Placing concrete encasement on a duct bank can be a challenge. People add concrete encasement as a safety barrier for a duct bank. This way, if someone digs around the duck bank, they will hit the concrete encasement before they hit the conduits. The conduits have high-voltage wiring running through them. Calculating the amount of concrete needed for a duct bank depends on the size of the duct bank.
Determine the conduit formation. Most duct banks are either a square or rectangle formation. An example would be a three-by-three formation, which means three conduits high and three conduits wide. This equals a total of nine conduits.
Calculate the total width of the duct bank including the encasement. The conduits should be spaced 3 inches apart, and the encasement should extend 3 inches beyond the conduit on the outside of the duct bank. This means that for a three-by-three duct bank, the width would be 3 inches times 4 plus the number of horizontal conduits times the outside diameter of one conduit. If the outside diameter of the conduit is 4 inches, then the total width is 3 inches times 4 plus 3 conduits times 4 inches, which equals 24 inches.
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Calculate the total height of the duct bank. Repeat step 2 using the number of vertical conduits instead of the number of horizontal conduits. Since this is a three-by-three duct bank, its height will also be 24 inches.
Calculate the cross-sectional area of the duct bank. Multiply the total width and total height. In this three-by-three example, that would equal 24 inches times 24 inches or 576 square inches. Now subtract from this number the total area occupied by the conduit. The conduit area would be the cross-sectional area of one conduit times the number of conduits. The conduit area is pi times the radius squared times the number of conduits. For a three-by-three, that is 3.14 times 4 times 9, which equals 113.04 square inches. Subtract this number from the total cross sectional area of the duct bank. So, 576-square inches minus 113.04 square inches equals 462.96 square inches. This is the cross-sectional area occupied by the concrete alone.
Convert the cross-sectional area occupied by the concrete from square inches to square feet by dividing that number by 144. By dividing 462.96 by 144, the result is 3.215 square feet. Now multiply this number by the total length of the duct bank. If the duct bank is 100 feet, then the total volume of concrete needed is 3.215 square feet times 100 feet, which equals 321.5 cubic feet of concrete.