Anyone who uses pigments to colour precast concrete elements knows the challenges: exact dosing, uniform colour effect, standard requirements for reinforced components. Small errors in the process can have a big impact here. In our technical application information for colouring precast concrete elements, ATI for short, we have put together a practical guide:
The colouring of precast concrete elements is achieved by adding synthetic inorganic black, white and coloured pigments (Bayferrox® -iron oxide pigments, titanium dioxide, chrome oxide green, HEUCODUR® - mixed oxide pigments) possible.
Efficiency meets safety
In many precast concrete plants, the paint is usually dosed manually due to the lack of an automatic dosing system. In this case, the weighing process represents both an additional process and a potential source of error. For this reason, we recommend that you have the pigment to be used packaged by our small packaging service. Then all you have to do is empty the pigment pack into the concrete mixer at the right time (to aggregate the rock). Liquid paint can also be filled in portions in small hobbocks. Attention: In our “ATI Principles of Concrete Colouring”, it is described that pigment and aggregate should be mixed together for about 30 seconds before the cement enters the mixer. There is often no provision for a cement delay in the automatic mode of the mixer control system. The system should therefore be operated manually.
We can offer further assistance by packaging the solid pigment forms (powder, granulate and compact pigment) in water-soluble bags. This makes it unnecessary for the user to pour out the bag. The complete, sealed bag can be thrown into the cement mixer. There is no dust and, as a rule, no contact with the pigment.
Quality in accordance with standards — for reinforced concrete
Precast concrete elements are often reinforced components. Pigments that are used to colour steel-reinforced concrete and must therefore meet special requirements in accordance with DIN EN 12878: The proportion of water-soluble components of the pigments used for this purpose must not exceed 0.5% and the proportion of chloride must not exceed 0.1%. This prevents corrosive salts from entering the concrete as a result of the pigment, which could attack the reinforcement. Pigment preparations such as liquid dyes, granules or compact pigments contain water-soluble additives in an amount greater than 0.5%. However, in accordance with DIN EN 12878, these pigment preparations may be used to color reinforced concrete if these additives comply with the concrete additive standard EN 934-2 and do not exceed a water-soluble content of > 5%. Pigments and pigment preparations which meet these requirements are classified as “Category B” (can be used in reinforced concrete) in accordance with DIN EN 12878. Only Category B products should be used to color reinforced concrete. Products that do not meet these strict limits or are not subjected to these tests comply with Category A (can be used in unreinforced concrete).
We provide users with a variety of Bayferrox® powder pigments and granules that meet these requirements. In the area of liquid paints, we have developed a product line that is specifically suitable for colouring reinforced concrete in accordance with DIN EN 12878: Scholz F-BB liquid paints. Appropriate test certificates from independent testing institutes are available for all these suitable product groups. Of course, all our products that are used to colour concrete come with a declaration of performance and the CE mark.
Surfaces with character
Coloured precast concrete elements require uniformity in surface finish and colour scheme to a greater extent than with other concrete products. In order to achieve this, the manufacturer of concrete components is required to ensure the greatest possible consistency in terms of the raw materials used, the manufacturing process and the manufacturing conditions (temperature, humidity). Large-format prefabricated parts in particular with distinctive visible surfaces, which are made from more than one concrete mix, do not forgive differences in water/cement value or fluctuations in raw materials. It is also important to ensure that mold release agents (shallot oil) are applied evenly and extremely thinly. The metal vibrating tables or molded parts must be demagnetized, particularly when using color pigments. Black iron oxide is magnetic and when magnetic fields such as those built up by pieces of magnetic metal exist, pigment segregation and thus inhomogeneous colors can occur.
Our pigments are generally considered chemically passive, which means that there are normally no chemical reactions with other substances contained in concrete. However, it has been shown that the use of high-performance plasticizers containing PCE can lead to undesired interactions between these substances and the pigments. These interactions influence the development of the colour force of our pigments. Particular attention should be paid to this when creating sample bodies or test surfaces. If necessary, the flow agent must be replaced by a compatible variant. In case of doubt, please contact our application engineers (see ScholzInfo Sheet “Contact Details”). For more information on this topic, please read our ATI Black Colouring II.
Even if you take the greatest possible care, there may be variations in colour or texture on the concrete surface. Fluctuations in consistency, efflorescence, streaks or cloud formation are the causes here. You can find more information about efflorescence in our ATI lime efflorescences. Just the following should be said here: efflorescence can occur when free calcium hydroxide, which is produced when the cement sets, reaches the concrete surface and reacts with carbon dioxide in the air to form the white lime deposit. This coating will be replaced by weathering over time, but depending on the weather conditions, it may take several years. If you want to get rid of efflorescence more quickly, you have to process the concrete surface mechanically or chemically. Sandblasting removes efflorescence, but also changes the surface condition. Commercially available lime removers are used for acidification. However, care should be taken that these acid-based agents can also attack the cement stone, especially when used in concentrated form. In any case, the manufacturer's processing and application instructions should be followed.
Cloud formation is caused by excess water. Excess water is the result of segregation, which can occur when the concrete is shaken. Only mechanical or chemical surface treatment, as described in the previous chapter, can help with this.
Even if manufacturers and processors take the necessary care, there may be variations in shade and colour intensity, which are ultimately acceptable to all parties involved. In order to clarify in advance what requirements must be placed on the concrete structure, the “Factblatt Sichbeton” published by the BDZ (Bundesverband der Deutschen Zementindustrie e. V.) and the DBV (Deutscher Beton- und Bautechnik-Verein e.V.) provides procedures and guidelines. Here, exposed concrete classes are formulated with technically clear criteria for the requirements for texture, porosity, colour uniformity and working and formskin joints.
Many architects and planners have recognized that absolute uniformity in surface finish and colour is more detrimental to the visual effect of a concrete structure than beneficial. Slight fluctuations make a building appear unique and lively or natural. As a result, some fluctuations are now even produced in a targeted manner. Even the use of different colours to produce multi-coloured finished parts is practiced.
Your partner for coloured precast concrete elements
Ob powder pigment, granules or liquid paint — we offer solutions that are tailored to the requirements of precast concrete plants. From advice on the correct application to detailed technical information, our Scholzianer to the side.