MICROPLANE MODEL FOR CONCRETE .1. STRESS-STRAIN BOUNDARIES AND FINITESTRAIN

The paper presents an improvement of the microplane model for concrete -a constitutive model in which the nonlinear triaxial behavior is char acterized by relations between the stress and strain components on a m icroplane of any orientation under the constraint that the strains on the microplane are the projections of the macroscopic strain tenser. T he improvement is achieved by a new concept: the stress-strain boundar ies, which can never be exceeded. The advantage of this new concept is that various boundaries and the elastic behavior can be defined as a function of different variables (strain components). Thus, whereas for compression the stress-strain boundaries are defined on the microplan es separately for volumetric and deviatoric components, for tension an additional boundary is defined in terms of the total normal strains. This is necessary to achieve a realistic triaxial response at large te nsile strains. For microplane shear, a friction law with cohesion is i ntroduced. The present model is simpler than the previous microplane m odel. Finally, the microplane model is generalized to finite, but only moderately large, strains. Verification and calibration by test data are left to a subsequent companion paper.