Cores taken to reveal the strength of a slab on grade sometimes reveal another problem. They may reveal a core that is thinner than the specification. Questions then arise about whether the entire slab is out of spec, and if so, how thin it is. The design engineer may include allowable tolerances for slab-on-grade thickness in the plans and specifications. However, when a tolerance isn't given, we recommend using the one in Standard Tolerances for Concrete Construction and Materials (ACI 117). The tolerance for slabs less than 12 inches thick is plus three-eighths inch, minus one-quarter inch.
A slab on grade should be considered acceptable if at least 95 percent of the slab has a thickness that equals or exceeds the design thickness minus one-quarter inch and the probability of error is 5 percent or less. If tests show that more than 5 percent of the thickness measurements aren't in compliance with specifications, the owner or engineer may want an estimate of the as-built slab thickness. This article includes several examples that illustrate how to use some of the tables included within the article.