Tokyo Metropolitan University Official web magazine
My Research Frontier - Vol. 5

Endeavoring to Establish an International Standard for CO2-fixing Carbon Pool Concrete

上野 敦准教授
Faculty of Urban Environmental Science / Department of Civil and Environmental Engineering / associate professor UENO Atsushi

Ueno, who holds a PhD in Engineering, graduated from the Department of Civil Engineering at the Tokyo Metropolitan University (TMU) Faculty of Engineering. Since then, he served in positions including as Research Associate at the Faculty of Engineering before arriving at his current position in 2007. He specializes in areas including concrete engineering and concrete materials science. His current focuses include the practical application of low environmental impact concrete and the characterization of concrete materials originated from by-products.


In the “My Research Frontier” series, we ask TMU faculty to talk about their newest research findings, as well as what makes their research interesting. For this volume, we spoke with Associate Professor Atsushi Ueno of the Faculty of Urban Environmental Science, who is part of a Green Innovation Fund Project promoted by NEDO (New Energy and Industrial Technology Development Organization).

Q. Tell us about your areas of expertise and your current research.

I am a materials specialist who studies the general topic of concrete. In particular, I research pavement materials used in roads and sidewalk, as well as pavement techniques using those materials. We study structures that give pavement materials the permeability, drainage, water retention, and other characteristics needed by different applications. Our focuses include techniques that control skids in rainy conditions and the heating of pavement surfaces by sunlight. Each of these research efforts are aimed at increasing the additional value of cement concrete.

Together with Assistant Professor Tomohisa Kamada, I currently am part of a NEDO (New Energy and Industrial Technology Development Organization) Green Innovation Fund Project aimed at establishing carbon recycling technologies in the fields of concrete materials. More specifically, the project is aimed at establishing a CO2-fixing technology called “carbon pool (CP) concrete,” which will reduce the CO2 emissions generated by concrete and cement production. TMU is in charge of researching the social implementation of this technology in pavement, while the University of Tokyo is in charge of building construction applications. The Central Research Institute of Electric Power Industry is researching its applications to the construction of civil engineering structures.

We have also developed technologies for creating continuous pores in conventional road pavements for the purpose of road surface drainage and recharging rainwater to the ground . These pores will likely also effectively function to absorb and fix atmospheric CO2, including exhaust gases from vehicles. Meanwhile, breaking up older concrete structures into gravel or sand-sized pieces and laying them under roads is a viable method of reusing materials. This fine demolished concrete which is referred to as recycled crushed stone, has a reuse ratio of over 90%. However, there is still need for deeper consideration of higher-levels of materials use . To that end, methods to use it as recycled aggregate in place of the aggregate particles used in the production of cement concrete have been studied and are now standardized by JIS (Japanese Industrial Standards). Despite this fact, these methods have actually not seen widespread adoption. While there are no regulatory issues with their use, there tends to be a higher demand for natural aggregate. It is true that when you take a closer look at the properties of recycled aggregate, it cannot be denied that it is easily deformed due to the cement paste that has attached to the original aggregate particles. That said, additional grinding of recycled aggregate is expensive, uses more energy, and increases the environmental impact of production of the aggregate. In this regard, the NEDO project will contribute to solving this and other existing issues.

「Conceptual Diagram of the Development of Technology for Producing Concrete and Cement Using CO2
Conceptual Diagram of the Development of Technology for Producing Concrete and Cement Using CO2
by NEDO (New Energy and Industrial Technology Development Organization) HP

Q. Please tell us more about the NEDO project.

The project, which is one of the Green Innovation Fund Projects, is entitled the “Development of Technology for Producing Concrete and Cement Using CO2.” In addition to institutes of higher learning such as TMU and the University of Tokyo, the project also involves so-called ”Super General Contractors” (The five biggest Japanese construction companies), and other entities. This inter-disciplinary initiative brings wisdom from all over Japan. By extension, the project can be said to be aimed at achieving carbon neutrality and carbon negativity with an eye on a decarbonized society.

One of the research and development themes of the project is the development of carbon pool concrete (CPC), which fixes CO2. Largely divided, we are taking two main approaches: the development of new recycled aggregates and the utilization of sludge powder, which is otherwise discarded as waste. Both approaches aim to achieve drive greater usage of concrete waste and by-products by utilizing CO2 on them.

Consider the example of cement concrete that is no longer used due to obsolescence or functional insufficiency and has become waste material. When crushed into granular recycled aggregate and acted upon by CO2, the cement paste adhering around the original aggregate particles returns to limestone rock. This results in the recycled aggregate particles becoming denser and stronger, greatly mitigating its characteristic of being deformable. What we are attempting to do is fix CO2 in the paving materials themselves at the stage before CO2 is fixed in cement concrete after paving, and by doing so, not only pool carbon, but also improve the characteristics of the materials.

The project involves several companies that work with ready-mixed concrete. This is to promote higher-level use of the surplus ready-mixed concrete left over from various construction projects, which has not been done in the past, by utilizing CO2 in the process of converting it into recycled aggregate. By using CO2 as a material rather than treating it as a troublesome gas, it is possible to turn crushed concrete into high-quality recycled aggregate, rather than simply crushing it and using it as-is. Specifically, we are developing a method to fix CO2 in surplus fresh concrete that has been granulated by putting a chemical admixture into the drums of agitator truck, and then use that resulting concrete as recycled aggregate.

In addition, every time fresh concrete has been agitated in a drum, the inside of the drum needs to be cleaned. Cleaning the drum produces a gray sludge of concrete and water. Since it is illegal to discharge untreated sludge, contractors transfer it into settling tanks, where it is agitated to prevent it from solidifying. After the solid particles in the sludge separate from the water, the supernatant water is reused in the next concrete mixing, while the layer of sludge containing fine particles is then filtered. The powder remaining after filtration has previously been disposed of as industrial waste. However, this powder contains cement and fine aggregate. This deposited and filter pressed solid is called sludge cake. If sludge cake is fixed with CO2, it can be turned into a replacement for cement. This initiative to reuse sludge powder is very significant because it will allow something that was previously disposed of to be used to fix CO2.

By the way, my lab is looking not only at the process of utilizing CO2 on recycled aggregate before paving, but also at processes to utilize and fix CO2 after paving. The method we are imagining is akin to spraying water containing CO2 on the surface of pavement. Though we expect the fixation rate to be faster when utilizing CO2 as a gas, the use of water as a CO2 carrier will reduce the amount of CO2 that escapes into the atmosphere. We believe that this will ultimately result in higher CO2 fixation efficiency. To prove this, we are preparing to conduct experiments to verify both the efficiency of CO2 fixation, as well as the amount.

Sample of Pervious Cement Concrete

Q. What is TMU’s mission in this project?

We expect that fixing CO2 in recycled aggregate will have a positive effect on its properties as aggregate. TMU's mission is figuring out what tests will best quantify and demonstrate the qualities of recycled aggregate acted upon by CO2. We will establish and verify test items based on existing standardized test methods and, if necessary, develop new testing and evaluation methods.

Our emphasis is on making the methods sufficiently easy and sensible that producers will actually use it. If the evaluation criteria we choose are excessively strict, producers will say they are infeasible for real-world use. It is essential that we consult with producers to decide on standards that reliably maintain quality but are also within the scope of what they can actually produce. We have to produce a well-balanced standard.

An essential step in this direction is building test pavements with prototype carbon pool concretes. Aiming to ultimately put the standard into service by 2030, we will be verifying the amount of CO2 fixation, and the change in characteristics of the pavement surface over time, all while revising evaluation criteria along the way.

These experimental devices make this TMU facility look like a civil works site!
These experimental devices make this TMU facility look like a civil works site!

Q. What motivates you, and what are your hopes for the future?

One goal of the NEDO project is fixing CO2 through the development of carbon pool concrete. As a material specialist, however, my biggest joy is seeing the potential for higher-level use of recycled aggregate and other pavement materials. One of these materials, sludge powder, is something of an unknown material , so researching it is quite rewarding. While I feel responsibility for establishing a standard for working with this material, exploring its new possibilities makes the research enjoyable.

Naturally, we are pursuing more than just the establishment of CO2 fixation technology to help realize a decarbonized society. We also have to pursue the basic properties required of pavement concrete, such as driving and walking safety. I have a growing expectation that we will be able to gain knowledge that can be applied to existing research, such as safety in inclement weather (e.g., rain and snow), and measures to deal with high pavement surface temperatures in severe heat.

Q. Do you have a message for current and potential students?

I have always been of the mindset that no matter who or what I am dealing with, I am going to take things seriously, and I am going to try to thoroughly enjoy myself. This has caused me to realize something. The field of mechanics, which once interested me (I have loved gears since childhood), and the civil engineering and concrete fields, which are now my life's work (I sometimes get so wrapped up in my work that I forget to eat) share one thing in common: failure to pay attention to the details can lead to catastrophe. I always take my research seriously, because defects on the pavement surfaces that cars drive on and people walk on can be life-threatening.

My advice for students is that you should actively listen to people around you and go with the flow, but be serious about everything you do. Observe things closely and think about them carefully in your own way. Remember the importance of having unyielding convictions while also paying attention to details.

上野 敦准教授