## 3 Results and Discussion
### Board Properties
#### 3.2.1 Non-Destructive Tests  
Mean values of axial dynamic elastic moduli (\(E_{\text{a}}\)) and moisture content (MC) of 'exposed' control and TM boards are shown in Figure 8 for the measurement intervals. During weathering, levels of average MC were considerably higher and ranged between 13-23% for control boards and 8-12% for TM boards. These variations between spring and autumn measurements were smaller for TM than control boards, due to the improved hygroscopic properties of Thermo-D spruce wood [1; 12]. Mean values of \(E_{\text{a}}\) for both sample sets were between 13.5-15 and 11-12.5 GPa for Tof and resonance-based measurements, respectively (Figure 8). This difference in level of \(E_{\text{a}}\) between dynamic test methods is in line with the literature, which reports that axial dynamic stiffness in timber is typically overestimated 10-20% by Tof [25; 74], and previous test results on 100 control and 100 TM boards tested 'directly' [22]. Levels of mean \(E_{\text{a}}\) and MC of control and TM boards were inversely related (Figure 8a), and are consistent with established \(E_{\text{a}}\)-MC relationships for unmodified timber [33; 75; 76]. Significant differences in mean values of \(E_{\text{a,tof}}\) and \(E_{\text{a,res}}\) were found between measurement intervals for control boards (one-way ANOVA, p \(<\) 0.001), but not for TM boards (one-way ANOVA, \(p\)\(>\) 0.05) (Figure 8b). After adjusting dynamic elastic moduli of control boards to 12% MC (Figure 8c), no significant differences in mean \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Treatment** & \multicolumn{3}{c}{**Control**} & \multicolumn{3}{c}{**TM**} \\ \cline{2-7}
**Exposure** & **Direct** & \multicolumn{2}{c}{**Exposed**} & **Direct** & \multicolumn{2}{c}{**Exposed**} \\ \cline{2-7}
**Interval** & **–** & **Before** & **After** & **–** & **Before** & **After** \\ \hline \multicolumn{7}{c}{NON-DESTRUCTIVE TESTS} \\ \hline \multirow{3}{*}{\(E_{\text{a,res}}\) (MPa)} & **12,623** & **12,523** & **12,488** & **12,293** & **12,173 a ** & **11,880 a ** \\  & 1864 & 1975 & 2051 & 1876 & 1995 & 1908 \\  & 15\% & 16\% & 16\% & 15\% & 16\% & 16\% \\ \hline \multicolumn{7}{c}{BENDING TESTS} \\ \hline \multirow{3}{*}{MC (\%)} & **12.9 c** & **13.5 a ** & **12.3 a,c**  & **4.7 c** & **5.5 a ** & **7.0 a,c** ** \\  & 0.7 c  & 0.4 & 0.3 c  & 0.6 & 0.4 & 0.5 \\  & 5\% & 3\% & 2\% & 12\% & 7\% & 7\% \\ \hline \multirow{3}{*}{\(f_{\text{m}}\) (MPa)1  & & **23.1 C**  \\  & 10.7 & & 12.1 & 6.9 & \multirow{3}{*}{N.A.} & **6.5** \\  & 24\% & & 27\% & 28\% & & 28\% \\ \hline \multirow{3}{*}{\(t\) (s)} & **211** & \multirow{3}{*}{N.A.} & **212** & **145 c**  & & **133 c  \\  & 39 & & 43 & 28 & \multirow{3}{*}{N.A.} & **24** \\  & 18\% & & 20\% & 19\% & & 18\% \\ \hline \hline \end{tabular}  
* \({}^{1}\) 5 th percentile value of bending strength (\(f_{\text{m,05}}\)) given in parenthesis. a,a,a Significance exposed before vs. after (dependent \(t\)-test). b,b Significance direct vs. exposed before (F-test, independent \(t\)-test). c,c,c Significance direct vs. exposed after (F-test, independent \(t\)-test). Significance levels: lower case letters \(p\)\(<\) 0.01 (e.g., a), Greek letters \(p\)\(<\) 0.05 (e.g., a), upper case letters \(p\)\(<\) 0.10 (e.g., A).  
\end{table} 
Table 4: _Cont._values of \(E_{\text{a,res,12}}\) and \(E_{\text{a,tof,12}}\) between measurement intervals were found (one-way ANOVA, \(p>0.05\)). Adjustment factors for TMT have not been established, but it has been shown that the influence of MC on static bending properties is less for TM small clear wood specimens compared to unmodified reference specimens [77]. This is consistent with smaller fluctuations of \(E_{\text{a,res}}\) and \(E_{\text{a,tof}}\) for TM boards compared to control boards, and no other trends were observed in Figure 8. Overall, the axial dynamic stiffness of TM and unmodified spruce timber did not change during 30 months of weathering in South Sweden.