The trough-shaped mixing chamber of an open-top mixer is equipped with mixing tools that rotate about a horizontal axis. Fig. shows a single-shaft mixer with individual mixer arms attached to the shaft in a helical arrangement.
Twin-shaft mixers are used more frequently in concrete plants than single-shaft mixers. The two horizontal shafts of a twin-shaft mixer synchronously counterrotate. The mixing volumes of the two shafts overlap in the centre of the trough, thus creating a region with a very high mixing intensity. Twin-shaft mixers are used in ready-mixed concrete plants and precast facilities.