The Potential of Producing High Added Value Structural Timber from Lamellae Waste. Classification and Visual Grading

This study is the continuation of the first part (Horváth et al. 2023), in which density, bending strength and modulus of elasticity of 100 oak lamellae generated as small-sized production waste were investigated. In this part of the study series, the classification of our sample set is carried out according to the EN 338. A visual pre-grading is presented, to remove the worst specimens and achieve a better final result and a standardized visual grading is also shown. Our results are compared with literature values of clear specimens as well. Amount of 80% of the specimens were found to be suitable for further structural use. The total sample set is classified in strength class D35 (average density 712.6±72.5 kg/m3; average MoRadj 65.4±16.2 MPa; average MoEadj 13.4±3.1 GPa), while the visually pre-graded part with better average test results is classified in strength class D45. Industrial wood residues contain a sufficiently high proportion of elements for further processing, so that after sorting it can be used as raw material for glued-laminated load-bearing timber.

The potential of producing high added value structural timber from lamellae waste. Test results and analysis

The research was based on the analysis of the density, bending strength and modulus of elasticity of 100 oak lamellae generated as small-sized production waste. In this part of the study series, the test results were presented in detail and analysed, in particularly the density distribution. Correlations between some test results have been shown. The dynamic and static test results were also compared. Despite the poor quality lamellae, the average density of the sample set corresponds to literary values and the distribution of density is normal. Specimens with low density are unsuitable for further use. But the density alone cannot be used for classification. Between static and dynamic modulus of elasticity can be found a good relationship. The relationships between density and both static and dynamic modulus of elasticity of the specimens can be considered as good, too. The best correlation is in bending tests between the deflection of the specimens in the elastic range and the bending strength.

The effect of the relaxation time on the mechanical properties of longitudinally compressed wood

Longitudinal compression makes natural wood easier to bend. The relaxation after compression results in much improved bending properties. During a bending test, the maximum deflection increases with the relaxation time, while the needed force to reach the same deflection decreases, similarly to the modulus of elasticity (MoE). The modulus of rupture (MoR) of the compressed wood does not change considerably compared to the untreated wood, except at the long-time relaxed samples. The ideal relaxation time is 1 minute. After that the change of the important properties slows down. Of course with special demands, the relaxation time can be also very long. In this case the process leads to a wood sample with pronounced flexible properties. Samples were left to rest between normal circumstances for 1 day, but this resting period did not have a significant effect on its mechanical properties.