# Mechanical behavior of Glulam samples subjected to thermal and moisture gradients

Hussein Daher<fnref n="1" />,<fnref n="2" />, Sabine Caré<fnref n="2" />, Gilles Forêt<fnref n="2" />, Loïc Payet<fnref n="1" />

## ABSTRACT

Glued laminated timber (Glulam), renowned for its sustainability and structural performance, is increasingly used in construction. However, during fire exposure, the structural integrity of buildings and the safety of occupants may be significantly compromised. One of the current challenges is to better understand the mechanical properties of non-carbonized wooden structural elements subjected to temperatures up to 200-250°C.

Research conducted at CSTB and the Navier laboratory aims to investigate the effects of temperature and moisture content on the mechanical properties of wood [1]. Initially, the mechanical properties of samples were determined under homogeneous thermal and hydric conditions using three protocols, referred to as P₁, P₁bis and P₂ (see [2] for more details). However, such conditions may not accurately represent real fire scenarios. Consequently, this paper presents tests under more representative conditions, incorporating thermal and moisture gradients within the wood samples. Additionally, the thermal recovery behavior after exposure to elevated temperatures is investigated to assess the structural performance of timber elements upon cooling.

The experiments were conducted on samples with three different dimensions (S₁, S₂ and S₃) to evaluate the scale effects on compressive strength. Two experimental protocols were implemented:

* Protocol P₃: This protocol evaluates the immediate changes in mechanical properties after heating. It was applied to all three sample dimensions (S₁, S₂, and S₃) at temperatures ranging from 60°C to 250°C and for five heating durations (15, 25, 40, 50, and 60 minutes).

* Protocol P₄: This protocol investigates post-heating recovery after a 24-hour cooling period at ambient temperature, applied to S₁ samples heated for 60 minutes.

As shown in the Figure 1, results from Protocol P₃ demonstrate significant compressive strength losses for all tested temperatures and heating durations. These losses were consistent across the three sample dimensions (S₁, S₂ and S₃) and align with findings from prior research. For example, earlier studies report strength reductions of up to 65% at 230°C after three hours for samples with dimensions similar to S₁ [3]. These results will be discussed in light of thermal gradients measured by thermocouples embedded in the samples and the moisture content evaluated under all test conditions.

<figure><img src="image_1.png" /><figcaption>Figure 1: Evolution of the compressive strength fc in relation to the oven temperature for S₂ samples (97x92x200 mm³) according to the protocol P₃ (with 5 heating durations).</figcaption></figure>

<footnote n="1">¹Centre Scientifique et Technique du Bâtiment (CSTB), 84 Avenue Jean Jaurès, 77420 Champs-sur-Marne, France</footnote>

<footnote n="2">²Laboratoire Navier, Gustave Eiffel University, ENPC, CNRS (UMR 8205), F-77455 Marne-La-Vallée, France</footnote>
Regarding the tests conducted after cooling (protocol P₄), Figure 2 demonstrates that the compressive strengths are nearly identical to their initial values. Given that the average moisture content remains of the same order of magnitude before and after the cooling period, it can be inferred that the thermal effects are largely reversible. These findings will be further analysed in conjunction with the results from Protocols P₁, P₁bis, and P₂, particularly under conditions of low moisture content.

<figure><img src="image_2.png" /><figcaption>Figure 2: Evolution of the compressive strength fc in relation to the oven temperature (between 60°C and 250°C) for S₁ samples (50x50x100 mm³) according to the protocol P₃ (after 60 minutes in the oven) and the protocol P₄ (after a 24-hour cooling period). The average moisture content is indicated for each obtained result. At room temperature (20°C), the moisture content is about 12%.</figcaption></figure>

In conclusion, our results indicate that the mechanical properties of wood decrease with prolonged heating durations and are influenced by the average moisture content, as well as by temperature and moisture content gradients. These insights contribute to improving the design and fire safety of wood-based structures, fostering the development of resilient engineering solutions tailored to extreme environmental conditions.

**Keywords:** Compressive strength, Fire exposure, Non-carbonized wooden structural elements, Thermal recovery.

## REFERENCES

[1] Daher, H. (2024) 'Comportement thermo-hydro-mécanique du bois soumis à des sollicitations accidentelles d'incendie', Thèse de doctorat, École des Ponts ParisTech. (In French).

[2] Daher, H., Caré, S., Payet, P. & Forêt, G. (2024) 'Temperature and moisture content effects on wood compressive properties', 11th European Conference on Wood Modification (ECWM11), Florence, Italy, 15–16 April 2024. To be published in Springer Proceedings in Materials series (2025), 8 pp.

[3] Marinquez, M.J. & Moraes, P.D. (2010) 'Influence of the temperature on the compression strength parallel to grain of parica', Construction and Building Materials, 24(1), pp. 99–104.