Question: I am planning a pour for a 15-foot-high wall and am concerned about form pressures. We're not actually using a self-consolidating concrete but will be using a super-plasticizer to get a slump of about 10 inches. The owner is trying to get some points for a “green” building, so there will be 15% fly ash replacement of the cement. We'll be pouring in May, and I'm planning for three 5-foot lifts.

Answer: In 2001, ACI Committee 347 issued new guidelines for determining formwork pressures. The updated formwork standard provides two pressure formulas, one for walls and one for columns. It also introduces weight and chemistry coefficients, CW and CC, that make it possible to apply the formulas to a variety of mixes and concrete unit weights.

ACI 347 defines a wall as a vertical element with at least one plan dimension greater than 6.5 feet, and a column as a vertical element with no plan dimension larger than 6.5 feet. Although pressure at any given point within the form varies with time, the designer usually does not need to know the specific variation, since the equations indicate the maximum pressure the forms experience.

To determine the form pressure to use to design your forms, you'll first have to know your rate of placement and the concrete temperature at the time of placement. You can then use the formulas, or you can get the base value for lateral pressure directly from a table. Within the ACI 347 guide there are three tables. Table 1 gives values of CW and CC. Table 2 gives base values of lateral pressure on column forms—that is, pressures that can be used when both CW and CC are equal to 1 (as defined in Table 1). Table 3 gives base values for lateral pressure on wall forms—again pressures that can be used directly when both weight and chemistry coefficients are 1. These tables (and the formulas, in case you like doing calculations) are available on our Web site at www.concreteconstruction.net (search on “form design” then click on the article “Pressure on Wall and Column Forms” by Mary Hurd from October 2002).

For your case, if we assume you're using normal-weight concrete (between 140 and 150 pcf) then the unit weight coefficient, CW, is equal to 1. With some fly ash and a water reducer that we'll assume will slightly delay the set, the chemistry coefficient, CC, is 1.4. If we assume a pour rate of 5 feet per hour and that the concrete will be at about 60° F when placed, we could calculate the base pressure value using the formula for walls or take it directly from Table 3. Either way, we would come up with 1107 pounds per square foot as the base value for formwork pressure. We then multiply that times the two coefficients (1.0 and 1.4) to get a design pressure of 1549 pounds per square foot. If the concrete is warmer than 60° F or if your pour rate is slower than 5 feet per hour, this value will be less.