An Evaluation of Several Techniques for Measuring Air-void Content in Asphalt Concrete Specimens

Abstract

Abstract Precise and accurate measurement of air-void content is critical to the evaluation of asphalt-aggregate mix specimens taken in the field or compacted in the laboratory. It is an integral part of the performance-based testing and analysis methods developed by the Strategic Highway Research Program Contract A-003A (SHRP A-003A). Standard methods for drying specimens after coring and/or cutting from a larger compacted mass, and for measuring bulk specific gravity present problems for testing of the same specimen using equipment developed by SHRP A-003A, or whenever the specimen must be fixed to the testing equipment by means of an adhesive. This paper presents a comparison of the standard methods of air-void content measurement, measurement of bulk specific gravity using unsealed specimens and using specimen sealed with paraffin wax (ASTM D 1188, Test Method for Bulk Specific Gravity and Density of Compacted Bituminous Mixtures Using Paraffin-Coated Specimens), with three alternative methods for sealing the specimen: dusting with zinc stearate powder, wrapping with a waxy elastic paper called Parafilm™ (American National Can), and covering with a rubber membrane. The results indicate that each of the methods produces different measured air-void contents. The waxy elastic paper results were closest to those obtained from specimens sealed with paraffin, and it is suggested that this is a suitable alternative to paraffin wax when the specimens have smooth (cut/cored) surfaces and are to be tested for performance afterward. The use of an air gun to dry specimens after coring and/or cutting was compared with the standard oven-drying method (ASTM D 2728, Practice for Paving Uses and Application Temperatures for Road Tars). It was found that the use of a 210 kPa (30 psi) air gun produced similar results to those found with the standard method and was nondestructive, thus allowing further testing of the specimen.