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Nanomaterial-inspired building materials, especially those involving both concrete and carbon-based nanomaterials, have gathered a lot of interest recently. Even over the course of the last year, the field has developed significantly with the benefits being documented by several companies about how graphene can be used in concrete. However, while graphene has hit the headlines for nanomaterial-enhanced concrete, carbon nanotubes (CNTs) are also presenting themselves as an alternative option for improving the properties of concrete.
Carbon nanotubes (CNTs) have been a material of much discussion over the years. Like graphene, they were dubbed a wonder material. However, interest faded quite quickly (and for quite a while) not long after being dubbed a wonder material. This was because people could not work out how to use them.
Now that many people have become aware of the potential and commercial viability of graphene and other carbon-based nanomaterials, there has been a resurgence and interest in using CNTs for a range of applications, including fundamental research and in commercial settings.
While there are very differentiating structures of CNTs and graphene, at their most-hyped point, they were both touted for a lot of the same applications, especially structural applications. CNTs can bring beneficial properties to concrete when integrated into its matrix, but the effects and benefits are much different from those experienced with graphene and other nanomaterials. It could be entirely possible that we will see different nano-enhanced concrete materials on the market that are suitable for different scenarios and locations.
Why CNTs are Integrated into Concrete
The constituents inside concrete are not perfectly bonded together. While this is the case for many composite materials, the effects can be prominent in hardened concrete and range from pores to pre-cracks (that can develop into cracks under an applied load). Many of the techniques—known as reinforcing techniques—use filler materials to fill the pores and chemically connect the different constituents within the concrete matrix.
In this area, there is a separate sub-area known as fiber-reinforced concrete, which is when fibrous materials are used as the filler. In many concrete matrices, the fibers integrated into the concrete typically range from millimeters to centimeters in length. While these fibers can fill most of the voids in the matrix, the size of them means that the nano-sized pores within the matrix are left unfilled.
CNTs have emerged as an option to fill these voids within the concrete and develop a more ‘complete’ bonding of the constituents inside the concrete matrix. CNTs are known for their structural properties and ability to reinforce materials. In concrete matrices, the CNTs interact with calcium silicate hydrate nanoparticles that are already present in the concrete mix. A large number of carbon atoms in the nanotubes form an interface with the atoms in the calcium silicate hydrate.
These interfaces enable stress-induced forces to be transferred more efficiently, and the inherent flexibility of the nanotubes enables them to form molecular bridges across the micro- and nano-sized cracks in the matrix. This increases the overall mechanical and physical properties of the concrete.
There are several ways in which the addition of CNTs has beneficial effects on the properties of the concrete. However, there are also some challenges that need to be ironed out at very high temperatures. For example, CNT-concrete shows excellent physical properties at room and elevated temperatures, however, the effects become negative compared to other reinforced concretes at very high temperatures.
Improved Flexural Strength
The addition of CNTs can significantly improve the flexural strength of concrete. Like many nanomaterial additives, only a small amount (0.05%) of CNTs are needed to induce benefits. At normal temperatures, the CNT-enhanced concrete has a much better load capacity and ductility properties thanks to the CNT-calcium silicate hydrate interface.
This interface enables the flexible properties of the CNT to have a greater impact across the composite, increasing the concrete’s ductility and flexural strength. CNTs are also responsible for controlling any shear strains across the matrix and can deal with higher load capacities.
Even at elevated temperatures, CNT-enhanced concrete shows better ductility properties compared to other reinforced concretes. While the ductility properties do worsen by some degree at high temperatures (400-600 °C), the properties decrease gradually and by a much lesser degree than other reinforced concretes which tend to lose their properties very quickly (and by a much greater degree).
Issues with Crack Resistance at High Temperatures
CNTs also improve the crack resistance of concrete, and in particular, the flexural crack resistance. At room temperatures, the crack resistance properties can be enhanced by up to 35%, however, these properties start to deteriorate at higher temperatures. At around 400 °C, there are small improvements in performance over other reinforced concrete. However, beyond this, the performance is worse than other reinforced concrete materials.
At very high temperatures (600 °C), the crack resistance properties of CNT-concrete begin to fall well below that of other reinforced concretes because the CNTs de-bond from the matrix. While CNT-concrete is excellent at room temperature, and still good at temperatures of up to 400 °C, once you get to very high temperatures, other reinforced concretes perform better in terms of crack resistance. However, despite the failures at these high temperatures, the CNT-concrete can still support higher loads than other reinforced concrete.
Even though graphene has recently taken the headlines for creating environmentally-friendly concrete that uses less material, the offerings from CNT-reinforced concrete offer something different to other nano-enhanced concretes. They offer very high flexural strengths and enhanced ductility, with no deterioration in other mechanical properties. However, there are limitations, especially at high temperatures.
While the ductility is still much better at high temperatures compared to other concrete, the high temperatures cause the matrix to de-bond, increasing the chance of cracks appearing. While there is a lot of potential for CNT-enhanced concrete, special attention is needed if the concrete is to be used in fire hazardous buildings.
References and Further Reading
Elkady H. and Hassan A. (2018), Assessment of High Thermal Effects on Carbon Nanotube (Cnt)-Reinforced Concrete, Scientific Reports, https://doi.org/10.1038/s41598-018-29663-5