Characterization of stress-deformation behavior of concrete and rocks have been a subject of active research for a long time. Linear elastic, nonlinear (piecewise) linear elastic, elastic-plastic and endochronic models have been proposed and used by various investigators and the literature on the subjects is very wide. A review of various models together with their implementation is numerical (finite element) procedures is presented in Ref. (77). The primary objective of the present study is to develop a generalized constitutive model based on the theory of plasticity. Although such a model can be used for a wide range of materials, in this dissertation its applications to plain concrete and rocks are emphasised. One of the main objectives of this dissertation is to study constitutive behavior of concrete and soapstone under multiaxial load histories by using a truly triaxial or multiaxial testing device. The truly triaxial device is capable of applying a general three-dimensional state of stress. Samples can be tested along any three dimensional stress path. Therefore, constitutive behavior of concrete and soapstone can be studied under all possible states of stress. The conventional, octahedral, proportional loading and circular stress test series are conducted using the truly triaxial cubical device. For meaningful results, samples with consistent initial properties are essential. In order to produce samples with uniform initial properties such as density, equipment and procedures are developed to standardize the sample preparation process. The test data is used to determine the material constants associated with the proposed constitutive model. The model is then verified by back-predicting the stress-strain curves obtained from the laboratory.

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