According to the Portland Cement Association (PCA), the freezing and thawing of concrete is one of the main causes of spalling. The other two main reasons are high water/cement ratio and finishing concrete too early.
As we all learned in science class, water is one of the few substances that expands as it freezes. When water lands on concrete and then freezes, it fills in the pores of the concrete and builds pressure which weakens the concrete. The more often this occurs, the quicker the concrete will begin to fall apart.
Deicing chemicals, such as rock salt and calcium chloride, increase the number of freeze-thaw cycles because they lower the freezing point of water. As the snow or ice melts and forms a brine, the liquid mixture decreases the concentration of the deicer. If the solution is diluted enough, it will refreeze at a higher temperature and will need an additional application. One way to minimize the number of freeze-thaw cycles is to use a product with a low practical temperature limit. A deicer with a low practical temperature will be more likely to remain in a liquid form once it has formed a brine. The chart below shows the lowest temperature that typical deicers can work effectively. This is not the lowest temperature possible for deicers; that is called the Eutectic Temperature.
To give you an idea of average temperatures around the Midwest, take a look at the chart below. It shows average high and low temperatures in January, the coldest month of the year. Northern areas like Minneapolis and Milwaukee rarely have days that reach up to 20°F, the lowest effective temperature of rock salt. In addition, southern snow areas such as St. Louis and Kansas City typically have lows below 20°F. Therefore, rock salt will be highly ineffective in the North, and though it will work in southern areas, refreeze overnight is very likely.
* Data from Applied Geographic Solutions, Inc. (AGS) 2007
However, the coldest month of the year isn’t the only time freeze-thaw cycles occur. The chart below marks months that have average high temperatures above freezing and average low temperatures below freezing, thus creating the possibility for daily freeze-thaw cycles if precipitation is present. For half of the year, temperatures in Denver waver above and below freezing levels. While significantly less, Minneapolis is still affected for two full months a year.
Months with average highs above freezing and average lows below freezing:
* Data from Weather.com historical data
In the end, PCA states that good-quality concrete is your best defense against freeze-thaw cycles. However, if, for some reason, you know your concrete has a few flaws, you may want to take special care to limit the number of freeze-thaw cycles in an effort to prolong its lifespan.