Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- 2017 (3)
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- Beitrag zu einem Tagungsband (3) (entfernen)
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- Fire (3) (entfernen)
Unilateral thermal exposure of concrete building components induces moisture transport processes that have a significant influence on the spalling behaviour of dense high-strength concrete (HSC). These transport processes are based on evaporation and condensation mechanisms of liquid and gaseous water in the pores as well as the chemically bound water within the concrete. The low permeability of HSC and the formation of a saturated zone within building components (also known as a moisture clog) leads to high water-vapour pressures, which contributes to explosive spalling. The formation of these pressures has already been verified by means of pore-pressure measurement techniques. In addition, the redistribution of the moisture within concrete specimens subject to unilateral thermal exposure has been demonstrated on fractured surfaces. Investigations by means of the nuclear magnetic resonance (NMR) relaxometry technique and neutron radiography have shown one-dimensional changes in moisture distribution during thermal exposure.
However, none of these methods has been able to depict the moisture distribution in three dimensions (3D), so the link between pore size, concrete micro-structure and moisture content is missing. The research project presented in this paper aims to fill this gap by developing a new multi-level test methodology to characterise non-destructively the temporal course of spatial moisture distribution during unilateral thermal exposure. The procedure used during this programme included the collection of X-ray 3D-computed tomography (CT) measurements using a miniaturised specimen subjected to in-situ thermal exposure and the comparison of those CT results with the results of one-dimensional NMR-relaxometry before and after the heating process.
In the first step, a mobile heating device was developed, built and tested. To simulate a unilaterally-heated construction component, a cylindrical specimen made of HSC (Ø = 40 mm, L = 100 mm) was cast into an impermeable glass ceramic shell. The ceramic shell ensured a one-dimensional moisture flux and limited the thermal expansion of the concrete. An additional high-temperature wool (HTW) insulating shell ensured a one-dimensional heat flux. The heating device, which operated using infrared radiation (IR), allowed the unilateral heating of the specimens up to 300 °C using variable heating regimes.
In the second step, the mobile heating device was integrated into the CT-scanner, which enabled the collection of measurements before, during and after heating. By subtraction of successive 3D-CT images, X-ray attenuation differences could be resolved three-dimensionally in the specimen and interpreted as changes in the moisture content.
Initial results show that this test methodology can monitor the 3D changes of moisture content inside the specimen during thermal exposure. It enables the researcher to visualise areas with moisture accumulation as well as dehydrated areas inside the specimen. Comparative one-dimensional NMR-relaxometry measurements confirm the results of the CT image analysis.
A probabilistic approach for finite element analysis (FEA) for tunnel linings exposed to the nominal fire is presented. The probabilistic FEA accounted for the uncertainties distributions tied to the conductivity and specific heat as well as of the compressive strength, tensile strength, Young’s modulus, and ultimate strain in compression. To get an understanding on the influence of different probability density functions on the distribution of maximum displacements of the tunnel lining, a sensitivity analysis was performed. Four sets of FEAs were carried out with different probability distributions of the conductivity, the specific heat, and the compressive strength of the concrete, respectively. An experimental design based on a Latin Hypercube Sampling algorithm was performed to define the input parameters which describe each analysis case. A reliability analysis was executed considering a limit state function based on the temperature-dependent ultimate strain. The results show that, depending on the distribution adopted, the standard deviation of the maximum displacements can vary up to 47,4% of the minimum standard deviation. The large standard deviation is associated with the possibility of a greater displacement and, hence, to a structure more vulnerable to fire.
Probabilistic thermo-Mechanical analysis of a concrete tunnel lining subject to fire The probability distributions of the parameters related to the thermal analysis was considered in order to study the variability of the results and to carry out a reliability analysis. This assessment considered as random variables the thermo-mechanical properties of the concrete, the maximum heat release rate (HRR), the duration of the period of maximum HRR, the convective coefficient, the emissivity at the surface exposed to the fire, the air velocity within the tunnel, and the initial fire radius. The temperature-time curve was described by a correlation. An experimental design based on a Latin Hypercube Sampling algorithm was performed to define the input parameters to each analysis case. The definition of a limit state function based on the punctual strain status has permitted to carry out a reliability analysis.