WO2014200364A1

WO2014200364A1 - Formwork of reducing thickness due to loading of slab cast in-situ

Info

Publication number: WO2014200364A1
Application number: PCT/NZ2014/000115
Authority: WO
Inventors: Fabio Parodi
Original assignee: Fabio Parodi
Priority date: 2013-06-11
Filing date: 2014-06-11
Publication date: 2014-12-18
Also published as: US20170159314A1; US20160122998A1; EP3008262A4; CN105452581A; EP3008262A1; JP6416239B2; JP2016524060A

Abstract

An formwork apparatus (1,5) and a method of use of the apparatus, for forming a concrete slab (17). The apparatus comprising a support element (6,7) having an active layer (10) preferably bonded on each side to a backing sheet (8,9). The layer (10) chosen to slowly compress over time when placed under a load, the rate of compression chosen such that the active layer continues to support the slab until such time as the slab becomes self-supporting (i.e. the concrete has cured/set - preferably over a period of 2 to 10 days). The backing sheet (8) has a surface which is preferably of a relatively low coefficient of friction. When two support elements (6,7) are laid one on top of the other with the surfaces (8) in contact, the upper support element (6) may slide over the lower support element (7), for example during a seismic event.

Description

FORMWORK OF REDUCING THICKNESS DUE TO LOADING OF SLAB CAST IN-SITU

STATEMENT OF CORRESPONDING APPLICATIONS

This application is based on the Provisional specification filed in relation to New Zealand Patent Application Number 611841 , the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a component that forms an upper surface of a formwork and a method for its use in construction of concrete slabs. In particular it may relate to situations where the concrete slab is used in conjunction with seismic base isolators, although this should not be seen as limiting.

BACKGROUND ART

When constructing structures such as large buildings, some components, such as concrete floor slabs, are typically formed as a single structure supported on a load bearing members of the structure, e.g. such as pillars, beams, foundations, seismic base isolators or whatever mechanism is used to carry the load. . This usually requires a temporary formwork to be created to contain and support the concrete slurry until it hardens. Once the concrete has acquired sufficient structural integrity to at least support its own weight, the temporary formwork may be removed. At this time the total load of the slab is transferred to the load bearing members.

This is common practice when forming slab floors in situ in a multi-storey building, for example. The slab to be formed is supported on formwork (typically wooden or metal panels) supported from below on a temporary scaffold. The slab is tied into the load bearing columns, foundations etc. as appropriate, of the structure. When the concrete slab has hardened the temporary scaffold and formwork are removed leaving the slab fully supported on the load bearing members.

However, in order for this system to work there needs to be sufficient space available to erect and (if necessary) remove the formwork. In traditional buildings this is seldom a problem as the slab is usually formed over a foundation having a base and wall supports which provide space to erect the formwork. In some cases the foundation can be a basement of the building, or it may be a slab floor formed on the ground, with accompanying walls.

A different situation can arise with many modern buildings which include seismic base isolation of at least the ground floor, either as part of a new construction or as a retrofit to an existing structure. Installing base isolation to a new ground floor slab to be poured over a basement foundation is generally not a problem as the base isolators can be placed on top of the walls or columns of the foundation and the formwork put in place as usual prior to forming the slab.

However, it can be a problem when the foundation is an existing concrete slab formed on the ground and the intention is to form the new slab such that the two slabs are separated by the base isolators only. The height of many conventional base isolators is such that, while formwork for the top slab could be erected around the base isolators, it would be very difficult to remove the formwork after the top slab is poured.

A further problem with this arrangement is that there is often a regulatory, or at least a good practice, requirement to inspect many types of base isolator during use. This requirement places a limit on the spacing between the slabs - namely there needs to be sufficient height to allow access to a person between the slabs in order to make the inspection (or the base isolators are located around the outside of the structure so that inspection can be carried out externally).

One solution is to form a structure with low pillars over the ground slab foundation. Base isolators can be located at various places on the top of these pillars. The pillars can be made sufficiently high to enable a person access to inspect the base isolators, albeit possibly crawling, between the foundation slab and the top slab.

A drawback of this solution is that it incurs additional costs in time, labour and materials to form the web structure, as well as adding design constraints from the need to provide access between the slabs and the additional height of the structure.

Some base isolation systems, for example those that do not deteriorate with time or as a result of a seismic event, may not require inspection and therefore there is no need to provide human access between the slabs. An example of such a base isolation system is a device known as a slider: essentially a slider may be any device composed of two flat surfaces having good load bearing capacity but low horizontal resistance to motion. Another form of slider, commonly referred to as a "friction pendulum", typically uses a curved surface to provide a restoring force.

Use of a slider is often combined with other types of base isolation, collectively called dumpers, which dump the seismic energy, restrict the displacements of the components to within an acceptable range, and in some cases to restore the structure to its original configuration following a seismic event. The friction pendulum slider referred to above is designed to act as a dumper by providing a restoring force as well as a dumping effect as a consequence of the curved surface geometry.

An example of dumpers are steel plates which are formed in a "U" shape (UFP).

When sliders and dumpers are used the upper slab may be located in relatively close proximity to the ground foundations or existing floor slab, as the gap between them only needs to accommodate the relatively small height of the slider. Indeed, it can be that the gap between the two slabs is too small to be readily accessed by a person, making it difficult to remove the formwork for the upper slab.

A solution in such circumstances may be to lower the formwork into place from above (once the seismic base isolators, glide bearings, sliders or whatever are in place) and then pour the concrete over the formwork, with the intention of leaving the formwork in place. Unless a structural formwork (e.g. a prefabricated structure placed on top the isolators and able to bear the weight of the wet concrete) is used, this is not necessarily a good solution from an engineering point of view as the formwork continues to bear at least some of the load of the slab, rather than transferring it to the desired load bearing components. Furthermore, structural formwork is relatively expensive, adding to the cost of construction. This solution has another significant problem in that, in the event of a seismic attack, there can be considerable frictional forces set up between the formwork that is left in place and the slab floor which is moving over it. These frictional forces, which depend on the downward load on the formwork, interfere with the movement of the structure and can limit the ability of the seismic isolators, sliders, or other bearings, to perform to their design capabilities. The formwork can also get jammed against the base isolators, adding to the problem.

It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.

All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country. Throughout this specification, the word "comprise", or variations thereof such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

DISCLOSURE OF THE INVENTION

The present invention discloses an apparatus, and a method of use of the apparatus, for use in a formwork system to form a concrete slab in situ, primarily in situations where the formwork remains in place after the slab has been poured. Once the concrete has set the load of the slab is to be supported on a load bearing member, with a much reduced load, or essentially no load, remaining on the formwork under the slab. The invention is intended to be used in situations where removal of the formwork is not required (e.g. may remain hidden) or where it is not possible, impractical or not convenient to remove the formwork after the slab has been formed.

One aspect of this invention is to provide an apparatus and method of forming a concrete slab for use with some form of seismic base isolation where the formwork is to remain in place after the slab has been formed, whereby the friction between the slab and a supporting surface of the formwork is significantly reduced from that of prior art apparatus and methods. A feature of the present invention is to form a formwork for a concrete slab using a layer of active material that slowly compresses over time when placed under a load,

The general concept is that the formwork, including the layer of active material, is initially capable of supporting the weight of the freshly poured concrete slab. The weight of the poured slab exerts a compressive force on the layer of active material, causing it to reduce in thickness. While many materials compress under a compressive load, the rate at which this happens varies. In the present invention the rate of compression is such that the layer of active material continues to support the slab until such time as the slab becomes self- supporting (i.e. the concrete has cured/set) on the load bearing members, while maintaining a thickness such that the height of the contact surface of the formwork (i.e. the upper surface of the formwork onto which concrete is poured) is comparable to the height of the load bearing structure. For the purposes of clarity, a comparable height should be understood to be ± 5 mm.

According to one aspect of the present invention there is provided a support element for use as a component of a formwork for producing a concrete slab, the support element including a layer of active material attached to a backing sheet, the active material having a thickness which reduces slowly over 2 to 10 days under the load of the poured concrete slab.

In a particularly preferred embodiment the thickness of the layer of active material reduces over the time it takes for the poured concrete to set and harden. In most instances this can be around 2 - 10 days, after which time the slab can support its own weight (on the relevant support members) without sagging or requiring support from the formwork.

In a preferred embodiments the backing sheet has a coefficient of friction less than 0.4.

In some embodiments the backing sheet includes thermal insulation. In a preferred embodiment the layer of active material is sandwiched between two backing sheets.

Preferably at least one to the two backing sheets has a relatively low coefficient of friction, preferably less than 0.4.

The use of a backing sheet having a relatively low coefficient of friction may provide benefit during a seismic event. For example, the low friction backing sheet may enable the slab to slide relatively easily over the support element. When two low friction backing sheets are used the support element may also move relatively easily over the formwork supporting it.

The inclusion of thermal insulation in the backing sheet may reduce heat loss from the slab, which may reduce heating costs within the building. The thickness of the layer of active material may vary depending on a number of variables, examples of which may include one or more of, but should not be limited to: the type of slab that is to be supported; the mass of the slab to be supported; the compressibility of the active material; the rate at which the thickness of the active material reduces; and/or the desired set time of the supported structure. In a preferred embodiment the thickness of the active material is in the range from 10 mm to 50 mm.

In a preferred embodiment the active material includes an enclosed void.

In a preferred embodiment the enclosed void contains a fluid.

In a preferred embodiment a surface of the enclosed void is flexible. In a preferred embodiment a surface of the enclosed void is permeable to the fluid within the enclosed void when under a load. In preferred embodiments the enclosed void includes a surface which is permeable to the fluid contained within the enclosed void, so that the fluid is released from within the enclosed void over time when a load is applied. In the present invention the permeability of the surface of the enclosed void is chosen so that, under the load of concrete of the poured slab, at least some of the fluid escapes over a period lasting from 2 to 10 days.

Furthermore, in preferred embodiments the enclosed void includes a flexible surface, so that loss of fluid from the enclosed void results in a reduction in volume of the enclosed void, which, under load, results in the enclosed void being squashed, thus reducing the thickness of the layer of active material. In an especially preferred embodiment the active material includes a layer formed from a plurality of enclosed voids. Preferably the voids are filled with fluid. Preferably the fluid is air.

In a preferred embodiment the layer of active material is formed from an array of air filled enclosed voids.

In a preferred embodiment the array of air filled enclosed voids is formed from a plastics material.

In a preferred embodiment the plastics material is polyethylene film.

A layer of active material as described above may be similar to a layer of the material known as Bubble Wrap™. As with Bubble Wrap™, a layer of active material according to the present invention may consist of a single layer of air-filled enclosed voids, or it may consist of any number of single layers stacked on top of one another to make a multi-layered structure of whatever thickness is required.

Furthermore the permeability of the plastic film used to form the voids may be varied to control the rate at which air permeates through the film when under load. In the present invention the permeability of the plastic film is such that the volume of the void reduces slowly over 2 - 10 days.

Reference will be made throughout the remainder of this specification to a layer of active material as a layer (or multi-layer) of bubble wrap. However, those skilled in the art will appreciate that the key concept of the present invention is the slow reduction of thickness of the active material under a load, and that it is anticipated that other materials will have this property so that reference to the active material as bubble wrap only should not be seen as limiting. Indeed it is anticipated that foam materials (e.g. (without limitation) blown

polyurethane foam materials) which compress slowly may also be used with the invention. According to another aspect of the present invention there is provided a formwork for a concrete slab, wherein the formwork includes a layer of active material having a thickness, wherein the formwork is configured to support the load of the poured concrete and the active material is configured to decrease in thickness under the load over the time it takes for the concrete to set.

In a preferred embodiment the layer of active material is oriented substantially parallel to a plane of a support surface of the formwork, wherein the support surface is the surface of the formwork which, in use, supports and is situated closest to the poured concrete. In some instances the support surface may be in direct contact with the poured concrete, but in others there may be a sheet of material over the support surface, such as a sheet used to form a damp proof membrane (DPM).

In a preferred embodiment the formwork includes a structural element substantially as described above.

In a preferred embodiment a surface of the structural element forms the support surface of the formwork. Preferably the surface of the structural element that forms the support surface is a backing sheet.

An advantage of using a backing sheet as the support surface is that it may protect the active material from scratches or other damage that could occur when forming the formwork or when pouring concrete onto or around the formwork. Other advantages may include lowering the frictional forces between the slab and the support element and between the support element and the remaining formwork. Additional thermal insulation from a suitably chosen backing sheet, as well as adding to the integrity of the structural element under load, may help to protect the active material from the heat released by the concrete during curing and setting, as well as reducing heat loss through the slab. In a preferred embodiment the formwork includes a plurality of structural elements.

In a preferred embodiment the plurality of support elements are placed on top of one another such that the backing sheets of each structural element having a coefficient of friction less than 0.4 contact one another.

In a particularly preferred embodiment each outer surface of the support element is formed by a backing sheet wherein a smooth side of the backing sheet has a coefficient of friction less than 0.4 and the other, rough, side of the backing sheet has a coefficient of friction greater than 0.4. In a preferred embodiment a first surface of the support element is formed by a smooth side of the backing sheet.

In a preferred embodiment a second surface of the support element is formed by a rough side of the backing sheet. In a preferred embodiment two support elements are stacked on top of one another with the first, smooth, sides in contact with one another, and the second, rough, sides on the outer sides of the two support elements.

This embodiment may provide an advantage during a seismic event as the outer rough sides of the backing sheet may grip onto the slab (on a top side) and a foundation(on a bottom side) while the two support elements may slide over one another due to the two smooth sides of the backing sheets being in contact. This may further reduce any frictional force between the slab and the support elements.

In another aspect of the present invention there is provided a method of forming a concrete slab to be supported on a top surface of one or more load bearing members, wherein the formwork used to support the concrete when poured includes a support element having a layer of active material which compresses slowly under load, wherein a surface of the support element forms a support surface of the formwork onto which concrete is poured to form the concrete slab, the method including the steps of:

• preparing the formwork to cover a space around the load bearing structure, including under the concrete slab when poured;

• sealing any space between the top surface of the load bearing structure and the support surface to form a continuous surface onto which concrete can be poured; and

• pouring the concrete over the continuous surface, wherein the step of preparing the formwork includes forming the formwork such that an initial level of the support surface is substantially at the same level as the level of the top of the load bearing structure, and wherein the thickness of the active material of the support element reduces slowly over 2 to 10 days under the load of the poured concrete slab. Preferably the method includes the step of placing reinforcing elements in the volume to be filled by the slab prior to the step of pouring the concrete. In a preferred embodiment the top of the one or more load bearing members includes a base isolation device.

In a preferred embodiment the base isolation device is a slider.

In a preferred embodiment the slider is a friction pendulum slider.

The present invention may have a number of advantages over prior art formworks and methods, including:

• production of a low to zero friction interaction between the concrete slab and the formwork by significantly lowering the normal force supported by the formwork, allowing the slab to move relatively easily over the formwork during a seismic event;

• providing a method for producing two concrete slabs one on top of the other in close proximity, where they slabs are held apart either by a device such as a seismic isolator or by the formwork of the present invention, and where the slabs can move relative to one another with relatively low frictional forces between them.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings in which:

Figure 1 shows a support element according to one embodiment of the present invention;

Figure 2 shows a formwork according to one embodiment of the present invention;

Figure3 shows a) - an exploded perspective view of a method of forming a slab

according to one embodiment of the present invention; b) - a cross sectional view soon after the slab is poured; and c) - after hardening of the slab;

Figure 4 shows a cross sectional schematic view of a foundation and formwork

according to one embodiment of the present invention;

Figure 5 shows a cross sectional schematic view of an apparatus for forming a

seismically isolated slab according to the embodiment of the present invention using the formwork illustrated in Fig. 4; Figure 6 shows views of an experimental arrangement for testing the method of the present invention - a) perspective view and b) - cross sectional view; and

Figure 8 shows the results of the experiment using the arrangement of Figure 7.

BEST MODES FOR CARRYING OUT THE INVENTION With reference to Figure 1 there is shown a support element according to one embodiment of the present invention, generally indicated by arrow 1 , including a layer of active material in the form of a layer of Bubble Wrap™ 2. In other embodiments the active material may be any layer of compressible material, and preferably a compressible material where the rate of change of the thickness of the layer of material (i.e. creep) under load is relatively slow (i.e. takes up to 10 or more days).

The embodiment illustrated in Figure 1 shows a layer of active material formed from five layers of bubble wrap 2 placed one on top of another. In other embodiments the layer of active material may have any number of layers of bubble wrap, one on top of another, depending on the desired load carrying capacity and desired rate of compression, among other factors. For example, a relatively larger number of layers may be required to support a building structure having a large mass and to compensate for compression of the support element due to the additional weight. In other situations the support elements to be used with relatively light building structures may only require 1 or 2 layers of active material.

The size, shape and permeability of the bubble wrap layer may be chosen to suit the particular application.

The layers of bubble wrap 2 in Figurela are sandwiched between a first backing sheet 3 and a second backing sheet 4. Backing sheet 3 has an outer surface formed from aluminium foil and includes an interior layer of insulating material such as XPE foam. A thermally active layer may provide insulation, or may be configured to reflect heat produced as the concrete sets back into the concrete. In some embodiments backing sheet 4 also includes a thermally active layer. Further thermal insulation between the backing sheets 3 and 4 is provided by the air bubbles in the layers 2 of bubble wrap.

Backing sheet 4 has an outer surface having a relatively low coefficient of friction, such as a PE coated or laminated Kraft paper foil. In other embodiments a structural element may include only a single backing sheet rather than two. Still other embodiments may have any number of backing sheets or intermediary backing sheets between layers of active material/bubble wrap. A structural element having one or more internal backing sheets may provide greater cohesion between adjacent layers, particularly for those structural elements having a relatively large number of layers of bubble wrap.

It will be appreciated that in some embodiments the surfaces of the bubble wrap may suffice for use as a backing sheet or sheets. It will be appreciated that the properties of the support element 1, and of the backing sheet 3 and 4, may vary depending upon the substrate upon which the support element is to be placed. For example, if the substrate is a substantially rigid, flat, structure the support element 1 may be flexible and the backing sheets 3 and 4 only serve to prevent the flow of concrete into the active material. In other situations the support element 1 may be supported on an irregular or partially open surface, such as scaffolding or a temporary support structure. In such cases the first backing sheet 3 may be formed from a rigid material of the required thickness, or an additional rigid layer may be used to provide support to the concrete.

The layer of active material 2 has an initial thickness, being the distance between the interior faces of backing sheets 3 and 4, i.e. the thickness of the five layers of bubble wrap when not under load (other than atmospheric pressure).

A feature of the present invention is that the thickness of the active layer, i.e., the combined thickness of the five layers of bubble wrap 2 shown in Figure 1 , decreases when a load is applied to the surface (3 or 4) of the structural element 1. Initially the load will flatten the bubbles leading to an initial, relatively fast decrease in thickness of the structural element as the layers of bubble wrap adjust to supporting the load.

However it is a feature of bubble wrap (and other active materials according to the present invention) that the thickness continues to decrease under the continued application of the load. In the case of bubble wrap, after the initial flattening the bubbles slowly lose air due to the permeability of the polyethylene film forming the bubbles of bubble wrap when under load. The rate of air loss, and hence the rate of flattening of the film for a given load, may be controlled by choosing a suitable permeability for the bubble wrap material. This may involve choosing a different material to form the bubbles of the bubble wrap, or increasing (or decreasing) the thickness of the walls of the bubbles to decrease (or increase) the rate of compression of the layer of bubble wrap. A feature of the present invention is to use the load bearing properties of the layer of bubble wrap to support the load while the concrete sets, but to do it in a way whereby the load progressively transfers from the bubble wrap to other load bearing structures or devices. In the case of forming a concrete slab, the rate of compression is chosen to provide the required decrease in thickness over the time that the concrete forming the slab sets (typically 2 to 10 days).

A formwork according to a preferred embodiment of the present invention is generally indicated by arrow 5 in Fig. 2. In this embodiment the layer of active material consists of two support elements generally indicated by brackets 6 and 7. Each support element is bonded to a backing sheet having a smooth outer surface 8 and a rough outer surface 9. The smooth surface is indicated by a bold line in Fig. 2, while the rough surface is indicated by the short vertical lines extending from the bold line. Several layers of bubble wrap 10 (3 layers in Fig. 2) are sandwiched between the two backing sheets.

The support elements 6, 7 of the formwork 5 are in turn supported on a rigid platform indicated by 18 in Fig. 2. This may be a concrete slab of the foundation, or it may be a rigid platform of a traditional framework. The formwork in the latter case may also include scaffolding etc. as in the formwork of the prior art.

Concrete is poured onto the rough surface 9 of the upper support element 6 (see Fig.3a) to form a slab 17.. Figure 3a shows an expanded view of a formwork 16 as used in the construction of a seismically isolated concrete slab 17 formed on a foundation 18. The seismically isolated concrete slab 17 is supported on seismic devices in the form of sliders each having a slider base 19 and a slider plate 20, where the slider plate 20 and an upper surface 21 of the slider base 19 are designed and configured to slide readily across one another during a seismic event.

In other embodiments the seismic device may be any other suitable device as well known in the art, such as lead-rubber base isolators.

The formwork 16, which is essentially as illustrated in Fig. 2, includes a support element 22 which is placed over the foundation 18 and around the slider base 19 of the slider. This is achieved by forming apertures 23 through the formwork 16 to coincide with the location of the slider base 19. The lateral dimensions of the apertures 23 are such that they are larger than the lateral dimension of the slider base 19, and less than the lateral dimensions of the slider plate 20. This arrangement provides an empty space around each slider base (see 29 in Fig. 4) which may provide a buffer zone if the support element moves relative to the slider base during a seismic event. Furthermore, when the upper end of the aperture 23 if covered by a slider plate 20, the slider plate rests on all sides on the formwork 16, essentially providing a continuous surface (once taped etc.)

In preferred embodiments the support element 22 is in the form of a sheet as illustrated for example in the layer 6 of Fig.2. In alternative embodiments the support element 22 may be in the form of a panel or a tile. In embodiments where the support elements are in the form of panels or tiles an aperture may not be required as the panels or tiles may be arranged to predominantly fill the space over the foundation and around the bases of the sliders. Once in position a contact surface 24 of the support element 22 is positioned at substantially the same height as the top surface 21 of the slider base 19. A slider plate 20 is positioned over each aperture 23. The slider plates 20 provide an interface between the isolated slab 14 and the top surface 21 of the slider base 19. The slider plates 20 also function to create, with the support element 22, a formwork that the isolated slab 14 is formed on. Adjacent panels, tiles or sheets of support element 22 are joined with tape to provide a waterproof seal.

A layer 25 of damp proof membrane (DPM) is placed over the layer formed by the support element 22 and the slider plates 20, sealing any space between the top surface of the load bearing member and the support surface to form a continuous surface onto which concrete is poured. The DPM may provide protection to the outer surface of the formwork 16 from the abrasive concrete slurry (when poured) as well as forming a damp proof barrier below the isolated slab 17.

Prior to pouring the concrete slurry the slider bases 19 and slider plates 20 need to be fixed relative to one another and to the foundation slab 18. This is achieved by pinning the slider bases and the slider plates to the foundation 18 and to the isolated slab 17 (when poured) respectively, as illustrated schematically in Fig. 4. This cross section shows two rebar pins, 26, which are used to secure the slider base 19 in position on the foundation 18. A fixing plate 27 is placed over the slider plate 20 and two further rebar pins 28 are located in channels formed through the fixing plate 27, DPM 25, slider plate 20 and into the top of the slider base 19, thus fixing these components in place relative to one another. Fig.4 also shows the open space 29 between the support element 16 (and hence the formwork) and the slider base 19. It is also clear from Fig. 4 that the slider plate 20 overlaps with the top surface of the formwork.

The isolated slab 17 is formed on top of the support element 16 of the formwork by pouring concrete slurry into the formwork, as shown in Fig. 5. The support element 16 compresses under the weight of the concrete such that typically the height of the lower face of the isolated slab 17 is below the upper surface of the slider plate (see arrows 30). However, there is sufficient load carrying capacity in the active layer of the formwork to support the weight of the concrete slurry as it sets. As time goes on the bubble wrap 10 forming the active layer (7, 8) continues to slowly lose air from within the bubbles due to the permeability of the film containing the bubbles when under pressure due to the weight of the concrete being supported. This causes the thickness of the active layer to slowly decrease as the concrete sets. As the concrete sets there is a natural tendency for the load to be progressively transferred to any load bearing members under the slab - in this case the slider bases 19 on top of the load bearing members. When hardened it is anticipated that 90% or more of the total weight of the suspended slab will be carried by the slider bases 19/support members, and less than 10% by the formwork. This reduction in load bearing may reduce the frictional force between the slab 17 and the formwork by as much as 90% or more. A further reduction may be expected as a result of the low coefficient of friction surfaces 12 and 13 (Fig. 2) between the two structural elements forming the active layers 6, 7 of the formwork 16.

When the concrete has hardened sufficiently the pins 28 are removed from the fixing plate 27 as the slider plate 20 will be fixed in the slab 17. The shafts that retained the pins 28 can be closed over and finished off at the surface.

Support for the behaviour of a formwork including an active layer may be tested using the apparatus shown schematically in Figure 7, which shows an experimental arrangement for testing the load bearing performance of a support element in accordance with one preferred embodiment of the present invention. The experimental arrangement includes a formwork 31 including a wooden boxing 32 (500 x 500 x 250 mm) which is positioned on top of a support element 33 having a foil layer 34 and 6 layers of bubble wrap 35. The support element 33 is located on a foundation 36 including a flat timber panel 37 and a weighing scale 40. The weighing scale thereby measures the weight of timber 37, support element 33 and, most significantly, the weight being supported by the support element 33. Four rebars 41 are positioned at the corners of the formwork 31. The rebars 41 pass through the timber panel 37 and support element 33 and sit directly on the concrete floor 38. The rebars 41 were monitored for creep to determine whether the support element 33 reduces in thickness too quickly. If the support element 33 reduces too quickly the rebars 41 will be pushed up through the timber panel 37 and support element 33. The wooden boxing 32 erected on top of support element 33 was filled with wet concrete slurry 42 (aggregate (0.06 m³⁾ + cement (20 kg) + water (8 I)).

The level of the concrete was marked on the rebars 41 at the point 43 at which they entered the concrete.

The weight borne by the scale 40 was monitored over a period of 260 hours. The aim of the experiment was to measure any change in the weight supported by the support element 33 as the concrete hardened.

The results are illustrated in Figure 8, which shows the weight measured by the weighing scales 40 as a function of elapsed time, in hours, since the concrete was poured.

Measurements were taken every hour. After 260 hours the scale 28 measured 14.5 kg (or 10% of the original weight).

After 260 hours the creep on the bars 29 was checked and a variation of about 1 mm was measured. This correlates with the known degree of shrinkage of concrete during the setting process, indicating that the concrete had hardened sufficiently during the time spent supported by the bubble wrap 35 for the concrete to develop sufficient structural integrity to maintain its own weight.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof as defined in the appended claims.

Claims

WHAT I CLAIM IS:

1. A support element for use as a component of a formwork for producing a concrete slab, the support element including a layer of active material attached to a backing sheet, the active material having a thickness which reduces slowly over 2 to 10 days under the load of the poured concrete slab.

2. A support element as claimed in claim 1 wherein the backing sheet has a coefficient of friction less than 0.4

3. A support element as claimed in either one of claims 1 or 2 wherein the backing sheet includes thermal insulation.

4. A support element as claimed in either one of the preceding claims wherein the layer of active material is sandwiched between two backing sheets.

5. A support element as claimed in any one of the preceding claims wherein the thickness of the active material is in the range from 10 mm to 50 mm.

6. A support element as claimed in any one of the preceding claims wherein the active material includes an enclosed void.

7. A support element as claimed in claim 6 wherein the enclosed void contains a fluid.

8. A support element as claimed in claim 7 wherein the fluid is air.

9. A support element as claimed in any one of claims 6 to 8 wherein a surface of the

enclosed void is flexible.

10. A support element as claimed in claim 9 wherein the surface of the enclosed void is formed from a plastics material.

11. A support element as claimed in claim 10 wherein the plastics material is polyethylene film.

12. A support element as claimed in any one of claims 9 to 11 wherein the surface of the enclosed void is permeable to the fluid contained within the enclosed void when under a load.

13. A support element as claimed in claim 12 wherein the permeability of the surface of the enclosed void is such that, under the load of the poured concrete slab, at least some of the fluid escapes over a period lasting from 2 to 10 days.

14. A support element as claimed in any one of the preceding claims wherein the active layer includes a plurality of enclosed voids.

15. A support element as claimed in claim 14 including a plurality of layers of enclosed

voids.

16. A support element as claimed in either one of claims 14 or 15 wherein the layer of active material is formed from an array of air filled enclosed voids.

17. A support element as claimed in any one of the preceding claims wherein a surface of the structural element forms a support surface of a formwork onto which concrete is poured to form the concrete slab.

18. A support element as claimed in claim 17 wherein the support surface is a backing sheet of the support element.

19. A method of forming a concrete slab to be supported on a top surface of one or more load bearing members, wherein the formwork used to support the concrete slab when poured includes a support element having a layer of active material which compresses slowly under load, wherein a surface of the support element forms a support surface of the formwork onto which concrete is poured to form the concrete slab, the method including the steps of:

• preparing the formwork to cover a space around the load bearing

structure, including under the concrete slab when poured;

• pouring the concrete over the continuous surface, wherein the step of preparing the formwork includes forming the formwork such that an initial level of the support surface is substantially at the same level as the level of the top of the load bearing structure, and wherein the thickness of the active material of the support element reduces slowly over 2 to 10 days under the load of the poured concrete slab.

20. A method as claimed in claim 19 including the step of • placing reinforcing elements in the volume to be filled by the concrete slab prior to the step of pouring the concrete.

21. A method as claimed in either one of claims 19 or 20 wherein the surface of the

structural element that forms the support surface is a backing sheet.

22. A method as claimed in any one of claims 19 to 21 wherein the formwork includes a plurality of structural elements.

23. A method as claimed in any one of claims 19 to 22 wherein the formwork includes two or more support elements placed on top of one another.

24. A method as claimed in claim 23 wherein the support elements are placed on top of one another such that the backing sheets of each structural element having a coefficient of friction less than 0.4 contact one another.

25. A method as claimed in any one of claims 19 to 24 wherein the top of the one or more load bearing members includes a base isolation device.

26. A method as claimed in claim 25 wherein the base isolation device is a slider.

27. A method as claimed in claim 25 wherein the slider is a friction pendulum slider.

28. A method as claimed in claim 25 wherein the base isolation device is a lead-rubber base isolation device.