Upon appearance, concrete may seem like a very dense material. However, in reality, it is like a sponge. It can and does absorb water. This can be easily observed on a summer day. Pouring water on a sidewalk, you may actually observe the water penetrate (be absorbed by) the surface of the concrete. Quality concrete should contain air voids.
Generally, all deicers work in the same way. They depress the freezing point of ice or snow and turn the mixture into a liquid or slush. Salts cut through ice or snow and form a salt water (brine) solution. This brine spreads under the ice or hard-packed snow and breaks the bond to the concrete surface. Once loose, the ice or snow can be easily removed by mechanical means. Or, in many cases, associations may apply the material in anticipation of ice or snow. This prevents the bond to the surface and melts the snow or ice as it comes in contact with the brine.
The melting action of the material allows water to enter the concrete. If the temperature then drops and the water freezes, the ice crystals can explode the concrete surface. This surface defect is commonly referred to as spalling or scalling.
The first measure of a deicer's effectiveness is the range of temperatures in which it can provide deicing action (in a reasonable time period). The lowest temperature limits for these materials is defined as effective (it melts the ice) within 15-20 minutes of application. Calcium Chloride works to temperatures to minus –25 degrees F. Sodium Chloride (Rock salt) works only to temperatures of 20 degrees F° resulting in a higher potential of freeze/thaw to occur and the crystal growth or expansion discussed above if temperatures fall below 20 degrees.
Don't Pour it On
Salt is often harmful to concrete surfaces, more frequently harmful on lower grades of concrete. During the first winter, deicers should not be used on concrete as the concrete may still be curing and containing excess water and as such less voids. The additional absorption of water from the ice melt and a freeze cycle may be just enough to cause a problem as the voids are full. Even when the concrete has cured, depending upon the mix (discussed below) a reaction may take place when rock salt is used. Some damage to plants can occur from excessive use of this material and care should be taken when applying the material.