HOW SALT DAMAGES CONCRETE
In today's blog we have decided to provide some information on a common problem that has become much more prevalent in the Treasure Valley these last few years. This problem is damage from salts and ice melts being used on driveways and sidewalks. We out like the types of damage and causes below. In many cases where concrete damage, often called spalling, occurs there are few if any options to "repair" the concrete. Hopefully this article will educate, explain, and help you avoid this type of damage.
Salt damage is most commonly due to exposure to de-icing salts. However, any chemical containing chlorides, including sodium chloride, potassium chloride, calcium chloride etc. found in “safe de-icing chemicals,” fertilizers, ocean water, etc. present a danger to the concrete. All are mildly acidic, and attack the bonds that hold concrete together.
While the exact mechanism of salt damage is not known, a number of known, related factors instigated by exposure to salt contribute to the deterioration of concrete.
- First, salt, a mild acid, lowers the pH in the concrete. The acidic reaction attacks the concrete paste and aggregate, increasing the pore size and allowing additional water and chemicals into the concrete which can exacerbate the freeze/thaw cycle damage.
- Secondly, salt is hygroscopic, meaning that salt attracts and retains water. When salts are applied to concrete, they attract additional water (up to 10% more) into the pore structure of the concrete. This process leaves less room for expansion in the pore structure which, in turn, creates more pressure inside the concrete when it freezes, causing the surface to chip, flake, and pop (typically called “spalling”).
- Third, salt accelerates the carbonation process. Carbonation is a process that slowly reduces the pH levels in concrete through contact with Carbon Dioxide (air) and contributes to the corrosion of reinforcing steel. Most salts are neutral to mildly acidic (pH of 4.5 – 7), and absorption of the salts (brine) by the concrete will accelerate the pH reduction of the concrete.
- Fourth, salts are the primary source of chlorides introduced into concrete. Chlorides are also a major player in the corrosion of reinforcement, as they pierce the thin, protective iron oxide layer surrounding the reinforcement and initiate a corrosive reaction in the steel.
Sulfates are, quite simply, a salt of sulfuric acid. They are found in all natural waters, and are a major dissolved component of rain. Concrete is exposed to sulfates in two ways, externally sourced and internally sourced. Most commonly, sulfate attack occurs where concrete is exposed to high sulfate content in the soil, in areas of run-off, and in wastewater. Secondly, Portland cement contains a small percentage of gypsum (calcium sulfate dihydrate). The sulfate in the Portland cement is an internal source, and is activated when the internal temperature of the curing concrete reaches 160 degrees Fahrenheit, a condition which can and should be avoided.
Sulfates react in the alkaline environment of the concrete paste and create highly expansive crystals called Ettringites. Ettringite is calcium aluminum sulfate. Aluminum is also frequently found as a component of Portland cement in trace amounts. The formation of these expansive crystals creates a great deal of pressure inside the concrete and cause the concrete to crack and spall.
In addition, sulfates are acidic, break down the alkaline environment in the concrete and deteriorate the concrete paste through acidic attack. This acidic reaction causes the concrete to weaken and crumble.