by Dr. Christopher R. Kelson, Ph.D., P.G., Senior Geologist, Atlantic Testing Laboratories
Concrete (derived from the Latin word “concretus”, which means “to grow together”) is the most widely-used and most manufactured product, by volume, in the world today. It is a complex, man-made mixture of naturally-occurring materials and air first developed and utilized by the Romans as a building material (opus caementicium) over 2,200 years ago. Unlike other building materials, the basic nature of concrete changes over time as a wide-range of internal physiochemical reactions occur which can ultimately prove either beneficial (e.g., overall increase in strength) or detrimental (e.g., cracking) to its longevity and/or suitability for a specific use. Many external factors such as temperature changes, exposure to chemicals, minerals within the soil or groundwater, etc. may also cause concrete to deteriorate and possibly fail.
CRACKING can be caused by alkali-aggregate reaction (AAR) which denotes a detrimental chemical reaction between alkalis in the cement, water, and certain aggregate types. The most common form of AAR is ASR (alkali-silica reaction) which produces a gel that volumetrically-expands when wet, thus causing the concrete to expand and crack in a random “map” pattern. Cracking can also be caused by a similar reaction between the alkali-rich cement and dolomitic aggregate rock types (alkali-carbonate reaction, ACR).
FLAKING or SCALING of concrete may be caused by water within the concrete and/or certain aggregate types repeatedly freezing (expanding) and thawing, therefore causing the concrete to repeatedly expand and contract. It may also be caused by exposure to deicing salts or caustic solutions, or by premature finishing or inadequate curing.
CORROSION and OXIDATION of embedded steel items such as rebar and pipes within the concrete can cause the concrete to expand and crack, since the iron oxide that forms from these reactions is volumetrically-larger than the metal itself.
Contact Dr. Chris Kelson, Ph.D., P.G. (CKelson@atlantictesting.com) at 315-386-4578 to discuss your project and testing needs.