Time-Dependent Behavior of Reinforced Concrete Columns Subjected to High Sustained Loads

Several catastrophic progressive collapses of RC buildings, initiated by column failures, occurred in the past under sustained gravity loads long after the initial construction. To study the near-failure behavior of aged concrete columns under sustained high stresses and the effects of column transverse reinforcement on nonlinear creep, 13 columns were tested after 200 days of concrete casting. Test variables included sustained load level, age at loading, eccentricity ratio, and transverse reinforcement ratio. Two plain concrete and five RC columns were subjected to sustained concentric loads ranging from 76% to 110% of nominal short-time strength that neglected the confinement effects provided by transverse reinforcement. One plain concrete column, as a control specimen, was also tested under concentric loading to failure in a short time. Five RC columns were tested under sustained eccentric loading that initially caused the bending moment at the critical section to reach 77% to 100% of nominal flexural capacity. The aged columns showed high resistance to heavy sustained loads; however, one concentrically loaded and one eccentrically loaded column failed under the sustained loads. Higher column transverse reinforcement ratio decreased concrete creep during the early stage of concentric loading and increased flexural stiffness during sustained eccentric loading, thereby reducing the risk of failure due to second-order effects. Moreover, the tests indicated that Poisson's ratio of the cover concrete at the extreme compressive fibers is a suitable indicator of high sustained load levels.