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Slab Thickness Tolerances

ASCC Position Statement #9

Most project specifications, including ACI 301-99, “Specifications for Structural Concrete,” reference ACI 117-90, “Standard Specifications for Tolerances for Concrete Construction and Materials.” This document allows the same thickness tolerance—+3/8 in. (+10 mm) and -1/4 in. (-6 mm)—for both slabs on ground and elevated slabs 12 in. (300 mm) thick or less. These tolerances give owners, engineers, and architects false expectations because no measured and reported slabs have ever met these criteria.

An article by Armand Gustaferro (“Are Thickness Tolerances for Concrete Floors on Grade Realistic?” Concrete Construction, April 1989) gives slab thickness data from seven projects representing more than 2000 cores. On these projects, the standard deviation for measured slab-on-ground thickness varied from 0.47 to 0.90 in. (11.9 to 22.8 mm). In a later interview, Eldon Tipping, a floor consultant for Structural Services, Inc., and Armand Gustaferro indicate that the ACI 117-90 tolerances for slabs-onground

thickness are “not reasonable” and “ridiculous” (“Slab Thickness Tolerances: Are They Realistic?” Concrete Construction, June 2000).

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