Exerpt from the December 2008 issue of Concrete Plant International by Volker Gretschel
For some years now, the trend in precast construction has been towards satisfying the desire for concrete without air voids and other cavities. There are many factors having a major influence on the appearance and quality of formed concrete surfaces. Once the concrete's recipe has been perfected, the key to high-class concrete surfaces is to be found in employing the right release agent.
Release agent types
A great variety of different concrete release agents are to be found on the market. The first release agents were made from mineral oils, which were then increasingly replaced by mineral oil additives. A further step brought about an advance in processing by adding solvents. Solvents improved penetration onto the formwork, and quantity dosage was simplified by the solvent's evaporation. Beginning with the nineties onwards, more concentrated work was carried out, together with colleges, on the development of emulsion products. These have been represented in many manufacturing operations for over 10 years now.
Make-up and mode of operation
All release agents, which are used in larger quantities, have both a physical and chemical separating effect. The physical separation is assured by the oil proportion and the chemical by the additive part. The parts active in the separation process are generally fatty acids or ester. In the case of vegetable oils, there exist no strict dividing lines (physical/chemical) in the fatty acids they possess, i.e., just the oil phase alone can generate a chemical separation. Utilizing paraffin, naphthenic or aromatic oil bases makes little difference from a technical viewpoint but, nonetheless, have considerably more potential drawbacks concerning health and olfactory issues.
Chemical separation generally occurs between the COOH groups in the fatty acids and the calcium hydroxide in the concrete. The product thus created which causes separation is so-called calcium soap.
Further influences come into play through the degree of compaction, the method of compaction, temperature and time. The greatest challenge for release agents is currently the vibratory compaction plants, which have been introduced in the last few years. Although these plants are very quiet, the contact surface is very abrasive and demands a release agent with a very strong adhesive force and excellent chemical separation. The formwork release agents designed for vibratory compaction are very frequently too reactive for agitation compaction or for self and easy compacting concretes. The result is extreme "powdering".
Application method and amount
The quantity of release agent is a very important factor in achieving a presentable concrete surface. It is affected by the application method, the type of release agent, the processor and the nozzle used. The objective is to have a thin and even film of release agent. Systems without solvent agents require very carful processing as the applied with layer is also the residual separating layer. The case is quite different with emulsion-based release agents or those with solvents, as only about one-third of the wet layer is left as a separating film. This means that a consistent reproduction can be obtained even with different layer thicknesses.
A high-pressure sprayer continues to be the most common means of application. Its main benefit lies in its ability to be used precisely and everywhere but this advantage can also be a disadvantage as the result depends very much on the processor and his experience. This can be contrasted with gantry sprayers, which are ideal for large surface formwork, but only of limited use in spraying partial areas. Plant engineering developments are varied here. Plants with heated reservoirs are especially commendable even as they permit consistent viscosity, and thus, enable the release agent to be sprayed constantly even at varying atmospheric temperatures. When solvent-free release agents are employed and heated to 35° C, a very fine spray jet can be generated which, in turn, favors fine application. No further remarks will be made at this point on the utilization of rags, mops, etc., as these methods can only be taken into consideration for special cases.
Spraying with rotation sprayers poses problems in manual processing as the equipment is relatively heavy and fatigue symptoms quickly crop up. However, in combination with stationary spraying equipment, rotation sprayers are a good alternative to high pressure plants. Provision must nonetheless be made for spraying vertical formwork parts separately.
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