Engineering Corner


Limits on free fall of concrete

Q. Is there a maximum acceptable free-fall distance for concrete placement?

A. The American Concrete Institute’s two main standards governing building construction, ACI 318-05 and 301-05, don’t directly address the issue of free fall of concrete. Other ACI documents currently provide the following information on the free fall of concrete:

Section 5.4.1 of ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete,” cautions:

“Arrange equipment so that the concrete has an unrestricted vertical drop to the point of placement or into the container receiving it. The stream of concrete should not be separated by falling freely over rods, spacers, reinforcement, or other embedded materials. 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. Concrete should be deposited at or near its final position because it tends to segregate when it has to be flowed laterally into place.”

Section 3.5.6 of ACI 336.1-01, “Specification for the Construction of Drilled Piers,” cautions:

“Guide placement of free-fall concrete so as not to hit the reinforcement, hole sides, or anchor bolt assemblies. Vibration of concrete free falling more than 20 ft is not required.”

In the Specification’s Notes to Owner’s Representative, ACI 336.1-01 has two other pieces of information:

  1. If some pier diameters are less than 30 in. (750 mm), review the Specification and modify. If pier diameters are less than 30 in. (750 mm), certain elements of the Specification can be inappropriate, such as the permitted use of free-fall concrete and any requirements for physical downhole inspection. The risk of free-fall concrete scraping the sides of the shaft while falling increases dramatically as the shaft diameter decreases below 30 in. (750 mm), and physical bottom inspection of pier diameters less than 30 in. (750 mm) is impractical.
  2. Specify if free-fall concrete is not permitted or if the free-fall height is limited. Recent research on free-fall concrete has confirmed that free fall does not cause segregation, at least for fall heights up to 60 ft (18 m) and pier diameters as small as 3 ft (1 m) with 10 in. (750 mm) diameter cages. Even accidentally hitting the reinforcing bar cage does not appear to result in measurable segregation (ADSC-FHWA report on “The Effects of Free-Fall Concrete in Drilled Shafts,” [1994]); however, hitting the reinforcing bar cage may displace the cage and should be avoided. Thus, free-fall limits may be desirable in small diameter shafts deeper than 60 ft (18 m).

ACI 336.3R-93, “Design and Construction of Drilled Piers,” states:

“It is also permissible to allow free fall of concrete as long as it can be directed vertically on the centerline of the shaft, and it does not hit the sides of the shaft or the reinforcement cage.”

In the June 2001 issue of ACI’s magazine, Concrete International, an article by Suprenant cites several references summarizing the effects of free fall. He concludes that free fall of concrete directly over reinforcing bar or at high slumps doesn’t cause segregation or reduce compressive strength, but restricting free-fall height does decrease concrete production rates and increase owner costs.

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