Ask the Builder
Deicing Salts And Concrete
By Tim Carter
©1993-2010 Tim Carter
Summary: Rock salt can be damaging to concrete driveways, walkways and patios if
the concrete installation was not originally done correctly. Concrete's
compressive strength, air entrainment, placement and finishing are all
important to concrete's ability to resist freeze / thaw cycles and salt.
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Concrete Sealant Manufacturers
DEAR TIM: We just moved into our new house. Our builder has advised us not to use rock salt on our concrete sidewalks and driveway for snow and ice removal. He told us that it will damage the concrete. I don't believe him, because our city uses it on our streets every winter. Does my builder know what he is talking about? S. D.
DEAR S. D.: Your builder's advice is accurate. He is well informed on this subject. Rock salt can be one of concrete's worst enemies. Avoid using it on your concrete sidewalks, driveways, and patios if they have not been installed correctly.
Concrete is a magnificent material. As a paving material, it can be one of the longest lasting surfaces that you can use. However, its useful life can be significantly reduced if you do not recognize its weaknesses.
Concrete has high strength when it is compressed, or 'squeezed'. However, it is extremely weak when it is subjected to tension, or 'pulled'. Rock salt can take advantage of this weakness.
Believe it or not, while concrete appears to be a very dense material, it is in fact quite like a blotter. It can and does absorb water. You can actually see this happen on a hot summer day. Sprinkle some water on your sidewalk or driveway and look very closely. You can actually see the water penetrate the surface of the concrete.
When you spread rock salt on your concrete to melt snow and ice, the salt dissolves the snow and makes a salt water mush. The melting action of the salt allows water to enter the concrete. If the temperature then drops and the water freezes, the growing ice crystals can blast apart the concrete.
Salt is also hygroscopic. It attracts water. It can cause concrete to become more saturated with water than it would otherwise. The presence of this extra water in freezing conditions can spell trouble. The volume of water increases by 9 percent when it freezes within the concrete matrix. The pressure of the growing ice crystals can cause the surface of the concrete to fail. It usually spalls off.
Freshly poured concrete is most susceptible to damage. Concrete placed in the late fall needs at least 30 days of drying time. This young concrete is still highly saturated with water. The water within the concrete can freeze and cause the surface to pop off. However, if enough cement was in the initial mixture and this cement was not diluted by the addition of water, the concrete will be able to resist the damaging forces of the freezing water.
Always order concrete that will attain a minimum compressive strength of 4,000 pounds per square inch. Be sure that is air entrained as well. These two things are a good defense against salt attack. Concrete which is ordered, placed, finished, and cured properly can resist years of contact with rock salt. Concrete surface failures such as spalling can almost always be traced to workmanship errors.
The placement and finishing of the concrete is critical as well. The upper surface of concrete can be severely weakened by poor workmanship. Sometimes workers add water to concrete at the jobsite or use it as a finishing aid. These practices dilute the amount of cement at the surface of the concrete. The cement is the ingredient in concrete that holds everything together. To resist the freeze/thaw action of water, you need to have strong concrete at or near the surface.
There is an alternative to using rock salt. You can use sand. The sand will not melt the snow and ice, but it will provide you with traction.
You can also treat your concrete with clear coatings that minimize or eliminate the possibility of water being absorbed by your concrete. Some of these clear coatings contain silanes and siloxanes. These ingredients allow the clear coatings to breathe. Avoid using products that contain silicone or paraffin. These can produce a surface film. A surface film does not allow the concrete to breathe. Concrete soaks up water from the soil. This water passes through the concrete and eventually evaporates. However, if you trap this water at the surface with film forming sealants, you may cause spalling. Be careful!
