BS 8500-2:2023 Concrete – Specification for constituent materials and concrete does not provide a time limit on from when the water is added to cement and discharge. A 2-hr period was included in previous versions of this standard e.g. BS 8500-1:2015, Clause 14.2.

Fresh concrete has a limited ´shelf life´ when casting an element or structure. The 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 may be considerably shorter or longer and will depend on the ambient and concrete temperature, cement type, mix proportions, the use of admixtures and consistence. Provided the concrete remains sufficiently workable for final compaction and finishing there is no time limit.

The time was advised as an approximate, not a definite. Some specifications 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. The use of consistence retention admixtures in combination with cement type, temperature etc. gives the specifier a wide range of ´open time´.