Moisture conditions in rain exposed wood joints - Experimental methods and laboratory measurements

Sammanfattning: This thesis concerns the moisture conditions in rain exposed wood structures, i.e. wood exposed to high moisture levels. The focus was on the microclimate (the climate at the wood surface) and the material climate (the wood moisture content) in joints. Methods for determination of microclimate, i.e. the duration of moisture on a wooden surface and the duration of water trapped in gaps between two boards, as well as a method for moisture content determinations close to surfaces and joints were developed. In addition, a theoretical study was performed to investigate the influence of the distance to a surface and specimen size on resistive moisture content measurements.
The relationship between microclimate and moisture content in wood joints was studied by exposing three different types of joints to artificial rain in the laboratory. The microclimate was varied by varying the size of the gap between the two boards. The measurements were performed on Norway spruce (Picea abies (L.) Karst.) heartwood and sapwood. Both slow grown wood from northern Sweden and fast grown wood from southern Sweden were used in the experiments. The duration of high moisture contents was evaluated for all joints. In addition, the measured moisture contents were used to evaluate differences in expected service life for the different joints using three decay models from the literature. End grain water absorption was also studied separately using computed tomography.
Modelling of moisture transport at high moisture levels requires knowledge of sorption properties also in the high moisture range. However, since the method used in the high moisture range was originally designed for desorption experiments there is a lack of absorption isotherm data in the high moisture range. A method for absorption experiments based on the pressure plate technique was therefore developed. Measurements were performed at one pressure level for Norway spruce sapwood and the results were compared to results from absorption experiments at lower relative humidity levels as well as a pressure plate desorption experiment.