BRI Research Paper


Drying Shrinkage Mechanism of Building Materials.

A.Baba; March, 1978. 79p.


This paper is about the theoretical and experimental study on the drying shrinkage mechanism and its prediction of various building materials. Wood, concrete, plastered or sprayed materials, and board materials such as asbestos coment board, fiber board and wood-wool cement board change their dimension and moisture content very much according to the fluctuation of temperature and humidity of the atmosphere. This behavior of properties is commonly observed in building materials, and thus characterizing them.

Materials showing such behavior of properties are called moisture containing materials or drying shrinkage materials. When this type of materials are used as structural materials, apart from drying shrinkage, such behavior of properties as drying creep and strength increase due to drying are also observed. Therefore, examination on behavior of properties due to drying is required, when structural application of such materials is considered. Fundamental knowledge of drying shrinkage mechanism is necessary in order to examine this behavior of properties.

Further, drying shrinkage is one of the main causes of cracks of reinforced concrete structure, and so many studies have been made to prevent cracks. And many profitable knowledges have been obtained on drying shrinkage of concrete. Yet, however, many problems are left unsolved or crack prevent on. Therefore, it is very significant to study such a traditional problem as crack prevention in view of the drying shrinkage mechanism.

The purpose of this paper is to examine fundamentally and physically the drying shrinkage of various building materials from the viewpoint of its mechanism. In general, capillary tension mechanism is acting on the drying shrinkage within the range of high humidity where building materials are in service. However, most of the previous studies on capillary tension mechanism did not examine the relation between its mechanism an external force acting on internal pores and drying shrinkage as a strain of materials by external forces. The autor, therefore, induced the relation between capillary tension and drying shrinkage by proposing the internal structure model of materials. This model is called 'Capillary Tension Model'. This model was constructed by means of Self-Consistent Approximation method which is an advanced method employed in materials science. According to the 'Capillary Tension Model' drying shrinkage is also treated as a kind of mechanical phenomenon. That is, the rate of drying shrinkage is the rate of strain in accordance with 'stress density of drying shrinkage' as a hydrostatic pressure by capillary tension in the material. Comments have been also made that 'stress density of drying shrinkage' is a kind of potential density of stress, and that it was considered to influence such mechanical properties as strength and creep to a certain extent.

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