New Method for Detecting Asphalt Contamination within Fine Aggregate Medium through Chemical Testing

Abstract

A method has been developed using gel permeation chromatography (GPC), also known as size exclusion chromatography (SEC), for examining extracted material from fine aggregate to identify asphalt contamination within the fine aggregate. This method development was initiated after the authors’ research group was contacted by a major contractor who voiced concern regarding potential asphalt contamination of fine aggregates. The problem with fine aggregate contamination concerns alteration of optimum asphalt content and the resulting change in asphalt performance grade. Contamination is brought about because of the presence of small amounts of asphalt cement that are undetectable by the unaided eye. This study evaluated two sands of known origin that were contaminated with PG64-22 asphalt and one additionally contaminated with PG82-22 asphalt, all at very low percentages. A traditional method of asphalt content testing, the AASHTO ignition oven method, was compared with the newly developed method. For GPC sampling, Soxhlet extraction was performed on fine aggregates using the asphalt solvent tetrahydrofuran (THF). The recovered material was then injected in solution into the GPC for evaluation. The GPC method yields convincing evidence of asphalt contamination and a more accurate estimation of percent asphalt contamination within fine aggregates.