To keep concrete in place while it sets, the forms must be designed to withstand the concrete's pressure. The American Concrete Institute (ACI) recommends several formulas for calculating the pressure of concrete as it is poured. You will need to evaluate the temperature of the concrete (T), the rate of pour in cubic feet per hour (R), the weight of the concrete (W), the unit weight coefficient (Cw), the chemistry coefficient (Cc) and the height to which you are pouring the concrete (H).
Open Microsoft Excel and create a new workbook. Create the headings across the top in separate columns: "Maximum Pressure" "Cw" "Cc" "R" "T" "W" and "H."
For a columns and walls less than 14 feet high enter the following formula in the cell under "Maximum Pressure" (which should be B1) without quotation marks: "=B2_C2_(150+9000_(D2/E2))" This will yield the result of the ACI recommended formula: "Pmax = Cw_Cc[150+9,000*R/T]"
Enter the values for Cw, which is the weight of your concrete in pounds per cubic foot divided by 65.8 Kilogram per cubic foot. Typically concrete will be between 65.8 and 68 Kilogram per cubic foot, so Cw will usually be 1. Enter the value of Cc, which is based on the blend and composition of your concrete. Standard concrete has a Cc of 1, but if retarders are added it may rise to 1.2 or if you use a "blend containing more than 70% slag or 40% fly ash" the Cc may be as high as 1.4. . Enter the rate of pour (R) in feet per hour. Enter the temperature (T) in degrees Fahrenheit. Enter the weight (W) of the concrete in pounds per cubic foot and enter the height (H) of the concrete pour in feet.
Change the formula in cell B2 if your concrete will be poured over 14 feet high and at a rate less than 15 feet per hour. Enter (without quotes): "=B2_C2_((150+43400/E2)+(2800_(D2/E2)))" If the rate is over 15 feet per hour change the formula to: "=F2_G2" without quotation marks.