Q&A: Spalling Concrete Floors

A.Jay Meunier, contracting specialist at S.T. Griswold and Co., a ready-mixed concrete supplier in Williston, Vt., responds: I am not aware of any cure for spalling slabs. Spalling can be caused by poor weather conditions during finishing, improper finishing techniques, or damage from de-icing salts.

Spalling slabs are more common in climates with frequent freeze-thaw cycles. Such climates require the use of air-entrained concrete (concrete with small bubbles of air to provide room for expansion when moisture freezes). Assuming that air-entrained concrete was used, most spalling problems can be traced to errors during the pour. Either the slab dried too quickly, or the concrete slab was worked too early, trapping the bleed water just below the surface.

When a slab is poured on a hot, windy day, it can dry too quickly. When that happens, some workers apply small amounts of water to the surface of the concrete to get the cream back to a workable condition. This reduces the strength of the concrete surface. The weaker surface layer can later spall when moisture freezes and pops the cream.

If the slab is worked too early, bleed water can be trapped in the concrete, since the concrete just below the cream on the surface has not yet set up. A weak layer in the concrete will form where the bleed water was trapped. Again, when the slab later freezes, the concrete can pop along the line at the weak layer.

Choose a concrete contractor with a reputation for quality work. Don’t pour a slab that is open to the weather on a hot, sunny, windy day. Finally, always apply a curing-sealing compound to allow the slab to cure more slowly and seal the top from contaminants like road salt.