How Does Salt Affect Concrete?

Water Absorption

• Salt causes ice and frost to melt. The slow release of moisture from the melting ice is absorbed by the concrete. Concrete constantly absorbs moisture because of its porous nature. Low winter temperatures cause the water to refreeze while trapped in the concrete. Salt also attracts more water, leading to saturated concrete. De-icing salts contain minerals like magnesium and calcium to add to their ice-melting capabilities, but some of these minerals have a corrosive effect on concrete.

Spalling

• When the water absorbed by the concrete re-freezes, cracks and chips occur in the material (a process known as spalling), says Hottmann Construction. Even if you don't see this damage on the surface of the concrete, the sharp edges of your snow shovel will find any weaknesses hidden below the top of the concrete. Keeping water out of your concrete when temperatures are low is crucial. The corrosive and water-attracting properties of salt make it a poor choice for de-icing concrete because it allows ice to accumulate as well as melting it.

Types

• Some types of de-icing salt have less of a damaging effect on concrete. Magnesium chloride is a common salt used for home de-icing, but it produces the worst corrosive damage, according to the Iowa Center for Transportation Research and Education. Calcium chloride is also damaging. If you must use de-icing salts, plain sodium chloride, common table salt, causes the least corrosion. You must still protect the concrete in some way from the freezing damage, however.

Protection

• Concrete needs to cure completely before freezing, so pour fresh concrete early enough in the fall that it has 30 days of drying before the first frost. Adding the right sealant to your finished concrete product also protects it from damage from both water absorption and corrosion. Look for a sealant that contains paraffin or silicon. These ingredients allow the concrete to breathe and release any moisture that it absorbs through the ground.