Plywood is an important construction material yet prone to water uptake, which can decrease strength and increase decay risk. To predict service life and improve fit-for-purpose design, it is crucial to understand the moisture behavior and structural changes of plywood. In this research, several plywood specimens were exposed outdoors for approximately one year. During this period, the moisture distribution in different layers of the exposed plywood specimens was monitored continuously and detailed field weather information was collected simultaneously. The internal structure of the specimens was also investigated by periodically scanning using 3D X-ray CT. The moisture distribution throughout the different plies is not always homogeneous. The second layer can accumulate a significant amount of water in outdoor weathering conditions, giving rise to high Time of Wetness (TOW) and long rainfall events can keep wetting the inner layers of plywood. TOW, moisture dynamics and wood species used are the main factors causing structural changes of the plywood veneers in service mainly occurring as cracks. Most internal structural changes were found in the second veneers of plywood specimens. The glue line between veneers can hardly be ruptured after exposing outdoors for one year. Plywood with veneers showing a slow water sorption and fast water desorption could effectively avoid internal moisture accumulation and cracks in service. Based on the knowledge of the relationship among weathering data, internal moisture behavior and structural changes in service, the dedicated plywood could be designed by optimizing the characteristics, i.e. veneer wood species, veneer thickness, glue type and such. The above knowledge could also contribute to the service life prediction of plywood.

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