An incremental three-dimensional constitutive relation for concrete has been developed. The linear anisotropic and path-dependent behavior is modeled by updating the stiffness matrix at each load increment. The material is assumed incrementally elastic and the six elastic moduli E11, E12 .... E33 are expressed in terms of both the tangential hydrostatic and deviatoric stiffness whereas the three tangential shear moduli are expressed in terms of the deviatoric stiffness only. The hydrostatic and deviatoric stiffness are determined from uniaxial stress-strain relationships by employing the space truss concept. The unaxial stress-strain relationships are in a sense the stress-strain relationships of the members of the truss, and they were based on a rheological stochastic model developed earlier. The predictions of the model compare favorably with experimental data reported by various investigators. Complex loading paths are reproduced with acceptable accuracy as is demonstrated in the second part of this paper.

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