Range of finishes
There are two main types of finish:
n Those produced direct from the formwork, often called 'as struck' finishes, comprising plain, smooth finishes, and textured and profiled finishes such as board-marked concrete and ribbed or striated and modelled surfaces
n Those produced indirectly by further treatment after the formwork is removed, including exposed-aggregate and tooled finishes.
The two types are often combined to produce, for example, a striated and tooled finish, or a modelled exposed-aggregate finish (Figures 31 and 32).
Standard of finish
Concrete can be produced within close dimensional tolerances, but the inherent variations in colour and the presence of some small air bubbles or blow-holes on the surface produce a finish which is seldom entirely blemish-free; it is unrealistic to expect perfection in appearance. It is essential to have a full-size section of part of the structure produced as a sample in order to avoid later dispute about the quality of the work. Guidance on the standards of surface finish is given in the BCA series of publications Appearance Matters and Concrete Society Technical Report 52,
Plain formed concrete finishes. To judge the quality of the finish, the sample should be viewed at the same distance from which the job will be seen.
To provide a reference to the surface finishes Types A and B as specified in BS 8110, sets of panels have been manufactured and installed at seven regional centres throughout the UK. They can be visited freely and should reduce the conflict that can occur due to differing interpretations of the standard.
Influence of formwork
A concrete surface will reproduce every detail of the form surface from which it was cast. The formwork must remain watertight against the pressure of concrete in order to prevent leakage. Any loss of water from the fresh concrete will result in a dark area on the completed surface, which extends into the concrete and cannot usually be removed by tooling - in fact more often than not tooling makes the blemish more obvious. Caskets may be necessary to maintain watertightness at the joints and, in the case of exposed-aggregate and tooled finishes, the formwork joints may be sealed with adhesive tape, joints in the formwork, even well made ones, will show in the finished concrete; the position of formwork joints should therefore be planned.
The standard of finish specified and the number of uses required for the formwork usually affect the choice of facing material. The surface characteristics of the formwork have a profound effect upon the appearance of the concrete. Unsealed plywood - because it is slightly absorbent - will cause relatively few blow-holes, but variations in the absorbency of the timber grain will produce corresponding variations in the colour of the concrete. On the other hand, impermeable formwork will give rise to more blow- holes, although the colour may be more uniform. A further complication is that concrete cast against smooth impermeable surfaces may have a dark, almost black, surface (as described under Surface treatment on page 43). The colour variation may be reduced if the formwork has a matt surface which will retain the release agent, rather than a smooth shiny one from which the release agent is removed during placing and compaction of the concrete.
A thin coating of release agent should be applied to the formwork each time before the concrete is placed to prevent adhesion, and thus make formwork removal easier when the concrete has hardened. Release agents must be applied sparingly otherwise the surface of the concrete could be adversely affected. There are several different types of release agent and it is important to use one that is suitable for the form material in question. Recommendations of types and applications are given earlier (page 43) under the heading Surface treatment.
The quality of surface finish may be significantly improved with the use of a controlled-permeability formwork (CPF) system; see Types of formwork page 43. Here a patent woven synthetic fabric is securely fixed to the form face and the concrete cast in the normal way. Bleed water that would cause unsightly marking of vertical surfaces is drained away and blow holes of entrapped air are also absorbed into the fabric. The resulting concrete surface benefits both from a blemish-free appearance and, by virtue of having been effectively dewatered, the free water/cement ratio in the cover zone is reduced, making the concrete more durable.