Automated Determination of Entrained Air-Void Parameters in Hardened Concrete

Abstract

Air-void against are used to improve freezing and thawing resistance of concrete. Depending on the size of aggregates used in the concrete mixture, the volume of air required for optimum resistance against frost damage varies from 4.5% to 7% by volume of concrete. This article describes the development of an automated linear traverse system for determination of air-void parameters in hardened concrete. The image analysis system is used for acquisition of surface images through a microscope, there is a methodology for detection and identification of entrained air voids, and a computer-controlled two-dimensional motorized actuator for bringing the sampling points in line with the microscope. In addition to the sample preparation procedures involved in ASTM C 457, the automated technique requires an additional step that includes pigmenting of the sections to create color contrast between the voids and the concrete matrix. The authors report that, following the sample preparation steps, the process requires approximately 15 min for the complete determination of air-void parameters along a 2642 mm (104 in.) traverse length. The system automatically analyzes the data and reports on air-void parameters in terms of air content, specific surface, spacing factor, void frequency, and other parameters of importance. The authors report on their evaluation of the system with a series of experiments that involved nine concrete specimens with various levels of entrained air contents.