To make ready mix concrete, large trucks (called ready-mix trucks) pick up sand, gravel, and other aggregates at a materials plant. The materials are dropped into a rotary cement mixer along with Portland cement. Then, water is added to the mixer, which creates a slurry. The slurry is then pumped from the mixer and into a silo, which is connected to the top of the mixer. Ready mix concrete is delivered in large bags or cases. It is usually sold by the cubic yard or cubic metre.