Q.: I'm designing a concrete foundation to support a large industrial furnace. The furnace vendor says concrete under the furnace may reach 400° F. What special precautions should I take to prevent damage to the concrete?

A.: Depending on the concrete moisture content, bare concrete surfaces exposed to 400° F temperatures probably will spall. That's because steam generated within the pores of the hydrated cement paste and the aggregate doesn't have time to escape before enough pressure develops to fracture the paste, aggregate particles or both. The longer the concrete dries, the more resistant it is to damage caused by spalling, but the required drying time to prevent such damage may be months or even years.

Some investigators have added polypropylene fibers to high-performance concrete to prevent steam buildup. The idea is that when the fibers melt, they create passageways that allow steam to escape. Research showed that this technique helped to prevent spalling of high-strength concrete columns subjected to high temperatures (Ref. 1).

Reference 2 discusses other factors that affect concrete resistance to high temperatures. For example, at 400° F there may be some degradation of the calcium silicate hydrates in the hardened paste.

1. T.A. Hammer, "High Strength Concrete, Phase 3, SP6 Fire Resistance--Report 6.2, Spalling Reduction Through Material Design," SINTEF Report STF70 F92156, Trondheim, Norway, 1992.

2. Peter Smith, "Resistance to Fire and High Temperature," Significance of Tests and Properties of Concrete and Concrete-Making Materials, STP 169C, ASTM, West Conshohocken, Pa., 1994, pp. 282-295.