APA - The Engineered Wood Association

Concrete Form Maintenance & Surface Characteristics

Stripping

Metal bars or pry bars should not be used on plywood because they will damage the panel surface and edge. Use wood wedges, tapping gradually when necessary. Plywood’s strength, light weight and large panel size help reduce stripping time. Cross-laminated construction resists edge splitting.

Cleaning and Release Agent Application

Soon after removal, plywood forms should be inspected for wear, cleaned, repaired, spot primed, refinished and lightly treated with a form-release agent before reusing.

Handling and Storage

Care should be exercised to prevent panel chipping, denting and corner damage during handling. Panels should never be dropped. The forms should be carefully piled flat, face to face and back to back, for hauling. Forms should be cleaned immediately after stripping and can be solid-stacked or stacked in small packages, with faces together. This slows the drying rate and minimizes face checking.

Coatings and Agents

Protective sealant coatings and release agents for plywood increase form life and aid in stripping. “Mill-oiled” Plyform panels may require only a light coating of release agent between uses. Specifications should be checked before using any release agent on the forms.

Plywood form coatings, such as lacquers, resin or plastic base compounds and similar field coatings sometimes are used to form a hard, dry, water-resistant film on plywood forms. In most cases the need for application of release agents between pours is reduced by the field-applied coatings.

Effect of Admixtures on Forming Panels

Admixtures are chemicals added to a concrete mix to change the properties of a basic mix of cement, water and aggregate. They can speed or retard setting times, increase workability, increase air content, decrease water permeability, increase strength, etc. Admixtures include pozzolans such as silica fume, blast-furnace slag and fly ash.

The use of admixtures has become relatively common and many of these additives increase abrasiveness and/or alkalinity of the concrete. While wood and phenolic overlays are very resistant to alkaline solutions and abrasion, the use of admixtures may significantly decrease the “normal” life of a concrete-forming panel. The examples of reuse life in this publication assume standard concrete mixes with minimal or no use of admixtures.

Concrete Surface Characteristics

Surface dusting of concrete has occasionally been observed in concrete poured against a variety of forming materials, including plywood. There appears to be no single reason – the soft, chalky surface has been traced to a variety of possible causes, including excess oil, dirt, dew, smog, unusually hot, dry climactic conditions, and chemical reactions between the form surface and the concrete.

Various means of rectifying the problem have been successful. Preventive measures include proper form storage (cool, dry conditions) and cleanliness (avoiding needless exposure to dust, oil and weathering). For more information, see APA Technical Topic: Dusting of Concrete Poured Against Plywood Forms, Form TT-029.

Staining is occasionally observed on concrete poured against HDO plywood forms. The reddish or pinkish stain is a fugitive dye, and usually disappears with exposure to sunlight and air.

On rare occasions, other discolorations have been observed in new concrete. For example, iron salts resulting from iron sulfides and ferrous oxides in slag cement have been found to stain concrete a greenish-blue color, particularly when large, continuous, smooth and airtight form surfaces are used.

Ferrous sulfides in the coarse aggregate, such as pyrite and marcasite, can cause rust-colored stains on the concrete. For more information, see APA Technical Topics: Staining of Concrete Poured Against Plyform, Form TT-059.