No, it’s not margarrita time, but winter is over where salt most likely met the pavement. Unfortunately, the results do not resemble the Margarittaville picture that Jimmy Buffet sings about. We are entering the summer months here in Colorado, and with it comes discovering the results of a winter of de-icers and salt. Every year when this season approaches, Colorado Hardscapes receives many calls on how to take repair damaged concrete that seems to be spalling or whiting, for “no apparent reason”. Salt and de-icers are extremely harsh on concrete surfaces and not recommended. But why does salt do so much damage to concrete?
When salt is spread onto the concrete sidewalks it drops the surface temperature. The brine produced from the salt and melted snow mix has a very low freezing point. The brine is absorbed into the concrete where it is diluted further by the moisture in the concrete. At the depth of about 5mm, the solution reaches a freezing point near the 0° C mark.
The top layer of concrete is very sensative to minor temperature changes, especially within the first year of being placed. Not only does this narrow the freezing zone, it also reacts to any and all temperature changes. For example, if the temperature drops even by one degree, the thin top layer of concrete will freeze. Any temperature change, regardless of how small can cause the freeze-thaw cycles to start, thus damaging a thin layer of the concrete surface. Repeated cycles will drive the damage deeper and deeper. As drastic as it may sound, temperature changes caused by a passing cloud, an open door or even the slightest change from body heat are just a few of the factors that cause minor fluctuations every day. Each minor change is half of a freeze-thaw cycle.
Freezing water expands about 10%, and if restrained, creates the type of stress that no concrete can withstand. Imagine a full water bottle that is placed in the freeer and expands to the point of bursting. De-icing salts (or any de-icing material) merely increase the number of times that the concrete freezes. The effect of the salts are physical rather than chemical. It does not produce hydrochloric acid as some have mentioned. The damage caused by salt is due to disruptive hydraulic pressures within the concrete.
Colorado Hardscapes only recommends sand to be placed on the concrete. However, if de-icing salts or materials have to be placed on the concrete sidewalks (though we strongly recommend against this) they should be washed off the concrete as soon as possible. De-icing materials of any type could have an effect on the warranty to the concrete hardscape installed on your project.
There are sealers that can be applied to the surface of cured concrete that can help minimize the risk, but nothing is bullet-proof to the harsh reactiveness of de-icers and freeze-thaw cycles.
So, before you put away the summer shades and shorts at the end of this season, be sure to have de-icing plan in mind for the winter months. If possible, leave the salt off the pavement and allow the concrete to be the durable finish it is known for.