APPLICATION OF LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS) AND CHEMOMETRICS TO EXTEND PETROGRAPHIC STUDIES TO RAPID, REAL-TIME CONSTRUCTION AGGREGATE ANALYSIS

Petrographic methods have been proven to be reliable and useful in determining the mineralogical and textural features of construction aggregate materials. They have not, however, been useful in screening aggregates for quality control during quarrying or concrete production process. Such quality control would require the development and deployment of a monitoring system that could determine aggregate quality in the field and in real-time. LIBS analysis is a new technique for rapid determination of aggregate mineralogy that has the potential to extrapolate the results of petrographic studies to real-time quality control. In LIBS, materials are pulsed with a high power laser, causing ablated atoms to burn in a short-lived, high-temperature plasma. As the plasma cools, atoms emit photons resulting from the energy loss when electrons decay from excited orbitals. The light is collected by fiber optic and diffracted by spectrometer to form a LIBS spectrum that records compositional information from major, minor, and trace elements. Because LIBS spectra are geochemically complex, they are an accurate fingerprint of aggregate chemistry. Spectra are analyzed by the multivariate technique PLS-1.

LIBS spectra were used in three studies. These studies included the use of: limestones with varying amounts of insoluble residue to assess the frictional properties of aggregates in New York; D-cracking susceptible limestones from Kansas, and the discrimination between chert and quartz in a study of alkali silica reactivity in Texas. In all studies, LIBS spectra were obtained from aggregates provided by state DOTs and correlated by multivariate chemometric analysis to the results of relevant ASTM or AASHTO tests or petrographic analysis. In the New York study, a calibration of % insoluble residue was constructed that allows this parameter to be measured rapidly on-site by LIBS. In Kansas, LIBS spectra were successfully used to predict whether an aggregate might pass or fail tests that are used by Kansas to identify D-cracking susceptible aggregates. In the Texas study, models were generated to identify the percentage of alkali-silica reactive chert, The results provide convincing data to suggest that LIBS has the potential for use in aggregate quality control monitoring.