Sulfates are a naturally occurring mineral salt compound. When soil becomes wet from irrigation or rainfall, sulfates dissolve the water and seep into porous (less dense) concrete.  The sulfates eat away the cement “paste” that holds the concrete together. Once the deterioration starts, the damage to the concrete cannot be reversed and progressively worsens over time. However, if the problem is addressed in its early stages, substantial damage can be avoided.

Like sulfates, chlorides dissolve into ground moisture and are drawn into concrete.  Chlorides, however, are highly corrosive to metal. Accordingly, chlorides affect the anchor bolts and post-tension cables of the homes, which creates an extremely high potential for damage. If the anchor bolts and post-tension cables are compromised the home can experience severe structural damage.

Sulfate damage typically appears in the form of hairline cracks called “etching,” or white, powdery stains referred to as “efflorescence.”  The first places to look is on the flatwork (walkways and sidewalks), along the exterior foundation, or the block wall.  As the sulfate attack worsens, other parts of the home’s foundation or building’s foundation will show signs of efflorescence and etching.  Chloride damage will appear as rust on the metal hardware or post-tension cable ends.

As the sulfates infiltrate concrete, it combines with the C-S-H, or concrete paste, and begins destroying the paste that holds the concrete together.  As sulfate dries, new compounds are formed, often called ettringite.  These new crystals occupy empty space, and as they continue to form, they cause the paste to crack, further damaging the concrete.  The concrete weakens and the paste continues to separate and crack.  Consequently, the concrete becomes increasingly permeable, allowing in more and more water filled with sulfates.  Over time, the process accelerates dramatically.  The excess water content also begins to rust away the anchor bolts that tie the home’s structure to the foundation.

Eventually, the concrete foundation is severely compromised.  This condition, coupled with the rusting structural ties and anchor bolts, puts a home/building and its occupants at greater risk from high winds, earthquakes, and other destructive forces of nature.  According to the National Association of Home Builders, the useful life of concrete should be about 150 years; however, left unattended; sulfate attack can cut that life span to 15 years or less.

Since 1979, Building Codes have required that a special Type V cement be used in high-sulfate soils, and then a lower water-to-cement ratio be used to mix the cement to maintain a proper level of density in the concrete after curing.  Dense concrete retards groundwater from seeping in; it’s the water that carries the sulfates.  According to most code, the water-to-cement in high-sulfate soil conditions should be no more than 0.45.  However, in many cases that we have investigated, the ratio is more like 0.65 to 0.70, or 50% more than the maximum allowed by code.  The result is concrete with loosely interconnected pore structure, and therefore not dense enough.

Certainly, many builders comply with code sulfate requirements. However, much concrete used in the construction of homes built on high-sulfate soils since the mid-1980s do not meet Building Code standards, creating a defect that can result in the concrete’s decomposition.  One potential reason is that contractors and suppliers of concrete ignored the code requirements.  Compounding the problem, those contractors who poured concrete at building sites may have mixed in additional water to make the concrete less thick, which resulted in quicker and easier installation.

Before beginning construction, builders are required to conduct soil tests that measure sulfate and chloride content.   Reports on such tests should be on file with local building departments or environmental management agencies.  If a housing project is newer, the builder may still retain a copy of the report.