Concrete pads must be able to withstand hundreds of pounds of force from the supporting structures placed on them. To calculate the load that a concrete pad can support, you need to know its width, length, depth and its strength.
Determine the Concrete's Strength
Not all concrete is the same strength. Before trying to determine how strong a pad is, you must first know how strong the concrete is. This is normally measured as compression strength, measured by pounds per square inch, or psi. A common bag of concrete, for example, may have a compression strength of 4,000 psi, based on a thickness of 2 inches after it has set for 28 days. Other concrete may have a compression strength of only 550 psi.
Determine the Pad's Area
Measure the length and width of the top surface of the concrete pad in inches and then multiply them to get the pad's area in square inches. For example, the length is 20 inches and the width is 15 inches, the area is 300 square inches.
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Determine the Pad's Thickness
Knowing a concrete pad's thickness or depth is vital if you need to determine the load it can withstand. If the concrete pad is two-inches thick and the concrete manufacturer's stated compression strength is based on that thickness, then you can simply multiply the area by the stated psi to determine how much weight the pad can bear. If the pad is of a different thickness, you will need to use an engineering table, or learn some complex equations, to calculate the weight it can withstand.
For example, a 6-inch pad with a compression strength of 700 psi can support 1,105 psi. If it was 7 inches thick, it could support 1,194 psi, and if it was 12-inches thick, it could support 1,563 psi.
TL;DR (Too Long; Didn't Read)
Consult the manufacturer for the stress capacity of your specific concrete before use.
Reinforcing concrete with rebar will increase its strength.