Bamboo as Sustainable Building Materials: A Systematic ...

6,7,

Dendrocalamus hamitonii

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,

Bambusa tulda

,

Bambusa nutans

, and

Bambusa balcoa

, and found that

Bambusa nutans

has the highest compressive and bending strength of 98.24 MPa and 7.669 MPa, respectively.

Bambusa tulda

has the strongest tensile strength of 226.28 MPa, and the highest shear strength of 19.56 MPa was obtained by

Dendrocalamus hamiltonii

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. Krause et al. [

Dendrocalamus gigantus

is dictated by the fiber volume fractions and independent of the position of the culm wall. Cui et al. [5,21,

The use of bamboo culms in the construction industry presents an environmental advantage because of the reduction in carbon dioxide generation compared to conventional structural materials [ 1 ]. Bamboo exhibits numerous notable physical properties that play a vital role in its diverse applications. These properties include size, color, dimension, texture, grain, density, moisture content, thermal conductivity, and absorption. The dimensions and length of bamboo can differ based on the species and the maturity of the plant [ 2 ]; in comparison to timber, bamboo achieves maturity at a faster rate, typically taking only three to four years to reach maturity [ 3 ], while the former ages more slowly over 6 years and lacks its strength-up to 12 years old [ 4 ]. Physical properties can vary considerably among different species due to their distinct cellular structures and varying fiber proportions, which can influence their density. Some bamboo species may have a higher density, while others may have a lower density [ 5 8 ]. Remarkable mechanical properties characterize bamboo, rendering it a versatile and sustainable material. With an impressive strength-to-weight ratio, bamboo possesses excellent flexural ductility compared to wood by 3.06 times [ 5 ]. Singh et al. [ 9 ] tested the mechanical properties of different bamboo species such as, and, and found thathas the highest compressive and bending strength of 98.24 MPa and 7.669 MPa, respectively.has the strongest tensile strength of 226.28 MPa, and the highest shear strength of 19.56 MPa was obtained by. Krause et al. [ 10 ] mentioned that the tensile behavior ofis dictated by the fiber volume fractions and independent of the position of the culm wall. Cui et al. [ 11 ] observed that the tensile strength of bamboo fiber bundles gradually decreases with increasing temperature, which is mainly influenced by the decrease in the relative content of cellulose and hemicellulose. Wu et al. [ 12 ] confirmed that the tensile strength of an outer bamboo sliver treated with alkali at 60 °C and an inner bamboo sliver treated with alkali at 40 °C will increase up to a maximum of 86.6% and 132%, respectively, compared to untreated bamboo. Lorenzo et al. [ 13 ] performed both an experimental test and finite element simulation on Moso bamboo and found that the nonlinear behavior is mainly caused by the incremental development of cracks to the locations where the circumferential tensile capacity of bamboo is exceeded, leading to the eventual failure of the pole. Bamboo material is also excellent in compressive strength. Awalludin et al. [ 14 ] confirmed that the compressive strength of bamboo increases as the moisture content decreases. Bahtiar et al. [ 15 ] developed the structural grading of Guadua culms under axial compression and concluded that the strongest coefficients of determinations for structural grading were related to density. Five different bamboo species were studied by Drury et al. [ 16 ] such as Moso, Guadua, Tali, Black Java, and Madake. The compressive strength of these species was found to be greater than 50 MPa, which makes them excellent for construction use. Gauss et al. [ 17 ] stated that the presence of nodes has a local stiffening effect with respect to compression and shear. Shawana et al. [ 18 ] investigated the strength characteristics of bamboo along the length of the culm and observed that the size of the culm governs the ultimate load capacity. The larger culm and thicker wall obtained more load. Lo et al. [ 19 ] conducted a strength analysis of Mao Zhu and Kao Zhu bamboo species that are basically used for scaffoldings in China. They observed that the compressive strength of both species ranges from 45 to 65 MPa, and the top section of the culm has higher strength than its bottom section. Suarez et al. [ 20 ] studied the influence of weathering on the mechanical properties of bamboo culm from Guadua species with and without nodes. There was a 9% decrease in the compressive strength, but they had no influence on the tension and bending performance when the silica layer was placed on the compression face. Also, nodes have a positive effect on the stiffness near the points of load application when tested for bending. Bamboo material is also outstanding in flexural strength [ 1 22 ], allowing it to bear substantial loads and stresses. Its good elasticity enables efficient absorption and distribution of impact energy. Bamboo&#;s mechanical properties vary based on factors such as species, age, and culm regions [ 23 ]. It is highly regarded for its strength, flexibility, and resilience, making it suitable for various applications, including construction, furniture, and engineered products. Bamboo composite or engineered bamboo is widely explored to optimize the utilization of bamboo. These mechanical properties of engineered bamboo products can be influenced by processing methods [ 24 ].

In this paper, we introduced a bamboo longitudinal flattening technology and analyzed the effects of the softening&#;flattening process on the physical and mechanical properties of moso bamboo. This is a newer bamboo processing technology that can enhance the utilization and reduce pollution compared with traditional bamboo-based products. Results showed that the parenchyma cells distorted and compacted due to the flattening process. The hemicellulose and cellulose content decreased, while the content of lignin presented an increasing tendency. As expected, the dimensional stability of moso bamboo enhanced due to the decrement of hemicellulose. The softening&#;flattening process positively contributed to the micro-mechanical properties of treated bamboo specimens. For example, the hardness and modulus of elasticity of the untreated bamboo sample increased from 0.58 and 15.7 GPa to 0.8 and 17.5 GPa, respectively. In addition, the changes in cellulose crystallinity and mechanical properties were also investigated in this paper. The cellulose crystallinity increased from 37.5% to 43.2%, significantly. However, the modulus of rupture of the flattened bamboo board decreased from to MPa due to the grooves made by the flattening roller. The MOE of flattening bamboo board showed the same decreasing tendency.

1. Introduction

As an abundant biomass material, bamboo has attracted more attention due to its short growth cycle, excellent mechanical property, and easy harvest [1,2,3,4]. In addition, bamboo can be widely applied in the areas of decoration, furniture, construction, and so on. However, the inner drawbacks restrict bamboo&#;s application, such as dimensional instability, hydrophilic property, and ease of decay [4,5,6,7,8]. For practical application products, bamboo scrimber, laminated bamboo board, bamboo rotary cutting veneer, oriented strand board, fiberboard, and ply bamboo have received increasing attention in the past decades as a substitute for wood materials. For bamboo culm, nodes and internodes are the basic constituents. Thus, bamboo cannot be processed like wood because of its different structure. Furthermore, it is hard to obtain a large original bamboo board with elegant texture through the saw cutting process. Bamboo flattening technology is a useful technology for solving this difficulty [9].

Bamboo culms consist of hemicellulose, cellulose, lignin, ash, and so on [10]. The main chemical compositions mentioned above are containing amorphous phases. When exposed to high temperature and high-pressure conditions, these chemical components exhibit viscoelastic and plastic behaviors. Normally, scientists use glass transitions to describe this thermal softening behavior. The polymers become rubbery and soften when the temperature is above glass transitions [11,12,13,14]. Bamboo culms can be flattened into flattened bamboo board in this &#;rubbery&#; state. Like wood materials, bamboo can be softened by heat treatment medium (hot air, hot water, high-frequency heating, microwaves, and hot oil) and chemical agents (NaHCO3, NaOH, and KOH). However, treatment with chemical agents is not environmentally friendly and affects the natural and sustainable character of bamboo. Through reviewing some reported studies and patent [11,12,13,14,15,16,17,18] and after visiting some industries in China, bamboo softened by saturated steam in a sealed equipment is shown to be most the eco-friendly and cost-efficient method now. Compared with traditional softening treatment mediua (oil, hot water, hot air, etc.), saturated steam can provide a mild environment consisting of high moisture content and high pressure.

Bamboo flattening technology has been developed over 30 years. In , Zhang first reported bamboo flattening technology. Zhang softened bamboo culms through hot water and then flattened half-tubular bamboo culms via a hot press. Although the flattened bamboo board was obtained, unfortunately, many deep cracks were shown in the bamboo surface. Parkkeeree and his colleagues reported a flattening process on black sweet bamboo (Dendrocalamus asper) with hot oil [15]. Bamboo culms were flattened by two plates. However, the efficiency of the process was very low. Liu et al. did flattening tests on 6-year-old moso bamboo (phyllostachys pubescens) with a special device [16]. The bamboo splices were flattened by applying the horizontal forces. However, the process is controlled manually with very low efficiency, and the width of the flattened bamboo board is limited. In recent years, some scholars reported &#;notched flattening technology&#; [17]. &#;Notched flattening technology&#; can flatten bamboo culms through making non-penetrating indentations in order to increase the circumference of the inner layer so that the softened bamboo culms can be flattened without cracks. However, this &#;notched flattening technology&#; cannot flatten long bamboo culms because of the different thickness of the bamboo wall along the longitudinal direction. Many entrepreneurs and scientists are working on the invention of bamboo longitudinal flattening technology, which can broaden the field of application of flattened bamboo board. Recently, a device that can flatten long bamboo culms has been created by scientists ( ). This bamboo flattening equipment consists of a set of rollers with knives. The long bamboo culms can be flattened gradually with the increasing width of flattening rollers. However, few literatures focused on this newer bamboo longitudinal flattening technology.

In this paper, we reported a bamboo longitudinal flattening technology and focused on the effect of the softening&#;flattening process on the micro-morphology, chemical composition, physical properties, and mechanical properties of bamboo via X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR), the wet chemistry method, and nanoindentation (NI). In addition, we proposed the best softening parameters by means of flatten tests at different temperatures and times.

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