Title: Log Double Power Law for Concrete Creep
Author(s): Zdenek P. Bazant and Jenn-Chuan Chern
Publication: Journal Proceedings
Appears on pages(s): 665-675
Keywords: coefficient of variation; computer programs; concretes; creep properties;dynamic modulus of elasticity; humidity; loads (forces); measurement;strength; stresses; stress relaxation; temperature.
An improved law of creep of concrete at constant humidity and temperature is proposed. The well-known double power law gives too high a final slope of creep curves compared to available test data. this is remedied by a new formula which exhibits a continuous transition from a power curve to a straight line in the logarithm of creep duration. The straight line has the same slope for all ages at loading, and the higher the age at loading, the longer is the duration at which the transition occurs. The exponent of the initial power curve is higher than that used in the double power law and is much too high in comparison with the existing test results for very short load durations in the dynamic range. This penalty, however, is outweighed by etter extrapolation to very long load durations. The new formula significantly restricts the occurrence of divergence of creep curves at various ages at loading but does not eliminate it completely unless closeness of data fit is sacrificed. The new formula also greatly reduces the occurrence of negative values at the end of calculated stress relaxation curves. Extensive statistical analysis of most test data available in the literature reveals a relatively modest improvement in the overall coefficient of variation for the deviations of the formula from test data and significant improvement for the deviations of the final slope from its measured value. The same improvements were achieved in an earlier study in which the transition from the power law to the logarithric law was abrupt, with a discontinuity in curvature. The continuity of curvature in the present formulation is desirable for applications in data extrapolation.