Current Practice Sheet 134. Achieving quality

Summary

The correct concrete must be specified to achieve a good quality concrete surface, and in addition the quality of workmanship in the formwork production, the application of release agent and mixing, placing, compacting and finishing are all contributory factors. BS 8500-1: 2002 addresses the specification of finishability. Advice is given on concrete composition ( including sand and aggregate sizes ), workability and water:cement ratio. The lack of a fine sand fraction ( see BS EN 12620: 2002 ) may lead to bleeding and sand scouring, whilst incorrectly-sized coarse aggregate may form surface holes by bridging. Concrete should be remixed prior to placing in layers of 400-500 mm thickness, with each layer being fully compacted before the next is placed. Dark lines at the tops of walls and columns may be avoided by preventing drying of the concrete whilst the formwork is in position. Formwork may contribute to colour variation, but this can be avoided by preventing the surfaces from drying out and by correct curing. Weathering may also lead to variations in visual appearance. 12 refs.

Primary author(s): Day R; Roberts D

Source: Concrete

             37, No.10, p.45-46

 

                             

 

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