Vjekoslav Živković

Wood as a building material is a good thermal insulator, but in thin wood coverings this is not an obstacle to heating when the heating system is placed directly under the top layer. This study investigated the influence of a safety extra-low-voltage power supply (12 V and 24 V) on the surface temperature, response time and heat distribution of wood covering elements with an integrated heating system. Sixteen oak–plywood elements with copper heating wires 3 mm below the walking surface were tested in daily heating cycles; two of them were insulated with reflective foil or aerogel. At 12 V, the surface reached 29.1 ± 0.5 °C (about 5.6 K above ambient), within the comfort limit for floors, while at 24 V it reached 38.6 ± 1.3 °C (p < 0.001) and local checks appeared on the oak surface. The differences between elements were three times larger at 24 V (4.3 K compared with 1.5 K), but the relative non-uniformity was the same (CV of 8.5% at 12 V and 9.0% at 24 V; r = 0.98). Insulation under the element increased the temperature rise of the surface by 15–19%, while the type of insulation had little effect. A 12 V power supply is therefore a safe and sufficient solution for heating with wood coverings in nearly zero energy buildings.

The study aimed to investigate the thermal storage potential of oak tree bark as a natural and easily accessible material and to examine the possibility of incorporating this raw material into external prefabricated walls. The U-values of oak tree bark were compared with mineral wool (MW), which is a standardized and well-known insulating material. Oak tree bark, a residual material, is mainly used as an energy source. However, in light of actual green policies at the European Union (EU) level, raw wood and its components, e.g., bark, should be used for long-lasting products and not as an energy source. According to the Croatian Technical Regulation on the Rational Use of Energy and Thermal Insulation in Buildings, the maximum U-value for external walls in Croatia is 0.30 W/m²K. Here presented test results show that the U-values of the experimental wall compositions were 0.22 W/m²K for the wall with mineral wool (MW) and 0.29 W/m²K for the wall with oak tree bark. Despite mineral wool having a thermal conductivity (λ, W/mK) 53% lower than the experimental tree bark, walls with bark fulfill the national insulation requirements; therefore, they present an alternative and sustainable insulation option. Furthermore, the Blower door test (n50 value) of the bungalow was measured to be 3.75 h^−1, proving the potential of tree bark as an alternative eco-friendly insulation material in exterior walls. However, further investigations are necessary in order to create and optimize the panel, its thickness, density, adhesive technology, the size of bark particles, production parameters, etc., as these influence properties of the final product to be placed on the market.

The world’s expanding population presents a challenge through its rising demand for wood products. This requirement contributes to increased production and, ultimately, the high-quality and efficient utilization of basic materials. Detecting defects in wood elements, which are inevitable when working with a natural material such as wood, is one of the difficulties associated with the issue above. Even in modern times, people still identify wood defects by visually scrutinizing the sawn surface and marking the defects. Industrial scanners equipped with software based on convolutional neural networks (CNNs) allow for the rapid detection of defects and have the potential to accelerate production and eradicate human subjectivity. This paper evaluates the suitability of defect recognition software in industrial scanners against software specifically designed for this task within a research project conducted using Adaptive Vision Studio, focusing on feature detection techniques. The research revealed that the software installed as part of the industrial scanner is more effective for analyzing knots (77.78% vs. 70.37%), sapwood (100% vs. 80%), and ambrosia wood (60% vs. 20%), while the software derived from the project is more effective for analyzing cracks (70% vs. 65%), ingrown bark (42.86% vs. 28.57%), and wood rays (81.82% vs. 27.27%).

Various surface modification techniques have been developed to improve synthetic polymer surfaces’ wetting, adhesion, and printing by adding various functional (polar) groups. UV irradiation has been proposed as a suitable procedure to achieve adequate surface modifications of such polymers, which can be of further use to bond many compounds of interest. The activation of the surface, the favourable wetting properties, and the increased micro tensile strength of the substrate after short-term UV irradiation suggest that such pretreatment can also improve the bonding of the wood-glue system. Thus, this study aims to determine the feasibility of UV irradiation for pretreatment of wood surfaces before gluing and to determine the properties of wooden glued joints prepared in this way. UV irradiation was used to modify variously machined pieces of beech wood (Fagus sylvatica L.) before gluing. Six sets of samples were prepared for each machining process. Samples prepared in this manner were exposed to irradiation on a UV line. Each radiation level had a certain number of passes through the UV line, the more passes, the stronger the irradiation. Thus, the radiation levels were as follows: 1, 5, 10, 20, and 50 passes. The dose (energy delivered on the wood surface) in one pass was 2.36 J/cm². A wetting angle test with glue, a compressive shear strength test of lap joints, and designation of main failure patterns were used to evaluate the properties of wooden glued joints. Wetting angle test was performed according to EN 828, while the compressive shear strength test samples were prepared and tested following the ISO 6238 standard. The tests were conducted using a polyvinyl acetate adhesive. The study found that UV irradiation before gluing improved the bonding properties of variously machined wood.

This study analysed the bonding mechanisms and strength between wood and non-wood implants in producing laminated oak-wood beams. The suitability of different types of adhesives, namely for load-bearing and general purpose, was also analysed. Three different types of non-wood implants—carbon fibres, glass fibres, and aluminium were glued with epoxy resin (ER), thermoplastic 1-k polyurethane adhesives (PUR), structural polyurethane adhesives (PUR 2 and PUR 3), and polyvinyl acetate (PVAc) adhesives and bonds were tested for shear strength (SS) according to ISO 6238:2018. Results of the bond quality expressed as the ultimate load to failure and displacement were recorded using the universal mechanical testing machine in combination with the digital image correlation (DIC) method. Before the shear test, all the samples were conditioned in dry and wet climatic conditions. Test results indicated that the application of PUR adhesives for bonding carbon and glass fibres with oak wood could sufficiently replace two-component ER, which is generally recommended for such purposes but is very challenging to utilise in industrial conditions. PVAc adhesives proved efficient only for combination with AL implants and in dry conditions. Aluminium sheets were shown to require surface pre-treatment, such as sanding and degreasing or a different type of adhesive to achieve sufficient adhesion.

In this study, beech wood (Fagus silvatica L.) has been chemically modified with citric acid (Acidum citricum) and sodium hypophosphate (SHP) as the catalyst and gradually thermo-condensed in the dryer. Afterwards, wetting angle, surface energy, and shear strength of glued joints of modified and unmodified wood were determined. Testing of the bond strength according to standard EN 204 and comparison between modified and unmodified samples were executed. The adhesive used for bonding samples was polyvinyl acetate (PVAC), commonly used for gluing solid wood panels. Testing material was divided into three groups (dry, wet, and wet conditioned samples), within which statistical analysis was performed, and the significance of the differences between the modified and unmodified samples was determined. Surface energy is correlated with the bond strength, indicating that modification with citric acid negatively affects the adhesive properties of beech wood. A reduction in the bond strength of modified wood glued with PVAC glue compared to unmodified wood was determined. All the results indicate that the modified samples do not meet the minimum requirements for EN 204 bonded with PVAC glue. Therefore, it will be necessary to conduct further studies using other types of adhesives to investigate whether modified wood might be suitable for gluing.