GB2206637A - Collapsible shuttering to absorb ground heave - Google Patents

Abstract

Shuttering for use in casting a slab/beam 13 over a substrate 5 comprises a surface 1 for supporting the cast material and a support structure comprising walls 2, 3 which, in use, is located on the substrate to support the surface 1 above the substrate. Following casting, a predetermined heaving movement of the substrate (5) causes the walls (2, 3) to break up leaving a void between the substrate and the cast slab/beam. <IMAGE>

Description

Shuttering for use in casting slabs or beams The present invention relates to shuttering for use, in the construction industry, in casting slabs or beams over a substrate.

The invention relates, more especially, to shuttering for use in casting floor slabs or ground beams over a substrate in which movement, particularly heaving movement, is expected. A floor slab or ground beam cast directly on such a substrate (for example, clay) would be at risk from cracking as a result of excessive movement in the substrate but the risk is eliminated if a void can be provided, between the floor/beam and the substrate, into which this movement can take place.

It has, therefore, already been suggested that some form of support structure, or shuttering, should be used, onto which the slab/beam can be cast, the support structure being of a temporary nature so that it will, in the course of time, leave a void between the substrate and the slab/beam (which is now self-supporting).

For example, a known form of shuttering is intended to be destroyed by moisture emanating from the substrate or introduced deliberately after the slab has been cast. This known form has the disadvantage that it can be destroyed prematurely by moisture from other sources, for example by rainwater.

Other forms of shuttering have been proposed, which are intended to be compressed by upward movement of the substrate. Shuttering of this type is intended to take-up movement of the substrate and thereby to prevent damage to the cast slab/beam but, in practice, it is difficult to ensure that a damaging load is not applied to the slab/beam if the substrate heaves.

It is an object of the present invention to provide a shuttering structure for use in casting slabs or beams, which enables disadvantages of the known structures to be overcome and which enables a void of predetermined depth to be obtained between cast floor slab/ground beam and the substrate.

The present invention provides shuttering for use in casting a slabXbeam over a substrate, comprising a surface for supporting cast material and a support structure locatable on,the substrate to support the surface in a position above the substrate, the support structure being such that it will support the weight of the cast material but will break up, at least partly, under a predetermined compressive force, due to heaving movement of the substrate, less than that which will cause movement in the slab/beam.

The support structure may be formed from a material which will break up under the said redetrined load.

In an embodiment of the invention described herein, the support structure comprises a plurality of spaced-apart support members each of which will break-up, at least partly, under the said predetermined compressive force. The support members are formed from a material which, in the direction perpendicular to the plane of the surface, has comparatively good compressive strength and a comparatively low transverse flexural strength.

The present invention accordingly also provides shuttering for use in casting a slab/beam over a substrate, comprising a surface for supporting cast material, and a support structure locatable on the substrate to support the surface in a position above the substrate, the support structure comprising a plurality of spaced-apart supports having, in the direction perpendicular to the plane of the surface, comparatively good compressive strength and a comparatively low transverse flexural strength. To assist in obtaining the comparatively low flexural strength, the transverse dimension of each support may be small in comparison with its dimension in the direction perpendicular to the plane of the said surface (ie. the height of the support when the shuttering is in use). The spacing between the supports may be large in comparison with the transverse dimension of the supports.

Advantageously, the support structure is formed from a material in which creep occurs when the structure is subjected to a compressive load. The support structure may, for example, be formed from a rigid expanded plastics material. In an embodiment of the invention described herein, the support structure is formed from expanded polystyrene.

The support structure may be of cellular construction: it may, for example comprise a plurality of spaced-apart walls defining cells of generally cuboidal form.

The shuttering may be in a panel form.

In use, the support structure of shuttering in accordance with the invention is located on a substrate and a slab/beam is cast over the supporting surface of the shuttering. The support structure may then break up, at least partly, under a compressive ore due to heaving movement of the substrate to form a void between the slab;beam and the substrate.

The present invention accordingly also provides a method of casting a slab/beam over a substrate comprising the steps of locating shuttering on the substrate to provide a supporting surface for cast material; casting the slab/beam over the surface, and allowing the shuttering to break up, at least partly, under a compressive force due to heaving movement of the substrate to form a void between the slab/beam and the substrate.

Alternatively, when a slab/beam has been cast over the supporting surface of shuttering constructed in accordance with the invention, the support structure of the shuttering can be broken up by a force applied specifically for this purpose, to form a void between the slab/beam and the substrate.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a side view of a shuttering panel constructed in accordance with the invention; Fig. 2 is a view, taken in the direction of the arrow II in Fig. 1, of the underneath of the panel; Fig. 3 is a vertical section illustrating the panel in use, and Fig. 4 is the same vertical section but shows the arrangement after the panel has collapsed.

The panel shown in Figs. 1 and 2 has a rectangular top 1 comprising a sheet of rigid material to the underneath of which are attached a plurality of downwardly extending walls 2,3. The plane of each wall is perpendicular to the plane of the top 1 and some (2) of the walls run parallel to the long sides of the top while the others (3) run parallel to t short sides.

The walls 2, 3 are spaced aparat regularly in both directions, forming a cellular construction.

The walls 2 extend continuously along the whole length of the top 1 while the walls 3 extend across the whole width of the top but are not continuous because they are interrupted at intervals by the walls 2. The cells defined by the walls 2, 3 are substantially square in plan.

The rectangular top 1 is formed from any suitable rigid material: it may, for example, be a timber product such as plywood.

The walls 2, 3 are formed from expanded polystyrene, ideally British Standard grade SD (N) V manufactured from low density bead, and are bonded to the top 1 in any suitable manner, for example by an impact adhesive. The walls are required to have a comparatively good compressive strength in a direction perpendicular to the plane of the top 1 and a comparatively low transverse flexural strength and, as indicated in Figs. 1 and 2, are comparatively thin for their height and are also thin in comparison with the distance between them. The reason for this will be explained below.

The dimensions of the shuttering panel are determined by its intended use but, for laying a suspended floor slab as described below, the rectangular top 1 is typically 2.44m long and 1.22m wide. The thickness of the top 1 is, typically, within the range of from 0.5cm to 2.0cm. The walls 2, 3 are spaced apart at a distance between centres which is typically within the range of from 10cm to 30cm and are typically from 10cm to 30cm high and from 2cm to 5cm thick. In the panel shown in Figs. 1 and 2, the top 1 is 1.8cm thick and the walls 2, 3 are 20cm high and are spaced apart at a distance, between centres, of approximately 23cm.

The manner in which the panel is used in laying a suspended floor slab of a building over a substrate is illustrated in Figs. 3 and 4. The normal surfacelevel of the substrate is indicated at 5 and one of the piles that are sunk into the substrate to support the building is indicated at 6. A conventional ground beam 7 of reinforced concrete extends along the top of a line of piles 6, on suitable bedding material 4, to support a respective one of the walls 8 of the building between which the floor slab is to be suspended.

The substrate over which the floor slab is to be constructed is excavated to the required depth (determined in a manner described below) and the surface 9 is made level. Shuttering panels 10, each as shown in Figs. 1 and 2, are then laid edge-to-edge to cover the prepared surface 9 completely, with the joins between adjacent panels being covered over, for example with tape.

If any make-up pieces of shuttering are required to ensure that the prepared surface 9 is completely covered, these pieces can be cut, using a saw, from a full-size panel. The lower edges of the walls 2, 3 of the panels rest on the prepared surface 9, and the outer sides of the panel assembly (ie, adjacent the support walls 8) are finished with a plain strip of expanded polystyrene (not shown in Figs. 3 and 4).

Conventional steel reinforcement (not shown) for the suspended floor panel is then secured over the panels 10, being spaced slightly above the tops 1 of the panels by conventional spacers (not shown).

Concrete is then laid over the support panels 10 and vibrated in the normal way and, 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 panels 10 but, as the concrete cures, the floor slab 13 becomes self-supporting between the walls 8.

If heaving movement occurs in the substrate, a vertical compressive force is exerted on the walls 2, 3. Initially creep occurs in the polystyrene material and, if the heaving movement in the substrate is extensive,the walls 2, 3, begin to flex. If the compresive force on the walls 2, 3 is such that the flexing movement exceeds a predetrmined limit, the polystyrene material fails and the walls 2, 3 break up instantaneously. This is the situation illustrated in Fig. 4 which shows that there is now a layer 11 of polystyrene rubble lying on the substrate surface 9, with the remains 12 only of the polystyrene walls 2, 3 still attached to the underside of the panel tops 1. The walls 2, 3 are designed to ensure that they break up before the heaving movement in the substrate causes any upward movement in the floor slab 13.Following the break-up of the walls, a void 14 is formed between the substrate and the floor slab 13, into which any subsequent movement of the substrate can take place without affecting the slab. It will be appreciated, however, that this void 14 is formed only if sufficient heaving movement occurs in the substrate to bring about the break-up of the walls 2, 3 and, until such movement occurs, the shuttering 10 will remain intact.

Once the void 14 has been formed, its depth will vary as the substrate shrinks and expands.

It has been found that expanded polystyrene of the type known as low density bead board is particularly suitable for the walls 2, 3, especially the British Standard grade SD (N! V mentioned above. This material, when used with the dimensions specified above for the walls 2, 3, has good compressive strength in the plane of the board but a low transverse flexural strength and will creep under compression before it breaks up so that it is less likely to have any adverse effect on the floor slab 13 than a material which fails without creep. However, any other material with similar characteristics could be used and it is also possible, although less desirable, to use a material which breaks up without creep, for example rigid plastics sheet or thin hardboard.

Generally it is envisaged that the shuttering panels would be formed from materials that are, inherently, not attacked by moisture. However materials that are affected by moisture could be used provided that they are given some form of coating that prevents the entry of moisture and that, in the case of the walls 2,3, this coating does not affect the break-up of the walls in response to a predetermined heaving movement of the substrate.

The thickness, number and/or lay-out of the walls 2, 3 can be varied, having regard to the conditions under which the walls are required to break up and bearing in mind that a change in the thickness and number of the walls will alter the surface area over which the walls contact the substrate. For example, the size of the cells defined by the walls can be decreased by increasing the number of shorter walls 3 from elevenfas shown in Figs. 1 and 2) to twelve or thirteen and/or increasing the number of longer walls 2 from six (as shown in Figs. 1 and 2) to seven.

Alternatively, or in addition, the shape of the cells could be altered. Generally, the ratio of the thickness of the walls to the spacing between them (ie distance between centres) should be within the range of from 1:2 to 1:15.

The height of the walls 2, 3 is determined by the required depth of the void 14, which should be sufficient to accommodate the expected substrate movement (which can be determined by soil analysis) together with the depth of the rubble 11 (Fig. 4). The wall height of 20cm specified above may not be suitable in all cases and could, when appropriate, be reduced or increased. The height of the walls 2, 3 in turn determines the depth to which the substrate should be excavated in preparing the surface 9 on which the shuttering is placed. The height of the walls also determines, for any particular material, the maximum permissible thickness of the walls if they are to break-up in a reliable manner when a predetermined heaving movement occurs in the substrate. Generally, the ratio of the thickness of a wall to its height should be within the range of from 1:2 to 1:15.

The shuttering panel shown in Figs. 1 and 2 can, if required, be modified further by adding a second rectangular sheet of rigid material, similar to the top 1, at the bottom of the walls 2, 3 to form a lower supporting surface which, in use, would be placed on the prepared surface 9 of the substrate. It can also be modified by replacing the walls 2, 3 by spacedapart support members of some other form: for example, instead of the walls 2, 3, a plurality of spaced-apart pillars could be used to support the top panel 1 above the substrate provided that the shuttering will support the weight of concrete during casting and that the pillars will break-up as described above when a predetermined heaving movement occurs in the substrate.

However, a support structure that comprises interlocked walls (s,as as that shown in Figs. 1 and 2) is preferred because it has greater stability.

While it is envisaged that the shuttering described above would normally be produced in the form of rectangular panels of a standard size, as shown in Figs. 1 and 2, this is not essential and the shape and size of the panels can be altered as required.

It is possible, for example, to use shuttering of the type described above to provide the support on which the ground beam 7 is cast (the shuttering then replacing the bedding material 4) and, in this case, panels of a different size are required. Typically, shuttering panels for use in casting ground beams are 2.4m long and have a width in the range 45cm to 60cm. Because these panels are substantially narrower than those used in casting floor slabs, the lay-out of the polystyrene walls 2, 3 will differ from that described above with reference to Figs. 1 and 2, to ensure stability when the concrete is being laid: for example, the longer walls 2 will be more closely spaced so that the cells defined by the walls will be rectangular rather than substantially square. In addition, the panels should be designed to support a greater depth of concrete.

Shuttering of the type described above can be used in many situations in which concrete slabs or beams are cast over a substrate and it is desired, when the casting process has been completed, to allow for the formation of a void between the slab/beam and the substrate, into which movement of the substrate can take place. The polystyrene walls 2, 3 in the shuttering serve to insulate the concrete and thereby to accelerate the curing of the concrete especially in cold weather.

Insulation is also provided, following the collapse of the shuttering, by the layer of polystyrene rubble 11 on the substrate (Fig. 4) and this is of importance, especially, underneath a floor slab.

Shuttering of the type described above can also be used when it is required to form a void beneath a floor slab/ground beam for other purposes, for example to accomodate services or to permit the evacuation of methane gas. In this case, when the casting process has been completed, the shuttering is deliberately broken up by an externally-applied force to form the required void. In this case, the force need not be applied in the same direction as that due to heaving movement of the substrate but can, for example, be applied in a direction generally parallel to the substrate provided that it causes the shuttering to break up to form the required void.

To facilitate the handling and, particularly, transport, of the shuttering and to reduce the risk of damage,the individual panels can be wrapped in a suitable material for example polythene. The wrapping need not be removed from the panels before use provided, of course, that the wrapping does not have a substantial effect on the characteristics of the shuttering and, in particular, does not hinder the break-up of the shuttering in response to heaving movement of the substrate.

If the shuttering structure is formed from materials that are not attacked by moisture, there is no need to ensure that the wrapping excludes moisture so that, for example, a perforated wrapping could be used. However, by encasing the shuttering panels completely in a wrapping that prevents the entry of moisture the possibility arises of using, in the shuttering structure, materials that might otherwise be discounted because they are adversely affected by moisture.

Claims (26)

CLAIMS:

1. Shuttering for use in casting a slab/beam over a substrate, comprising a surface for supporting cast material, and a support structure locatable on the substrate to support the surface in a position above the substrate, the support structure being such that it will support the weight of the cast material but will break up,at least partly, under a predetermined compressive force, due to heaving movement of the substrate, less than that which will cause movement in the slabgbeam.

2. Shuttering as claimed in claim 1, in which the support structure is formed from a material which breaks up under the said predetermined force.

3. Shuttering as claimed in claim 1 or claim 2, in which the support structure comprises a plurality of spaced-apart support members.

4. Shuttering as claimed in claim 3, in which the dimensions of each support member are such that the support member breaks up, at least partly, under the said predetermined force.

5. Shuttering for use in casting a slab/beam over a substrate, comprising a surface for supporting cast material, and a support structure locatable on the substrate to support the surface in a position above the substrate, the support structure comprising a plurality of spaced-apart support members having, in the direction perpendicular to the plane of the surface., comparatively good compressive strength and a comparatively low transverse flexural strength.

6. Shuttering as claimed in claim 4 or claim 5, in which the transverse dimension of each support member is small in comparison with its height.

7. Shuttering as claimed in claim 6, in which the ratio of the transverse dimension of the support member to its height is within the range of from 1:2 to 1:15.

8. Shuttering as claimed in any one of claims 4 to 7, in which the transverse dimension of each support member is small in comparison with the spacing of the support members.

9. Shuttering as claimed in claim 8, in which the ratio of the transverse dimension of the support member to the spacing of the support members is within the range of from 1:2 to 1:15.

10. Shuttering as claimed in any one of the preceding claims, in which the support structure is formed from a material in which creep occurs when the structure is subjected to a compressive load.

11. Shuttering as claimed in any one of the preceding claims, in which the support structure is formed from a material that is not attacked by moisture.

12. Shuttering as claimed in any one of the preceding claims, in which the support structure is formed from a rigid expanded plastics material.

13. Shuttering as claimed in claim 12 in which the rigid expanded plastics material is expanded polystyrene.

14. Shuttering as claimed in any one of the preceding claims, in which the support structure is of cellular construction.

15. Shuttering as claimed in claim 14, in which the support structure comprises a plurality of spacedapart walls defining cells of generally cuboidal form.

16. Shuttering as claimed in claim 15, in which the plane of each wall is perpendicular to the plane of the supporting surface.

17. Shuttering as claimed in any one of the preceding claims, which is in the form of a rectangular panel.

18. Shuttering as claimed in any one of the preceding claims, in which the supporting surface is a sheet of rigid material.

19. Shuttering as claimed in claim 18, in which the supporting structure is attached to the sheet of rigid material.

20. Shuttering as claimed in any one of the preceding claims, in which the support structure is located on a substrate and a cast slab/beam is located over the surface.

21. A method of casting a slab/beam over a substrate, including the steps of locating shuttering on the substrate, the shuttering being as claimed in any one of the claims 1 to 19, and casting a slab/beam over the supporting surface of the shuttering.

22. A method as claimed in claim 21, further including the step of breaking up the support structure of the shuttering, at least partly,to form a void between the cast slab/beam and the substrate.

23. A method of casting a slab/beam over a substrate comprising the steps of locating shuttering on the substrate to provide a supporting surface for cast material; casting the slab/beam over the surface, and allowing the shuttering to break up, at least partly, under pressure due to heaving movement of the substrate to form a void between the slab/beam and the substrate.

24. A method as claimed in claim 23, in which the shuttering is as claimed in any one of claims 1 to 19, the support structure being located on the substrate.

25. A method as claimed in any one of claims 21 to 24, substantially as described herein.

26. Shuttering for use in casting a slab;beam substantially as described herein with reference to and as shown in the accompanying drawings.