What Cracks to Repair

Hotline Question

We have constructed a reinforced concrete skip joist floor (interior exposure) that has developed some through-slab cracks parallel to the joists. We believe the cracking is not likely to continue. Crack widths range from hairline to nearly 1/8 inch. Are there any standards that can tell us which cracks must be repaired?

Hotline Answer

There are no ACI standards or recommendations that give a "yes" or "no" answer to crack repair based on crack width and other factors. The significance of cracking depends on many factors, including crack orientation with respect to reinforcement, cover thickness, and exposure. As described in "Causes, Evaluation, and Repair of Cracks in Concrete Structures" (ACI 224.1R), repair methods also consider whether the cracking is due to one time or repeated effects. Since the cracking appears to be caused by unique events, epoxy injection is a logical choice.

Experienced injection crews can repair very fine cracks, often finer than 5 mils (0.005 inch), if this is needed. It often isnt, and economics then will dictate that a wider limit be set as the basis for repair. Often, this limit is about 25 to 30 mils. This would probably be sufficient on your job. However, epoxy injection consultant Peter Barlow of Seattle, Washington, recommends that cracks as fine as 8 to 10 mils be injected if there is available budget and a likelihood that movement may continue at the cracks. If movement continues, the crack edges will tend to ravel, and future repairs may be needed.

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