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Off-shutter Concrete
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Beton Brut
Off-shutter concrete is a raw concrete look that's created by removing the temporary shuttering used to contain the concrete while it sets. The shuttering, usually made of wooden planks, leaves an imprint of its texture in the concrete.
Rough and fair face concrete
Concrete as undisguised material has its origins in the early 1900s in France with the work of Tony Garnier and Auguste Perret, in Switzerland with the famous bridges of Robert Maillart, and in Italy with the magnificent structures of Pier Luigi Nervi. Perret was one of le Corbusier's teachers. The latter's use of beton brut is well known. Maillart experimented with the omission from the structure of material that did no work, leading to elegant and daring designs.
Rough structural concrete is usually covered with a layered finish like plaster, tiles or masonry. Such concrete is not cast very accurately, and a measure of surface imperfection is allowed.
Undisguised concrete is known as fair face concrete. Such concrete is usually accurate, of good quality, and patching is not allowed.
The success of fair face concrete depends on various factors: quality of shuttering, aggregate, mix proportion, vibration and finish.
Fair face concrete can be left unfinished, hacked, sandblasted, brushed or painted.
The following factors should be considered in the design and execution of fair face concrete:
Edges and joints
Sharp edges of slabs, columns and construction joints are difficult to cast straight, are prone to discolouration due to greater exposure, and break off easily. Corners should therefore be chamfered. Special plastic corner fillets are available for this purpose.
Construction joints occur at the beginning or end of a day's casting, or as necessitated by the limitations of the shuttering. Walls and columns are normally cast in storey heights. This eliminates expensive joints, and the greater mass of the plastic concrete eliminates air bubbles rising to the surface, creating weak joints. Construction joints typically occur at bottom and top edges of slabs, columns and walls.
(Wegelin 2009:168-169)