Patent Application
20160122998
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.
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., 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-story 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 that 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 that 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, labor, 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, 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 the case 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. Accordingly, an improved formwork may be beneficial. Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.
Certain embodiments of the present invention may provide solutions to the problems and needs in the art that have not yet been fully identified, appreciated, or solved by current formwork technologies. For example, some embodiments of the present invention pertain to 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. Some embodiments of the invention are 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 the present 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. The friction between the slab and a supporting surface of the formwork is significantly reduced from that of conventional apparatuses and methods. A feature of some embodiments 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 some embodiments of 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 includes a layer of active material attached to a backing sheet. The active material has a thickness that reduces slowly over 2 to 10 days under the load of the poured concrete slab. In certain embodiments, 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 some embodiments, the backing sheet has a coefficient of friction less than 0.4. In some embodiments, the backing sheet includes thermal insulation. In certain embodiments, the layer of active material is sandwiched between two backing sheets. At least one of the two backing sheets may have a relatively low coefficient of friction, for instance, less than 0.4.
The use of a backing sheet having a relatively low coefficient of friction may provide certain benefits 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, but are not 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 some embodiments, the thickness of the active material is in the range from 10 mm to 50 mm. In certain embodiments, the active material includes an enclosed void. In some embodiments, the enclosed void contains a fluid. In certain embodiments, a surface of the enclosed void is flexible. In some embodiments, a surface of the enclosed void is permeable to the fluid within the enclosed void when under a load.
In some embodiments, the enclosed void includes a surface that 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 some embodiments of 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 certain 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 some embodiments, the active material includes a layer formed from a plurality of enclosed voids. The voids may be filled with a fluid, such as air. In some embodiments, the layer of active material is formed from an array of air filled enclosed voids. In certain embodiments, the array of air filled enclosed voids is formed from a plastics material. In some embodiments, 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 some embodiments of 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 some embodiments of 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 some embodiments 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) that compress slowly may also be used with some embodiments of the present invention.
According to another aspect of the present invention, there is provided a formwork for a concrete slab. The formwork includes a layer of active material having a thickness, where 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 some embodiments, the layer of active material is oriented substantially parallel to a plane of a support surface of the formwork. The support surface is the surface of the formwork that, 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 some embodiments, the formwork includes a structural element substantially as described above. In certain embodiments, a surface of the structural element forms the support surface of the formwork. The surface of the structural element that forms the support surface may be 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, but are not limited to, 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 some embodiments, the formwork includes a plurality of structural elements. In certain embodiments, 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 some embodiments, each outer surface of the support element is formed by a backing sheet where 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 some embodiments, a first surface of the support element is formed by a smooth side of the backing sheet. In certain embodiments, a second surface of the support element is formed by a rough side of the backing sheet.
In some embodiments, 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. Such embodiments 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. The formwork used to support the concrete when poured includes a support element having a layer of active material which compresses slowly under load, where 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 includes the steps of: (1) preparing the formwork to cover a space around the load bearing structure, including under the concrete slab when poured; (2) 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 (3) pouring the concrete over the continuous surface. 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. 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.
The method may include the step of placing reinforcing elements in the volume to be filled by the slab prior to the step of pouring the concrete. In some embodiments, the top of the one or more load bearing members includes a base isolation device. In certain embodiments, the base isolation device is a slider. In some embodiments, the slider is a friction pendulum slider.
Some embodiments of the present invention may have a number of advantages over prior art formworks and methods, including, but not limited to: (1) 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; and (2) 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 some embodiments of the present invention, and where the slabs can move relative to one another with relatively low frictional forces between them.
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:
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
The embodiment illustrated in
The layers of bubble wrap 2 in
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 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).
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
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.
A 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).
A formwork according to an embodiment of the present invention is generally indicated by arrow 5 in
The support elements 6, 7 of the formwork 5 are in turn supported on a rigid platform indicated by 18 in
The formwork 16, which is essentially as illustrated in
In some embodiments, the support element 22 is in the form of a sheet as illustrated, for example, in the layer 6 of
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 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
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
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 (
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 behavior of a formwork including an active layer may be tested using the apparatus shown schematically in
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 1)).
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
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.
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.
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.
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.
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 611841 | Jun 2013 | NZ | national |
This application is a U.S. national stage application under 35 U.S.C. §371 of PCT Application Number PCT/NZ2014/000115 filed on Jun. 11, 2014, which claims the benefit of New Zealand Provisional Patent Application Number 611841 filed on Jun. 11, 2013. The subject matter of these earlier filed patent applications is hereby incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NZ2014/000115 | 6/11/2014 | WO | 00 |
