For this week's educational news article we'll highlight an excellent article written by Sidney Freedman, Director, Architectural Precast Division, Prestressed Concrete Institute; Chicago IL. You can download the full article here.
A form release agent must do several jobs:
- Permit clean release of formwork
- Protect the formwork
- Help produce a hard, non-powdery, stain-free concrete surface
- Prevent corrosion of steel forms
- Release agents should have a reasonably long and stable storage life and should not be susceptible to damage from extreme temperature changes or from rough or repeated handling.
- The manufacturer's recommendations on the rate of spread and the method of application should be sought and followed.
- Any of various application methods can be used, depending on the type of agent. Spray or rolling methods are most commonly used because they are inexpensive and the produce uniform films. When spraying, a low-pressure, fine fog fanning out from the nozzle is most desirable.
- Whatever method of application is used, the release agent should be applied evenly in a very thin film without bubbles or streaks.
Agents commonly used with various forms
- Wood and plywood forms - Most of the commercial oils are satisfactory, straight refined, pale, paraffin-based mineral oil and oil-phased emulsions have been successfully used.
- Metal forms - Release agents for steel forms should contain a rust inhibitor and be free of water.
- Fiberglass forms or plastic form liners - An oil-phased emulsion or a high quality household wax containing carnauba wax are the preferred release agents.
- Rubber form liners - Most rubber mattings do not require applications of release agents if the surface is thoroughly cleaned and moistened with water just before concrete is placed.
- Plastic foams - Paraffin oil generally is lightly sprayed over plastic foam forms or form liners.
- Concrete forms - Light colored petroleum oils or oil emulsions of various types have been used successfully.
Choosing the form release agent
The safest approach is to evaluate several different commercial release agents under actual use conditions, either on a test panel or on a non-architectural portion of the concrete on the project.
(Photo from the National Precast Concrete Association)
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