LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS) AS A TOOL FOR DETECTING ALKALI-SILICA REACTIVITY OF HIGHWAY AGGREGATES

Many engineering tests for quality control of highway aggregates are time-consuming, limiting the number of samples that can be used to monitor aggregate quality. Laser-Induced Breakdown Spectroscopy (LIBS) is a rapid analytical tool that can be used to determine many engineering properties within minutes rather than weeks or months for traditional analysis. In LIBS analysis, a high-energy pulsed laser ablates a sample, forming a high-temperature plasma in which electrons are excited. As the plasma cools, the electrons move to lower orbitals, emitting energy in the form of photons. The photons are captured by fiber optic, diffracted by a spectrometer, and recorded on a CCD camera. LIBS spectra contain information about the concentrations of most elements in the periodic table and isotopic information, creating a detailed and unique spectral fingerprint of the material. In this work, LIBS spectra and multivariate analysis were used to measure specific gravity and to assess aggregates for alkali-silica reactivity (ASR).

The standard test method for ASR, also known as the Mortar-Bar method, assigns a C-1260 value to aggregates based on their percent expansion when submerged in sodium hydroxide (ASTM International, 2006). The test takes about three weeks to complete. LIBS spectra were correlated to C-1260 values using the multivariate statistical technique PLSR (Projection to Latent Structure Regression). Although the PLSR model does not calculate the C-1260 value, it does successfully separate samples into the three groups defined by ASTM International (2006): 1) C-1260 < 0.10% (little reactivity); 2) C-1260 > 0.20% (reactive samples); and 3) C-1260 between 0.10% and 0.20% (need further testing). The PLSR models are accurate for 96% of the samples studied.

The success of LIBS and multivariate analysis for rapid analysis of engineering properties such as ASR reactivity and specific gravity has potential for increasing the number of samples in quality control studies. Development of a Sample Laser Targeting (SLT) system permits rapid analysis of aggregates on site at quarries, aggregate companies, and construction sites.