Causes of the Spalling of Concrete

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You shouldn't see the rocky layer beneath the first layer of concrete.
You shouldn't see the rocky layer beneath the first layer of concrete. (Image: Thinkstock Images/Comstock/Getty Images)

Concrete surfaces have two layers: smooth and rocky. The smooth surface is the visible part of concrete and the most fragile. When the smooth concrete layer becomes chipped, scaled or flaked, contractors say it is spalling. This makes concrete walkways, stairs, driveways and sidewalks unappealing and in need of repair to eliminate the deterioration. Although spalling usually affects outside structures, it also may happen to interior concrete. There are three main causes of spalling.

Water Damage

Freezing water, such as melted snow or rain, damages concrete in two ways. Water seeps into concrete, causing cracks to form. In addition, during winter months, water freezes and causes the concrete to break apart.

Rust damage is the second way water causes concrete spalling. Water seeps into the concrete and corrodes the rebar. The rust expands, causing spalling. In some instances, you can see the metal bars protruding from the concrete.

Heat

Extreme heat causes spalling. Chunks of concrete begin separating when exposed to high temperatures -- either over time or suddenly. For example, fire can cause the concrete to break up, then fall from the foundation. However, heat-related spalling usually doesn’t affect homeowners because the enormous amount of heat needed to cause damage is rarely achieved outside of fire, according to ReliableRemodeler.com.

Pressure

Pressure may work in conjunction with water damage or alone. The constant freeze/thaw cycle during winter months causes water trapped in the cement to expand, then deflate, deteriorating cement.

In addition, if you apply large amounts of deicer and rock salt, some of it may collect under the smooth surface of the concrete. As the salt crystallizes, pressure builds up and causes the concrete to flake or chip.

Prevention

The best way to prevent spalling is to use quality products, such as cement mix, and apply it correctly.

To prevent spalling on existing structures, use a concrete sealer. A sealer forms a protective barrier between the concrete and water, preventing such items as deicer and water from seeping into the concrete. Apply the sealer by spraying, brushing or rolling it onto the concrete. The concrete sealer typically dries in two to four hours, according to Concrete Treat.

However, concrete damaged too extensively by spalling may require repair by a professional certified in concrete masonry.

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