Posted on May 12, 2022
One way to address concrete’s sustainability goals is to craft it to serve multiple masters. To a significant degree, it already does. For example, thanks to carbonation, exposed concrete meets structural needs while also serving as a carbon sink.
What if it is possible for concrete to take on other tasks? What if it could generate electricity or heat? What if it could store electricity? Researchers have developed special cement-based composites (CBCs) that exhibit electrical properties without undermining structural performance. They’ve tried various conductive materials, including nanocarbon black and carbon fibers.
Concrete and Electricity
Nanocarbon black is at once conductive, available, and affordable. Researchers found that an even distribution of the right concentration of nanocarbon black particles makes concrete conductive.
Research into nanocarbon doping of concrete is the result of a multi-year effort. Researchers at MIT's Concrete Sustainability Hub (CSHub) collaborated with a team at the French National Center for Scientific Research (CNRS).
The “percolation threshold” is the point at which concrete can carry a current. The team at MIT found that it was possible to reach the percolation threshold with sufficient infusion and distribution of nanocarbon black. Selected formulations also ensure that electrical currents remain at levels safe for humans.
Ideal conductivity
To improve conductivity in concrete, the team sought to optimize a metric called “tortuosity.” The goal is to make the conductive pathways as efficient as possible. It’s like comparing distances “as the crow flies” versus actual road routes. The latter is always longer than the former. The longer the path, the higher the tortuosity. In other words,
In conductive concrete, optimum tortuosity requires a balancing of the amount and dispersion of carbon. A tortuosity value of two is ideal. This means that the electron path is only twice the length of the sample. Too little nanocarbon black fails to achieve the desired conductivity. At the same time, too much actually sabotages the effort.
Potential Uses of Conductive Concrete
Concrete that conducts electricity offers an array of tantalizing possibilities. It can make paving more sustainable. It can make concrete buildings self-sensing. Conductive concrete may also allow for more efficient slab heating. Finally, it may reduce a building’s energy consumption by a significant percentage.
Make paving more sustainable
Conductive pavement could reduce concrete’s carbon footprint in important ways. For example, less use of deicing chemicals reduces road damage. This delays repairs and/or replacements. It also limits the emissions of heavy salt trucks. Reduced salt use improves the environment. Airport runways and hilly roads are among the potential beneficiaries. Iowa State researchers have examined the deicing potential of concrete infused with recycled carbon fibers.
Become sensors for structural health
Nanocarbon-infused concrete may also serve as a self-sensing mechanism for monitoring structural health. For example, it could predict a concrete structure’s remaining service life. This approach reduces the need for traditional sensors and their external power sources.
Provide slab heating
Thanks to the Joule effect, the current also generates heat. This is the thermal manifestation of electrical resistance. At low five-volt levels, surface temperatures could rise as much as 100 degrees. This makes radiant floor heating another possible application. Conductive concrete could offer more even heat distribution than current systems.
Reduce energy consumption
The triboelectric effect is the result of friction between two non-conductive objects. Contact electrification occurs as material becomes electrically charged. Thanks to the triboelectric effect, potential energy sources range from waves and wind to rain and even footsteps.
Another research team focused on carbon fibers rather than nanocarbon black. The team included engineers associated with several South Korean universities. The results of their research appeared in the journal Nano Energy in November 2021.
In discussing the use of carbon fibers, team member Seung-Jung Lee said, “we decided to use it with conductive fillers as the core conductive element for our CBC-TENG system.”
They deployed a CBC-based capacitor in a lab-based structure. They found that a 1% carbon-fiber formulation was ideal. In their concrete formulation, carbon fibers become a mechanical energy harvester called a triboelectric nanogenerator (TENG). The South Korean researchers estimate the formulation could reduce a building’s energy consumption by up to 40%.
With every new innovation in cement/concrete, the scalability question inevitably arises. MIT researchers specifically selected nanocarbon black because it is a lower-cost material that is readily obtainable.
Carbon-based Materials: Related Uses
Researchers identify other ways to enhance concrete’s properties with carbon-based materials.
Cement-based batteries
In the journal Buildings, a study details rechargeable cement-based batteries. Carbon fibers improve the conductivity of the cement-based electrolyte. The design combines an iron and zinc anode with a nickel-based oxide cathode. Researchers examined cement-based electrodes produced by 1) powder-mixing, and 2) metal-coating. The latter delivered the best performing Ni-Fe battery.
CFRP concrete
Concrete with carbon fiber-reinforced polymer (CFRP) is one of many fiber-reinforced polymer concrete formulations. Carbon fiber delivers strength and stiffness while the polymer is a bonding agent.
CFRP concrete is corrosion and alkali resistant. High strength-to-weight ratio. High fatigue resistance. Short curing times translate into quicker project completion. Unlike CBC concrete, it has low electrical and thermal conductivity. It is also non-magnetic. Contractors use CFRP concrete to extend the life of bridges and other critical infrastructure.
About PACA
The Pennsylvania Aggregates and Concrete Association (PACA) reports on innovations through this website. These posts inform industry members and the general public alike.
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