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Spall Protection for Structural Tunnel Liners Subjected to Blast or Fire
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2020-02-22
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[PDF-5.75 MB]
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Abstract:Concrete is prevalently used in modern tunnel construction; however, concrete spall due to extreme fire events and blast diminished the integrity, serviceability and safety. Because of the relatively high requirements on conducting blast-related study and low vulnerability of tunnel liner compared to tunnel drop ceilings, this study concentrates on spall protection for structural tunnel liners subjected to fire. Twenty-four normal weight concrete panel specimens were experimentally subjected to high-intensity single-sided thermal loading (applied via custom gas-fired radiant burner, delivering near ASTM E1529 flux) under constant uniaxial load. The inventory of 150-mm thick panel specimens included three mix designs, two steel reinforcement configurations, and three curing processes. The three mix designs produced compressive strengths of 38, 40, and 47 MPa with nominal cover of 38.1 mm and reinforcement ratios of either 0.006 or 0.008. Two mix designs produced very similar water permeable void ratios (defined by ASTM C642) around 13%, while the third mix expressed 9.2% void by volume. Three levels of axial load were applied at 2%, 16%, and 28% of the nominal concrete compressive strength. Moisture content, internal relative humidity, and ambient temperature were recorded prior to testing. Twenty specimens explosively spalled, while the remainder expressed surface cracking. Specimens with 13% void ratio spalled at 16% or greater axial load, over a wide range of moisture content, ranging 2.4 to 4.6%. When the applied load was reduced to 2% spalling behavior was not observed. The 9.2% void ratio specimens spalled at MC at or above 3.5%, below this threshold however, spalling occurred at lower relative humidity (below 25%) but was avoided at higher relative humidity (above 65%). The results of this study further reinforce the premise that explosive spalling in rapidly heated concrete is influenced by the interaction and summation of thermal stress, pore pressure stress, and mechanical stress from applied load.
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