Concrete @ your Fingertips

Limits to time in transit

BS 8500:2023 has dropped the 2-hour ´Time of transport´ limitation clause found in previous revisions. The allowance of new cement types, admixture technology working life, modern computerised production plants and mixer efficiency have all aided improving concrete placement without unneccessary rejection of perfectly good workable concrete due to this prescripitive time constraint, but it still should be considered as a reasonable rule of thumb for a conventional concrete.

Previously, BS 8500-2:2015+A2:2019 Concrete - Complementary British Standard to BS EN 206 Clause 14.2 Time of transport, required concrete to be delivered "within 2 hour 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.

In recognition that concrete may be still compactable after a time in transit, depending on temperature, cement type, use of admixtures etc., ASTM C 94-21 Specification for Ready Mixed Concrete RMC dropped its 90-minute rule – more onerous than BS 8500’s 120 minutes. Introduced in 1935, this limit required the completion of concrete discharge within 90 minutes after the introduction of mixing water to the cement and aggregate. The ASTM sub-committee validated that a discharge time, up to 150 minutes, had no significant impact on the fresh properties, hardened properties, and durability. By reducing rejected loads, it was considered that environmental impacts and greenhouse emissions would be substantially reduced as well as lowering costs associated with waste, unnecessary testing and corrective work.

Note that in BS 8500, the time of transport did not include the waiting period/delay on site prior to discharge. Importantly, it is recognised in the previous and current version of BS 8500-1 that extended delay between the placement of one layer followed by another may produce a cold joint. Also that, “...with the use of retarding admixtures the set may be delayed for 12hours or more.”