Bamboo – in the form of small diameter whole culm (bars) or split bamboo (splints or round strips) – has gained some attention as a potential alternative to steel used for reinforcing concrete. As well as being much cheaper than steel, bamboo is also seen as a more sustainable replacement for concrete reinforcement. However, an international team of researchers has demonstrated that bamboo is a poor substitute for steel reinforcement, and often fails to deliver the sustainability benefits it has come to be associated with.
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The authors of the study, published in the journal Materials and Structures, are from the University of Bath and Coventry University in the U.K., Zurich University in Switzerland, Pittsburgh University in the U.S., and Arup, a multinational consultancy firm managing construction and infrastructure projects worldwide.
They found that, while bamboo is a material with remarkable mechanical properties, it does not have enough durability, strength, or stiffness to replace steel in concrete reinforcement applications.
The Differences Between Steel and Bamboo Concrete Reinforcement
Concrete behaves differently when reinforced with bamboo, compared to conventional steel reinforcement. These fundamental differences have to be taken into account when comparing the materials.
First, bamboo is essentially elastic and brittle. Compared to steel, which is ductile, bamboo’s allowable stress in engineering applications is more limited. The margin of error in terms of engineering safety is much narrower as a result.
Steel’s ductility is what has made it such a ubiquitous material in demanding applications. Ductile steel can be used to design statically indeterminate structures safely, applying the lower bound theory of plasticity. Bamboo reinforced concrete, being less ductile, is inadequate for seismically active regions. It is also likely inappropriate for any statically indeterminate structures, said researchers.
Second, characteristic values of bamboo’s longitudinal tensile modulus and strength are generally less than a tenth of steel. Tensile modulus ranges from 7.5 to 13 GPa while characteristic strength is generally around 40 MPa. Bamboo also typically has a 12% moisture content. This results in allowable design strengths of around 16 MPa. The low modulus causes significant serviceability considerations (deflections and crack control) which must be carefully designed against.
A third challenge is bamboo’s anisotropic nature. This leads to complex interactions with the concrete around bamboo reinforcement. These include issues with bamboo’s coefficient of thermal expansion (CTE). Concrete and steel both exhibit similar CTEs, while bamboo’s CTE is different.
Bamboo’s CTE is also much greater in the transverse direction than it is in the longitudinal direction. This affects the compatibility of bamboo with surrounding concrete and significantly impacts the behavior of composite bonds in the final material. Bamboo’s anisotropic structure also makes it dimensionally unstable, requiring some form of treatment to resist moisture transmission. Anisotropy in bamboo means that dimensional stability is not uniform across different directions.
The fourth challenge for bamboo reinforcements is that, unlike steel, bamboo is susceptible to degradation after exposure to hygrothermal conditions and in high-alkali environments. Both of these conditions prevail when bamboo is embedded in concrete. On the other hand, bamboo is not threatened by corrosion, which degrades steel reinforcement.
The final challenge that scientists noted was bamboo’s susceptibility to termite and fungal attacks. This means that it degrades quickly after exposure to high levels of moisture. The authors found no published or industry guidance to suggest that bamboo is protected against rot when embedded in concrete. Even coating bamboo reinforcements with waterproofing products may have no effect.
Does Bamboo Have Any Applications in Construction?
While the authors conclude that bamboo’s applications in concrete reinforcement are misguided, they do cite some instances where bamboo has been successfully used in construction. They note, however, that such applications are beyond the scope of their study and so did not comment on the suitability of bamboo for these applications.
Small cane or bamboo splints can be used as an alternative for crack control reinforcement in concrete slabs on grade (concrete cast directly into the ground). These slabs are designed to stay uncracked or are built with control designs that only permit controlled cracking in the concrete.
Light cement bamboo frame (LCBF) panels are also a well-established construction technique. The use of these panels, also known as bahareque construction, is a modern construction technique that uses composite shear panels. The composite panels are made up of a wall matrix of bamboo or metal lath that is nailed onto a bamboo framing system. The matrix is plastered with cement or lime mortar render. The method only works because the wall matrix’s stresses are very low, but it is recognized and promoted by standardization, falling under the ISO 22156 standard.
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Small culm or bamboo splints can also be used in reinforcement applications for masonry construction. Masonry reinforcement works differently from concrete reinforcement, and so bamboo reinforcement has been proposed for use in hollow-core masonry in non-seismic environments.
Heat-treated, densified engineered bamboo composites have also been proposed for concrete reinforcement. These composite strips have high reported strength and are promising considering they may overcome the obstacles of bamboo reinforcement highlighted in the study. However, the authors pointed out the additional processing, energy, and resin requirements that would have an impact on the material’s sustainability.
In any case, using bamboo in construction necessitates a number of processes and treatments to counteract the issues of durability, dimensional stability, and bond highlighted in the recent paper. These processes often work against bamboo’s sustainability credentials, making bamboo products more environmentally harmful as well as less appropriate for critical applications.
References and Further Reading
Archila, H., S. Kaminski, and K.A. Harries (2018). Bamboo reinforced concrete: a critical review. Materials and Structures. Available at: https://doi.org/10.1617/s11527-018-1228-6.
Moroz, J.G., S.L. Lissel, and M.D. Hagel (2014). Performance of bamboo reinforced concrete masonry shear walls. Construction and Building Materials. Available at: https://doi.org/10.1016/j.conbuildmat.2014.02.006.