Petrographic examination of concrete aggregate is visual examination and analysis in terms of both lithology and properties of the individual particles. The procedure requires use of a hand lens and Petrographic and stereoscopic microscopes; less commonly, X-ray diffraction or differential thermal analysis is used to supplement the microscopical examination. By Petrographic examination, the relative abundance of specific types of rocks and minerals is established; the physical and chemical attributes of each, such as particle shape, surface texture, pore characteristics, hardness, and potential alkali reactivity, are described; coatings are identified and described; and the presence of contaminating substances is determined. As will be discussed subsequently, Petrographic examination contributes in several ways to the investigation, selection, testing, and control of aggregates. Consequently, the method is being progressively more widely applied. Since 1936, all aggregates used in concrete construction by the Bureau of Reclamation have been examined petrographically as a part of the basis for their selection [1]. The method has been applied similarly by the Corps of Engineers since before 1940 [2]. Coarse aggregates proposed for use in either portland-cement concrete or bituminous concrete are examined by Petrographic methods in laboratories of the Ontario Department of Highways, and specifications governing acceptance are based upon the results [3]. Petrographic examination of aggregates is performed also by several other agencies of the U.S. government and state highway departments, and may be obtained through some commercial laboratories. In 1952, the ASTM accepted a Tentative Recommended Practice for Petrographic Examination of Aggregates for Concrete (C 295), which was adopted as a standard in 1954. Minor modifications were made in 1964. Petrographic examination is cited in the ASTM Specifications for Concrete Aggregates (C 33).