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Concrete Casting News from the Hill and Griffith Company

Specifications for Structural Concrete: Form Release Agents

Posted by Hill and Griffith Company on Jun 11, 2020 4:36:46 PM

Excerpt from the ACI 301 Field Reference Manual

4.9 Form release agents

4.9.1 General - Release agents are materials applied to the form sheathing to prevent the bonding of concrete to the sheathing, keep the formwork clean, and assist the successful production of high-quality architectural surfaces.

4.9.2 Selection - Release agents help produce the concrete surfaces specified in the design reference sample, contract documents, and mockup. Additionally, the following should be considered:

  • Compatibility of the release agent with the form or form liner, admixtures in the concrete mixture and, if used, the form sealer or coating;
  • Possible interference with the adhesion of other materials such as sealants, architectural coatings, and curing compounds to the hardened concrete surface;
  • Allowable amount of any discoloration or staining and the permissible number and size of bugholes on the concrete surface;
  • Effect on stripping time, ease of stripping, and cementitious buildup on the form;
  • Effect of seasonal temperature extremes on application procedures when the concrete placing portions of the project overlap more than one season, which may affect both concrete color and bughole blemishes on the surface;
  • Effect with accelerated curing procedures (especially steam) on stripping and the appearance of the concrete surface;
  • Uniformity of appearance: the same release agent should be used for all the architectural concrete surfaces;
  • Local and federal and federal environmental regulations, especially on volatile organic compounds (VOCs);
  • Dew point of water-borne materials; and
  • Entrapped air migrations in the consolidation process.
Release Agent Applied to Forms 

The safest approach to evaluate several different release agents is under actual use conditions on a test panel, mockup, or non-architectural portion of the project concrete. Information should also be obtained from the release agent manufacturer as to the kind of form surface for which the product is intended and the proper method of application to produce the desired surface appearance because the thickness of the application may affect the quality of the finished surface and air voids.

4.9.3 Types of release agents - Release agents fall into two main classes: barrier and chemically active. Barrier types are water-insoluble materials that include oils without additives (neat oils), diesel oil, paraffin wax, and silicone oils. The EPA prohibits use of uncut or straight diesel oil as a release agent. Barrier-type release agents are not recommended for architectural effects. They tend toward more stains, bugholes, and difficulty with releasing in both very cold and very hot weather, and they can cause problems with adhesion of coatings and other construction materials to the hardened concrete.

Chemically active release agents are the most common for architectural concrete surfaces. Fatty acids chemically react with the basic materials in concrete and produce soap. Soap is a better lubricant than oil for the removal of entrapped air in fresh concrete.

The formation of the soap film from the ingredients in the cement paste and the chemically active release agent prevents the concrete from bonding to the formwork. Applied at the right rate recommended by the manufacturer, the chemical reaction only consumes a very small quantity of the free lime from the fresh concrete. During consolidation, the soap film on the form face is an excellent channel for the migration of the entrapped air out of the fresh concrete.

In vertical casting, undesirable striping effects are sometimes produced when an immersion vibrator is improperly placed very close to the release agent. It is caused by over application of the release agent. The excess release agent is consumed by the basic materials in the concrete raising the w/cm at the points of tangency as the vibration stimulates the reaction. At the secant points, there is not sufficient stimulation of the vibration to change the x/cm; consequently, a striping effect is created. This striping effect will not bleach out. For this reason, control of vibrator intersections is critical to the overall appearance. Other unrelated causes of striping effects exist, such as shadows of reinforcement, porous form facings, and overly wet concrete mixtures.

Each brand of release agent exhibits its own fingerprint of final surface color, although vibration and form surface texture also have a pronounced effect. Using the same release agent throughout a project is recommended for achieving uniform color.

The two common categories of chemically active release agents include both buffered reactive (partially reactive) and fully reactive types.

Buffered for release agents tend to produce an improved soap film that not only helps remove entrapped air but may promote better flow of a thin skin of cement paste at the very surface of the form. This may help explain why, in vertical castings, these release agents tend to minimize or eliminate the striped effect from vibrator insertions.

Fully reactive form release agents can provide a good basic soap film that, depending on the brand, works well in most cases. Because buffered and fully reactive release agents are similar and proprietary, specifying absolute differences between them is difficult. Generally, the buffeted release agents produce a slightly different type of soap film that, with some brands, assists in improving the visual impact.

Properly formulated, both oil-based and water-based form-release agents can meet the Federal Volatile Organic Content regulations of 450 g/L and even the more restrictive value of 250 g/L required in some areas.

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