Time limit between placing fresh concrete layers
There is often debate about the time limit between placing fresh concrete against fresh concrete already placed and compacted i.e. minimising the risk of a cold joint being formed.
The Specification for Highway Works clause 1710 section 3 takes a rigid approach in stating:
Fresh concrete shall not be placed against in situ concrete that has been in position for more than 30 minutes unless a construction joint is formed.....
BS 8500-1:2015+A2:2019 Concrete – Complementary British Standard to BS EN 206, clause A.9.1 takes a more flexible approach in stating:
Concrete should be placed into its final position as soon as practicable. Extended delays in placing can lead to a significant loss of consistence with time making it impossible to place with satisfactory compaction. Where there is an extended delay between the placement of one layer of concrete and the following layer then there may be a cold joint between layers. For this to occur the delay should be sufficient for the lower layer to develop an initial set or harden. The time for this to occur depends on the concrete temperature, cement type, mix proportions, the use of admixtures with set retarding properties and consistence of the concrete.
BS 8500-2: 2015 +A2:2019 clause 14.2 requires concrete to be delivered within 2 hours after the time of loading where transported in truck mixers or agitators or within 1 hour after the time of loading where non-agitating equipment is used, unless a shorter time is specified or a longer time permitted by the specifier. These limits are sufficient for normal UK temperatures.
Where required, the specification of a maximum time between placing layers of concrete should take account of the likely temperature of the concrete, cement type, mix proportions, the use of admixtures with set retarding properties and consistence of the concrete.
Ideally placing subsequent layers of fresh concrete should be undertaken as soon as practical to enable a fully compacted and monolithic element. The time a surface remains ‘open’ is not easily defined without taking account of;
- the time the concrete has taken to be discharged after batching
- the ambient temperature
- the cement type
- the admixture(s) used
- the time it takes to be compacted.
A note to BS 8500-1 clause A.9.1 above states:
Where placed concrete remains compactable by vibration then the continued placement of concrete does not produce a cold joint.