If you mean for railway construction, that's often the case, but sleepers are still used in some places today. I saw brand new ones being laid in the ground 2 years ago at Göteborg C.Tombrittberg said:
There is also a difference between load-bearing and non-load-bearing parts. Load-bearing parts cannot/should not lose their function, which they do if they rot away. Load-bearing parts also tend to be the ones that are most difficult to replace and often more or less hidden, so any damage is not as easy to detect in time.
Timber quality is crucial for moisture resistance. Densely grown pine with extremely small growth rings means that the wood is packed (naturally impregnated) with the wood's own moisture protection of resins and other substances. (Those found in tar, etc.) In the past, one could also go into the forest and select suitable trees several years in advance and damage them in the bark mechanically or with fire so that they produce large amounts of resin as a healing effect, which provides the internal moisture protection desired. Such wood is called "fetved." It was used for the most extremely moisture-exposed parts, such as window frames, doors, thresholds, sills, etc.
Further south in the country, the trees (conifers) grow too quickly to have good built-in moisture resistance. However, there was/is oak, which is the most moisture-resistant timber that grows in Sweden. (For oak timber, like pine, the moisture-resistant timber belongs to the heartwood. The sapwood is useless from a moisture perspective.)
My impression is that larch grown here in Skåne does not become super wood. Larch requires exactly the same conditions as pine to become good wood, i.e., grow slowly, which it does in very cold climates and on poor soils such as northern Norrland and Siberia. For the forest owner, it is, however, not profitable to have to wait 100-200 years to get a tree ready for harvest. Here at the southern end of the country, conifers are ready for harvest after about 40 years, but then you get balsa wood that is only suitable for pulpwood.
Today's industrial forestry does not consider timber quality. Therefore, you must assume that all standard wood you find in the lumberyard is trash wood in terms of moisture resistance. The next more moisture-resistant alternative is pressure-treated wood. It is also standard wood but treated to withstand moisture a little better. It is cheaper/more profitable to do so than to manually select good wood at the root and fell, saw, and dry it adapted to become really quality wood.
Good heartwood of pine in a well-considered dimension is as good as pressure-treated wood.
Heart oak in a well or very well-considered dimension is better. There you could probably get up to 100 years (untreated and exposed to moisture). I have myself found old fence posts, about 50x50 mm, in oak around our pastures in good condition. They have thus been lying on the ground for at least the 20 years we have lived here, and reasonably stood as fence posts/poles for at least 20 years before since the previous owner also had horses/pastures here. The posts have more or less rotted off at the ground surface, but the part (about 1 m) that was above ground is only gray about 2-3 mm on the surface. Underneath, the wood is completely healthy. However, around knots and cracks, the wood is also a little affected.
Timber quality is crucial for moisture resistance. Densely grown pine with extremely small growth rings means that the wood is packed (naturally impregnated) with the wood's own moisture protection of resins and other substances. (Those found in tar, etc.) In the past, one could also go into the forest and select suitable trees several years in advance and damage them in the bark mechanically or with fire so that they produce large amounts of resin as a healing effect, which provides the internal moisture protection desired. Such wood is called "fetved." It was used for the most extremely moisture-exposed parts, such as window frames, doors, thresholds, sills, etc.
Further south in the country, the trees (conifers) grow too quickly to have good built-in moisture resistance. However, there was/is oak, which is the most moisture-resistant timber that grows in Sweden. (For oak timber, like pine, the moisture-resistant timber belongs to the heartwood. The sapwood is useless from a moisture perspective.)
My impression is that larch grown here in Skåne does not become super wood. Larch requires exactly the same conditions as pine to become good wood, i.e., grow slowly, which it does in very cold climates and on poor soils such as northern Norrland and Siberia. For the forest owner, it is, however, not profitable to have to wait 100-200 years to get a tree ready for harvest. Here at the southern end of the country, conifers are ready for harvest after about 40 years, but then you get balsa wood that is only suitable for pulpwood.
Today's industrial forestry does not consider timber quality. Therefore, you must assume that all standard wood you find in the lumberyard is trash wood in terms of moisture resistance. The next more moisture-resistant alternative is pressure-treated wood. It is also standard wood but treated to withstand moisture a little better. It is cheaper/more profitable to do so than to manually select good wood at the root and fell, saw, and dry it adapted to become really quality wood.
Good heartwood of pine in a well-considered dimension is as good as pressure-treated wood.
Heart oak in a well or very well-considered dimension is better. There you could probably get up to 100 years (untreated and exposed to moisture). I have myself found old fence posts, about 50x50 mm, in oak around our pastures in good condition. They have thus been lying on the ground for at least the 20 years we have lived here, and reasonably stood as fence posts/poles for at least 20 years before since the previous owner also had horses/pastures here. The posts have more or less rotted off at the ground surface, but the part (about 1 m) that was above ground is only gray about 2-3 mm on the surface. Underneath, the wood is completely healthy. However, around knots and cracks, the wood is also a little affected.