Shuttering
Shuttering is the oldest method of shaping concrete in modern times. Made of untreated softwood, the boards can only be used between two and four times before they have to be replaced. The first shutters were plain, rough boards, inserted as needed, old next to new, with no special attention paid to moisture content or wood type. They did not provide a very good seal, but this remained largely without effect because the tamped concrete then common had no liquid mortar content. Concrete surfaces were rough, but there was no bleed-through or pore-formation on the surface as the wood absorbed air and water bubbles near the surface. Sharp edges and corners were not possible, nor required. Eventually planed tongue-and-groove boards started to be used; and they still are in some areas of construction (e.g. bridge-building).
When working with wooden shuttering, it is important to coat the inner face first with wet cement or concrete to prevent the wood sugar impairing the surface of the concrete. Also, it must be borne in mind that the absorptive properties of the shuttering decrease with each application, and that varying moisture conditions can give rise to deformation of the wood. Producing high-quality visual surfaces with wooden shuttering is especially difficult and a job for contractors with extensive experience. The desired result should be thoroughly tested and sampled in advance. After the end of the 1960s, however, rough shuttering boards virtually disappeared from new builds with the advent of framed formwork.
Smooth formwork
Smooth formwork is formwork that produces consistently smooth surfaces. The features of these surfaces are the joints between formwork panels and, on walls, the holes for the formwork ties. Further design options for smooth concrete surfaces are restricted largely to joint patterns and colour (see below). Smooth formwork with a non-absorptive skin does not automatically produce blemish-free surfaces, however. In projects where very specific criteria are required of the final finish, proper planning and testing is important.
The danger with visual concrete is that contractors will opt for the same kind of formwork sheets that they use for ordinary concrete. The designer should therefore get involved at an early stage in the choice of sheet and release agent. Only after successful testing should the parameters be fixed and purchases made.
Formliners
Formliners are flexible plastic liners which imprint a surface texture onto the concrete. A range of standard textures is available, but special one-off designs are possible. For this a plastic cast is made which then forms the mould for the concrete. If carefully handled plastic liners can produce consistent results for up to 100 times. The liners consist of a rubber or plastic mat, 8-10 mm thick, plus the depth for the surface profile. For a more robust product, the flexible liners are fixed to a foam-plastic backing board with adhesive. This is especially suited to deep surface textures and it also simplifies handling as it gives added strength. The strengthened liners are then either stuck onto or inserted into the formwork.
Filter mats
Filter mats are drainage mats applied directly to the inner face of the formwork. Excess water and air bubbles close to the surface can transpire through this layer. The resulting surfaces are pore-free and display a fine, even colour. However the colour appears darker because the fine texture of the mat is imprinted on the surface of the concrete. The additional cost of filter matting is compensated by reduced outlay on formwork lining, release agents and time-consuming tests.
Oriented strand board
Oriented strand board (OSB) is mostly used as additional sheeting in framed formwork. The characteristics of OSB have an effect on the concrete surfaces. The chips in the material remain clearly visible on the concrete and the absorption of OSB gives rise to areas with no pores. The fresh wood in OSB introduces unevenness in the curing process, thus generating rough, darker areas. The formwork swells slightly by taking up moisture from the concrete, causing joints to close. No wet cement can escape, thus colour continuity is retained.
Exposed aggregate finishes
In exposed aggregate finishes, a thin layer of cement and fines is removed to reveal the aggregate. This was popular between 1965 and 1980 but has now almost entirely disappeared, except in cast stone components. Nevertheless this type of finish still offers interesting design potential and low-relief textures are currently being explored. The best quality finish is achieved in the factory, on precast components. Handling the wash water needs careful consideration. A prerequisite for the finish is the application of a retarder to the formwork. No release agent is needed. The depth of the surface is generally controlled to between one and six millimetres. Aggregate sizes in the concrete mix are chosen to match the desired effect. Often in prefabricated components, a facing with an exposed aggregate finish is attached to a structural concrete core.
Photographic effects
Concrete can be made to look as if a photograph has been embedded in it, by applying retarder to a backing foil in line with the variations in the photograph, and then washing out the concrete surface to generate a greyscale copy of the original. Different thicknesses of retarder give rise to slightly different depths in the final washed surface, thus marking out the details in the photograph. Because of the need for precision, this process is only attempted in the factory on precast components.
Acid-treating
Applying an acid solution to concrete surfaces also offers interesting design possibilities. The acid loosens the wet cement which is then removed mechanically. Texture and depth are easily controlled. In terms of colour, acid-treated surfaces have good brilliance, so the technique tends to be used mainly on lighter surfaces. This process is also restricted to factory-made components.
Blasting
Blasting finished concrete with solid abrasives is a cheap option for surface design. Sand is the most common abrasive, and dry-blasting the most convenient method. However, the final result is difficult to predict. In particular problems occur with in-situ concrete (columns and walls) in terms of evenness, because mixture strength and aggregate distribution varies around the perimeters. When planning blasted surfaces particular care has to be taken over corners, edges, recesses etc., which could suffer irreparable damage. Tests are recommended in advance. Also advisable is to ensure that all the blast work takes place on concrete at the same stage of curing – in the case of in-situ concrete, this should be late, at around ten to twenty days, for precast concrete, after two or three days.
Surface dressing
Surface dressing, such as bush-hammering, boasting, grinding and polishing, is offered by stonemasons specialising in concrete finishes. All involve removing part of the outer layer of concrete. With proper consultation, trials and sampling in advance the results are mostly satisfactory and predictable. Bush-hammered finishes are produced with a hammer fitted with different heads. The finished surface can be up to 20 mm deep or very fine. Suitable areas for working are level surfaces without any major defects. Cracks are not removed by this process. Coarser textured finishes tend to be reserved for exterior components where they disguise the effects of weathering. Fine bush-hammering is suitable for interior applications, where it gives a quality feel. A mottled effect can be achieved on coloured surfaces. It is also a way of “rescuing” smooth visual concrete surfaces which did not succeed. In boasting a flat tool is used instead of a pointed one. The marks made by the tool are used as a feature of the design. This technique is often employed in combination with bush-hammering, particularly around edges and other accented areas. Grinding the surface reveals the aggregate and the concrete matrix but does not break it. Ground surfaces are generally smoother than the original surface and they look like a cross-section of the concrete. Polishing is carried out on an already finely ground surface to produce a “mirror” finish. However, this high-gloss effect wears off after about a year.
Colour design
Basically there are two ways of introducing colour to visual concrete: either through pigmentation of the concrete mix, or colour-treating the finished surface. To simply lighten the surface colour, white colour pigment (titanium oxide) can be added. For very light or white surfaces, white cement must be used, and if the whole mass has to be white, white pigments are also added. Coloured pigments are added to achieve coloured finishes. For brighter colours (yellow, red, green, blue etc.) the basic concrete mix is made with white cement, but for darker shades (ochre, brown, black) light grey cement is used. Ordinarily between 3% and 6% of the cement content is added as colour pigments. Careful weighing is required, also a longer mixing time and prewashing of mixing tools and transport vehicles. Material costs for coloured concrete are considerably higher than for normal concrete. To colour treat a finished surface the only option at present is a permanent colour glaze. This is also useful for treating areas of smooth visual concrete which are unsatisfactory.
