8.1 : Mixing Concrete

Concrete mixing ensures a homogenous blend where aggregates are well-coated with cement paste. Concrete mixing is typically done using two main types of mixers: batch and continuous. Batch mixers handle one batch at a time, thoroughly combining materials before discharging and receiving the next batch. In contrast, continuous mixers receive a steady flow of ingredients, mixing them consistently and discharging without interruption. Within batch mixers, tilting drum mixers mix with internal blades and discharge by tilting, while non-tilting drum mixers use a chute or reverse rotation for emptying. Pan-type mixers, with paddles rotating about a vertical axis, and dual drum mixers, allowing sequential mixing in two drums, are also common in batch operations. Concrete mixers are available in a wide range of capacities ranging from 1.5 cubic feet to 17 cubic yards. Mixing a volume that is only a minor portion of the mixer's total capacity is inefficient and can lead to inconsistent mixes. Similarly, overloading the mixer beyond its recommended capacity by more than 10% can also result in a non-uniform mix. To verify the mixer's efficiency in yielding uniform mix across the batches, sampling is conducted at various discharge points and evaluated for consistency according to set criteria such as density, air content, slump, and compressive strength.