APPLICATION OF LASER- INDUCED BREAKDOWN SPECTROSCOPY (LIBS) TO KANSAS HIGHWAY AGGREGATE ANALYSIS: RAPID IDENTIFICATION OF HIGH-QUALITY AGGREGATES

Carbonate highway aggregates in Kansas undergo several tests that identify quarry beds that will minimize D-cracking in concrete. If an aggregate passes three tests (modified freeze-thaw, KTMR-21; percent expansion, KTMR-22; and durability factor, KTMR-22), it may be used in highway concrete. These tests are performed on aliquots that are small relative to the amount of aggregate used and can take several months to complete. LIBS is a rapid and simple spectroscopic technique that can be used to predict whether or not an aggregate would pass or fail the KTMR tests.

In LIBS, a high-power pulsed laser ablates the sample, resulting in a short-lived, high-temperature plasma. As the plasma cools, excited electrons decay to lower-energy orbitals, releasing energy in the form of photons that is collected by fiber optic and diffracted by spectrometer. The resulting LIBS spectrum consists of the intensities of wavelengths between 200 and 1000 nm and contains information on major, minor, and trace elements as well as certain molecules. Thus, LIBS spectra are a rich fingerprint of a material's composition.

In this study, the multivariate technique PLS-1 (partial least squares regression to one variable) was used to regress LIBS spectra to pass/fail criteria based on the KTMR-21 and KTMR-22 tests. Although the KTMR tests are performed on bulk samples and measure physical responses to various stresses, the responses are largely tied to aggregate mineralogy. The LIBS spectra record chemical composition, which is also a function of mineralogy. Because of this, LIBS spectra are able to predict whether a given aggregate will pass or fail the KTMR tests for 93% of 140 examples. This high success rate suggests that LIBS could be employed real-time in quarries or concrete production processes to minimize utilization of aggregates that cause D-cracking.