Deformation and thermal characteristics of concrete throughout the fire burning and decay phases

Title: Deformation and thermal characteristics of concrete throughout the fire burning and decay phases
Author(s): E.A. Jensen and M. Joshi
Paper category : conference
Book title: 2nd International RILEM Workshop on Concrete Spalling due to Fire Exposure
Editor(s): E.A.B. Koenders and F. Dehn
ISBN: 978-2-35158-118-6
e-ISBN: 978-2-35158-119-3
Publisher: RILEM Publications SARL
Publication year: 2011
Pages: 401 - 410
Total Pages: 10
Nb references: 15
Language: English

Abstract: Adequate knowledge of the deformation and thermal characteristics of concrete throughout the fire burning and decay phase is paramount to predict the deformation history of concrete structures subjected to fire. This paper presents new experimental data on the deformation of axially loaded plain concrete prisms as well as the thermal diffusivity of plain concrete slabs subjected to fire conditions. The concrete is designed as a normal strength, structural concrete with a design compressive strength of 50 MPa.
When investigating the deformation characteristics the variables are: 1) stress ratio (0, 17, 33 and 50 %); defined by the ratio of applied axial stress and the cold compressive strength; and 2) maximum core temperature when cooling is induced (200, 400, 600 and 800!C).
Deformations considered are the total free expansion as well as the total deformation comprised of thermal and transient strains, of loaded concrete. While the specimens are about twice that used in earlier studies, the development of free thermal expansion, total thermal strains and transient strains during heating are generally in agreement with results reported in the literature.
During cooling, the specimen continues to carry load up to a predefined time limit, after which the unloaded specimen continues to cool. The rate of deformation with decreasing temperature was observed to be substantially higher when the specimen was loaded. This indicates that the combined effect of maximum temperature before cooling as well as the magnitude of load significantly affects the total deformation. At the same time, the thermal diffusivity was found to significantly decrease with increasing temperature whereas it remained constant during cooling. The value decreased from 4.8x10-7 m2/s at 20-100!C temperature to 3.6x10-7 m2/s at 360°C. This demonstrates that the concrete temperature and the reinforcement temperature may be grossly underestimated if this behavior is omitted in the predictions.
Furthermore, the residual compressive strength was significantly affected by the stress ratio during cooling as well as the maximum core temperature. The residual strength was about 90% for specimen subjected to 200ºC; 80 to 66% (for increasing stress ratio) for those subjected to 400ºC, and as low as 50-40 % for the specimen subjected to 600ºC.

Online publication: 2012-01-17
Publication type : full_text
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