Colour shade

Colour shade of the product is influenced by the following factors:
Dosing pigments
has a principle influence on the resulting colour shade. It is essential to keep to the same pigment dosing into concrete mixture and use pigment of the same colour shade.
Colour of cement
Grey cement exists in many shades. The colour oi the product depends on uniformity of the colour. The dependence on the colour of cement is higher in case of lower dosage of a colouring pigment.
Cement-water ratio »w«
The cement-water ratio is a ratio between the amount of batch water in concrete and amount of cement symbol »w« is used for it. Batch water is all water, which is added in the concrete mixture (water in aggregates, water in liquid pigments, free added water, which is calculated for the given »w«). When »w« increases, the colour becomes more distinctive. The cause of this is a better hydration of cement and by this way also creation of better crystallised hydration products, which form the colour phase in concrete. This phenomenon can be influenced more significantly by using plasticising agents, which support hydration of cement and by this way also colour distribution in the system.
•Compacting concrete.
A correct compacting increases dislincliveness oi the colour shade and, on the contrary, insufficient compacting can cause lower the value of the resulting shade.
Time of mixing and order of addition of components in the mixer
With an increasing time of mixing the colour shades (mainly red and brown) tend to go yellower. That is why it is necessary to keep to the same time of mixing in order to have the same shade. There are a few recommendations: Pigment is added at the same time together with or in the follow-up to the addition of aggregates and ingredients, the mixture is mixed for about 30 seconds. In the follow-up admixtures are added (as recommended by the supplier), then cement and after about 60 seconds water. Agitating wet mixture for a time of further about 60 seconds is satisfactory for an optimum homogenisation of individual constituents.
Effect of additives for concrete - these are mostly liquids, which enable a better pressing, decrease sticking of the mixture to the form and improve appearance (also colour) and physical and mechanical properties. It is necessary to keep to recommendations by manufacturers of additives.

The resulting quality of manufactured concrete and concrete products are influenced mainly by the following factors:

•Technological discipline of producing equipment operators
•Quality, suitability and way of storing input materials for the manufacture
•Uniform quality of input materials, above all that of aggregates
•Suitability of the manufacture equipment
•Conditions of ageing of final products and following storage of final products

Concretes with various ability to be processed are manufactured fordifferent technologies of manufacture of concrete products. In case ofmanufacture by means of vibropressing machines, so called wet concretewith minimum content of batch water is used - the cement-water ratioranges within 0.30 - 0.32. Very soft concrete with the cement-water ratioabout 0.48 is used for the manufacture of products gained by »casting«(vibropressed) in forms. In case of higher water contents, where w>0.55,the quality of the concrete is significantly decreased. The amount of water, which is necessary for cement hydration, is about 25%of the total cement amount. The rest of water is added in order to improveability to be processed. Nevertheless, the water is evaporated within atime andpores remain in cement. The higher is the porosity of the cement and by thisway also of concrete, the lower is its strength, compactness and resistance todestruction by aggressive components (salts, sulphates) etc.

The amount of batch water also influences the intensity of colour shade of products.Application of efficient additives, which help to meet optimum ability ofprocessing at relatively low content of water in fresh concrete, isrecommended. Concrete becomes more compact and has higherresistance to negative influences. Ingredients in wet concrete have toimprove its compactness at an optimum dose of water so that productscould not change its shape, when made more compact, and material doesnot stick to bottom die. The surface of the product should be closed asmuch as possible.

Information has been developed by PRECHEZA a.s.