Researchers study bamboo for engineeredbuilding material23 July 2014, by Jennifer Chu

Researchers in Gibson's lab have collected bamboosamples of various thicknesses to analyze bamboo'smicrostructure. Credit: Jennifer Chu/MIT

Bamboo construction has traditionally been ratherstraightforward: Entire stalks are used to createlatticed edifices, or woven in strips to form wall-sized screens. The effect can be stunning, andalso practical in parts of the world where bamboothrives.

But there are limitations to building with bamboo.The hardy grass is vulnerable to insects, andbuilding with stalks—essentially hollowcylinders—limits the shape of individual buildingcomponents, as well as the durability of thebuilding itself.

MIT scientists, along with architects and woodprocessors from England and Canada, are lookingfor ways to turn bamboo into a constructionmaterial more akin to wood composites, likeplywood. The idea is that a stalk, or culm, can besliced into smaller pieces, which can then bebonded together to form sturdy blocks—much likeconventional wood composites. A structuralproduct of this sort could be used to construct

more resilient buildings—particularly in places likeChina, India, and Brazil, where bamboo isabundant.

Such bamboo products are currently beingdeveloped by several companies. The MIT projectintends to gain a better understanding of thesematerials, so that bamboo can be more effectivelyused structurally. To that end, MIT researchershave now analyzed the microstructure of bambooand found that the plant is stronger and denserthan North American softwoods like pine, fir, andspruce, making the grass a promising resource forcomposite materials.

"Bamboo grows extensively in regions where thereare rapidly developing economies, so it's analternative building material to concrete and steel,"says Lorna Gibson, the Matoula S. SalapatasProfessor of Materials Science and Engineering atMIT. "You probably wouldn't make a skyscraper outof bamboo, but certainly smaller structures likehouses and low-rise buildings."

Gibson and her colleagues analyzed sections ofbamboo from the inside out, measuring thestiffness of each section at the microscale. As itturns out, bamboo is densest near its outer walls.The researchers used their data to develop a modelthat predicts the strength of a given section ofbamboo.

The model may help wood processors determinehow to assemble a particular bamboo product. AsGibson explains it, one section of bamboo may bemore suitable for a given product than another: "Ifyou wanted a bamboo beam that bends, maybeyou'd want to put the denser material at the top andbottom and the less dense bits toward the middle,as the stresses in the beam are larger at the topand bottom and smaller in the middle. We'relooking at how we might optimize the selection ofbamboo materials in the structure that you make."

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Gibson and her colleagues have published theirresults in the Journal of the Royal Society:Interface.

For their experiments, the researchers analyzedspecimens of moso, the main species of bambooused in China. Like most types of bamboo, mosogrows as hollow, cylindrical stalks, or culms,segmented by nodes along the length of a stalk.Bamboo can reach heights of 20 meters—as tall asa six-story building—in just a few months. The stalksthen take another few years to mature—but stillmuch faster than a pine tree's statelier, decades-long growth.

"One of the impressive things is how fast bamboogrows," Gibson notes. "If you planted a pine forestversus a bamboo forest, you would find you cangrow far more bamboo, and faster."

Researchers used electron microscopy to obtainimages of the bamboo microstructure and createcomplete, microscale cross-sections of the entireculm wall at different heights along the stalk.

The resulting images showed density gradients ofvascular bundles—hollow vessels—that carry fluid upand down the stalk, surrounded by solid fibrouscells. The density of these bundles increasesradially outward—a gradient that seems to growmore pronounced at higher positions along a stalk.

The researchers cut sections of bamboo from theinside out, noting each sample's radial andlongitudinal position along a culm, then gauged thestiffness and strength of the samples by performingbending and compression tests. In particular, theyperformed nanoindentation, which uses a tinymechanical tip to push down on a sample, to gainan understanding of bamboo's material propertiesat a finer scale. From the results of thesemechanical tests, Gibson and her colleagues foundthat in general, bamboo is stiffer and stronger thanmost North American softwoods commonly used inconstruction, and also denser.

This images shows a vascular bundle. You can see it ismade up of the vessels (large dark holes, empty looking)and supporting fibers (somewhat dark very solid lookingregions). The parenchyma (light circular cells) surroundthe vascular bundle (vascular bundle refers to the overallclover shaped structure).

The researchers then used the stiffness anddensity data to create a model that accuratelypredicts the mechanical properties of bamboo as afunction of position in the stalk. Gibson says woodprocessors that she works with in Canada may usethe model as a guide to assemble durable bambooblocks of various shapes and sizes.

Going forward, the processors, in turn, will send theMIT team composite samples of bamboo tocharacterize. For example, a product may beprocessed to contain bamboo along with othermaterials to reduce the density of the product andmake it resistant to insects. Such compositematerials, Gibson says, will have to be understoodat the microscale.

"We want to look at the original mechanicalproperties of the bamboo culm, as well as howprocessing affects the product," Gibson says."Maybe there's a way to minimize any effects, anduse bamboo in a more versatile way."

Oliver Frith, acting director of programme for theInternational Network for Bamboo and Rattan,headquartered in Beijing, says that very fewspecies of bamboo have been classified, and the

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lack of knowledge of the material's microstructurehas impaired efforts to design efficient, optimalstructural products.

"MIT's work is very timely and has great potential tosupport development of the sector," says Frith, whowas not involved in the research. "While bamboohas similarities to wood, as this study shows, thematerial also has very distinct properties. Althoughcurrent approaches to developing structuralengineered bamboo have tended to focus onmimicking engineered wood products, the futurewill probably lie in innovating new approaches thatcan better enhance the natural advantages of thisunique material."

Provided by Massachusetts Institute ofTechnologyAPA citation: Researchers study bamboo for engineered building material (2014, July 23) retrieved 9July 2018 from https://phys.org/news/2014-07-bamboo-material.html

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