Q.: We do finish work for a pool contractor who insists on using #4 rebar in all his flatwork. We get hairline cracks above the rebar locations except where concrete is 6 inches thick. We have suggested that he use welded wire mesh but he refuses. What can we do to stop cracks?
A.: Your suggestion of using wire mesh (which has a smaller diameter than #4 bars)--and your observation that cracking doesn't occur when the steel is deeper in the concrete--both agree with the recommendation of the ACI 302, Section 2.2.2, which appears in Concrete International, June 1980.
The first paragraph of that section says:
"Plastic settlement cracking over rebars is caused by inadequate concrete cover over steel, larger rebar size and/or higher slump concrete or a combination of these. To help avoid these problems, concrete cover should be at least 2 inches (50 millimeters) where possible. Other measures to avoid this are smaller bar size, low slump concrete (2 inches or 50 millimeters) and/or revibration of concrete."
This recommendation is based on data presented by Fadh. H. Dakil, Philip D. Cady and Roger E. Carrier in the article "Cracking of Fresh Concrete as Related to Reinforcement," ACI Journal, August 1975, pages 421-428.
ACI 302 also says (Section 11.1.3):
"Some cracking may occur before the concrete has hardened. This may complicate the finishing operations considerably. Some examples are: . . . (2) cracking from settlement of concrete around reinforcing bars or other embedments."