Hygro-thermal behaviour of high performance reinforced concrete at elevated temperature during fires
Student thesis: Doctoral Thesis
The prediction of moisture migration and pore pressure build-up in concrete at high temperatures is of great concern in assessing the safety of high-rise buildings that are exposed to fires. Under severe thermal loads, the induced gradients of temperature and pore pressure in concrete provoke internal stresses that can cause micro-cracks and explosive spalling. Thus, the prediction of the hygro-thermal behavior of concrete at elevated temperatures is very important. The hygro-thermal behavior and variable thermal properties of highperformance concrete (HPC) at elevated temperatures have been extensively studied. A three-dimensional mathematical model for predicting changes in temperature, moisture content, and pore pressure in concrete at elevated temperatures is developed in this study. All of the important phase changes of water – adsorption-desorption, condensation-evaporation, and chemical reactions such as hydration-dehydration – and the related heat and mass sources (or sinks) are considered. The proposed model does not take into account mechanical aspects: that is, the solid skeleton is considered to be rigid. The governing equations of the coupled heat and mass transfer model are transformed into non-orthogonal coordinates, thus making the model applicable to a wide range of geometries. The experimental tests performed by Kalifa (2000) on a concrete specimen (30 x 30 x 12 cm3 ) at high temperatures and the numerical results obtained from the present model presented are compared, and good qualitative agreement is attained. The spalling of moisture-laden concrete exposed to elevated temperatures can be attributed primarily to two mechanisms – thermal stress and pore pressure. Previous fire tests and studies have verified that pore pressure build up is the primary cause of spalling. Due to moisture clogging and the high values of the pore pressure build up in the clogged region, explosive spallings of concrete are expected as the pore pressure exceeds the concrete’s tensile strength. The numerical results precisely predict the phenomenon of “moisture clogging”. The influence of the initial moisture content and heating rate on moisture clogging is investigated here through parametric studies. Finally, the numerical results indicate that the presence of steel reinforcement impedes moisture movement and produces quasi-saturated zones in cover concrete where significant pore pressure develops.
- Reinforced concrete, Fire testing, Thermal properties