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Free Fall Of Concrete

ASCC Position Statement #17

Concrete placing operations are often planned to allow for the free fall of concrete. This planning must also consider any segregation that might occur when the concrete free falls into place. Techniques such as placing concrete with drop chutes or through windows in wall forms can minimize the effects of concrete free fall. Using these measures unnecessarily, however, can increase concreting costs without improving the in-place quality of the concrete. 

Sometimes specifiers and inspectors dictate the maximum free-fall distance of concrete because they believe limiting free fall is necessary to minimize concrete segregation. Usually they limit the free-fall distance to 3 to 5 ft (0.9 to 1.5 m), but occasionally the limit is as little as 2 ft (0.6 m). Neither
ACI 301- 99, “Specifications for Structural Concrete,” nor ACI 318-02, “Building Code Requirements for Structural Concrete,” limit the maximum distance concrete can free fall. 

ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete,” states that “if forms are sufficiently open and clear so that the concrete is not disturbed in a vertical fall into place, direct discharge without the use of hoppers, trunks, or chutes is favorable.” ACI 301, 304, and 318, however, all require placing the concrete at or near its final position to avoid segregation due to flowing. At least four field studies have shown that free fall from great distances doesn’t reduce concrete quality:

  • “Investigation of the Free-Fall Method of Placing High-Strength Concrete in Deep Caisson Foundations,” C. N. Baker, Jr., and J. P. Gnaedinger, 1960 report; 
  • “Unconfined Free-Fall of Concrete,” C. D. Turner, ACI JOURNAL, Dec. 1970, pp. 975-976;
  • “Concrete Free Fall Tested in Alabama Highway Department Project,” S. Litke, Foundation Drilling, June-July 1992, pp. 14-16; and
  • The Effects of Free Fall on Concrete in Drilled Shafts, STS Consultants Ltd., report to the Federal Highway Administration, 1994.

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