Properties Of 10 Ghanaian High Density Lesser-Used-Species Of Importance To Bridge Construction – Part 1: Green Moisture Content, Basic Density And Shrinkage Characteristics

Abstract

Sixty trees of ten high density Lesser Used Species (LUS) of potential importance to bridge construction were extracted from four forest reserves - Bobiri, Pra-Anum, Nueng, and Subri River (in four different ecological zones). Logs from the trees were converted on a horizontal bandmill to 27 and 53 mm thick boards. Specimens were prepared from the green boards for the determination of the green moisture content, basic density, and dimensional shrinkage of each of the ten species: Celtis mildbraedii, Celtis zenkeri, Combretodendron africanum (Essia), Cynometra ananta (Ananta), Lophira alata (Kaku), Nauclea diderrichii (Kusia), Nesogordonia papaverifera (Danta), Piptadeniastrum africanum (Dahoma), Strombosia glaucescens (Afina), and Sterculia rhinopetala (Wawabima). Mean green moisture content was lowest in Kaku (45%) and highest in Celtis zenkeri (93%). Mean basic density was highest in Kaku (840 kg/m3) and lowest in C. zenkeri (560 kg/m3). Mean green moisture content and mean basic density of the species negatively correlated with a correlation coefficient of -0.87. Mean total tangential shrinkage varied from a low of 6.2% for Dahoma to a high of 9.9% for Kaku. Based on the mean tangential shrinkage from green to 12% moisture content the dimensional shrinkage of the species were classified as ‘small’ for Dahoma, Kusia, Ananta and Danta; ‘medium’ for Celtis mildbraedii, Afina, C. zenkeri, Essia and Kaku; and ‘large’ for Wawabima. Tangential to radial shrinkage (T/R) ratio was ‘very low’ in Kaku, ‘low’ (or fairly close to 1.5) in Danta, Afina, C. zenkeri and C. mildbraedii, ‘pronounced’ (> 1.5) in Ananta and Dahoma; and ‘very pronounced’ (>> 1.5) in Kusia, Wawabima and Essia. The pronounced differential shrinkage ( = 1.5) in these woods is likely to cause wide splits, checks and distortions if precautions are not taken during the kiln drying of these heavy wood species. Total longitudinal shrinkage was excessive and exceeded the normal figure of 0.2% in all ten species, and was higher than the rare figure of 0.4% in Kaku and Ananta.

Keywords: High Density Species, Bridge Construction, Green Moisture Content, Basic Density, Shrinkage