Prospective Graduate Students / Postdocs
This faculty member is currently not actively recruiting graduate students or Postdoctoral Fellows, but might consider co-supervision together with another faculty member.
Stavros Avramidis
Professor
Research Classification
Research Interests
Wood-water relations and sorption thermodynamics.
Fluid flow, heat transfer, and molecular diffusion
Modeling steady-state and dynamic non-isothermal diffusion in wood.
Application of fractals and machine learning to modeling wood physical properties.
Molecular modeling of the wood nano-pore network in the cell wall.
Industrial wood kiln drying optimization and development of new drying strategies.
Radio frequency vacuum (RFV) heating and drying of wood and wood products.
Dielectric wood phytosanitation.
Wood thermal modification.
Relevant Thesis-Based Degree Programs
Research Options
I am available and interested in collaborations (e.g. clusters, grants).
I am interested in and conduct interdisciplinary research.
Graduate Student Supervision
Doctoral Student Supervision
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Fractal modeling of moisture diffusion in wood (2022)
The mechanisms of moisture diffusion in wood are not yet fully understood, due to the complex and hierarchical structure of the wood cell wall constituents. In order to investigate this mechanism in this hierarchical structure, fractal geometry analysis was used as proper tool. This study has three major objectives: first, to investigate and understand water sorption, pore structure, and fractal property of two softwood and two hardwood species; second, to develop a theoretical fractal moisture diffusion model for wood cell wall by taking into consideration its structural geometry using the tortuous capillary bundle model and fractal theory; and third, to upscale that model to gross wood by employing electrical resistance modeling and validation. The proposed fractal diffusion model is a function of both pore and tortuosity fractal dimensions, porosity, and pore size distribution of the wood cell wall. The water vapor sorption behaviors of various wood types were studied using the dynamic sorption method. Their pore structure and fractal characteristics were investigated using nitrogen and mercury intrusion porosimetry. The pore size distribution ranged from 1.4 nm to 350 um, and porosity ranged between 58 and 76 %. The pore and tortuous fractal dimension values ranged from 2.5 to 2.98, and from 1.034 to 1.076, respectively, with a higher degree of pore complexity for larger pores. The derived fractal diffusion model was validated using experimental and data calculated by a past published model. The trends for diffusion coefficients predicted by the fractal model were similar to the experimental and calculated data and successfully predicted the diffusion coefficients at low moisture contents. Pore size ratio, pore, and tortuous fractal dimensions were negatively correlated to fractal diffusivity, while the porosity was positively correlated. When the pore and tortuosity fractal dimensions were close to 2 and 1, respectively, the diffusion coefficient values approached to the minimum. The findings of this study contribute to the creation of a decision support system that would allow predicting wood geometric properties and moisture diffusivity properties based on wood structural and ultrastructural attributes.
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Estimating the final moisture content variation in kiln-dried pacific coast hemlock (2021)
Kiln drying is indisputably a significant value-adding step in timber processing where the importance of predicting moisture within a dried batch cannot be overemphasized. Among timber drying quality indices, the uniformity of final moisture content within a drying timber batch is crucial. Lack of such uniformity leads to undesirable moisture ranges, thus producing large percentages of over-dried and/or under-dried timber sub-populations which in turn, result is significant degradation and value downgrade. Because it is a cumbersome task to collectively correlate various pre-dry timber factors to moisture variation of the post-dry timber population, predicting the value and variability of final moisture is still a great challenge. Part I of this study is dedicated to predicting and characterizing that moisture variation based on the initial and target moisture values using polynomial models. Four polynomial models (PM) are used to correlate initial and final moisture characteristics. While one model failed due to discontinuity three models successfully characterized final moisture variation with the best one showing an R2 > 96% for the goodness of fit. The robustness of the three best models is analyzed and a closed formula is proposed to evaluate the final moisture coefficient of variation based on the target moisture (setpoint) and initial moisture coefficient of variation. In part II, five artificial intelligence (AI) approaches including multilayer perceptron (MLP) and radial basis function (RBF), group method of data handling (GMDH), adaptive neuro-fuzzy inference system (ANFIS), and support vector regression (SVR) are employed to create a comprehensive predictive model that connects selected initial wood attributes (basic density, initial weight, initial moisture, and target moisture) to the final moisture. Seven configurations are constructed, i.e., A (4 attributes); B, C, and D (3 attributes); E, F, and G (2 attributes). As a result, GMDH showed the best performance in predicting final moisture followed by SVR and MLP, especially when it came to configuration A. Neural networks and polynomial models have their own advantages and limitations. Neural networks are capable of predicting final moisture for every single dried timber using numerous factors while polynomial models provide a closed formula for estimating the final moisture variation.
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Experimental investigations of shear connections with self-tapping-screws for cross-laminated-timber panels (2020)
Advances in the areas of engineered wood products, such as Cross-laminated Timber (CLT), and wood connection solutions such as Self-tapping Screws (STS), as well as supporting legislation have created new possibilities for the structural application of timber in mid-rise construction. CLT structures and their connections need to be designed for appropriate capacity, stiffness, and if applied in seismic zones – ductility; however, current North American design standards contain no provisions for STS in CLT. The research developed in this dissertation first examined the withdrawal resistance of STS in CLT for different screw diameters, effective screw embedment lengths, and angles of the screw axis relative to the wood grain. The work demonstrated that the existing product approval equation which was developed for Glulam can be used to predict the withdrawal resistance of STS in CLT. Subsequently, the performance of CLT-STS joints was investigated under monotonic and reversed cyclic tests. Different conventional joint types were tested, namely surface spline with STS loaded in shear and half-lap with STS loaded in either shear or withdrawal. The research further investigated novel CLT joints combining STS loaded in withdrawal with STS loaded in shear, and butt joints with double inclination of STS. The joint performance was evaluated in terms of capacity, stiffness, yield strength, and ductility, and it was shown that joints with STS in shear exhibited high ductility but low stiffness, whereas joints with STS in withdrawal were found to be stiff but less ductile. Combining the shear and withdrawal action of STS led to joints exhibiting high stiffness and ductility. Varying number of screws in one joint allowed evaluating group effect of STS for joint capacity, stiffness, and ductility, under both monotonic and cyclic loading. E.g. for joint capacity, group effect can be expressed as neff = 0.9∙n for all joints under static loading, where n, and neff are actual and effective number of screws respectively. For cyclic loading, more pronounced group-effect was observed that can be expressed as neff = n0.9. The data and analyses presented in this dissertation will provide guidance to structural engineers and builders for designing CLT-STS shear connections.
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Water sorption hysteresis and wood cell wall nanopore structure (2017)
The origin of sorption hysteresis in the wood-water system is still under debate. In this study, cell walls are considered as micro-mesoporous materials and capillary condensation in the entire hygroscopic region is proposed as an alternative sorption mechanism. Initially, the pore connectivity was investigated by observing five experimentally generated hysteresis patterns at 25 and 40oC. Consistent patterns were found for the species-temperature combinations. Further, the satisfactory congruency and wiping-out properties indicate the dominance of independent cell wall pores. After this experimental phase, the geometric interpretations derived from the Preisach model, the mathematical form of the independent domain model, was used tο explain the observed hysteresis patterns. Additionally, a modification to the aforementioned model was suggested that involves a numerical implementation, which avoids the use of unknown parameters. The low prediction errors and well-maintained wiping-out property support the suitability of our approach. In the next phase, grand canonical Monte Carlo (GCMC) technique was applied in a simplified wood-water system to simulate sorption isotherms and hysteresis at 25 and 40°C. In the simulation system, wood is represented by a cell wall model that is composed of solid substances and evenly distributed independent cylindrical nanopores with sizes in the range of 0.6 – 2.2nm. Two types of pore-wall compositions regarding polysaccharides and lignin have been considered. The hydroxyl groups are modeled as negative energy pits attached to walls whereas water is represented by the SPC/E model. Results demonstrated that hysteresis can be well explained by the existence of metastable states associated with capillary condensation and evaporation of water in cell wall pores. The alternative sorption mechanism driven by capillary condensation is also strongly supported by the simulation. In the last phase, the cell wall pore size distributions in the hygroscopic range were explored for the three species from a “trial and error” calculation approach. This approach was indirectly examined by comparing derived volumetric strain of cell walls and the density of adsorbed water in the hygroscopic range with literature data. The qualitative agreement indicates the soundness of assumptions made on the cell wall swelling process and proposed calculation procedures.
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Unified and heterogeneous modeling of water vapour sorption in Douglas-fir wood with artificial neural networks (2010)
The objective of this study was firstly to investigate and understand sorption properties of earlywood, latewood, annual rings and gross wood. Secondly, to develop a heterogeneous sorption model for earlywood, latewood and annual rings by taking into consideration unified complex interactions of anatomy, chemical composition and thermodynamic parameters. Thirdly, to upscale the annual ring level model to gross wood by applying artificial neural networks (ANNs) modeling tools using dimensionally reduced inputs through dimensional analysis and genetic algorithms. Four novel physical models, namely, dynamical two-level systems (TLS) model of annual rings, sorption kinetics, sorption isotherms and TLS model of physical properties and chemical composition were derived and successfully validated using experimental data of Douglas-fir. The annual ring’s TLS model was capable to generate novel physical quantities, namely, golden ring volume (GRV) and golden ring cube (GRC) to which the sorption properties are very sensitive, according to the validation tests. A new heterogeneity test criterion (HTC) was also derived. Validations of the TLS sorption models revealed new evidence showing a transient nature of sorption hysteresis in which boundary sorption isotherms asymptotically converged to a single isotherm at large time limit. A novel method for the computation of internal surface area of wood was also validated using the TLS model of sorption isotherms. The fibre saturation point prediction of the model was also found to agree well with earlier reports. The TLS model of physical properties and chemical composition was able to reveal the self-organization in Douglas-fir that gives rise to allometric scaling. The TLS modeling revealed existence of self-organizing criticality (SOC) in Douglas-fir and demonstrated mechanisms by which it is generated.Ten categories of unified ANNs Douglas-fir sorption models that predict equilibrium moisture content, diffusion and surface emission coefficients were successfully developed and validated. The network models predict sorption properties of Douglas-fir using thermodynamic variables and parameters generated by the four TLS models from chemical composition and physical properties of annual rings. The findings of this study contribute to the creation of a decision support system that would allow predicting wood properties and processing characteristics based on chemical and structural attributes.
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Modeling Wood Shrinkage Response to Tensile Stresses in Convective Drying (2009)
No abstract available.
Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Assessment of some wood properties by near infrared spectroscopy (2020)
Near-infrared spectroscopy (NIRS) is a suitable technique for characterizing many materials, including wood, and has been used to predict several wood properties. However, existing reports on this use of NIRS have paid little attention to the effect of wood surface condition and grain orientation. This study therefore used NIRS to assess wood density, modulus of elasticity, modulus of rupture, grain angle, and annual ring width, studying whether and how surface condition and grain orientation affected the measurement of these properties.The research focused on using NIRS coupled with partial least squares regression (PLS-R) to predict the properties of two softwoods (Western hemlock and Douglas-fir). PLS-R models were calibrated and validated using the test-set validation method. The predictive accuracies based on grain orientation (quarter-sawn and flat-sawn) and wood surface condition (rough and smooth) were compared. Models developed using reduced wavelengths also showed the possibility of predicting these properties using a narrow spectral range.The results of this study showed that calibrations based on mixed sets, which included both cross-sections, were inferior to those based on these cross-sections separately. Promising predictive models were obtained for density (Rp² = 0.66), modulus of elasticity (Rp² = 0.78), and modulus of rupture (Rp² = 0.82), with poor correlations for grain angle and annual ring width (Rp² ≤ 0.50). Further, the rough surface predictions outperformed those from the smooth surface for all properties. The quarter-sawn sections also showed better predictive ability than the flat-sawn sections for both surface conditions. The only exception was for modulus of rupture, where the trend was reversed. The results therefore show the potential for using NIRS as a non-destructive technique to predict the properties of wood.
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Thermo-hydro-mechanical densification of western hemlock (tsuga heterophylla) (2020)
Thermo-Hydro-Mechanical densification (THM) is a controlled wood compression at high temperatures that results in improving some of its properties. This process has not been used until now to produce a commercial product, and still, remains a laboratory-scale process. Therefore, more research is needed to explore possible applications of THM with local species. This thesis tests the hypothesis that THM will increase wood density and accordingly, improve mechanical properties such as hardness, strength, and abrasion resistance. Therefore, low-to-average density wood species could be proper candidates for densification. In addition, interactions among three important factors (pre-treatment, densification time, and temperature) on THM were examined to find the best combination of factors in process. Western hemlock (Tsuga heterophylla) as one of the most common and important local timber species was selected for THM. Because this species is used mostly in construction, thus, the development of other uses could create new markets and extra profits to the industry. Kiln-dried hemlock boards were subjected to THM process. Two different specimen pre-treatments were selected before starting the compression of wood. The independent variables considered were 12% moisture content conditioned sample surfaces with and without hydration by water spraying, three densification times (5, 10, and 15 minutes), and three temperatures (120, 160, and 200°C). Tests performed on densified and control samples by means of a hot press to evaluate density, hardness, spring-back, color change, and abrasion resistance. Data analysis shows that temperature, densification time, and moisture pre-treatment were significant factors. Compared to untreated samples, THM significantly improved density, hardness, and abrasion resistance of hemlock by 197%, 386%, and 437%, respectively. As a desired feature, this process darkened the hemlock around 200% as well. In addition, after 200 hours, spring-back reached to a steady state (less than 2% change). The optimum treatment took place for pretreated samples with water spray that were densified for15 minutes at 160°C (WS-15-160).
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Thermal modification of western hemlock (Tsuga heterophylla) (2018)
Thermal modification is a controlled degradation process of wood cell wall material at high temperatures that results in improving some of its properties. This process, albeit quite developed in Europe, is still in its infancy in Canada. So, use of such method might add value to local wood species and thus, allow the development of new products and markets. Western Hemlock (Tsuga heterophylla) is a locally abundant species used mostly in construction, so the development of other uses could bring extra revenues to the industry. This research focuses on exploring thermal treatment levels and their effect on some material properties in order to achieve the optimum combination for hemlock. In this regard, kiln-dried western hemlock boards with two cuts (flatsawn and quartersawn) were subjected to the Thermowood® process at three different maximum treatment temperatures (170, 212, and 230°C) for 2hrs. Samples were cut from both the treated and untreated boards for property evaluation tests including basic density, equilibrium moisture content, water absorption, anti-swelling efficiency, color change, Janka hardness, and dynamic modulus of elasticity. Data analysis revealed that there was no significant difference between the two cuts and temperature was the only factor that affected the wood properties. Basic density, equilibrium moisture content, and water absorption were lower at higher treatment temperatures, while dimensional stability considerably increased. The impact of higher treatment temperatures on hardness and stiffness of samples was hardly noticeable, but it visually influenced the color of samples and made them darker. Within the scope and limitations of this study, the optimum treatment temperature, namely, the one that enables improved dimensional stability and provided a darker color without significantly affecting the wood strength suggested establishing at 212°C. Further research is required to fully determine the performance of thermally modified wood for interior and exterior applications.
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Kiln Drying Optimization for Quality Hem-Fir Lumber (2016)
Western hemlock is a dominant coastal species in British Columbia, Canada. This species is commonly marketed with amabilis fir as Pacific Coast Hemlock or hem-fir. Hem-fir is difficult to dry, mostly because of the existence of wetwood and large initial moisture content variation. The dried lumber will likely end up with a large final moisture content difference resulting in increased drying defects and decreased lumber quality and factory productivity. In this study, application of green chain moisture-based sorting coupled with drying schedule modifications were considered as ways to improve final moisture content variation within and between kiln dried hem-fir lumber. There were two research phases. The first (without sorting), aimed to develop a modified drying schedule whereas in the second, the developed schedule was used along with a standard industrial schedule. Additionally, there was a green moisture content pre-sorting component in the second phase where freshly cut specimens were sorted based on their initial moisture content into three groups, i.e., mixed, low, and high moisture. To assess the specimen kiln dried quality, final moisture content variation, moisture content gradient, drying rate, warp, surface and internal checks, shrinkage, and casehardening were assessed. Data analysis revealed that there was no significant difference between the drying runs in terms of final moisture content variation, except in the high initial moisture content group. High initial moisture content sorting helped to reduce the final moisture content variation. The modified schedule, when there was a high initial moisture content sorting, also improved the uniformity of final moisture content in comparison to the industrial schedule. Moreover, neither the moisture sorting nor the drying schedule did affect the final moisture content variation for the low and mixed initial moisture content groups. Therefore, the green moisture-based pre-sorting was statistically effective just in the sorted group with high initial moisture content and where the modified schedule was used.
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Internal moisture movement in hem-fir timbers exposed to ambient conditions following kiln drying (2013)
This study aimed to investigate the changes of internal moisture content distributions of high-value hem-fir timbers after kiln drying while exposed to two different local outdoors seasonal conditions for a period of time of few weeks.Hem-fir is the most abundant species in coastal British Columbia, Canada, and high-quality thick hem-fir timbers used as construction material are one of the most important, and profitable products due to international market demand especially in Japan. Presently more-and-more of those houses are pre-fabricated and dimensional stability is paramount. Internal moisture profiles after kiln drying and their behaviour as a function of weather exposure in storage can result in dimensionally unstable products and consequently compromise quality. Conventional kiln drying of thick timbers is relatively difficult and requires long drying time to reduce final moisture content variation. Fast drying will result in steep moisture content gradients which may result in undesirable dimensional changes when products are used in normal service conditions. Thus, it is important to understand moisture behavior after drying and how that is affected by the environment.In this study, 90 x 90 mm in cross-section hem-fir timbers were dried to three different target moisture contents. Thereafter, stickered packages were stored under two diverse seasonal coastal environments thus emulating outdoor timber storage in a local sawmill. Moisture contents at 25 mm and 45 mm depths were continuously monitored for a period of three weeks.The results showed that moisture movement was observed between at 25 mm and at 45 mm depths regardless seasonal conditions while no significant net moisture content reduction took place during the cold-wet season after kiln drying. Also, regardless seasonal condition or target moisture content, moisture movement between at 25 mm and at 45 mm depths slowed down when differential moisture content between them was below 2.5%. In particular, at high target moisture content of 22%, moisture content values both at 25 mm and at 45 mm depths remained constant at moisture content difference value of 2.5% after two weeks regardless seasonal conditions, and no further drying or no further moisture content equalization were observed after that point.
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Radio frequency heating pre-treatment of sub-alpine fir to improve kiln drying (2010)
The objective of the study was to reveal the effect of RF heating at different power densities and time combinations as pre-conventional kiln treatment on the drying characteristics and quality of sub-alpine fir lumber. As a consequence of this objective, the study hypothesis was formulated as: “if RF heating improves the permeability of sub-alpine fir, then upon kiln drying, final moisture content variability between and within lumbers as well as drying defects will decrease”. In this research, thirteen groups of one meter long, 51 x 102 mm sub-alpine fir lumbers were RF heated with two power densities (27 and 72 kW/m³) and eight time combinations (15, 30, 45, 60, 75, 90, 105 and 120 minutes), but all to the same final temperature level (100°C) before kiln drying in a two-phase experimental design. Two groups (one for each phase) served as controls. Permeability tests were also performed in the second phase of the study. The effect of RF heating on permeability, drying rates, moisture content gradient, final moisture content variability between pieces and drying defects was evaluated and analyzed. Treatment effect on total energy consumption (sum of RF heating and kiln drying energies) was also assessed to ascertain the feasibility of applying this technology in industrial setting. Data analysis revealed in phase 1 that, not all treatments reduced moisture content variability within and between specimens and improved drying rates above and below fiber saturation point, compared to the control. Defect appearance also did not significantly reduce, and all except two treatments increased total energy consumption. In phase 2 however, permeability improved in all except one treatment but was found not to be statistically significant. Treatments also improved moisture gradient as well as drying rates above and below fiber saturation point but the moisture gradient was found not to be statistically significant. Compared to the control, not all treatments reduced moisture content variability between wood samples. Defects did not significantly improve between treatments and control, and total energy consumption was relatively higher in treatments. Results obtained within the limitations of this study led to the rejection of the hypothesis.
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