Abstract
Bamboo can be processed into engineering materials with excellent properties by reasonable processing methods. In this study, the performance of mould-pressed bamboo (MBP) veneer products was examined. The physical mechanical properties and connection properties of MPB were tested, and the application performance of the MPB was analyzed. The results show that MPB has a comprehensive property of high internal bonding and good dimensional stability, and its density and mechanical properties are similar to those of wood dimensional stock. The overall bending strength, bending modulus, and compression strength of MPB were 29.0 MPa, 6.83 GPa, and 15.6 MPa, respectively. While the overall carrying capacity was relatively low, the connection performance of BPM was good. Thus, it can be used as a connector or substructure.
Download PDF
Full Article
Physico-Mechanical and Joint Performance of Bamboo Veneer Products Manufactured by Mould Pressing
Wenfu Zhang,a,b Shaohua Gu,a Cuicui Wang,a Haitao Cheng,a and Ge Wang a,*
Bamboo can be processed into engineering materials with excellent properties by reasonable processing methods. In this study, the performance of mould-pressed bamboo (MBP) veneer products was examined. The physical mechanical properties and connection properties of MPB were tested, and the application performance of the MPB was analyzed. The results show that MPB has a comprehensive property of high internal bonding and good dimensional stability, and its density and mechanical properties are similar to those of wood dimensional stock. The overall bending strength, bending modulus, and compression strength of MPB were 29.0 MPa, 6.83 GPa, and 15.6 MPa, respectively. While the overall carrying capacity was relatively low, the connection performance of BPM was good. Thus, it can be used as a connector or substructure.
Keywords: Bamboo moulds-profile; Dimension stock; Physical mechanics properties; Connection properties
Contact information: a: International Centre for Bamboo and Rattan, Beijing, P.R. China, 100102; b: Zhejiang Forestry Research Institute, Hangzhou, Zhejiang, P.R. China, 310023;
* Corresponding author: wangge@icbr.ac.cn
INTRODUCTION
Bamboo is a natural engineering material with high strength, good toughness, and good wear resistance. It offers the advantages of green environmental protection and special cultural elements, suggesting that it can serve as a suitable substitute for building materials such as bricks and rocks (Janssen 2000; Lugt et al. 2016). The unique structural characteristics of bamboo equip it with excellent longitudinal mechanical properties and processing properties. According to the longitudinal characteristics of bamboo, it can be processed into bamboo strips to make bamboo floors (Sharma et al. 2015; Fang et al. 2018), as bamboo bundles to make reconstituted materials (Chen et al. 2017), and as bamboo slivers to make bamboo slivers-based panels (Feng et al. 2013; Deng and Wang 2018). Consequently, its characteristics and good longitudinal mechanical properties can be fully utilized. However, bamboo easily splits longitudinally and has no transverse organizational unit, which results in a weak transverse structure and great difficulty in horizontal processing. By processing bamboo into bamboo veneers, a mould-pressed bamboo (MPB) structure can optimize the characteristics of a layered structure. Using a mould pressing system, bamboo is processed into a bamboo composite material with reasonable structure that conforms to building material specifications.
In recent years, the development speed of light wood structure houses in China has been relatively fast; however, their number is still below 1% of the annual construction volume of other countries such as North America, Europe, and Japan. Most frame materials that are being incorporated into light wood structure houses in China are almost exclusively imported from Canada and the United States. The design and construction technology of wood structures also comes from abroad. Both the promotion and localization of light wood structure houses are limited (Yuan et al. 2010). MPB retains the characteristics of good mechanical properties and natural appearance of bamboo, and it avoids the defects of high density of bamboo engineering materials that have caused problems in the past. MPB can replace wood specifications as light bamboo and wood structure building materials. MPB alleviates the current situation that the materials of light wood structure houses in China depend on imports from overseas, reduces the cost of raw materials of light wood structure houses, and promotes the development of light wood structure houses.
The properties of wood specification and bamboo structural material (Zhao 2010; Guo et al. 2011; Bhavna et al. 2015; Zhong et al. 2015, 2016) and the joining properties of wood specification, plywood, and reconstituted bamboo (Fei et al. 2008; Xu et al. 2011; Zhang et al. 2012; Zhou et al. 2012; Zhou et al. 2016) are well known. However, as a new type of building material, MPB has not been reported in the context of light wood structure. There is less information available concerning the physical and mechanical properties of MPB that it is unsafe in building structures without establishing the suitable design mechanical performances. In this paper, the physical mechanical properties and connection properties of MPB were studied, and the performance characteristics of MPB were analyzed to provide reference for its practical application.
EXPERIMENTAL
Materials and Equipment
A commercial bamboo veneer (Phyllostachys pubescens) with a moisture content ranging from 6% to 8% and urea formaldehyde resin (UF) with a solid content of 48% were respectively supplied by Dashan Bamboo Industry Co., Ltd. (Fujian Province, China) and Taier Co., Ltd. (Beijing, China). The experimental equipment included a mechanical testing machine (Instron 5582, Instron, Norwood, MA, USA), Jinan gold assay mechanical testing machine (WDW-300E, Jinan, China), field emission environment scanning electron microscope (FEG-ESEM; XL30, ThermoFisher Scientific, Hillsboro, OR, USA), body microscope with INFINITY analysis software (Lumenera Corporation, Ottawa, Canada), sliding table saw (FESTOOL-CS70, Festool, Wendlingen, Germany), dicing saw (J1G-355, DongCheng company, Qidong, China), electric drill, oven, water bath kettle, and electronic digital caliper.
Profile Preparation
The processing of MPB is shown in Fig. 1(a). The MPB were hot pressed with a removable core mold and moulded platen at a platen temperature 130 °C for 10 min.
The cross-section specification of MPB was 90 × 40 mm. The length of MPB depended mainly on the mould and processing procedure. The MPBs of different lengths were produced by lengthening the mould or by intermittent mould pressing procedure. Figure 1 shows that the wall structure of the materials was composed of nine layers, and the adjacent layers were assembled in the form of billets having a rectangular cross-section.
In the process of billet assembly, splicing or butt joints were adopted to connect the bamboo veneers. Generally, splicing gaps occurred easily in the overlapping longitudinal billet layers, as shown in Fig. 2(a), layers 1, 3, 5, 5, 7, and 9. Transverse billet layers were jointed at the corner or easily broken at the corner, as shown in layers 2, 4, 6, and 8 in Fig. 2(b).
Fig. 1. Process and structure of MPB. (a) Process, (b) Structure
Fig. 2. Structure defects of MPB. (a) Lap defects, (b) docking defects, and (c) accumulative defects
Moisture Equilibration
The MPB were put in the lab with a constant temperature of 20 C and relative humidity of 65% for at least two weeks. For each experimental condition, 6 and 12 specimens were tested for physical, mechanical, and connection properties, respectively.
Physical Properties Test
The volume density (i.e., bamboo profiles are recognized as solid wood to calculate density), material density (i.e., the density calculated by removing the hollow volume of bamboo profiles), and the wall thickness swelling rate and cross-sectional width dimensions swelling rate were measured according to GB/T 17657 (2013).
Mechanical Properties Test
The bending and compression properties of bamboo profiles were tested according to GB/T 28987 (2012). The bending strength, bending modulus, and compression strength of the whole material (including the hollow volume) and the solid material (removing the hollow volume) were calculated. Referring to ISO/TR 22157-2:2004(E) (2004), the shearing strength of materials was tested (Deng et al. 2016). The test of the internal bonding (IB) strength of the material followed the GB/T 17657 (2013) standard.
Connection Properties Test
According to GB/T 50329 (2002), the test symmetrical double shear single bolt connection properties of the material were tested. An M 10 bolt was selected for connection. The bolt connection strength was calculated using Eq. 1,
σ = Fmax / (D * t) (1)
where σ, Fmax, D, and t represent the bolt connection strength (MPa), maximum failure load (N), hole diameter (mm), and main material thickness (mm), respectively. The screw holding force and screw holding force of the material were evaluated according to GB/T 17657 (2013).
RESULTS AND DISCUSSION
Physical Properties of MPB
Table 1 shows the volume density, material density, wall thickness swelling extent, and the cross-sectional width dimensions swelling rate of MPB. The average volume density of MPB was 0.370 g/cm3, which is similar to the air-dry density of SPF Chinese fir (Guo 2007) and meets the density requirement of a light wood structure. The material density of MPB was 0.687 g/cm3, which was lower than that of raw bamboo (0.81 g/cm3), ply-bamboo, and reconstituted bamboo (Cheng et al. 2009; Feng et al. 2013). The MPB consisted of bamboo veneers with layer-by-layer assembly, and the cladding layer material around MPB was ply-bamboo. The extent of wall thickness swelling of the MPB was 4.40%, which was superior to the performance of ply-bamboo (Gao et al. 2012). The cross-sectional width dimensions swelling extent of MPB was 3.03%, which was superior to the performance of wood dimension stock and achieved a better dimensional stability.
Table 1. Physical Properties of MPB
SD, standard deviation; CV, coefficient of variance
Mechanical Properties of MPB
Table 2 presents the mechanical properties of MPB. The overall bending strength and bending modulus of materials were 29.0 MPa and 6.83 GPa, respectively, which are lower than those of larch and Chinese fir (Zhao 2010; Guo et al. 2011). Compared with No. 1 Chinese fir, the bending strength and bending modulus of materials were decreased by 33% and 35%, while the bending strength and bending modulus of MPB were 41.4 MPa and 19.7 GPa, respectively, which are not less than reported values. This result showed that the bending capacity of MPB is lower than that of wood materials; however, MPB has structural advantages that improve the volume bending performance of materials. The overall compression strength of MPB was 15.6 MPa, which is lower than that of larch and Chinese fir (Zhao 2010; Guo et al. 2011). Compared with No. 1 Chinese fir, the compression strength of MPB decreased by 49%, while the solid compression strength of MPB was 39.0 MPa, which is higher than that of larch and Chinese fir.
Table 2. Mechanical Properties of MPB
Figure 3a shows that when MPB was subjected to bending load, there was no dislocation shear failure on both sides of the profiles and no obvious damage on the upper and lower surface layers. The hollow structure was broken at the loading position on the upper surface, but it did not completely lose its bearing capacity. Figure 3b shows that the failure modes of profiles under compression along the grain differed from those of wood profiles. Wood profiles were mainly crushed under compression. For MPB, there was structural damage under compression, and the hollow structure expanded outward, resulting in the circumferential force failure of bamboo veneers in all layers. The MPB properties and characteristics of bending failure and compression failure showed that the billet structure and hollow structure are more reasonable. Furthermore, the bonding between layers was closer, which improved the overall mechanical properties of profiles. This result indicated that MPB can be used to replace wood specifications as columns or secondary components.
Fig. 3. Test destruction examples of MPB. (a) Bending failure; (b) Compression failure
Table 2 and Fig. 4 show the shear testing of MPB hollow structures. Four relative shear planes were formed on both sides of the profiles. The shear strength of MPB was 7.70 MPa. During the test, no apparent damage occurred on the four shear sides, and only a certain depth of indentation formed on the upper and lower end faces. This result showed that the longitudinal and transverse billet structure of MPB hinders the transmission of shear force along the grain and improves the shear failure resistance of the material along the grain.
Fig. 4. Shear test destruction of MPB. Shear properties (a) test schematic and (b) specimens
Table 2 shows the bonding properties of MPB. The bonding strength of MPB was 1.58 MPa, which is superior to that of ply-bamboo (Gao et al. 2012) because the thickness of each bamboo veneer unit used in MPB was 0.5 to 0.7 mm. The interlayer pavement was uniform, with the surface adhesives uniformly distributed, and there was no lack or leakage of adhesives.