Abstract

In this research, the variation of thermal conductivity with temperature of a limestone and its relationship with porosity is studied. Samples from Prada formation, a lower Cretaceous limestone from the Catalan Pyrenees (Spain), obtained from the Tres Ponts road tunnel were subjected to temperatures of 105, 300, and 600 °C and then cooled at a slow rate by air-cooling to laboratory temperature. Open porosity tests were determined before and after heating to evaluate the porosity increase and the micro-cracks growth. Complementarily, thermal conductivity was measured in the rock samples before and after the application of a thermal treatment by means of C-Therm TCi device, a Modified Transient Plane Source (MTPS). This is a non-invasive, quick, and precise method, when compared with other steady-state laboratory alternatives, widely used to directly determine thermal properties of rock samples. A clear decrease in the thermal conductivity of above 10% was observed for samples heated at 600 °C, probably due to a dramatic increase in porosity. The obtained results could be of great interest for the incorporation of the effect of temperature on rock in numerical models, to evaluate the potential impacts induced by eventual fires developed inside the Tres Ponts tunnel.

Introduction

The determination of the ground thermal parameters is crucial in different fields of engineering such as geothermal energy [1], underground infrastructures [2], or sustainable construction [3]. The thermal conductivity coefficient of a material represents its ability to transfer heat by conduction. It represents the amount of heat flowing per time and surface area unit (J/t·m2) when unit temperature gradient is applied (K/m). Thus, the thermal conductivity λ is expressed as W/(m·K). Similarly to other properties, thermal conductivity is characterized by a large range of factors such as temperature, porosity, degree of saturation, pore fluid, dominant mineral phase, texture and anisotropy [1]. Such properties show dramatic variations with temperature that condition the thermal conductivity of the thermally treated rocks. Research on the effects of high temperatures on the physical, mineralogical, and mechanical properties of rocks is a topic of interest, and a number of authors have specifically focused on limestones. Thus, Yavuz et al. [4] described a marked decrease on microstructure and effective porosity at 400 °C. Zhang et al. [5] determined that from 200 to 500 °C porosity and pore size rapidly increased. Later, Zhang and Lv [6] described a strong relationship between the mineral content and thermal damage in limestones (China). However, few research is focused on the variation on thermal conductivity of limestones after subjected to high temperatures.

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