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Bamboo pith ring (BPR) is regarded as a particular tissue affecting bamboo processing and is usually discarded, resulting in waste and low utilization of bamboo. To improve the utilization of bamboo and make full use of BPR, the fundamental properties, including morphology, chemical properties, and mechanical properties of BPR by confocal laser-scanning microscope, SEM, IR-image, and nanoindentation, were investigated in this study. On the macroscale, the stone cells in BPR are round or square and closely arranged, and the average thickness of BPR is 404.6 μm. On the cell scale, with the increase in distance from the pith cavity, stone cells show different shapes and sizes. On the cell wall scale, the stone cell in BPR show a multi-layer structure with alternating thick and thin walls, and there are dense pits on the wall layer. Stone cell is mainly composed of cellulose, hemicellulose, and lignin, and hemicellulose and lignin contents are higher than parenchyma cell. The elastic modulus and hardness of the stone cell wall were 6.98 GPa and 491.8 MPa, respectively. Studying BPR morphology, chemical, and mechanical properties are expected to lay a foundation for, among others, bamboo gluing, mechanics research, and drying cracking. [ABSTRACT FROM AUTHOR]

As a natural bamboo structure, bamboo pith ring (BPR) is commonly discarded from bamboo composites because it influences the gluing process and might cause complex handling. This study founds that the cells in BPR and bamboo middle (BM) are vertically staggered, forming vertical interlacing layers at the bonding interface in Arc-shaped Bamboo Natural Laminated Lumber (ABNLL). The role of BPR in the bonding interface was analyzed from macroscopic to cell wall scale. The retention of BPR can reduce the longitudinal, radial, and tangential deformation of bamboo blocks and composites during shrinkage and swelling. This phenomenon may be since the deformation direction of the cells in BPR and BM is vertical in the process of water absorption and dehydration, and the existence of BPR inhibits the deformation of BM. The deformation of cells in different directions in BPR and BM results from microfibrils' arrangement and chemical properties. The arrangement direction of microfibrils in BPR and BM is perpendicular to each other, leading to the vertical deformation of cell deformation in water absorption and water loss. BPR has low crystallinity (24.01%), a large microfibril angle (16.22°), high hemicellulose content, and is more sensitive to water, so BPR has a faster deformation rate and larger deformation range than BM. BPR has little effect on the mechanical properties of bamboo composites. In addition, retaining BPR increased the mass and thickness of bamboo by 7.43% and 9.40%, respectively. • Retention of bamboo pith ring can improve the dimensional stability of bamboo composite. • The cells of the bamboo pith ring and bamboo middle are arranged vertically staggered. • The deformation direction of the bamboo pith ring and bamboo middle were perpendicular to each other under the action of moisture. [ABSTRACT FROM AUTHOR]