Engineered bamboo scrimber: Influence of density on the mechanical and water absorption properties
Anuj Kumar
a,b,⇑, Tomáš Vlach
a,b, Lenka Laiblova
a,b, Martin Hrouda
a, Bohumil Kasal
c,d, Jan Tywoniak
a,b, Petr Hajek
a,baCzech Technical University in Prague, Faculty of Civil Engineering, Department of Building Structures, Thákurova 7, 166 29 Prague 6, Czech Republic
bUniversity Centre for Energy Efficient Buildings of Technical University in Prague, Trˇinecká 1024, 273 43 Bušteˇhrad, Czech Republic
cDepartment of Organic and Wood-Based Construction Materials, Technical University of Braunschweig, Hopfengarten 20, Braunschweig 38102, Germany
dCentre for Light and Environmentally-Friendly Structures, Fraunhofer Wilhelm-Klauditz-Institut WKI, Bienroder Weg 54E, Braunschweig 38108, Germany
h i g h l i g h t s
Influence of bamboo scrimber density on the tensile strength and tensile modulus properties.
Effect of bamboo scrimber density on the compressive strength and compressive modulus properties.
The flexural strength and flexural modulus of bamboo scrimber were evaluated.
Long term water absorption by bamboo scrimber.
a r t i c l e i n f o
Article history:
Received 21 July 2016
Received in revised form 7 October 2016 Accepted 9 October 2016
Available online 14 October 2016 Keywords:
Biocomposite Mechanical properties Strength
Ultrasonics Bamboo scrimber
a b s t r a c t
Engineered bamboo scrimber has processed from the raw bamboo culm into a compressed or laminated product with thermosetting resin in the density range of 800–1200 kg/m3. The present work investigates the mechanical properties of commercially available engineered bamboo scrimber and compares the results of present work with the existing results in the literature. The main aim of this work is to inves- tigate the influence of bamboo scrimber densities on the mechanical properties. The strength and mod- ulus properties in tensile, compression, shear and flexural were evaluated using the specimens of three different densities. The dynamic modulus of elasticity of bamboo scrimber beams were evaluated using ultra-sonic pulse method. According to the present results it can be concluded that the density have sig- nificant influence on the mechanical properties of bamboo scrimber. The long term water absorption was also conducted to analysis the weight gained by the bamboo scrimber having different densities and the results revealed that density of specimen’s influences the water absorption.
Ó2016 Elsevier Ltd. All rights reserved.
1. Introduction
Bamboo and bamboo-based panels would be the ideal source, to fulfil the demand of wood in construction sector. Bamboo is a rapidly-renewable resource; it regenerated for harvesting in three to eight years[1,2]. Due to the hollowness and the longitudinal fibers direction makes bamboo as an efficient natural structural design[3]. In the recent years bamboo attracted significant scien- tific research for the sustainable building material development since it possesses similar mechanical properties to those of
structural wood products and bamboo is more renewable compare to wooden products due to its fast growth rate[3].
Bamboo is an orthotropic material with high strength along and low strength transversal to its fibers[4]. Bamboo having compara- ble tensile strength, compressive strength, Young’s modulus and shear strength on a weight to-weight basis to conventional mate- rials such as low-carbon steel and glass-reinforced plastics[5–8].
Bamboo fibers have various applications in the material develop- ment such as bio-composites with polymer matrix [9,10], rein- forced material for concrete[11,12]. There are mainly three types of engineered bamboo products those being used as structural building material: laminated bamboo, reconstituted densified bamboo, and bamboo board[13]. Reconstituted densified bamboo also known as strand woven bamboo or bamboo scrimber. The word ‘‘Scrimber’’, originally proposed by Coleman [13], means http://dx.doi.org/10.1016/j.conbuildmat.2016.10.069
0950-0618/Ó2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding author at: Czech Technical University in Prague, Faculty of Civil Engineering, Department of Building Structures, Thákurova 7, 166 29 Prague 6, Czech Republic.
E-mail addresses:[email protected],[email protected](A. Kumar).
Contents lists available atScienceDirect
Construction and Building Materials
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o n b u i l d m a t
numerous wood splinters bonded together, latter it was adopted for crushed bamboo culms bonded together with adhesives (phe- nol formaldehyde resin) and compacted together up to twice of its density using cold and hot pressing[13]. The density of a bam- boo scrimber can be increased to 1050–1250 kg/m3, to improve the bonding strength and the bamboo scrimber has been successfully commercialized and rapidly developing in China[14,15].
Yu et al. [16] described the fabrication and characterisation bamboo scrimber (1150 kg/m3) with different weight percentages of phenol formaldehyde [PF] resin and different heat treatments of bamboo bundles. They revealed that the influence of PF resin loading on water swelling and mechanical properties of bamboo scrimber. Sharma et al.[17]investigated the commercial bamboo scrimber mechanical properties with density 1163 kg/m3and com- pared the mechanical properties with laminated bamboo sheets.
They investigated the mechanical properties of bamboo scrimber in parallel to fiber direction only and do not evaluates the modulus of elasticity. Yu et al.[18]investigated the mechanical properties of bamboo fiber reinforced composite (BFRC) comparing with those of commercial bamboo scrimber and laminated bamboo lumber (LBL). They presented the tensile, compressive and shear strength of bamboo scrimber of 1100 kg/m3density. Shangguan et al.[19]
developed a 2D model for compressive parameter of bamboo scrimber and the model predicted the influence of compressive loading angle to fiber direction on compressive property of scrim- ber. Guan et al.[20] fabricated the composite bamboo scrimber using two different bamboo species i.e. Moso bamboo and Muli bamboo (Melocanna baccifera) and compare their mechanical prop- erties. The existing literature has presented limited data on the mechanical properties of bamboo scrimber and has not considered the influence of density on properties of bamboo scrimber as the density varies between 800 and 1200 kg/m3[19].
The present study investigates the influence of density on the mechanical properties such as tensile, compressive, shear, and flex- ural strength as well as modulus of elasticity of commercial bam- boo scrimber and evaluates long term water absorption.
2. Materials and methodology 2.1. Materials
In the present study commercially produced bamboo scrimber also known as strand woven bamboo supplied from China was used. The bamboo scrimber are produced commercially using Moso bamboo (Phyllostachys pubescens) along with phenol formaldehyde resin, and the thermal treatment using saturated steam (SST) was also employed during fabrication process.Fig. 1 shows the schematic presentation of bamboo scrimber commercial production; the bamboo culms were process minimally to remove the few millimetre of outer and inner skin. The supplied bamboo scrimber having 40 mm40 mm620 mm dimensions and
density varies between 1000 and 1220 kg/m3. The received bam- boo scrimber were conditioned in a chamber at 65% ± 5% relative humidity and 20°± 2°C temperature for one month to maintain the final moisture content around 7%. Further, the conditioned bamboo scrimber were trimmed into different dimensions as per standard requirement for mechanical property analysis and again conditioned for two weeks prior to testing in chamber at 65% ± 5% relative humidity and 20°± 2°C temperature.
The foil linear strain gauges (type 20/120 LY41, temperature response matched to steel with
a
= 10.8 [106/K]), with the later applications of bridge for connection the cables, were purchased from the company HBM (Hottinger Baldwin messtechnik) GmbH.2.2. Mechanical properties testing
The mechanical properties were measured with a universal testing machine (LabTest 4.100SP1, Czech Republic) using load cell of 100 kN capacity.
2.2.1. Dynamic modulus of elasticity (ultrasonic pulse method) Ultrasonic impulse measurement was conducted using Tico apparatus. The measurements were conducted in both longitudinal (parallel to grain) and perpendicular (across the grain) directions, as shown inFig. 2. The CˇSN 73 1371[21]was employed to deter- mine the modulus of elasticity non-destructively and calculation process is given below and the dimensions of bamboo scrimber 40 mm40 mm620 mm was used.
q
¼mV ðkg=m3Þ ð1Þ
The transit speed of impulse:
Vi¼L
T ðK:m=sÞ ð2Þ
where; T = t1tcorr.
Coefficient of dimensionality of the environment:
K¼Vi
f ðmmÞ ð3Þ
- - consideration of unidirectional environment?k = 1 MOED¼
q
V2iK ðMPaÞ ð4Þ
2.2.2. Compressive strength
The compressive strength of engineered bamboo scrimber were evaluated in parallel to grain directions as per Cˇ SN 490110[22]and perpendicular to grain directions as per CˇSN 490111[24] stan- dards. Two foil linear strain gauges were pasted using epoxy resin on the mid-span of samples for accurate measurement of force and linear displacement for calculation of modulus of elasticity. The Nomenclature
ftk tensile strength parallel to grain direction [MPa]
ft\ tensile strength perpendicular to grain direction [MPa]
Etk tensile modulus parallel to grain direction [GPa]
fck compressive strength parallel to grain direction [MPa]
fc\ compressive strength perpendicular to grain direction [MPa]
Eck compressive modulus parallel to grain direction [GPa]
s
k shear strength parallel to grain direction [MPa]ff flexural strength 3-point bending [MPa]
Ef flexural modulus 4-point bending [GPa]
MOEDk dynamic modulus of elasticity parallel to grain direction [GPa]
MOED\ dynamic modulus of elasticity perpendicular to grain direction [GPa]
CBT control bamboo treatment oven dry at 85°C SST saturated steam treatment
HDAT hot dry air treatment
THMDB thermo-hydro-mechanically densified bamboo PSL parallel strand lumber
compressive strength was measured perpendicular to grain direc- tion as per CˇSN 490112[23]standards.Table 1shows the samples dimensions and compression testing setup used.
2.2.3. Tensile properties
The tensile strength and modulus of elasticity of scrimber sam- ples were evaluated in parallel to grain direction according to Cˇ SN 490113[24]standards. The modulus of elasticity parallel to grain was calculated from the linear part of the load curve using the strain gauge. Cˇ SN 49 0114[25] standard was used to calculate the tensile strength perpendicular to fiber direction.Table 1shows the samples dimensions and tensile testing setup used. An elec- tronic strain gauge was adhered in the middle surface of the spec- imen along its longitudinal direction to measure the corresponding strain. Tensile forces were loaded at each end of the specimen along its longitudinal direction. The strain in the middle section,
the load, and the stretch value of the specimen were recorded simultaneously. The loading speed was controlled by force at the rate of 1 kN/min for tensile modulus testing. Let the load increase to 15 kN from 0, then unloading it to 5 kN, and then reloading it up to 15 kN. Repeating the loading–unloading cycle in the range of 2–
10 kN for five times hereafter, the mean value of the data recorded from the last four cycles were adopted to evaluated the tensile modulus in the parallel-to-grain direction.
Etk¼D
r
D
e
¼btDFDte
ð5Þwhere Etk is the tensile modulus in parallel-to-grain-direction of bamboo scrimber;D
r
andDe
are the increments in stress and strain in the middle of the specimen respectively;DFtis the increment in the loading;bandtare the width and thickness of the middle sec- tion of specimen respectively. After the last cycle reverse loading at Fig. 1.Industrial fabrication process of engineered bamboo scrimber.Fig. 2.Schematic presentation for ultrasonic pulse measurement.
the rate of 0.5 mm/min until the sample broken occurs. The tensile strength of the specimen was evaluated by:
ftk¼Ft
bt ð6Þ
whereftkis the tensile strength of bamboo scrimber in parallel-to- grain direction;Ftis represents the ultimate tensile load applied to sample.
Table 1
Details about testing setup and sample dimensions.
Number of samples (each density) Loading speed Mechanical property Dimensions and testing setup
10 2.5 mm/min Tensile strength and moduluskgrain
10 1 mm/min Tensile strength\grain
10 1 mm/min Shear Strengthkgrain
10 2 mm/min Compressive strength bothkand\to grain directions
10 0.5 mm/min Compressive moduluskto grain directions
10 – Water absorption
2.2.4. Shear strength
The shear strength of bamboo scrimber parallel to grain direc- tion was evaluated as per CˇSN 490118 [26] standard and the dimension of sample and testing setup is shown inTable 1.
2.2.5. Ultimate flexural strength
The flexural strength of bamboo scrimber was estimation using 3-point bending test according to Cˇ SN 490115[27]. The sample dimensions and testing setup has shown inTable 1.
2.2.6. Flexural modulus
The flexural modulus of bamboo scrimber was evaluated using non-destructive four point bending test as per Cˇ SN 490116[28]
standards. The strain gauges were fixed on the samples mid-span for measuring the strain deflection as shown inFig. 3along with testing setup. The tested specimen was loaded by constantly increasing force till 400 N with a loading speed of 25 N/s and con- tinually unloaded to 100 N. The same process was repeated four continued load cycles and measured the deflection at the moment when load force reaches 100 N and 400 N in the time span of 10 s.
The modulus of elasticity is calculated as follows:
E¼
r
e
ð7ÞE,the elastic modulus determined from linear part of the load curve [MPa].
r
, stress [MPa].e
, strain [–].r
¼MW ð8Þ
M,the maximum moment acts on the beam [N.mm].
W,the section modulus of the sample [mm3].
Moment:
M¼DF 2 L
3 ½Nmm ð9Þ
DF, variance of the forces at the beginning and the end of one linear part of the load cycle curve [N].
L,the span of the beam between the supports = 70 mm [mm].
DFfrom the last four load cycles:
DF¼F1F2 ð10Þ
F1, the value of the force at the beginning of the one linear part of the load cycle curve [N].
F2, the value of the force at the end of the one linear part of the load cycle curve [N].
Section modulus:
W¼1
6bh2 ð11Þ
b,the mean width of specimen [mm].
h,the mean height of specimen [mm].
e
¼Dllo ð12Þ
Dlthe difference betweenl1andl2[mm].
loinitial known length of strain gauge [mm].
Dlfrom the last four load cycles:
Dl¼l2l1½mm ð13Þ
l1, the value of the track where the linear part of the one load cycle curve starts [mm].
l2, the value of the track where the linear part of the one load cycle curve ends [mm].
Flexural modulus:
Ef ¼ mlo
WD0l¼D
0Fl 6 lo
WD0l ½MPa ð14Þ
D0F, the mean value calculated fromDF [N].
D0l, the mean value calculated fromDl [mm].
Fig. 3.Four-point bending testing setup.
2.2.7. Compressive modulus
The compressive modulus of bamboo scrimber was evaluated using non-destructive parallel to grain direction according to Cˇ SN 49 0111 [29] standard. The sample dimensions and test setup details are shows inTable 1. The compressive modulus calculation process same as mentioned in Section2.2.6, only difference is in the maximum loading force (4000 N) and unloading force (1000 N) rest of process is same.
2.3. Water absorption
The water absorption experiment was conducted according to Cˇ SN 49 0104 [30]standard. The rectangular prism samples with dimensions of 20 mm20 mm20 mm were dipped half to the thickness as shown inTable 1; the weight of samples were mea- sured every 24 h until the constant weight.
2.4. Optical microscopy
T0 understand the resin distribution in the cross-section of bamboo scrimber, the surface morphological images was captured using optical microscopy [Olympus BX 41] at different magnifica- tion in range 50–500.
3. Experimental results
All of the mechanical properties were calculated in the linear elastic region according to the standards.Table 2listed the mean and the standard deviations of ten specimens tested for each destructive and non-destructive test.
3.1. Dynamic modulus of elasticity
The modulus of elasticity in dynamic mode of engineered bam- boo scrimber beams was evaluated using ultra-sonic pulse method in both parallel (MOEDk) and perpendicular (MOED\) to grain direc- tion.Table 2shows the mean values and standard deviations of the calculated modulus of elasticity for both grain directions; in paral- lel to grain direction the MOE varies from 16 to 20 GPa along with the density of specimens. Similarly in the perpendicular to grain the MOE varies from 5 to 7 GPa along with the density of specimens.
3.2. Tensile properties
The tensile strength (ftk) of bamboo scrimber in parallel to grain orientation was influenced by density of specimens; the specimens with density 1054 kg/m3having mean tensile strength 111 MPa and 145 MPa for specimens having density 1215 kg/m3as shown in theTable 2andFig. 4a, b . Similarly in the perpendicular to grain direction the tensile strength (ft\) varies from 4 to 7 MPa along
with the density of specimens as given inTable 2andFig. 4c shows the tensile stress–strain curve in perpendicular to grain direction.
The tensile modulus (Etk) parallel to grain direction was calculated from the measured strain using linear strain gauges; the density of specimens have less influence on theEtkthe mean values are var- ies 12 to 14 GPa as given inTable 2.
3.3. Compression properties
The compressive strength of engineered bamboo scrimber in both parallel (fck) and perpendicular (fc\) grain direction were eval- uated using 30 mm20 mm20 mm specimens. The compres- sive strength (fck) of specimens increases with the density i.e. the 1054 kg/m3 have the mean values (n = 10)105 MPa and speci- mens with density 1215 kg/m3have the meanfck116 MPa (see Table 2andFig. 5a, b). Similarly, the compressive strength (fc\) influenced by the density of specimens, the lower density speci- mens have 50 MPa compressive strength and higher density specimens have77 MPa as shown inFig. 5c andTable 2.
The compressive modulus of engineered bamboo scrimber in parallel (Eck) to grain direction was evaluated using 80 mm20 mm20 mm specimens. The mean calculatedEckof specimens was significantly influenced by the density, the 1054 kg/m3specimens have the meanEck12 GPa, the specimens with higher density such as 1127 and 1215 kg/m3having 15 GPa and 17 GPa compressive modulus respectively (seeTable 2).
3.4. Flexural properties
The ultimate flexural strength of engineered bamboo scrimber was evaluated using 3-point bending setup and the specimens dimension 80 mm20 mm20 mm. The flexural strength of bamboo scrimber slightly improved along with the specimens’
density the 1054 kg/m3, 1127 kg/m3 and 1215 kg/m3 have 132 MPa,155 MPa and 166 MPa flexural strength respectively as shown inTable 2andFig. 6a, b.
The flexural modulus of scrimber samples was evaluated using 4-point bending testing setup (seeFig. 3). The strain was measured by linear strain gauges adhered on the central span of specimens by successive loading and unloading during testing. The flexural modulus varies between 14 GPa and 19 GPa, the higher the density for the specimens better the flexural modulus as shown inTable 2.
3.5. Shear strength
The shear strength parallel to grain direction influences by the specimens densities. The lower density specimens having around 12 MPa and the higher density specimens having 17 MPa shear strength as shown inTable 2andFig. 7.
Table 2
Mechanical properties of engineered bamboo scrimber.
Density Tension Compressive strength Compressive
modulus
Shear strength
Flexural strength
Flexural modulus
Dynamic MOE q(kg/m3) ftk(MPa) Etk(GPa) ft\
(MPa)
fck(MPa) fc\(MPa) Eck(GPa) sk(MPa) ff(MPa) Ef(GPa) MOEDk
(GPa)
MOED\ (GPa) 1215 ± 10 144.75
(10.21)
14.02 (0.33)
6.7 (0.77)
115.7 (2.74)
77.00 (6.30)
17.05 (0.81)
17.00 (0.44)
166.5 (4.54)
18.65 (0.59)
19.95 (1.49)
7.00 (0.80) 1127 ± 10 115.5
(7.59)
13.22 (0.83)
5.55 (0.85)
113.4 (3.10)
64.16 (4.4)
15.24 (0.42)
14.25 (1.34)
155.33 (5.22)
16.39 (0.39)
17.22 (1.09)
6.40 (0.73) 1054 ± 10 111.00
(8.15)
12.18 (0.63)
4.18 (0.45)
104.71 (4.39)
49.33 (5.5)
12.95 (0.45)
11.89 (0.60)
131.83 (7.47)
14.68 (0.39)
16.06 (1.34)
5.1 (0.57)
⁄Values given in parentheses are the standard deviations (n = 10) of specimens per testing.
3.6. Water absorption
Fig. 8a shows the water absorption percentage by bamboo scrimber specimens after 500 h dipping into water. The density of specimens significantly influences the water absorption, higher the density means more compacted are the bamboo fibers together. The lower density specimens showing around 34% water in comparison to 25% by higher density specimens.Fig. 8b shows the growth of molds and fungi during the water absorption experiment.
4. Discussion
Bamboo scrimber is an engineering material, where bundles of bamboo fiber are arranged in parallel, with fibers glued to each other using adhesive under a hot press or cold press plus heat acti- vation of resin after heat drying at high temperature (aprox.
170°C)[31–33]. The bamboo scrimber used in the present study was the 5th generation strand-woven bamboo (SWB).Fig. 9shows the bamboo culms cross-sections, in the 3rd SWB only the central portion of bamboo culms has been used, the inner and out layers are removed during the processing and, which created lots of bam- boo wastage. In the 5th generation SWB only >1 mm inner and outer layers are removed during the processing and used almost entire bamboo culms are used. The mechanical properties (tensile and compression) of bamboo specimens reduce along with the
distance from the outer layer towards the central portion of culms [4,34]. The outside region of bamboo culms has higher strength in comparison to inner region of bamboo, due to the density the outside region has densely compact fibers and inner has loosely compact fibers[4,34].
According to actual industrial production and previous studies [35–37], two available thermal treatments, namely saturated steam treatment (SST) and hot dry air treatment (HDAT), were employed for the bamboo bundles. In SST treatment 0.35 MPa sat- urated steam for 150 min is used and drying to 10% moisture con- tent at 85°C. In HDAT process the green crushed bamboo bundles were dried to 2.5% moisture content at 85°C and the dried bamboo bundles were treated with hot dry air (180°C) for 10 h. The flexural strength and modulus properties of bamboo scrimber are influ- enced by the thermal treatment process;Table 3shows the differ- ence in the flexural properties and shear property of bamboo scrimber prepared by different thermally treated bamboo bundles [16]. Recently, Dixon et al.[33]reported the influence of thermo- hydro-mechanical densification on the flexural behaviour of bam- boo stripes and the modulus of rupture and modulus of elasticity improved significantly. So, the mechanical properties of engi- neered bamboo scrimber influenced by various processing param- eters. Table 3 demonstrated the mechanical properties data collected from the literatures. In the next sections we discuss and compared the mechanical properties of present study with literature data.
Fig. 4.(a) Tensile properties of bamboo scrimber in parallel to grain direction; Stress-strain curves of tensile property of bamboo scrimber (b) parallel to grain and (c) perpendicular to grain.
4.1. Influence of density on tensile properties
The tensile strength in both parallel and perpendicular to grain directions influences by the specimen’s density means higher the density better the tensile strength (see Table 2).
Table 3demonstrated mechanical properties data collected from the literature. The tensile strength of bamboo scrimber parallel to grain, according to literature data [17,18] shown in Table 3, the tensile strength influence by the density of specimens and the values were 120 MPa, 115.73 and 41 MPa for 1160 kg/m3, Fig. 5.(a) Compressive properties of bamboo scrimber in parallel to grain direction; Stress-strain curves of compressive property of bamboo scrimber (b) parallel to grain and (c) perpendicular to grain.
Fig. 6.(a) Flexural properties of bamboo scrimber and (b) Stress-strain curves of flexural strength of bamboo scrimber.
1100 kg/m3 and 801 kg/m3 densities respectively. The tensile strength values of current are in very good agreement with the literature data, especially the 1127 kg/m3 density specimens and the highest values was 144.5 MPa for 1215 kg/m3specimens (Fig. 4a, b and Table 2). The perpendicular grain tensile strength (ft\) values according to literature [17], the ft\ values decreases with density (see Table 3). But results from current study are not in the agreement with literature data; the ft\
increases with the density of specimens (see Table 2 and Fig. 4c). The fracture of specimens during tensile loading was brittle in both parallel to grain and perpendicular to grain directions (seeFig 4b, c).
The tensile modulus parallel to grain direction (Etk) values of bamboo scrimber influences by the density of specimens, theEtk
increases along the density of bamboo scrimber (see Table 1 andFig 4a). The current work evaluated theEtkof bamboo scrim- ber first time, which is calculated from the strain values mea- sured by linear gauges. The mean and standard deviations of estimated values ofEtk are given inTable 2, those are varies in between 12 and 14 GPa along with the densities of bamboo scrimber.
4.2. Influence of density on compressive properties
The compressive strength in both parallel and perpendicular to grain direction increases with density are presented in the results section (seeTable 2). The parallel to grain compressive strength is almost double to the perpendicular to grain, because bamboo fibers strength higher in parallel direction.Fig. 10shows the pho- tos demonstrated the fracture phenomenon during compression loading, in parallel to grain loading the bonded bamboo fibers are de-bonded from resin matrix and resisting the loading force like a ductile material. In perpendicular direction the specimens are compressed together (seeFig. 10). The tensile strength parallel to grain of present study for density 1127 kg/m3 is 115.7 MPa which is in good agreement with literature, which have density 1163 kg/m3and tensile strength is 120 MPa [17]and 115.7 MPa for 1100 kg/m3[18]respectively. According to literature[17]the tensile strength perpendicular to grain direction for density 1163 kg/m3have lower values (3 MPa) even in compare to current results for density 1127 kg/m3 sample values (5.5 MPa) [see Table 3], this variation in results might be due to the different sam- ples dimensions used in the testing the current study and literature [17].
The compressive modulus parallel to grain direction of bamboo scrimber was evaluated using bigger samples (seeTable 1) com- pare to compressive strength, because the linear strain gauges were employed to measure the exact strains during continues loading and un-loading of specimens.Fig. 11a shows the loading and un-loading curves for individual sample from each set of den- sity, it is clear that the higher density specimens have bigger linear strain compare to lower strain in low density specimens. The com- pressive modulus of high density (1215 kg/m3) bamboo scrimber is 14 GPa and low density (1054 kg/m3) is 12 GPa and this variation in the values follow the pattern of natural bamboo compressive property, denser the fibers higher the compressive properties [38,39]. Interestingly the collective literature does not evaluated the compressive modulus of bamboo scrimber, except Li et al.
[39] estimated the compressive modulus of laminated bamboo and the values from studies are in good agreement with the values given by them.
4.3. Influence of density on flexural properties
The flexural strength was evaluated using 3-point bending setup (see Table 1). The calculated values of specimens are Fig. 7.Stress-strain curves of shear strength parallel to grain direction of bamboo
scrimber.
Fig. 8.Bamboo scrimber specimen’s water absorption results after 500 h dipping into water (a) and (b) specimens during water absorption test with some growth of molds and fungi.
demonstrates inTable 2, and flexural strength was greatly influ- enced by the density of samples. The flexural strength values are in good agreement with literature[16,18]especially for 1127 kg/
m3 specimens. The flexural strength of bamboo scrimber (1100 kg/m3) specimens estimated by Sharma et al.[17]is lower (119 MPa) than the estimated values of 1054 kg/m3density sample (132 MPa) (seeTables 2 and 3). Guan et al.[20]fabricated the dif- ferent bamboo species and mixed bamboo scrimbers with varying densities from 1050 kg/m3 to 1350 kg/m3. The estimated values presented by them are >200 MPa for even low density sample also, and the highest value for flexural modulus in current study was
166 MPa (seeTable 3).Fig. 12a shows the fractured sample during 3-point bending flexural test, the delamination mainly caused due to the presence of inter-nod section of bamboo culms, which might not be fully crushed during the processing[40].
The flexural modulus was estimated using 4-point bending setup (details given is Section2.2.6). Fig. 11b shows the loading and un-loading strain curves measured by strain gauges to esti- mate the flexural modulus. The calculated values also vary with densities of samples given inTable 2, but fully in the agreement with values given in the literature (Table 3). In the literature also density influences the flexural modulus. The samples dimensions Fig. 9.Bamboo cross-section, the selected section of bamboo culms used to produce the bamboo scrimber.
Table 3
Survey about the mechanical properties estimated in the existing literature and comparison with present results.
Reference Density Comments Tension Compression
strength
Compressive modulus
Shear Strength
Flexural strength
Flexural modulus
qkg/m3 ftk
(MPa) Etk
(GPa) ft\
(MPa)
fck(MPa) fc\
(MPa)
Eck(GPa) sk(MPa) ff(MPa) Ef(GPa)
Kumar et al. 2016 (present work)
1215 SST 144.75 14.02 6.7 115.7 77.00 17.05 17.00 166.5 18.65
Kumar et al. 2016 (present work)
1127 SST 115.5 13.22 5.55 113.4 64.16 15.24 14.25 155.33 16.39
Kumar et al. 2016 (present work)
1054 SST 111.00 12.18 4.18 104.71 49.33 12.95 11.89 131.83 14.68
Yu et al. 2015[16] 1150 CBT – – – – – – 19.2f 150g 10.5g
SST 18.0f 150g 14.3g
HDAT 10.1f 75g 12.3g
Sharma et al. 2015[17] 1163 120a – 3a 86b 37b – 15b 119c 13
Yu et al. 2014[18] 1100 115.73i – – 77.9j – – – 149.93g 11.1h
Shangguan et al. 2014 [19]
1210 – – – 143.9e – – – – –
880 – – – – 11.2e – – – –
Guan et al. 2012[20] 1090 Moso natural – – – – – – – 202.73 11.4
1240 Muli natural 265.88 15.21
1350 Muli-
carbonized
257.73 14.08
1290 Muli-Mixed 229.65 13.08
Dixon et al. 2016[33] 997 ± 124 THMDB – – – – – – – 100–250 5–20
Liu and Lee[41] 680 PSL Southern Pine
– – – 54.2
(6.0)
– – 8.4 (1.7) 80.2 (15.1) 11.89 (1.4)
Liu and Lee[41] 620 PSL Yellow Poplar
– – – 48.6
(6,0)
– – 8.4 (1.7) 87.5 (11.5) 10.96 (1.17)
Kurt et al[42] 460 PSLPopulus
deltoids
– – – 48.23
(7.34)
– – – 68.62 (4.60) 7.073 (0.43)
Ahmad and Kamke[43] 783 PSL Calcutta bamboo
– – – 66.3
(8.2)
– 7.8 (1.6) – 133 (32.6) 12.3 (1.7)
aASTM D143.
b BS 373.
c BS EN 408.
dChinese standard JB/T 1927-1943.
eChinese National Standard GB 50005-2003: Code for Design of Timber Structures.
f ASTM D2344.
gASTM D-1037.
h ASTM D3500.
i ASTM D3500 type A.
j ASTM D3501.
are smaller than the standard dimensions presented by Sharma et al.[17]but the flexural modulus value is quite similar to the reported value.
The shear strength of bamboo scrimber with different densities in parallel to grain direction is in good agreement with values given in literature[16,17].Fig. 12b shows the ruptured sample from shear test; the fracture surface is not looks uniform delamina- tion of bamboo fibers rather it looks like a sudden delamination.
The selected region inFig. 12b looks darker in comparison to uni- formly delaminated surface; it may be the outer portion of bamboo culm, which is very dense in general compare to inner portion of bamboo culm. The resin might not be uniformly penetrated the
denser bamboo bundles and that might be the reason for sudden ruptured of bamboo scrimber during shear loading.
4.4. Influence of density on dynamic modulus of elasticity
The ultrasonic impulse method was employed to estimate the dynamic modulus of elasticity (MOED) of bamboo scrimber beams in both parallel and perpendicular to grain directions. The MOEDk and MOED\ bamboo scrimber were influenced by the density;
Table 2shows the estimated values. In the both direction MOED
increases along with the density of specimens. In earlier literature, no data was available about the non-destructive ultrasonic impulse Fig. 10.Fracture behavior of bamboo scrimber during compressive loading in perpendicular and parallel to grain direction.
Fig. 11.Strain measurements using linear strain gauges during the loading and unloading of bamboo scrimber specimens for calculating the E-modulus (a) compressive modulus and (b) flexural modulus.
Fig. 12.Shows the fracture surface of bamboo scrimber (a) flexural testing and (b) shear testing.
evaluation of bamboo scrimber MOE. The MOEDkof bamboo scrim- ber almost three times the MOED\for every set of density.
Interesting point to be noted in the results given inTable 2that the modulus of elasticity of bamboo scrimber parallel to grain for compressive, tensile, dynamic and flexural testing almost showing the similar values for every set of densities. Sharma et al.[17]have compared the mechanical properties of bamboo scrimber with laminated bamboo and laminated veneer lumber (LVL) and dis- cussed the prospects for application in construction. The bamboo scrimber required a high quantity of thermoset resin (PF resin)
> 15%, which is not sustainable for long run and possible emissions of formaldehyde or other volatile organic compounds.
Fig. 13shows the optical micrographs of bamboo scrimber’s cross-section with different densities. The vascular bundles are fully saturated with the phenol formaldehyde resin (dark brown colour), the resin distribution in the bamboo vascular bundles are very similar for each density. So it is hard to claim the resin loading influences the density of prepared bamboo scrimber and Yu et al.[16]) was analysed the influence of different resin loading of PF resin from 4% to 18% on the mechanical and water absorption properties of bamboo scrimber. They found that the resin loading have no significant influenced on the bending strength, bending modulus and shear strength of bamboo scrimber, but they reported that resin loading significantly improved the water absorption and thickness swelling properties of bamboo scrimber. However, the processing parameters such as heat treatment significantly improved the mechanical properties of bamboo scrimber[16].
4.5. Comparison with parallel strand lumber (PSL)
PSL is a composite of wood strand elements with wood fibers primarily oriented along the length of the member. These strands are clipped from veneers similar to those used to produce lami- nated veneer lumber (LVL) or plywood[41]. Engineered bamboo scrimber also have similar orientation of fibers along length of
the member, but have higher density in compare to parallel strand lumber. PSL generally have density below 800 kg/m3[41,42]. The compressive strength of PSL produced from Southern Pine strands with density 680 kg/m3was 54.2 MPa [41], which is half of the compressive strength (115.2 MPa) of bamboo scrimber with den- sity 1215 kg/m3as shown inTable 3. PSL have showed half flexural strength compare to dense bamboo scrimber and as well as lower flexural modulus. Similarly the Yellow poplar PSL (620 kg/m3)[41]
has lower values of compressive and flexural properties in compare to Southern pine (680 kg/m3), here also density influences the mechanical properties of final product like engineered bamboo scrimber showed.
Kurt et al[42]. produced PSL usingPopulus deltoidsstrands with low density 460 kg/m3the compressive strength of produced PSL are similar to PSL made of Yellow poplar strands (620 kg/m3) as shown in theTable 3, but have lower flexural strength and flexural modulus. In another study, Ahmad and Kamke[43]fabricated Cal- cutta bamboo based PSL with density 773 kg/m3, the flexural prop- erties of bamboo PSL are quite similar to result presented in current study for 1054 kg/m3density bamboo scrimber, but the compressive strength bamboo PSL is much lower compare to bam- boo scrimber as shows inTable 3.
5. Conclusion
The present work evaluated the influence of bamboo scrimber density and bamboo fiber grain orientation on the mechanical properties such as compressive strength, tensile strength, shear strength, flexural strength and elastic modulus for compression, tensile, flexural. The long term water absorption and dynamic modulus of elasticity were also estimated. The results revealed that all the mechanical properties of bamboo scrimber significantly vary with the density of bamboo scrimber as well as with the fibers orientation. The dynamic modulus of elasticity for bamboo Fig. 13.Optical microscopy images of bamboo scrimber cross-sections with different densities.
scrimber was evaluated first time in both parallel and perpendicu- lar fiber direction. The long term water absorption result also sig- nificantly varies with density of specimens. So, the present results will be very useful for further development for uniform density bamboo scrimber production and applications. According to the long term water absorption results, fungal and molds growth were noticed, so proper preservation of bamboo fibers also needed for futuristic exterior application of bamboo scrimber.
There are still no standard and code are available to character- ized the engineered bamboo composites. So, we have compared the mechanical properties of engineered bamboo scrimber with the literature, those have used different standards.
Acknowledgements
We acknowledge the support of Ministry of Education, Youth and Sports, Czech Republic, within National Sustainability Pro- gramme I (NPU I), project No. LO1605 – University Centre for Energy Efficient Buildings – Sustainability Phase. Authors would like to thank the Fraunhofer WKI for Financial support to Dr. Anuj Kumar.
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