Approximate Relaxation Function for Concrete

Presented is an approximate algebraic formula for calculating the relaxation function for aging concrete. The formula is general; it applies to any form of the creep function. Compared to the previously used effective modulus method, the formula reduces the error from up to 37% to within 2% relative to the exact solution according to the superpositon principle. Calculation of long-time relaxation does not require knowledge of the elastic modulus and of the creep function for load duration below one day. The formula enables direct calculation of the age-adjusted effective modulus from the creep function, and thus it allows dispensing with the table of the aging coefficient. This is particularly useful in case of a more sophisticated creep model which reflects the fact that the creep functions for different humidities and sizes have different (nonproportional) shapes.