In the study, we assessed the condition of 111 wooden piles made of different wood species (spruce, fir, red pine and sweet chestnut) and protective treatments (impregnation with Silvanolin and thermal modification) after 54 months of exposure in the 4th use class. The piles were exposed at three different locations - in Ljubljana and in Bilje, in a orchard field with irrigation and in a field without irrigation. The results showed that after 54 months, all chestnut piles and piles impregnated with the copper-ethanolamine preparation Silvanolin were completely preserved, while the control piles made of fir, spruce and red pine mostly failed. Protection with thermal modification proved to be ineffective, as almost 30% of the piles failed. Exposure location had a significant influence on the results—the highest number of failed piles was recorded in the irrigated field, where conditions for fungal decay were most favorable. Measurements of density, mechanical properties and proportion of decayed cross-section confirmed that microlocation (bottom of the pile) is a key factor for the rate and efficiency of decay. It was concluded that chestnut wood and wood treated with Silvanolin exhibit sufficient durability for use in demanding Use Class 4 conditions.
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