QUESTION: An engineer told me that the tolerance for the thickness of a floor slab-on-ground is +3/8 inch and -1/4 inch. That’s ridiculous. Is it even possible?
ANSWER: You’re in luck, because that tolerance is out of date. Many industry experts agreed that that tolerance was unrealistic (see Concrete Construction, June 2000, Slab Thickness Tolerances: Are they Realistic?), so when they revised the tolerance’s specification, it got changed. The new tolerance for thickness of a concrete floor slab-on-ground is -3/8 average and -3/4 for a single sample—there is no plus tolerance. This is found in the Section 4.5.4 of the 2010 issue of ACI 117, Specification for Tolerances for Concrete Construction. If you don’t have a copy of ACI 117-10, get one and read how the thickness is measured and how the average is calculated. When you sign a contract for a slab, make sure that this newer tolerance is what you will be required to achieve.