The effect of weathering on the surface moisture conditions of Norway spruce under outdoor exposure

Understanding wood surface moisture variations is fundamental to the modelling of moisture content. Exposure to sunlight, fluctuating temperature and rain leads to superficial deterioration and reduced surface hydrophobicity. Here, the effect of weathering was studied through monitoring the surface and global moisture content of axially matched pre-weathered and planed sets of specimens exposed outdoors over 18 months. The difference in surface conditions was characterised by analysing the rain-induced peaks. The results suggest that, following precipitation, weathered surfaces remain wet over a longer time period. No effect was observed through the global moisture content. After one year of exposure, the difference between pre-weathered and planed surfaces was small to insignificant. In service life modelling, it is therefore unnecessary to consider the unweathered state and simulations should target the behaviour of weathered wood. Numerical simulations were able to capture the general behaviour of the surface and global moisture content, but discrepancies over individual peaks were observed. Finally, the study demonstrates how peak analysis and signal analysis can be used for isolating subtle differences between time-series of surface moisture content. The same techniques can be used in future studies to characterise other factors influencing surface conditions, such as wood species and detailing.