The present invention relates to the manufacture of concrete slabs and more specifically to a tray-like mould utilised during said manufacture.
In a method, developed during the last few years, for the production of slabs from a granulate of stone material and a cement binder - which method is described and claimed in Italian Patent No. 1,288,569 in the name of Marcello Toncelli - it is envisaged that the starting mix is prepared using particular ratios of the granulate and cement binder, with a suitable fluidifier for cement mixes being added to the latter.
The mix is loaded into container-like or tray-like moulds, forming a layer of predetermined thickness. The mould is then subjected to a first - very intense but short - vacuum step intended to remove the air which occupies the spaces between the granules of stone material and is retained therein.
During a second step, the mix is subjected to a vibratory movement under a vacuum which is less intense than that of the previous step.
The purpose of these two different degrees of vacuum is to prevent the mix water from boiling and therefore generating steam which would hinder a perfect compaction.
This step is then followed by the slab setting and hardening steps.
It is clear from the preceding short notes that the tray-like mould is subject to significant stresses during application of the vibratory movement, so that hitherto it has been made of metallic materials of considerable thickness, which obviously results in drawbacks in particular with regard to handling.
In addition, it must be pointed out that, for the industrial implementation of the setting and hardening steps, which are of a relatively long duration, a considerable amount of tray-like moulds must be made available.
Finally, the tray-like moulds must be made of material which does not absorb the water from the "green" (i.e. not yet hardened) slabs which are formed therein, since the slab would thus have an insufficient water content with harmful effects on the mechanical properties of the end product. In fact, it should not be forgotten that, in the method outlined above, the mix has a water content which is strictly controlled and slightly greater than the theoretical quantity required for the setting and hardening reactions.
During implementation of the method in question it is essential that the "green" (i.e. not yet hardened) slab or tile, at the end of the compaction step, should not lose moisture, with the result that hitherto the individual tray-like moulds have been introduced into a steam-saturated curing chamber and kept there for 24 hours.
It is worth mentioning a number of prior documents representing the state of the art.
EP-A-0 461 331 discloses a mould used in the manufacture of items where concrete needs to be aerated during the drying (setting) step. The mould is made of plastic material with a flat bottom and the side walls defining the cavity externally provided with vertical wings which are adapted to abut on the peripheral edge of another mould when several moulds are vertically stacked.
DE-A-2 909 491 discloses a mould used to make thin items of concrete, such as wall tiles. It is made of plastic foam and has a peripheral edge consisting of two portions with staggered heights to permit various moulds to be vertically stackable.
BE-A-0 537 141 discloses a mould used to make items of concrete. The mould cavity has a number of internally directed ribs which correspond to an extensive portion of the bottom wall and have a not negligeable height with respect to the side walls of the mould.
In view of these problems and drawbacks, the present invention provides a solution which not only is able to achieve the substantial elimination thereof, but also provides undoubted advantages from a technical and industrial point of view.
The present invention consists of a tray-like mould for the manufacture of concrete slabs having a bottom wall and a plurality of side walls and is obtained from a plastic material with good mechanical strength properties and impermeable, so as to prevent the absorption of even a small amount of water from the concrete mix in contact with the said walls and characterized in that it is provided at the outer surface of said bottom wall with several stiffening ribs and is furthermore provided with a peripheral edge shaped so as to allow several tray-like moulds to be stacked on top of each other in a steam-tight manner.
In the preferred embodiment, the bottom of the tray is provided with zones which are depressed by a predetermined amount so as to produce in the resulting slab a plurality of projections acting as supporting feet for the subsequent processing operations, in particular during the sizing step of the rough-formed slab.
Still with reference to the preferred embodiment, devices for stacking and simultaneously centring the individual tray-likes with respect to those located immediately above and below are associated with the outer edges of the tray-like moulds.
The particular aspects and advantages of the present invention will emerge more clearly from the detailed description which follows, with reference to the accompanying drawings in which :
With reference first of all to Figures 1-4, a tray-like mould according to the present invention has a bottom wall 10 and four side walls 12 which define an internal moulding cavity 14.
The outer surface of the bottom wall 10 is provided with projecting stiffening ribs 16 (A, B and C), of which the ribs 16A and 16B are arranged perpendicular to each other (namely parallel to the side walls 12 of the mould), as can be easily understood from Fig. 2, while the ribs 16C are arranged diagonally. In this way the mould has a high mechanical strength and at the same time is sufficiently lightweight to allow handling thereof during the various processing steps.
The arrangement of the ribs is also so designed to allow the penetration of the forks of forklift trucks for handling operations. However, the outer edge also has a form which allows manual gripping.
In fact, as can be readily seen from Figures 3 and 4, the external edge 18 of the tray-like is suitably shaped so as to allow both stacking and the above mentioned gripping.
With reference to Figures 8 and 9, the first of these figures shows three tray-like moulds stacked on top of each other, while the second one shows a partially sectioned and enlarged detail of the stacked and centred arrangement of the tray-like moulds.
In particular from Figures 8 and 9 it can be seen that the edge 18 consists of two portions 20 and 22 which have staggered heights so that, during stacking, the bottom of the overlying tray-like mould is arranged in the outer area defined by the portion 20 and is enclosed by the portion 22.
It is worth noting that, during manual stacking onto the last tray-like, a lid (which may also consist of an empty tray) for closing the cavity 14 is provided onto the uppermost tray-like mould.
Fig. 5 shows a particular detail of the bottom of the tray according to Fig. 1. In predefined and equidistant positions along its own periphery, the bottom wall 10 of the tray-like mould has a depression 21 with a height which is very small but sufficient to form in the final slab a plurality of projections that are able to act as supporting feet for the rough-formed slab when the latter is transferred (after the hardening step) to the stations for the subsequent processing step, in particular during the sizing step.
In the embodiment shown in Fig. 1, the depressions 21 are eight in number, namely two on each side of the bottom wall 10 of the tray-like mould. From Fig. 1 it can also be seen that these depressed zones have a circular shape so that the resulting projections in the final rough-formed slab have the shape of true supporting feet.
Preferably the tray-like mould is obtained by injection-moulding from a suitable plastic material such as polyurethane or ABS resin having the desired technical characteristics.
By way of conclusion, with the tray-like mould according to the invention the following advantages may be achieved:
In particular the ability of staking and of providing a sealing action against undesirable water evaporation, especially during the setting step of the mix, are to be appreciated.
It is understood that mechanically equivalent modifications and variations are possible and may be envisaged within the coverage of the appended claims.
In particular, modifications shall be expected as regards the arrangement of the reinforcing and stiffening ribs and the shape of the edges so as to allow stacking and simultaneous sealing of several tray-like moulds.