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When a load is applied to a body, the body is deformed. For a particular body loaded in a particular environment the amount of the deformation depends upon the magnitude of the load, the rate at which it is applied, and the elapsed time after the load application that the observation is made. Different materials vary widely in their response to load. This response is known as rheological behavior. While instantaneous effects and time-dependent effects are not entirely separable, it is common to consider them separately as elastic properties (instantaneous) and creep (time-dependent). A knowledge of the rheological properties of concrete is necessary to compute deflections of structures, to compute loss of prestress in prestressed structures, to compute stresses from observed strains, and, when working stress design procedures are employed, to proportion sections and to determine the quantity of steel required in reinforced concrete members. Although a vast amount of work has been done on the rheology of materials, much of it is not applicable to concrete. Because of the peculiar “gel” structure of cement paste, concrete behaves quite differently under applied load than does a crystalline material.
Philleo, Robert E.
Civil engineer, Office of Chief Engrs., Washington, D. C.