Model formulations for numerical creep calculations for concrete

The numerical errors obtained using various creep models and calculation procedures are compared in order to identify suitable creep models to use in the simulation of time-dependent behavior of concrete-framed structures subjected to variable sustained loads. A hereditary integral (memory) model and a nonhereditary (state) model were investigated using various formulations and alternative integration procedures. The alternative models are compared in terms of convergence, accuracy in relation to number of time steps, and the efficiency of the numerical integration methods. While the error in all of the formulations is seen to decrease with the number of time steps used, calculations that use the mean stress in each time interval give much more accurate results than those that use the stress at the commencement of the time step. Comparable accuracies are obtained from state and memory models, although numerical instabilities arise if linearization of the exponential functions in the state model is attempted.