Triple power law for concrete creep

An improved creep law for concrete at constant temperature and water content is proposed. It gives the creep rate as a product of power functions of the load duration, the age at loading and the current age of concrete. This law exhibits a gradual smooth transition from the double power law for veiy short load durations to the logarithmic law for very long load durations. The higher the age at loading, the longer the load duration at the transition. Determination of creep compliance requires evaluation of a binomial integral, which can be carried out either with the help of a truncated power series or by replacement of certain integrals with sums. A table of values from which interpolation is possible is also given. Extensive fitting of creep data from the literature reveals only a modest improvement in the overall coefficient of variation of the deviations from test data; however, the terminal slopes of creep curves are significantly improved, which is especially important for extrapolation of creep measurements. Compared to the previous double power-logarithmic law, the present formulation has an advantage of continuity in curvature, and compared to the log-double power law, the present formulation has a greater range of applicability involving also very short creep durations, including the dynamic range. The new formulation also significantly limits the occurrence of divergence of creep curves, and permits even a complete suppression of this property, although at the cost of a distinct impairment in data fits.