Relationships between Some Anatomical, Physical and Durability Properties of the Wood of Some Lesser Utilised Ghanaian Hardwoods

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This thesis is the result of a comprehensive study of some anatomical structures as well as some of the properties of wood utilization of selected species and the interrelationship amongst them. The natural durability of ten lesser-known West African Species, namely Albizia ferruginea (Guill. & Perr.) Benth. (Awiemfosamina), Amphimas pterocarpoides Harms (Yaya), Antiaris toxicaria Lesch. (Kyenkyen), Blighia sapida Koenig (Akye), Canarium schweinfurthii Engl. (Bediwonua), Celtis zenkeri Engl. (Esa), Cola gigantea A. Chew. (Watapuo), Petersianthus macrocarpus (P. Beauv.) Liben (Esia), Sterculia oblonga Mast. (Ohaa), Sterculia rhinopetala K. Schum. (Wawabima), and as reference Teak (Tectona grandis), were evaluated by the field test according to EN 252 (1989) for a period of 6 months. Structural size samples were tested for their mechanical properties according to EN 408 (2003). Their water sorption properties were determined at relative humidity conditions of 30, 45, 60, 75, and 90 % at a temperature of 25oC and compared with Albies alba, Picea albies, Fagus sylvatica (European species). Three of the ten species, Albizia ferruginea, Blighia sapida, and Sterculia rhinopetala were selected for anatomical investigations based on their performance in the durability and mechanical strength tests. Microscopic sections of the transverse and tangential planes were made on a Leica sliding microtome, stained and permanently mounted in Canada Balsam. The micrographs were analysed using the stereological technique described by Ifju (1983) and Steele et al. (1976). Maceration was also done. The results from the field test indicate that some of these less utilized species could be used as substitutes for the more standard commercial species in their end use applications. It was found that Albizia ferruginea was very durable, Sterculia rhinopetala was durable and Blighia sapida was moderately durable. A general trend found in the ten species was that, the denser the species the less water/moisture it takes up and the more durable it is, combined probably with specific polyphenolic substances. An exception to this trend was Albizia ferruginea, having the highest sorption of the three mentioned species but at the same time the highest durability and Sterculia rhinopetala the least. The best correlations found between the mentioned properties/features are between density and bending strength, followed by density and sorption, sorption and durability and to a lesser extent density and durability. As could be expected, durability (as a result of natural impregnation with protective substances) was in fact not completely correlated with density: Albizia ferruginea although the lightest of the three specially tested species, had the relatively highest durability according to the graveyard test. Blighia sapida was found to have crystals which could be harmful for working tools. This could explain its brittle behaviour under stress. It had a bending strength of 61.4 N/mm2 at a moisture content of 29 % with an average density of 0.899 g/cm3. A peculiar form of cell wall thickenings has been found in the vessels of this species with the help of scanning electron microscopy. In a third position, Albizia ferruginea had lowest bending strength of 50 N/mm2 at a lower average density of 0.740 g/cm3. In conclusion, Sterculia rhinopetala was used for a pedestrian bridge constructed at the KNUST campus in Kumasi due to its performance in most of the tests carried out and even though most of the tested lesser utilized species were not durable; their durability could be enhanced by impregnation with protective substances in order to be used for external applications.
A thesis submitted to the Department of Wood Science and Technology at the Kwame Nkrumah University of Science and Technology, Kumasi in partial fulfillment of the requirements for the Degree of Doctor of Philosophy