Q.: Would a natural gas heater 3 feet above the floor cause blistering? We supplied concrete for a garage floor and the flatwork contractor had trouble finishing it in the vicinity of the heater. He said concrete near the heater felt like bread dough during finishing, and afterward he found blisters in that area. The area was only about 2 feet square.

A.: It's likely that the blisters were caused by the gas heater. Moderately rapid evaporation of bleedwater makes the surface ready to be troweled while the underlying concrete is still bleeding or still plastic and releasing air. Troweling seals the surface so that bleedwater or air from below can't get through. Blisters form sometime after the first troweling. Cold concrete or a cold subgrade would aggravate the problem. Low temperatures retard setting and increase the bleeding period of concrete beneath the warmed surface. Air-entrained concrete with a high cement content or made with fine sand is most likely to blister. Thick slabs and slabs placed on a polyethylene vapor barrier also are more likely to blister.

There's an excellent discussion of blistering available from the National Ready Mixed Concrete Association. It's called Concrete Blisters: What, Why, and How. To order copies write to NRMCA, 900 Spring Street, Silver Spring, Maryland, 20910.