Q: We have undulating lines of honeycomb on some walls. Is this because of improper vibration?

A: Very likely it was caused by separation of coarse aggregate during placement. This happens either when concrete is not delivered vertically into the form or when aggregate separates and accumulates at the bottoms of the slopes.While the concrete is being deposited it moves laterally toward the middle of the placement interval and some of the clustered aggregate becomes strung out along the form at the edge of the previously placed layer. This happens again during vibration. This lateral movement occurs en masse in whatever degree of separation the concrete is in while being placed and is not objectionable. Such movement would not of itself cause honeycomb joint line separation.During the ensuing period all slump is lost and initial set of the lower layer is imminent. If there has been separation during placement, it is likely that a streak of honeycomb will appear. The longer the time between layers the more likely it will be. Even when there is no honeycomb a mark will inevitably be left on the formed surface. Although it is customary to refer to such marks as cold joints, cores drilled from concrete with such honeycomb have shown fully monolithic concrete.To be sure of eliminating such imperfections one should:

  • schedule placement operations so that each layer is placed while the previous layer is still soft
  • make sure that no coarse aggregate separates during placement (If it occurs as a result of stacking, keep a vibrator running in the center of the drop point so that a hill does not build up.)
  • use the largest and most powerful vibrators that can be operated in the work
  • see that vibrators penetrate deeply (at least 6 inches) into the layer below

    In many cases successive adjacent direct placement without hoppers or drop chutes has eliminated problems with placement at intervals, with better results and in less time.