Impregnation of railway sleepers - Process optimisation by using an oily wood preservative and a mechanical pre-treatment
von Michael Starck
Datum der mündl. Prüfung:2021-11-10
Erschienen:2022-05-31
Betreuer:Prof. Dr. Holger Militz
Gutachter:Prof. Dr. Holger Militz
Gutachter:Dr. Markus PD. Euring
Gutachter:PD Dr. Christian Brischke
Gutachter:Prof. Dr. Kai Zhang
Dateien
Name:Dissertation_Michael_Starck_final_ediss_no_CV.pdf
Size:6.28Mb
Format:PDF
Zusammenfassung
Englisch
The use of creosote as a wood preservative has a long history. For more than 170 years wooden railway sleepers are impregnated with creosote. After initially using vacuum pressure processes for impregnation, empty cell processes have been developed quickly and are used until today. Because of political developments and its alarming properties against human health and environment, creosote will probably be banned from the European market in the near future. Today, most of the sleepers in track are concrete sleepers, but wooden sleepers are still essential for particular applications such as tracks with narrow curve radii, mountain tracks with uneven underground conditions and low ballast bed thicknesses, for switches, for railway bridges and for shunting stations. Without a successor product, wooden sleepers have to be installed either again without adequate protection against wood destroying organisms or will possibly be replaced by sleepers made from alternative materials like concrete, steel or polymers. To ensure the use of wooden sleepers as part of the track superstructure after a possible ban of creosote, two different types of process optimisation for sleepers made from European beech (Fagus sylvatica L.) have been evaluated regarding their potential. The first type of process optimisation was carried out by a mechanical pre-treatment namely incising. It was evaluated regarding its influence on seasoning speed, check formation and dimensional stability. Incising reduced the formation of checks in length, width and depths for sleepers made from European beech until they were sufficiently seasoned. No decrease in seasoning duration as well as no influence on the moisture distribution inside the sleeper was shown due to incising. An increased dimensional stability during seasoning was also not achieved. The second type of process optimisation focused on reaching a complete penetration of the sleeper cross section using an alternative oily wood preservative as possible replacement for creosote. The penetration behaviour of different alternative oily products after pressure impregnation is hardly investigated so far. Therefore, research has been carried out in regard of hydrophobic carrier substances and their viscosity and penetration behaviour for Beech sleepers. By increasing the temperature, the investigated hydrophobic carrier substances showed decreasing viscosities, also with the addition of copper hydroxide and a co-biocide as biocidal components. A macroscopically investigation of the penetration behaviour of three hydrophobic carrier substances in Beech and Scots pine sapwood (Pinus sylvestris L.) showed incomplete penetration of the cross section for Beech and complete penetration of the cross section for Scots pine sapwood, when end grains were sealed.A microscopic analysis of two hydrophobic carrier substances showed similar penetration pathways compared to creosote. For Beech, most of the penetration of the hydrophobic carrier substances took place in longitudinal direction via vessels and fibers, whereas less penetration seemed to occur within the wood rays. For Scots pine sapwood, penetration in longitudinal direction took place through the lumen of the late- and earlywood, but also through resin canals. The impregnation of European beech using an alternative oily wood preservative did not result in complete penetration of the specimen cross section, using empty cell and full cell processes. In case of empty cell processes, retentions were de- and increased by variations of air pressure intensity and fluid pressure duration. However increased retentions did not improve the penetration depth. Modified versions of the empty cell process including pre-heating of the specimens did also not improve preservative penetration at comparable retentions. Measurements regarding pressure gradients during the impregnation of Beech sleepers were carried out, excluding a possible negative influence of compressed air as cause for the insufficient penetration of the alternative oily wood preservative. However, a substantial influence of the longitudinal direction on the air pressure distribution (up to 700 mm) became apparent. Furthermore, a delay in pressure distribution in transversal direction was also observed. Additional evaluations regarding the influence of the three anatomical directions on the preservative penetration, showed that preservative penetration is considerably more effective in axial direction during an applied fluid pressure. To take advantage of the axial preservative penetration, the existing drilling pattern, which improves the penetration of creosote in Beech sleepers, was adapted to the penetration behaviour of the alternative oily wood preservative. Although, results showed a considerable improvement in preservative penetration, a complete penetration of the cross section was not reached. Further process optimisations using water-based, chromium-free and copper-based preservatives resulted in complete penetration of the sleeper cross section even without attached drilling pattern and incising. Hereby, the insufficient penetration of the sleeper cross section by the alternative oily wood preservative was compensated. Therefore, the potential of being the first step of a double impregnation in combination with the alternative oily wood preservative became apparent. During the double impregnation a positive effect of incising on retention and -penetration of the alternative oily wood preservative was occurring in the area of the incision. Furthermore, it became evident, that a gross weight of at least 950 kg/m³ was needed for penetrating the peripheral area of the sleepers, to achieve an additional homogenous envelope treatment by the alternative oily wood preservative. Both types of process optimisation showed great potential for improving the properties of Beech sleepers and ensuring the use of wooden sleepers even beyond a possible ban of creosote.
Keywords: wood preservation; railway sleepers; creosote; oily wood preservatives; double impregnation; incising; check formation; moisture content monitoring