Q.: We're placing concrete for a job with 10-foot-high columns, 18x18 inches in cross section. The columns contain four #8 vertical bars. The fastest way to place the column concrete is by direct discharge because there's an unobstructed path to the bottom of the column. After we'd placed several columns this way, the engineer said we had to use a tremie hose because concrete shouldn't free fall more than 3 to 5 feet.
ACI 301 is the specification for this job and I can't find any mention in the specification of a height limit for concrete free fall. We've already stripped columns and there's no evidence of segregation or honeycombing. Is it necessary to use a tremie under these conditions? And if so, what's the maximum permitted height for dropping concrete without a tremie?
A.: Some publications used to give a recommended free fall for concrete. However, ACI 301 makes no mention of a 3- to 5-foot limit.
ACI's Guide for Measuring, Mixing, Transporting, and Placing Concrete (ACI 304R-89) in Section 5.4.1 gives some precautions for placing. The guide says equipment should be arranged so that the concrete has an unrestricted vertical drop to the point of the placement. The stream of the concrete shouldn't be separated by permitting it to fall freely over rods, spacers, reinforcement, or other embedded materials. If forms are sufficiently open and clear so that the concrete isn't disturbed in a vertical fall into place, direct discharge without the use of hoppers, trunks, or chutes is usually desirable.
PCA's Design and Control of Concrete Mixtures (13th edition, page 104) specifically addresses your question. This source says the height of free fall of concrete need not be limited unless a separation of coarse particles occurs (resulting in honeycomb). If honeycomb does occur, a limit of 3 to 4 feet may be adequate.
The PCA book cites a study in which concrete was dropped vertically 50 feet into a caisson. There was no significant difference in aggregate gradation between control samples as delivered and free-fall samples taken from the bottom of the caisson. This showed there was no segregation.