AU2010246376B2

AU2010246376B2 - Environmentally degradable void former

Info

Publication number: AU2010246376B2
Application number: AU2010246376A
Authority: AU
Inventors: John D'amici; Garyth Wignall
Original assignee: Superslab Tech Pty Ltd
Current assignee: Superslab Tech Pty Ltd
Priority date: 2009-11-23
Filing date: 2010-11-23
Publication date: 2014-07-24
Anticipated expiration: 2030-11-23
Also published as: US20110120036A1; AU2010246376A1; NZ589460A; AU2011201876A1; AU2011201876B2

Abstract

There is disclosed herein a void forming and suspension system (10), which generally consists of a plurality of void forming elements (12, 16) arranged to extend 5 over a ground surface (22) upon which the building is to be constructed. As will become more apparent in the following description, the void forming system (10) functions to act as an interface between the ground surface (22) and a concrete slab (20) of the building to be constructed, while the slab is being formed. Once the slab (20) is formed and the wet concrete for forming the slab has hardened, the void forming 10 system degrades to create a cavity into which swelling ground can expand. c:\nrportbI\syddocs\jphX1292183_1 doc f414 * .f

Description

I AUSTRALIA FB RICE & CO Patent and Trade Mark Attorneys Patents Act 1990 SUPERSLAB TECH PTY LTD COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Environmentally degradable void former The following statement is a full description of this invention including the best method of performing it known to us:- 2 Cross Reference to Related Applications This application is related to Australian Provisional Patent Application No. 2009905738, the entire disclosure of which is incorporated herein by way of reference. 5 Field of the Invention The present invention relates to a void forming apparatus and suspension system for a building/construction and in particular to an apparatus and method for creating voids under a structural concrete slab of a building/construction such that the 10 building/construction is suspended above the ground and is able to withstand changes in the level of the underlying ground support. Background Art 15 Buildings, such as residential homes or commercial buildings, generally rely upon a solid footing to support the considerable weight of the structure. The footing generally provides a flat and level base upon which the building is supported, with the base being in communication with the underlying ground surface. Many modem buildings are built upon a flat, concrete slab that provides both a base for the structure 20 as well as the bottom floor for the building. The concrete is typically placed over a prepared ground surface and is shaped to the desired dimensions to form the footings of the building. This may be achieved through laying pre-cast components of concrete, or through pouring wet concrete into a mould and allowing the concrete to set in position. Typically such concrete slabs consist of a system of structurally engineered concrete 25 beams and platforms, which together form what is collectively referred to as a slab. A common problem with concrete slabs supported on a ground surface is that large areas of land available for residential development have soil profiles with high clay content. Such soil profiles are not considered stable, as the surface level of the soil 30 can change as the moisture content of the soil changes. This is due to the volume of the soil being directly dependent upon the soil's moisture content. In particular, as the moisture content of the soil increases, the volume of the soil increases, resulting in the surface level of the soil changing. Such a phenomena causes uplifting of a concrete slab that may be supported on the surface of the soil. When this uplifting of soil 35 occurs, the concrete slab formed on this surface is said to have "heaved". With such c:\nrportbl\syddocs\jph\1292183_1.doc 3 soils, when their moisture content reduces, the soil reduces (shrinks or settles) in volume. Heaving and settling of soil due to moisture content change therein tends to 5 occur at varying rates across the surface plane of the soil. This variance is termed differential movement. As a concrete slab being supported on the surface of such soil will heave and settle accordingly, this differential movement can result in a concrete slab supported on such a soil surface experiencing a variety of loading forces over the life of a building construction. 10 The factors contributing to variations in soil moisture content, which generates such concrete slab movements, are various and complex. Some common factors include: natural moisture variations due to seasonal considerations such as wet and dry seasons; soil drying due to trees growing or being introduced into a zone of influence of 15 the soil; soil wetting due to the removal of trees and other related flora such that previously drier soils re-hydrate resulting in the phenomena of "rebound" occurring in the soil; garden watering (or lack thereof) by owners/occupiers of a subject site; or a combination of any of these sources. Any or all of these factors can significantly alter the soil moisture content, resulting in a concrete slab deflecting or becoming damaged. 20 This can result in damage to the super-structure supported on the slab, such as a residential dwelling or the like, requiring time-consuming and expensive corrective action. Typically, soil moisture content is not consistent and may vary across regions, 25 resulting in further differential shrinking/swelling of the soil. Historically, construction areas have been chosen to avoid soils having a high clay content to reduce the occurrence of soil heave. However, as populations increase, more and more land having such high clay content soil is being used for development. In this regard, areas which have previously been avoided such as beach/river frontages; steeply sloping 30 hinterland areas beyond the coastlines; and the grazing plains (some of which are flood prone) which surround most cities and towns, are increasingly becoming the site for a variety of constructions and developments that have previously been considered unsuitable for such a soil base. 35 A number of methods have been proposed to address the soil heave phenomena and to prevent highly expansive soils from damaging the structures supported on such c:\nrportblNsyddocsjph\1 292183_1.doc 4 soil. Typically, these methods are employed prior to construction and rely upon chemical treatment of the soil; engineered fill and compaction of the soil; and/or the formation of a void space beneath the concrete slab to accommodate expansion of the soil into these voids without damaging the structure. 5 Most void systems previously proposed have been arranged such that the beams of the concrete slab are placed between the void forming elements. Conventionally, the beam regions of the concrete slab do not have a void space and hence are more susceptible to soil heave. 10 Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority 15 date of each claim of this application. Summary of the Invention Throughout this specification the word "comprise", or variations such as 20 "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. In a first aspect, the present invention provides an apparatus for creating a void 25 beneath a structural concrete slab, comprising: a body having a first surface adapted to support wet concrete for forming at least a portion of the concrete slab and a second surface adapted to contact a ground surface; wherein the body is partly or fully formed of a non-cellulosic biodegradable polymer, and wherein the biodegradable polymer retains sufficient structural strength to 30 support the structural slab until the concrete has cured sufficiently to be self-supporting, but thereafter degrades to create a cavity into which swelling ground can expand. In one embodiment, the first surface is located remote from the second surface such that the at least a portion of the concrete slab is separated from the ground surface. 35 In one form, the first surface and the second surface are separated by one or more wall members. The one or more wall members may extend from said first surface at a 5 proximal end terminating at a distal end. In this regard, the second surface of the body may define the distal end of the one or more wall members. Alternatively the apparatus may comprise an enclosed form. The apparatus may include one or more cavities or may be a solid block of low density material or water degradable material or water 5 soluble material or include a combination of these features. The body may include at least one cavity extending at least partially from the second surface toward the first surface. The at least one cavity may be open to the ground surface to accommodate any rise that may occur in the ground surface 10 following positioning of the member beneath the slab, or may be closed by a continuous floor member in which case rise in the surface may be accommodated by degradation of at least part of the body. The at least one cavity may be formed by the one or more wall members and the first surface. 15 The body may have a thin-walled cellular structure, which may be formed by thermoforming or vacuum forming. One or more supporting struts may be provided within the cavity. The struts may extend from the first surface at a proximal end to the second surface at a distal 20 end. In this regard, the distal end of the struts may contact the ground surface to provide support to the first surface or may be supported on a floor member closing the cavity. The one or more struts may be regularly spaced within the cavity. In one form, the struts are deformable such that any rise of the ground surface into the cavity will cause deflection of the struts. Once the body begins to degrade, deflection or 25 disintegration will occur when the ground level raises. The body is therefore preferably sacrificial, and will collapse rather than transmit ground movement to the slab. According to a second aspect, the present invention provides a system for creating a suspended structural concrete slab, comprising: 30 at least one pile member, the or each pile member having a first end and a second end, the first end adapted to be embedded into a ground surface to be in contact with load bearing ground beneath said ground surface and the second end arranged to extend a position at or near said ground surface; and at least one void forming element, the or each void forming element being 35 formed of a non-cellulosic biodegradable polymer and being arranged to substantially 6 cover a ground surface upon which the structural slab is to be placed and to define a surface onto which wet concrete for forming the structural slab is to be poured; wherein said at least one pile member is adapted to support the structural slab above the ground surface such that said at least one void forming element is positioned 5 between the structural slab and the ground surface; and wherein the biodegradable polymer retains sufficient structural strength to support the structural slab until the concrete has cured sufficiently to be self-supporting. According to a third aspect, the present invention provides a method of forming 10 a suspended structural concrete slab, comprising: installing at least one pile member, the or each pile member having a first end and a second end, the first end adapted to be embedded into a ground surface to be in contact with load bearing ground beneath said ground surface, and the second end arranged to extend to a position at or near said ground surface; 15 installing at least one void forming element, the or each void forming element being formed of a non-cellulosic biodegradable polymer and being arranged to substantially cover a ground surface upon which the structural slab is to be placed, the pile members extending above at least part of the surface formed by the at least one void forming element; and 20 pouring wet concrete to form the structural slab, whereby the structural slab is supported above the ground surface by the at least one pile member such that the at least one void forming element is positioned between the structural slab and the ground surface, wherein the biodegradable polymer retains sufficient structural strength to 25 support the structural slab until the concrete has cured sufficiently to be self-supporting. In embodiments of any of the first to third aspects above, the void forming elements may be formed by injection moulding, rotational moulding or vacuum moulding the biodegradable polymer. The biodegradable polymer may be adapted to be degraded by 30 contact with water and/or soil. The biodegradable polymer may be compostable. The biodegradable polymer may be a manufactured biopolymer, such as a thermoplastic starch polymer (eg. the biopolymer sold under the trade name PlanticTM). Alternatively, the biodegradable polymer may be PLA, Mater-bi, Biograde, or a range of biodegradable polyesters such as PHA, PBAT, or PBSA. A barrier may be provided 35 between the biodegradable polymer and the structural slab. The barrier may take the form of a film of moisture inhibiting material or a moisture inhibiting coating on the 7 biodegradable polymer. Alternatively, or in addition, a thickness of the biodegradable polymer may be selected to provide the biodegradable polymer with a desired structural longevity. The biodegradable polymer is preferably adapted to retain sufficient structural strength to support the concrete slab for at least 12 hours, more preferably at 5 least 24 hours, and in some embodiments for at least 48 hours. In a dry, undegraded, granular state, the biodegradable polymer preferably has a density of more than 1 g/cm3, and more preferably between 1 g/cm3 and 2 g/cm3. In a dry, undegraded, granular state, the biodegradable polymer preferably has a maximum tensile stress of between 10 MPa and 100 MPa, and more preferably between 15 MPa and 50 MPa. In 10 a dry, undegraded, granular state, the biodegradable polymer preferably has a Young's Modulus of between 500 MPa and 5000 MPa, and more preferably between 500 MPa and 3000 MPa. The biodegradable polymer may degrade into a biocompatible component of soil underneath the concrete slab. 15 In one embodiment of the second aspect, the at least one void forming element is an apparatus according to the first aspect. The at least one void forming element may be arranged such that the second end of the at least one pile member extends therethrough. 20 The pile members may engage the structural slab. The engagement may be in the form of a wider portion located in the upper part of the pile such that the concrete formed over the pile surrounds and engages the widened portion and a narrower portion beneath it. The wider portion may comprise a rod or bar extending horizontally through a hole in a vertical portion of the pile located above the lower extremity of the 25 structural slab. The piles may be located in beam portions of the structural slab and the rod or bar may extend longitudinally of the beam portions.

8 In another embodiment, the at least one pile member is a screw pier foundation pile. The second end of the at least one pile member may be raised above the ground surface to a predetermined height. Where more than one pile member is employed, the pile members may be arranged in a grid to form a consistent and level array of loading 5 points above the ground surface. The structural slab may be a concrete slab consisting of a plurality of platforms and beams. The structural slab may be supported above the ground surface by the beams receiving the second end of the at least one pile member. The depth to which 10 the second ends of the piles are received by the beams is determined by the bearing capacity of the building/construction. In this regard, the installed height of the pile member above the ground enables the second end of the pile members to penetrate to a desired depth into the beams of the structural slab. In this regard, the piles are subsequently structurally connected to the concrete slab within the beam. 15 Brief Description of the Drawings By way of example only, embodiments of the present disclosure are now described with reference to the accompanying drawings, in which: 20 Fig. 1 is a perspective view of a void forming and suspension arrangement of the present disclosure; Fig. 2 is a cross sectional side view of the void forming and suspension 25 arrangement of Fig. 1, supporting a concrete slab; Fig. 3 is a plan view of the slab of Fig. 2; Fig. 4 is a perspective view of an embodiment of a spacer void forming element 30 of the void forming arrangement of Fig. 1; Fig. 5 shows a plan view of a second embodiment of a spacer void forming element of the void forming arrangement of Fig. 1; 35 Fig. 6 shows an elevation view of the embodiment of a spacer void forming element of Fig. 5 with a first height; c:\nrportbl\syddocs\jph\1 292183_1.doc 9 Fig. 7 shows an elevation view of the embodiment of a spacer void forming element of Fig. 5 with a second height; 5 Fig. 8 shows (a) plan, (b) elevation and (c) end elevation views of an embodiment of a shallow void forming element of the void forming arrangement of Fig. 1; Figs. 9a shows a underside perspective view of an alternative embodiment of the void forming elements of Figs. 5-7, and Fig. 9b shows a plurality of the void forming 10 elements of Fig. 9a grouped together in abutting relationship; and Figs. 10a and l0b show underside perspective and cross-sectional views, respectively, of a further alternative embodiment of the void forming elements of Figs. 5-7. 15 Detailed Description of an Exemplary Embodiment of the Present Invention A void forming and suspension system 10 in accordance with the present disclosure is shown in Fig. 1. 20 The void forming and suspension system 10 generally consists of a plurality of void forming elements 12, 16 arranged to extend over a ground surface 22 upon which the building is to be constructed. As will become more apparent in the following description, the void forming system 10 functions to act as an interface between the ground surface 22 and a concrete slab 20 of the building to be constructed, while the 25 slab is being formed. Once the slab 20 is formed and the wet concrete for forming the slab has hardened, the void forming system is superfluous and can be allowed to collapse, reducing the risk of swelling ground heaving the slab. Prior to positioning the void forming system as shown in Fig. 1, a plurality of 30 load bearing compression and tension piles 14 are driven, drilled, jacked, tied or otherwise fixed into the deeper soil or rock of high bearing capacity to provide stability to the structure which is to be built. The piles 14 may be in a variety of forms, such as screw piles which communicate directly with the load bearing ground beneath the ground surface 22. The piles 14 perform in both tension and compression and are 35 raised above the ground surface to a predetermined height. In this regard, the piles are c:\nrportbi\syddocs\jph1 292183_1.doc 10 arranged in a grid to form a consistent and level loading region above ground upon which to found the beams of the finished concrete slab. The depth to which the piles 14 are installed is determined in relation to where 5 they will achieve the engineered and designed bearing capacity for the said building/construction. The installed height of each pile 14 above ground will allow the end of the piles 14 to penetrate to an engineered depth into the finished beams of the concrete slab above. The piles 14 are subsequently structurally connected to the concrete slab within the beam. It will be appreciated that the size and shape of the slab 10 can be readily adapted to suit the size and shape of the building to be constructed. As such, the placement location and number of piles 14 utilised is variable, though based on the relevant modular system engineered design for that project. Following the positioning of the piles 14 in accordance with the design of the 15 structure to be built, void forming elements 12 are positioned over and around the piles 14. The void forming elements 12 are in the form of elongate shallow members which may be designed to effectively absorb extreme ground movement in the short term, as well as allow the piles 14 to pass therethrough. In the long term, the void forming elements 12 will disintegrate due to their construction from environmentally degradable 20 material, as discussed in detail below. As the void forming elements 12 extend along the piles 14, they are typically arranged in parallel rows in accordance with the specific design of the structure to be built. It will be appreciated that the arrangement as shown in Figs. 1 to 3 represents a substantially rectangular footing wherein the piles are regularly spaced and positioned in rows and hence the void forming elements 12 extend 25 in rows, however it will be appreciated that the footing structure may vary in accordance with the specific design of the building to be constructed. Between each of the rows of shallow void forming elements 12, there are provided spacer void forming elements 16. The spacer void forming elements 16 are in 30 the form of elongate void former structures, which are also partially or fully formed of an environmentally degradable material and may also be deformable to accommodate short term movement of the ground surface. Over time, the spacer void forming elements 16 will degrade, due to contact with moisture or soil, and collapse, leaving the concrete slab 20 supported substantially entirely by the piles 14. The spacer void 35 forming elements 16 extend between each of the shallow void forming elements 12, as shown, to provide an additional surface, elevated from the ground surface, offering a c:\nrportbl\syddocs\jph\1292183_1.doc 11 holistic void forming body upon which the concrete slab (platform regions and beams) can be placed. In this regard, the platform and beam regions of the concrete slab are supported upon the spacer void forming elements 16 and the shallow void forming elements 12 and are suspended from the ground surface upon placement. 5 The void forming elements 12, 16 are formed by injection moulding the environmentally degradable material. The environmentally degradable material is a biodegradable, manufactured biopolymer, preferably a thermoplastic starch polymer formulated for injection moulding. The material is also compostable under ambient 10 conditions, such as those in a conventional vegetable waste compost environment. A suitable material is sold under the trade name PlanticTM. PlanticTM is renewable sourced biodegradable polymer made from non-genetically modified corn starch. Several forms of PlanticTM are available, some of which are water dispersible and/or injection mouldable. In a dry, undegraded, granular state, the environmentally 15 degradable material has a density of between 1 g/cm 3 and 2 g/cm 3 , a maximum tensile stress of between 15 MPa and 50 MPa, and a Young's Modulus of between 500 MPa and 3000 MPa. Other suitable environmentally degradable materials are PLA, Mater biTM, BiogradeTM, or a range of biodegradable polyesters such as PHA, PBAT, or PBSA. In any case, the environmentally degradable material has a degradation rate 20 appropriate to provide sufficient structural strength to support the structural slab until the concrete has cured sufficiently to be self supporting. The timeframe for the slab 20 to become self-supporting varies based on a number of factors, including the nature of the concrete mix for forming the slab, the weather conditions and the slab design. Typically, however, the slab will be self-supporting in around 24 hours. In order to 25 slow the degradation rate of the environmentally degradable material, a barrier, in the form of a plastic film, may be provided between the void forming elements 12, 16 and the slab 20 to inhibit passage of moisture from the slab, especially in the first 24 hours after it has been poured. In other embodiments, the barrier may take the form of a coating on the environmentally degradable material. Alternatively, or in addition, the 30 thickness of the environmentally degradable material may be selected to provide the desired structural longevity. After the slab has sufficiently cured, the degradable material degrades into a biocompatible component of soil underneath the concrete slab. It will be appreciated that prior to setting and positioning the void forming 35 system 10 of Fig. 1, the area upon which the void forming system is placed is prepared. Such preparation typically comprises excavating the site to form a level surface upon c:\nrportbl\syddocs\jph\1 292183_1.doc 12 which the concrete is to be placed. Trenches can also be dug to position deep edge beams and the like to define the perimeter of the foundation to enable the piles 14 to be positioned as discussed above. The void forming elements 12, 16 can be positioned as shown in Fig. 1, for placement of the concrete. 5 The environmentally degradable material of the spacer void forming elements 16 and the shallow void forming elements 12 can be readily shaped or cut to fit around plumbing pipes and the like, through the use of a hand or power saw, Generally, this may not be necessary as the spacer void forming elements 16 and shallow void forming 10 elements 12 can be custom made and delivered to the building site to suit the building plans provided. It will be appreciated that whilst the void forming elements 12, 16 of one embodiment of the invention are made from an environmentally degradable material, the void forming elements may be made in a variety of shapes and from a variety of materials to enable the void forming system 10 to perform its function, as 15 will be appreciated below. Prior to placement of the concrete slab 20, reinforcing mesh and bar 18 (as shown in Fig. 2), and/or steel fibres, may be placed over the void forming elements 12, 16 to further reinforce the slab 20. Such a mesh material may be particularly applicable 20 if the concrete is to be poured over the void forming elements 12, 16 to form the slab 20. As is clearly evident in Fig. 2, the slab 20 is fully supported above the surface of the soil 22 by the void forming elements 12, 16 and piles 14, thereby completely isolating the slab 20 from the ground soil 22. In this regard, following setting/positioning of the slab 20 and construction of a building over the slab 20, the 25 slab is suspended above the ground surface 22, with the load of the structure being supported by the piles 14 which are in turn supported by the deeper soil or rock of high bearing capacity. It will be appreciated that in this arrangement, the piles 14 have a direct/positive engagement with the slab 20 and the ground, thereby offering load bearing capacity for the concrete slab in both tension and compression. The 30 engagement will typically be in the form of a wider portion located in the upper part of the pile such that the concrete formed over the pile surrounds and engages the wider portion and a narrower portion beneath it. Preferably, the wider portion comprises a rod or bar 15 extending horizontally through a hole in a vertical portion of the pile located above the lower extremity of the structural slab. Preferably also, the piles are 35 located in beam portions 13 of the structural slab and the rod or bar 15 extends longitudinally of the beam portions. cAnrportbl\syddocs\jph\1 292183_1.doc 13 Fig. 3 shows a plan view of one such slab arrangement 20 where the shallow void forming elements 12 are arranged to form two internal parallel beams which extend the length of the slab 20. Perimeter beams 24 are also formed by placement of 5 shallow void forming elements 12 about the perimeter of the site such that the parallel beams work in conjunction with the perimeter beams 24 to form a grid arrangement upon which the slab 20 is supported, separate from the ground surface. It will be appreciated that the size and shape of the slab 20 can be readily adapted to suit the size and shape of the building to be constructed. As such, the void forming and suspension 10 system 10 is a modular arrangement that can be easily assembled to accommodate a variety of sized and shaped slabs. In this regard, a variety of sizes are provided in both the void forming elements 12, 16 to accommodate differing dimensions of the slab design and positions and lengths of the piles 14 to accommodate varying depths of soil penetrated to achieve the appropriate bearing capacities required. 15 Referring to Fig. 4, one embodiment of the structure of the spacer void forming element 16 is shown. The void forming element 16 is in the form of a box-like element having four vertical side surfaces 26 and a top surface 28. The surfaces 26, 28 of the void forming element 16 are configured to interface with the concrete slab 20 in the 20 manner as shown in Fig. 2. The underside 30 of the void forming element 16, namely the side which is supported on the ground surface 22, is open thereby providing a cavity within the void forming element 16. The cavity is defined by the inner walls of the surfaces 26, 28, and in the embodiment as shown in Fig. 4, represents a substantially rectangular space. 25 A plurality of strut members 32 are provided within the cavity and extend from the inner surface of the top surface 28 to the ground surface 22 when the void forming element 16 is positioned thereon for use. The strut members 32 support the top surface 28 of the void forming element 16 along its length such that the top surface 28 is able 30 to support the load of the reinforced concrete during placement of the slab 20, as well as the load of workers or machinery used during the placement process. In this regard, the shape of the struts 32 and their location within the cavity defines the strength of the void forming element. It will be appreciated that the arrangement and shape of the struts 32 can be altered in accordance to the specific construction, Such an 35 arrangement of struts 32 provides structural integrity and enables the void forming elements 12, 16 to carry higher loads without failing or disintegrating during the c:\nrportbl\syddocs\jph\1292183_1.doc 14 formation of the structural slab 20. However this configuration of walls and individual struts also provides an ability to cope with any soil heave that may occur over time as the struts and walls will collapse under pressure such that movement is not transmitted to the slab. Moreover, the strut members 32 may be formed from an environmentally 5 degradable material, such that, like the void forming elements 12, 16, the strut members 32 degrade over time. Again, the strut members 32 may be shielded from moisture by a barrier such as a coating or plastic film, or be formed from an appropriate thickness of environmentally degradable material, in order to increase their structural longevity if required. Whilst not shown, it will be appreciated that the shallow void forming 10 elements 12 may also be constructed in the same manner as the spacer void forming element 16 of Fig. 4. It will be appreciated that the structure and material of the void forming elements 12, 16 ensure that they degrade over time to leave cavities into which soil 15 supporting the slab may expand without heaving the slab, with the void forming elements 12, 16 preferably also being adapted to compress under load during the construction process without deforming or failing should ground swelling occur at this time. In this regard, in the event of soil expansion in the direction of arrows A of Fig. 2, the soil is able to expand into the cavities remaining after degradation of the void 20 forming elements 12, 16 (and the struts 32 if formed from degradable material), where appropriate. In embodiments where the struts 32 are not degradable, since the void forming elements 12, 16 are made from an environmentally degradable material, in the event of soil expanding into the cavities, the struts 32 break away from the body of the degraded void forming elements 12, 16 to further accommodate the expanding soil. 25 Referring to Figs. 5 and 6, a more preferred embodiment of the structure of the spacer void forming element 16 is shown in plan and elevation respectively. The void forming element 16 in this embodiment is in the form of a box-like element having four vertical side members 46, 47 and a top platform member 49 forming the concrete 30 supporting surface 48. Lower portions 46 of the side walls are corrugated and while upper portions 47 of the side walls are corrugated on an inner surface and have the corrugations filled on the outer surface. The members 46, 47 49 of the void forming element 16 are configured to interface with the concrete slab 20 in the manner as shown in Fig. 2. As with the previously described embodiment, the underside 30 of the void 35 forming element 16, namely the side which is supported on the ground surface 22, is open thereby providing a cavity within the void forming element 16. The cavity is c:\nrportbl\syddocs\jphX 292183_I.doc 15 defined by the inner surfaces walls of the members 46, 49, and in the embodiment as shown in Figs. 5 and 6, again represents a substantially rectangular space, although in this instance with corrugated walls. 5 Figs 7 show an elevation of an alternative version of the spacer void forming element 16 having differing heights to that of Fig. 6. In other respects the embodiments shown in elevation in Figs. 6 and 7 are similar to one another. A plurality of strut members 42 are provided within the cavity and extend from 10 the inner surface of the platform member 49 to the ground surface 22 when the void forming element 16 is positioned thereon for use. The strut members 42 provide support for the platform member 49 of the void forming element 16 along its length such that the top surface 48 is able to support the load of the reinforced concrete during placement of the slab 20, as well as the load of workers or machinery used during the 15 placement process. In this regard, the shape of the struts 42 and their location within the cavity defines the strength of the void forming element. In the figure 5 embodiment the struts have a substantially rectangular cross section with two opposing concave sides. It will be appreciated that the arrangement and shape of the struts 42 can be altered in accordance to the specific construction. 20 Fig. 8 illustrates an embodiment of the structure of the shallow void forming element 12. The shallow void forming element 12 in this embodiment is also in the form of a box-like element having four vertical side members 56 (in this case not corrugated) and a top platform member 59 forming the concrete supporting surface 58. 25 The height of the shallow void forming element 12 is preferably the same as the height of the corrugated lower portion 46 of the walls of the spacer void forming element 16. Therefore the overall height of this shallow void forming element 12 is shorter than that of the spacer void forming elements 16 to allow the formation of concrete beams between the spacer void forming elements 16. The members 56, 59 of the shallow void 30 forming element 12 are also configured to interface with the concrete slab 20 in the manner as shown in Fig. 2. A single internal strut 52 is provided extending from the inner surface of the platform member 59 to the ground surface 22 when the void forming element 12 is positioned thereon for use and the strut 52 divides the internal space of the shallow void forming element into two cavities 60. Each cavity is defined 35 by the inner surfaces of walls of the members 56, 59 and the strut 52, and in the embodiment as shown in Fig. 6, again represents a substantially rectangular space. c:\nrportbl\syddocs\jph\1 292183_1.doc 16 Figs. 9a and 9b show an alternative embodiment of the void forming elements 12, 16 of Figs. 5-7, where corresponding reference numerals indicate corresponding features with corresponding functionality. The void forming elements 12, 16 of Figs 9a 5 and 9b include a thin-walled cellular base structure defined by a network of cell walls 70 formed by thermoforming or vacuum forming. A solid top 72 is provided over the cellular base structure to support the curing concrete slab 20. The top 72 may or may not be formed from the environmentally degradable material. It will be appreciated that a smaller amount of environmentally degradable material is required to be used to form 10 the void forming elements of Figs. 9a and 9b as compared to those of Figs. 5-7, which facilitates cost savings. The thin-walled construction of the void forming elements of Figs. 9a and 9b may also assist in crumpling of the void forming elements 12, 16 due to soil heave in situations where the void forming elements have not completely degraded. As shown in Fig. 9b, the void forming elements 12, 16 of Fig. 9a may be grouped 15 together in abutting relationship to define a void form of a desired size for supporting a concrete slab during curing. Figs. 10 a and 10 b show a further embodiment of the void forming elements 12, 16 of Figs. 5-7. The embodiment of Figs. I a and 10b has many features in common with the 20 embodiment of Figs 9a and 9b, where corresponding reference numerals indicate corresponding features with corresponding functionality. As with the void forming elements of Figs. 5-7 and 9a and 9b, the void forming elements 12, 16 of Figs. 10a and 1Gb are formed by injection moulding the environmentally degradable material. Void forming elements 12, 16 as shown in Figs. 10a and 10b may be grouped together, as 25 shown in Fig. 9b, to define a void form of a desired size for supporting a concrete slab during curing. In yet further embodiments (not shown), the void forming elements 12, 16 may have a lattice construction, similar to an inverted milk crate. In such embodiments, the top of 30 the void forming elements 12, 16 may be solid, or of lattice construction with an overlaying solid layer for supporting the curing concrete slab 20. In other embodiments, not shown, the void forming system is formed from a solid block of environmentally degradable material, which degrades over time due to 35 contact with moisture and/or soil to leave a void into which swelling substrate ground 20 can expand without heaving the slab 20. c:\nrportbl\syddocs\jph\1292183_1.doc 17 It will be appreciated that the system described in the embodiment above comprises a concrete slab supported on deep footings, typically screw piles, drilled piles and piles designed for compression and/or tension. The slab is designed to be 5 suspended between the deep footings and the void forming elements allow concreting to be easily accomplished during construction and serves as a compressible/crushable/degradable zone during the life of the slab as the underlying soil expands. 10 It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above described embodiments without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. cAnrportbl\syddocs\jph\1 292183_1.doc

Claims

1. An apparatus for creating a void beneath a structural concrete slab, comprising: a body having a first surface adapted to support wet concrete for forming at least 5 a portion of the concrete slab and a second surface adapted to contact a ground surface; wherein the body is partly or fully formed of a non-cellulosic biodegradable polymer, and wherein the biodegradable polymer retains sufficient structural strength to support the structural slab until the concrete has cured sufficiently to be self-supporting, but thereafter degrades to create a cavity into which swelling ground can expand. 10

2. An apparatus according to claim 1, wherein the void forming elements are formed by one of: injection moulding, rotational moulding, and vacuum moulding the biodegradable polymer.

3. An apparatus according to claim 1 or claim 2, wherein the biodegradable polymer is adapted to be degraded by contact with water and/or soil. 15

4. An apparatus according to any one of the preceding claims, wherein the biodegradable polymer is compostable.

5. An apparatus according to any one of the preceding claims, wherein the biodegradable polymer is a manufactured biopolymer.

6. An apparatus according to claim 5, wherein the biodegradable polymer is a 20 thermoplastic starch polymer.

7. An apparatus according to any one of claims 1 to 4, wherein the biodegradable polymer is one of: polyactide (PLA) and a biodegradable polyester.

8. An apparatus according to any one of the preceding claims, comprising a barrier between the biodegradable polymer and the structural slab. 25

9. An apparatus according to claim 8, wherein the barrier comprises a film of moisture inhibiting material or a moisture inhibiting coating on the biodegradable polymer.

10. An apparatus according to any one of the preceding claims, wherein a thickness of the biodegradable polymer is selected to provide the biodegradable polymer with a 30 desired structural longevity.

11. An apparatus according to any one of the preceding claims, wherein the biodegradable polymer is adapted to retain sufficient structural strength to support the concrete slab for at least 12 hours.

12. An apparatus according to claim 12, wherein the biodegradable polymer is 35 adapted to retain sufficient structural strength to support the concrete slab for at least 24 hours. 19

13. An apparatus according to any one of the preceding claims, wherein the biodegradable polymer has a maximum tensile stress of between 10 MPa and 100 MPa.

14. An apparatus according to any one of the preceding claims, wherein, in a dry, undegraded, granular state, the biodegradable polymer has a Young's Modulus of 5 between 500 MPa and 5000 MPa.

15. An apparatus according to any one of the preceding claims, wherein the biodegradable polymer is adapted to degrade into a biocompatible component of soil underneath the concrete slab.

16. An apparatus according to any one of the preceding claims, wherein the body 10 has a thin-walled cellular structure.

17. A system for creating a suspended structural concrete slab, comprising: at least one pile member, the or each pile member having a first end and a second end, the first end adapted to be embedded into a ground surface to be in contact with load bearing ground beneath said ground surface and the second end arranged to 15 extend to a position at or near said ground surface; and at least one void forming element in the form of the apparatus of any one of the preceding claims, the or each void forming element being arranged to substantially cover a ground surface upon which the structural slab is to be placed and to define a surface onto which wet concrete for forming the structural slab is to be poured; 20 wherein said at least one pile member is adapted to support the structural slab above the ground surface such that said at least one void forming element is positioned between the structural slab and the ground surface.

18. A method of forming a suspended structural concrete slab, comprising: installing at least one pile member, the or each pile member having a first end 25 and a second end, the first end adapted to be embedded into a ground surface to be in contact with load bearing ground beneath said ground surface, and the second end arranged to extend to a position at or near said ground surface; installing at least one void forming element in the form of the apparatus of any one of claims I to 16, the or each void forming element being arranged to substantially 30 cover a ground surface upon which the structural slab is to be placed, the pile members extending above at least part of the surface formed by the at least one void forming element; and pouring wet concrete to form the structural slab, whereby the structural slab is supported above the ground surface by the at least one pile member such that the at 35 least one void forming element is positioned between the structural slab and the ground surface. 20

19. An apparatus for creating a void beneath a structural concrete slab, said apparatus substantially as hereinbefore described with reference to any one embodiment, as that embodiment is shown in the accompanying drawings.

20. A system for creating a suspended structural concrete slab, said system 5 substantially as hereinbefore described with reference to any one embodiment, as that embodiment is shown in the accompanying drawings.

21. A method of forming a suspended structural concrete slab, said method substantially as hereinbefore described with reference to any one embodiment, as that embodiment is shown in the accompanying drawings.