Excerpt from the book, Formwork for Concrete Structures by Kumar Neeraj Jha
Mold surfaces of wood and steel in due course, after repeated use, warp and rust, respectively, making the molds unserviceable if unprotected. It is common to treat the sheathing materials with some coating or release agent for easy removal. Only a few special form face materials, such as expanded polystyrene, do not need release agents.
The coating or release agents are temporary coatings and composed of fatty acids that react with the alkali in cement. The reaction produces a soap-like substance on the contact surface, which helps remove forms easily. Coating agents can also help enhance the mold life. The form surface remains smooth, provides good abrasive resistance, makes the wooden surface moisture resistant and retards the rusting of steel.
There are many release agent chemical compounds that have been developed for use as the coatings for wood and steel. It is, therefore, important to select the right one.
The three most common types of release agents are:
1. Neat oils with surfactants: mainly used on steel faces, but can also be used on timber and plywood.
2. Mold cream emulsions: for use on timber and plywood – a good general-purpose release agent.
3. Chemical release agents: can be used on all types of form face recommended for all high quality work.
These release agents could be oils, emulsified wax, oil-phased emulsions with water globules, petroleum-based products, catalyzed polyurethane foam, etc. Waste oil is also used as a release agent.
The type of composition of the coating or release agent depends on the following:
1. type of sheathing materials
2. conditions under which it is applied
3. type of concrete
4. finish quality
5. form area
6. ease of application
The primary objective of the coating and release agents is to ensure easy removal of the form material without damaging the form and concrete. The concrete surface should also not get any stain from the application of release agents. It should be possible to apply the form release agent in an even manner on the form surface. The form release agent should not react with the concrete and produce some undesirable substance in the process.
Coatings on all type of forms are employed with the following objectives:
1. Protection of the form for durability
2. On timber, it reacts with organic constituents and provides a uniform surface on each use. Penetration of the chemicals helps control grain or edge effect as well as fill pores in the wood.
3. Chemically active coating reacts with free lime from the fresh concrete and produces water-insoluble soaps. When dried, these soaps act as positive concrete release agents. When wet, they help move air voids along the form.
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