Scratching the surface

Post-finishing techniques can create a variety of textures in exposed concrete. Elaine Toogood outlines the main approaches.

The as-struck surface texture of concrete is an exact impression of the formwork facing material or lining that it is cast against. Form liners are available or can be created to order with an almost infinite range of patterns and surface reliefs. Further opportunities for pattern or texture are offered by the removal of all or part of the concrete’s surface – the result depending on how much of the concrete is removed, and the method.

 Acid etching is a common post-finishing technique employed for architectural precast elements, applied a few days after striking in the factory. By removing the smooth outer surface, or laitance, the fine grain and colour of the aggregate in the mix starts to be revealed. With a deeper etch, a different tone and more defined texture emerges, showing more of the aggregate, which means that consistency is important if a more uniform appearance is required. Precast manufacturers are able to provide samples with a range of different finishing techniques and variants.

Abrasive blasting can be used to create a similar effect to etching, depending on the size of grit or shot used and the duration of application. This may also be applied to in-situ concrete. Advance testing is essential for agreeing the blasting technique to be used, and to define the appearance required. Light abrasive blasting uses grit sized 0.2-1.5mm, removing just a little of the surface mortar, and is best carried out 3-7 days after the concrete is poured. A heavy abrasive blast, on the other hand, will remove all of the surface laitance, back to about one-third of the depth of the coarse aggregate and should be carried out when the surface of the concrete is still relatively weak, say 1-2 days after pouring, depending upon the strength gain of the concrete. The use of stencils or templates with abrasive blasting allows the creation of a permanent pattern in the surface.

Surface retardants offer an alternative, controlled means of revealing the aggregate. These can be applied to the face of the formwork or to freshly struck concrete, although the latter is more common for unformed faces. In both situations, the retardant prevents the surface laitance from hardening, so it can simply be washed or brushed away once the concrete is struck. The depth of laitance, or cement matrix, removed, can be controlled by product selection. One patented technique prints digital images or patterns in retardant onto a paper form liner. This allows very precise, repeatable articulation of the surface texture by contrasting aggregates exposed by the retardant, with smooth untreated surfaces adjacent. Even large photographic images can be recreated in the surface of concrete.

Tooling, or breaking the hardened surface of the concrete with tools, can be used to create a rougher surface texture, either with or without broken aggregate. This can be carried out by either hand or machine. In practice, such techniques take time and labour to execute safely, and are therefore far less common in contemporary construction.

Polishing the formed face of concrete is also a mechanical process, but can be safely and cost-effectively carried out over large horizontal surfaces. Manufacturers of architectural precast elements use an automated bridge polisher for large flat areas, progressively grinding and polishing to achieve an ever smoother, shinier surface. The more concrete that is removed, the wider the cross-section of aggregate on show. Smaller, less accessible areas, such as window reveals, can be polished using hand-held machines.

Selection of aggregate is clearly critical to the final appearance of any concrete, where texture is to be created by exposing the aggregate. Designers should consult precast and ready-mix suppliers to establish what aggregates might  be available. Since exposed concrete is at the core of architectural precast products, these manufacturers offer the widest range of concrete mixes and textures – but with an understanding of the techniques, creative possibilities open up for creating textures in all forms of concrete.

Photos: Daici Ano

For further information on post-finishing see Visual Concrete: Finishes, from The Concrete Society