Weathering of concrete finishes

The weathering of exposed visual concrete is affected by the local microclimate, the concrete finish itself and the detailing used to control the flow of rainwater over the surface. It is virtually impossible to ensure that all sides of a building are equally exposed, as inevitably there will be a prevailing wind and rain direction which determines the weathering pattern. It is therefore likely that weathering effects will differ on the various elevations of any building. Some elevations will be washed regularly, whilst others may suffer from an accumulation of dirt which is rarely washed. However, this broad effect is less likely to

Fig. 3.21 Tools for indirect visual concrete finishes

Point tool Bush hammer Needle gun

Fig. 3.21 Tools for indirect visual concrete finishes

Polished finish

Fig. 3.22 Selection of visual concrete finishes

Polished finish

Fig. 3.22 Selection of visual concrete finishes cause unsightly weathering than the pattern streaking on individual facades.

The choice of concrete finish can have a significant effect on the weathering characteristics. Good-quality dense uniform concrete is essential if patchy weathering is to be avoided, and generally a rougher finish is likely to perform better than a smooth as-cast finish. Profiling and the use of exposed aggregates have the advantage of dictating the flow of rainwater, rather than letting it run in a random manner, but dirt becomes embedded in the hollows. Dark aggregates and bold modelling minimise the change in appearance on weathering but, generally, exposed non-absorbent aggregates are likely to give the best weathering performance. Horizontal surfaces may be subject to organic growths and this effect is increased by greater surface permeability.

Careful detailing is necessary to ensure a dispersed and controlled flow of water over the washed areas. The water should then be collected or shed clear by bold details to prevent, pattern staining below. Water collected onto horizontal surfaces should not be allowed to run down facades below, so copings, sills and string courses all should be provided with drips to throw the water off the building face; alternatively water should be removed by gutters. Multistorey facades should be articulated with horizontal features to throw the water off, at least at each storey height. Only on seriously exposed facades where strong winds are likely to cause rain to be driven upwards, should small horizontal drip projections be avoided. Where concrete is modelled, due consideration should be given to the direction of flow and the quantity of rainwater anticipated.

0 0

Post a comment