Bamboo – in the form of small-diameter whole thatch (bars) or split bamboo (splints or round strips) – has attracted attention as a potential alternative to steel used to reinforce concrete. In addition to being much cheaper than steel, bamboo is also considered a more durable substitute for reinforcing concrete. However, an international team of researchers has demonstrated that bamboo is a poor substitute for steel reinforcement and often fails to deliver the durability benefits it is associated with.
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The authors of the study published in the journal Materials and Works, come from the University of Bath and the University of Coventry in the United Kingdom, the University of Zurich in Switzerland, the University of Pittsburgh in the United States and Arup, a multinational consulting company which manages construction and infrastructure projects around the world.
They found that although 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 should be considered when comparing materials.
First, bamboo is essentially elastic and brittle. Compared to steel, which is ductile, the allowable stress of bamboo in engineering applications is more limited. The margin of error in terms of technical security is therefore much narrower.
Steel’s ductility is what has made it such a ubiquitous material in demanding applications. Ductile steel can be used to safely design statically indeterminate structures, applying the theory of the lower limit of plasticity. Bamboo-reinforced concrete, being less ductile, is unsuitable for seismically active regions. It is also likely inappropriate for any statically indeterminate structure, the researchers said.
Second, the characteristic values of longitudinal tensile modulus and strength of bamboo are generally less than one tenth of those of steel. The tensile modulus ranges from 7.5 to 13 GPa while the characteristic strength is generally around 40 MPa. Bamboo also typically has a moisture content of 12%. This results in allowable design strengths of approximately 16 MPa. The low modulus results in significant service considerations (deflections and crack control) that must be carefully designed.
A third challenge is the anisotropic nature of bamboo. This leads to complex interactions with the concrete around the bamboo reinforcements. These include issues with the coefficient of thermal expansion (CTE) of bamboo. Both concrete and steel have similar CTEs, while the CTE of bamboo is different.
The CTE of bamboo is also much higher in the transverse direction than in the longitudinal direction. This affects the compatibility of the bamboo with the surrounding concrete and has a significant impact on the behavior of the composite bonds in the final material. Bamboo’s anisotropic structure also makes it dimensionally unstable, requiring some form of treatment to resist moisture transmission. The anisotropy of bamboo means that the dimensional stability is not uniform in different directions.
The fourth challenge for bamboo reinforcements is that, unlike steel, bamboo is susceptible to degradation after exposure to hygrothermal conditions and in strongly alkaline environments. These two conditions prevail when the bamboo is embedded in concrete. On the other hand, bamboo is not threatened by corrosion which degrades the steel frame.
The final challenge the scientists faced was bamboo’s susceptibility to termite and fungal attack. This means that it degrades quickly after exposure to high levels of humidity. The authors found no published or industry guidelines suggesting that bamboo is protected from rotting when encased in concrete. Even coating the bamboo reinforcements with waterproofing products may have no effect.
Does bamboo have applications in construction?
Although the authors conclude that the applications of bamboo in concrete reinforcement are flawed, they cite some cases where bamboo has been successfully used in construction. They note, however, that such applications are outside the scope of their study and therefore did not comment on the suitability of bamboo for these applications.
Small cane or bamboo splints can be used as an alternative for reinforcing crack control in concrete slabs at grade (concrete poured directly into the ground). These slabs are designed to remain uncracked or are constructed with control designs that allow only controlled cracking in the concrete.
Lightweight 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. Composite panels consist of a wall matrix of bamboo or metal slats that is nailed to a bamboo framing system. The matrix is coated with a plaster of cement or lime mortar. The method only works because the stresses of the wall matrix are very low, but it is recognized and promoted by standardization, falling under ISO 22156.
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Small thatch or bamboo splints can also be used in reinforcement applications for masonry construction. Masonry reinforcement works differently than concrete reinforcement, and so bamboo reinforcement has been proposed for use in hollow core masonry in non-seismic environments.
Densified and heat-treated engineered bamboo composites have also been proposed for concrete reinforcement. These composite strips have high strength and are promising as they can overcome the barriers of bamboo reinforcement highlighted in the study. However, the authors pointed out the additional processing, energy, and resin requirements that would impact the durability of the material.
In any case, the use of bamboo in construction requires a number of processes and treatments to counter the issues of durability, dimensional stability and adhesion highlighted in the recent article. These processes often go against bamboo’s sustainability benchmarks, making bamboo products more harmful to the environment and less suitable for critical applications.
References and further reading
Archila, H., S. Kaminski and KA Harries (2018). Concrete reinforced with bamboo: a critical assessment. Materials and Structures. Available at: https://doi.org/10.1617/s11527-018-1228-6.
Moroz, JG, SL Lissel and MD 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.