Polarizing light microscope used to petrographically analyze concrete
Concrete, a man-made mixture comprised of rocks and minerals (aggregates) within a binding medium, has been utilized as a building material for thousands of years. Petrography, a branch of geology that describes and classifies rocks and minerals via microscopic examination, has been an integral part of all geological research, especially since the polarizing light microscope first appeared in the early 1850s. The obvious and inevitable application of petrography to study concrete and its constituent materials began with examining cement clinkers in 1882 and hardened concrete in 1915. Since then, technological advances with the polarizing light microscope and the advent of newer, more powerful instruments, including the electron microprobe and scanning electron microscope, have proved invaluable helping us understand the basic physical and chemical nature of concrete via the science of concrete petrography.
Concrete petrography can help answer many basic questions about hardened concrete, especially if no mix design for it is available. This includes identifying its constituent aggregates, estimating the approximate apparent water to cement (w/c) ratio of the paste, determining the abundance and types of pozzolanic material in the paste, presence of deleterious materials, depth of carbonation of the paste, and approximate amounts, types, and distribution of air voids.
Concrete petrography has also evolved into an indispensable forensic tool to help determine why concrete fails. The obvious, easily-observed and outward signs of concrete failure (cracking, delaminating, spalling, efflorescence, etc.) are most often caused by physical and chemical reactions occurring on a microscopic scale and consequently impossible to identify without petrographic examination. Concrete may fail for many reasons:
- Chemical reactions between its aggregates, cement paste, and water
- Reactions with chemicals or solutions such as deicers, seawater, or sewage
- Extreme changes in temperature due to freeze-thaw or fire
- Human error related to mixing, finishing, or placing the concrete
One of the most common causes of concrete failure identified by concrete petrography is ASR (alkali-silica reaction). ASR can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition. There are many projects that could benefit from the information and data generated by petrographic analysis.
To learn more about petrography and other services offered by ATL, please visit our website at atlantictestin.wpengine.com or contact Dr. Christopher Kelson, Ph.D., P.G. at ckelson@atlantictesting.com.
Distribution and sizes of entrained air voids within cement paste
Extent of carbonation within paste beneath surface of concrete sidewalk
