Scaling Concrete

DEAR TIM: My 9 month old concrete driveway has begun to deteriorate. The surface finish is scaling. My contractor has blamed the defect on my use of deicing salt and that brought onto my driveway by my cars. He refuses to replace/repair the damaged concrete. Is it true that deicing salt is the cause of my problem? Do you think my contractor has no responsibility in this issue? A. R.

DEAR A. R.: Based upon your description of the problem, I am quite certain that your contractor is liable for the damage to your driveway. The deicing salt excuse is often used by concrete contractors who do not install concrete properly or who install it too late in the season. Concrete which is properly formulated, installed, and finished will withstand many years of exposure to freezing temperatures and deicing salts.

Deicing salts melt snow and ice which in turn creates salty meltwater. This meltwater, as well as regular winter rainwater, can and does saturate the upper layers of concrete surfaces. As the temperature drops, the meltwater can freeze within the concrete. Water expands approximately 9 percent in volume when it freezes. This expansion can blast apart concrete which is not formulated and installed to withstand these freezing forces.

Concrete can be ordered from ready mix plants which contains microscopic air bubbles (air entrained). The air within the bubbles acts like mini pressure relief valves. The expanding ice compresses the air within the bubbles. The air absorbs the impact instead of the cement, sand, and gravel Your contractor is also able to purchase concrete with varying amounts of cement and water. Cement is the glue which holds the sand and stone particles together in concrete. Concrete that will experience freezing temperatures requires a minimum of 564 pounds of cement per cubic yard. This is sometimes referred to as a 6 bag mix.

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The amount of water present during the mixing and finishing processes is critical to the final strength of the concrete. Weak concrete can not adequately resist repeated freeze-thaw cycles. If you add too much water while mixing concrete, it dilutes the fixed amount of cement "glue" that is present in the mix. Sprinkling or spraying water on the surface of concrete or troweling in bleed water during the finishing process dilutes the amount of available cement in the top most layer, that place where it is needed the most!

Concrete formation and strength development continues for many months after the concrete is placed and finished. Ongoing chemical reactions within the slab require water. Thus, it is important for concrete to be cured with a liquid compound, moist cured with water, or tightly covered with plastic to retard or stop moisture loss from the slab for a period of approximately 7 days.

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Concrete must also be allowed to dry out after the initial curing period. Concrete poured too late in the year still retains water from the mixing process. This water coupled with rain or meltwater can overwhelm the tiny pressure relief air bubbles. Before exposure to deicing salts or extended periods of freezing and thawing, concrete slabs should be allowed to air dry for a minimum of 30 days.

Millions of cubic yards of concrete exist in my city, as well as yours, that have been through years and years of winter weather. These roadways, drives, and sidewalks get repeatedly wet and are frequently exposed to deicing salts. These same concrete surfaces show no signs of scaling. I suggest that you contact your contractor and the ready mix company. They can produce the delivery ticket that will indicate the strength of the concrete, whether or not air was included, and how much water may have been added to the mix once the concrete arrived at the jobsite.

Read my article on  Concrete and Masonry Sealers in the July 2, 2009 Newsletter.