U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
The Aggregate and Petrographic Laboratory characterizes and classifies aggregate types based upon their mineralogical makeup and physical characteristics of the rock types, and provides quantitative data for each rock type present in the aggregate in accordance with procedures set in American Society for Testing and Materials (ASTM) C 295. If aggregates are found to be of marginal or poor quality by petrography, additional durability, and performance testing is recommended in the analysis report.
Figure 1a. Stereomicroscope photograph. Dense chert particles.
Figure 1b. Stereomicroscope photograph. Porous and moderately soft chert particles.
Figure 1a and b. Stereomicroscope photographs of representative dense and porous chert particles from the materials retained on the No. 8 sieve size of a fine aggregate. Length of field of view from left to right in each image is about 25 mm. Some chert materials in aggregates can affect performance in concrete, with respect to freeze-thaw damage or Alkali-Silica Reaction (ASR).
Figure 2a. Photomicrograph. Fibrous Chalcedonic chert.
Figure 2b. Photomicrograph. Fibrous Chalcedonic chert.
Figure 2a and b. Photomicrographs. Transmitted light (cross-polarized light), thin section photomicrographs of potentially ASR-reactive chalcedonic chert particles (microcrystalline quartz) from a fine aggregate. Note: accessory plate (gypsum) was inserted in b. Length of field of view from left to right in 2a is about 2 mm.
The Aggregate and Petrographic Laboratory provides troubleshooting of performance problems and inferior quality, and investigates degradation and distress mechanisms of concrete and other materials from highway materials and concrete structures in accordance with procedures set in ASTM C 856. The laboratory also performs condition assessment and evaluates condition of: (a) concrete from existing structures by analyzing cores extracted from representative areas of the structure(s), and (b) concrete cylinders, prisms, and mortar bars subjected to strength and durability testing regimes.
Figure 3. Photomicrograph. Stereomicroscope photomicrograph of lapped and polished concrete section showing a translucent ASR gel at the interface of a reactive chert coarse aggregate with the surrounding paste (shown by arrows).
Figure 4. Photomicrograph. Transmitted light (plane-polarized light) thin section photomicrograph showing a crack filled with ASR gel extending from the reactive chert coarse aggregate particle into the surrounding cement paste (shown by arrows).
The Aggregate and Petrographic Laboratory also evaluates and measures concrete air-void properties in accordance with procedures set in ASTM C 457. Air content and other air-void parameters (including spacing factor and specific surface) are critical to predict the freeze-thaw durability of concrete structures that will be or were exposed in different weathering regions.
Figure 5a. Photomicrograph. Lapped section of a concrete core showing the air-void system in a hardened concrete. The length of field of view from left to the right is approximately 9 mm.
Figure 5b. Photomicrograph. Stereomicroscope photograph of lapped section of concrete after surface treatment was applied to perform RapidAir C457 air-void analysis. The white spherical shapes are air voids in a hardened concrete.
The Aggregate and Petrographic Laboratory also helps in forensic investigations of problems related to aggregates in asphalt pavement, polishing/slipperiness issues of pavement surfaces, as well as examining sources and character of mineral fillers and other additions (e.g., lime) in asphalt pavement mixtures.
Figure 6a. Photomicrograph. Very smooth and polished coarse aggregate from the middle portion of a core extracted from an asphalt pavement.
Figure 6b. Photomicrograph. Polished Aggregates (shown by arrows).
Figure 6a and 6b. Stereo-optical photomicrographs showing polished and smooth exposed aggregates along the top surface of a core extracted from an asphalt pavement. Scale shown in both images is in one-sixty-fourths-of-an-inch increment.
Figure 7a. Photomicrograph. Note the portion of the aggregate shown by red arrows is still covered with asphaltic material.
Figure 7b. Photomicrograph. Middle portion is still covered with asphaltic materials (shown by red arrows).
Figure 7a and 7b. Partially polished limestone aggregates (shown by yellow arrows). Broken yellow lines mark an inferred outline/edge of the aggregate particles in the portion where it is still covered by asphaltic material. Scale shown is in one-sixty-fourth-of-an-inch increment.
Figure 8a and 8b. Example Stereophotomicrographs showing aggregate particles in the top asphalt-rich layer of a pavement that are different from aggregates observed in the first lift beneath it (mixture of chert, siliceous carbonate rock, and carbonate rock particles).
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Turner-Fairbank Highway Research Center
6300 Georgetown Pike
McLean, VA 22101-2296
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