Question: I hope that you may be able to shed some light on what an “acceptable” crack is in a residential slab. I have read and heard that the one truth in concrete is that it will crack. The only thing that we can do as contractors is control where it will crack. That said, when it does crack, what is the industry standard for an acceptable crack?
Answer: You don't mention whether your cracks are on exterior slabs, slab-on-grade foundations, or on basement floors, so we will mention each situation—including foundation walls.


Foundation Walls: Jim Baty, technical director for the Concrete Foundations Association (CFA):
The Residential Concrete Work Committee ACI 332 does not provide an allowable crack width. In our CFA Standard, we specifically call out that the maximum allowable crack for a foundation wall width is 1/8 inch because water and dampproofing can easily span that width.

Exterior Slabs: Average concrete shrinks about 0.06%, so unless there are control joints, cracking is inevitable. There are few guidelines that we could find that define when a crack is too wide. The Illinois Ready-mix Concrete Association published a guideline statement for residential flatwork in 2002. In it they say that exterior flatwork shall be deemed acceptable when it does not display major cracking, defined as “an uncontrolled crack with a width of 1/8 inch or more that covers more than 10% of the total length of all the joints and edges of the slab, and the total length is no more than 20% of the perimeter length of any one panel.”

Prevention is better than the cure, however, so we suggest that you get a copy of the guideline statement for designing control joints from the American Concrete Pavement Association (847-966-2272 or ).

Slab-on-grade foundations: They are usually reinforced with rebar or post-tensioned tendons. So cracks, if they occur, should be hairline only

Basement Floors: Cracking, though unsightly, depends on the use that the floor will have. If it's to be finished and used as a living area, then cracks shouldn't be of a size that they will reflect through a finished floor covering.