# Parallel-to-Grain Compressive and Tensile Behavior of Paulownia Wood at Elevated Temperatures
## 3 Experimental Results and Discussion
### Temperature-Dependent Strengths and Deformations  
Figure 11a,b show the average parallel-to-grain compressive and tensile strengths of the Paulownia wood at different temperatures. Figure 11c,d show the normalized compressive strength and tensile strength, respectively. As the temperature increased from 20 \({}^{\circ}\)C to 220 \({}^{\circ}\)C, the average compressive strength generally decreased. The average compressive strength of the specimens at 120 \({}^{\circ}\)C was 88% of that at room temperature. Moreover, the average compressive strength increased significantly from 120 \({}^{\circ}\)C to 140 \({}^{\circ}\)C. Consequently, the part of the curve at 140 \({}^{\circ}\)C was almost overlapped by that at 20 \({}^{\circ}\)C, as shown in Figure 10a. As described before, this was mainly attributed to the hardening of the dry lignin of the wood between 120 \({}^{\circ}\)C and 140 \({}^{\circ}\)C. At 220 \({}^{\circ}\)C, the retention rate of the average compressive strength was only 38%. The average tensile strength decreased significantly as the temperature increased from 20 \({}^{\circ}\)C to 100 \({}^{\circ}\)C. When the temperature increased from 100 \({}^{\circ}\)C to 140 \({}^{\circ}\)C, the average tensile strength gradually increased. It is interesting to find that the tensile strength and stiffness increased at a lower temperature (100 \({}^{\circ}\)C) than that (120 \({}^{\circ}\)C) for the compressive strength and stiffness. This can be explained by the free water in the tensile specimen evaporating faster than the compressive specimen, since the tensile specimen is much thinner than the compressive specimen. Hence, the hardening degree of the dry lignin of the tensile specimen is greater than the compressive specimen. At 140 \({}^{\circ}\)C and above, the average tensile strength gradually decreased due to the degradation of the polymer components of the wood. The average tensile strength at 220 \({}^{\circ}\)C was 42% of that at room temperature. Table 3 summarizes the average compressive strength and tensile strength, as well as the corresponding retention rate. Figure 10: Typical stress-displacement curves: (**a**) compression and (**b**) tension. The average parallel-to-grain compressive and tensile strengths of the Douglas-fir wood and Larch wood at elevated temperatures from 20 \({}^{\circ}\)C to 250 \({}^{\circ}\)C are also depicted in Figure 11a,b, respectively. At each target temperature, the average compressive (tensile) strength of the Larch wood was the highest, while that of the Paulownia wood was the lowest. This can be understood because the density of the fast-grown Paulownia wood (353 kg/m\({}^{3}\)) is much lower than the Douglas-fir wood (486 kg/m\({}^{3}\)) and Larch wood (596 kg/m\({}^{3}\)). According to EN 1995-1-2, the normalized parallel-to-grain compressive strength \(P_{\rm c}\) and the tensile strength \(P_{\rm t}\) can be calculated as shown in Equations (1) and (2), respectively:  
\[P_{\rm c}=\left\{ \begin{array}{llll}-\frac{3}{320}\text{T}+\frac{19}{16}& \text{if 20 }^{\circ}\text{C}\ \leq\ \text{T}\ \leq\ 100\ ^{\circ}\text{C}\\ -\frac{1}{800}\text{T}+\frac{1}{8}&\text{if 100 }^{\circ}\text{C}\ <\ \text{T}\ \leq\ 300\ ^{\circ}\text{C}\end{array} \right. \tag{1}\]  
Figure 11: Strength degradation of the wood samples [10,20,21]. (a) compression; (b) tension; (c) normalized compressive strength; and (d) normalized tensile strength. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & \multicolumn{2}{c}{**Compressive**} & \multirow{2}{*}{**Tensile Strength**} & \multicolumn{2}{c}{**Compressive**} & \multirow{2}{*}{**Tensile Deformation**} & \multirow{2}{*}{**Tensile Deformation**} \\ \cline{2-3} \cline{5-8}  & **Strength** & & & & & & **Deformation** & & \\ \cline{2-3} \cline{5-8}  & **(kN)** & **(\%)** & **(kN)** & **(\%)** & **(mm)** & **(\%)** & **(mm)** & **(\%)** \\ \hline
20 & 24.5 & 100 & 43.6 & 100 & 1.27 & 100 & 1.78 & 100 \\
60 & 21.8 & 89 & 33.4 & 77 & 1.01 & 80 & 1.40 & 78 \\
100 & 21.5 & 88 & 2