Incremental model formulation of age-dependent concrete character and its application

Abstract

An incremental format for an age-dependent constitutive equation was derived to account for the persistent change in creep-causing stress. This derivation was achieved by expanding the total form of the constitutive equation by a first-order Taylor series, with respect to the stress, creep, and shrinkage strains, and the elastic modulus of concrete. The development of the creep strain was depicted using a two-way parallel creep curve that was derived to remedy the disadvantage of the rate-of-creep method. The resulting incremental constitutive equation was defined using three basic equations for the creep, shrinkage, and development of the elastic modulus. Laboratory experiments were carried out on unreinforced and reinforced cylindrical concrete test specimens. The performance of the creep model was identified by creep tests on cylindrical specimens with stepwise loads, and compared with the performance of the effective modulus method, the age-adjusted effective modulus method, the rate-of-creep method, and the step-by-step method. The long-term behaviors of an existing three-span prestressed continuous double-T beam were analyzed using the presented age-dependent constitutive equation, and the results were compared with those calculated by the other age-dependent constitutive equations that were based on the creep models of the effective modulus method, the age-adjusted effective modulus method, the rate-of-creep method, and the step-by-step method.