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 types present in the aggregates in accordance with procedures set in American Society for Testing and Materials (ASTM) C 295. If aggregates are found marginal or poor quality by petrography, additional testing is recommended in the report.
Figure 1a. Dense chert particles.
Figure 1b. 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 the fine aggregate. Length of field of view from left to right in each image is 26 mm.
Figure 2a. Chalcedonic chert.
Figure 2b. Chalcedonic chert.
Figure 2a and b. Transmitted light (crossed-polarized light), thin section photomicrographs of potentially reactive chalcedonic chert particles (microcrystalline quartz) from a fine aggregate. Length of field of view from left to right is 5 mm in 2a and 2 mm in 2b.
The Aggregate and Petrographic Laboratory provides troubleshooting of performance problems/inferior qualities and investigates degradation (distress) mechanisms of concrete from highway and other concrete structures in accordance with procedures set in ASTM C 856. The laboratory also performs condition assessment and evaluates conditions of quality control concrete cylinder and mortar bars.
Figure 3a. Stereo-optical image of a portion of a lapped concrete section showing a crack filled with alkali-silica reaction (ASR) gel bridging the two reactive chert particles (shown by arrows). Length of field of view from left to right is approximately 4.5 mm.
Figure 3b. Transmitted light (cross-polarized light) thin section photomicrograph showing a crack filled with alkali-silica gel (ASR) extends from the reactive chert particle to the 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 4. 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.
The Aggregate and Petrographic Laboratory also helps in forensic investigations of problems related to asphalt pavement polishing/slipperiness, as well as sources of admixtures (e.g., lime) in hot-mix asphalt pavement.
Figure 5a. Very smooth and polished coarse aggregate from the middle portion of a core extracted from an asphalt pavement.
Figure 5b. Polished Aggregates (shown by arrows).
Figure 5a and 5b. 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 6a. Note the portion of the aggregate shown by red arrows is still covered with asphaltic material.
Figure 6b. Middle portion is still covered with asphaltic materials (shown by red arrows).
Figure 6a and 6b. 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.
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Turner-Fairbank Highway Research Center
6300 Georgetown Pike
McLean, VA 22101-2296
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|»||Pavement and Materials Discipline|
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