a concrete mixer having a mixing drum constructed of high density crosslink polyethylene material . the drum includes a bottom supported by a conventional rigid metal pan secured to the external surface thereof to rigidify the plastic drum and extend the life expectancy of the plastic drum by enabling the concrete mixer to be used to complete a mixing job at a job site even though movement of the concrete mix within the drum during repetitive mixing cycles may ultimately wear a hole through the bottom of the plastic drum . paddle assemblies are positioned interiorly of the drum and oriented to maintain minimum splashing during the mixing operation . a rigid ring gear is attached peripherally of the drum in spaced relation to the metal supporting pan with the drum , ring gear and metal bottom pan being uniquely related to facilitate assembly and disassembly of the components to facilitate easy replacement of the components .