Novel engineered wood and bamboo composites for structural applications: State-of-art of manufacturing technology and mechanical performance evaluation

Highlights

• A-state-of-art review was conducted on novel engineered wood and bamboo composites.

• Attentions were paid to their manufacturing technologies, modeling approaches, and mechanical properties.

• Mechanical properties of the engineered wood and those of the engineered bamboo were compared.

• Several structural application cases of these engineered wood/bamboo composites were illustrated.

Abstract

Both wood and bamboo are renewable anisotropic materials with a long application history in human society. Engineered wood or bamboo aiming to mitigate the variability of the natural material can provide better material properties and structural performance, compared to original sawn lumber or raw bamboo. In this paper, a state-of-the-art review was conducted on three types of novel engineered wood composites, namely fiber reinforced polymer (FRP) reinforced glulam, cross-laminated timber (CLT), and wood scrimber, as well as three types of novel engineered bamboo composites, namely laminated bamboo lumber (LBL), glued-laminated bamboo (glubam), and bamboo scrimber, with particular attentions to their manufacturing technologies, modeling approaches, and mechanical properties. Then, for these novel engineered wood/bamboo composites, a comprehensive comparison was conducted on their mechanical properties and on their densities. Finally, several cases of structural applications were respectively illustrated, in which these aforementioned engineered wood/bamboo composites were adopted as main building materials. Potentials of applying these engineered wood/bamboo composites for structures were confirmed, and their possible existing drawbacks were also discussed.

Introduction

A new interest on sustainable materials for structural applications is being raised around the world, owing to a series of severe challenges (e.g., energetic crisis, environmental pollution, resource depletion, etc) and their resulting regulations. On the other hand, the choice of structural materials can have a significant effect on the global environment (i.e., the greenhouse effect). The greenhouse effect is causing increasing global concern as the surface temperatures of the earth rise. It is owing to excessive emission of carbon dioxide and other gases into the atmosphere. Both wood and bamboo can absorb carbon dioxide from the atmosphere as they grow. The net accumulation of carbon in the atmosphere is generally 5.5 ton of carbon per hectare (tCO₂/ha) approximately for European forestry [1]; that value is about 4.0 tCO₂/ha for the forestry in Southern England [2]. For moso bamboo (phyllostachy pubescens), the net accumulation of carbon in the atmosphere is about 36.44 tCO₂/ha [3]. Besides, both wood and bamboo structures can storage carbon for many years. Therefore, wood and bamboo species are pretty potential for the future construction industry, considering that their derivative products with high sustainable rates are reusable, recyclable, and naturally renewable [1], [3].

Due to the excellent strength-to-weight ratios, ideal thermal insulating and acoustical properties, wood is suitable for different kinds of applications as structural beams and columns, insulating envelopes, shear walls, flooring materials, etc. [4]. In comparison to wood, bamboo’s history being used as structural materials can be traced back even longer, making bamboo indispensable in human life. One advantage for bamboo is its rapid growing rate. It can reach the full height of 15–30 m within a period of 2–4 months by diurnal growth rates of about 20 cm up to 100 cm [5]. Furthermore, bamboo forests have up to four times the carbon storage (per hectare) of spruce forests over the long term [6]. Therefore, when used as structural materials, bamboo is more economical and more effective to mitigate the greenhouse effect. Meanwhile, both wood and bamboo are anisotropic materials; from a seismic view point, both of them are suitable for the buildings or bridges built in high seismicity regions, owing to their characteristics of lightness, viscoelastic properties, and deformability, compared to steel or concrete.

Wood is composed of cellulose, hemi-cellulose, lignin, and extractives [7], and several important factors that affect its mechanical properties can be concluded as knots, spiral grain, and wood twist, etc. [8]. For raw bamboo composed of a hollow culm, the culm length is divided by the nodes; besides, within the culm thickness, the fiber content is distributed non-uniformly. Overall, for the naturally grown wood and bamboo, high variability in their mechanical properties can be found, due to the differences in species, growth pattern, grown features; which hampers their extensive structural applications. To mitigate the effects from these aforementioned defects, a series of engineered wood or bamboo products have been developed for structural applications. Within their manufacturing process, some defects (e.g., knots) are removed or distributed randomly, which makes the engineered wood or bamboo products have the advantages of enhanced dimensional stability, more homogenous mechanical properties, and better durability.

In the last decades, a series of engineered wood suitable for structural applications and satisfying the modern structural design methods have been developed. It makes structural design or construction industries embrace wood as one kind of dominant material for modern structures. From wood harvested from forest to dimensional lumber veneers/strands for engineered wood manufacturing, at least three basic processing steps are required, namely peeling or cutting, drying, and strength grading. Within the step of peeling or cutting, barks and surface defects of the round wood are removed, and the processed round wood is subsequently peeled or cut into lumber veneers/strands or dimensional lumber, respectively. Then, the processing step of drying should be conducted, because wood commonly with a moisture content of 100% or even more is vulnerable to fungal degradation; besides, drier lumber can provide a more receptive substrate for gluing [9]. Based on the processing step of drying, an upper limit of 20% moisture should be satisfied for the wood with structural applications [10]. Finally, the strength grading is required for each piece of dimensional lumber, using the approaches of visual strength grading (VSG) or machine strength grading (MSG) according to EN 14081 [11]. The step of strength grading ensures the engineered wood can provide expected and stable mechanical properties for structural applications. These processed lumber veneers/strands or dimensional lumber combined with adhesives can be manufactured into various engineered wood products, with the advantages of increased dimensioned stability, more stable mechanical properties, greater durability, etc.

Engineered bamboo products are mainly manufactured in regions where bamboo is a locally available natural resource, including South America, Africa, and Southeast Asia. China is currently the world’s largest exporter of engineered bamboo, accounting for over half of the global trade [12]. The manufacturing process of most engineered bamboo products learns from the methods and technology used for manufacturing the engineered wood. Although the manufacturing process depends on the specific type of the manufactured engineered bamboo, the common processing steps for most engineered bamboo products can be concluded as splitting, gluing and pressing, and post-processing (e.g., heating or cutting). The splitting is a processing step within which the bamboo culms are split to half-split culms, strands, and strips (the bamboo-related nomenclature is in accordance with Liu et al. [13]). Then, within the processing step of gluing and pressing, these randomly mixed bamboo strands or strips are glued using resin matrix, and normal- or hot-pressed into bamboo sheet, bamboo lumber, and other engineered bamboo. The longitudinal bamboo fibers are maintained and oriented along the same direction, whereby the inherent strength of raw bamboo can be remained. Finally, the post-processing step including heating or cutting is required to form an engineered bamboo product with a regular shape and more stable properties. Currently, the engineered bamboo is of increasing interests from structural design or construction industries, owing to its shape standardization and relatively low variability in material properties.

Families of the common engineered wood for structural applications can be listed as glued-laminated wood (Glulam), cross-laminated timber (CLT), wood scrimber, laminated veneer lumber (LVL), parallel strand lumber (PSL), etc. Based on a survey of the literature from the Web of Science Database, the numbers of published studies related to each aforementioned engineered wood (within the last two decades) were counted. It is found that wood scrimber, as one kind of reconstituted wood developed in recent years, has relatively limited published studies (i.e., around 25), compared to other engineered wood. Glulam with widely structural applications has attracted continuous relatively high study interests, resulting in the largest publications (i.e., around 610). However, glulam beams with some dominant inherent properties of wood always have a larger deflection [14], compared to steel or concrete ones. Under the bending moment, the tensile zones of the glulam beams are prone to premature brittle damages. For improving their bending performance, fiber reinforced polymer (FRP) is adopted as reinforcement for the glulam beams, forming relatively novel FRP reinforced glulam. Published studies on FRP reinforced glulam account for 10% of glulam-related publications approximately. For the CLT as an innovative engineered wood suitable for mid- and high-rise buildings, it is achieving more focus recently, resulting in almost the same publications as glulam. In Europe and Canada, CLT is mainly made from Spruce-pine-fir (SPF), Douglas fir-Larch, and southern pine. Considering the demand for CLT has increased many fold times in recent years, various wood species have been investigated to find their potentials for CLT manufacturing; meanwhile, for making CLT manufacturing more efficient, several kinds of structural modified CLT have been developed. Finally, considering the already existing literature review papers on some kinds of these aforementioned engineered wood composites [15], [16], [17], [18], [19], [20], [21], this state-of-the-art review paper will focus on three relatively novel engineered wood products, namely FRP reinforced glulam, CLT made from relatively lower grade wood or structural modified CLT, and wood scrimber, mainly in terms of mechanical properties and structural application cases.

For engineered bamboo, laminated bamboo lumber (LBL), glued-laminated bamboo (glubam), and parallel strand bamboo (PSB) are currently the three commonly used engineered bamboo products for structural applications. These engineered bamboo products are mainly manufactured in China, and being used for the construction of buildings or bridges. Based on counting the papers from the Web of Science Database, publications on engineered bamboo are significantly less than engineered wood, whereas, more design or construction industries have started to embrace engineered bamboo as a structural material. Therefore, it is necessary to comprehend the state of the art for engineered bamboo. This state-of-the-art review paper will focus on LBL, glubam, and PSB, mainly in terms of manufacturing process, mechanical properties, and structural application cases. Physical and mechanical properties will also be compared between the engineered wood and the engineered bamboo, with the hope of providing direct and useful reference information for structural researchers or designers.