Your recent Problem Clinic item on the pressure of wall concrete on wall forms, April 2006, p. 58, refers readers to my article on form pressure that appeared in Concrete Construction, October 2002, and is now available on your Web site. The article was based on ACI 347-01, but should no longer be followed because the current ACI Formwork Standard (ACI 347-04) changes the formulas recommended for determining maximum lateral pressure on wall forms.
There are now two wall pressure formulas (Section 2.2.2). One applies to walls where the placement height is 14 feet or less and the rate of placement is less than 7 feet per hour. The second applies to walls with a placement rate less than 7 feet per hour and having a placement height more than 14 feet, and for all walls with placement rates of 7 to 15 feet per hour. The unit weight and chemistry coefficients are the same as described in the 2002 article.
Because of the combination of wall height and placement rate, your example happens to be in agreement with the second of the two formulas, except for one detail—the slump of the concrete is given as 10 inches. ACI 347-04 limits application of the wall pressure formulas to concrete with a slump of 7 inches or less, and recommends for higher slump concretes the basic equivalent hydrostatic head, p = wh, with no chemistry or weight coefficients to modify it.
To learn more, refer to the new seventh edition of ACI SP-4, Formwork for Concrete, or ACI Standard 347-04. Lateral pressure calculations are also performed in the spreadsheet program made available to ACI by William C. Epstein, professor of construction management at California State Polytechnic University, San Luis Obispo. To use the spreadsheets, go to www.concrete.org and click on “free resources” shown in the box on the right. Then go down the list and click on “form design spreadsheets,” and select “walls.”
— M.K. Hurd, Engineered Publications