The use of an appropriate release agent, correctly applied, is critical to creating a good, consistent surface quality
by Elaine Toogood
Originally published in the Winter 2018 Issue of Concrete Quarterly.
Applying a release agent to the face of the formwork is like greasing a cake tin. Without it, the formwork cannot easily be removed or released from the hardened concrete.
According to the Concrete Society publication Visual concrete – Planning & Assessment, there are essentially two types of release agent: barrier and reactive. These are further divided into eight categories, though developments in this field mean that some products do not easily fall within these categories.
Not all release agents are suitable for creating visual concrete and selection will be based on many factors, including compatibility with form-facing material, concrete specification and expected site conditions. A good release agent will also help maximize the number of times that a form can be used.
4 Pancras Square by Eric Parry Architects, where the post-tensioned slabs are exposed as soffits. The concrete contractor specified a concrete mold oil release agent for use with the plastic-faced ply lining of the formwork system.
Photo credit: Rory Gardiner
Barrier release agents work by creating a layer between the form face and the concrete. Oils with surfactants are general-purpose release agents for many different types of formwork, including steel, and are often used in the precast industry. None of the other barrier types such as neat oils, mold cream emulsions, water-soluble emulsions and barrier paints are recommended for high-quality finishes. Although barrier paints may be part of a preparatory treatment for timber and plywood before it is first used to extend the life of the formwork.
Most of the reactive types of release agents are suitable for visual concrete and are categorized as chemical release agents, surface retardants and other specialist release agents, including those based on vegetable oil (VERA). They allow the formwork to be struck by creating a very thin layer of unhardened concrete or “soap” on the surface that must be brushed away when the formwork is struck. Chemical release agents are a popular choice but can lead to dusting if over-applied. Although recommended for high-quality finishes, they may not be appropriate for concrete containing silica fume or high amounts of admixtures. VERA fall under the category of other specialist release agents and are recommended for visual concrete, with the added advantage of being non-toxic and biodegradable, and reportedly with a low incidence of blowholes and blemishes.
Design professionals do not need to specify the release agent that is to be used, but rather provide a performance specification. This directs the contractor to select one appropriate for creating high-quality visual concrete surfaces, that has little or no detrimental impact on the appearance of the concrete and that suits the choice of formwork facing and concrete used. Consultation with the manufacturer and form-facing material supplier will be necessary to select the best product and method of application for the specific project. Biodegradable and non-toxic products may also be identified for health and safety reasons.
The selected product or products should then be tested with a full-scale mock-up panel or in a non-critical location of the structure to review the results and trial the method of covering the formwork before final selection. Release agents should be applied in accordance with manufacturers’ guidance – they are often sprayed or applied with a soft brush – to create a thin, uniform and complete coating. Once satisfactory results have been established, the standard of workmanship must be rigorously maintained to ensure consistency. Incorrect application, whether too much or too little, can lead to abrupt color or tone changes in the concrete surface. The surface of the formwork should be cleaned and a coating of release agent applied before every use.
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