[0001] The present invention relates to a process for manufacturing a concrete slab having
a surface formed by granules of natural stone embedded in a matrix of cement stone,
in which process concrete is prepared at least on the basis of said granules of natural
stone and cement, this concrete being applied in a mould and enabled to harden in
order to form a slab, and said surface of the hardened concrete slab being made rough.
[0002] From DE-OS-37 16 344, such a concrete slab is known. Upon manufacturing this known
concrete slab, there is started with concrete composed of grains of granite, grains
of diabase, cement and water. The purpose is to obtain a concrete slab of granite
which looks as much as possible like natural slabs of granite. With slabs of granite,
it is known that they obtain a silky shine when their surface is made rough. According
to DE-OS-37 16 344, a same effect is obtained upon making rough the concrete slabs
of granite comprising also grains of diabase. Making rough of the surface of the slabs
is performed by so-called "granulating" of the slabs wherein the surface of these
slabs is cut by means of a "granulating machine" with several cutters.
[0003] The invention has now however as object to provide a process enabling to manufacture
concrete slabs of natural stone with another type of look.
[0004] According to this object, the process according to the invention is characterised
in that said concrete is prepared on the basis of granules of a soft natural stone
and said surface of the hardened slab being treated by means of a milling cutter so
as to cleave grains of said natural stone present therein and to obtain a rough surface
which is partially formed by the cleaved surfaces of the grains of natural stone.
[0005] In the process according to the invention, a rough surface is thus obtained which
is considerably formed by the natural cleaved surfaces from the grains of natural
stone. This is achieved by the combination of the use of soft granules of natural
stone and of a milling treatment for making rough the surface.
[0006] In DE-OS-37 16 344 use is on the contrary made of hard granules of natural stone,
i.e. of granite with a Mohs hardness from 6 to 7 and of diabase with a Mohs hardness
of 6, the grains of which are made rough upon "granulating" the slabs in order to
obtain the desired silky shine. Only a limited number of the hard grains of natural
stone will be cleaved in this way.
[0007] Milling the surface of concrete slabs is known per se front DE-OS-39 27 601. This
German patent application proposes as alternative for "granulating" the concrete slabs
to treat the surface thereof with a milling cutter. In this way, an old and weathered
outlook would be obtained. An essential aspect of the invention according to DE-OS-39
27 601 consists in the fact that the milling treatment, in contrary with "granulating",
can be performed onto a non hardened slab. From DE-OS-39 27 601 it appears thus nowhere
that by milling the concrete slabs another surface effect would be obtained than by
a "granulating treatment".
[0008] As already indicated hereinabove, such a new effect is obtained according to the
invention by the combination of the use of a soft natural stone in combination with
the milling treatment.
[0009] Preferably, this natural stone has therefore in the process according to the invention
a Mohs hardness of 2 to 5 and in particular a Mohs hardness of 2.5 to 4 and preferably
of about 3.
[0010] In an efficient embodiment of the process according to the invention, said concrete
is prepared on the basis of granules of natural stone having a granulometry between
0 and 6 mm.
[0011] In this way, a good bound can be obtained between the grains of the granules and
the matrix of cement stone causing most of the grains upon the milling treatment to
be cleaved instead of to be knocked out, in particular if use is made of a rotary
milling cutter. An additional advantage is further that the application of this small
granulometry contributes to a larger amount of the surface to be formed by the granules
of natural stone which contributes on its turn that formation of moss and soil of
the surface of the slabs are avoided.
[0012] Preferably, said concrete is prepared on the basis of granules of natural stone having
a sufficient quantity of fine fraction to form said cement stone.
[0013] In this way, the small quantity of cement stone remaining visible can be given the
same colour as the granules of natural stone itself. In case of a light natural stone,
use is for example made of white cement causing the colour of the natural stone to
be maintained.
[0014] The invention relates further also to a concrete slab manufactured by application
of the process according to the invention and comprising a roughened surface formed
by granules of natural stone embedded in a matrix of cement stone. In the concrete
slab according to the invention, the granules of natural stone are formed by a soft
natural stone and said surface is a rough milled surface with grains therein of said
granules of natural stone which are cleaved as a result of the milling treatment.
[0015] Further details and particularities of the invention will become apparent from the
following description of a particular embodiment of the process and the concrete slab
manufactured in this way according to the invention. This description is however only
given by way of example and is clearly not intended to limit the scope of the invention.
The reference numerals used in this description relate to the annexed drawings, wherein
:
Figure 1 shows a perspective view on a concrete slab according to the invention;
Figure 2 shows a perspective view on the milling part of a possible rotary milling
cutter which can be applied in the process according to the invention, with omission
of the milling cutter discs; and
Figure 3 shows on an enlarged scale a shaft of the milling part shown in Figure 2
onto which milling cutter discs are mounted.
[0016] In the process according to the invention for manufacturing a concrete slab 1 with
a surface 2 formed by granules of natural stone embedded in a matrix of cement stone,
firstly concrete 3 is prepared at least on the basis of the granules of natural stone
and cement. This concrete 3 is then applied in a mould and is hardened in order to
form a slab. After the concrete 3, also a further layer of concrete 4, made of sand,
cement and gravel is however mostly as usual applied in the mould. This layer 4 has
for example a thickness of 3 cm while the layer 3 with the granules of natural stone
has for example a thickness of 1 to 2 cm.
[0017] Before hardening, the concrete layers 3 and 4 are, as known, vibrated at high frequency
and pressed at for example 500 tons.
[0018] According to the invention, the surface 2 of the slab 1 is treated, after hardening,
by means of a milling cutter, more specifically in such a manner that a number of
grains from the natural stone present therein are cleaved and a rough surface is obtained
being partially formed by the cleaved surfaces of the grains of natural stone. By
the selection given hereinafter of soft granules of natural stone and by using a milling
cutter, natural cleaved surfaces are essentially obtained being practically not disturbed
by the working of the milling cutter. In this way, a surface having a natural look
is obtained wherein the cleaved surfaces of the grains of natural stone contribute
to a special effect. The remaining part of the surface is formed by the cement stone
and the uncleaved grains of the granules of natural stone. Since these granules of
natural stone are particularly obtained by grinding larger pieces of natural stone,
the grains of these granules have already on their own cleaved surfaces. In order
to obtain this effect, it is preferable to prepare the concrete 3 on the basis of
granules of natural stone having a granulometry between 0 and 6 mm.
[0019] Upon preparing the concrete 3, it is essential that use is made of granules of a
soft natural stone, such as the known French natural stone or the Burgundian natural
stone such as Comblanchien. Of course, also other varieties of soft natural stone
can be applied which deliver the desired rough structure with cleaved surfaces, when
they are treated by means of a rotary milling cutter. The Mohs hardness of the used
granules of natural stone is efficiently between 2 and 5, in particular between 2.5
and 4 and is preferably about 3.
[0020] The granules of natural stone comprise preferably a sufficient quantity of fine fraction
for forming a cement stone wherein the grains of natural stone are embedded. In this
way, no sand or other fine granulated material must thus be used for preparing the
concrete 3, so that a uniform colour can be given readily to the surface 2, which
is already considerably formed by the cleaved surfaces of the granules of natural
stone. The concrete 3 is for example prepared from 450 kg white cement P50, 1600 kg
Comblanchien ( 0 - 6 mm, hardness 2 to 3 Mohs), 3.7 l Superplasticizer and 250 litre
water per m
3.
[0021] To obtain the rough surface with cleaved grains of natural stone described hereinabove,
a milling cutter 5 is used according to the invention, more particularly a rotary
milling cutter.
[0022] In Figure 2, a part of such a rotary milling cutter 5 is shown, more particularly
the milling part itself. This milling part has a number of shafts 6, for example six
shafts, which are positioned at equal distance of a central geometric rotation axis
7. The shafts 6 are for example fixed at the extremity to two circular discs 8. These
discs 8 are beard in the milling machine in a non illustrated manner and can be driven
by means of a drive belt 9, for example by an electrical motor.
[0023] As can be seen in Figure 3, a number of milling cutter discs ("flails") 10 are rotatably
mounted on each of the shafts 6 in such a manner that the shafts 6 extend through
the centre point of these milling cutter discs. The milling cutter discs 10 are provided,
at their periphery, with a number of protrusions 11, in particular five protrusions,
by means of which the surface of the slab is milled. These protrusions 11 are therefore
provided, at their extremities, with a hard cutting element 12, for example from wolfram
carbide.
[0024] To keep the milling cutter discs 10 at a certain mutual distance, circular rings
13 are for example applied therebetween. Possibly, additional circular discs 14 can
be slid between the discs 8 over the shafts 6 in such a manner that the milling cutter
discs 10 on the different shafts 6 are always in line and that therefore a certain
groove structure is obtained in the surface 2 of the slab 1. Due to the special construction
of the rotary milling cutter 5, this groove structure is not formed by fine ground
grooves, but it pertains globally to the rough structure of the granules of natural
stone of the surface 2.
[0025] In this respect, it has been found that it is preferable to use a rotary milling
cutter 5 as described hereinabove, of which the mutual distance between two neighbouring
cutting elements 12 on the milling cutter discs 10 being practically equal to 1/3
to 1/5 of the distance between the shafts 6 and the geometric rotation axis 7. The
shafts 6 can further be positioned therefore in particular at 5 to 10 cm from the
rotation axis 7 and more particularly at 7 to 8 cm from this rotation axis 7.
[0026] It has been established that a rotational speed of 1500 to 2500 rpm and in particular
of about 2000 rpm is preferable in view of obtaining the desired rough structure of
granules of natural stone. To obtain an optimal effect, the milling cutter is preferably
moved at a speed of about 1 m/min with respect to the slab.
[0027] From the description given hereinabove of some particular embodiments of the process
and the concrete slab according to the invention, it will be clear that several modifications
can be applied to these embodiments without falling outside the scope of the annexed
claims.
1. A process for manufacturing a concrete slab (1) having a surface (2) formed by granules
of natural stone embedded in a matrix of cement stone, in which process concrete is
prepared at least on the basis of said granules of natural stone and cement, this
concrete being applied in a mould and enabled to harden in order to form a slab, and
said surface (2) of the hardened concrete slab (1) being made rough, characterised
in that said concrete is prepared on the basis of granules of a soft natural stone
and said surface (2) of the hardened slab (1) being treated by means of a milling
cutter (5) so as to cleave grains of said natural stone present therein and to obtain
a rough surface (2) which is partially formed by the cleaved surfaces of the grains
of natural stone.
2. A process according to claim 1, characterised in that said natural stone has a Mohs
hardness of 2 to 5 and in particular a Mohs hardness of 2.5 to 4 and preferably of
about 3.
3. A process according to claim 1 or 2, characterised in that said concrete is prepared
on the basis of granules of natural stone having a granulometry between 0 and 6 mm.
4. A process according to anyone of the claims 1 to 3, characterised in that said concrete
is prepared on the basis of granules of natural stone having a sufficient quantity
of fine fraction to form said cement stone.
5. A process according to anyone of the claims 1 to 4, characterised in that use is made
of a rotary milling cutter (5) for treating said surface of the hardened slab.
6. A process according to claim 5, characterised in that use is made of a rotary milling
cutter (5) having a number of shafts (6) positioned at equal distance of a central
geometric axis (7) and rotating around said axis (7) and on each of which a number
of milling cutter discs (10) are rotatably mounted, said shafts (6) extending through
the centre point of these milling cutter discs (10).
7. A process according to claim 6, characterised in that said milling cutter discs (10)
are provided around their periphery with a number of protrusions (11), in particular
five protrusions (11), by means of which said surface (2) of the hardened slab (1)
is milled.
8. A process according to claim 7, characterised in that a milling cutter (5) is used
of which the distance between the extremities of two neighbouring protrusions (11)
being equal to 1/3 to 1/5 of the distance between said shafts (6) and said central
geometric axis (7).
9. A process according to anyone of the claims 6 to 8, characterised in that a milling
cutter (5) is used said shafts (6) of which being positioned at 5 to 10 cm of said
central geometric axis (7), and in particular at 7 to 8 cm of this geometric axis
(7).
10. A process according to anyone of the claims 1 to 9, characterised in that said milling
cutter treatment is performed with a rotational speed of 1500 to 2500 rotations per
minute and in particular with a rotational speed of about 2000 rotations per minute.
11. A concrete slab manufactured by application of the process according to anyone of
the claims 1 to 10 and comprising a roughened surface (2) formed by granules of natural
stone embedded in a matrix of cement stone, characterised in that the granules of
natural stone is formed by a soft natural stone and that said surface (2) is a rough
milled surface with grains therein of said granules of natural stone which are cleaved
as a result of the milling treatment.
12. A concrete slab according to claim 11, characterised in that said natural stone has
a Mohs hardness of 2 to 5 and in particular a Mohs hardness of 2.5 to 4 and preferably
of about 3.