STUDY OF HEAT AND MOISTURE TRANSPORT THROUGH CONCRETE EXPOSED TO ELEVATED TEMPERATURES

Abstract

Study of concrete exposed to elevated temperatures such as fire finds many engineering applications such as safety of nuclear reactors, accidental fire in long bridges, tunnels, high rise buildings etc. As the mechanical damage of concrete structures is directly related to the human safety, research in this study has attracted many researchers in the past two decades. Many experimental research works involving laboratory scale model to real structures have been carried out in order to understand the behavior of concrete exposed to high temperatures. High strength concretes are widely used for multi-storey buildings, nuclear reactors, tunnels etc., however, the important issue with high strength concrete is it has low porosity compared to normal strength concrete. Hence when such huge concrete structures are exposed to high temperatures due to fire etc., the severe heating of concrete results in thermal spalling and failure of structure. As concrete is an unsaturated porous media consisting of solid, liquid, water vapor and air, the severe heating of concrete results in coupled heat and moisture transport phenomena within concrete and this takes place within a short duration of exposure of concrete to fire. Hence the study of heat and moisture transport in concrete exposed to high temperatures will enlighten the cause of transport mechanisms that lead to thermal spalling of concrete. A detailed literature survey has been carried out in this research work to understand the various factors that contribute for the coupled heat and moisture transport within concrete exposed to high temperatures. Many literature on experimental and numerical research have been studied in detail and found that the transient heat and moisture transport processes within concrete that take place during the initial one hour duration contributes for the onset of thermal spalling of concrete. It is observed that due to immense evaporation of free water by bulk supply of heat from fire, the pore pressure inside the concrete increases. The increase is mainly because the vapor generated is not able to diffuse into the concrete due to low porosity of high strength concrete. Due to heating the water vapor generated also increases and its saturation temperature also increases with increase in vapor pressure. It is observed from literature that no clear explanation has been given for the formation of dry zone, saturation and sub-cooled regions in the concrete as it is exposed to high temperature. The processes behind the viii formation of these zones need to be understood. Further it is observed not many studies have focused on the transport of free water and bound water within concrete. Other than concrete heating by fire, there may be other methods of heating such as constant rate heating like the one followed in the laboratory studies and constant fluid temperature exposure. Hence in the present research it is proposed to study heat and moisture transport within concrete exposed to a fluid heated by fire, by constant rate heating and by constant fluid temperature. The effect of these heating methods on transport of free water, bound water, thermal conductivity and porosity of concrete are also investigated in detail. In the present research work a mathematical model for the study of heat and moisture transport through concrete has been developed using conservation equations for liquid water, water vapor, air along with energy equation in which evaporation of free water and bound water also have been accounted for. The resulting coupled nonlinear equations are recast in terms of useful primary variables, temperature, vapor content and pore pressure. Galerkin’s weighted residual finite element method has been implemented to obtain the solution for the field variables on the nodal points in the computational domain which has been discretized using isoparametric formulation. Fully implicit time marching algorithm has been used to discretize the time domain and mid-interval time scheme is followed to handle the non-linearity of the governing equations. Simulation results have been obtained using the properties of a concrete HSC M100. The computer code developed was initially validated with experimental and numerical results available in the literature. A mesh sensitivity study also has been carried out to make sure that the simulation results are independent of the size of the discretization of computational domain. The results obtained are discussed for the analysis of heat and moisture transport through concrete for different heating methods with the help of distribution of temperature, vapor content and pore pressure plots. Results for the transport of free water and bound water are obtained from post-processing of the primary variables and these results are also analyzed for the three heating methods. Finally the variation of thermal conductivity and porosity of concrete is also investigated in detail. From the simulation results it is observed that the evaporated free water forms a dry zone as long as the temperature of concrete is greater than the saturation temperature corresponding to the vapor pressure developed due to the storage of water vapor. When the vapor pressure becomes equal to the saturation pressure corresponding to the temperature of ix concrete a saturation zone is established, resulting in the onset of condensation of water vapor and this increases the free water content in concrete away from the dry zone. This inter-relation of temperature and pressure varies with different rate of heating, causing the movement of peak pore pressure at different depths inside the concrete. It is found that concrete exposed to fire experiences higher pore pressure followed by constant fluid temperature heating.