The rationale for the no-tolerance approach is that a crack is an indication of a construction flaw. Poor curing, inadequate sawcut depth for contraction joints, late joint sawing or a number of other reasons may be cited. This is an example of unrealistic expectations for concrete construction. Some owners, architects and engineers believe that perfection is possible in concrete construction. Other owners know perfection isn't possible but look for ways to keep some of the contractor's retainage.
It's interesting to note that two of the most authoritative American Concrete Institute documents, excerpted below, state specifically that some random cracking should be expected.
The Foreword to "Guide for Concrete Floor and Slab Construction (ACI 302.1R-04)" includes the following:
Application of present technology permits only a reduction in cracking and curling, not elimination. Even with the best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Consequently, every owner should be advised by both the designer and contractor that it is normal to expect some amount of cracking and curling on every project, and that such occurrence does not necessarily reflect adversely on either the adequacy of the floor's design or the quality of its construction.
The synopsis for "Guide to Design of Slabs-on-Ground" (ACI 360R-10) contains almost identical wording:
Even with the best slab designs and proper construction, it is unrealistic to expect crack-free and curl-free floors. Every owner should be advised by the designer and contractor that it is normal to expect some cracking and curling on every project. This does not necessarily reflect adversely on the adequacy of the floor's design or quality of construction.
Section 18.104.22.168 of ACI 302.1R-04 gives more detail regarding the amount of cracking to expect:
For unreinforced, plain concrete slabs, joint spacings of 24 to 36 times the slab thickness, up to a maximum spacing of 18 feet (5.5 meters), have produced acceptable results. Some random cracking should be expected; a reasonable level might be random visible cracks to occur in 0 to 3 percent of the surface area floor slab panels formed by saw-cutting, construction joints, or a combination of both.
Despite these cautions, concrete contractors are often told to repair all cracks or remove and replace cracked panels, even though cracked floors may perform their intended function. If such requirements are contained in the specifications and the contract has been signed, there is little the contractor can do. But an attachment to the bid can take exception to the more stringent crack requirements by citing the ACI documents. And the documents may also be used to establish that the contractor following a normal standard of care does not preclude some random cracks.
Ward R. Malisch, P.E., is technical director for the American Society of Concrete Contractors. He can be reached at firstname.lastname@example.org. Bruce A. Suprenant, P.E., is the president of Concrete Engineering Services. He can be reached at email@example.com.