###### Abstract  
Composite slabs with steel decking profiles are widely used in building construction. However, the literature on the fire resistance of lightweight concrete (LWC) composite slabs with steel decking is limited. In this work, the thermo-structural performance of LWC composite slabs with trapezoidal steel decking was studied under fire conditions. A total of 12 experimental fire tests were carried out using specimens of 160 mm thickness, 1120 mm width and 2030 mm length, in which nine composite slabs were made of LWC and the remaining three slabs were made of normal concrete (NC) to serve as a benchmark for comparison. All the samples were tested in a furnace following EN 13381-5, applying the standardized time-temperature curve and constant load. During the experimental tests, phenomena such as the vaporization of the free water inside LWC, debonding between steel decking and concrete and changes in material properties affected the thermo-structural performance of composite slabs. The test results show that the load-bearing capacity of lighter slabs does not assure the minimum structural behavior of R30. However, the lighter the concrete is, the lower the thermal transmittance, improving the slabs' thermal performance under fire conditions. Advanced nonlinear numerical models were developed to predict the thermal and structural performance of the studied LWC composite slabs in terms of temperature and time-displacement. The influences of key factors such as vaporization, thermal strains and debonding were included using material properties and a thermal contact conductance interlayer. Finally, the nonlinear models and the experimental results were compared. The difference between the experimental and numerical values was less than 15%, showing that the numerical results were in good agreement with the experimental results. The results of this study also compared the performance of LWC composite slabs with the NC composite slabs, giving rise to interesting conclusions from a practical point of view. lightweight concrete; composite slabs; fire resistance analysis; numerical simulation; FEM 2022 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 222 22 22 22 22 22 22 22 22 22 22 22 22 22 22 222 22 22 22 22 22 22 2 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 22 2 22 22 22 22 22 22 22 22 22 22 22 22 22 222 22 satisfactory serviceability performance and strength requirements for commercial and residential buildings [4]. Concrete is usually reinforced to control the effects of shrinkage and to distribute the effect of concentrated loads. Reinforcement also increases the sagging moment capacity that helps to meet fire requirements [5]. Possible alternative solutions to traditional reinforcement are steel, polymer or glass fibers. The use of fibers reduces the formation of cracks caused by shrinkage and improves the tensile strength of concrete as well as its fire resistance [6]. LWC is a sustainable material with great benefits due to the significant reduction in self-weight compared to NC [7]. Many studies have been conducted to examine the structural performance of trapezoidal steel decking using LWC instead of NC at ambient temperatures [8; 9; 10]. Since LWC is a product of high-temperature sintering, which results in more internal holes inside the aggregate, it has good thermal stability, low thermal conductivity, and low thermal expansion coefficient [11]. Therefore, the LWC should have a better fire resistance performance than NC and thus the study of the fire resistance of LWC composite slabs is highly essential. However, few studies are currently available on the structural behavior of LWC composite slabs in fire [12]. The complex behavior of composite slabs under fire conditions and the high cost and time required in laboratory tests make numerical methods an interesting way to analyze this kind of problem. In the last decade, the use of FEM has been implemented to reveal the behavior of any structural element in a fire situation.