Hygroexpansion behaviors of bamboo in response to moisture absorption and desorption

Abstract

As a natural cellulosic material, bamboo is hygroscopic. Its dimension changes in response to moisture variations. The purpose of this study aimed to investigate the variations of hygro-expansion, moisture absorption and desorption behaviors of bamboo’s internodes and nodes, as well as the effect of sample length on the moisture absorption and desorption. The results showed that the rate of moisture absorption and desorption of the nodes was relatively higher than that of internodes at initial stage, and then tends to converge the same, which follow a two-stage exponential model and experience rapid and slow stage for both internodes and nodes. An anisotropic hygro-expansion existed during absorption and desorption, with significantly larger deformations in transverse direction than in longitudinal direction. It was observed that swelling in radial direction were relatively larger than in tangential direction, while shrinkage was just opposite. The longitudinal deformations were less than 0.6% and can be ignored in many situations. A second-order effect of moisture absorption on dimensional changes and anisotropic behavior was confirmed by the facts that transverse dimensional changes were more during desorption compared to adsorption. The swelling and shrinkage behaviors of the external, middle and internal layers in radial direction of samples are different, but no significant difference was detected. The dimensional changes in radical and tangential directions were significantly different between the nodes and internodes. The sample length has no effect on the rate of moisture absorption and desorption. The moisture absorption capacity of the oven-dried samples is much lower than that of green samples. The results can provide knowledge and ideas for the improvement of drying technique in new-typed bamboo structural products, as well as for the evaluation of deformation and structural stability of modern bamboo constructions.