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Most of the important properties of hardened concrete are related to the quantity and the characteristics of the various types of pores in the paste and aggregate components of the concrete. The engineering properties of concrete, such as strength, durability, shrinkage, and permeability, are directly influenced or controlled by the pores in the concrete and the relative amounts of the different types and sizes of these pores. The pores can exert their influence on the properties of the concrete in various ways. As regards the strength and elasticity of the concrete, it is primarily the total volume of the pores that is important, not their size or continuity. The permeability is influenced by the volume, size, and the continuity of the pores. Shrinkage, at least that part of drying shrinkage that is reversible, is largely a function of changes in surface energy at the solid to pore interface and, therefore, depends upon the nature of the solid surface and the total surface area of the pore system. Irreversible drying shrinkage may involve capillary phenomena. The resistance of concrete to freezing and thawing and deicer scaling is controlled by the volume and the characteristics of entrained air voids. It is, therefore, not surprising that there has been considerable interest in developing ways of measuring and characterizing the different types of pores in concrete and in elucidating the various mechanism by which they influence the properties of the concrete. In order to understand the significance of the various pore types and their characteristic effects and interrelationships in connection with some of these engineering properties, it is important to understand the manner in which these pores originate, the factors that affect them, and the limits within which their quantities may vary.
Director of research, Portland Cement Assn., Skokie, Ill.