Drying Shrinkage - Đó là giải thích trước đó rằng xi măng cứng được đặc trưng bởi một cấu trúc xốp, với một độ xốp tối thiểu của khoảng 28%, đạt được khi tất cả các lỗ chân lông mao mạch trở nên hoàn toàn đầy gel xi măng (xem phần ). Điều này có thể xảy ra, về mặt lý thuyết ít nhất, trong một dán được chữa khỏi với một nước với tỷ lệ (W / C) xi măng khoảng hoặc ít hơn. Nếu không, độ xốp dán là do hydrat hóa không đầy đủ và sử. | Chapter 7 Drying Shrinkage . INTRODUCTION It was explained earlier that hardened cement is characterised by a porous structure with a minimum porosity of some 28 which is reached when all the capillary pores become completely filled with the cement gel see section . This may occur theoretically at least in a well-cured paste made with a water to cement W C ratio of about 0-40 or less. Otherwise the porosity of the paste is much higher due to incomplete hydration and the use of higher W C ratios. In practice and under normal conditions this is usually the case and a porosity in the order of some 50 and more is to be expected. The moisture content of a porous solid including that of the hardened cement depends on environmental factors such as relative humidity etc. and varies due to moisture exchange with the surroundings. The variations in moisture content generally referred to as moisture movement involve volume changes. More specifically a decrease in moisture content . drying involves volume decrease commonly known as drying shrinkage or simply shrinkage . Similarly an increase in moisture content . absorption involves a volume increase known as swelling . In practice the shrinkage aspect is rather important because it may cause cracking see section and thereby affect concrete performance and durability. Swelling on the other hand is hardly of any practical importance. Hence the following discussion is mainly limited to the shrinkage aspect of the problem. In this respect it should be pointed out that although shrinkage constitutes a bulk property it is usually Copyright 1993 E FN Spon Fig. . Schematic description of volume changes in concrete exposed to alternate cycles of drying and wetting. measured by the associated length changes and is expressed quantitatively by the corresponding linear strains A 0. . THE PHENOMENA A schematic description of volume changes in concrete subjected to alternate cycles of drying and wetting is given in .