Knowing the load-bearing capacity of your concrete is essential to make sure that what you plan to have supported by the wall or slab will not be too heavy and cause it to crack and break. While cracks in a slab may not seem like a big deal, the smallest crack represents a large amount of downward pressure that may be crushing pipework beneath the slab. The only way to specifically calculate the load capacity of concrete is to have a licensed structural engineer process the formulas and consult a myriad of tables that all apply to the specific design of your project. You can, however, calculate the load capacity of concrete in a quick and general way. This will tell you if you if there may be a problem in the project plan that needs the expensive attention of an engineer. This is also an acceptable method for figuring out the load-bearing capacity for small home projects, like a driveway or a garage slab.
Look at the general instructions at the front of your project plans. Under the heading "Concrete" you will see a specific note of the pounds per square inch--psi--strength that the concrete must reach. This note reflects the specific mix you will order throughout your project. Write this number down. This is the base load-bearing strength of your concrete. If you are doing a smaller home project with no special accelerants added to your mix, your load-bearing strength per concrete yard will be 3,000 psi. This is the standard psi of a common mix. If accelerants are called for, you must consult a licensed structural engineer for the load-bearing calculation.
Look in your project plans for any notes on additional rebar or wire mesh that has been added to your concrete pour. If additional re-enforcement has been added, then you will add 1,000 psi to the load-bearing amount of your concrete yard.
Add the base psi you found in step 1 to any additional psi provided by added re-enforcment in step 2. This is the load-bearing capacity of your concrete overall.
Mark your project plans with a highlighter wherever the psi amount you calculated in Step 3 is found. Note that any portion of your slab or wall on your plans that has additional depth or re-enforcement added to it must be recalculate. Add 1,000 psi for every layer of rebar installed. For example, if you have a slab with a single mat that changes to a double mat where a refridgerator goes, in the area where there is a double mat the load-bearing capacity would be your base psi plus 2,000.
Mark each section of you plan where the psi changes and double-check this plan against the architectual plans that show exactly where appliances and utilities are placed. Make sure that the extra load-bearing planned matches the placement of appliances in the architectural drawings. The structural and architectural drawings are not drawn by the same company, and it is rare that they are compared for error before issued for building.
Notify the structural engineer on the project or the architect of record immediately if you notice any discrepency between the structural load-bearing plans and the architectural ones. If you do not, you run the risk of having to disrupt your project schedule to fix this mistake.