Abstract: An asphalt mix storage bin outlet is surrounded by a rim member having a flat bottom surface in which there is an annular groove that extends around the outlet. An outlet closure comprises a plate-like edgewise slidable gate member mounted on rollers that ride on parallel rails at opposite sides of the outlet. The rails have long, straight portions along which the gate member rides to and from its open position with its top surface spaced below the bottom surface of the rim member, but as the gate member nears its closed position it rides up onto wedge-like portions of the rails that cam it towards snug engagement of its top surface with the bottom surface of the rim member. For an airtight seal with the gate member closed, grease is forced into the groove in the rim member, to be distributed around the outlet by the groove and thence forced into any space between the opposing surfaces of the rim member and gate member.