Fresh concrete working life
Fresh concrete has a limited shelf life when casting an element or structure. A 2-hour shelf life has been a historic general rule of thumb that should not be exceeded based on setting time test data from cement manufacturers. In practice the time required will depend on the ambient and concrete temperature, cement type, mix proportions, the use of admixtures and consistence.
BS 8500-2:2015 Concrete – Specification for constituent materials and concrete gives a 2-hr period from the point water is added to the cement. This is advised as an approximate not a definite. Some specifications also call for a tighter time frame between loads placed to ensure that the risk of cold joints or lift lines is reduced. However, provided the concrete remains sufficiently workable for full compaction and finishing there is no definitive time limit within which the concrete can be cast. It may be considerably shorter or longer.
For applications such as piling and mass pours, an extended working life is often necessary to ensure the sound casting of an element so as not to affect the structural integrity of the concrete. Similarly, if delays in transportation or extended travel distance are expected, which will reduce the useful life of the fresh concrete on site for placing and compacting, the 2-hr rule of thumb may be inappropriate. A reputable concrete supplier will be able to provide a concrete mix that meets the specifier’s request.