When building any type of structure, it is important to make sure that the materials you are using are structurally sound to guarantee the integrity and longevity of the structure. Some of the most common materials that are used in modern construction are concrete products. Testing the integrity of concrete and other concrete products is referred to as petrographic testing. A full petrographic testing procedure is composed of two separate tests: the petrographic analysis and the air void analysis.
Petrographic analysis testing is performed on samples of hardened concrete or concrete products from construction sites, or existing concrete that has been exposed to natural elements. There are several reasons that a petrographic analysis may be necessary. They include the determination of:
• The condition of concrete in construction
• Causes of inferior quality, distress, or deterioration of concrete
• Probable future performance of the concrete
• Whether cement-aggregate reactions have taken place and their effects on concrete
• Whether the concrete has been subject to chemical attacks or the effects of freezing and thawing
• Potential safety concerns in the structure
• Whether concrete that has been subjected to fire is damaged
• Factors that caused a given concrete to serve satisfactorily in the environment in which it was exposed
• The presence and nature of surface treatments
• Investigation of the performance of the coarse or fine aggregate in the structure
Samples of the concrete product are taken by sawing off an appropriately sized piece (approximately one 6-inch diameter core) from the concrete at the field site. The procedure for a petrographic analysis includes a visual examination of the sample, followed by an additional examination using a stereomicroscope. If a conclusion cannot be drawn from the information gathered in the first tests, further testing may be done using petrographic or metallurgic microscopes, x-ray diffraction, or other chemical / physical tests.
A report on the findings is then prepared. If the concrete sample was being examined because of structural failure, this report details the interpretation of why the concrete failed based on the findings.
These procedures and reports are done by a concrete petrographer. Concrete petrographers need to be knowledgeable on concrete making materials, the processes of batching, mixing, handling, placing, and finishing of hydraulic cement concrete, composition and microstructure of cementitious paste, interaction of constituents of concrete, and the effects of exposure of concrete to a variety of conditions.
The second test that is used for the full petrographic testing procedure is the air-void analysis. This test is used to determine the air content, specific surface, void frequency, spacing factor, and paste-air ratio of the air-void system of the concrete sample. Examining these factors can help determine whether the concrete was damaged by the freeze / thaw cycle.
When concrete is exposed to the elements, water is likely to settle in the air-gaps. When the water freezes and expands, it could harm the structural integrity of the concrete. Therefore, it is important to calculate the size and frequency of the air-gaps to determine whether or not the concrete would be acceptable to use in construction.
The petrographic testing process is essential to the integrity of any concrete or concrete product structure. Without it, the concrete could be subject to structural failure that could have been prevented. If you need construction materials testing and inspection, including petrographic testing, contact Civil Laboratory Supervisor Jeremy Lake at (716) 592-3980 ext. 133 or jlake@encorus.com.
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