Change Leader Interview: Quantifying the Usability of Recycled Concrete
Yahya Kurama is a professor and associate chair of the University of Notre Dame’s Department of Civil & Environmental Engineering & Earth Sciences. A recent research project focuses on the usability of recycled concrete for building structures rather than just as road base. But to make recycled concrete a legitimate choice as a recycled building material for new construction, more needs to be known about its properties.
Research has focused on classifying old crushed concrete for use as coarse aggregates in new construction, with an eye on quantifying its effects on the strength, durability and long-term deformations of building structures. The recycled material is used to replace the natural aggregates, such as crushed stone and gravel, in the concrete mixture. Experimental and analytical research is being applied to understand the economic and environmental benefits of recycled concrete as well as many performance factors. Ultimately, the project will develop performance-based requirements, mix-design guidelines, acceptance tests and structural design/analysis/construction guidelines to achieve acceptable performance of concrete members using recycled concrete aggregates instead of natural coarse aggregates.
I2: Our Change Leader profile focuses each month on someone who is trying something new and leading a positive change for the community. Sustainability tends to factor to some degree, and that’s certainly true of your work to recycle concrete.
Kurama: Everybody’s talking about sustainability these days. To talk about sustainability without bringing concrete into the discussion is very hard. It’s such a resource-demanding product, and it permeates every part of our lives. That’s the reason I got into it.
My graduate education is in reinforced concrete structures, but I had not done anything related to sustainability before. And then when I thought about what I could do to reduce the environmental impacts of concrete, I realized there are many ways that you can help with that problem. I wanted to look at aggregates because much has already been done in the area of cement, but there is little prior research on reducing the demands for natural aggregates.
I2: I’m familiar with recycled concrete as a road-base material, but you’re focused on making it a structural component for new buildings. Is that the way to make the most of the material?
Kurama: What we’re doing isn’t new in terms of the idea of recycling concrete—that has been going on for a long time in the United States and abroad. That’s not really utilizing the material to its maximum potential. When you look at the material properties of recycled concrete, there is a lot of variability, and that is one of the engineering constraints with the material. Our research tries to address that variability, and also elevate the use of recycled concrete for structural applications.
If you quantify the properties and establish a set of guidelines to select among the variations within the material, with a hierarchy of quality, then you’re getting a recycled material that could be used in the superstructure. In other words, we’re trying to elevate the usability of the material to a wider range of applications, taking it out of the ground and putting it above the ground.
I2: If you retain the viability of the material for buildings, then you also retain the value, is that correct?
Kurama: There are definite economic benefits for that. We currently use the existing material in road-base applications primarily, flattening and leveling ground to carry the roadway above it. The demands on that ground aren’t very high. But when you try to make beams and columns and slabs out of it, that’s a completely different story. The consequences of failure are much more important, and the load and service demands are much higher.
That’s why when you start using this material in structural applications (in the case of buildings, the superstructure above the ground), then you have to investigate the variability of the material, and its mechanical properties and durability. There are a whole suite of things that come into play.
I2: Is the idea to recycle the materials onsite, or would you use the material in a precast mode where it’s more of a manufacturing environment with more controls? What is the best scenario for recycling concrete for superstructures?
Kurama: There are many different possibilities. The precast concrete application is something that we’re pursuing right now, and I don’t know if anybody has ever looked at this in research. We couldn’t find it in any of the published literature, but it could be going on in practice. I have spoken with a number of precast concrete producers, and they have certainly thought about it.
The precast producers I spoke with were very excited about being able to reuse their materials. Because they produce quite a lot of waste from rejected products, some own a recycling facility, but they do not currently use that material back into their own products. It takes up space in their facilities, which is not desirable. And eventually, it’s again used for road base or another low-grade application.
We are currently working with two precast producers, one in Michigan and the other in Indiana. We’ve collected materials and are looking at their effects on concrete strength and prestressing strand bond characteristics. You’ve got to know the effect of recycled materials on strength performance.
We’re doing some of these more fundamental tasks, but ultimately we will be looking at the service and ultimate behavior of precast-prestressed beams utilizing the mix designs that these precast producers readily use (so we’re not trying to change their mix designs), but essentially taking all or a large portion of the natural aggregate out of the mixture and putting their own recycled products back in.
There are a lot of benefits to doing this. One is that you know the material that’s coming back into your product. You know the mixture design, materials, and strength of the original concrete, and you know it’s not a mixture of different sources and that it’s not inferior concrete from a very old or deteriorated structure. Those are the advantages. The quality of the recycled material is higher when you get it from a precast plant.
Our previous project looked at recycled aggregates obtained from 16 different construction demolition and recycling facilities in the Midwest. We were one of the first research groups to look at variability this extensively. Many researchers had identified variability as an issue, but nobody had actually gathered materials from a large number of sources to look at the properties of the aggregate. You can’t simply go to a recycling facility and use any material in a new building. There has to be some prequalification of the material. We looked at the means to prequalify and determine the limits of that prequalification. With precast concrete, the prequalification is a lot easier, because you know the properties of it.
I2: Do you get into the recycling methods and ways to screen the recycled materials for best qualities?
Kurama: This question gets to the heart of economic viability. There are sustainability goals to using recycled materials of course, but, at the end of the day, you can’t sell the idea if you can’t make it economical. What we’re trying to do is minimize processing of the material so that it’s more economically appealing than getting natural aggregate. Other researchers have looked at different ways of batching concrete with recycled materials. For example, there are ways of quantifying how much mortar is in the recycled aggregate. You can quantify the amount of mortar on average and can incorporate that number in your mix design. This process makes the mix design much different than what we would do in regular practice. We also saw significant issues with workability, so we avoided that.
What we’re promoting is a direct volume replacement. You replace an equal volume of natural aggregate with recycled aggregate. You have exactly the same volume being replaced. All concrete mixture designs are volume based. So the practicality is very simple from that aspect.
In terms of preparing the material, you have to prequalify it and take some measurements, which you would do normally for the absorption of the material. The other important property we identified is the amount of deleterious substances. Inevitably, you’re going to have some non-concrete materials such as brick and asphalt and wood chips, etc. The amount of that on average needs to be quantified and needs to be below a certain value. This becomes a non-issue, of course, if you’re using materials from a precast plant, which won’t have much deleterious substances.
The material then has to be washed, but this is not different than natural aggregates. Some people have looked at getting rid of the deleterious substances. If you put this as a requirement, you will increase the processing costs significantly, and it will be a lot harder to push the reuse of these materials into higher utilization.
We can simply have guidelines where recycled concrete with high amounts of deleterious substances get pushed to road base use and those with low amounts of deleterious substances are suitable for superstructures. It would be an enormous amount of work to try and separate brick and asphalt out of an existing source. We’re trying to keep it simple.
I2: I would think that precast with the source close to its place of reuse would score points on energy and emissions given that it doesn’t have to travel far. Is that a factor in its economic viability?
Kurama: Absolutely. And, we’re working to expand our efforts into other regions of the U.S. and also into looking at economic comparisons. Our previous study was limited to the Midwest region. In our current project funded by the National Science Foundation, we have partnered with researchers at New Mexico State University and the University of Texas at Tyler. They have been collecting materials from the south, east, and west of the country. They are doing a similar study on mechanical properties of recycled materials.
We developed some predictive models based on the absorption and amount of deleterious substances of materials from the Midwest. But fOur partners are looking at our predictive equations to see if they work for materials from other parts of the country. The reason this is important is that the concrete mixture designs and types of original aggregates used in different parts of the country may be different. For example, in the Midwest, we have limestone, but if you go to the east, you may have more granite. There may be other differences as well.
Through these new partnerships, our research group is also looking into lifecycle costs to quantitatively show that it is more cost effective to use recycled materials than natural aggregates. It’s not obvious to people that recycling old concrete into buildings makes economic sense. Everyone understands that this is good from an environmental perspective, but people sometimes question the economic benefit. We are making the case that it’s beneficial to use old concrete structures in new construction, and precast would score very high on reduced transportation costs for sure.
I2: I like that you’ve created decision-support tools. Having formulas and tools to quantify local sources is probably key to gaining wider adoption, correct?
Kurama: Yes, this is very important. We have spent quite a bit of time on that, because otherwise you wouldn’t know which recycled aggregate source is suitable for a building application. Most, if not all, of these materials may be suitable for road base, but not all are suitable for a superstructure application. We’re also doing durability studies with our partner universities.
There needs to be a greater effort researching on recycled concrete materials in the United States. What we’ve found through our literature search is that, in this country, recycled concrete is something that has been pushed aside. If you look at Europe and Asia (especially China), they’re way ahead of us in terms of using old concrete as a resource for new construction. You can’t simply adopt what others have found, because materials and quality-control procedures are very different. We have to do our own research, and, unfortunately, there are very few research groups in the United States looking at this problem. We’re making a contribution, but there’s a lot more that could be done.
I2: Is there a band of viability on how old the concrete can be?
Kurama: This issue is related more to durability. Certain types of aggregates cause an undesirable reaction with cement, called Alkali-Silica Reaction (ASR). ASR causes expansion at the aggregate/cement interface and could internally deteriorate your concrete. In the old days, when this phenomenon wasn’t known, they may have used materials that are highly susceptible to ASR. The newer concretes don’t have that. That’s the main limitation in terms of age.
Other than that, concrete strengths were relatively weaker in the old days, but this is something that can be quantified. In other words, you can engineer around it. You might end up with concrete strengths that are a little bit lower, but those may be suitable for other applications. On the other hand, if you have recycled concrete that is going to internally deteriorate due to an undesirable chemical reaction inside the new concrete, then it’s obviously not the material you want to use. Again, the limitation is more on the chemical makeup of the old concrete. This chemical makeup can be related to the materials used in the old concrete as well as subsequently introduced from the service of the old structure, such as salts from bridges and parking garages.
I2: How are you using your work on this research in the classroom?
Kurama: The University of Notre Dame hosted the Great Lakes Regional Conference of the American Society of Civil Engineers (ASCE) last year, which involves concrete canoe racing, steel bridge testing, and materials testing, among other competitions. They incorporated the use of recycled materials in the material competitions. Every school that hosts the Conference has some leeway on what they want to incorporate into the contest. Our objective was to get the word out and make young people think about sustainability related to concrete.
Additionally, graduate students and undergraduates working on the project are being educated. The fact that we have three universities involved makes it for the education of a broader and more diverse group of students and faculty. There has also been a good amount of interest within the University of Notre Dame as well. It seems that sustainability is on everyone’s mind.
I2: There’s so much going on in the material science world, and I wonder if this work crosses over to that?
Kurama: I’m a structural engineer, so I’m interested in how materials behave so that structures will behave in manners that you design and predict them to behave. This doesn’t just involve ultimate strength. What we’ve found is that recycled aggregate doesn’t have the biggest impact on ultimate strength; the bigger effects are on the service behavior of the structure, for example resulting in more deflections.
What this means is that if you want to design a long-span beam using recycle concrete aggregates, you may be restricted or have to use a deeper beam, which will obviously impact the economy issue. You don’t want to make a deeper beam just for the objective of using recycled aggregates. On the other hand, if you have shorter spans where deflections aren’t governing, then you certainly can use this material if the predicted deflections are within the normal limits without an economic penalty. The whole goal is to understand what the effects are and then design around those effects.
The other thing is the durability issue, and a lot more study needs to be done on that, including the material science area. I should note that we aren’t promoting the use of this material in exterior applications. For example, using recycled concrete in a bridge application would not be a good idea. The material is generally more absorbent than natural aggregates, so with a lot of water and roadway salts and chemicals around, it will pull in more of these substances and deteriorate the concrete, and eventually they could reach the reinforcement and cause it to deteriorate as well.
I2: What kind of volumes are involved with stockpiles of recycled materials?
Kurama: When we went to recycling yards, we saw piles and piles of recycled concrete aggregate. We often get asked why we need to use these materials in buildings, because they are already being used in road applications. The answer is that people need to go and look at the recycling yards in their communities and understand that there is plenty of excess material around. In fact some of the recycling yards we visited were no longer admitting new material, because they couldn’t get rid of their existing piles.
The amount of demolished old concrete will further increase as we replace our deteriorating infrastructure. Using recycled concrete for new building applications would be a way to increase demand to match the supply. This could also give recycling yards guidance on the quality of their material. If you give more value to higher-quality aggregate, then the market will develop for higher-quality recycled materials. Then, the different materials would be sorted according to their properties, and they would be used for different applications.