The free drying shrinkage characteristics of Moso bamboo

Bamboo shrinkage is influenced by its structural characteristics and drying stress. Understanding the free shrinkage characteristics of bamboo is essential for analyzing stress variation patterns and mechanisms during the drying process, which can help minimize drying defects. In this study, small tangential and radial layered bamboo bars were allowed to shrink freely at 25 degrees C, while sliced specimens experienced restricted shrinkage. Under low relative humidity (RH) conditions (< 90 %), there were no moisture content (MC) gradients between the layered specimens or within bamboo slices, preventing new drying stress. Early-stage residual drying stress might mitigate subsequent stress development. The restricted shrinkage observed in the tangential direction of the bamboo slices was attributed to gradient structural changes in the tangential tissue, leading to differences in shrinkage. For tangential layered specimens, the gradients of the tangential free shrinkage coefficient (K) and free absolute dry shrinkage ratio (ymax) decreased from the outer layer (30.03, 5.84 %) to the inner layer (12.86, 2.90 %). In contrast, the K and ymax values for the radial layered specimens were nearly identical, with no significant differences (p > 0.05), averaging 13.21 and 2.72 %, respectively. The fiber saturation point (FSP) values derived from both the tangential and radial layered specimens, as well as the sliced specimens, showed no significant differences (p > 0.05), with an average value of 20.51 %, despite the variation in shrinkage across the tangential layers.