GB2030890A

GB2030890A - Improvements in concrete building products

Info

Publication number: GB2030890A
Application number: GB7838068A
Authority: GB
Original assignee: Redland Technology Ltd
Current assignee: Redland Technology Ltd
Priority date: 1978-09-25
Filing date: 1978-09-25
Publication date: 1980-04-16
Also published as: GB2030890B

Abstract

A concrete building product, for example, a roof tile is manufactured as follows. A substrate of a concrete mix is shaped to form a roof tile and a layer of cementitious slurry between 25??? and 400??? thick is applied to the uncured substrate. The substrate and slurry layer are then cured and finally a coating of polymer emulsion preferably between 5??? and 50??? thick is applied to the tile.

Description

SPECIFICATION Improvements in concrete building products This invention is concerned with improvements in concrete building products and is more particularly concerned with polymer finishes for slurry coated concrete building products, for example, roofing tiles.

For some considerable time it has been the practice of some tile manufacturers to provide concrete roofing tiles with a surface finish to give the tiles an aesthetic appeal. The surface finish may be a polymer coating over a cementitious layer applied as a slurry.

Such practice may obviate the need to include pigment in the tile body thereby reducing the cost per unit. This is because pigment may be provided only in the slurry layer and/or in the polymer layer of the surface finish.

The short term effect of applying surface finishes of this type to concrete roofing tiles is more than satisfactory. However, after a few years there may be a marked degradation of the tiles due to cracking of the surface finish, i.e. the slurry layer.

One other problem which polymer layers are intended to inhibit is that caused by efflorescence.

Efflorescence is caused by leaching out of calcium hydroxide from the tile body or slurry layer. The calcium hydroxide mixes with carbon dioxide in the air to give calcium carbonate, i.e.

chalk, viz, efflorescence. However, while the short term efflorescence problem may be overcome the complete breakdown of the surface finish of the tile occurs as the tile epands and contracts during freeze-thaw cycles because of differential movement between the main tile body and the slurry coating due in part to the greater aggregate content of the tile body. Another contributory factor to the cracking of the slurry layer is the effect of curing the tile which causes differential shrinkage due to the different water content of the tile body and the slurry layer.Once the cracks in the slurry layer and the polymer coat are sufficient to allow water into the body of the tile by capillary action then the rate of degradation of the tile increases and subsequent freeze thaw cycles in inclement weather conditions will have a snowball effect thereby causing the surface coatings to flake off.

This damage to concrete roofing tiles is obviously quite unsatisfactory since the tiles on a roof become less aesthetically pleasing.

Of course, it is realised that cracking of the slurry layer and crazing of the polymer layer to a certain degree cannot be avoided. What it is intended, however, is that any cracking which occurs will be of an acceptable level and not have any deleterious side effects.

Concrete roofing tiles are generally manufactured by extruding a ribbon of a concrete mix from an extruding machine onto a continuous stream of pallets. The mix passes under a shaped roller which imparts the upper surface shape to the tile; the undersurface being shaped by the configuration of the pallet.

It was found by experiments that a polymer layer would have to be of substantial depth in order to cater for the uneven surface of an extruded tile ribbon. This is due to the grain size of the aggregate in the extruded ribbon which grain size would be of the order of 3 mm resulting in a microscopically rough surface on the extruded ribbon.

However, the thicker the layer of polymer applied to the extruded tile ribbon the more costly the product and a point is soon reached where it become uneconomical to increase the depth of the coating.

Thus tile technology progressed by utilising a relatively cheap cement slurry coating on the extruded ribbon to provide a smooth interface between the tile body and the polymer coating.

In one arrangement a layer of cementitious slurry was applied to "green" state extruded concrete tiles, produced as aforesaid, and the tiles were thereafter cured in known manner.

After the tiles were removed from the curing chamber a post cure coating of a polymer emulsion was applied onto the smooth slurry coated surface of the tile while the tile still retained some of the heat from the curing chamber.

The thickness of the slurry coat in the said arrangement was approximately 600 microns and the thickness of the polymer coating was approximately 40 microns.

Such depths of coating were originally provided so that, even after the polymer coating was lost, it would be some considerable time before the slurry-coating was completely lost due to washing away and atmospheric attack.

From further experiments we have carried out into ways of overcoming the problem of excessive crack formation encountered in surface finishes of the type just described, we have discovered that coatings of greatly reduced thickness have improved performance characteristics over products presently available and examples given in prior art disclosures.

The invention provides a method of manufacturing a concrete building product comprising applying to a substrate of concrete mix, an intermediate layer of cementitious slurry and subsequently applying a coating of a polymer emulsion in which the thickness of the slurry layer is between 25,u and 400u.

Preferably the thickness of the slurry layer is between 50,u and 1 50,u.

The thickness of the polymer coating may be between 5y and 50,u and is preferably between 15 and 25.

The slurry layer is preferably applied to an uncured substrate, the substrate and slurry layer subsequently being cured before application of the polymer coating.

The method preferably includes the further step of performing the substrate into a predetermined shape, for example that of a roof tile.

The invention also provides a concrete building product produced by the method described above.

Preferably the product is shaped to form a roof tile.

It was found that the number of freeze-thaw cycles which tiles coated in this way could withstand was at least twice the number for tiles coated to much greater depths with slurry and polymer.

In one example of coated concrete tiles the constituents of the body of the tile and the surface layers were as follows: Tile body mix of aggregate, i.e. siliceous material, silt, ordinary Portland cement pigment and water in appropriate quantities.

Slurry layer or ordinary Portland cement pigment and water in appropriate quantities.

Polymer layer of styrene acrylic emulsion, silica and water.

In this example, the thickness of the slurry layer was 1 O9,u and the thickness of the polymer layer was 27it.

Very fine sand may be added to the slurry layer and the grain size of the silica in the polymer layer should not exceed half the depth of the proposed polymer layer.

In producing tiles according to the present invention a slurry layer is applied by any convenient means to a "green" state tile produced as aforesaid and the tiles are thereafter cured in a curing chamber.

Subsequently, and preferably just after the tiles are removed from the curing chamber, the polymer coat is sprayed onto the smooth slurry layers of the cured tiles while they are still near their elevated cure temperature.

Although the manufacture of tiles described herein usually has pigment provided in the body of the tile and in the slurry layer, and the polymer layer is a clear layer, in other methods in accordance with the invention pigment may be provided in the slurry layer and/or in the polymer layer.

Whereas the process described envisages the slurry layer being applied pre-cure and the polymer layer post cure, in other methods in accordance with the invention the coating layers may both be applied as a post cure treatment.

Claims

1. A method of manufacturing a concrete building product comprising applying to a substrate of concrete mix, an intermediate layer of cementitious slurry and subsequently applying a coating of a polymer emulsion in which the thickness of the slurry layer is between 25zt and 4out.

2. A method as claimed in claim 1 in which the thickness of the slurry layer is between 50 and 1 50et.

3. A method as claimed in claim 1 or claim 2 in which the thickness of the polymer coating is between 5 and 50st.

4. A method as claimed in claim 3 in which the thickness of the polymer coating is between 1 5 and 25zit.

5. A method as claimed in any one of the preceding claims in which the slurry layer is applied to an uncured substrate, the substrate and slurry layer subsequently being cured before application of the polymer coating.

6. A method as claimed in any one of the preceding claims in which the substrate is preformed into a predetermined shape, for example, a roof tile.

7. A method of manufacturing a concrete building product substantially as hereinbefore described.

8. A concrete building product manufactured by a method as claimed in any one of the preceding claims.

9. A roof tile manufactured by a method as claimed in any one of claims 1 to 7.