full scale fire test on a post tensioned...
TRANSCRIPT
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Full Scale Fire test on a Post Tensioned Slab
By Dr. Fergal Kelly
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Introduction
Background– Increasing workload with PT slabs – Concerns voiced by suppliers
Historical research– Surveys, large scale tests, numerical
modelsFire test on PT Slab
– Configuration, resultsOverview and Conclusions
– Further Testing
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Background
Development of concrete cover– BRE Study 1936-46 – 13-64 mm cover (CP114 1957)– Increased cover = Increased fire resistance– Increased cover = Increase in spalling– More spalling with continuity– Spalling at moisture content as low as 2.3%
Spalling – cause & effect– Moisture content & permeability– Stress, Aggregate type, etc– Loss of section, exposed rebar – Flat slabs and columns most vulnerable
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Changes in Modern Concretes
Composition change– Early strength rates
Permeability– Higher strength concrete– Lower permeability
Structural configurationPT SlabsFE design
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Historical Research
PT Slabs– Herberhen and Van Damme tests– Value of recent tests questionable (PT and Precast)
Columns– 15% loss of area = 34% loss of strength– Modern concrete may be more susceptible
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Post tensioned slab fire test
Slab Configuration– 8.5 x 3.6 m slab, 250 mm thick– RC40 with OPC and Thames Valley Gravel, MC 4.6%– Bonded tendons, 4 per duct, 40 cover – 2 hour fire resistance– 34 Type K thermocouples
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Post tensioned slab fire test
Test Configuration– Standard fire test temperature curve– 20 type K thermocouples, plate (control) and wire– Restrained test, 1200 kN force, 10% preset– Twin hydraulic rams with spreader beams– Deflection via 3 LVDT gauges at mid-span– Load ratio of 0.5
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Test Results
Summary– 11 mins - Spalling began– 18.5 mins - Max restraint force reached– 20 mins - PT ducts visible– 28 – Spalling reduces, steady increase in deflection– 66 mins - Collapse
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Test Results
Vertical Deflection
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Test Results
Expansion
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Test Results
Temperature Profile in Slab
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Test Results
Temperature of PT Duct (Bottom)
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Test Results
Temperature of PT Duct (Side)
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Test Results
Temperature of PT Duct (Spalled)
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Overview
PT Slab OverviewPT stress +2 N/mm2
Restraint forces significant, ≥ 1.3 N/mm2
PT ducts vulnerable to spallingConcrete cover questionable in terms of duct temperature even with minor spallingAdditional btm mesh should be consideredAdditional large scale testsInformation on long-term moisture content
Questions