Salt and Concrete – The Real Story
Salt and concrete have a bad reputation. Rock salt is often used on roads and driveways during cold weather to melt ice and snow. The salt is blamed for damaging concrete surfaces but is that the whole truth? Is there a way to prevent damage to concrete? Here's the real story
Ice melters lower the freezing point of water. Applying them to surfaces melts snow and ice. The resulting water won’t turn back to ice until the temperature falls below these colder temperatures. When temperatures rise again, treated residue changes the ice back to water. When temperatures fall, it refreezes. These swings are called freeze-thaw cycles.
Rock salt, or sodium chloride, lowers the freezing point from 32o F (0o C) to 25o F (-3.9o C). Potassium Chloride lowers it to 15o F (-9.4o C). Magnesium Chloride lowers it to -15o F (-26.1o C). Calcium chloride lowers it to -25o F (-31.7o C)
Custom blends of different ice melters like calcium magnesium acetate are available for reduced costs but are less effective than pure products. Ammonium products are also available are a but these chemicals attack concrete directly and should never be used.
Indirectly, yes, salt and concrete mean damage. After spreading salt to melt snow or ice, the top surface of concrete often spalls or scales. But salt is not directly at fault. It does however play a role in winter concrete damage. In fact, ALL ice melters have the capability to damage concrete.
When the snow and ice melt with the help of rock salt, they enter the concrete surface as salted water. Salt attracts more water and increases the pressure of frozen water.
When temperatures fall below 25 degrees Fahrenheit, the water refreezes and expands with added pressure causing damage if the concrete is not strong enough. Fresh or low-strength concrete is more susceptible to damage because it holds more water already. Concrete pavers are extremely strong and not very porous so not affected by freeze thaw cycles.
The increased freezing water creates an upward pressure resulting in “pitted” or spalled surfaces leaving the aggregate mixture beneath visible. It appears in large sections or can be scattered in smaller areas.
The first line of defence to protect against the damaging reaction between salt and concrete is during construction. It is important to not add too much water when mixing concrete or use a water reducer to help make the concrete stronger.
In addition to quality construction, you can add additional protection by sealing the concrete to prevent further salt damage.
Read more about concrete sealing here.
To eliminate the risk of salt damage, you can use an environmentally friendly alternative like sand. Studies on natural deicers like beet juice indicate a successful reduction of the freezing point but seem to be more harmful to aquatic species.