Q. We placed concrete for the footings and floor slab of an industrial building. The interior column footings are 18 inches deep and contain a bottom and top mat of reinforcing bars. The 6-inch-thick floor slab contains welded wire fabric.

In a timely fashion, we sawed contraction joints along the column lines in both directions, but after 1 week, the floor slab has developed a fine crack -- circular in plan view -- that circumscribes the interior column footings. Is this crack caused by drying shrinkage, and could we have prevented it by moving the sawcut joints away from the column grids and interior footings?

A. The interior columns may not be properly isolated from the floor. Isolation is typically achieved by using diamond-shaped or round blockouts, or by cutting joints in a pinwheel pattern around the steel columns and wrapping the columns with a preformed joint filler. If the columns aren't isolated and the column footings settle, they pull the floor slab down with them, creating a tension crack in the slab around the column. You should consult a structural engineer if the cracking worsens and an elevation difference appears inside the crack perimeter.