Double power law for basic creep of concrete

The dependence of creep on load duration (t-t′) as well as age at loading t′ is described by the law [1+φ{symbol} ₁ t′^-m (t-t′) ⁿ ]/E ₀ in which m, n, φ{symbol} ₁ E₀=material parameters which are determined from test data by optimization techniques. The law is limited to basic creep, but with different values of material parameters it can also describe drying creep up to a certain time. The previous formulations are extended by introducing the age dependence. This also enhances the reliability in long-term extrapolation of creep data. Substituting t-t′=0.001 day, the law also yields the correct age dependance of the conventional elastic modulus, E. If E₀, which is much larger than E, were replaced by E (as implied by previous power laws without age dependence), the age dependence of creep curves obtained by data analysis would be more scattered, the age dependence of E would have to be described by a separate formula, and more material parameters would be necessary to fit test data. The simplicity of the double power law is a major advantage for statistical evaluation of test data.