## 3 Results and Discussion
### Board Properties  
Table 4 shows mean, standard deviation and coefficient of variation (CoV) values for various properties of control and TM boards tested directly, and before and after exposure. Average board density (\(\rho\)) was approximately 460 kgm\({}^{-3}\) for control boards and 420 kgm\({}^{-3}\) for TM boards. Moisture content (MC) at the time of testing was significantly different after exposure compared to boards tested directly or before exposure for both control and TM boards, whereas all boards were stored under similar conditions prior to testing. These differences were not larger than approximately 0.5% for control boards, but almost 2% for TM boards. The reason for this may be the increase in equilibrium moisture content (EMC) of thermally modified wood after exposure to high humidity levels [1]. Mean values of modulus of elasticity (MOE) and bending strength (\(f_{\rm m}\)) ranged between 10-13 GPa and 41-43 MPa for control boards and 10-13 GPa and 23-25 MPa for TM boards, respectively (Table 4). The levels and variation of \(\rho\), MOE and \(f_{\rm m}\) of control boards shown in Table 4 are typical for Norway spruce timber coming from Sweden at 12% MC [70; 71; 72]. Treatment has a significant effect on mean \(\rho\) and \(f_{\rm m}\) of timber (independent \(t\)-test, \(\alpha=0.05\)), which are approximately 8-10% and 40-45% lower for Thermo-D spruce timber compared to unmodified spruce timber (Table 4). Similar reductions in material properties were obtained by others for thermally modified timber of spruce, pine and beech [71; 73]. The non-destructive test (NDT) and 4-point static bending (4 PB) properties of the 100 control and 100 TM boards tested 'directly' (see again Figure 1) were compared in detail by [PERSON] et al. [21; 22], and was not within the scope of the present study. With exception of acoustic velocity from Tof (\(v_{\rm a,res}\)), no statistical differences in NDT properties were found between boards tested directly and before exposure for both control and TM boards (Table 4). That is, sample sets 'direct' and 'exposed' were comparable before weathering.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Treatment** \\ **Exposure** \\ **Interval** \\ \end{tabular} } & \multicolumn{2}{c}{**Control**} & \multicolumn{2}{c}{**TM**} \\ \cline{2-6}  & **Direct** & \multicolumn{2}{c}{**Exposed**} & **Direct** & \multicolumn{2}{c}{**Exposed**} \\ \cline{2-6}  & **Direct** & \multicolumn{2}{c}{**Exposed**} & **Direct** & \multicolumn{2}{c}{**Exposed**} \\ \cline{2-6}  & **Before** & **After** & \multicolumn{2}{c}{**–**} & **Before** & **After** \\ \hline \hline \multicolumn{6}{c}{NON-DESTRUCTIVE TESTS} \\ \hline \multirow{2}{*}{\(\rho\) (kgm\({}^{-3}\))} & **463** & **457** & **457** & **422** & **417 a** & **422 a** \\  & 39 & 39 & 39 & 33 & 35 & 36 \\  & 8\% & 9\% & 9\% & 8\% & 8\% & 9\% \\ \hline \multirow{2}{*}{\(v_{\rm a,tof}\) (ms\({}^{-1}\))} & **5560 b** & **5682 a,b** & **5665 a** & **5735 b** & **5852 a,b** & **5755 a** \\  & 231 & 265\% & 270 & 258 & 279 & 272 \\  & 4\% & 5\% & 5\% & 5\% & 5\% & 5\% \\ \hline \multirow{2}{*}{\(E_{\rm a,tof}\) (MPa)} & **14,374** & **14,786 a** & **14,723 a** & **13,929** & **14,356 a** & **14,029 a** \\  & 1997 & 2079 & 2117 & 1916 & 2047 & 1976 \\  & 14\% & 14\% & 14\% & 14\% & 14\% & 14\% \\ \hline \multirow{2}{*}{\(v_{\rm a,res}\) (ms\({}^{-1}\))} & **5208** & **5222 a** & **5211 a** & **5384** & **5384 a** & **5291 a** \\  & 254 & 271 & 296 & 293 & 319 & 313 \\ \cline{1-1}  & 5\% & 5\% & 6\% & 5\% & 6\% & 6\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Mean value (upper and bold), standard deviation (middle) and coefficient of variation (lower) for various properties of control and TM boards tested directly, and before and after exposure.