Question: We supplied concrete for a 4-inch-thick unreinforced slab on grade for a new building. The specifications called for maximum control-joint spacings of 30 feet for slabs to receive flooring finish materials, and 15 feet for all other floors. Several shrinkage cracks about 10 to 12 feet apart have appeared in the 30x30-foot floor panels, and some of the 15x15-foot panels also have cracked.

The building owner and contractor both say our concrete caused the cracking. However, we think the joints are too far apart. Are the joint spacings normal for a 4-inch-thick slab?

Answer: ACI 302.1R-96, "Guide for Concrete Floor and Slab Construction," states that joint spacings of 24 to 36 times the slab thickness up to a maximum spacing of 18 feet have generally produced acceptable results for unreinforced slabs. For a 4-inch-thick slab, this would produce suggested joint spacings ranging from 8 to 12 feet, considerably less than the 30-foot spacings for your floor and slightly closer together than the 15-foot spacing. ACI 302.1R also states that even with the suggested joint spacings, some random cracking should be expected, with a reasonable level being random cracks forming in 0% to 3% of the floor slab panels. With the joint spacing you describe, the amount of cracking isn't surprising.

The editors of Concrete Construction magazine have developed a field training kit that your customers can use to train finishers on techniques that minimize cracks in floors and slabs. The kit includes a set of laminated picture cards showing various cracking problems and explanations, in both Spanish and English, of how to avoid future failures.

The kit, entitled "Preventing Random Cracking in Concrete Flatwork," is available through Aberdeen's online bookstore at