Joris Van Acker

Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents

Thermal modification at relatively high temperatures (ranging from 150 to 260 °C) is an effective method to improve the dimensional stability and resistance against fungal attack. This study was performed to investigate the impact of heat treatment on the mechanical properties of wood. An industrially-used two-stage heat treatment method under relative mild conditions (< 200 °C) was used to treat the boards. Heat treatment revealed a clear effect on the mechanical properties of softwood species. The tensile strength parallel to the grain showed a rather large decrease, whereas the compressive strength parallel to the fibre increased after heat treatment. The bending strength, which is a combination of the tensile stress, compressive stress and shear stress, was lower after heat treatment. This decrease was less than the decrease of only the tensile strength. The impact strength showed a rather large decrease after heat treatment. An increase of the modulus of elasticity during the bending test has been noticed after heat treatment. Changes and/or modifications of the main wood components appear to be involved in the effects of heat treatment on the mechanical properties. The possible effect of degradation and modification of hemicelluloses, degradation and/or crystallization of amorphous cellulose, and polycondensation reactions of lignin on the mechanical properties of heat treated wood have been discussed. The effect of natural defects, such as knots, resin pockets, abnormal slope of grain and reaction wood, on the strength properties of wood appeared to be affected by heat treatment. Nevertheless, heat treated timber shows potential for use in constructions, but it is important to carefully consider the stresses that occur in a construction and some practical consequences when heat treated timber is used.RésuméLa modification thermique du bois à des températures relativement élevées (entre 150 et 260 °C) présente une méthode efficace pour améliorer la stabilité dimensionnelle et la résistance aux attaques de champignons. Ce travail porte sur les effets du traitement thermique sur les propriétés mécaniques du bois. Les planches ont été soumises à un traitement thermique à des températures relativement modérées (< 200 °C) selon un procédé industriel en deux phases. Il s’est avéré qu’un tel traitement influe nettement sur les propriétés mécaniques des bois résineux. La résistance à la traction dans la direction parallèle au fil du bois est diminuée de manière assez importante, tandis que, dans la même direction, la résistance à la compression est augmentée. La résistance au fléchissement, qui intègre la résistance aux efforts de traction, de compression et de cisaillement, était plus réduite après le traitement thermique. Cette diminution était moins importante que celle de la résistance à la traction considérée seule. La résistance aux efforts de choc a diminué de manière importante. Les tests de flexion ont permis de constater également une augmentation du module d’élasticité à la suite du traitement thermique. Des changements et/ou des modifications au niveau des principaux composants du bois semblent être impliqués dans les effets d’un traitement thermique sur les propriétés mécaniques. Les effets potentiels de dégradation et de modification d’hémicelluloses, de dégradation et/ou de cristallisation de cellulose amorphe, et de réactions de polycondensation de lignine sur les propriétés mécaniques de bois traité thermiquement ont été discutés. Le traitement thermique influait également sur l’effet de défauts naturels, tels que nœuds, poches de résine, déviations anormales du fil du bois et bois de réaction, sur les propriétés mécaniques du bois. Le bois ayant subi un traitement thermique présente malgré tout un bon potentiel en applications en structure. Néanmoins, il reste important de prendre en compte soigneusement les efforts mécaniques ainsi que quelques implications pratiques qui jouent un rôle lors de l’utilisation de bois traité thermiquement dans une construction.

Three-Dimensional X-Ray Imaging and Analysis of Fungi on and in Wood

As wood is prone to fungal degradation, fundamental research is necessary to increase our knowledge aiming at product improvement. Several imaging modalities are capable of visualizing fungi, but the X-ray equipment presented in this article can envisage fungal mycelium in wood nondestructively in three dimensions with submicron resolution. Four types of wood subjected to the action of the white rot fungus Coriolus versicolor (Linnaeus) Quélet (CTB 863 A) were scanned using an X-ray-based approach. Comparison of wood volumes before and after fungal exposure, segmented manually or semiautomatically, showed the presence of the fungal mass on and in the wood samples and therefore demonstrated the usefulness of computed X-ray tomography for mycological and wood research. Further improvements to the experimental setup are necessary to resolve individual hyphae and enhance segmentation.

X-ray tomography as a tool for detailed anatomical analysis

Abstract• Wood identification, anatomical examination and retrieval of quantitative information are important aspects of many research disciplines. Conventional light microscopy with a camera and (semi)-automatic image analysis software is an often used methodology for these purposes. More advanced techniques such as fluorescence, scanning electron, transmission electron, confocal laser scanning and atomic force microscopy are also part of the toolset answering to the need for detailed imaging.• Fast, non-destructive visualization in three dimensions with high resolution combined with a broad field of view is sought-after, especially in combination with flexible software.• A highly advanced supplement to the existing techniques, namely X-ray sub-micron tomography, meets these requirements. It enables the researcher to visualize the material with a voxel size approaching < 1 μm for small samples (< 1 mm). Furthermore, with tailor-made processing software quantitative data about the wood in two and three dimensions can be obtained. Examples of visualization and analysis of four wood species are given in this paper, focusing on the opportunities of tomography at micron and sub-micron resolution.• X-ray computed tomography offers many possibilities for material research in general and wood science in specific, as a qualitative as well as a quantitative technique.Résumé• L’identification du bois, l’observation anatomique et l’obtention d’informations quantitatives sont des aspects importants dans différentes disciplines scientifiques. La microscopie optique conventionnelle couplée à l’acquisition et au traitement semi automatique des images est souvent utilisée pour atteindre ces objectifs. Des techniques plus récentes comme la fluorescence, la microscopie électronique par balayage ou par transmission, la microscopie confocale ou encore à force atomique constituent une panoplie d’outils répondant à ces besoins d’imagerie fine.• Il y a une forte demande d’outils non destructifs de visualisation 3D à haute résolution combinés à un large champ de vision et surtout avec des logiciels flexibles.• En complément avancé à ces techniques la tomographie RX submicrométrique remplit ces conditions. Elle permet au chercheur de visualiser le matériau avec une taille de voxel inférieure au micron pour de petits échantillons dont la taille est inférieure au mm. En outre, à l’aide de logiciels de traitement adaptés, des données quantitatives peuvent être obtenues pour le bois en deux et trois dimensions. Dans ce papier on présente des exemples de visualisation et d’analyse pour quatre essences en focalisant sur les possibilités de tomographie aux échelles micrométrique et submicrométrique.• La tomographie RX offre de nombreuses possibilités pour la recherche en science des matériaux en général et en sciences du bois en particulier, que ce soit pour les approches qualitatives ou pour les approches quantitatives.

Fluctuations of cambial activity in relation to precipitation result in annual rings and intra-annual growth zones of xylem and phloem in teak (Tectona grandis) in Ivory Coast

BACKGROUND AND AIMS Teak forms xylem rings that potentially carry records of carbon sequestration and climate in the tropics. These records are only useful when the structural variations of tree rings and their periodicity of formation are known. METHODS The seasonality of ring formation in mature teak trees was examined via correlative analysis of cambial activity, xylem and phloem formation, and climate throughout 1·5 years. Xylem and phloem differentiation were visualized by light microscopy and scanning electron microscopy. KEY RESULTS A 3 month dry season resulted in semi-deciduousness, cambial dormancy and formation of annual xylem growth rings (AXGRs). Intra-annual xylem and phloem growth was characterized by variable intensity. Morphometric features of cambium such as cambium thickness and differentiating xylem layers were positively correlated. Cambium thickness was strongly correlated with monthly rainfall (R(2) = 0·7535). In all sampled trees, xylem growth zones (XGZs) were formed within the AXGRs during the seasonal development of new foliage. When trees achieved full leaf, the xylem in the new XGZs appeared completely differentiated and functional for water transport. Two phloem growth rings were formed in one growing season. CONCLUSIONS The seasonal formation pattern and microstructure of teak xylem suggest that AXGRs and XGZs can be used as proxies for analyses of the tree history and climate at annual and intra-annual resolution.

High-resolution proxies for wood density variations in Terminalia superba.

BACKGROUND AND AIMS Density is a crucial variable in forest and wood science and is evaluated by a multitude of methods. Direct gravimetric methods are mostly destructive and time-consuming. Therefore, faster and semi- to non-destructive indirect methods have been developed. METHODS Profiles of wood density variations with a resolution of approx. 50 µm were derived from one-dimensional resistance drillings, two-dimensional neutron scans, and three-dimensional neutron and X-ray scans. All methods were applied on Terminalia superba Engl. & Diels, an African pioneer species which sometimes exhibits a brown heart (limba noir). KEY RESULTS The use of X-ray tomography combined with a reference material permitted direct estimates of wood density. These X-ray-derived densities overestimated gravimetrically determined densities non-significantly and showed high correlation (linear regression, R(2) = 0·995). When comparing X-ray densities with the attenuation coefficients of neutron scans and the amplitude of drilling resistance, a significant linear relation was found with the neutron attenuation coefficient (R(2) = 0·986) yet a weak relation with drilling resistance (R(2) = 0·243). When density patterns are compared, all three methods are capable of revealing the same trends. Differences are mainly due to the orientation of tree rings and the different characteristics of the indirect methods. CONCLUSIONS High-resolution X-ray computed tomography is a promising technique for research on wood cores and will be explored further on other temperate and tropical species. Further study on limba noir is necessary to reveal the causes of density variations and to determine how resistance drillings can be further refined.

End-use related physical and mechanical properties of selected fast-growing poplar hybrids (Populus trichocarpa × P. deltoides).

This study focused on physical and mechanical properties of fast-growing poplar clones in relation to potential end uses with high added value. A total of 14 trees from three different clones, all P. trichocarpa × deltoides (T×D) hybrids, were felled in a poplar plantation in Lille (Belgium): six ‘Beaupré’, four ‘Hazendans’ and four ‘Hoogvorst’. Growth rate was found to have no significant influence on the physical mechanical properties. Although the investigated clones are genetically closely related, important variations in physical and mechanical properties were observed. Specific features such as spatial distribution of tension wood and dimensional stability are the main quality factors. It was concluded that ‘Beaupré’ is suitable for a wide range of high value added applications, such as plywood or construction wood. ‘Hazendans’ and ‘Hoogvorst’ will need adapted technology in processing. Further research is needed to characterize clonally induced variation in properties and to assess adequate processing strategies for multiclonal poplar stands.RésuméCette étude porte sur les caractéristiques physiques et mécaniques du bois de clones de peupliers à croissance rapide, en fonction de l’aptitude à l’emploi. Quatorze arbres ont été étudiés, provenant d’une plantation de peuplier à Lille (Belgique), appartenant à trois hybrides différents de P. trichocarpa × deltoides, à savoir six “Beaupré”, quatre “Hazendans” et quatre “Hoogvorst”. Les caractéristiques de croissance n’ont pas affecté de manière significative les propriétés physiques et mécaniques. Bien que les clones étudiés soient génétiquement rapprochés, des variations importantes ont été constatées dans les propriétés physiques et mécaniques. Des caractéristiques spécifiques telles que la distribution spatiale du bois de tension et la stabilité dimensionnelle du bois sont des propriétés importantes affectant sur la qualité du produit final. On peut conclure que le bois de “Beaupré” est apte à la fabrication de panneaux contreplaqués et de bois de sciage. Une adaptation de la technologie de transformation sera nécessaire pour les clones ‘Hazendans’ et ‘Hoogvorst’. Des recherches approfondies seront requises afin d’évaluer la variabilité induite par l’effet clonal ainsi que pour identifier des stratégies adaptées à la transformation du bois de peuplements multiclonaux.

Impact of organosilicon treatments on the wood-water relationship of solid wood.

The interaction of wood with water is a primary factor for degradation of the substrate. Organosilicons are known to behave like a water repellent, thus they are in discussion as protecting agents for wood used outdoors. In the present paper, the influence of water on organosilicon treated material was investigated. Solid Scots pine sapwood was impregnated or treated superficially with low concentrations of organosilicons and evaluated for several water related characteristics. The results confirm the water repellent properties of organosilicons as the polysiloxane film increases the contact angle between water and the wood substrate. However, they do not influence the total uptake of moisture but can change the rate at which moisture is taken up, and as such, prolong the service life of the substrate. In general, impregnated specimens behave better than dipped ones. To gain more insight in the exact behavior, continuous moisture uptake experiments are needed in real outdoor exposure tests.

Forest structure and soil fertility determine internal stem morphology of Pedunculate oak: a modelling approach using boosted regression trees

This study aims at the explanation of internal stem morphology of vital (co)dominant Pedunculate oak (Quercus robur L.) trees in homogeneous even-aged high-forests by the factors tree age, forest structure and site quality, using boosted regression trees as a powerful modelling technique. The study area covers the region of Flanders (Northern Belgium), which is characterised by the absence of strong topographic and climatic gradients. For 76 adult sample trees covering the entire productivity range of Pedunculate oak, morphological characteristics were derived from measurements of ring width or heartwood area on wood cores. Forest structure, soil physicochemical properties, humus quality, vegetation indices and litter nutrient contents were quantified at each sample location. Model predictive performance and generality are good. Tree age effects correspond to expected trends in age-related radial growth and heartwood portion. Even if management of oak trees in even-aged high-forests is rather similar over Flanders, forest structure is the most important factor determining ring width, followed by soil fertility. Heartwood portion is determined by soil fertility and crown structure. Effects of topsoil and humus physicochemical characteristics, litter nutrient contents and water supply mainly confirm autecological knowledge on oak. However, variables related to soil water availability are only occasionally relevant, and always of lower importance than soil fertility. The low importance of water availability in the models contradicts results from other studies, and the potential effect of confounding is discussed. The observed growth reduction at low litter N/P ratios might be indirectly linked to early litterfall.

Methodology to assess both the efficacy and ecotoxicology of preservative-treated and modified wood

Abstract• Wood used in outdoor conditions out of ground contact is susceptible to weathering, inducing both fungal decay and leaching of components to the environment.• This paper presents a methodology to determine these two parameters for untreated, preservative-treated and modified wood. Therefore, the wood was first leached and subsequently exposed to fungal decay of the most prominent wood-rotting fungi. The crustacean Daphnia magna was exposed to the leachates to provide information on their impact on the environment.• Combining both parameters reveals that preservative-treated wood and modified wood are capable of protecting the wood adequately for application under use class 3 conditions without posing a threat to the environment.• This proves the suitability of the concept of combining efficacy and ecotoxicology for the evaluation of new types of wood treatments.Résumé• Le bois utilisé à l’extérieur sans contact avec le sol est susceptible de s’effriter, induisant des dégradations fongiques et le lessivage de composés dans l’environnement.• Cet article présente une méthodologie pour déterminer ces deux paramètres pour du bois non traité, traité avec des agents de protection et modifié. Ensuite le bois a été tout d’abord lessivé puis exposé à des attaques fongiques du plus important champignon lignivore. Le crustacé Daphnia magna a été exposé aux lixiviats de manière à évaluer leur impact sur l’environnement.• La combinaison des deux paramètres fait apparaître que les bois traités et modifiés confèrent une protection suffisante pour les usages de classe 3 sans danger pour l’environnement.• Cela prouve la pertinence du concept qui combine efficacité et écotoxicité pour l’évaluation de nouveaux types de traitement des bois.

Wood Specific Gravity Variations and Biomass of Central African Tree Species: The Simple Choice of the Outer Wood

Context Wood specific gravity is a key element in tropical forest ecology. It integrates many aspects of tree mechanical properties and functioning and is an important predictor of tree biomass. Wood specific gravity varies widely among and within species and also within individual trees. Notably, contrasted patterns of radial variation of wood specific gravity have been demonstrated and related to regeneration guilds (light demanding vs. shade-bearing). However, although being repeatedly invoked as a potential source of error when estimating the biomass of trees, both intraspecific and radial variations remain little studied. In this study we characterized detailed pith-to-bark wood specific gravity profiles among contrasted species prominently contributing to the biomass of the forest, i.e., the dominant species, and we quantified the consequences of such variations on the biomass. Methods Radial profiles of wood density at 8% moisture content were compiled for 14 dominant species in the Democratic Republic of Congo, adapting a unique 3D X-ray scanning technique at very high spatial resolution on core samples. Mean wood density estimates were validated by water displacement measurements. Wood density profiles were converted to wood specific gravity and linear mixed models were used to decompose the radial variance. Potential errors in biomass estimation were assessed by comparing the biomass estimated from the wood specific gravity measured from pith-to-bark profiles, from global repositories, and from partial information (outer wood or inner wood). Results Wood specific gravity profiles from pith-to-bark presented positive, neutral and negative trends. Positive trends mainly characterized light-demanding species, increasing up to 1.8 g.cm-3 per meter for Piptadeniastrum africanum, and negative trends characterized shade-bearing species, decreasing up to 1 g.cm-3 per meter for Strombosia pustulata. The linear mixed model showed the greater part of wood specific gravity variance was explained by species only (45%) followed by a redundant part between species and regeneration guilds (36%). Despite substantial variation in wood specific gravity profiles among species and regeneration guilds, we found that values from the outer wood were strongly correlated to values from the whole profile, without any significant bias. In addition, we found that wood specific gravity from the DRYAD global repository may strongly differ depending on the species (up to 40% for Dialium pachyphyllum). Main Conclusion Therefore, when estimating forest biomass in specific sites, we recommend the systematic collection of outer wood samples on dominant species. This should prevent the main errors in biomass estimations resulting from wood specific gravity and allow for the collection of new information to explore the intraspecific variation of mechanical properties of trees.