Now, to answer the questions. Regarding 4-inch or 5-inch: 4-inch-thick slabs work just as well as five in residential applications IF all of the above items are adhered to. If subgrade is suspect and vehicle loads will be present you are better off with a 5-inch slab. For slabs that will only see foot traffic, certainly 4-inch is fine, but again, proper preparation is important.
I once saw plans for a new fire station. The slab was designed 10 or 12 inches thick with two mats of rebar. If the subgrade was really weak, i.e., an old peat bog or bay mud, this structural section might have been warranted. However, this fire station was to be located on a stable, mildly sloping hillside with soil bearing capacities in excess of 2,000 pounds per square foot. I think the structural engineer got a little carried away; I would have designed a slab half that thick with half the rebar.
The best way to eliminate surface cracking is by doing what I recommend in 1-4 above. Also, it is a good idea to place a couple 3-foot long sticks of #4 rebar diagonally at every reentrant corner because you know a crack will emanate from that location. Place the first stick about 3 inches from the corner and the next about 9 inches from the corner, both at mid-height of slab. You’ll still get a crack, but it should be hairline at most.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author and software producer for the building industry. Send e-mail to email@example.com. Tim reads every one.
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