Congested reinforcement can lead to many problems of placing and compacting concrete. In extreme cases this can result in poorly compacted areas into which no concrete has penetrated. Typically heavily congested reinforcement occurs in beam/beam and beam/column junctions and in bridge desk slabs. Concrete can be placed in such locations but this requires both good detailing (see separate entry Reinforcement detailing) on the part of the designer and good planning and control on the part of the contractor.
Designers and detailers should be aware of and follow the rules given in Eurocode 2 (which supersedes the now withdrawn BS 8110) for the maximum amounts and minimum spacing of reinforcement in a section. Remember that a line on a drawing representing, say, a 25mm bar is not to scale and that a nominal 25mm diameter bar is in practice nearly 30mm in diameter due to the deformations and the use of tying wire. Where problems are likely to occur, a scale drawing (or CAD image) should be prepared, to check whether the poker vibrator can pass through the congested reinforcement.
Practical advice for detailers and contractors is given in Concrete Advice No. 04, Congested reinforcement: effects on placing and compacting concrete.