10;~679~
1 This invention relates to a method of manufacturing pre-cast concrete building slabs or panels, and relates partic-ularly though not exclusively to a method of manufacturing pre-cast concrete building panels for use as wall cladding.
There are a number of pre-cast concrete building slabs and panels commercially available at the present time, and these known slabs and panels come in a variety of sizes and designs.
For example, it is known to provide a building slab or panel with cavities in order to reduce the overall weight of the panel, and to improve its insulation properties. However, in providing such cavities there is a tendency for the panel or slab to lose some of its rigidity and strength, and if it is a smaller panel of a nature which can be carried manually and readily, then the panel thickness must be maintained at a minimum thickness, but with wire reinforcement if the panel is to have sufficient strength.
Therefore, on the one hand, it is desirable to reduce the thickness of the slab or panel to the minimum in order to save costs, but this reduction in thickness has the effect of reducing the strength, and on the other hand, therefore, care must be taken to ensure that the panel strength is not so im-paired as to make the slab or panel unusuable.
It is known to include wire reinforcement in a panel to improve the strength, but this has the disadvantage of in-creasing the thickness of the panel to provide the necessary cover to the steel therefore increasing the weight of the panel and increasing its cost.
One solution which I have already proposed to overcome the problem is to reinforce the rear of a thin concrete skin with hardboard or the like sheet material, and this solution has given improved results. The present invention provides a method -1- ~
. q' 10367g7 1 for the manufacture of such panels so that the results are improved even further, and the panels produced thereby are of even and higher strength, without the weight being increased.
In accordance with the method and the present invention, a pre-cast concrete panel is manufactured in the following steps:
a) a reinforcing sheet of hardboard or the like is soaked in water;
b) it is supported substantially horizontally in a mould;
c) the hardboard or the like is covered with a thin skin of concrete whilst the hardboard or the like is still wet;
d) a cavity in the mould extending peripherally of the hardboard or the like is filled with concrete, before or after applying the concrete skin to the hardboard so that hardboard edge regions become embedded in the concrete; and e) the concrete and hardboard or the like are caused or allowed to dry sufficiently to enable the panel to be removed from the mould.
It has been found, using this method, that the hardboard is firmly anchored to the skin of concrete when dry and gives the panel, in the region of the concrete and hardboard, consider-able strength, sufficient that, for normal wall cladding uses, no reinforcement is required in the concrete skin.
Furthermore, the section of the panel defined by the hardboard and concrete skin has surprising ductility and behaves more like a ductile metal than a concrete laminate. This effect is achieved due to the fact that the hardboard shrinks as it dries out, and during initial shrinkage of the hardboard, the concrete, being still wet and having virtually no inherent strength, moves with the hardboard. After an initial period, however, the concrete sets sufficiently to resist the continuing ~0;~67g7 1 hardboard shrin~age, and the hardboard becomes tensioned, and the concrete compressed, and hence pre-stressed. Thus, when the panel is loaded on the concrete skin, and the skin and hardboard deflect, the effect is not to tension one side of the skin as might be expected but only to release the pre-stressing, and the skin remains in compression; it is well known that concrete is very strong in compression and hence thQ reason why the panel has more strength than would be expected.
It may be desirable to locate wire or rod reinforcement in the cavity which is filled with concrete to reinforce the concrete placed therein, which forms a peripheral rib to the finished panel.
In a preferred case, the skin is faced by tamping aggregate thereinto whilst it is still wet.
The aggregate also adds to the strength of the panel, and contributes to the fact that the skin can be made of a minimum thickness whilst maintaining a high strength also contributing to the fact that it is not necessary for the skin to have any wire reinforcement therein. Indeed, in some examples the skin will be so thin that it would not be possible to insert any wire reinforcement.
Preferably, the rib will have bolt holes to enable securing bolts to be passed therethrough for the fixing of the slab or panel either to an adjacent slab or panel or to a struc-tural beam or the like. Wooden or other fixing fillets may be let into the rib during casting to provide suitable nailing or screwing surfaces for example for an interior lining to be secured to the rib so as to overlie the hardboard or the like sheet.
Also, according to the invention, there is provided a pre-cast panel formed in accordance with the method as aforesaid.

103679q 1 An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:
Fig. 1 is an elevation of a slab or panel constructed in accordance with the invention;
Fig. 2 is a fragmentary sectional view of a slab or panel constructed in accordance with the invention, showing the panel in its mould, Fig. 3 is a fragmentary sectional side view showing how two similar panels each formed in accordance with the method of the invention may be constructed simultaneously; and Fig. 4 is a pian view of the arrangement shown in Fig~
3.
Referring to the drawings, and in particular to Figure

2, the slab or panel comprises a pre-cast concrete skin which defines a major face of the panel, and on its inner face the skin is reinforced by means of a sheet of hardboard or the like material 1. The slab or panel illustrated may be approximately 8' x 3'4" x 3" so as to be capable of being carried by one man or certainly by two men. The slab or panel has a peripheral rib 2 which is of approximately 2" square section. The thickness of the concrete skin 3 which is reinforced b~ the hardboard or like panel 1 may be in the region of 3/4". This is a very thin skin, and because of the thinness of the skin there is no steel rein-forcement therein. The reinforcement is provided by the hard-board or the like sheet 1, and aggregate which is tamped into the outer surface of the skin 3 in performing the method of the invention which will be explained hereinafter.
In order to manufacture the panel shown in Figure 1, a mould is constructed. This has a base plate Ml, side pieces 1 M2 and a recess former M3. The former M3 is smaller than the internal dimensions of side pieces M2 so that a peripheral cavity C is formed in the mould. Circular rods R are pushed through aligned holes in the former M3 and side pieces M2 to de-fined bolt holes 7 in the finished panel as shown in Figure 3.
Metal wire rod or bar 6 reinforcement is located in the cavity C as shown and the hardboard 1, after being thoroughly soaked in water is placed on former M3 as shown. Concrete is vibrated into the cavity ~ to fill sa~e and next the concrete is skimmed over the hardboard 1, whilst it is still wet, to form the concrete skin 3. The concrete skin may be laid on the hard-board before cavity C is filled with concrete, if desired. Whilst the concrete of the skin is still wet, further surface treatment is carried out by laying the desired type of aggregate 4, by machine or by hand as desired, and in the desired layout, and subsequently tamping in the aggregate with a broad timber or the like, to the depth and distribution required. The aggregate, in addition to contributing to the strength, presents an attractive surface finish. As the hardboard and concrete dry out, so the ~ hardboard binds to the concrete skin 3 and pre-stresses the same as explained hereinbefore.
In efect, it will be seen that the slab or panel is hollowed to the rear or has a rear recess 5 as shown in Fig. 3.
This makes the panel light in weight, and its strength is main-tained as explained herein by the provision of the hardboard or the like sheet 1.
The peripheral rib 2 is reinforced by means of the metal wire or rod 6 and is also provided with the bolt holes or slots 7 to enable securing bolts to be passed therethrough. In addition, the peripheral rib may be provided with wooden or 1 other fixing fillets during the casting of same for the attachment of additional components or sheets to the rib as desired.
In Figures 3 and 4, two panels or slabs each as described in the above, are shown joined together by a peripheral concrete joint 8 provided with air gaps 9, the entity forming a hollow block which may be used for partition building or wall building. Heavier reinforcement may be incorporated in the rib to make the units more load bearing and the inner suxace of the recess 5 of each panel or slab may have a coating of plaster applied thereto.
In soaking the hardboard with water, it may be desirable to introduce additives with the water in order to achieve improved bonding between the hardboard and concrete skin.

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