Question: Has there been experience or research in wall or mass footing placements relative to yield loss or densification and segregation in high-lift walls (20 feet high) or mass concrete?
Answer: We are aware of three possible sources of yield loss in high-lift walls. First is the possibility common to all walls: over-vibration. Over-vibration can result in loss of air. Each percent of air lost is equivalent to a loss in yield of 1 percent. With normal amounts of vibration this loss should be negligible.The second possibility is an apparent yield loss caused by bulging of forms. When forms bulge it takes more concrete to fill them and the yield appears to have diminished. The amount of bulging will depend on the pressure of the concrete, which will be greater in high lifts, and on the stiffness of the forms. The concrete pressure actually depends on many factors, as discussed by ACI Committee 622 in "Pressures on Formwork," Journal of the American Concrete Institute, August 1958, pp. 173-190.The third possibility is the yield loss caused by compaction of air from concrete pressure. If nothing but the hydrostatic pressure of the concrete were considered, ignoring the effects of restraint by formwork and setting of cement, a loss in yield of about 1.5 percent from this cause might be expected in a lift of 20 feet.
The effects of restraint and setting would reduce this loss somewhat. This value was arrived at as follows: For concrete that weighs 150 pounds per cubic foot it takes 14.1 feet of fresh concrete to produce an increase in pressure of 1 atmosphere. The volume of air in the concrete will decrease with this increasing pressure. At the bottom of the lift the air volume, in percent, will be: = volume of air at bottom of lift, in percentVi = initial volume of air in fresh concrete, in percenth = height of the lift in feetIf the air content in the fresh mix were 5 percent and the lift were 20 feet, The air content at the top of the lift is Vi. The average air content in the lift is the average of Vb and Vi, or 3.5 percent.Reduction in air volume for the whole mass is the difference between Vi and 3.5, or 1.5 percent. The concrete has diminished in volume by the same amount to make a yield loss of 1.5 percent (ignoring restraints by forms and by setting of cement).On the other subject raised, that of segregation, it should be said that tall forms may give more opportunity for segregation because of increased likelihood of the concrete hitting the sides of the forms on its way down. Anything that can be done to insure a vertical drop without impact on the sides before the concrete comes to its final resting place will be helpful.