Is That Concrete Bad?

In older homes with exposed foundation walls, or concrete basement floors, we sometimes see chunks of concrete flaking and breaking away (spalling).  Typically, we would also see a white, powdery crust called efflorescence. 

What’s going on?  

The white deposits are minerals.  The minerals came from either the soil on the exterior of the foundation, or from contaminants within the concrete.  These minerals were dissolved by water in the soil on the exterior of the foundation wall.  That water, along with the dissolved minerals, can then migrate through the foundation.  While concrete seems impenetrable, water can migrate through it.  The water does not damage the concrete.  Concrete maintains its strength characteristics even if totally submerged.  The problems arise when the water-with the minerals - moves through the concrete foundation wall. When the moisture reaches the interior surface of the foundation and evaporates into the basement or crawlspace - the minerals won’t.  The minerals will no longer be dissolved (since the water has evaporated) and will reform as the solid minerals they once were when in the soil.

These powdery deposits would appear incapable of damaging concrete, but when the minerals crystalize in the microscopic pores of the concrete, and expand, a large tensile force can develop just below the surface of the concrete.  While concrete can hold up a house – a compression load – concrete is comparatively weak when it’s being pulled apart (tension).

Efflorescence is not a problem in itself, and can often be noted on newer foundation walls without any accompanying damage due to spalling.  But efflorescence does indicate that moisture is migrating from the exterior to the exterior.

As I said, the minerals migrate through the concrete in water.  So, the real source of the problem is the water!  Since Colorado is generally dry, especially on the Western Slope, the water issues is almost always due to either poor roof drainage, or irrigation systems.  If landscaping is planted too close to the structure or if a sprinkler leak goes unnoticed adjacent to a foundation, the soil next to a foundation can become saturated.  Poor guttering and downspouts that drain next to the house can concentrate rainwater next to a foundation.  Maintaining a slope away from the foundation is also essential for protecting the foundation from excessive moisture.

Keeping water away from a foundation is important for many reasons.  Some soils will expand when wet - and with tremendous force - causing significant damage to a building’s foundation.  Some soils will subside when exposed to water.  Subsiding soils can leave the foundation unsupported by the soil below.  And as I have detailed above, efflorescence, and damaging spalling, are caused by excessive soil moisture.

Ensure proper slope away from your foundation, check that gutters and downspouts move water away from the house, and keep an eye on the sprinklers!