Concrete Finishes Smooth finishes

In direct as-cast concrete, the surface texture and water absorbancy of the formwork , or any formwork lining, directly determines the final exposed fairfaced finish. A high level of quality control is therefore required to ensure a visually acceptable finish. Hard shiny non-absorbent formwork materials such as steel glass-fibre reinforced polyester (GRP) or plastic-coated plywood can give surfaces which suffer from map crazing due to differential shrinkage between the surface and underlying bulk material. Additionally, blow-holes caused by air bubbles trapped against the form face may spoil the surface if the concrete has not been sufficiently vibrated. Where the absorbency of the formwork varies, because of the mixing of new and reused formwork, or variations within the softwood timbers, or because of differing application of release agent to the formwork, permanent colour variations may be visible on the concrete surface. Release agents prevent bonding between the concrete and the formwork, which might cause damage to the concrete on striking the formwork. Cream emulsions and oils with surfactant are typically used as release agents for timber and steel respectively. Formwork linings with controlled porosity can improve the quality of off-the-form finishes, by substantially reducing the number of blow-holes. The linings allow the escape of air and excess moisture but not cement during vibration. A good-quality direct-cast concrete should exhibit only a few small blow holes and modest colour variation.

The application of paint to off-the-form concrete will emphasise the surface blemishes such as blow holes. These become particularly noticeable if a light colour gloss paint is used. Surface defects must therefore be made good with filler before priming and subsequent painting of the concrete.

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