U.S. patent application number 13/082019 was filed with the patent office on 2011-07-28 for method and plant for the manufacture of building products.
This patent application is currently assigned to Monier Technical Centre Limited. Invention is credited to Caroline Joyce BENNETT, Peter SROSTLIK, Gregor VOGT.
Application Number | 20110183023 13/082019 |
Document ID | / |
Family ID | 34674010 |
Filed Date | 2011-07-28 |
United States Patent
Application |
20110183023 |
Kind Code |
A1 |
BENNETT; Caroline Joyce ; et
al. |
July 28, 2011 |
METHOD AND PLANT FOR THE MANUFACTURE OF BUILDING PRODUCTS
Abstract
A plant and method for the manufacture of cementitious products,
viz. cast concrete roof tiles (T), the plant comprising a mixing
station where the constituents of a cementitious composition, less
an air entraining agent, are admixed under atmospheric pressure and
further mixed under a partial vacuum of 200 to 340 mBar after the
addition of an air entraining agent, the plant also comprising a
moulding station where the tiles (T) are cast in a gang-mould
before being cured in a conventional autoclave for up to 10 hours
at 45.degree. C. The cured concrete roof tiles (T) are de-moulded
after curing and conveyed to a surface preparation zone where a
primer coat of a water based epoxy and a surface coating of a water
based emulsion paint are applied to at least an upper surface (in
use) of the cast concrete roof tiles (T). The admixed cementitious
composition has a Ford Cup flow of between 40 and 60 seconds and a
slump test of between 180 and 220 mm. In an alternative method,
instead of applying a primer coat, the upper surfaces, in use, of
the roof tiles T are subjected to abrasion by grit-blasting to
remove a thin layer from the upper surfaces thereby facilitating
the subsequent application thereto of a surface coating.
Inventors: |
BENNETT; Caroline Joyce;
(Sussex, GB) ; SROSTLIK; Peter; (Maintal, DE)
; VOGT; Gregor; (Bruchsal, DE) |
Assignee: |
Monier Technical Centre
Limited
|
Family ID: |
34674010 |
Appl. No.: |
13/082019 |
Filed: |
April 7, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11912808 |
May 9, 2008 |
|
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PCT/GB2006/001540 |
Apr 26, 2006 |
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13082019 |
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Current U.S.
Class: |
425/91 |
Current CPC
Class: |
B28B 5/028 20130101;
C04B 28/02 20130101; B28B 11/04 20130101; Y02W 30/92 20150501; C04B
2111/00594 20130101; C04B 41/71 20130101; C04B 41/009 20130101;
Y02W 30/91 20150501; C04B 41/52 20130101; C04B 28/02 20130101; C04B
14/06 20130101; C04B 18/08 20130101; C04B 40/0028 20130101; C04B
40/0089 20130101; C04B 40/0263 20130101; C04B 2103/304 20130101;
C04B 2103/32 20130101; C04B 2103/54 20130101; C04B 41/52 20130101;
C04B 41/0072 20130101; C04B 41/4853 20130101; C04B 41/502 20130101;
C04B 41/53 20130101; C04B 41/52 20130101; C04B 41/0072 20130101;
C04B 41/48 20130101; C04B 41/502 20130101; C04B 2103/40 20130101;
C04B 41/009 20130101; C04B 28/02 20130101 |
Class at
Publication: |
425/91 |
International
Class: |
C04B 41/81 20060101
C04B041/81 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 28, 2005 |
GB |
0508652.5 |
Claims
1. A plant for the manufacture of a concrete roof tile, the plant
comprising: a) a mixing station for admixing a cementitious
composition comprising water, sand, pigment, reactive fine filler,
cement, a super-plasticiser and additives; characterized in that
the mixing station comprises a1) means for mixing the water, sand,
pigment, reactive fine filler, cement and an air entraining agent
under atmospheric pressure; a2) means for adding the
super-plasticiser to the cementitious composition; and a3) means
for thereafter subjecting the cementitious composition to a partial
vacuum; b) a molding station where the admixed cementitious
composition is fed into a re-usable mold to form a green-state roof
tile; c) a curing station where the green-state roof tile is cured;
d) a de-molding station where the cured roof tile is de-molded, e)
a surface preparation zone including conveying means for conveying
the cured roof tile through the surface preparation zone, and means
for preparing the upper surface, in use, of the cured roof tile to
facilitate subsequent application thereto of a surface coating,
wherein the surface preparation zone comprises a first coating zone
for the application of a primer coat to said upper surface; and, f)
a second coating zone where the cured roof tile is provided with a
surface coating on its prepared upper surface
2. A plant according to claim 1, wherein the surface preparation
zone includes a first curing zone for heating and drying the primer
coat to cause cross-linking thereof and to reinforce said surface
to provide a more receptive surface for said surface coating.
3. A plant according to claim 1, wherein the second coating zone
also comprises a curing zone for the application of heat to dry the
surface coating applied to said prepared upper surface.
4. A plant according to claim 1, wherein the partial vacuum is
between 240 and 320 mBar.
5. A plant according to claim 1, wherein the feeding station
comprises means selected from means to feed the cementitious
composition into the reusable mold under pressure and means to feed
the cementitious composition into the reusable mold by gravity.
6. A plant according to claim 1, wherein the feeding station
comprises a funnel device through which the cementitious
composition is fed into the re-usable mold.
7. A plant according to claim 1, wherein a plurality of half molds,
arranged in a gang mold, are provided for the simultaneous molding
of a plurality of roof tiles between respective pairs of said of
adjacent half molds of the gang mold.
8. A plant according to claim 7, the arrangement being such that,
when molding the roof tiles, the half molds are oriented on end
within the gang mold.
9. A plant according to claim 8, wherein the de-molding station
includes means to rotate the gang mold to orient the half molds and
the cured roof tiles in a substantially horizontal condition for
de-molding.
10. A plant according to claim 9, wherein the de-molding station
includes means to de-mold each cured roof tile together with a next
inline below half mold, and separation means for separating the
cured roof tiles from their respective half molds.
11. A plant according to claim 10, wherein conveyor means are also
provided for conveying the separated cured roof tiles to the
coating station and the separated half molds to a cleaning station
whereat the half molds are prepared for a subsequent molding
operation.
12. A plant according to claim 1, wherein the surface preparation
zone further comprises abrasion means, whereby said upper surface
of the cured roof tile is subjected to abrasion to remove surface
detritus to thereby further facilitate a subsequent application
thereto of said surface coating.
13. A plant according to claim 12, wherein said abrasion means
comprises a grit blaster.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional application of U.S.
Ser. No. 11/912,808 filed May 9, 2008, which application claims
benefit and priority of PCT International Application No.
PCT/GB2006/001540 filed Apr. 26, 2006 and United Kingdom
Application No. 0508652.5 filed Apr. 28, 2005, all of the foregoing
applications being incorporated herein in their entirety.
BACKGROUND OF THE INVENTION
[0002] This invention is concerned with a method for the
manufacture of building products and an appropriate plant for
effecting the method for the manufacture of the building
products.
[0003] By building products where used herein, it is meant concrete
roof tiles and concrete parts and fittings therefor.
[0004] A conventional practice for the manufacture of concrete roof
tiles is described in GB 1352627, where the tiles are manufactured
by an extrusion method, in which a stiff concrete mortar is fed
from a hopper onto a series of tile pallets passing beneath the
hopper where the mortar is formed on the pallets by extrusion
between the pallets and an overlying roller and slipper. The
surface finish of extruded roof tiles is very rough. Therefore, the
surface of the tiles uppermost in use is coated with pigmented
coating compositions in order to enhance the surface finish and the
aesthetic appeal of the finished product.
[0005] A principal drawback of forming roof tiles by an extrusion
method is that the shape and configuration of the upper surface of
the extruded roof tile is dictated by the cross-section of the
extrusion rollers and slippers that are conventionally used for
their production. Thus, it is not possible to provide tiles with a
headlock for increasing the rain-tightness thereof when the tiles
are assembled on a roof. Also, tiles with exotic and aesthetic
shapes cannot be produced with the known extrusion process. This
presents a serious commercial disadvantage as the demand for
roofing products with such shapes is strongly increasing in many
countries.
[0006] In order to overcome or mitigate the disadvantages described
above, casting processes were developed. Casting of concrete roof
tiles facilitates the choice of shape and configuration for the
upper surface in use of the concrete roof tiles made by this
method.
[0007] Known casting processes can be distinguished in two types:
those using re-usable moulds and those using lost moulds.
[0008] A casting process for roof tiles using re-usable moulds is
described in EP 0437672, in which process a series of half moulds
is formed into a gang mould so that a plurality of roof tiles can
be cast at one time. Due to the surface smoothness of the half
moulds the cast roof tiles receive a much smoother surface than
extruded roof tiles. However, there are two serious disadvantages
in such a process. Firstly, the half moulds need to be cleaned and
provided with a release agent after each casting process in order
to allow a proper shaping and de-moulding of cured roof tiles.
Secondly, it is not possible to apply surface coatings to the
surfaces of the cured roof tiles because any residual presence of
the release agent will prevent a proper adhesion of the surface
coating. Application of a surface coating is essential to resist
the hazards of inclement weather conditions and for the supply of
roof tiles with different surface colors and finishes.
[0009] WO 03/008166 describes a manufacturing process for concrete
bodies which prevents the complicated handling and treatment of
re-usable moulds by using lost moulds. The moulds are made from a
material, e.g. moulding sand; which can be aggregated by
compression and subsequently broken up. Moulds made from such a
material have a rough surface and, consequently, the cast concrete
bodies will also have a rough surface necessitating the application
of a surface coating thereto.
[0010] Summarizing, it is a fact that none of the roof tile
manufacturers have managed to launch a cast roof tile in the market
regardless of the increasing demand for different shapes of tiles
and the existence of casting processes. This is because they are
not able to provide roof tiles with surface coatings capable of
resisting the hazards of inclement weather conditions.
[0011] During their work the Applicants have become aware that
there are two reasons for the difficulty to coat the roof tiles.
Firstly, because the surface of the tile has a weak layer
consisting of fine particles not "glued" together with cement. This
weak layer is easily removed over a period of time so any coating
applied directly to the surface will have poor adhesion. Secondly,
because the surfaces of the roof tiles contain residua of the
release agent, which serves to further weaken the tile surface
layer that may prevent penetration of a primer coat.
SUMMARY OF THE INVENTION
[0012] It is therefore one of the objects of the invention to
provide a plant and a method of manufacturing for roof tiles, and
fittings therefor, which plant and method facilitates a high
flexibility in the tile shaping and provides excellent surface
qualities on the visible surfaces, in use, of the roof tiles.
[0013] The present invention thus provides a method for the
manufacture of building products as hereinbefore defined comprising
the steps of: a) admixing a cementitious composition comprising:
water, sand, pigment, reactive fine filler, cement and additives;
b) feeding the final cementitious composition into re-usable moulds
to form green-state building products; c) curing the green-state
building products in a curing station; d) de-moulding the cured
building products, e) conveying the cured building products through
a surface preparation zone where upper surfaces, in use, of the
products are prepared to facilitate subsequent application thereto
of a surface coating, wherein the surface preparation of said upper
surface includes the application of a primer coat thereto; and, f)
conveying the cured building products through a coating zone where
the cured building products are provided with a surface coating on
their prepared upper surfaces.
[0014] Conveniently, the method of admixing the cementitious
composition includes the addition of an air entraining agent and/or
a super-plasticiser.
[0015] Preferably, in admixing the cementitious composition, water,
sand, pigment, reactive fine-filler e.g. fly ash, cement and the
air entraining agent are mixed under atmospheric pressure; and a
super-plasticiser is then added to the cementitious composition,
which is thereafter mixed in a second step in a partial vacuum.
[0016] Preferably, the mixed cementitious composition has the
properties of: a Ford Cup flow of between 35 and 90 seconds; and a
slump test of between 160 and 220 mm.
[0017] Conveniently, the cementitious composition is mixed in the
second step under a partial vacuum of between 240 and 320 mBar to
produce the cementitious composition having a Ford Cup flow of 40
to 60 seconds.
[0018] In effecting the method provided by the present invention,
the moulds are filled under pressure or by gravity and the filled
moulds are conveyed to a curing zone where the green-state building
products are subjected to heating at between 40 and 60 degrees
centigrade for a period of between 8 and 12 hours.
[0019] Preferably, the moulds are prepared after the removal of the
cured building products for their re-use by cleaning and applying a
release agent.
[0020] The green state building products may be cured by conveying
them to a separate curing station, wherein the temperature is
increased sufficiently to effect curing. Alternatively, curing
occurs in the moulding station and the cured building products are
then conveyed to the surface preparation zone.
[0021] Conveniently, the surface preparation of the upper surfaces,
in use, of the cured building products includes the application of
a water based primer coat to the said upper surfaces.
[0022] In one convenient method provided by the present invention,
after application of the water based primer coat to the cured
building products, they are conveyed to a first curing zone in
which the primer coat is heated to effect cross-linking of the
primer coat and to reinforce the surfaces of the cured building
product to provide receptive surfaces for a subsequently applied
surface coating and, after application of the surface coating
thereto, the cured building products are conveyed to a second
curing zone in which the water based emulsion surface coatings are
heated to reduce the tendency for the cured building products to be
damaged during packaging.
[0023] Preferably, the building products are selectively warmed up
in a first heating zone before the application of the water based
primer coat to the upper surfaces, in use, of the building products
and that primer coated building products are selectively warmed up
in a second heating zone before the application thereto of a water
based surface coating.
[0024] In a preferred method, provided by the present invention,
the primer coat comprises a water-based epoxy.
[0025] In one embodiment of the invention, the upper surfaces, in
use, of the cured building products are also subjected to abrasion
in the surface preparation zone to further facilitate the
subsequent application thereto of a surface coating. Conveniently,
the abrasion of the said upper surfaces, in use, of the cured
building products is effected by grit-blasting of said
surfaces.
[0026] The present invention further provides a plant for the
manufacture of building products as hereinbefore defined, the plant
comprising: a) a mixing station for admixing a cementitious
composition comprising water, sand, pigment, reactive fine filler,
cement and additives; b) a moulding station where the admixed
cementitious composition is fed into re-usable moulds to form
green-state building products; c) a curing station where the green
state building products are cured; d) a de-moulding station where
the cured building products are de-moulded, e) a surface
preparation zone where the cured building products are conveyed by
conveying means of the plant through the surface preparation zone
where upper surfaces, in use, of the cured building products are
prepared to facilitate subsequent application thereto of a surface
coating, the surface preparation zone comprising a first coating
zone for the application of a primer coat to said upper surface;
and f) a second coating zone where the cured building products are
each provided with a surface coating on their prepared upper
surfaces.
[0027] Conveniently, the surface preparation zone comprises an
applicator for the application of a water based primer coat to the
upper surfaces, in use, of the cured building products.
[0028] Preferably, the surface coating comprises a water based
emulsion.
[0029] Conveniently, the second coating zone includes a curing zone
for the application of heat to dry the surface coating applied to
the prepared upper surfaces, in use, of the cured building
products.
[0030] In one preferred embodiment of the invention the surface
preparation zone comprises a first curing zone for heating and
drying the primer coat to cause cross-linking thereof and to
reinforce the surfaces of the building products to provide a
receptive surface for a subsequently applied water based emulsion
surface coating, the plant also comprising a second curing zone for
heating and drying the surface coating to give a strong outer
surface to the building products.
[0031] Preferably, after the addition of a super-plasticiser, the
cementitious composition while in the mixing station is subjected
to a partial vacuum of between 240 and 320 mBar to produce the
cementitious composition having a Ford Cup flow of 40 to 60
seconds.
[0032] Conveniently, the cementitious composition is fed into the
re-usable moulds under pressure or by gravity through a funnel
device.
[0033] Preferably, when the plant is in use, the green-state
building products are cured in the curing station for between 8 and
12 hours at a temperature of between 40.degree. C. and 60.degree.
C.
[0034] A plurality of half moulds, arranged in a gang mould, are
conveniently provided for the simultaneous moulding of an
appropriate number of building products between each pair of
adjacent half moulds of the gang mould and, when moulding the
building products, the half moulds are preferably oriented on end
within the gang mould.
[0035] Conveniently, means are provided at the de-moulding station
for rotating the gang mould, for example through 90 degrees, to
orient the half moulds and the cured building products in a
substantially planar condition for de-moulding, and the cured
building products are each de-moulded together with a next inline
below half mould, the de-moulding station preferably comprising
separation means for separating the cured building products from
their respective half moulds.
[0036] The invention further comprises conveyor means for conveying
the separated cured building products to the coating station and
the separated half moulds to a cleaning station whereat the half
moulds are prepared for a subsequent moulding operation.
[0037] Preferably, the surface preparation zone of the plant also
comprises an abrasion means whereby the upper surfaces, in use, of
the cured building products are subjected to abrasion to further
facilitate the subsequent application thereto of a surface coating.
Conveniently, the abrasion means is provided by grit-blasting
means.
[0038] The present invention conveniently provides a building
product manufactured by the method hereinbefore described when
effected in the plant as described above.
[0039] Conveniently, the building product is a roof tile, or a
fitting for use with a roof tile.
[0040] Other features and advantages of the present invention will
become apparent from the following description of the invention
which refers to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] There now follows by way of example of the invention, a
detailed description, which is to be read with reference to the
accompanying drawings in which:
[0042] FIG. 1 is diagrammatic representation of mixing and moulding
stations of the invention and,
[0043] FIG. 2 is a diagrammatic representation of cured
cementitious product and half-mould separation station and product
preparation, coating and heat treatment zones of the invention.
[0044] The plant for the manufacture of building products, viz.
concrete roof tiles T, generally comprises: a mixing station 2
whereat the constituents of a cementitious composition are admixed;
a moulding station 4 whereat the cementitious composition is fed to
gang-moulds 6 to be cast into roof tiles T; a curing zone, not
shown, where the cast roof tiles T, in their respective gang-moulds
6 are cured; a de-moulding station 8, where the cast roof tiles T
are removed from the gang-moulds 6; a cleaning station 10 for
preparing gang-moulds 6 for successive casting operations; a
surface preparation zone 12 whereat the cured roof tiles T are each
subjected to a surface preparation operation and provided with
surface coatings to at least their upper surface in use on a roof;
and a stacking and packaging station 14 where the coated roof tiles
T are stacked and packaged for storage and distribution.
[0045] The mixing station 2 is of conventional design and includes
a mixer 16 wherein, when the plant is in use, the constituents of a
cementitious composition are admixed as hereinafter described and
subjected to mixing regimes including mixing in a partial
vacuum.
[0046] The moulding station 4 has provision for locating a
succession of gang-moulds 6, each of which comprises a series of
half-moulds 7 for the simultaneous production of twenty-four
concrete tiles T. The moulding station is provided with funnels,
not shown, through which, when the moulding station is in use, a
cementitious composition is fed simultaneously and under pressure
into twenty-four mould cavities defined by the half-moulds 7.
[0047] The curing zone is of conventional design and has provision
for control of the temperature and humidity therein.
[0048] The de-moulding station 8 is provided with means, not shown,
for separating the half moulds 7 from the cast concrete roof tiles
T and feeding the half moulds 7, on edge, to a washing and cleaning
station 10 whereat the half moulds are prepared for a subsequent
moulding operation, see FIG. 2.
[0049] The de-moulding station 8 is also provided with means,
illustrated diagrammatically in FIG. 2, for feeding the cast
concrete roof tiles T seriatim to the surface preparation zone 12,
whereat, when the plant is in use, the cast concrete roof tiles T
are coated with a primer coat and a surface coating.
[0050] The surface preparation zone 12 comprises a conveyor 20, a
heating zone 22 for selective warming cast concrete roof tiles
passing there-through and a first coating zone 24 whereat, when the
surface preparation zone 12 is in use, the primer coat is applied
to the concrete roof tiles T. By "selective warming" in this
context we mean that at least those surfaces of the concrete
roofing tiles which are to receive the primer coating are
heated.
[0051] The surface preparation zone 12 also comprises a first
curing zone 26 for effecting cross-linking between the cementitious
body of each roof tile T and the primer coat.
[0052] The surface preparation zone 12 also comprises a second
heating zone 27 for selective warming the primer coated concrete
roof tiles before application of the surface coating thereto. By
"selective warming" in this context we mean that at least those
surfaces of the primer coated concrete roofing tiles which are to
receive the subsequent surface coating are heated.
[0053] The surface preparation zone 12 further comprises a second
coating zone 28 whereat, when the surface preparation zone 12 is in
use, a surface coating is applied to the primer coated concrete
roof tiles T, the surface preparation zone 12 also being provided
with a second curing zone 30 for effecting cross-linking between
the primer coats and the surface coatings of the roof tiles T.
[0054] The conveyor 20 extends through the first heating zone 22,
the first coating zone 24, the first curing zone 26, the second
heating zone 27, the second coating zone 28 and the second curing
zone 30 to convey the coated concrete roof tiles T to the stacking
and packaging station 14.
[0055] In manufacturing concrete roof tiles T by a first method
provided by the invention the constituents of a cementitious
composition are admixed in the mixer 16 of the mixing station 2,
the admixture comprising water, reactive fine-filler, e.g. fly ash,
small aggregate, e.g. sand of 0.3 mm to 3 mm grain size, pigment,
cement and an air entraining agent. These constituents are admixed
at atmospheric pressure to form a precursor admixture after which a
super-plasticiser is added to the precursor admixture, which is
then mixed in a partial vacuum to produce a cementitious
composition having a Ford Cup flow of 50 seconds and a slump test
of 200 mm.
[0056] The benefit of using a partial vacuum during mixing of the
constituents, and especially after admixing the super-plasticiser,
is that the amount of entrained air is controlled to give a
required consistency to the admixed cementitious composition.
[0057] The admixture of the cementitious composition is then fed
under pressure to moulding station 4 and into the cavities formed
between the half moulds 7 of the gang-mould 6 to form twenty-four
concrete roof tiles T. At this time the gang-mould 6 is oriented so
that the half moulds 7 are positioned on end and thus, the cavities
formed by the half moulds 7 are also oriented so that the concrete
roof tiles T are cast in a vertical plane.
[0058] From the moulding station 4, the gang-mould 6 is conveyed to
the curing zone where the concrete roof tiles T are subjected to a
curing regime of between 40 and 60 degrees C. for ten (10)
hours.
[0059] When the concrete roof tiles T are fully cured in their
respective gang-mould 6, the gang-mould 6 is conveyed to an
orienting station, not shown, where the gang-mould 6 is rotated
through 90 degrees to orient the half moulds 7 and the cast
concrete roof tiles T in a substantially planar position.
[0060] This reorientation of the half moulds 7 and roof tiles T
facilitates the ease with which the tiles T may be separated from
the half moulds 7. Thereafter, the half moulds 7 are conveyed to
the cleaning station 10 where they are cleaned and made ready for a
subsequent moulding operation by applying a release agent to the
moulding surfaces thereof.
[0061] At the same time, the cast concrete roof tiles T are placed
on the conveyor 20 with their upper sides in use uppermost, the
tiles T being conveyed seriatim (i) through the heating zone 22
where the tiles are selectively warmed prior to having a primer
coat applied thereto, (ii) through the first coating zone 24 where
a primer coat of a water based epoxy primer coat is applied to the
at least the upper surface in use of the roof tile T, the primer
coat being capable of cross-linking with the cement rich surface of
the roof tile to provided a receptor surface for a top coat, (iii)
through the first curing zone 26 where the primer coat is cured,
(iv) through the heating zone 27 where the primer coated concrete
roof tiles are selectively heated before application of a surface
coating thereto, (v) through the second coating zone 28 where a
surface coating of water based emulsion paint is applied onto the
primer coat (vi) through the second curing zone 30 where the
surface coating of water based emulsion paint is cross-linked with
the water based epoxy primer coat, and (vii) to the stacking and
packaging station 14 where the coated concrete roof tiles T are
stacked and packaged for storage and eventual distribution.
[0062] Although it is described herein that the cementitious
composition is fed into the mould cavities under pressure, in a
modified method the cementitious compositions may be fed under
gravity.
[0063] In a second method provided by the present invention for the
manufacture of concrete roof tiles T, prior to applying the primer
coat to the upper surfaces, in use, of the cured building products,
viz. the roof tiles T, those surfaces are subjected to the abrasion
action of grit-blasting means, not shown. The action of the
grit-blasting means removes a thin, possibly release oil
contaminated layer of the building products, thereby facilitating
and improving the subsequent application of surface coatings
thereto.
[0064] In effecting the second method provided by the present
invention, the grit-blasting operation may be carried out at an air
pressure of between 0.5 and 3.0 bar using alumina particles having
an aspect ratio 75 to 150 microns.
[0065] Although the present invention has been described in
relation to particular embodiments thereof, many other variations
and modifications and other uses will become apparent to those
skilled in the art. It is preferred, therefore, that the present
invention be limited not by the specific disclosure herein, but
only by the appended claims.
* * * * *