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The purpose of the research reported in this paper is to provide a rapid means for determining the entrained air content of hardened concrete by using scanning electron microscope photographs of fractured faces of samples from hardened concrete. The method that has been developed and verified at a reasonable level of confidence consists of an array of low magnification scanning electron microscope photographs of fractured faces of concretes of known air contents. The air contents range from 0 to 20% in increments of approximately 1%. At a magnification of ×80, air-entrained bubbles interrupted by the fracture face are visible as portions of hollow spheres. To determine the air content of a concrete a random series of photographs is made of fractured faces, and these are compared as a group to the array of known air content photographs until a best match is made. With untrained personnel, 1 σ precision, where σ is the standard deviation, of less than 1.5% has been obtained in selecting the correct air content for a concrete with an unknown air content.
Highway engineer, North Carolina Department of Transportation,
Professor of civil engineering, North Carolina State University, Raleigh, N.C.
Stock #: CCA10178J