a mixing fin assembly adapted to mix and guide materials in a rotatable mixing drum of a type used in a transit concrete mixing vehicle includes a spiral mixing fin designed to extend transversely into the drum constructed of a lightweight , moldable polymeric material which is flexible and resistant to abrasion , the mixing fin includes a plurality of conjoined sections that are fit together , preferably utilizing fin locking tabs and mating fin locking sockets . a preferred way of mounting the mixing fin into mixing drums uses a spiral fin mount rail track adapted to be fixed to the interior surface of the mixing drum . a locking tab locks the upper part of the rail track into the fin structure .