A discharge window defines the time allowed for completing discharge of ready-mixed concrete. It puts an explicit time boundary around delivery, making elapsed time a condition of the transaction between purchaser and producer.
From ninety minutes to an agreed limit
In its August 2024 explanation of the ninety-minute rule, the American Concrete Institute describes the earlier requirement in ASTM C94/C94M, Standard Specification for Ready-Mixed Concrete. The specification used to require completion of discharge from a concrete truck within 90 minutes of batching.
ACI states that, starting with the 2021 edition, the rule was replaced by an alternative in which the purchaser and the concrete producer establish a time limit.
What extended-discharge testing examined
A study published on July 26, 2024 in Construction and Building Materials, Extended discharge time of ready-mixed concrete: Myth or necessity?, evaluated three different concrete mixtures. The programme examined both fresh and hardened concrete properties over a sequence of elapsed times.
Evaluation at batching supplied the initial observation, followed by tests at 60, 90, 120 and 150 minutes. The schedule includes an interval before the earlier ninety-minute limit, an interval at that limit, and intervals beyond it.
The mixtures were maintained with admixtures as needed to preserve workability without altering the mix design. The study describes a high-range water reducer (retarder or stabilizer) and air-entrainment admixtures. The comparison is consequently about elapsed time with workability maintained in the manner described, rather than elapsed time with no provision for maintaining workability.
The fresh-property measurements were air content, slump and temperature. The hardened-property measurements were compressive strength, freeze-thaw resistance and surface resistivity.
The reported result and its scope
The authors reported that discharge time had no significant impact on the fresh and hardened properties, even through 150 minutes. That finding describes the three mixtures evaluated with the admixtures incorporated as needed.
It does not supply a separate agreed discharge limit for a transaction between a purchaser and producer.
Time limits and measured properties
A time limit defines the allowed period for completing discharge; the tests describe how the evaluated concrete performed over the periods examined. A finding about measured properties does not itself change the terms under which a load is accepted.
The broader discussion of fresh and hardened concrete testing explains the measurements used to describe concrete, while delivery geography places the discharge window within the movement of a load from plant to site.