When a truck mixer doesn't produce well-mixed concrete, the lack of uniformity is usually caused by failure to set up the flow down the spiral blades and the counter flow up the drum axis. Other causes include the development of head packs and cement balls.
High mixing speed helps to set the desired flow pattern in the drum. Preferable speed is 20 to 22 rpm but not more than 25 to 27 rpm. Cement balls are evidence of improper loading or batching sequences. Head packs occur when sand is loaded before coarse aggregate, and packs in the head of the mixing drum. It then breaks loose during discharge, causing variations in slump, air content and strength. Proper batching and loading techniques will prevent both cement balls and head packs.
Another cause of variations in truck-mixed concrete is mixer blade wear. Blades should not be worn down by more than 10% of the original height.
ASTM C 94, Standard Specification for Ready Mixed Concrete, contains uniformity testing procedures and requirements. Samples taken from two parts of a batch are tested for slump, air content, strength, coarse-aggregate content, and air-free unit weight of concrete and mortar.