Extended discharge time of ready-mixed concrete: myth or necessity?

A significant volume of pre-mixed concrete is frequently sent back to batching plants for disposal due to various factors such as excessive slump loss during transportation, overproduction, and experimentation with new batches. It is estimated that approximately 3 % of the total concrete production becomes waste. Rejecting numerous truckloads of concrete is typically based on strict adherence to the concrete discharge 90-minute time limit. This study investigates the effects of extending the discharge time on the properties of fresh and hardened ready-mixed concrete, as well as its durability. To emulate typical field construction conditions, we formulated representative concrete mixtures and sourced, batched, and poured them from a local ready-mix concrete supplier. To maintain the concrete’s workability without altering the mix design, high-range water reducer (retarder or stabilizer), and air-entrainment admixtures were incorporated as needed. The fresh and hardened concrete properties were evaluated at batching and at time intervals of 60, 90, 120, and 150 minutes for three different
concrete mixtures. Fresh concrete properties included air content, slump, and temperature, while hardened concrete properties encompassed compressive strength, freeze-thaw resistance, and surface resistivity. Remarkably, the discharge time had no significant impact on the fresh and hardened concrete properties even up to 150 minutes. This indicates that the strict discharge time limit is overly cautious. Embracing an extended discharge time offers numerous economic and sustainable advantages.