Concrete Cracking in Basement

[TiltonBB]

It would be interesting to know the age of the concrete and the spacing and depth of the control joints.

Concrete is sold by it's compressive strength, not it's tensile strength. When concrete comes out of the truck it is at it's highest volume. As it dries, it shrinks, just like your clothes. A typical rule of thumb is the volume and surface will contract about 1/4 inch for every 20 feet. The control joints put in right after placement are just to show it where to crack and will look much better than a jagged crack. The control joints should be cut 1/3 of the depth of the concrete. Sometimes floors without joints are connected by rebar to the walls and have no way to absorb the shrinkage. The resulting cracking from contraction is pretty bad.

In cold climates the concrete is entrained with air, usually 4 - 6%. That is, microscopic invisible air bubbles to help with expansion and contraction. That was not done with concrete many years ago and led to a lot of cracking and spalling. If the concrete was placed with internal steel rebar that could be rusting and causing cracks also.

There are many companies that will repair concrete with epoxy that has some ability to expand and contract. If you have identified the cause, and eliminated it, you may be able to hide the cracks with epoxy and forget about them.