Unlike the vibration of slabs and beams, the vibration of columns and walls requires holding the vibrator only in the vertical position in most cases. It is thus the force of gravity alone that makes concrete flow around the reinforcement driving out the entrapped air. Since the air escapes more easily throughout the uncompacted concrete, it is highly desirable to avoid passing the vibrator down through concrete just deposited in the column. It is therefore recommended to begin the consolidation process by dropping the vibrator down to the bottom of the formwork, placing the concrete on top of it and withdrawing it slowly upwards through the newly deposited concrete. Another difficulty in consolidating is caused by dense reinforcement at a column base in which the footing has mat reinforcement running in two mutually perpendicular directions. It is good practice, therefore, to prohibit the rushing of placement and consolidation of fresh concrete in the clearer area of the base; to make sure by working and observing that the concrete is being compacted onto the base (not flowing away too fast) and being packed tightly against the inner surfaces of the dowel bars and beginning to ooze through them; to thrust the vibrator in to the bottom of the concrete that rest on the bottom bars, making the concrete flow though the bars and compacting it below them; and to keep placing and consolidation until the concrete of the whole base has been brought to the same level as that within the cage of the dowel bars, watching to ensure that the concrete completely surrounds the steel and bonds to it. For depositing fresh concrete for walls and bulkheads to the wall, the plastic mass should be distributed evenly. Otherwise, if the slump is high, the concrete will flow and coarse aggregate will separate from the mortar and cause honeycombing. Such flow can be especially noticeable when the depth of wet, flowing concrete becomes less than two or three times the nominal size of the coarse aggregate. An effective way to fix this type of segregation is to use a dry mix that will not flow so easily. Use of such a dry mix permits, in addition, early removal and reuse of the wall forms.