Field of the invention

The present invention relates to a composite concrete panel formed of an aerated concrete mixture and a method of manufacture.

Background

Panels formed of aerated concrete, which are sometimes referred to as cellular concrete or foam concrete, are known but have limited application due to limitations in strength and surface hardness.

It is desirable to use panels formed of aerated concrete as they can be efficiently and cost effectively formed in a factory. Such panels can also be lightweight compared to regular concrete panels, have excellent fire resistance and good sound attenuation properties.

One commonly used panel formed of aerated concrete is known as Hebei and which is formed using aerated concrete that is cured in an autoclave under steam. Such a process is commonly referred to as Autoclaved aerated concrete (AAC) and by introducing the panel into an environment of steam, curing can be performed rapidly. Hebei panels are formed with internal reinforcement to increase their strength, though this adds weight and cost to the panels. Ultimately the lack of strength of a Hebei panel is a limiting factor and another limitation of Hebei panels is their surface hardness, which can be low so that the external surfaces can be prone to damage.

The lack of strength in an autoclaved lightweight concrete panel can be attributed to weak tensile bonds within the material due to large air bubbles being present. In this regard, concrete surrounding the air bubbles within the material can be stretched too thin at certain places, leading to areas of weakness that can crack, leading to panel disintegration. The location of areas of weakness are impossible to identify due to a lack of uniformity of air bubble size. The lack of uniformity of air bubble size is thought to arise due to the forming process in which chemical reactions take place within the concrete mixture to generate the gas bubbles within the panel. Unless this process is tightly controlled, the size distribution of gas bubbles within the panel can vary across the panel, leading to areas of weakness.

It is desirable to provide a lightweight concrete panel with improved strength and surface hardness, while retaining the properties of aerated concrete and not causing the price of the panels to rise.

There is a need to address the above, and/or at least provide a useful alternative. Summary

According to the present invention there is provided a method of manufacturing a composite concrete panel, comprising the steps of: forming an aerated concrete composition by adding a foaming agent to a concrete mixture and mixing the composition to substantially evenly distribute the foaming agent through the composition; providing a cavity in which the panel is to be formed; inserting the aerated concrete composition into the cavity; and pressurizing the panel during curing. whereby air bubbles present in the aerated concrete mixture are compressed to a smaller and more uniform size.

According to preferred embodiments, the panel is pressurised by contacting external surfaces of the panel against the cavity during curing. Preferably, sides of the cavity are advanced toward each other to apply pressure to the panel. Preferably, the panel is pressurised to bring the density of the panel to within a range of 300 to 800 kg/m3. Preferably, the panel is pressurised up to approximately 3 atmospheres. The method can further include the step of placing an internal core within the cavity.

Preferably, the internal core has a porous external surface. The internal core may be engaged with a strengthening element. The strengthening element may be a rib formed of metal, timber or fibreglass. Preferably, the core is formed of a polymer. The polymer may be polystyrene or polyurethane.

Preferably, the concrete mixture includes cement, sand, water and the said foaming agent. Preferably, the concrete mixture further includes strengthening additives, which may include epoxy resin. The concrete composition can further include foam glass or pumice glass.

According to another aspect of the present invention, there is provided a composite concrete panel formed with a method of the above described type.

Brief description of the drawings

In order that the invention may be more easily understood, an embodiment will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1: is a cross sectional view of a panel formed according to a preferred embodiment of the invention;

Figure 2: is a very close cross section of the panel showing air bubbles; and

Figure 3: is another very close cross section of the panel showing the air bubble size close to a surface interface. Detailed description

A composite concrete panel 10 according to a preferred embodiment of the invention is illustrated in Figure 1. The panel 10 is formed of an aerated concrete mixture 12 and has an internal stiffening core 14, though in other embodiments the core may be omitted. The panel 10 is pressurised within a mould during manufacturing to reduce the size of air bubbles within the mixture and increase panel strength. Pressurising the panel 10 within the mould during manufacture also increases the surface hardness of the concrete at each surface interface, i.e. an outer surface and an inner surface where the aerated concrete mixture 12 contacts the core 14.

In one example, the panel 10 is pressurised by contacting external surfaces of the panel 10 against a mould cavity during curing. To achieve this, walls of the mould may be moveable to squash the panel 10 within the cavity. Alternatively, the aerated concrete mixture can be injected under pressure, just like in injection moulding. In such embodiments, complicated moulds with moving parts can be avoided. In Figure 1, an example of an injection port 20 is illustrated. The inventor has found that the strength of an aerated concrete panel is proportional to the size of air bubbles that are introduced into the mixture 12. Without wishing to be bound by any particular theory, the inventor believes that on a macroscopic level, it is the thickness of the concrete mixture or walls between the air bubbles that makes a significant contribution to the strength of the panel, due to the prevention of weak tensile bonds at localised locations. As the air bubble size increases, the sections of concrete between the air bubbles become longer and thinner, and therefore capable of carrying less load to the detriment of the overall strength of the panel. This is illustrated in Figure 2, whereby the thickness of the mixture 12 between air bubbles 16 is shown, along with the varying thickness of the mixture 12. Owing to this effect, it is important that gas bubbles be evenly distributed within the panel, otherwise areas of weakness can be introduced. With prior art panels that rely on a chemical reaction to generate gas bubbles, control of the bubble distribution within the panel, and therefore their size once compressed, can be difficult to achieve.

To ensure a relatively even distribution, a foaming agent is introduced into the concrete mixture and the mixture stirred well, prior to the concrete composition being introduced into the mould. This ensures a generally even distribution within the mixture so that once pressurised, the gas bubbles are uniformly small and the strength of the panel improved. The foaming agent may be any suitable commercially available foaming agent, organic or chemically derived, that creates air bubbles in the concrete mixture. The amount of air bubbles added to the mixture governs the density of the panel.

On compressing the panel 10, the air bubble size can be reduced and the size of bubbles across the panel made more consistent so that areas of thin concrete between adjacent bubbles and the associated areas of weakness can be avoided so that a designer can more accurately predict the strength of the panel.

Figure 3 illustrates the size distribution of air bubbles within the mixture toward an interface surface 18 where the mixture contacts the mould and also a second interface surface where the mixture contacts the core 14. By compressing the panel during manufacturing, at the external surface of the panel 18 and also at the interface between the panel and the core, the panel exhibits greater strength due to air bubbles being eliminated or reduced to a very small size. This not only provides increased surface hardness, but also greater resistance to damage at the interface of the core. It can be seen that the air bubble size increases away from the interface surface 18, where the material is compressed to a lower level.

By compressing panel 10 within the mould, the surface hardness of the panel 10 increases, providing an external panel surface that is less prone to damage, particularly during transportation. This may allow the panel 10 to be coated with paint or render at manufacture and then transported to a building site, thereby significantly reducing manufacturing costs.

Although issues with low surface hardness can be solved in the prior art by bonding an outer skin, in the present disclosure a thick and strong outer skin is obtained on the external surface without creating an interface or transition zone between two components that can be vulnerable to failure.

This is achieved because the air bubbles that are closer to the surface of the panel, both the external surface and the interface with the core, are pushed together under greater force than those within the middle of the panel 10. To understand such an outcome, an analogy of a large crowd can be used. In the event that the crowd rushes toward a fence or immovable object, those at the object are impacted more than those away from it. With the disclosed invention, an analogues outcome is obtained, i.e. greater compression at the external surface of the panel and the interface with the core. This may be due to the viscosity of the aerated concrete mixture 12, but in any event, the result is that the surface of the panel 10 is harder and stronger than within the panel 10.

In preferred embodiments, the internal core 14 has a porous external surface. Another advantage of the present disclosure is that by pressurising the panel 10, greater engagement or bonding between the panel 10 and the internal core 14 is obtained as the mixture is forced into the pores, as can be seen in Figure 3, thereby further adding to the strength of the panel. The aerated concrete mixture 12 includes cement, sand, water and a foaming agent or air entrainer that is added prior to the mixture being added to the mould. In other embodiments, the aerated concrete mixture 12 can further include strengthening additives. The strengthening additives can include epoxy resin or fibres for example. Fine particles such as foam glass or pumice glass may also be added to the mixture 12. As such materials are incompressible, the remaining mixture within a mould is subjected to greater pressure, thereby increasing the presently disclosed advantages. In some embodiments, the internal core 14 is provided with a strengthening element (not shown) in engagement with the core 14. The strengthening element may be a longitudinal rib running within the core or along an outer edge, and may be formed of metal, aluminium, timber or fibreglass.

In preferred forms, the core 14 is formed of a polymer such as polystyrene or polyurethane. Preferably the polymer has a porous surface for the reasons discussed above. During manufacture, the panel 10 is formed by providing a cavity in which the panel is to be formed, placing the internal core 14 within the cavity, inserting the aerated concrete mixture 12 into the cavity; and pressurizing the panel 10 within the cavity during curing, whereby air bubbles present in the aerated concrete mixture 12 are compressed to a smaller and more uniform size. Previously, it was not possible to have any control on the size of air bubbles within the mixture 12. A significant issue with this is that air bubbles often join together, creating larger bubbles and an area of localised weakness in the panel.

Preferably, the mixture is pressurised upon injection into a mould, similar to as is done with injection moulding. In other examples, the panel 10 is pressurised by contacting external surfaces of the panel 10 against the cavity during curing. To achieve this sides of the cavity may be advanced toward each other to apply pressure to the panel.

The panel 10 is pressurised from a lighter density to bring the density of the panel to within a range of 300 to 800 kg/m ³ . In other embodiments, the pressure may be increased up to 1600 kg/m ³ though it will be appreciated that applying large pressures to the mould can be difficult and expensive. To achieve a density in the range of 300 to 800 kg/m ³ , the panel is pressurised to 0.5 to 10 atmospheres, preferably approximately 3 atmospheres. The described embodiments provide a lightweight panel having a number of uses, particularly in the building industry. One example is intertenancy parting walls where non- combustible materials are required, and it is desirable that the walls have good noise attenuation without costing more than previously available panels. The described panels can also have high impact resistance The described panels may also be used for the construction of a low-cost single storey dwelling.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.