Computation of Kelvin chain retardation spectra of aging concrete

A method of computing the age-dependent spectra of a Kelvin chain model from given data on creep of concrete is presented, along with a full listing of a FORTRAN IV subroutine. The method is based on a Dirichlet series expansion of creep rate as a function of the duration of creep test at constant current time. The inverse retardation moduli are approximated as a Dirichlet series in concrete age and as a quintic polynomial in the logarithm of the retardation time. These approximations serve at the same time as a smoothing device. They also enable explicit expressions for creep functions. The retardation moduli are determined by the method of least-squares. Numerical examples demonstrate that the retardation spectra completely characterize concrete creep because the original creep function can be recovered from them with a negligible error. The retardation spectra provide a rate-type creep law, which is necessary for analyzing large structural systems.