Q. We placed concrete for a basement floor with plastic hoses embedded in it for radiant heating. The concrete was placed directly on expanded-polystyrene insulation laid over plastic sheeting. A lot of bleedwater collected at the surface, so we used propane heaters to dry it up and then ran the heaters all night to help cure the concrete. The next day we applied two coats of sealer. Two weeks after the floor was placed, the owners reported that the surface was wearing away under foot traffic. They want to put a tile floor over the concrete but are concerned that the tile adhesive won't bond. What caused the problem, and what can we do to correct it?

A. The problem is called dusting, and is caused by carbonation. If a combustion heater isn't vented, it emits exhaust gases with the heated air. The exhaust gases contain carbon dioxide, which reacts with water on the fresh concrete surface to form carbonic acid. The acid then combines with calcium hydroxide, a byproduct of cement hydration, to form weak calcium carbonates instead of the strong calcium silicate hydrates that are normally produced (Ref. 1). The soft layer caused by carbonation is usually only a small fraction of an inch deep. For reasons that aren't fully understood, dusting doesn't always occur when unvented combustion heaters are used. But to be on the safe side, you should use vented combustion heaters or electric heaters during the first 24 hours after concrete flatwork is placed (Ref. 2).Wet grinding with a terrazzo grinder is one way to remove the weakened layer without creating dust. You can also rent grinders with a vacuum attachment that collects the dust. Remove the surface until you've reached sound concrete, and clean all remaining dust before floor tile is laid.


  1. Moira Harding, "Portable Heaters for the Jobsite," Concrete Construction, October 1994, pp. 800-808.
  2. Steven H. Kosmatka and William C. Panarese, Design and Control of Concrete Mixtures, 13th ed., Portland Cement Association, 1988, rev. 1992, p. 160.

Reader Response:

In answer to the June 1996 Problem Clinic question concerning the cause of a dusting floor (pp. 510-511), you respond that the problem is due to a combustion heater's unvented gases, which react with bleedwater to form carbonic acid.However, you fail to explain why the contractor had a lot of bleedwater on the floor surface. The excess bleedwater was due to the improper placement--directly beneath the slab--of the plastic sheeting, or vapor diffusion retarder (VDR), and the expanded polystyrene (EPS). Relocating the VDR and EPS prevents excess bleedwater and the subsequent need for heaters to dry the slab.

Gene Leger
Leger Designs
New Boston, N.H.

Editor's Response:

We disagree that it's improper to place EPS insulation board or a vapor retarder directly beneath a concrete slab. In our experience, freezer floors are always placed directly on the insulation board, and radiant-heated slabs (such as the slab in question) are placed directly on the insulation board or vapor retarder.Leger recommends relocating the insulation board or vapor retarder, presumably by placing a granular blotter layer directly beneath the slab. This may cause heat to flow downward toward the cooler blotter layer instead of upward into the room to be heated.However, we should have warned the contractor that bleeding time increases when concrete is placed directly on a nonabsorbant base. Thus, a low-slump, low-water-content concrete is needed to shorten the bleeding period.