Masters Degrees (Forest and Wood Science)

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    Improved pallet base designs for citrus export: a structural and airflow study
    (Stellenbosch : Stellenbosch University, 2023-12) Rust, Michiel; Wessels, Coenraad Brand; Berry, Tarl; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: The overall aim of this study was to investigate and develop pallet base designs for the citrus export industry that have sufficient mechanical strength and stiffness to handle the rigors of the export process. Pallet designs should also facilitate good airflow penetration for an efficient cold chain during shipping. Six existing pallet designs were tested for strength and stiffness performance to obtain a baseline data set. Airflow penetration was also modelled for these designs. An existing pallet testing rig that has the capability of testing various aspects of pallet strength and stiffness, was used to evaluate racking bending strength/stiffness, forklift bending strength/stiffness, and impact strength. Results from these tests assisted with the identification of weaknesses in each of the pallet designs. The information was used to create improved pallet designs which were subsequently manufactured and compared to the original designs. In designs where the thickness of the structural members were increased, it resulted, as expected, in a stiffer pallet. The most breakages occurred in the stringers and especially where there were knots present on the stringer, indicating that this specific member should be a focus of improvement. The orientation of the deck boards also played a role in the stiffness of the pallet as it affected the orientation of the corner blocks. If the corner blocks were parallel to the length of the pallet, they effectively reduced the span of bearers and therefore increased the pallet stiffness. In the improved designs, an extra nail was added on the corner block for better impact performance. This inadvertently increased pallet stiffness by aiding the connections between the bearer and the corner blocks. Mixed species pallets, where Eucalyptus grandis stringers were used with other members being Pinus, also resulted in stiffer pallets. The accuracy of the nailing pattern had a large effect on the performance of the pallet during impact testing. Airflow of all the improved pallets could be enhanced by better slat spacings. Apart from improving pallet designs, it is recommended that the manufacturers and buyers of pallets should implement more stringent quality control measures. A citrus pallet quality control working group consisting of users and manufacturers of pallets can aid in this process and should be considered by the South African citrus industry.
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    Out-of-plane strength and stiffness prediction of SA pine cross-laminated timber
    (Stellenbosch : Stellenbosch University, 2023-03) Jacobs, M. J.; Wessels, Brand C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: South Africa recently adopted a slightly adapted version of ANSI PRG 320:2019 as a standard for performance-rated cross-laminated timber (CLT) – with a local code number of SANS 8892:2020. In ANSI PRG 320:2019 and SANS 8892:2020 there is a table containing unfactored resistance values for standardized CLT layups manufactured with different grades of timber in both the major and the minor strength directions. The grades of timber used in the laminations to produce the unfactored resistance tables are those available in the United States of America and, therefore, this table does not apply to the South African standard as different species of timber under different conditions are used here. Most importantly, South Africa uses a different structural timber grading system. The use of locally grown species to manufacture CLT will ensure that CLT becomes economically feasible and competitive as a high-value green alternative for reinforced concrete. Optimal design will only be possible if sufficient knowledge of local species, and how they perform in the context of CLT, is available. In this study, the locally grown SA pine was used to manufacture CLT specimens which were evaluated for bending and shear stiffness and strength, in both the major and minor strength directions. Bending evaluation was done at a span:thickness ratio of thirty with a 4-point bending test. Shear evaluation was done at a span:thickness ratio of six with a 3-point bending test. The specimens tested consisted of two 3-layer layups with respective thicknesses of 66 mm and 138 mm; these layups relate to layer thicknesses achievable by using the smallest and largest commonly available timber-lamella thicknesses in South Africa. A total of ten specimens for each layup, test and orientation combination were evaluated. The experimentally obtained results were then compared to analytically predicted stiffness results from the most widely used analytical predictive models, namely the shear-analogy and gamma methods. Where possible, all the input parameters for the predictive modelling were determined beforehand to make predictions as specific and accurate as possible. It was found that the shear-analogy method is more broadly applicable and can be used for all span:depth ratios, while the gamma method only produced accurate predictions of bending stiffness for long span:thickness ratios where the percentage of shear deformation was small. Based on the comparisons presented in this thesis, the shear analogy was chosen and used to create an unfactored resistance table for 3- and 5-layer layups using South African-strength class and lamination thickness as inputs. The thinner 66 mm CLT specimens had significantly higher bending and shear strength, and higher bending stiffness when compared to the 138 mm CLT. High rolling-shear strength values were found when compared to values found in literature for European spruce. The main outcomes of this work were (a) the proof that the shear-analogy method was a more broadly applicable prediction model for CLT lay-ups and (b) the creation of unfactored resistance tables for 3- and 5-layer layups using South African pine CLT. Future work should include the determination of rolling- shear modulus and strength of the SA pine resource, with a larger number of specimens from various regions and grades. Furthermore, the influence of layups on mechanical properties should be explored when comparing 5-layer layups to the 3-layer layup. Ways of increasing the effective stiffness of CLT plates in the out-of-plane configuration should be explored, with one such example being the addition of ribs in the long-span direction to increase stiffness.
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    Investigating a new green edge laminating process for CLT manufacturing from twist-prone SA pine sawn timber
    (Stellenbosch : Stellenbosch University, 2023-03) Favis, Jesse; Wessels, Brand C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: There is potential for fast-grown, low quality raw material to produce high-value cross laminated timber (CLT). South African Pine plantations are harvested in shorter-age rotations to increase productivity, which results in greater proportions of corewood in the timber. Corewood tends to have a lower density and highly variable physical properties, leading to severe deformation after the timber is dried. Twist is the drying defect that causes the most rejection of dried wood for structural applications in South Africa. It is increasingly necessary for the sawmill industry to recognise the importance of conversion efficiency as it increases profits, reduces waste and extends the supply of standing timber. Twist in sawn timber can have a large effect on the conversion efficiency of logs, affecting both the volume and value recovery. The processor can manage twist in two ways: first, by predicting twist propensity in sawn timber from specific log resources, and second, by devising strategies to reduce the twist in the sawn timber from these resources. This study investigated both the prediction of twist and the use of an alternative processing method to reduce twist. In the first sub-study, the relationship between grain angle (spiral grain) and twist deformation of fast-grown South African Pine was evaluated. The highly significant p value (2.77 x 10⁻¹⁵) association identified between grain angle and twist had a relatively low coefficient of determination (R² = 0.28). Grain angle alone could not predict this deformation; other influential properties, including variation in density, microfibril angle and curvature of growth rings should be investigated further, to be able to model twist in seasoned boards accurately. In the second study, timber boards for CLT panel layers were green-edge bonded with a one-component polyurethane adhesive (1C-PUR), before seasoning, to reduce twist deformation. The potential of this manufacturing process to improve the conversion efficiency of low-quality South African Pine wood into CLT was investigated. The new manufacturing process reduced twist by 60.1% and increased volume recovery by 7% compared with the conventional manufacturing method of manufacturing. The perfectly flat panel layers would require less pressure to be made into CLT, which would ultimately reduce the cost of the mass timber material.
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    Utilization preparation of Senegalia mellifera to produce animal fodder
    (Stellenbosch : Stellenbosch University, 2023-03) Neethling, Alwyn; Tyhoda, Luvuyo; Gorgens, Johann F. ; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: Senegalia mellifera, a tree species associated with significant bush encroachment, has been noted to cause great harm to the environment, the agricultural sector and inevitably the greater economy in Namibia. This is due to Namibia having one of the most severe cases of bush encroachment caused by Senegalia mellifera, which occupies about 70% of all arable land. The severe loss in arable land causes a reduction in available grazing for livestock. Furthermore, clearing these encroached areas could increase available grazing areas for livestock. In addition, due to droughts occurring in Namibia, farmers struggle to keep livestock alive and tend to reduce the number of livestock on their farms. As these droughts intensify, animal feed prices increase, making it unaffordable. Thus, by de-bushing encroached areas and making use of Senegalia mellifera to produce animal fodder, one could aid farms in keeping livestock alive during droughts, while addressing a significant ecological, environmental and biodiversity threat. Furthermore, in the event of animal feed being produced from Senegalia mellifera, farmers tend to only use the small, nutritious branches. By making use of these branches, the farms produce a form of feed that requires additives and they do not solve the problem of bush encroachment caused by Senegalia mellifera. Thus, by harvesting and utilising the complete tree for animal-fodder, one could establish whether the nutritional characteristics are influenced when proportions of stem material are added, and how various pre-treatment methods could overall affect the material digestibility. The objectives of the study were to: (1) evaluate the chemical composition and nutritional characteristics of a mixture of Senegalia mellifera wood that consisted of twigs leaves and mature stems; (2) comparatively study the use of different pre-treatment methods, namely steam explosion (160-220°C, 2-10 min), alkaline catalysed steam explosion (160-220°C, 2-10 min, 1-7 wt.% NaOH) and aqueous ammonia soaking (60-120°C, 1-6 hours, 5-15 wt.% NH3) to increase the material digestibility and available metabolizable energy of Senegalia mellifera. The chemical compositional analysis of Senegalia mellifera raw material indicated a high concentration of lignin (32.06 ± 0.054 %DM) and low levels of cellulose (27.19 ± 0.094 %DM). Nutritional analysis indicated a low level of crude protein (CP) (4.9 ± 1 %DM) in the animal feed mixture. Finally, the feed also had a low digestibility (EDOM) (50.02 g/kg DM) and available metabolizable energy (0.46 MJ/kg DM MEEDOM & 1.32 MJ/kg DM NviroTek), which can be attributed to a high degree of lignification and cellulose crystallinity. After the raw material had been pre-treated, a significant improvement (p < 0.05) on material digestibility and available metabolizable energy was observed in all samples. The effectiveness of the individual treatments in terms of material digestibility was as follows: The most effective method was alkaline catalysed steam explosion, followed by the auto catalytic steam explosion, and the least effective method was aqueous ammonia soaking. Alkaline catalysed steam explosion had the most significant influence on material digestibility by improving it 11-fold to 580.30 g/kg DM (58 %DM) and increasing the available metabolizable energy by just over 15 times to a maximum of 7.19 MJ/kg DM in the most severe conditions (220°C, 10 min, 7 wt.% NaOH). Auto catalytic steam explosion also increased the material digestibility 5-fold to 254.7 g/kg DM (25.47 %DM), while increasing the available metabolic energy (7.8-fold) to 3.59 MJ/kg DM in treatment condition (220°C, 2 min). Finally, the use of aqueous ammonia soaking was the least effective method as it only increased the digestibility twice and the metabolizable energy was increased almost 4-fold to 138.1 g/kg DM (13.81%DM) and 1.79 MJ/kg DM in the most severe treatment conditions (120°C, 6 hours, 15 wt.% NH3).One of the major concerns regarding proper rumen microbial function is the requirement of crude protein (CP). It was thus important to track this key nutritional factor throughout the pre-treatment process. Aqueous ammonia soaking was the most effective in preserving the crude protein content, followed by auto catalytic steam explosion, and the least effective method was alkaline catalysed steam explosion. Regarding the nutritional characteristics, it was observed that even though alkaline catalysed steam explosion improved material degradability and available energy, the already low level of CP was sacrificed, reducing it to 1.05 %DM, which is significantly lower (p < 0.05) when compared to the CP preservation by auto catalytic steam explosion and aqueous ammonia soaking. Finally, regarding crude protein, aqueous ammonia soaking increased the CP and the available nitrogen in fodder by 13.5-282.78 g/kg DM (1.35-28.28 %DM) due to the chemical binding of nitrogen to the prepared feed. This can be a possible reason for a false negative regarding the increase in crude protein noted during aqueous ammonia soaking. In conclusion, by comparing the in vitro enzymatic digestibility, the overall available metabolised energy and available crude protein of three different means of pre-treatment, this study showed that alkaline catalysed steam explosion provided the best overall increase in the digestibility of Senegalia mellifera and thus increased the available metabolic energy for ruminants, even though the crude protein fraction was sacrificed in the pre-treatment process. Regarding animal feed, it is plausible that the removal of a Senegalia mellifera as a whole bush consisting out of stem branches and leaves could be used as an animal feed after the optimal ACS pre-treatment.
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    Calibrating rotation age for eucalyptus pellita and eucalyptus grandis x urophylla to optimize market returns for transmission poles and plywood in Ghana
    (Stellenbosch : Stellenbosch University, 2023-03) Taziwa, Gerald; Ham, Cori ; Dash, Jonathan; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: There has been over-reliance on Tectona grandis and Ceiba pentandra for the manufacture of utility poles and plywood in Ghana. Miro Forestry has for the past eight years been growing various tree species including Eucalyptus pellita (Pellita) and E. grandis x urophylla (GU) on large- scale plantations for the first time in Ghana. However, optimum silvicultural regimes and growth and yield models are yet to be defined since the company is in its first full rotation. In this study, the determination of optimal rotation age for Pellita and GU for poles and plywood production was investigated. A combination of the market conditions, processing technology, silviculture regimes and Miro’s existing growth and yield functions were used to determine a suitable rotation age along with appropriate silvicultural treatments. Market analysis for pole size distribution and a plymill experimental study for peeler log processing were the benchmarks used to establish the optimum pole and peeler log specification to satisfy market demand and mill configuration. A taper function was used to calculate the tree size from which these logs can be produced. Once the optimal tree was ascertained, an investigation into the best regime to follow for poles and plywood ensued. This gave guidance on optimum silviculture regimes to grow the ideal tree for poles and plywood. Miro’s existing growth and yield functions within the Microforest software package were used to simulate different thinning scenarios to investigate growth response to thinning. Stocking, thinning intensity and frequency of thinning were altered to test their effect on the growth dynamics of the trees. Rotation age was defined by diameter growth and the time it took to reach the sizes of the optimum pole or peeler log. The results from this study showed that the optimum pole tree had a DBH of 23 cm and a height of 23.3 m. The optimum plywood tree had a DBH of 31 cm and a height of 30.8 m. The simulation results in Microforest showed that a two-thinning regime of 600 stems per hectare gave more favourable results and that GU would achieve diameters of 23 cm for poles at 8.5 years. Pellita would not be able to achieve those diameters even over a much longer rotation. Both species would not achieve the dimensions for an optimum plywood tree within the range of conditions tested. However, the plymill recovery for the smaller diameter class of 15-20 cm was not significantly different from the 20-30 cm class. A tree whose DBH is 21 cm and a height of 21.5 was also ideal for plywood and it produced logs whose average diameter fell within the 15-20 cm diameter class. A two-thinning regime would achieve these dimensions at six years for GU and 11 years for Pellita. GU had a biological and economic rotation of six and seven years respectively. The slower- growing Pellita had a biological rotation of 7.3 years, and the economic rotation was seven years. This study illustrates that when markets demand very specific product dimensions, the use of biological and economic rotation ages would be subservient to technical rotation age.