How to Calculate Concrete PSI

The load-bearing capabilities of concrete mixtures are expressed in terms of how many pounds per square inch (PSI) it can support before it cracks and fails. Standard concrete mixes designed for home use are rated at 3,000 PSI, or more. Test the PSI of a particular mixture before using it in a building project. This will require access to industrial equipment and a few mathematical equations.


Step 1

Mix the concrete according to the manufacturers instructions. Different mixes require different materials be added in specific quantities to achieve the desired consistency and PSI strength. In general, you must combine the concrete mix with water and sand before it will harden.

Step 2

Pour the wet concrete into a 12-inch cylinder mold with a 6-inch diameter. Allow the concrete to harden for 28 days to reach maximum hardness.

Step 3

Place a neoprene or sulfur mortar cap on the top of the concrete cylinder. This helps distribute the weight placed on the surface evenly.

Step 4

Place the concrete cylinder in a compression test chamber directly under the plunger.


Step 5

Start the machine and slowly increase the plunger force in increments of 0.05 inches per minute.

Step 6

Write down the amount of force the machine is exerting at the moment the concrete cylinder shows a crack. Write down the machine reading when the concrete cylinder breaks.

Step 7

Calculate the square of the concrete cylinder's diameter and multiply it by PI to find its surface area. In this case, 6^2 * PI = 113.1 square inches.

Step 8

Calculate the PSI for the initial failure reading. Divide the reading shown on the machine when the first crack appeared by 113.1 to calculate the concrete PSI rating for initial failure.

Step 9

Calculate the PSI for the catastrophic failure reading. Divide the reading shown on the machine when the concrete cylinder broke by 113.1 to calculate the concrete PSI rating for catastrophic failure.