Urbanization is a trend currently prevailing around the world and particularly in the growth markets including, India. This tendency is characterized by migration from country to town, rising populations in larger cities and an expansion of urban catchment areas. This is accompanied by an increase in demand for buildings and improved infrastructure. Concurrent to this phenomenon of urbanization, a growing need for more quality has made itself felt – more quality in living space design and thus also through the use of colour.
Coloured concrete materials, such as concrete roofing tiles, paving blocks and paving slabs today make an important contribution in making our environment more attractive, and are getting more and more accepted by the general public in India. With these materials, builders are able to combine technical functionality with an aesthetically pleasing appearance.
Nevertheless, concrete frequently has a rather negative image even though the outstanding technical properties of this all-purpose material are acknowledged by even the layman. Terms like “concrete jungle” and “as grey as concrete” are often used to describe an environment in which man does not feel particularly contented. The architectural design and, in particular, the color of a building tend to be the decisive factors in whether a project is viewed as successful or whether it is seen more as a dismal and monotonous structure.
There are many ways of giving concrete a Coloured appearance.The simplest method, is to paint the concrete surface, but the problem is that a coat of paint only has limited durability, and renewing it would in many cases be a particularly arduous task. Setting up the scaffolding and applying a new coat of paint not only involves considerable cost, it is also in many cases technically impossible.
Another method is to give the concrete a more lively appearance by using different aggregate materials. The possibilities for producing a colorful design with this method are nevertheless
In most cases, the method of choice is to integrally color the concrete, and a wide range of suitable pigments is nowadays available for this purpose. They enable almost any shade to be achieved, and have virtually unlimited durability.
The production of coloured concrete mixes does not basically differ from that of a grey concrete. We shall now look at what points need to be considered to produce attractive concrete surfaces through the addition of pigments.
The raw materials
a) The pigment
Due to the formation of calcium hydroxide, cement that is freshly made up with water is highly alkaline. One of the main demands made on the pigment is therefore that it is resistant to alkalis, in other words, the coloring effect of the pigment must not be impaired by the lime content of the cement.
Furthermore, the pigment must be neither destroyed nor washed out through the effects of the weather especially sunlight and the constant changeovers between heavy rain, heat and frost.
Many years of observation of coloured concrete products exposed to different climates in various parts of the world have shown that inorganic oxide pigments can satisfy the requirements expected of pigments for coloring concrete.
The builder has a choice of various colors. Iron oxide pigments (e.g. the Bayferrox grades) are available in red, yellow, black and brown. Greens can be obtained by using chrome oxide green pigment. White colors can be created with titanium dioxide pigments, and blues with lightfast pigments if the concrete is made with light-coloured cement.
b) The cement
It is wellknown that the various types of cement can also differ in their inherent color. Whereas, generally speaking, fluctuations in the color seldom occur within the production from one cement factory, there can be considerable differences in the shade of the cement from different manufacturers.
In practice, it is therefore normal – particularly with larger building projects – to purchase all the cement from the same supplier. This is strongly recommended when producing a coloured concrete, because it should largely eliminate any fluctuations in color due to differences in the color of the cement.
As mentioned before, white cement is naturally also particularly recommended for coloured concrete, because it is also possible to produce pastel shades.
c) The aggregates
The color of the sand and pebbles also affects the color of the final concrete. If the exposed concrete surfaces are to be subsequently treated by sandblasting, bush hammering etc., the color of the pebbles should not differ too much from the coloured concrete. The question regarding the choice of aggregate materials can be resolved by carrying out a few preliminary tests, which do not take up much time and are not particularly costly. Based on these trials, the manufacturer can see exactly which combination of sand, pebbles, cement and pigment produces the best color.
d) The water
Accurate control of the water supply in a concrete mixing unit is, of course, an integral part of efficient concrete production. For this reason, particular importance is attached anyway to ensuring that the volume of water added to the individual batches of concrete is precisely regulated.
However, apart from the technological properties, the color of the concrete is also dependent on the selected water to cement ratio. Excess water evaporates from the concrete and leaves behind cavities in the form of fine pores. These scatter the incident light and thus make the concrete lighter. In other words, the higher the water to cement ratio, the lighter the concrete looks, regardless of whether it is a grey concrete or one which has been coloured by the addition of pigments.
Production of the concrete mix
In practice, it has proved best to add the pigment dry to the aggregates in the mixer, and to premix it for around 30 seconds. Only then should the cement be added. After further premixing for about 30 seconds, the water can be added and the mixing process completed.
This method of adding the pigment during the mixing process is more the ideal than the norm, because it is sometimes simply not possible in practice where onsite concrete is concerned. However, one practical possibility for coloring onsite concrete is to add the pigment directly to the ready-mix trucks. This procedure is, of course, only advisable if the mixing action of the truck mixer is adequate and homogenous distribution of the pigment can be guaranteed. The question should be clarified by carrying out suitable practical trials before beginning the concreting work.
Application of the concrete
For working with coloured, ready-mixed concrete, the same principles apply as for producing exposed grey concrete surfaces. Care should nevertheless be taken with the choice of release agent. If too much release agent is applied, it will get on to the fresh concrete and stain the surface.
The above points, which need to be observed to obtain perfect results, should also be taken to heart for the production of non-coloured exposed concrete surfaces. Experience has shown that only a short time is needed for workers to become so familiar with the technique of coloring concrete that they can produce perfect results every time. The manufacturer will in any case be rewarded for the extra care by the success he achieves in complying with the wishes of the architects and building clients as regards the color of the concrete.
From what has been said so far, the impression may be gained that the process described for coloring concrete is something new. This is certainly not the case. The possibility of using color pigments for coloring architectural concrete has existed for some time now.
In most cases, it is this combination of design and color, which gives a building its unique flair. Nevertheless, it has to be admitted that color on its own will seldom be able to make up for a lack of attractive architecture. However, if the element of color can be used as a supporting element to an attractively designed building, the result will most certainly be well worth looking.
By Lutz Kohnert, Head of Global Competence Center Construction of the LANXESS Business Unit Inorganic Pigments