Constitutive Law for Nonlinear Creep of Concrete

Abstract

Endochronic theory, previously proposed and verified for multiaxial time-independent experimental data, is extended to nonlinear long-time creep at high stress and is compared with available uniaxial creep data. The extension is based on a Maxwell chain model, each unit of which is characterized by its own intrinsic time, an independent variable whose increments depend both on time and strain increments. The dependence on the latter involves the previously determined hardening and softening functions. Aging is included and the previously established Maxwell chain model for low-stress creep with aging is obtained as a special case. The theory also describes the decrease of strength with load duration when the compression is high, gives an increase when the compression is low, and yields the additional inelastic strain accumulation due to cyclic load. An effective and numerically stable algorithm for timestep integration of structural response, permitting the time steps to be increased with the load duration, is presented.