The mixer truck is a special truck used to transport concrete for construction; because of its shape, it is also often called a snail truck or an olive truck.  This type of truck is equipped with a cylindrical mixing drum to carry the mixed concrete. During transportation, the mixing drum will always be kept rotating to ensure that the concrete carried will not solidify.  After transporting the concrete, the inside of the mixing drum is usually flushed with water to prevent the residual concrete from hardening and occupying space, making the volume of the mixing drum smaller and smaller.