# Assessment of Lightweight Concrete Thermal Properties at Elevated Temperatures
## 1 Introduction  
Concrete is widely used as a primary structural material in construction due to its numerous advantages, such as strength, durability, ease of fabrication, and non-combustibility [1]. Concrete is grouped into different categories based on performance (conventional and high performance), weight (normal weight and lightweight), presence of aggregates (fibers, waste materials, and recycles aggregate concrete) [2, 3, 4], and strength (normal strength, high strength, and ultra-high strength). According to the International Energy Agency (IEA), the current demand for energy is 30% higher than it was in the 1990s [5]. Improving the thermal properties of concrete can significantly reduce heat loss in buildings and improve sustainability. Lightweight concrete (LWC) has been extensively used as both a structural and non-structural building material due to its important advantages, including a longer product life cycle than other construction materials. This type of concrete has low density, excellent behavior at elevated temperatures, and good thermal insulation due to its low thermal conductivity [6, 7, 8]. LWC is usually manufactured using lightweight aggregates such as perlite, pumice, vermiculite,or expanded clay [1; 9]. Recently, to mitigate some of concrete's environmental problems, LWC has also been manufactured using waste from different industries, such as fly ash and blast furnace slag, to substitute components and/or enhance the concrete's properties [10]. Fire is a severe environmental condition to which structures may be subjected. The behavior of a concrete element exposed to elevated temperature depends on its thermal and mechanical properties. The main thermal properties that influence temperature rises and distributions in concrete are thermal conductivity and specific heat. Previous experimental studies have reported that the thermal properties of concrete are highly influenced by aggregate type, moisture content, and porosity, as well as the composition of the concrete mix [4; 11; 12; 13; 14; 15; 16]. The thermal conductivity of normal concrete (NC) at ambient ranges is from 1.36 to 2 W/mK according to EN 1992-1-2 [17] and the standard EN 1994-1-2 [18]. Overall thermal conductivity, as well as the density of concrete, decrease with temperature. This trend is usually attributed to variations in moisture content with temperature [19]. The water contained within concrete starts to evaporate between 100 and 120 \({}^{\circ}\)C. Subsequently, the dissociation of the water of the hydrated calcium silicate (C-S-H) occurs between 150 and 400 \({}^{\circ}\)C. Finally, loss of mass occurs between 400 and 600 \({}^{\circ}\)C, weakening the concrete. The value of thermal conductivity may vary depending on the measurement method used [4]. Usually, homogeneous materials are studied using steady-state methods, such as the guarded hot plate method and laser flash analysis (LFA). Although these methods take more time, the values they achieve are more accurate than those achieved with transient methods. Transient methods are used for heterogeneous materials such as concrete. In addition, transient methods have some advantages, such as an easy and fast measurement procedure, the possibility of measuring different thermal properties simultaneously, and the absence of a calibration sample. Transient methods such as a transient plane source are usually favored for wet concrete due to its changes at elevated temperatures [4; 14; 20]. However, there is no discussion in the literature about the selection of a proper method for measuring thermal conductivity. The specific heat of NC at ambient temperatures ranges from 840 to 1800 J/kg\(\cdot\)K [1]. Specific heat is sensitive to various physical and chemical transformations that take place in concrete at elevated temperatures. During heating, free water vaporizes between 90 and 120 \({}^{\circ}\)C [4; 12; 21]. Between 180 and 300 \({}^{\circ}\)C, the loss of chemically bonded water occurs.