# Effect of Primary Spruce Lamella Aging on the Bending Characteristics of Glulam Beams
## 3 Results and Discussion
### MOR and MOE in Bending of Gluam Beams Versus Shear Strength of Glued Joints  
Comparing the bending characteristics of gluam beams with the shear strength (\(\sigma\)) of glued joints, certain significant links were observed only for the MOR; however, none were observed for the MOE (Figure 6). Generally, the artificial aging processes of spruce lamellas manifested in gluam beams as an apparent loss of MOR; however, they underwent less significant changes in their MOE (Table 1). This observation is in accordance with a greater negative effect of high temperatures above 160 \({}^{\circ}\)C or several aggressive chemicals on the wood strength than on the wood deflection or other deformation types [43]. As the bending characteristics of gluam beams depend not only on the used wooden materials-lamellas (species, quality, amount and location in beam, etc.) but also on the used glue (type, quality, amount, etc. ), the mechanical properties of gluam beams significantly depend on the \"wood-glue\" interfaces. Their quality is reflected in the adhesion strength, which, in practice, is usually determined by the shear strength. The shear strength (\(\sigma\)) of sets created from spruce lamellas and PUR glue was negatively influenced by the primary artificial aging of lamellas at high temperatures of \(\geq\)200 \({}^{\circ}\)C [55], as well as by the presence of inorganic preservatives, namely, CuSO\({}_{4}\times\)5H\({}_{2}\)O, ZnCl\({}_{2}\), H\({}_{3}\)BO\({}_{3}\) and (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\)[12]. In wood treated at high temperatures, there in polysaccharides and lignin are created new substances containing fewer polar -OH groups [43]. Such wood is more hydrophobic, has worse wettability and, finally, also forms weaker adhesion to polar glues. According to [PERSON] et al. [62], a reduction in the shear strength of glued joints created from thermally modified wood can be attributed to (1) a reduction in polar groups in the cell walls of wood due to the degradation of amorphous polysaccharides by heat treatment, resulting in fewer sites available for bonding; (2) the increased stiffness of cell walls after heat treatment, which results in a reduction in internal surfaces for the chemical bonding or mechanical interlocking of adhesives; and (3) a reduction in the wettability of wood, which may retard the proper penetration of glues. The formation of micro-cracks and checks, as well as the decrease in wood density due to heat treatment at temperatures above 180 \({}^{\circ}\)C [63], also might contribute to a decline in the shear strength of bonded wood. A decrease in the shear strength of glued joints for wood impregnated with chemicals can be explained by many factors, e.g., by the reduced penetration of glue into lumens and intercellular spaces in wood filled with chemicals, followed by reduced adhesion, as documented by [PERSON] et al. [64] and [PERSON] [65]. Generally, due to the action of high temperatures or aggressive chemicals with a low or high pH value, the oxidative character of the lignin-saccharide matrix of wood can possibly deteriorate, the individual wood cells can be defibrated and, finally, a decrease in the mechanical properties of wood occurs [40,66]. A decrease in adhesion strength also occurs owing to the presence of new improper, especially hydrophobic, substances on the surfaces of thermally aged wood, as well as owing to the lower penetration of glue into cell lumens filled with preservatives or other chemical substances [67, 68, 69]. Subsequently, a decrease in the mechanical properties of wood and changes in the wood-glue phase interface can negatively affect the strength of glued joints [48,65].