Creep behaviour of densified European beech and Scots pine under constant climate

ABSTRACT

Thermo-hydro-mechanical (THM) densification is a well-known procedure for improving the properties of low-density wood species, but the long-term behaviour of the densified wood under constant load has not been reported and may restrain its use in construction. This study has investigated the influence of THM densification on the creep behaviour in bending of beech and pine under constant climatic conditions. The specimens were loaded in three-point bending at applied stress level of 35% of the ultimate strength for 14 days at 20 ± 2°C and 65 ± 5% relative humidity (RH), and the bending deformation was registered. The THM densification doubled the density, increased the modulus of elasticity and the modulus of rupture, and reduced the equilibrium moisture content and creep compliance. The results demonstrate that THM densification can be an effective method to improve the short-term and long-term performance of wood in constructions.

KEYWORDS:

• Creep deformation
• rheological properties
• thermo-hydro-mechanical densification
• wood

Introduction

It is well known that there is a strong correlation between the mechanical properties of wood and its density. Thermo-hydro-mechanical (THM) densification, which involves a combination of heat, moisture and compressive force in the transverse direction of the wood, is a method of increasing the density and thereby improving the properties in the first place of low-density timber species (Navi and Sandberg 2012, Schwarzkopf 2021). Timber used in construction will be exposed to long-term loading, and will exhibit creep deformation and, in extreme cases, early failure of the structure may occur or, at least, its serviceability and safety during its service life may be impacted (CEN 2004). The viscoelastic behaviour of densified wood is very important, but it is not well understood. The purpose of the present study was to investigate and compare the creep behaviour of densified and non-densified beech and pine wood under constant climatic conditions.

Materials and methods

Kiln-dried European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.) sapwood with average densities of 650 and 500 kg m⁻³, respectively, were used. For each species, 20 straight-grained, knot-free and defect-free specimens 200 mm (longitudinal) × 20 mm (radial) × 20 mm (tangential) in size were prepared from a single piece of sawn timber. The specimens were randomly distributed into two different groups of 10 specimens each for beech and pine: reference (untreated) specimens and densified specimens. Each group was divided into sub-groups of five replicates for three-point bending and creep tests.

An open-system hydraulic hot-press (Langzauner “Perfect” LZT-UK-30-L, Lambrechten, Austria) was used for the THM densification. The specimens were compressed in the radial direction from 20 to 10 mm in the following stages: (I) the hot press was pre-heated to ca. 170°C, (II) the specimens were placed in the press, and a pressure of 4 MPa was applied for 3 min, (III) the temperature of the press was raised to ca. 200°C and kept for 2 min, and (IV) the press was cooled to ca. 60°C with the specimens remaining under compression for 5 min.

The specimens were conditioned at 20 ± 2°C and 65 ± 5% RH for 2 weeks, and then sawn to final dimensions of 200 mm (longitudinal) × 10 mm (radial) × 15 mm (tangential) before the bending and creep tests.

A three-point bending test (loaded in the radial direction) based on EN 408 (CEN 2010) was carried out using a universal machine Zwick UTM Z100 (Zwick GmbH & Co. KG, Ulm, Germany) with a 100 kN load cell to determine the modulus of elasticity (MOE) and modulus of rupture (MOR).

The creep test was carried out in a custom KAPPA Multistation (Zwick GmbH & Co. KG, Ulm, Germany), able to accommodate five specimens simultaneously. The specimens were loaded as in the three-point bending (radial load) with an applied stress at 35% of the mean MOR value (Table 1). Each test was run for 14 days at 20 ± 2°C and 65 ± 5% RH, in accordance with Service Class 1 as defined in Eurocode 5 (CEN 2004). The mid-span vertical deflection was recorded by an extensometer.

Table 1. Physical and flexural properties of un-densified reference (beech = BR; pine = PR) and densified (beech = BD; pine = PD) wood, and the results of the creep test.

Group	After 2 weeks conditioning at 20 ± 2°C and 65 ± 5% RH
	Density (kg m^–3)	EMC (%)	MOE (GPa)	MOR (MPa)	Di (GPa^–1)	Dc (14d) (GPa^–1)	ACE (14d) (%)
BR	654 (±8)	10.5 (±0.3)	12.5 (±1.5)	124.8 (±15.2)	0.85 (±0.12)	0.51 (±0.15)	/
BD	1090 (±24)	8.8 (±0.6)	20.0 (±1.7)	239.4 (±11.3)	0.53 (±0.09)	0.13 (±0.03)	74
PR	511 (±4)	9.4 (±1.6)	12.4 (±1.9)	101.5 (±10.4)	1.02 (±0.11)	0.36 (±0.07)	/
PD	940 (±40)	7.0 (±0.7)	14.0 (±3.4)	166.8 (±25.9)	0.57 (±0.11)	0.21 (±0.04)	41

To facilitate comparison, the creep is expressed as the initial compliance D_i and creep compliance D_c(t) at time t calculated according to: (1) $D_i={\displaystyle \frac{4b{d}^3}{F{l}^3}{\delta }_i}$(1) where b is the width (mm) of the specimen, d is the thickness (mm) of the specimen, F is the applied load (N), l is the span (mm) between supports and δ_i (mm) is the deflection after the loading reached the applied stress target, and (2) $D_{C}t={\displaystyle \frac{4b{d}^3}{F{l}^3}{\delta }_C}$(2) where δ_c (mm) is the creep deflection (mm) at time t.

The anti-creep efficiency (ACE) is a value which quantizes the ability of a given modification to reduce the creep deformation (Norimoto et al. 1992), and it is calculated as: (3) $ACE\left(t\right)={\displaystyle \frac{D_u-D_d}{D_u}}$(3) where D_u and D_d are the creep compliances of respectively untreated wood and densified wood.

Results

Table 1 shows the physical and flexural properties of the reference and densified wood. The density of both the beech and the pine were almost doubled after THM densification. The equilibrium moisture content (EMC) after conditioning at 20 ± 2°C and 65 ± 5% RH for 2 weeks of the BD and PD groups was slightly lower than that of their reference groups. The densification led to a higher MOE and MOR for both species, but the density, MOE and MOR of densified beech was higher than those of pine.

The creep compliance of both beech and pine under constant RH was dependent on the densification (Table 1). The initial compliance of the densified wood was about 60% of that of the untreated reference for both beech and pine, which can be attributed to the increased stiffness and the lower EMC. The creep compliance of all the groups showed a rapid increase in the first 2 days and thereafter gradually decreased with time (Figure 1). After 14 days, the creep compliance of the untreated beech and pine groups was 0.51 and 0.36 GPa⁻¹, respectively. The higher creep compliance of the untreated beech than that of the untreated pine could be due to the higher EMC of the untreated beech. Densification reduced the creep compliance by 74% and 41% for beech and pine, respectively. The densification thus reduced the creep deformation, is agreement with a previous study of the viscoelastic properties of densified wood using dynamic mechanical analysis, which showed a lower creep compliance of densified wood under constant RH (Kutnar et al. 2021). None of the specimens reached the tertiary creep stage, where the deformation rapidly increases leading to a failure of the material. The ACE of the densified beech was stable after achieving a peak point approximately two days after the start of the test. On the other hand, the ACE of the densified pine increased during the first day and thereafter decreased with time (Figure 2).

Figure 1. Average creep compliance as a function of time for un-densified references (R) and densified beech (BD) and pine (PD).

Figure 2. Anti-creep efficiency (ACE) of densified beech (BD) and pine (PD) as function of time.

Conclusions

The time-dependent creep of THM-densified wood showed a reduced under a long-term constant load at constant temperature and relative humidity (20°C/65% RH) was less than that of untreated wood. Both the initial compliance and the creep compliance of THM-densified wood after 14 days were less than that of untreated wood. The densified beech had a greater anti-creep efficiency (ACE) than densified pine, which did not decrease with time. These results demonstrate that THM densification not only increases the density, MOE and MOR but also decreases the creep, which are favourable properties for timber used in construction. To further advance the understanding of creep in densified wood, future studies should focus on how the moisture affects the bending creep behaviour, and the influence on the creep properties of the anatomical structure of non-treated and densified hardwoods and softwoods should be studied with regard to time-dependent deformations. This fundamental understanding of the performance of densified wood can lead to important advances in the use of low-density wood species as construction materials through wood densification.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors acknowledge the European Commission for funding the InnoRenew project (Grant Agreement #739574) under the Horizon 2020 Widespread-Teaming program and the Republic of Slovenia (investment funding of the Republic of Slovenia and the European Regional Development Fund) and the Slovenian Research Agency ARRS for funding project J4-3087 and IO-0035. The support of CT WOOD – a centre of excellence at Luleå University of Technology supported by the Swedish wood industry – is also gratefully acknowledged.