Double-power logarithmic law for concrete creep

An improvement of the double-power law for creep at constant temperature and moisture content is proposed. Comparisons with available test data indicate that the final slopes of long-term creep curves, as indicated by the double-power law, are predominanty on the high side. This is remedied by introducing a transition to a straight line in the logarithmic scale of load duration. The strain at the transition as well as the slope of the straight line are the same for all ages at loading. The strain and the slope at the transition point are continuous, while the curvature is discontinuous. The new law is also found to significantly limit the occurence of divergence of the creep curves and of negative values at the ends of the relaxation curves calculated by the superposition principle. Extensive statistical comparisons with test data from the literature justify the proposed law.