Slab thickness tolerances has long been a point of contention between contractors and specifiers. Typically a project specification will reference ACI 301, “Specifications for Structural Concrete” which in turn references ACI 117, “Specifications for Tolerances for Concrete Construction and Materials.” The slab thickness tolerance has finally changed in the newest version of ACI 117, published in 2006: For all slabs-on-ground there is only a minus tolerance, meaning that the specifier is not prohibiting thicker slabs. The tolerance states that no individual sample may be more than ¾ inch thinner than the specified thickness and the average of all samples can't be more than 3/8 inch thinner than specified. This is an improvement over the 1990 version of ACI 117, which had a thickness tolerance of +3/8 inch, -¼ inch.
The ACI 117-06 tolerance then is a simple average and a minimum thickness, both of which can certainly be achieved, although contractors should be aware of how much thicker to plan for the slab to be in order to make sure it is never more than ¾ inch thinner than specified. The commentary cautions specifiers to expect “localized occurrences of reduced thickness” and urges them to consider using a statistical approach to subgrade, thickness, and surface elevation, although the methodology is left up to the specifier. The important thing for contractors is to start the project with a clear idea of what the owner or specifier really wants and what you will have to do to accomplish that.
One good way for contractors to make their case is to use Position Statement #9, Slab Thickness Tolerances created by the American Society of Concrete Contractors (ASCC). Unfortunately, this position statement currently references ACI 117-90, although an update will soon be available and most of the information in the position statement remains relevant. And besides, many job specs will still be referencing ACI 117-90.
In June 2000, Concrete Construction ran an interview with two of the nation's top slab experts, Eldon Tipping, Structural Services Inc., and the late Armand Gustaferro. Both of them criticized the existing slab tolerances as unreasonable, ridiculous, and unachievable with current technology. Today, with the plus side of the tolerance gone, a slab can be built to satisfy the specification, although, as noted in the ASCC position statement, based on the variations in thickness with typical slab construction—even using a laser screed—you would need to plan for an average thickness 1½ inches thicker than the minimum in order to be statistically confident of meeting the specification. For a 6-inch slab, therefore, you would set your forms for a 6¾-inch thick slab. For a competent contractor working with a decent subbase, that thickness would easily keep the average above the 5 5/8 inches needed to satisfy the average thickness tolerance.
But do owners really want to pay for the extra concrete? The ASCC position statement, again, is a good tool to use in helping the owner, and specifier, make that decision. It notes that “Slab thickness variations don't usually lead to strength problems, since concrete and subgrade strengths are usually higher than those used in the design.” Contractors just need to make sure that they don't get stuck buying another ¾ inch thickness of concrete that wasn't in their bid.
In his editorial in the June 2000 issue of CC, Ward Malisch begged the industry to come up with a “method for determining compliance with slab thickness specifications,” noting that nothing had happened in the previous decade since publication of an article by Luke Snell and Robert Rutledge (see “A proposed method for determining compliance with floor thickness specifications,” CC, January 1989). Now nearly another decade has passed and there's still no generally accepted approach. But perhaps there's hope—Eldon Tipping is now chairman of ACI Committee 117 on Tolerances, and he is more familiar with this problem and its solution than anyone.