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High levels of sustained load can lead to time-dependent failure of reinforced concrete (RC) members. This in turn may lead to collapse of all or part of a building. Design errors, construction errors, and material deterioration may lead to concrete elements being subjected to high levels of sustained loads well exceeding typical service loads. Plain concrete can experience compressive failure when subjected to a high sustained stress (over 75% of its short-term strength). However, there is a lack of knowledge about the time-dependent strength and stiffness characteristics of RC members under high sustained loads. This paper presents the results of experimental testing of simply supported shear-controlled RC beams under high sustained loads. Two series of beams, consisting of 4 and 5 beams, were tested at concrete ages of 67 to 543 days to represent in-service concrete structures. The applied sustained loads ranged from 82% to 98% of the short-term capacity and lasted for 24 to 52 days. Test results indicated that high sustained load may eventually lead to failure (collapse); however, the level of load needs to be very close (~98%) to the short-term capacity. Under sustained load, all specimens experienced increased deflection with over half of the deflection increase occurring in the first 24 h. The sustained load increased the deflection at shear failure by 190% on average. The increase in the beam deflection may allow for load redistribution in redundant structural systems. A sharp increase in deflection due to tertiary creep occurred in a short time (~2 min) before failure, indicating little warning of the impending failure. 

Design and construction errors and material deterioration can lead to concrete elements being subjected to high levels of sustained stress well exceeding typical service levels. These high levels of sustained stress have led to structural collapses in the United States and around the world. However, the performance of shear-controlled concrete elements (beams and slab-column connections) under high sustained stress is not well understood. Under high sustained compressive stress (greater than 0.75fc’) concrete will suffer tertiary creep characterized by accelerated permanent strain, leading eventually to a failure. The bond of the reinforcing bars to the concrete is also affected leading to slip. This research presents the results of experimental tests on shear-controlled RC beams that were loaded to 81, 86, and 92 percent of their short-term capacity and observed for about four weeks. Deflection and strain measurements were recorded for each specimen throughout the sustained load test. Under high sustained stress the specimens showed continued deflection with time, with most of the deflection occurring shortly after the application of load. The failure of the specimens exhibited more flexural response than that of the control specimen. The test results show that high levels of sustained stress (up to 92% of their short-term capacity) can be sustained for a prolonged time; however, the deflections and cracking are increased and the ultimate failure mode may be changed. This information will help engineers identify elements nearing failure under high levels of sustained stress.