Chemical Influence on Concrete Deterioration

Concrete deterioration (degradation) can be from various chemical and/or physical causes. The leading cause of deterioration of concrete is when corrosion progresses on reinforcing steel and other embedded metals. When the corrosion process of steel within concrete begins, the resulting corrosion (rust) occupies a greater volume that the formed steel. This expansion of rust creates tensile stresses within the concrete, which over time and existing service environment conditions will eventually lead to further cracking, weakness of the concrete, as well as potential structural failure points on the concrete asset.

Chemical mechanisms cause the most significant deterioration to concrete and can be observed in a wide range of commercial, industrial, and natural environments. The concrete industry working in conjunction with the American Concrete Institute (ACI) has identified and recognized the following chemical mechanisms that are impactors when it comes to concrete deterioration (degradation), they are:

• Acid attacks: When newly hydrated concrete with a pH of 12.5 or greater value interacts with an acid pH lower than 7.0, the concrete components are dissolved and produce salts (chlorides, nitrates, sulfates), and as the cement paste is dissolved, coarse aggregates are exposed, producing an increase in porosity and a loss of concrete strength.
• Alkaline reactions: With newly hydrated Portland concrete has a pH value between 12 and 14, it will mostly remain unimpacted by alkaline solutions. Exposure to liquid materials that have a 25% or greater concentration of hot sodium hydroxide will start degrading concrete over time. There are other liquid materials that will also produce the same conditions.
• Alkali-aggregate reactivity: Also known in the industry as "AAR", which is a general category of aggregates that contain certain materials which react with alkali carbonates within concrete. AAR is very damaging to concrete due to expansion, also the most common form is a alkali-silica reaction (ASR).
• Carbonation: This naturally occurring reaction affects all concrete exposed to the atmosphere. It's the reaction of atmospheric carbon dioxide with hydrated materials with Portland cement paste.
• Chloride-induced: Structural reinforcing rebar and other steels within concrete will initially corrode and produce a tightly condensed oxide film over the surface of the steel. This passive protection film can be maintained by maintaining a high-alkaline condition with the hydrated Portland cement.
• Oils: Crude and other oils that have higher concentrations of sulfur can cause a number of deterioration conditions. Sulfur oxidizing and sulfate reducing bacteria (SOB and (SRB) can severely degrade concrete tanks used for crude oil storage, by attacking the hydrated cement paste.
• Sugar attack: Sugars imped setting and hardening of newly poured and fresh concrete. Sugars also cause hardened concrete to degrade as they interact with calcium hydroxide in the hydrated cement paste to form calcium saccharates.
• Sulfate attack: Naturally occurring and manufactured sulfate materials, including sulfates of sodium, magnesium, calcium, potassium can attack concrete.
• Chemical related erosion: Chemical attack, especially in acids from process manufacturing or sulfuric acid attacks within sewer collection systems.
• Plastic shrinkage cracking: Shrinkage cracks occur during new concrete finishing when surface moisture evaporates at a rapid rate than it can be replaced with "bleed water".

In the next post I will expand on most of the mechanisms listed above, so that you can visually identify various concrete defects that can manifest.

Source: American Concrete Institute (ACI) www.concrete.org