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Limits to time in transit

BS 8500:2015 +A2:2019 Concrete - Complementary British Standard to BS EN 206

BS 8500-2 Clause 14.2 Time of transport, requires 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.

Note that the time of transport does not include the waiting period/delay on site prior to discharge. Importantly, as stated in BS 8500-1 Clause A.9.1, “Where placed concrete remains compactable by vibration then the continued placement of concrete does not produce a cold joint.” Also that, “...with the use of retarding admixtures the set may be delayed for 12hours or more.

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 has 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.

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Fresh concrete working life