A MULTISCALE STUDY OF CONCRETE SUBJECTED TO ELEVATED TEMPERATURES
Deshpande, Alok Abhay
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Low aspect ratio, shear-critical, reinforced concrete walls are used in nuclear power plants to resist gravity and lateral forces, and some serve a containment function. Beyond design basis earthquake shaking has the potential to rupture reactor coolant pipes in containment structures in nuclear power plants, resulting in Loss of Coolant Accidents (LOCA). Design control documents filed with the United States Nuclear Regulatory Commission (NRC) indicate that a LOCA in a containment structure in a new large light water reactor could result in internal temperatures of up to 300°F [149°C]. Accordingly, it is important to understand whether the lateral stiffness and/or peak shear strength of reinforced concrete walls in reactor buildings are meaningfully affected by exposure to LOCA-related temperatures, which will help determine fitness for reactor re-start or the need for extensive, expensive repair or replacement.The seismic behavior of reinforced concrete walls at elevated temperatures was investigated through a first-of-a-kind experimental study. Four low-aspect ratio, reinforced concrete planar walls were subjected to reversed cyclic, inelastic loading after exposure to elevated temperatures of up to 450°F [223°C] in the heated and residual conditions. Details of the experimental program and results are presented. Materials-level tests were performed to a) characterize the effects of elevated temperature on the behavior of concrete of the type used to cast the walls, b) characterize the effects of elevated temperature on the behavior of mechanically damaged, normal strength concrete; and c) investigate the combined effects of moisture condition (unsealed, sealed and steamed) and elevated temperature on the behavior of normal strength concrete.Six recommendations are made for analysis and design of low aspect ratio, reinforced concrete walls, namely, 1) at levels of lateral force smaller than 30% of peak strength, the maximum reduction in initial stiffness of a wall due to exposure to temperature of 450°F [223°C] is approximately 30%, 2) at levels of lateral force greater than 30% of peak strength, any reduction in lateral stiffness due to exposure to temperature of 450°F [223°C] is masked by mechanical damage, 3) peak lateral strength is not affected by exposure to temperature of 450°F [223°C], 4) the cyclic backbone curve for a low aspect ratio wall exposed to temperature of 450°F [223°C] can be approximated by the piecewise linear relationship of ASCE 41-17, with linear response to peak strength, with co-ordinates given in NIST-GCR-17-917-45, 5) the effect of moisture condition (i.e., unsealed, sealed, or steamed) on the mechanical properties of concrete cylinders exposed to temperature of 450°F [223°C] for 90 minutes is negligible, and 6) the ACI 349-13 short-term temperature limit for concrete of 350° [177°C] in Section E.4.2 should be increased to 450°F [223°C].