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

Abstract

An apparatus, and a method of use of the apparatus, for forming a concrete slab are provided. A formwork may include a support element having a layer of active material bonded on each side to a backing sheet. The active material is chosen to slowly compress over time when placed under a load, and the rate of compression is chosen 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). Another feature is the inclusion of a surface of the backing sheet with a relatively low coefficient of friction. When two support elements are laid one on top of the other with the surfaces in contact, the upper support element may slide over the lower support element, for example during a seismic event.

Claims

1. 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 support surface onto which concrete is poured to form the concrete slab, the method comprising: providing a load bearing member to support the slab when cured; providing a formwork including a support element having a layer of active material that compresses progressively under the load exerted by the concrete over a time it takes for the concrete to cure; covering a space around the load bearing member, including under the concrete slab when poured, with the support surface of the support element facing towards where the slab will be formed; sealing any space between a 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 providing of the load bearing member comprises providing a load bearing member comprising at least part of a base isolation device, and a surface of the base isolation device forms the top surface of the load bearing member.

2. The method as claimed in claim 1, wherein the preparing of the formwork comprises forming the formwork such that an initial level of the support surface is at a same level as the level of the top of the load bearing structure.

3. The method as claimed in claim 1, wherein the formwork comprises two support elements and the covering of the space around the load bearing member comprises placing the two support elements on top of one another.

4. The method as claimed in claim 3, wherein an outer surface of each support element includes a smooth backing sheet having a coefficient of friction less than 0.4 with respect to a smooth backing sheet of another support element, and the covering of the space around the load bearing member comprises stacking the two support elements on top of one another with the smooth backing sheets in contact with one another.

5. The method as claimed in claim 1, wherein the base isolation device comprises a first plate that forms the top surface of the load bearing member.

6. The method as claimed in claim 5, wherein a first surface of the first plate is flat.

7. The method as claimed in claim 5, wherein a first surface of the first plate is curved.

8. The method as claimed in claim 5, wherein the base isolation device comprises a second plate having a second surface of complementary shape to the first surface of the first plate.

9. The method as claimed in claim 1, further comprising: placing reinforcing elements in a volume to be filled by the slab prior to the pouring of the concrete.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order that the advantages of certain embodiments of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. While it should be understood that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which:

[0037] FIG. 1 shows a support element according to one embodiment of the present invention;

[0038] FIG. 2 shows a formwork according to one embodiment of the present invention;

[0039] FIGS. 3(a)-(c) show (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;

[0040] FIG. 4 shows a cross sectional schematic view of a foundation and formwork according to one embodiment of the present invention;

[0041] FIG. 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;

[0042] FIGS. 6(a) and (b) show views of an experimental arrangement for testing the method of the present invention, where (a) is a perspective view and (b) is a cross sectional view;

[0043] FIG. 7 shows the results of the experiment using the arrangement of FIGS. 6a and 6b.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] Some embodiments pertain to a component that forms an upper surface of a formwork and a method for its use in construction of concrete slabs that relates to situations where the concrete slab is used in conjunction with seismic base isolators. With reference to FIG. 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, such as 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).

[0045] The embodiment illustrated in FIG. 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 application.

[0046] The layers of bubble wrap 2 in FIG. 1 a are sandwiched between a first backing sheet 3 and a second backing sheet 4. Backing sheet 3 has an outer surface formed from aluminum 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.

[0047] 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.

[0048] 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 of 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).

[0049] A feature of some embodiments 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 FIG. 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.

[0050] However, it is a feature of bubble wrap (and other active materials according to some embodiments of 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.

[0051] Another feature of some embodiments 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).

[0052] A formwork according to an 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.

[0053] 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 conventional formwork. Concrete is poured onto the rough surface 9 of the upper support element 6 (see FIG. 3a) to form a slab 17.

[0054] FIG. 3a shows an expanded view of a formwork 16 as used in the construction of a concrete slab 17 formed on a load bearing member in the form of foundation 18, the load bearing member including a part of a base isolation device in the form of a slider base 19 having an upper surface 21 that forms the top surface of the load bearing member. The slider includes a slider plate 20, which is attached to the slab once poured, 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. In other embodiments, the base isolation device may be any other suitable device as well known in the art, such as lead-rubber base isolators.

[0055] The formwork 16, which is essentially as illustrated in FIG. 2, includes a support element 22 that 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) that 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 is 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.)

[0056] In some 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.

[0057] 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.

[0058] 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.

[0059] Prior to pouring the concrete slurry, the slider bases 19 and slider plates 20 are 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.

[0060] 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.

[0061] 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 slabin 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.

[0062] 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.

[0063] Support for the behavior of a formwork including an active layer may be tested using the apparatus shown schematically in FIGS. 6a and 6b. This experimental arrangement includes a formwork 31 including a wooden boxing 32 (500500250 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.

[0064] Four rebars 41, which form the load bearing members, 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 m.sup.3)+cement (20 kg)+water (8 liters)).

[0065] 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 FIG. 7, 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).

[0066] 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.

[0067] 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.

[0068] 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.

[0069] It will be readily understood that the components of various embodiments of the present invention, as generally described and illustrated in the figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the detailed description of the embodiments of the present invention, as represented in the attached figures, is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention.

[0070] The features, structures, or characteristics of the invention described throughout this specification may be combined in any suitable manner in one or more embodiments. For example, reference throughout this specification to certain embodiments, some embodiments, or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases in certain embodiments, in some embodiment, in other embodiments, or similar language throughout this specification do not necessarily all refer to the same group of embodiments and the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0071] It should be noted that reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussion of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment.

[0072] Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.

[0073] One having ordinary skill in the art will readily understand that the invention as discussed above may be practiced with steps in a different order, and/or with hardware elements in configurations which are different than those which are disclosed. Therefore, although the invention has been described based upon these preferred embodiments, it would be apparent to those of skill in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention. In order to determine the metes and bounds of the invention, therefore, reference should be made to the appended claims.