The damage to concrete most people fear is actually caused by the freezing and thawing of water that soaks into the upper surface of the concrete. The use of deicing salts increases the amount of freeze-thaw cycles that a concrete sidewalk or driveway experiences. The volume of water increases by 9 percent when it freezes. This expansion creates internal pressures that can blast apart weak concrete.
Concrete that contains small air bubbles (air entrained), a minimum of 564 pounds of cement (6 bag mix) per cubic yard and a minimum amount of water when mixed (4 inch slump) can resist repeated episodes of ice expansion within the concrete. In addition, the concrete must be moist cured at or above 50 F for a minimum of seven days, produce a 28 day strength of 4,000 pounds per square inch and have a minimum drying time of 30 days before it is subjected to the first freeze-thaw cycle. These practices are commonly followed by experienced, professional concrete masons.
Deicing salts rarely cause problems on wood surfaces and asphalt. What little damage they may cause is far outweighed by the benefits of preventing personal injury as a result of a fall on some ice.
There are four primary deicing salts. All have different characteristics. The most common deicing salt is regular rock salt or sodium chloride. It is widely available and can melt snow and ice until the temperature drops to between 16 and 20 F. Below these temperatures the rock salt stops melting snow and ice. Rock salt also releases the highest amount of chloride ions when it dissolves. Chloride can pollute streams, rivers and lakes. The chloride also causes metal to corrode.
Calcium chloride is another deicing salt. Many people have seen these small rounded white pellets. It can continue to melt snow and ice as temperatures fall well below 0 F. It can cause skin irritation if your hands are moist when using it. Concentrations of calcium chloride can chemically attack concrete.
Potassium chloride is a deicing salt that available in some markets. It is not a skin irritant and does not harm vegetation. It only melts ice when the air temperture is above 15 F, but when combined with other chemicals it can melt ice at lower temperatures.
The newest deicing salt is magnesium chloride. It continues to melt snow and ice until the temperature reaches -13 F. In the photo, the air temperature was 0 F. The magnesium chloride melted through a 1/2 inch thick layer of ice in just 10 minutes. This salt releases about 40 percent less chlorides into the environment than either rock salt or calcium chloride. It is also less damaging to concrete surfaces of questionable or unknown quality. Magnesium chloride is less toxic to plants, trees and shrubs. It also does not leave a powder residue when tracked into your home.
Avoid the use of fertilizers as deicing and traction agents. Those that contain ammonium nitrate and ammonium sulfate can rapidly disintegrate concrete. Don’t take a chance if you don’t know what is in the fertilizer. If you don’t have salts available use sand to stop you from slipping and sliding.