1 Introduction
Bamboo, a fast-growing and renewable plant with notable characteristics such as lightweight nature, considerable strength, and flexibility, has been used for centuries in construction, decoration, textile, and paper production (Akinlabi et al.
2017). Recently, due to sustainability concerns in construction, bamboo has become a sought-after raw material. However, bamboo usage in construction is hampered by the natural variability in raw bamboo’s geometrical shape with a lack of standardisation (Harries et al.
2012; Sharma et al.
2015a; Hong et al.
2020). Therefore, engineered bamboo products with standardised shapes, less variable material properties, and improved weather resistance have attracted considerable interest from research and industry communities. Bamboo scrimber and glued laminated bamboo are commercially available engineered bamboo products increasingly utilised in various engineering applications (Sharma et al.
2015b). Bamboo scrimber is produced by pressing resin-impregnated crushed fibre bundles into a dense block, while glued laminated bamboo products are structural members formed by layering treated bamboo strips and glueing them under high pressure (Huang et al.
2019; Tan et al.
2021). Both engineered bamboo types provide versatile sizing for various structural uses while preserving bamboo’s natural strength through consistent fibre orientation, making them perfect for high-strength, lightweight structures.
As a hygroscopic material, bamboo’s mechanical properties are moisture-sensitive (Chung and Yu
2002; Xu et al.
2014). In outdoor applications, like landscaping and decking (Sharma and van der Vegte
2020; Lei et al.
2023), engineered bamboo products face varying humidity levels due to weather. As global warming is disturbingly real, higher air capacity for water vapour contributes to intensified or prolonged precipitation events (Tu and Lu
2022), thereby highlighting the importance of determining the effects of moisture content on the mechanical properties of engineered bamboo products. Generally, higher moisture content reduces bamboo’s mechanical properties (Chung and Yu
2002; Xu et al.
2014; Sánchez Cruz and Morales
2019; Zou et al.
2019; Sharma et al.
2021). Particularly in China’s southern regions, where springtime relative humidity can reach 100%, greater strength reductions were observed in natural ageing experiments than in the north (Zhang et al.
2022). Zou et al. (
2019) reported that the flexural strength and modulus of bamboo scrimber significantly decreased with increasing moisture content before reaching 19.9%. Beyond this value, the reductions in the flexural properties became limited with further increasing moisture content. A similar trend was observed in glued laminated bamboo by Sharma et al. (
2021), where increased moisture content lowered bending strength, elastic modulus, and shear modulus.
Most studies focus on the impact of moisture on the quasi-static mechanical properties of bamboo products. However, the loading state of materials under real-world conditions is significantly more complicated. High-loading rate tests can identify potential weaknesses that may not be visible under quasi-static loading conditions. In dry conditions, bamboo-based materials are sensitive to loading rates (Li et al.
2019; Qiu et al.
2021b), particularly in the cases where the load is perpendicular to fibre orientation (Li et al.
2020). Engineered bamboo’s strength and failure energy tend to be lower in the perpendicular direction (Sharma et al.
2015b; Li et al.
2020). Qiu et al. (
2021a) found that the peak force and the energy absorption of bamboo scrimber beams loaded perpendicular to the fibre orientation were greatly improved when the loading velocity increased from 2 to 10 m/s. Additionally, Qiu et al. (
2021b) proposed an orthotropic plate theory and composite failure criteria to predict the elastic flexural behaviour and progressive failure of bamboo scrimber plates. Despite the notable progress made in investigating the mechanical properties of engineered bamboo products under different loading conditions, the impact of humidity or moisture changes has not been considered concurrently. Given the increasing risk of damage from heavy rainfall and storms due to global warming, further investigation into the effects of moisture absorption and loading rate on the mechanical properties of engineered bamboo products is essential.
Currently, a notable research gap exists in understanding how engineered bamboo products respond to increasing loading rates, particularly up to 100 mm/min, under severe humidity exposure. This study was undertaken to address this research question, first by investigating the performance of the main elements, bamboo strips, across different loading rates and moisture contents. Gaining a comprehensive understanding of bamboo strip properties under the dual influence of varying loading rates and moisture contents is pivotal. Such insights would aid in the selection of appropriate glues and assembly methods tailored for products intended for use in environments with elevated moisture levels. This study investigated bamboo strips obtained after physical and chemical treatments of original bamboo, which are the fundamental components of glued laminated bamboo, to establish initial findings regarding engineered bamboo’s flexural behaviour changes in response to humidity exposure and higher loading rates. To this end, three-point bending tests were conducted on beam specimens with various moisture contents up to 112% at loading rates from 1 mm/min to 100 mm/min, determining the combined effects of moisture content and loading rate on flexural properties. The stress-strain behaviour, flexural strength, modulus, and failure strain of bamboo strips were characterised and discussed in detail. It is of great significance to consider both the products’ moisture contents and loading rates, which will aid in the better utilisation and performance prediction of engineered bamboo products in outdoor settings, amid increasing heavy precipitation events.
4 Conclusion
This study explored the effect of loading rates on the flexural properties of bamboo strips after different durations of severe moisture attack. The experimental results were analysed in terms of stress-strain behaviour, flexural strength, modulus, and failure strain, elucidating the combined influences of moisture content and loading rate on the flexural performance of bamboo strips. Based on the results, the following conclusions were drawn:
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Moisture absorption of bamboo strips was found to be proportional to the square root of exposure time at the initial stage for both 100% RH and water immersion conditions. A significantly higher moisture absorption rate was observed when the bamboo strips were immersed in water. A nearly saturated state with a moisture content of 112.0% was achieved after 14 days of water immersion.
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Non-uniform expansion was observed in bamboo strips during exposure to 100% RH and water immersion conditions. Negligible expansion occurred along the fibre direction, while width and thickness saw tenfold increases in expansion rates. Expansion rates in width, thickness, and volume notably rose from 0 to 20% moisture content but increased more slowly beyond this range.
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Throughout the bending tests, irrespective of moisture content or loading rates, all specimens experienced abrupt stress drops, accompanied by ruptures on the tension surface, i.e. the bottom outer surface. This underscores tension as the prevailing failure mode rather than local buckling on the compression surface.
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The most significant property changes occurred below 33.9% moisture content. After moisture exposure, bamboo strip’s flexural strength and modulus became more sensitive to loading rates, with the highest sensitivity observed at 33.9% moisture content. In terms of failure strain, sensitivity to loading rates increased only at a moisture content above 33.9%.
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Correlation analyses found strong correlations between flexural strength, flexural modulus, and volumetric expansion. Positive correlations between strength loss and volumetric expansion were established using linear regression fitting. The obtained slopes showed that a lower loading rate caused a faster strength loss with expansion.
These findings are practical for engineering applications in humid and rainy environments prone to potential impacts. Given the increasing frequency of heavy rainfall and storms due to global warming, monitoring engineered bamboo products’ properties in outdoor applications is essential for safety. This study has identified the specific moisture content level at which bamboo strips exhibit the highest sensitivity to loading rates. Moreover, it established a clear and positive correlation between the loss of strength and volumetric expansion in these materials. Such critical information serves as essential reference points for selecting appropriate loading rates and moisture content levels in future evaluations of bamboo products, especially those exposed to severe moisture conditions. However, this study only focused on a fundamental component of glued laminated bamboo within a limited range of loading rates. Deeper investigations on considering variations in glue types, density, and assembly methods will be carried out in future studies.
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