M RAs must be applied in a thin uniform layer covering the entire surface area of the unit, which is to either be filled or that will come into

contact with the concrete. One of the most economical ways of achieving a full uniform coverage would be to use a spraying unit with a flat spraying nozzle.

MRAs can also be brush or rag applied. However, this application is not the easiest or most economical way to apply MRAs, as under- or over-application of release oil can occur easily, which in turn, can have detrimental effects on the resultant cast concrete.

Some of the problems that can be caused from excessive amounts of release agent are discolouration, surface retardation and dusting on the surface, none of which will benefit the concrete and may result in units being scrapped or in need of repair, providing a great loss in both production time and an increase in wastage.

Water-based emulsion oilsWater-based emulsion release oils are now becoming more favourable within the industry due to their effectiveness and ease of use. They are produced by homogenisation, ie, mixing oil and water, which gives the emulsion oil a stable milky white appearance. Emulsion oils are also more environmentally friendly than the full oil- and solvent-based MRAs.

One of the many benefits of using this type of mould-release oil is that, because of the colour and recommended sprayed application, when sprayed onto the mould or panel surface, a white speckled residue is left showing where the oil has been applied. A further ten to 30 minutes after application, depending on temperature and relative humidity, the mould release agent will show a transparent appearance. Within this short time, the small oil drops will form a coherent demoulding agent film (self-levelling effect).

By using a water-based emulsion release oil, it can be seen how much MRA has been applied, reducing the uncertainties of whether there’s been full application or not. Correct application will produce aesthetically pleasing, problem-free products every time.

As good working practice, training and demonstrations showing the application of these emulsion MRAs should be provided by the company supplying the material. This will ensure that the advantages inherent in the product will be fully

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Mould-release oil application in precast concrete

Mould-release agents (MRAs) are a critical factor when it comes to precast concrete; they can determine both the appearance and durability of the finished units. There are many different types of MRAs on the market including solvent-based, full oil-based and water-based emulsion oils, all of which are used within the precast industry. The type of MRA used in production is dependent on many factors, including; preference, application and, by far the most scrutinised, price. Concrete report.

MRA application witha flat spraying nozzle.

Top: Too much release agent has been applied.

Above: Correct amount of release oil has been applied.

By using a water-based emulsion release oil, it can be seen how much MRA has been applied, reducing the uncertainties of whether there’s been full application or not.

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achieved. As with any release agent, correct application of the product is essential in order to maximise the emulsion oil’s potential. It is important not to overdose and in fact with certain emulsion types as little as 20g/m2 may be required.

A further benefit to this type of MRA is that, because it is visible upon the formwork when it is spray applied, after training, an operative applying this material consistently would be able to train his/her own eye and movement to ensure correct application of the product.

Test resultsSika has carried out extensive laboratory and field test evaluation of Sika Separol W water-based emulsion oil with a full oil mould-release agent. One method involved

using a wet precast mix design, which was placed into a transparent mould coated with the appropriate MRAs and poker vibrated for five to ten seconds.

Results from these trials showed that after only ten seconds the emulsion oil had aided the compaction of the concrete, whereas the full oil-based MRA needed further compaction in order to achieve the same standard. Further benefits in using this type of MRA included noise reduction, time saving, improved surface appearance and increased production capabilities.

In tests carried out by Sika, Separol W water-based emulsion oil MRA showed superior results when directly compared to full oil MRA. Whichever MRA is chosen, correct application is important. Too much mould oil will cause many different problems to both the aesthetic characteristics of the completed unit as well as the long-term durability of the actual concrete. It is therefore always important to seek the correct advice when using a new mould oil.

Follow the guidelinesProblems may not be initially seen while the concrete is being placed – possibly into thousands of moulds over a 24-hour period – but the problems will be evident after the concrete has set. Picture framing and retardation of the concrete are just two examples of problems arising from excessive mould-release oil application. Manufacturers’ guidelines should be followed, production trials should be carried out and even a tool-box talk and demonstration if available should be given to all staff involved with the day-to-day running of a concrete production line. Carrying out these simple steps will ensure the correct oil type is chosen. ●

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52 concrete APRIL 2011 www.concrete.org.uk

Below: After spraying,a white speckled residue

is left.

Below centre: After a few minutes the speckled appearance begins to

disappear.

Bottom: After a further ten to 30 minutes following application, depending

on temperature, relative humidity and wind, the

mould-release agent will show a transparent

appearance.

On the left of this picture, the mould was treated with traditional full oil MRA and on the right with emulsion-based MRA. After only ten seconds of vibration the emulsion oil has aided the compaction of the concrete, whereas the full oil-based MRA would need further compaction in order to achieve the same standard.

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