Characterization of the Physico-Mechanical Properties of the Different Zones of Borassus aethiopum (Mmaa Kube)

The suitability of Borassus aethiopum as a potential substitute for the primary timber species in the various timber applications was assessed by characterizing the physical and mechanical properties of the dermal zone, sub-dermal zone, central zone and the bulge area of Borassus aethiopum. Logs from five Borassus aethiopum trees extracted from Asempanaye were converted into boards. One-half of the boards were used in the green state and the other half air dried. The physical properties and mechanical strength test specimens were prepared and tested in accordance with the British Standard BS 373:1957. The range of mean basic density and density at 12% MC for the dermal, sub-dermal and the central zones were 480.4 - 752.7[582.5 - 957.7]kg/m3, 229.2 - 652.5 [266.4 -814.9] kg/m3, and 127.7 - 436.8 [145.9 - 525.7] kg/m3 respectively. The mean green moisture content for the dermal, sub-dermal and the central zones range were 30% - 82%, 37% - 134%, and 69% - 290% respectively. The ranges of mean moisture content, basic density, and density at 12% MC for the bulge area were 200-298%, 78.43-163.3kg/m3 and 89.0-187.7kg/m3 respectively. The range of mean strength values in the green and [dry] conditions for the dermal zone, sub-dermal zone, and the central zone were as follows: Modulus of Elasticity: 2971.0 – 20563.8 N/mm2 [5253.0 – 25871.8 N/mm2], 1407.3 – 16661.2 N/mm2 [18432 – 20323.1 N/mm2], and 118.7 – 4669.9 N/mm2 [206.0 – 5857.2 N/mm2], Modulus of Rupture: 31.1-156.1 N/mm2 [48.5 – 217.1 N/mm2], 10.6 - 98.2 N/mm2 [19.0 – 149.7 N/mm2], and 1.3 – 22.9 N/mm2 [2.1 – 31.9 N/mm2], Compression parallel to the grain: 16.9-91.9 N/mm2 [27.6 – 99.8 N/mm2], 2.3-61.5 N/mm2 [3.2 – 71.8 N/mm2], and 0.03-17.8 N/mm2 [0.9 – 18.7 N/mm2]. Shear parallel to the grain: 2.61-13.74 N/mm2 [5.47 – 19.75 N/m2], 0.43-10.40 N/m2 [1.38 – 14.13 N/mm2], and 0.09 -5.17 N/mm2 [0.20 – 6.23 N/mm2], Hardness: 2.74 - 12.23 kN [5.07 – 17.02 kN], 0.66- 10.12 kN [1.42 – 12.95 kN], and 0.04 -1.66 kN [0.1 – 3.13 kN]. The range of mean strength values for the MOE, MOR, Hardness, Compression and Shear Parallel to the grain for the bulge area were 70.27 – 188.46 N/mm2, 1.1 5- 3.30 N/mm2, 0.09 - 0.44 kN, 0.47 - 2.34 N/mm2 and 0.12 - 0.67 N/mm2 respectively. The overall order of decreasing strength properties and density for the various sections of Borassus aethiopum was as follows: dermal zone > sub-dermal zone > central zone > bulge area. The overall order of decreasing Moisture Content of the various sections of the tree was as follows: bulge area > central zone > subdermal zone > dermal zone. Analysis of variance (ANOVA) of the physical and mechanical iv properties of these zones indicated that there were significant difference at P<0.05 between the zones. The effect of stem height on Borassus aethiopum “wood” physical properties and mechanical properties for each of the zones were significant at P<0.05. There was a good correlation (97.1% ~ 99.9%, 94.9% ~ 99.7, 94.8% ~ 99.9% and 18.3%~89.3% respectively for the dermal zone, sub-dermal zone, central zone and the bulge area) between Density (X) and the various mechanical strength values (Y). Regression models in the form: Y = mx + c were derived with R2 values of 0.96 – 0.99, 0.95 – 0.99, 0.95 – 0.99 and 0.01 – 0.60 respectively for the dermal zone, sub-dermal zone, central zone and the bulge area. The mechanical properties of the dermal zone compare favourably with Afromosia (Pericopsis elata), Dahoma (Pepdiniastrum africanum), Kusia, Teak (Tectona grandis), and Sapele (Entandrophragma cylindricum). While that of the sub-dermal zone compares favourably with Mahogany. Hence, an indication that this monocot giant, Borassus aethiopum, is a good substitute for these timber species.

Source: KNUST