Time-dependent response of reinforced concrete elements near collapse

Reinforced Concrete (RC) structures may experience collapse under high levels of sustained gravity load. High levels of load are possibly due to errors in design and construction, material degradation, and abnormal loading. The evolution from local damage to large-scale collapse is time-dependent, and there is a lack of knowledge of the strength and stiffness characteristics of RC members under high levels of sustained loads. This research focuses on the impact of high levels of sustained gravity loads on the time-dependent strength and stiffness characteristics of RC isolated slab column connections and RC beams. Concrete is experiences creep under compressive load, and plain concrete has experienced compressive failure at load levels of 80 percent of its short-term strength (Rusch 1960). However, the behavior of reinforced concrete members under high levels of the sustained load is not well studied, yet there have been several previous collapses of RC structures under constant gravity loads. This research investigated the time-dependent behavior of RC beams and flat-plate connections under high sustained stresses through experimental testing of shear and flexure-controlled RC beams as well as punching shear in flat-plat connections. Two beam series consisting of 4 and 6 beams were tested at concrete ages of 67 to 543 under sustained loads ranging from 82 percent to 98 percent of the short-term capacity for time periods from 24 to 52 days, with one beam failing under sustained load within 84 minutes. Ten isolated slab-column connections with reinforcement ratios of 0.64 percent and 1 percent were tested. The specimens were 0.47 scale and tested at concrete ages from 175 to 402 days at load intensities of 83 percent to 97 percent, with one specimen failing under sustained load within 21 minutes. The research found that high sustained loads can lead to eventual failure (collapse) in these systems; however, the level of load needs to be very close (~95 percent) to the short-term capacity. The research also found that sustained load increased the deflection at peak load with greater increases in specimens that were more brittle under short-term loading. The increase in deflection could allow for load redistribution in redundant structural systems. The rate of increase in deflection followed the material level behavior of concrete under creep. Steel reinforcement strains increased at a similar rate to the deflection. The increase in steel strain under constant load indicated the redistribution of forces from the concrete as it deforms under creep. However, the sharp increase in deflection due to the tertiary phase of creep occurred in a short time (~2 min), leading to little warning of impending failure.