NONLINEAR CREEP OF CONCRETE - ADAPTATION AND FLOW

A nonlinear integral-type creep law is developed by generalizing the linear superposition integral for the creep rate rather than the total strain. At low service stress level there is a significant (though previously overlooked) nonlinearity that consists in gradual stiffening or adaptation to a sustained compressive stress. It is modeled by a stress-dependent acceleration of the age-dependence of stiffness, and by an adaptation parameter whose rate is a function of the stress and age. The high-stress nonlinearity that consists of a weakening of the stiffness, is essentially without memory and is described by an additive rate-type flow term. Its stress dependence and the flow rate decay is modeled by kinematic hardening. An extension to elevated temperatures, which agrees with recovery data, is indicated. Although uniaxial creep is of primary interest, a rational triaxial generalization involving proper stress invariants is derived.