## 4 Conclusions  
Thirty months of weathering had no significant influence on the static bending properties of thermally modified timber (TMT), but the number and size of checks were higher/greater in TMT than unmodified timber after weathering. In particular, checks along growth rings were deeper, longer and more common in TMT after weathering, especially on the pH side of boards. The maximum depth of these checks did not depend on board orientation (i.e., which side was exposed) and exceeded limits given in strength grading standards for 7% of the modified boards included in this study. On the bark side, checking occurred mainly across growth rings and was more common in TMT, but the depth was similar to unmodified timber and the area with checks smaller than in unmodified timber, especially in the board's centre. Axial dynamic stiffness determined at time intervals did not confirm the increase in the degree of checking of TMT. Dynamic stiffness was more stable over time for TMT compared to unmodified timber, because the variation in moisture content over time was smaller for TMT as well. The presence of checks from weathering did influence failure modes in TMT; horizontal shear failure \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{**Treatment**} & \multicolumn{2}{c}{**Control**} & \multicolumn{2}{c}{**TM**} \\ \cline{3-6} \multicolumn{2}{c}{**Bending Property**} & \multicolumn{1}{c}{\(E_{\rm m,g}\)} & \multicolumn{1}{c}{\(f_{\rm m}\)} & \multicolumn{1}{c}{\(E_{\rm m,g}\)} & \multicolumn{1}{c}{\(f_{\rm m}\)} \\ \hline \multirow{4}{*}{\(\rho\)} & direct & \(0.56\pm 0.14\) & \(0.16\pm 0.14\) & \(0.50\pm 0.15\) & \(0.17\pm 0.14\) \\  & exposed 2 & \(0.53\pm 0.14\) & \(0.35\pm 0.16\) & \(0.40\pm 0.16\) & \(0.10\pm 0.12\) \\  & exposed & \(0.50\pm 0.15\) & \(0.32\pm 0.16\) & \(0.39\pm 0.16\) & \(0.12\pm 0.13\) \\  & direct & \(0.80\pm 0.07\) & \(0.36\pm 0.16\) & \(0.82\pm 0.09\) & \(0.44\pm 0.15\) \\ \multirow{4}{*}{\(E_{\rm a,def}\)} & exposed 2 & \(0.79\pm 0.08\) & \(0.61\pm 0.13\) & \(0.80\pm 0.07\) & \(0.38\pm 0.16\) \\  & exposed & \(0.77\pm 0.08\) & \(0.58\pm 0.13\) & \(0.79\pm 0.08\) & \(0.36\pm 0.16\) \\ \multirow{4}{*}{\(E_{\rm a,res}\)} & directexposed 2 & \(0.87\pm 0.05\) & \(0.51\pm 0.15\) & \(0.90\pm 0.04\) & \(0.49\pm 0.15\) \\  & exposed & \(0.88\pm 0.05\) & \(0.70\pm 0.10\) & \(0.88\pm 0.05\) & \(0.46\pm 0.15\) \\ \cline{1-1} \cline{2-6}  & \multicolumn{1}{c}{} & \(0.88\pm 0.05\) & \(0.69\pm 0.11\) & \(0.87\pm 0.05\) & \(0.45\pm 0.15\) \\ \hline \hline \end{tabular}  
* \(R^{2}\)-values are significantly different from zero (F-test, p \(<\) 0.001). 2 NDT property was determined on ‘exposed’ control and TM boards before weathering.  
\end{table} 
Table 6: Coefficient of determination (\(R^{2}\)) incl. lower and upper bound at 95% confidence interval of relationships between static bending and non-destructive test (NDT) properties of control and TM boards tested directly and after exposure. 1. Figure 12: Relationships between static bending properties and dynamic elastic modulus of control and TM boards directly tested and after exposure: (**a**) \(E_{\rm a,res}\)–\(E_{\rm m,g}\) and (**b**) \(E_{\rm a,res}\)–\(f_{\rm m}\). became more frequent and some boards failed in compression. Longer exposure periods are required for accurate service-life predictions. Effects of checks on transverse dynamic stiffness, shear strength and the capacity of connections (at timber ends) of TMT, and possibilities for classifying/predicting the degree of checking of TMT based on raw material characteristics should be further investigated. Conceptualisation, J.v.B. and S.A.; Methodology, J.v.B. and S.A.; Software, J.v.B. ; Validation, J.v.B. ; Formal Analysis, J.v.B. ; Investigation--Laboratory work, J.v.B. ; Investigation - Field work, J.v.B. and S.A.A. assisted; Resources, S.A.; Data curation, J.v.B. ; Writing--Original Draft Preparation, J.v.B. ; Writing--Review and Editing, J.v.B., S.A. and S.A.A. ; Visualization, J.v.B. ; Supervision, S.A.; Project Administration, J.v.B.