A transit concrete mixer includes a drum which is mounted for discharge either forwardly or reversely of the vehicle and has two sets of flights for two-stage concrete loading and discharging. One set of flights is particularly useful for carrying the aggregate from the base of the drum upwardly along the major axis of the drum and a second set of flights is adapted for receiving the aggregate and transporting it through the remaining length of the drum to the discharge opening of the drum.the discharge opening of the drum is controlled in accordance with the slump of the discharging concrete mix or aggregate so that for relatively stiff and low order slump values the aggregate is dischargeable without impediment and, conversely, for more fluid, high slump aggregates.