Common Misunderstandings About Newly Placed Slab Foundations

1. If a newly placed slab-on-ground foundation cracks, there is a serious structural problem

Any competent structural engineer knows this is nonsense. All structural materials have specific characteristics and one of the material properties of concrete is that it cracks. As one of my engineer friends, Lowell Brumley, says: engineers have understood for hundreds of years that concrete cracks – that is why we put steel in it: to control the cracking.

Cracks that show up soon after the concrete is placed are normal. They are not due to overloading because the foundation has not been fully loaded until the slab is around seven years old.

2. My foundation is already not level

Many people think their new slab is level. Then they go by the slab after a rain and the puddles of water are clear evidence that the slab surface is not level. 

Construction tolerances for slab-on-ground foundations come in two varieties: overall levelness and local worst case. For overall levelness, the best you can hope for is that the largest elevation difference to be no more than 1.5 inches. For the local worst case, the tolerance is 1.25 inches over 10 feet.     

3. They poured my slab last week and they are already building the frame – isn’t too early

There are strict rules for how and when slab-on-ground foundations can be loaded, but they do not relate to when the framing can start. The framing can start as soon as the concrete is hard enough to walk on.

While the load from the framing material does go into the design calculations, it is not very significant. The first really high load occurs when the drywall is applied, and that does not happen until long after the concrete is effectively full strength.    

      

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