Weight-bearing pavement, such as a concrete driveway, requires a well-compacted sub-base to distribute the load from the pavement to the ground. Without an adequate sub-base, the regular loads will cause the pavement to warp and develop ruts. Industry specialists refer to hard, inert material (often processed waste and rubble from demolition) used in sub-bases as hardcore. Installing a proper sub-base before paving can save you expensive maintenance costs in the future. Calculating hardcore quantities allows you to purchase the correct amount necessary to install a proper sub-base.
Determine the minimum necessary depth of your sub-base. Multiply the size of the largest possible particle of hardcore by two. Set the depth to at least twice the size of the largest particle of hardcore to prevent a single particle from touching both the bottom and top of the sub-base, creating distortions.
Measure the length and width of the sub-base. Multiply the dimensions to determine the total area of hardcore required. For example, if a driveway must be 20 feet long by 10 feet wide, its area for hardcore is 200 square feet.
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Convert the necessary depth of sub-base to the same units as the length and width measurements. For example, if a driveway is measured in feet and must have a 4-inch deep sub-base, divide 4 inches by 12 inches per foot to get 0.333 feet deep.
Multiply the depth by the area. For example, a driveway that is 20 feet long by 10 feet wide with a 4-inch deep sub-base has a volume of 66.667 cubic feet.
Contact your hardcore supplier to find the density of hardcore in pounds per cubic foot. Divide by 27 to convert density in pounds per cubic yard to pounds per cubic foot. For example, 4,000 pounds per cubic yard equals 148.15 pounds per cubic foot.
Multiply the density of the hardcore by the volume of the sub-base. For example, multiplying 66.667 cubic feet by 148.15 pounds per cubic foot equals 9876.72 pounds of hardcore.
Divide by 2,000 to convert the weight of hardcore to tons. For example, 9,876.72 divided by 2,000 equals 4.95 tons of hardcore necessary for a driveway sub-base that is 20 feet long by 10 feet wide and 4 inches deep.