The present, Part I, of this two-part study presents a new microplane constitutive model for simulating the fracture and damage behavior of concrete materials subjected to various loadings. The concept of stress–strain boundaries (strain-dependent yield limits) is used to describe the inelastic behavior of concrete. To facilitate the constitutive formulation, the relation between the microplane strain components and the macroscopic strain tensor is firstly identified. A new concept of confinement-adjusted effective microplane tangential moduli is introduced. Based on the material experiment observations and physical concepts, some novel microplane stress–strain boundaries are developed. A new numerical algorithm for calculating the microplane stress components is also presented. The validation of the new model against experimental data will be presented in the second part of this study.