Cleaning fresh and hardened concrete from inside the mixer drum is an essential part of maintaining a ready mix truck. Up to 600 pounds of fresh concrete can be stuck to the inside of a 10-cubic-yard truck mixer at the end of the day. If this is not removed, it will harden and continue to accumulate.
A proper washout has two primary requirements: to remove concrete buildup on the inside of the mixer and to remove the film of cement. This can be accomplished by using a coarse aggregate slurry to scrub the inside of the drum.
Another technique uses special washout admixtures or wash water stabilizers. Instead of discarding leftover concrete, a special admixture is added to keep the concrete from hardening. The concrete is then left overnight and fresh concrete is added to the previous day's surplus. This reduces water costs, labor, concerns about water disposal, and conserves cement and aggregate as well.
Most mixers require chipping out at least once a year. This takes place when the amount of built-up concrete inside the mixer has reached unacceptable levels and must be removed. It is important to take every safety precaution possible when chipping out the mixer, including securing the drum, locking and tagging out the equipment, providing ventilation, and wearing protective gear.