising of the ground under the slab after construction (ground heave), venting of gas emerging from the ground under the slab, integrating the construction of the slab with the building of the walls, fitting a membrane, for example, a damp proof membrane (DPM) to prevent damp rising into the slab and ensuring good thermal insulation.

In practice, it is challenging to provide a construction which is able to meet some or all of these requirements and yet be cost-effective.

In a conventional arrangement the interface between the bottom face of a slab and the top face of shuttering over which it is cast is planar. It is, however, sometimes advantageous to provide grooves in the top face of the supporting layer in order to form strengthening ribs on the underside of the slab when it is cast.

It is an object of the invention to provide an improved arrangement for use in the construction industry in casting a slab over a ground base, and to provide an improved method of casting a slab over a ground base.

Summary of the Invention

According to a first aspect of the invention there is provided a construction comprising: a concrete slab; an upper layer of pre-formed plastics material having a top face over which the slab is cast, the top face being formed with a plurality of grooves therein; a supporting layer for supporting the upper layer over a top face of the supporting layer, a plurality of passageways and/or cavities being defined within the supporting layer; a former for defining the side edges of the slab which extends beyond the upper layer and has a lower edge portion on the outside of the upper layer.

Such a construction is able to provide a cost- effective and robust construction at the base of a building. The supporting layer is able to accommodate ground heave and/or provide for gas venting while the grooves in the upper layer of plastics material results in the slab being formed with strengthening ribs, enabling a relatively thin slab to exhibit substantial strength. Such strength is of particular advantage if ground heave is to be accommodated since the slab will then have to withstand some upward pressure before collapse of the supporting layer .

An upright wall may be provided on top of an edge of the slab. In an especially advantageous construction one or more upright walls are provided on top of the slab.

Thus the slab is held against upward movement. The upright wall(s) may be the inner wall(s) of one or more cavity walls . A concrete screed may be laid over the slab. An edge of the screed may abut the upright wall. A membrane may be disposed over the slab and the screed may be disposed over the membrane.

The former may be a generally '1/ shaped member which may be elongate. The '1/ shaped member may have an upright face that defines the edge to the slab. The former may be supported on a block of thermally insulating material. In the case of a generally '1/ shaped member, a horizontal limb of the member may rest on the block of thermally insulating material. The thermal conductivity of the insulating material is preferably less than 0.1 Wm_1K_1 and more preferably less than 0.06 Wm^K-1. The former may further comprise corner pieces and/or interlocking connectors for connecting an '1/ shaped member along one side of the slab to another similar member along an adjacent side, so that the former is continuous around the slab .

The top of the slab is preferably level with the top of the former. The former may thus provide a convenient way of defining the depth of the slab.

The advantageous features defined above enable the provision of a construction of a lightweight, thermally insulated, in-situ concrete floor slab cast on a layer able to deal with the effects of ground heave, ground gas or a combination of both. The construction can be simple and cost-effective for a builder to employ. Whilst it is preferred to include all the features recited above, it is within the scope of the invention to omit or vary certain features and the scope of the invention is defined in the claims .

The upper layer is preferably formed of a thermally insulating material. The thermal conductivity of the insulating material is preferably less than 0.1 Wm_1K·1 and more preferably less than 0.06 Wm“1K‘1. The upper layer acts as shuttering during casting of the slab but then remains in place and advantageously provides good thermal insulation to reduce heat transfer through the slab from or into a building above the slab.

The upper layer may be formed of one or more preformed panels. The panels may be of rectangular shape. By using preformed panels that may be placed side-by-side and/or end-to-end, a relatively large slab may be cast on a plurality of smaller panels; smaller panels are easier to handle and transport. In an embodiment of the invention described below panels have a length of 2.4m and a width of about 1.2m. When casting a slab, a plurality of panels may be placed adjacent to one another and sides and/or ends of such an array cut to provide a support surface matched to the size of the slab. The panels may be cut to the required size before they are supplied to the building site .

The one or more preformed panels may be made of a variety of materials but are preferably made of an expanded plastics material. The expanded plastics material is preferably an expanded polystyrene material. Such a material has good thermal insulation properties.

The grooves in the top face of the upper layer are preferably substantially parallel to one another and evenly spaced across the top face. In that case the ribs in the slab will be substantially parallel and evenly spaced, which is usually advantageous. The grooves preferably extend from one side/end of the panel to the opposite side/end; in that case panels may be laid next to one another with grooves continuing across adjacent panels. In embodiments of the invention described below, the grooves in the panels all extend in parallel directions but in some applications it may be preferred to further provide grooves extending in one or more other directions; in particular it may be preferred to provide a second set of grooves extending substantially perpendicular to the first set so as to define a square or rectangular grid of grooves.

The supporting layer may comprise one or more preformed supports. The supporting layer may comprise one or more preformed panels. In an embodiment of the invention described below the panels have a length of 2.4m and a width of about 1.2m. When casting a slab, a plurality of panels may be placed adjacent to one another and sides and/or ends of such an array cut to provide a support surface matched to the size of the slab. The panels may be cut to the required size before they are supplied to the building site.

The supporting layer may comprise one or more supports of expanded plastics material. The expanded plastics material may be expanded polystyrene. A plurality of passageways and/or cavities may be defined within the supporting layer. Such passageways and or cavities may have various functions.

According to a first function the supporting layer may comprise at least one collapsible support. The collapsible support may be provided by a single member or a plurality of members which may be abutting or spaced apart from one another. A support structure of the kind described may have a first supporting condition, in which it is manufactured, in which it can accommodate a given loading with very little compression of the material. The maximum loading at which the support structure is assured of remaining in the first supporting condition is referred to in commercial products as the "Safe Load" and during casting of a slab or beam this Safe Load should not of course be exceeded. The support structure may also have a second failed condition, in which the walls have failed. The minimum loading at which this is assured of having occurred is referred to in commercial products as the "Fail Load". As the loading on the product increases from zero towards the Safe Load, so its resistance to compression is maintained, but, at some stage after the loading exceeds the Safe Load, the "Fail Load" is reached and the resistance of the product to compression stops increasing and even reduces until the product is much reduced in depth.

Products of the kind referred to are used in a variety of applications and those different applications require different specifications of product. For a given application, there are two main kinds of variable to be specified: one is the amount of upward movement that the product is required to accommodate; the other is the value of the Safe Load that the product is required to accommodate, with the Fail Load preferably being only slightly higher than the Safe Load.

It is important that the slab is able to resist the upward load imparted by the supporting layer via the upper layer when there is heaving movement of the ground and, in particular, that the supporting layer reaches its Fail Load before the slab fails or suffers in some other way from the upward load imparted on it by the supporting layer. When a large thick slab is being cast, the strength of the slab is relatively great and the slab will not be affected adversely by the upward force imparted by the supporting layer even when it approaches its Fail Load. On the other hand in some other applications, of which a slab for a domestic dwelling would be a typical example, the slab may not be sufficiently strong to resist the upward force exerted by the supporting layer as it approaches its Fail Load and that may restrict the applications in which the arrangement can be employed.

By providing spaced apart collapsible supports it is possible to reduce the Fail Load at which the supporting layer collapses from that which would apply if the collapsible supports were continuous rather than spaced apart. In that way, the construction of the invention may be employed when casting relatively weak slabs, such as may be used for example in a domestic dwelling.

The collapsible supports may occupy less than 70% of the area under the upper layer. In that case, in the event of heave of the ground, for a given upward force on the slab, there is a significant increase in the vertical compressive force per unit area suffered by the supports from that which would apply if the supports occupied the whole of the area under the upper layer.

The spaced apart collapsible supports may take a wide variety of shapes but are preferably provided as a series of spaced apart strips. The strips are preferably substantially parallel to one another. The strips are preferably evenly spaced apart; that promotes the application of an evenly distributed load on the slab in the event of heave. Whilst it is possible for the strips to be formed as separate units it is generally preferred that a plurality of strips are defined in a single unit (panel). For example, the strips may be formed by cutting away material between adjacent strips or by moulding a panel with the strips defined in the panel.

Said at least one collapsible support may take any of a wide variety of forms but in a preferred embodiment of the invention comprises a plurality of spaced apart walls defining cells between the walls. The spaced apart walls may be upright walls. The walls may define cells of substantially hexagonal shape or of substantially rectangular shape, including square shape. Supports of this nature are disclosed in GB 2206637, GB 2241976, GB 2390390, GB 2417283, GB 2486722, GB 2486723, EP 2655749 and EP2655762. Such products are currently manufactured and sold by the Applicants under the trade mark CELLCORE.

In the case of a hexagonal structure, each substantially hexagonal cell may have the shape of a regular hexagon but in a preferred embodiment of the invention each cell has the shape of an irregular hexagon. Preferably the cell has some symmetry: more particularly, it is preferred that at least one pair of opposite walls of each substantially hexagonal cell are substantially parallel to one another. Said one pair of substantially parallel walls may be spaced apart by a distance less than the spacing of the other opposite walls of the substantially hexagonal cell. The difference in spacing is preferably small, and preferably less than 10%.

Preferably, all three pairs of opposite walls are substantially parallel to one another; it should be understood, however, that even in this case the hexagonal shape need not be that of a regular hexagon. In a regular hexagon, each interior angle is 120 degrees. In embodiments of the present invention, the interior angles of the hexagonal cells are preferably not all the same but preferably lie in the range of 110 to 130 degrees. In GB2486723 and EP2655762, shuttering having a continuous cellular collapsible support structure employing hexagonal cells is described and the various forms of cellular support structure described in that document may be employed in the present invention in said at least one collapsible support.

According to a second function, the supporting layer may have a plurality of openings over a bottom face and may comprise one or more passageways providing paths of fluid communication from the plurality of openings to one or more edges of the supporting layer. A supporting layer of this kind is able to vent any gases that may rise out of the ground to said one or more edges from where they may be vented. At the same time the membrane on top of the support layer may prevent gases passing upwards into the slab .

The passageways may provide paths of fluid communication to all edges of the supporting layer. The passageways may comprise one or more sets of parallel channels extending between opposite edges of the supporting layer. For example, there may be two sets of channels extending in perpendicular directions.

The supporting layer may comprise a multiplicity of upright pillars and the passageways may be defined by the spaces between the pillars. The pillars may be of square cross-section, of circular cross-section or of other cross-sections. A supporting layer of this nature is disclosed in GB2261002. Such products are currently manufactured and sold by the Applicants under the trade mark VENTFORM.

In some applications it may be desirable to provide both a heave compensation system (the first function referred to above) and a venting system (the second function just referred to). In that case the supporting layer may comprise at least two layers: a first of the at least two layers may comprise at least one collapsible support as defined above; a second of the at least two layers may have a plurality of openings over a bottom face and may comprise one or more passageways providing paths of fluid communication from the plurality of openings to one or more side edges of the supporting layer, as defined above; the second of the at least two layers may be open over a bottom face directly to the ground base or one or more elements may be interposed between the openings and the ground base, those elements allowing gas to pass from the ground base through the openings in the supporting layer; in an embodiment of the invention described below, a perforated sheet is bonded to the bottom face of the second of the at least two layers. The second layer may be placed above the first layer; in that case, the first layer preferably has openings in its top and bottom faces with passageways providing fluid communication through the first layer between the openings; such a construction is readily provided by a network of upright, spaced-apart walls. The first and second layers may be physically separate from one another and may simply be laid on top of one another or they may be fixed to one another or may be integrally formed with one another. Products of this kind are currently manufactured and sold by the Applicants under the trade mark CELLVENT.

The supporting layer may include a concrete blinding layer and/or a sand layer. The supporting layer may include a hardcore layer. The concrete blinding layer and/or the sand layer may be above the hardcore layer. The sand and/or hardcore layers may be provided in place of, or in addition to, the supporting layers described above.

The membrane layer may comprise a damp proof membrane for obstructing the passage of moisture upwardly into the upper layer. Such a damp proof membrane is usually required. The membrane layer may comprise a gas membrane for obstructing the passage of gas upwardly into the upper layer. The gas membrane may be provided to obstruct the passage of a variety of gases and its construction may be selected according to the gas(es) it is seeking to obstruct. For example, the gases may comprise Volatile Organic Compounds (VOCs) and/or radon, carbon dioxide, methane and/or other gases. Two separate membranes may be provided, one laid over the other, but it is preferred that the same membrane acts as both the damp proof membrane and the gas membrane. The membrane may take a wide variety of forms according to the particular application. For example, if it is provided simply as a DPM it may be of a simple construction and may be formed of polythene; for more complex applications, it may be a multi-layer membrane and the different layers may have different physical and/or chemical properties.

The slab may include reinforcing bars.

At least some of the reinforcing bars may be disposed in at least some of the grooves. In that case the ribs on the slab are themselves reinforced. Supports may be provided in the grooves for supporting and locating reinforcing bars away from the walls of the grooves during casting of the slab. That facilitates correct and precise positioning of the bars within the ribs. The supports may be spaced apart along the lengths of the grooves. The supports may be moulded, for example injection moulded, from plastics material. The supports may comprise integral deformable parts for deforming to accommodate a reinforcing bar and for subsequently retaining the bar in position in the support. The deformable parts may be arranged to obstruct movement of the bars both further into and out of the grooves. The supports may extend across substantially the whole widths of the grooves in which they are provided.

In that way the supports are located automatically in the correct positions in the grooves.

According to a second aspect of the invention there is provided a construction comprising an upper layer of preformed plastics material having a top face over which a slab is to be cast, the top face being formed with a plurality of grooves therein, and a supporting layer for supporting the upper layer over a top face of the supporting layer, wherein a membrane layer is provided between the supporting layer and the upper layer.

By interposing the membrane layer between an upper layer over which the slab is cast and a supporting layer for supporting the upper layer, as defined in the second aspect of the invention, any need to accommodate grooves in the surface on which the membrane rests, for example in the absence of a screed being provided over the slab, is eliminated. Preferably, the supporting layer has a top face that is substantially flat. Whilst the top face may be substantially continuous across all of its area, that is not a requirement. The provision of a flat top face makes it especially straightforward to place the membrane on top of the supporting layer. It is also preferred that the upper layer has a bottom face that is substantially flat.

If both faces are substantially flat, forces exerted on the membrane sandwiched between the upper layer and the supporting layer may be more evenly distributed across the membrane, reducing the peak pressure on the membrane. Such compressive forces may, for example, arise during casting of the slab or subsequently in the event that a ground base on which the supporting layer rests is subject to ground heave and rises up towards the slab.

The construction according to the second aspect of the invention may further comprise any of the features of the construction according to the first aspect of the invention. For example, the slab may extend beyond the upper layer and may have a lower edge portion on the outside of the upper layer. The bottom of the lower edge portion may be approximately at the level of the interface of the upper and supporting layers. The boundary of the edge of the slab may be formed by a generally '1/ shaped member. The member may act as shuttering during casting of the slab. It may then remain in place in the final structure .

The '1/ shaped member may be supported on a block of thermally insulating material. The thermal conductivity of the insulating material is preferably less than 0.1 Wm_1K_1 and more preferably less than 0.06 Wm_1K_1.

Whilst it is an essential feature of the second aspect of the invention that the membrane layer is provided between the supporting layer and the upper layer, it is within the broadest scope of the invention for the membrane to be omitted or placed elsewhere. For example, the membrane may be laid on top of the slab; in that case a further layer, for example a concrete screed, may be laid over the membrane, as in the first aspect of the invention.

Similarly the combination of features of the first aspect of the invention may not be present in the second, or some other, aspect of the invention. Thus according to a third aspect of the invention there is provided a construction comprising an upper layer of pre-formed plastics material having a top face over which a slab is to be cast, the top face being formed with a plurality of grooves therein, and a supporting layer for supporting the upper layer over a top face of the supporting layer, wherein the construction is further characterized by one or more of the following features : i. the upper layer is formed of a thermally insulating material having a thermal conductivity of less than 0.1 Wm-1K-1; ii. the supporting layer comprises collapsible supports which occupy less than 70% of the area under the upper layer; iii. the supporting layer has a plurality of openings over a bottom face and comprises one or more passageways providing paths of fluid communication from the plurality of openings to one or more edges of the supporting layer; iv. the supporting layer comprises at least two layers, a first of the at least two layers being according to any of claims 14 to 17 and a second of the at least two layers being according to any of claims 18 to 21; v. supports are provided in the grooves for supporting and locating reinforcing bars within the grooves; vi. both the upper layer and the lower layer comprise expanded plastics material; vii. the slab extends beyond the upper layer and has a lower edge portion on the outside of the upper layer; iii. the boundary of the edge of the slab is formed by a generally '1/ shaped member which acts as shuttering during casting of the slab and remains in place thereafter; and/or ix. the '1/ shaped member is supported on a block of thermally insulating material.

After casting the slab, a structure may be formed comprising any of the constructions defined above and side walls extending upwardly above the slab from around the slab .

The membrane may extend outwardly from the interface of the supporting layer and the upper layer and through the side walls.

The structure may be a domestic dwelling.

The present invention further provides a method of casting a slab over a ground base, the method including the step of building a construction as defined above on a ground base. Where the invention is according to the second or third aspect, the method may further comprise the step of casting a slab on the top of the upper layer of the construction.

In the case where the supporting layer provides a heave compensation system, if the ground base rises after casting of the slab, said at least one collapsible support may then collapse, so that excess pressure on the slab is avoided. Similarly, in the case where the supporting layer provides a venting system, if gases rise out of the ground base beneath the slab, they may pass upwardly into the supporting layer and be vented at one or more edges of the supporting layer.

It should be understood that features described in respect of one aspect of the invention may be employed in another aspect. For example, any of the further characterizing features of the third aspect of the invention may employ features described with reference to the first or second aspects of the invention.

Brief Description of the Drawings

By way of example, embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Fig. 1 is a sectional view of a portion of construction according to a first embodiment of the invention;

Fig. 2 is a bottom plan view of one of the layers of the construction of the first embodiment,

Fig. 3A is an end view of a first form of support for a reinforcing bar, for use in the construction of the first embodiment,

Fig. 3B is an end view of a second form of support for a reinforcing bar, for use in the construction of the first embodiment,

Fig. 4 is a sectional view of a portion of a construction according to a second embodiment of the invention,

Fig. 5 is an isometric view from below of one of the layers of the construction of the second embodiment,

Fig. 6 is a sectional view of a portion of a construction according to a third embodiment of the invention,

Fig. 7 is a sectional view of a portion of a construction according to a fourth embodiment of the invention,

Fig. 8 is a sectional view of a part of a building embodying the invention, and

Fig. 9 is a sectional view similar to Fig.l showing a modified version of the embodiment of Fig. 1.

Detailed Description of Embodiments

Fig. 1 shows a construction generally comprising a slab 1 cast over an upper layer 2 which in turn is supported on a supporting layer 3.

The upper layer 2 has a top face 4 in which parallel grooves 5 are formed. The grooves 5 extend the entire length of the upper layer 2. A damp proof membrane (DPM) 6 is disposed between the supporting layer 3 and the upper layer 2. The DPM comprises a sheet of polythene. Conveniently, the upper layer 2 and the supporting layer 3 are formed of rectangular panels of the same widths and lengths placed immediately adjacent to one another while the DPM is continuous across the whole of the interface between the layers. In one particular example of the invention each panel has a width of 1.2m and a length of 2.4m.

Referring also to Fig. 2, each panel of the supporting layer 3 below the upper layer 2 comprise a plurality of upright support walls 8 which together define a multiplicity of hexagonal cells 9. Each hexagonal cell 9 is bounded by a pair of opposite walls which extend parallel to one another and perpendicular to the sides of the support, and two other pairs of opposite walls. In the example of the invention shown in Fig. 2, the supporting layer 3 is provided across the whole of the area under the upper layer 2 and the size and shape of the hexagonal cells 9 is uniform across the supporting layer. As described below with reference to Fig. 9, the supporting layer 3 may alternatively occupy only a part of the area under the upper layer 2.

Each panel of the supporting layer is formed in its hollow form by a moulding process, from expanded plastics material. In the embodiment shown the panels are moulded from expanded polystyrene. As already mentioned, in GB2486723A shuttering having a continuous cellular collapsible support structure employing hexagonal cells is described and the various forms of cellular support structure described in that document may be employed in the supporting layer 3 of the first embodiment of the invention. Reference is made to GB2486723A for further details of the possible forms of hexagonal cellular structure that may be employed and for further details of how such a structure may be moulded from expanded polystyrene .

In the first embodiment shown in Figs. 1 and 2, the hexagonal cells are open at their bottom faces and closed at their top faces by a sheet of fluted polypropylene 10 that is bonded to the tops of the walls 8. The DPM 6 rests on the top of the sheet 10.

The depth of each panel of the supporting layer 3 is chosen according to the heave that is required to be accommodated in that particular application. Typically the depth is in the range of 85mm to 225mm.

Each panel of the upper layer 2 is of a regular cuboidal shape apart from the grooves 5 formed in the top face 4 of the panel. In this embodiment each panel comprises a sheet of expanded polystyrene. The grooves 5 are formed either by cutting material out of a cuboidal panel (which may itself be formed by moulding or cutting from a larger block) or by moulding the panel with the grooves .

In particular examples of the invention with a panel of the upper layer 2 having a length of 2.4m and a width of 1.2m, other significant dimensions of the panel are as follows :

Width of grooves 6: 75mm to 130mm Depth of grooves 6: 75mm to 225mm

Centre to centre spacing of grooves 6: 600mm to 1200mm Depth of panel: 150mm to 300mm

In use, a ground base 12 over which the floor slab is to be constructed is excavated to the required depth and the surface of the ground base, which may for example be clay, is made level. A layer of concrete blinding (not shown), or other material such as sand, may be placed over the clay substrate, if desired, to provide a flat ground base. Panels forming the supporting layer 3 are then laid edge to edge on the ground base. The joins between adjacent panels may be covered over, for example with a formwork tape. Full size panels may be cut to ensure the correct area for casting the slab is provided.

The DPM 6 is then laid over the top sheet 10 of the supporting layer 3. Panels forming the upper layer 2 are then laid edge to edge on the DPM 6. As with the supporting layer 3, the joins between adjacent panels 2 may be covered over, for example with a formwork tape and full size panels may be cut to ensure the correct area for casting the slab is provided.

Conventional steel reinforcement 13 for the floor is then placed over the panels 2, including in the grooves 5, and is spaced slightly above the tops of the panels by conventional spacers. As shown in Figs. 3A and 3B, supports are also provided for supporting reinforcement at intervals along the grooves 5. Referring first to Fig. 3A, a support 101A is shown which may be injection moulded from a plastics material and defines a generally 'U' shaped recess 102A in its top, in which a reinforcing bar (not shown in Fig. 3A) may be placed. Upstanding thin webs 103A and fingers 104A project into the recess 102A but are deformable to allow the reinforcing bar to be inserted into the recess 102A, the bar then being held in position from below by the webs 103A and from above by the fingers 104A.

In that way a variety of diameters of reinforcing bar can be located within the recess 102A. Fig. 3B shows a support 101B very similar to the support 101A but having two recesses 102B to accommodate two reinforcing bars (not shown). The parts in Fig.3B corresponding to those of Fig. 3A are designated by the same reference numerals but with a suffix B following the numeral.

After the reinforcement has been put in place, concrete is laid over the upper layer 2. As will be understood, during this procedure concrete enters and fills the grooves 5. When the top surface of the concrete has been finished, for example by tamping, the concrete is left to cure. During the laying and initial curing process, the concrete is supported by the upper layer 2 which in turn is supported by the supporting layer 3 but, as the concrete cures to form the floor slab 1, the slab becomes self-supporting. The floor slab 1 is formed with integral, reinforced, ribs 15 on its underside which provide further strength to the slab.

If heaving movement occurs in the ground base 12, a vertical compressive force is exerted on the supporting layer 3. Initially creep occurs in the walls 8 of expanded polystyrene material and, if the heaving movement of the ground is extensive, the supporting layer 3 begins to compress. If the compressive force on the walls 8 exceeds a predetermined limit, the walls will fail, the resistance of the walls to further compression will actually reduce and thus the resistance to further heaving movement will reduce. Thus the upward force on the slab 1 is restricted to the maximum force that the walls 8 can transmit.

Referring now to Figs. 4 and 5, the second embodiment of the invention will be described. The second embodiment differs from the first embodiment principally in respect of the nature of the supporting layer 3A. The other parts of the construction shown in the drawings are the same and are designated by the same reference numerals. The supporting layer 3A of the second embodiment again comprises panels but in this embodiment the lower part of each panel is defined by a multiplicity of feet or pillars 14 of square cross-section and arranged in a square grid as shown in Fig. 5, so as to define two sets of parallel channels 15 that are open along their bottom faces and extend perpendicular to one another. The channels 15 thus provide passageways that extend out to each of the edges of the supporting layer 3A. As may be seen in Fig. 5 the feet 14 may flare outwardly at their tops as they merge into a solid upper portion 16 of the panel; consequently, the cross-sectional shape of the channels 15 is an inverted U shape. The feet of the panel may be formed by cutting away material from a cuboidal panel (which may itself be formed from moulding or cutting from a larger block) or may be formed during moulding of the panel.

In one particular example of the second embodiment, each panel of the supporting layer has a width and length of 1.2m and a total depth of 80mm, the feet are of 40mm depth and 100mm square at their bottom ends. The channels 15 are of 50mm width at their bottom ends. If the panel is used in conjunction with panels of the upper layer 2 that are 2.4m long, then one panel in the upper layer 2 may be disposed above two panels in the lower layer 3. The panels of the supporting layer 3 may again be made of expanded polystyrene .

In use, the construction shown in Fig. 4 is built in the same way as the construction shown in Fig. 1. If gas rises up through the ground base 12, it enters the open bottoms of the channels 15 and passes along the passageways defined by the channels 15 to the edges of the supporting layer 3A and of the slab 1. The DPM 6 acts both as a damp proof membrane and as a gas membrane to prevent gas, which may pass through the upper portions 16 of the panels, from entering the slab. Suitable venting arrangements (not shown) may be provided around the slab to vent gas emerging from the ends of the channels 15, as is employed with the Applicant's venting product known as VENTFORM. In that way, gases such as radon, carbon dioxide, methane and VOCs that may rise out of the ground base 12 are prevented from reaching the slab 1.

Referring now to Fig. 6, there is shown a third embodiment which is a combination of the first and second embodiments. The third embodiment differs from the first embodiment principally in respect of the nature of the supporting layer 3B which in this case comprises two superimposed layers. The other parts of the construction shown in the drawings are the same and are designated by the same reference numerals. In the third embodiment, the supporting layer 3B itself comprises two layers: there is a heave compensation layer 31 which is the same as the supporting layer 3 of Figs. 1 and 2, and a venting layer 32 on top of the heave compensation layer which is similar to the supporting layer 3A of Figs. 4 and 5.

The heave compensation layer 31 comprises a plurality of upright supporting walls 8 which define hexagonal cells 9; the dimensions and shapes of those walls and cells may be as for the supporting layer 3 of Figs. 1 and 2.

The venting layer 32 comprises a multiplicity of cuboidal blocks 44 which are similar in size and placement to the feet 14 of the supporting layer 3A and thus define two sets of parallel channels 45 that are open along their bottom faces and extend perpendicular to one another. The channels 45 thus provide passageways that extend out to each of the side edges of the venting layer 32. A sheet 46 of, for example, fluted polypropylene is bonded to the tops of the blocks 44 and can be likened to the upper portion 16 of the supporting layer 3A of Figs. 4 and 5. The DPM 6 is laid, in use, on top of the sheet 46. A further sheet 47 of, for example, perforated fluted polypropylene is bonded to the bottom faces of the blocks 44 and also to the tops of the walls 8 of the heave compensation layer 31, so as to allow free gas movement from the hexagonal cells into the channels 45 whilst forming the venting layer 32 as a unitary structure. In one particular example of the third embodiment the sheets 46 and 47 are made of a fluted polypropylene material.

Both the heave compensation layer 31 and the venting layer 32 are formed of adjacent panels which in a particular example of the invention are 2.4m long and 1.2m wide .

As will be understood the supporting layer 3B is laid, in use, in substantially the same way as the supporting layers 3 and 3A. In the event of ground heave the heave compensation layer 31 collapses as described above for the supporting layer 3; similarly in the event of gas rising up through the ground base 12, the gas passes through the hexagonal cells 9 and the perforated sheet 47 and into the channels 45 and passes along the passageways defined by the channels 45 to the edges of the supporting layer 3B and of the slab 1.

Referring now to Fig. 7, the fourth embodiment of the invention will be described. The fourth embodiment differs from the first embodiment principally in respect of the nature of the supporting layer 3C. The other parts of the construction shown in the drawings are the same and are designated by the same reference numerals. The supporting layer 3C of the fourth embodiment comprises a layer of hardcore 51 laid on the ground base 12 and a layer of sand 52 laid over the top of the hardcore to provide a flat supporting surface on which the DPM and then the upper layer 2 can be laid.

The constructions described above can be used in many situations in which concrete slabs are cast over a ground base, for example, under reinforced concrete floors of domestic dwellings. The formation of reinforced ribs in the slab 1 enables a thinner slab to be used than would otherwise be possible. Difficulties with fitting of a DPM in the grooves that form the ribs can be avoided by placing the DPM 6 below the upper layer 2 of shuttering that forms the ribs and above the supporting layer 3. The DPM 6 is in that way allowed to be laid in a single horizontal plane across the whole width of the slab. Alternatively the DPM may be laid above the slab with a screed layer laid over the top of the DPM.

The upper layer 2, and in many cases also the supporting layer 3, also serves to insulate the concrete. That both accelerates the curing of the concrete, especially in cold weather, and improves the insulation of the final dwelling. Typically the expanded polystyrene of the upper layer 2 would have a much lower thermal conductivity than the concrete of the slab 1. For example the thermal conductivity of the expanded polystyrene of the upper layer 2 may be 0.038 Wm“1K_1 and the thermal conductivity of the concrete may be 2.3 Wm-1K-1. Thus it can be seen that the combination of the upper layer 2 and the slab 1 provides much better insulation across its combined thickness than would a concrete slab of that combined thickness .

Fig. 8 shows a sectional view through a side of the lower portion of a building incorporating a construction embodying the invention. A cavity wall of the building comprises an outer skin of bricks or blocks 81, an inner skin of blocks 82, a concrete slab 83 with reinforcement 84, a DPM 85, a cavity tray 86, a block 87 of low thermal conductivity resting on a foundation 88 and a galvanised steel angle 89 of '1/ shaped cross-section fixed to the top of the block. The angle 89 has an upright wall 90 that acts as shuttering to define the side edge and the level of the top of the cast slab. At the corners of the slab corner pieces and interlocking connectors (not shown) are provided so that the angle 89 is continuous around the slab. The inner skin of blocks 82 are supported on the slab 83 and therefore prevent the slab 83 from rising. A concrete screed is laid over the top of the DPM 85. The DPM 85 extends across the entire width of the slab, up the outside of the inner skin of blocks 82 and ends in the junction between two rows of blocks. The cavity tray overlies the DPM within the junction of the blocks and extends downwardly and across to the outer skin of blocks 81 in a conventional way.

Immediately inside the block 87 of low thermal conductivity, is, in this particular example, an upper layer 92 and a supporting layer 93 of the kind described above with reference to Figs. 1 and 2, although it should be understood that those layers could take any of the other forms described herein. It will be seen that the slab 83 extends downwardly at its side edges to a level corresponding approximately to the interface of the upper and lower layers.

In a modified arrangement of the building shown in Fig. 8, the DPM is positioned between the upper and lower layers 92 and 93, extends over the block 87 and then up the outside of the wall 90 of the angle 89. In that case a different system for fastening the angle 89 in place may be adopted to avoid perforating the DPM.

Whilst some particular embodiments of the invention have been described with reference to the drawings, many modifications may be made.

For example, one particular form of venting layer 3A is shown in Figs. 4 and 5 and another particular form of venting layer 32 is shown in Fig. 6. It would be possible to use a venting layer similar to the layer 32 in the embodiment of Figs. 4 and 5 or to use a venting layer similar to the layer 3A in the embodiment of Fig.6, and other forms of venting layer may also be employed.

There will often be options of bonding different layers in the construction together, forming them integrally with one another or of simply laying them on top of one another and different options will suit different applications .

The supporting layer 3 shown in Figs. 1 and 2 is formed of adjoining panels. Another possibility, shown in Fig. 9, is to provide the supporting layer 3 as a series of strips 60 that are spaced apart. Each strip 60 may be of the same hexagonal cellular construction as the supporting layer 3 but the strips 60 may for example occupy only about 60% of the area under the upper layer 2. In that case, in the event of heave, the upward force exerted by the ground base 12 on the slab 1 via the supporting layer 3 leads to a vertical compressive force per unit area in the strips that is substantially greater than it would be if the strips filled the entire area under the upper layer 2. Notionally, if the ground base is regarded as a solid body, the increase in the vertical compressive force is a factor of about 1.67 when the strips 60 occupy about 60% of the area under the upper layer 2. In Fig. 9 the strips are shown positioned under the grooves 5 but they need not be so positioned.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional.

Claims

Claims :

1. A construction comprising: a concrete slab; an upper layer of pre-formed plastics material having a top face over which the slab is cast, the top face being formed with a plurality of grooves therein; a supporting layer for supporting the upper layer over a top face of the supporting layer, a plurality of passageways and/or cavities being defined within the supporting layer; a former for defining the side edges of the slab which extends beyond the upper layer and has a lower edge portion on the outside of the upper layer.

2. A construction according to claim 1, in which an upright wall is provided on top of an edge of the slab.

3. A construction according to claim 2, in which the upright wall is the inner wall of a cavity wall.

4. A construction according to claim 2 or 3, in which a concrete screed is laid over the slab and an edge of the screed abuts the upright wall.

5. A construction according to claim 4, in which a membrane is disposed over the slab and the screed is disposed over the membrane.

6. A construction according to any preceding claim, in which the former is a generally '1/ shaped member.

7. A construction according to any preceding claim, in which the former is supported on a block of thermally insulating material.

8. A construction according to any preceding claim, in which the upper layer is formed of a thermally insulating material.

9. A construction according to claim 7 or 8, in which the upper layer is formed of one or more preformed panels.

10. A construction according to claim 9, in which the one or more preformed panels are made of an expanded plastics material.

11. A construction according to claim 10, in which the expanded plastics material is an expanded polystyrene material.

12. A construction according to any preceding claim, in which the grooves are substantially parallel to one another and evenly spaced across the top face.

13. A construction according to any preceding claim, in which the supporting layer comprises one or more preformed supports .

14. A construction according to any preceding claim, in which the supporting layer comprises one or more supports of expanded plastics material.

15. A construction according to claim 14, in which the expanded plastics material is expanded polystyrene.

16. A construction according to any preceding claim, in which the supporting layer comprises at least one collapsible support.

17. A construction according to claim 16, in which said at least one collapsible support comprises a plurality of spaced apart walls defining cells between the walls.

18. A construction according to claim 17, in which the spaced apart walls are upright walls and define cells of substantially hexagonal shape.

19. A construction according to claim 17, in which the spaced apart walls are upright walls and define cells of substantially rectangular shape.

20. A construction according to any of claims 1 to 15, in which the supporting layer has a plurality of openings over a bottom face and comprises one or more passageways providing paths of fluid communication from the plurality of openings to one or more edges of the supporting layer.

21. A construction according to claim 20, in which the passageways provide paths of fluid communication to all edges of the supporting layer.

22. A construction according to claim 20 or 21, in which the passageways comprise parallel channels extending between opposite edges of the supporting layer.

23. A construction according to any of claims 20 to 22, in which the supporting layer comprises a multiplicity of upright pillars and the passageways are defined by the spaces between the pillars.

24. A construction according to any of claims 1 to 15, in which the supporting layer comprises at least two layers, a first of the at least two layers being according to any of claims 16 to 19 and a second of the at least two layers being according to any of claims 20 to 23.

25. A construction according to claim 24, in which the second layer is above the first layer.

26. A construction according to any preceding claim, in which the supporting layer includes a concrete blinding layer.

27. A construction according to any preceding claim, in which the supporting layer includes a sand layer.

28. A construction according to any preceding claim, in which the supporting layer includes a hardcore layer.

29. A construction according to claim 26 or 27 and 28, in which the concrete blinding or sand layer is above the hardcore layer.

30. A method of casting a slab over a ground base, the method including the step of building a construction according to any preceding claim on a ground base.

31. A construction comprising an upper layer of pre-formed plastics material having a top face over which a slab is to be cast, the top face being formed with a plurality of grooves therein, and a supporting layer for supporting the upper layer over a top face of the supporting layer, wherein a membrane layer is provided between the supporting layer and the upper layer.

32. A construction according to claim 31, in which the supporting layer has a top face that is substantially flat.

33. A construction according to claim 31 or 32, in which the upper layer has a bottom face that is substantially flat.

34. A construction according to any of claims 31 to 33, in which the upper layer is formed of a thermally insulating material.

35. A construction according to claim 34, in which the thermal conductivity of the insulating material is less than 0.1 Wm-1^1.

36. A construction according to claim 34 or 35, in which the upper layer is formed of one or more preformed panels.

37. A construction according to claim 36, in which the one or more preformed panels are made of an expanded plastics material.

38. A construction according to claim 37, in which the expanded plastics material is an expanded polystyrene material.

39. A construction according to any of claims 31 to 38, in which the grooves are substantially parallel to one another and evenly spaced across the top face.

40. A construction according to any of claims 31 to 39, in which the supporting layer comprises one or more preformed supports .

41. A construction according to any of claims 31 to 40, in which the supporting layer comprises one or more supports of expanded plastics material.

42. A construction according to claim 41, in which the expanded plastics material is expanded polystyrene.

43. A construction according to any of claims 31 to 42, in which a plurality of passageways and/or cavities are defined within the supporting layer.

44. A construction according to any of claims 41 to 43, in which the supporting layer comprises at least one collapsible support.

45. A construction according to claim 44, in which said at least one collapsible support comprises a plurality of spaced apart walls defining cells between the walls.

46. A construction according to claim 45, in which the spaced apart walls are upright walls and define cells of substantially hexagonal shape.

47. A construction according to claim 45, in which the spaced apart walls are upright walls and define cells of substantially rectangular shape.

48. A construction according to any of claims 40 to 43, in which the supporting layer has a plurality of openings over a bottom face and comprises one or more passageways providing paths of fluid communication from the plurality of openings to one or more edges of the supporting layer.

49. A construction according to claim 48, in which the passageways provide paths of fluid communication to all edges of the supporting layer.

50. A construction according to claim 48 or 49, in which the passageways comprise parallel channels extending between opposite edges of the supporting layer.

51. A construction according to any of claims 48 to 50, in which the supporting layer comprises a multiplicity of upright pillars and the passageways are defined by the spaces between the pillars.

52. A construction according to any of claims 40 to 53, in which the supporting layer comprises at least two layers, a first of the at least two layers being according to any of claims 44 to 57 and a second of the at least two layers being according to any of claims 48 to 51.

53. A construction according to claim 52, in which the second layer is above the first layer.

54. A construction according to any of claims 31 to 53, in which the supporting layer includes a concrete blinding layer .

55. A construction according to any of claims 31 to 54, in which the supporting layer includes a sand layer.

56. A construction according to any of claims 31 to 54, in which the supporting layer includes a hardcore layer.

57. A construction according to claim 54 or 55 and 56, in which the concrete blinding or sand layer is above the hardcore layer.

58. A construction according to any of claims 31 to 57, in which the membrane layer comprises a damp proof membrane for obstructing the passage of moisture upwardly into the upper layer.

59. A construction according to any of claims 31 to 58, in which the membrane layer comprises a gas membrane for obstructing the passage of gas upwardly into the upper layer.

60. A construction according to claim 58 and 59, in which the same membrane acts as both the damp proof membrane and the gas membrane.

61. A construction according to any preceding claim, in which supports are provided in the grooves for supporting and locating reinforcing bars away from the walls of the grooves during casting of the slab.

62. A construction according to claim 61, in which the supports are moulded from plastics material.

63. A construction according to claim 61 or 62, in which the supports comprise integral deformable parts for deforming to accommodate a reinforcing bar and for subsequently retaining the bar in position in the support.

64. A construction according to any of claims 61 to 63, in which the supports extend across substantially the whole widths of the grooves in which they are provided.

65. A construction according to any of claims 31 to 64, further including a slab cast over the upper layer.

66. A construction according to claim 65, in which the slab includes reinforcing bars.

67. A construction according to claim 66, in which at least some of the reinforcing bars are disposed in at least some of the grooves.

68. A construction according to any of claims 65 to 67, in which the slab extends beyond the upper layer and has a lower edge portion on the outside of the upper layer.

69. A construction according to any of claims 65 to 68, in which the boundary of the edge of the slab is formed by a generally '1/ shaped member which acts as shuttering during casting of the slab and remains in place thereafter.

70. A construction according to claim 69, in which the '1/ shaped member is supported on a block of thermally insulating material.

71. A construction comprising an upper layer of pre-formed plastics material having a top face over which a slab is to be cast, the top face being formed with a plurality of grooves therein, and a supporting layer for supporting the upper layer over a top face of the supporting layer, wherein the construction is further characterized by one or more of the following features: i. the upper layer is formed of a thermally insulating material having a thermal conductivity of less than 0.1 Wm^K"1; ii. the supporting layer comprises collapsible supports which occupy less than 70% of the area under the upper layer; iii. the supporting layer has a plurality of openings over a bottom face and comprises one or more passageways providing paths of fluid communication from the plurality of openings to one or more edges of the supporting layer; iv. the supporting layer comprises at least two layers, a first of the at least two layers being according to any of claims 44 to 47 and a second of the at least two layers being according to any of claims 48 to 51; v. supports are provided in the grooves for supporting and locating reinforcing bars within the grooves; vi. both the upper layer and the lower layer comprise expanded plastics material; vii. the slab extends beyond the upper layer and has a lower edge portion on the outside of the upper layer; iii. the boundary of the edge of the slab is formed by a generally '1/ shaped member which acts as shuttering during casting of the slab and remains in place thereafter; and/or ix. the '1/ shaped member is supported on a block of thermally insulating material.

72. A construction substantially as herein described with reference to the accompanying drawings.

73. A structure comprising a construction according to any of claims 65 to 72 and side walls extending upwardly above the slab from around the slab.

74. A structure according to claim 73, wherein the membrane extends outwardly from the interface of the supporting layer and the upper layer and through the side walls .

75. A method of casting a slab over a ground base, the method including the step of building a construction according to any of claims 31 to 64 or 71 on a ground base, and casting a slab on the top of the upper layer of the construction.

76. A method of casting a slab over a ground base, the method being substantially as herein described with reference to the accompanying drawings.