Why Does Salt Do Damage to Some Concrete?
CEMENTIMAGINATION VOL. 21
Winter slush is unloved by millions. It is dirty, never seems to freeze,
and is very cold on the feet. It is like a mixture of salt in an ice
cream churn. The brine is actually colder than the ice it is made
from. The melting caused by the salt is an endothermic reaction that
consumes large amounts of heat.
When salt is spread on snowy and icy streets, it drops the temperature.
Furthermore, the brine produced has a very low freezing point -about
-14°c. The brine is absorbed into the curb or sidewalk concrete
where it becomes diluted with moisture. At a depth of about 5mm, the
solution is so weak that the freezing point is near the 0°C mark.
This brings us to the main question... Why does salt cause so much
damage to some concrete?
This top layer of concrete is very sensitive to minor temperature
changes. Not only is it a narrow freezing zone, it also reacts to
all temperature changes from -14° to 0° C. For instance, if
the temperature drops from -4° to -5°C, a very thin layer
will freeze.
The concrete that is saturated with enough salt to Have a freezing
point lower than -5° will remain unfrozen. We could make finer
divisions and say a drop of a three thousandth of a degree would freeze
a layer of concrete a thousandth of a millimetre thick. Any change,
no matter how small, between 0° and -15° will cause freezing
or thawing of a thin layer in that critical top 5mm. Temperature changes
caused by a passing cloud, an opened door, or even the infinitesimal
change caused by body heat are a few of the hundreds of factors that
cause minor fluctuations every day. Each change is a half of a freeze/thaw
cycle.
Northern cities like Sudbury used to boast of one freeze/thaw cycle
per year. With the advent of salting, they may have several daily.
Toronto spends much of the winter in the critical -6° to -1°
range and rarely is there a moment when salted concrete is not freezing
or thawing.
On freezing, water expands about 10% and, if restrained, creates stresses
that no concrete can withstand. De-icing salts merely increase the
number of times concrete freezes to an astronomical figure. Its effect
is physical rather than chemical. It doesn't produce hydrochloric
acid as is sometimes reported. However, there are fertilizers advertised
as de-icing agents that are chemically destructive. Ammonium sulfate
will damage concrete even in the summer.
The damage caused by salt is due to disruptive hydraulic pressures
in the concrete. Purposely entrained air bubbles act as reservoirs
for excess water forced into them by progressing ice fronts in capillaries.
Quality concrete air entrainment effectively prevents frost damage.
!
If you want to prevent this freeze - thaw damage to your concrete,
it simply must be sealed to prevent the moisture from soaking into
the cement. Be sure to choose a concrete sealer that will reject surface
water from infiltrating. Proper spread rates and application procedures
are necessary to achieve the desired results from a sealer. Some sealers
are "top down" meaning they help by sealing from the surface
first, then they penetrate down into the concrete. Other sealers are
more "bottom up" type sealers, meaning they start more deeply
in the concrete, but must these must be built up to achieve any surface
protection. If you don't achieve surface protection, you will NOT
be adequately protected against salt or freeze - thaw damage.
It is important to talk to the technical team at Vseal when choosing
your sealer so you can pick the best one for your application.
Thanks to St. Lawrence Cement, Ontario, Canada