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
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
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
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.