## 3 Discussion
### FT-IR Spectroscopy  
It is generally accepted that the rate of chemical reactions in solid polymers may change significantly under the influence of external or internal stresses. On one hand, the rate may change due to substantial changes in the structural and physical parameters of a polymer subjected to the action of mechanical stresses (molecular conformation, free volume in the polymer, changes in the permeability and diffusion of the low-molecular mass substances). On the other hand, stresses may directly affect the reactivity of deformed macromolecules, thereby altering the effective activation energies for chemical reactions [37]. Furthermore, the high testing temperatures may induce morphological changes in the polymeric product, which can affect the kinetics of oxidative degradation, leading to the formation of hydroxyls, carbonyl and carboxylic groups, ethers, peroxides and hydroperoxides. FT-IR spectroscopy was used to identify oxygen bearing groups, which may be formed during localized ageing processes in the immediate environment of slowly growing cracks in the HDPE under static loads (Figure 7). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample Number**} & \multicolumn{3}{c}{**Sample before Installing (Virgin)**} & \multirow{2}{*}{**Samples 20 Years after Installation (Aged)**} \\ \cline{2-2}  & \multicolumn{2}{c}{**(Declared Strength 55 kN/m)**} & \\ \cline{2-2}  & \multicolumn{2}{c}{**Mean Tensile**} & \multicolumn{2}{c}{**Mean Strain at**} & \multicolumn{2}{c}{**Mean Tensile**} & \multicolumn{2}{c}{**Mean Strain at**} \\  & \multicolumn{2}{c}{**Strength (kN/m)**} & \multicolumn{2}{c}{**Maximum Load (\%)**} & \multicolumn{2}{c}{**Strength (kN/m)**} & \multicolumn{2}{c}{**Maximum Load (\%)**} \\ \hline
1 & 60.77 & 9.55 & 52.18 & 7.41 \\
2 & 61.68 & 9.80 & 46.74 & 5.69 \\
3 & 60.79 & 9.32 & 50.48 & 6.68 \\
4 & 61.45 & 10.07 & 52.55 & 6.92 \\
5 & 60.28 & 9.66 & 42.63 & 5.28 \\ Mean & 60.99 & 9.68 & 48.92 & 6.4 Figure 7 shows the FT-IR spectra of new (virgin) and aged HDPE geogrids. For both materials (geogrid \"virgin\" and geogrid \"Radiowo\"), three absorption bands (characteristic of polyethylene) related to the vibrational modes of the C-H bond can be observed, namely, C-H stretching (2950-2850 cm\({}^{-1}\)), C-H bending (1350-1450 cm\({}^{-1}\)) and C-H rocking (near 700 cm\({}^{-1}\)). Besides the aforementioned C-H absorption bands, the FT-IR spectrum of the HDPE geogrid \"Radiowo\" shows an absorption band at approximately 1030 cm\({}^{-1}\). In Figure 8, the FT-IR spectra of the extended region 2750-3000 cm\({}^{-1}\) clearly show that there are no visible differences in the virgin and aged geogrid samples. In Figure 9a shows the spectra in the region of 800-1350 cm\({}^{-1}\) for the aged geogrid samples. Since these spectra were collected in reflective mode, the topmost spectrum shows the composition of the topmost layer of an aged geogrid sample. Subsequent spectra were collected downward from the topmost layers of the studied geogrids, which were removed systematically with a cutting knife. A decrease in the intensity of the band can be observed at approximately 1050 cm\({}^{-1}\), which can be attributed to the Si-O bond present in a typical fine sand (Figure 9b). These spectra suggest the slow penetration of fine sand particles into the geogrid polymer structure. After removal of several layers, a small absorption band at approximately 1150 cm\({}^{-1}\) can still be observed, which can be attributed to silica added as a filler to the HDPE matrix. Figure 8: Collected FT-IR spectra (2750–3000 cm\({}^{-1}\) region) of virgin geogrid samples and aged geogrid samples (Radiowo). Absorption bands originating from the oxidation of the HDPE matrix were not observed. Peaks from the OH groups (approximately 3300-3500 cm\({}^{-1}\)) and strong absorptions from the carbonyl C = O bonds (1700-1750 cm\({}^{-1}\) region) were not detected.