In construction, mixers are used to combine the materials to make concrete. Also known as cement mixers, concrete mixers use a revolving drum mechanism operated by petrol or electric-driven engines. Available in a number of sizes, the choice of mixer would depend on the scale of the construction work with the time restraints of using the concrete before it hardens. Industrial mixers are used in the production of precast concrete, when large quantities of concrete are needed. They have twin-shaft, vertical axis or drum mixers. Concrete mixing transport lorries have the interior of the drum fitted with a spiral blade. The concrete may be mixed en route or beforehand. A concrete mixer trailer is portable machinery set on a trailer to be pulled behind a vehicle. The smaller on-site or portable mixers are loaded by an hydraulically-operated hopper or manually. A self-loading concrete mixer is a drum mounted on a driven-chassis with a loading bucket. Concrete mixers are needed on most construction work whatever its scale. Industrial mixers are used exclusively in factories whilst larger pieces of machinery are used by professional builders. For householders interested in home improvement, a portable model would be sufficient for their needs.