Abstract: Time-dependent load carrying capacity of reinforced concrete (RC) box beams subjected to combined torsion by considering reinforcement corrosion is investigated in this paper. A modified constitutional relationship for reinforcement is derived by considering that the crack surface is not perpendicular to steel bar, as well as the bond slip effect between steel bar and concrete. Introducing the time-dependent degradation models of reinforcement and concrete, combined with a modified plate-truss model, the calculation for studying the time-dependent carrying capacity of RC box beams subjected to combined torsion is carried out. The test results of pure and combined torsional RC members verify the applicability of the modified constitutional relationship. The numerical results show that the torsional strength of the calculated corroded RC box beam model will decrease by approximately 15% after 100 years under the assumed environment and loading conditions.