Searching for materials to provide proper housing with less emission and low energy becomes an urgent demand with the ever-growing population. Bamboo has gained a reputation as an ecofriendly, highly renewable source of material. Parallel Strand Bamboo (PSB) is a new biocomposite made of bamboo strips which has superiority performances than wood products. It has attracted considerable interests as a sustainable alternative for more traditional building materials. But the mechanical performance study of PSB as construction materials is still inadequate. Also, the structural behavior of PSB is not quite understood as conventional construction materials, which results in the difficulties to predict the performances of PSB structural members. To achieve this purpose, 4-point bending experiments for PSB beams were carried out. The flexural performances, mode of failure in bending, and the damage mechanism of PSB beams were investigated in this paper.
In recent years, with the ever-growing population, searching for materials to provide proper housing with less emission and low energy becomes a challenge. Bamboo has gained a reputation as an ecofriendly, highly renewable source of material. Bamboo grows much faster than wood. Usually could reached maturity in 5 years. But raw bamboo can not meet the requirements as modern building materials because of their many varieties in mechanical properties and in geometrical shapes and sizes. To meet the requirements of current structural applications, many efforts have been made to bamboo to have stable properties and more uniform. Parallel Strand Bamboo (PSB) belongs to one of the engineered materials made with bamboo, which is fabricated by cutting bamboo into strips along parallel-to-grain direction by using adhesive in the lamination process parallel to each other into a prism often with a rectangular cross section [
Bamboo for the experiment came from Anhui province in China, which had been grown for 5 years. Bamboo was cut into many segments with a length of 2 m parallel to its grain. To study the mechanical performance of its different parts, three groups of samples were divided into the bottom, the middle, and the top part of the culm. Then every segment was split into strips (3 mm thickness, 10 mm width, and 2 m length). These strips gotten rid of the bamboo green and the bamboo yellow were grinded to be linked together in parallel-to-grain directions and unlinked in transverse-to-grain directions. In order to reduce the starch content, bamboo strips were carbonized in the steam oven with a pressure of 0.3 MPa for 90 min., as bamboo with low humidity is less prone to mould attacks. For its conservation, bamboo was air-dried in a drying house, and then these bamboo strips were soaked in a glue water pond for 10 min., air-dried again in the drying house to a humidity moisture content of about 12%, and loaded into an iron mould for thermoforming in the stove with a temperature of 120°C, at 150 MPa pressure for 8 hours. PSB beams (160 mm × 110 mm × 1880 mm) were fabricated by this method.
The 13 PSB beams were tested to investigate their bending performances. The cross sections of the beams were 80 × 110 mm2, and the lengths of them were 1560 mm. 5 PSB beams were fabricated from the bottom bamboo, 4 PSB beams were fabricated from the middle bamboo, and 4 PSB beams were fabricated from the top bamboo. Because there is no standard test method for the structural members of bamboo composites, the dimensions of specimens and the test setup were designed referring to ASTM D198-02 [
Test setup and photo of test.
The phenomena of the damage process could be summarized as follows. In the earlier stage of loading, the deflections in the middle span were linearly associated with the augmentation of load. When the load exceeded 1/3 to 1/2 of the ultimate load, some fine cracks within the moment span emerged and expanded along the parallel-to-grain direction below the neutral axis as the load increased, and bulking could be observed at the top surface of the beams in some cases. Finally the break occurred at the bottom of the beams when the loading reached the ultimate value (referring to Figure
Failure modes of specimens.
The bending strength of PSB is a complex problem. Bending strength depends on three main factors: the ratio of tension to compression strength of the material, nonlinear ductile behavior in the compression zone, and size-dependent brittle fracture in the tension zone. The tension strength of PSB was far greater than compression strength according to preliminary work of authors of this paper [
In summary, the bending damage mechanism of the beams could assume that (i) the beams were in the linear stage and could be idealized as a perfect elastic element at the beginning of loading; (ii) with the augmentation of loading, beams went into the nonlinear stage; the fibers in the top of the compressive zone, which were over the neutral axis of the cross section in the moment span, first reached the plastic state and the stress on them was gradually coming to the ultimate compressive strength from the outside extending to the inside of the beam; and (iii) the stress of the fibers in the outside of the tensile zone, which was below the neutral axis in the moment span, finally reached its ultimate (maximum) tensile strength and then broke.
Figure
Strain distribution in parallel-to-grain direction over the depth of section.
The relationship of strain changing with load was shown in Figure
Load-strain typical curve of PSB beam.
The direct results obtained from the bending tests are the load versus midspan deflection curves. Figures
(a) Load versus midspan deflection typical curves of PSB beams the tip.mid.bottom, respectively. (b) Load versus midspan deflection curves of most specimens.
The test result showed that tension modulus was slightly superior to the compression modulus. The load-deflection information derived from the 4-point bending tests can be analyzed using formulae for structural analysis. In this case, formulae for beams under bending loads are used. Assuming a perfect adhesion between PSB, in the proposed tests
Test results of specimens.
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Top | ||||
LT-1 | 87.81 | 12604 | 12734 | 12438 |
LT-2 | 96.56 | 12548 | 13029 | 13874 |
LT-3 | 93.07 | 12434 | 12736 | 12420 |
LT-4 | 88.08 | 12403 | 13792 | 13342 |
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Mean |
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Mid | ||||
LM-1 | 95.49 | 12464 | 12733 | 12863 |
LM-2 | 95.95 | 13037 | 15291 | 14425 |
LM-3 | 86.76 | 13981 | 13365 | 14409 |
LM-4 | 88.17 | 12313 | 13747 | 12614 |
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Mean |
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Bottom | ||||
LB-1 | 88.98 | 12112 | 12232 | 10933 |
LB-2 | 81.60 | 13765 | 13404 | 13506 |
LB-3 | 91.05 | 13639 | 14718 | 13002 |
LB-4 | 85.90 | 11341 | 11692 | 10083 |
LB-5 | 81.81 | 11882 | 12407 | 12398 |
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Mean |
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Total mean |
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Where
The bending strength and elastic modulus of upper PSB was about 10% higher than that of the bottom referring to Table
An overall analysis of the experimental results revealed the following. Mode of failure in bending was nonlinear ductile behavior in the compression zone and size-dependent brittle fracture in the tension zone. As compression yielding occurred, the neutral axis shifted toward the tension face, and tension stresses continued to increase until failure occurred as a rupture in the tension zone. The bending damage mechanism of the PSB beams could assume that (i) the beams were in the linear stage and could be idealized as a perfect elastic element at the beginning of loading; (ii) with the augmentation of loading, beams went into the nonlinear stage, as the fibers in the top of the compressive zone firstly reached the plastic state; and (iii) the stress of the fibers in the outside of the tensile zone finally reached its ultimate tensile strength before breaking. The PSB beams were accorded with the plane assumption. Therefore, the influence of the shearing effect on the bending of the beam may be ignored. Large nonlinear deformations were achieved and the ultimate deflections of PSB beams could reach about 1/35 span before breaking. The tension modulus was slightly superior to the compression modulus. The mechanical performances of PSB made from the bottom raw bamboo were inferior to that of the upper. The bending strength and Young’s modulus of PSB could be about 89 MPa, and 12000 MPa respectively.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The research was supported by the National Natural Science Fund of China (51378263), the Provincial Fund of Science of Jiangsu province (no. BK2012820), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.