This paper describes selected observations, measurements, and analysis from a series of large-scale experiments on cross-laminated timber (CLT) slabs that were exposed to fire from below, using four different heating scenarios, with a sustained mechanical loading of 6.3 kN m per metre width of slab. The deflection response and in-depth timber temperatures are used to compare the experimental response against a relatively simple structural fire model to assess the load bearing capacity of CLT elements in fire, including during the decay phase of natural fires. It is demonstrated that the ventilation conditions in experiments with a fixed fuel load are important in achieving burnout of the contents before structural collapse occurs. A mechanics-based structural fire model is shown to provide reasonably accurate predictions of structural failure (or lack thereof) for the experiments presented herein. The results confirm the importance of the ventilation conditions on the fire dynamics, burning duration, and the achievement of functional fire safety objectives (i.e. maintaining stability and compartmentation), in compartments with exposed CLT.

The results presented in this paper are part of a multi-partner collaborative project named 'The Épernon Fire Tests Programme', the purpose of which is to analyse the different behaviours of combustible and non-combustible loaded structures when they are tested under standard and 'natural' fires. The project has several objectives, such as quantification of the energy participation of combustible materials in fire tests, the influence of combustible surfaces and ventilation factors on the dynamics of compartment fires, and the thermo-mechanical behaviour of concrete and timber elements under natural fires. This paper focuses on the thermo-mechanical behaviour of cross-laminated timber (CLT) slabs under standard and natural fires. The objective is to provide new experimental data on the behaviour of unprotected CLT slabs exposed to fire. In addition, we present a detailed analysis of the thermo-mechanical behaviour of the slabs, particularly aimed at better understanding the mechanical failures of some of the slabs during and after fire exposure.

Concrete spalling is an important phenomenon to consider when evaluating the fire behavior of concrete, as this can sometimes have an impact on the structural capacity of the studied element. Spalling can be assessed experimentally using screening tests although it is influenced by the size, geometry, and boundary conditions of the tested element, among other factors. No standardized and systematic methods are yet available to assess concrete spalling sensitivity by testing. Plus, comparative results between screenings tests (small and medium scale) and full-scale tests to evaluate their representativity remain scarce in the literature. In this study, five different spalling tests-with different geometries and boundary conditions-that are used as screening tests were investigated. A concrete mix known to be sensitive to spalling was used to evaluate the representativity of two types of screening tests (material screening tests and intermediate-scale screening tests). The representativity of these test setups was evaluated by comparing the measured spalling depths to the spalling measured on a full-scale slab test using the same concrete mix. This comparative study confirmed that the presence of load and/or restraint was essential for a good representativity for a screening test but should always be implemented in large enough specimens.

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