Concrete and its reinforcement

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Concrete shall be cast so as to achieve the required design strength and durability.

The temperature of the concrete at the point of use should not be less than 5°C (41°F). Fresh concrete is susceptible to frost damage, and freezing can cause internal damage that is not immediately obvious.

Concrete should not be placed in or under water, unless it has been specially designed for that use.

Sufficient concrete should be mixed or ordered, so that it can be placed in a continuous process.

Concrete should be deposited as close as possible to its final location. Transportation on site should be as fast and efficient as possible in order to avoid segregation and to ensure full compaction of the placed concrete.

Site-mixed concrete should be placed within 30 minutes, and ready-mixed concrete within two hours, of water being added to the cement. Additional water should not be added to ready-mixed concrete unless under the supervision and approval of the supplier.

Concreting should, wherever possible, be carried out in one operation, taking account of:

  • weather conditions
  • available daylight
  • time to allow for surface finishing

Concrete cast in one operation (i.e. without construction joints) should always be as square in shape as possible and not greater than:

  • reinforced concrete 60m2
  • unreinforced concrete 16m2.

Construction joints should be formed only where unavoidable and in consultation with the engineer. These should not be positioned next to a return in the foundation. Before work continues beyond the joint, shuttering used to form the joint should be removed.

Reinforced concrete should be fully compacted using poker vibration unless the design states otherwise. Poker vibration should be carried out by experienced operators to ensure complete coverage and to avoid honeycombing. Vibrating beams or hand tamping may be used to consolidate slabs up to 150mm thick, unless the design details otherwise. Excessive use of vibration can cause segregation and prevent concrete reaching an adequate strength.