Q: We have what looks like a problem of durability with a concrete parking lot we put in last fall. Pitting of the surface showed up during the winter. At the time of construction the ready mix producer checked the air content and said it was all right. Now the architect wants a petrographic examination of the concrete to find the cause. What can we expect to learn from a petrographic examination and how sure can we be of the results?

A: A petrographic examination of hardened concrete starts with visual examination of the concrete. Usually cores are removed from the concrete and sawed to expose a flat cross-section. A polished cross-section is then examined under a microscope. The visual analysis may be supplemented by X-ray or spectrographic examination as well.In the visual examination, the laboratory checks the characteristics of the concrete most likely to be related to the problem.

In your case that might include:

  • Air entrainment: The total air will be checked throughout and examined at the surface to see if surface air was removed during finishing. As a contractor, the best procedure to follow during construction is to have your own air meter, preferably a pressure meter for normal concrete, and to make your own air tests at least twice during a day's pour. You can then prevent a problem caused by low air content for which your construction practices might be blamed.
  • Density of the surface mortar: The more porous the mortar is, the wetter the concrete probably was at the time it was finished and the less durable the surface is likely to be. Other finishing characteristics might also be identified.
  • Void space under the surface: The surface may have been sealed too soon, trapping bleed water beneath the surface.
  • Aggregate soundness: Soft, porous, or alkali-reactive aggregate particles may be the source of pitting. Whether there would be a continuing problem of deterioration would be determined by identifying the character and the quantity of such material in the concrete mix.
  • Frozen surface: This isn't likely in your case, but it can be identified if it is.
  • Chemical attack: This is also unlikely, but is possible to determine.

Cement content may also be estimated, but probably not accurately enough to determine whether or not it contributed to the problem. The accuracy for this test can be improved if samples of the aggregates and the cement used in the concrete mix can be analyzed separately in addition to the examination of the hardened concrete. Comparing a concrete sample from an area which shows no distress with a concrete sample from an area that does can also be helpful. You may find it useful to read American Society for Testing and Materials Standard Practice for Petrographic Examination of Hardened Concrete, ASTM C 856. Petrographic analysis is not as commonly understood as most other standard tests. For that reason ASTM C 856 states "It is assumed that the examination will be made by persons qualified by education and experience to operate the equipment used and to interpret the results obtained. " You will want to be sure that the analysis is made by someone with that kind of experience and knowledge. Petrographic examination provides information from which educated judgments may be made regarding the cause of a problem. Future performance of the concrete can sometimes be predicted. As it pinpoints the problem petrographic examination is also helpful in determining what action, if any, is necessary to make the concrete serviceable.