Q.: We have to cut a 2x2-foot opening in an existing elevated reinforced-concrete slab. An engineer has approved the location of the opening, but we're trying to decide the best way to cut it. In the past we've drilled core holes at the four corners of an opening, then cut from hole to hole. Coring eliminates overcutting and may reduce stress concentrations at the corners, helping to prevent cracks at the re-entrant corners.

Now that concrete chain saws capable of plunge cutting are available, we can make these openings without overcutting or drilling core holes. But is there still an advantage to drilling core holes to minimize the chance of cracking at re-entrant corners?

A.: In a reinforced-concrete slab, cracking at re-entrant corners shouldn't be a major problem since the reinforcing steel will prevent any cracks from widening appreciably. We'd opt for the cutting method that's most economical.