Allowable Crack Widths
(In the tensile face of reinforced concrete structures)
The project architect on one of my completes jobs is requiring me to remove an 8-inch thick, 10-foot high, 100-foot long reinforced concrete foundation wall because a few hairline cracks have developed. Is there any way to persuade him that these cracks are not detrimental and the wall should stay?
The question, at what width does a crack become a problem, arises often. Unfortunately, there is no definite answer. It may vary from one project to the next. The answer may also change with the persons perspective: what is acceptable to the engineer, owner, or contractor may be unacceptable to the architect.
ACI Committee 224 provides a guide for crack widths in the tensile face of reinforced-concrete structures (interior side for a foundation wall) for typical exposure conditions (see table). Although the table is titled "Tolerable Crack Widths," its intended use is as a design guide. Because this table is often misused (typically by attorneys), some would rather label it "Average Design Crack Widths." Designers could then use the table to determine if cracks will fall within these guidelines.
Observed crack widths, however, are not necessarily very good indicators of durability (the structures resistance to corrosion and deterioration). It may be (and ACI 224 states that is should be expected) that some of the cracks in a structure will be significantly wider than those in the table. The amount of concrete cover over the reinforcement may be a better indicator of corrosion resistance than crack width.
Another point to consider when using this table is that it applies only to structural reinforced concrete. These values should not be used to judge crack widths in unreinforced or lightly reinforced slabs on grade or in unreinforced basement walls. Also take into account the high variability in crack widths. Isolated locations where cracks are wider than those in this table 224 should not be grounds for rejection or reduced pay.