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Concrete is an important and widely-used constructional material because of its excellent combination of desirable properties, but it is subject to appreciable volume changes which may, under certain circumstances, result in poor performance, cause rupture, or aid the action of destructive agents through the development and growth of cracks. Volume changes in concrete due to variations of temperature, humidity, and stress are partly or entirely reversible, but volume changes due to destructive chemical and mechanical action are not reversible and are cumulative as long as the action continues. Unrestrained volume changes in concrete due to variations in temperature, moisture, and stress are generally of small concern. When volume changes are restrained by foundations, connecting members, or reinforcement, stresses are produced in the concrete which may cause distress and even failure. Since concrete is weaker in tension than in compression, restrained contraction are usually more important. While in a general way the causes of volume changes and the reactions of concrete to these causes are known, it is still not possible to build structures such as bridges, buildings, dams, and pavements with assurance that they will not crack. However, if proper attention is given to all of the many variables that influence the behavior of concrete, it is possible to build these concrete structures so that they are relatively crack free and satisfactorily resist the action of destructive agents.
Washa, G. W.
Professor of mechanics, University of Wisconsin, Madison, Wis.