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# What psi concrete is the typical warehouse / factory floor?

#### huleo

##### Hot Rolled
I do these calculations. Most of what you seek will be mostly spelled out per the ACI. As many have mentioned here, PSI of concrete has little bearing on actual bearing strength by itself. A typical slab on grade has virtually no bending strength, but for comparison that people should understand, when you drive over concrete, it act similar to beam loading in which the bottom of the slab is in tension, and for general reference concrete in tension is only about 10% of its compression rating. So 3000psi concrete in compression may break at only 300psi in tension.

The concrete works like a "plate" to distribute loading to the subgrade, at a general diverging angle of 45* in all directions. This is one way thicker concrete has an advantage. If little focus is made on subgrade, concrete can fail, which the concrete itself is usually suspected.

Concrete is usually specified at its "30 day strength", meaning it should achieve the rated compressive PSI at 30 days, but concrete can also be tuned for quicker cure, and concrete will continue to gain strength with age, sometimes an entire year!

I see many pours put on sand. Sand is NOT a base. I also see too much sand being used. In most cases, 1" or less is preferred on top.

Contrary to people calling them "expansion joints" they act as "contraction joints" in a green pour as the fresh concrete shrinks, building internal stress. Another poster is correct on the "slump" of the concrete which is of extreme importance in a good pour! High water content increases workability so labor guys like it, but it also decreases the strength of the concrete! As well, any good concrete company will know how to tune the concrete for the truck drive. It is very common to setup a higher water ratio in the truck so the "delivered slump" is on target. Remember that concrete is evaporating water and kicking off in the truck!

Remember that ANY non-tensioned rebar is dormant (does nothing) for the strength of your slab until it cracks, then the rebar aids in tension and shear loads to keep the pieces together. In thicker pours, it would be good practice to keep the bar at the bottom 1/3 of the pour to give best tension capacity.

One of the other big factors in a good pour is the curing process! It cannot be stressed enough! Concrete will dehydrate rapidly within the first week but it needs to do so at a controlled and even rate through the slab. Many use a "curing agent" to help retain water, but usually covering is the preferred method when practical. Pouring indoors is always good as you can control conditions better. Temperature is also related here. If concrete is poured in Summer, the top of the pour can increase in temp greatly compared to the bottom when the heat is being wicked by the subgrade. Good even weather temps are ideal. 60-70F.

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#### Speedie

##### Stainless
Fiber is what keeps concrete from cracking when curing and it also helps during the life of a slab. You should never pour less than 4000psi concrete as it is just too tender for a machine shop

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