Should We Be Recycling Concrete?

Illustration for article titled Should We Be Recycling Concrete?

Recently we’ve been looking into sustainable building materials, like skyscrapers made of wood. We’ve also been examining the slowly crumbling infrastructure, largely made of concrete. Perhaps it’s time to combine the two ideas—and look at recycling concrete.

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A team of scientists at Notre Dame are trying to figure out whether something can be done with old concrete structures besides tearing them down and dumping them somewhere. This is a problem that will only become more pressing. Concrete freeways and bridges see a lot of wear and tear. It’s only a matter of time before they have to be replaced—at which point we’ll need more concrete. It seems like a waste to toss concrete away, only to send out vehicles to quarry, crush, and deliver yet more concrete.

We already do recycle some concrete. Due to safety concerns, concrete that was once a bridge or a building has to be turned into something which doesn’t bear as much weight, like a sidewalk. The team at Notre Dame, in partnership with University of Texas and New Mexico State University, are looking at different sample of concrete and assessing the quality of the material, and whether use and the elements will deform it over time.

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If used concrete can stand up to both humans and the environment, we may see a day when we can re-use concrete over and over—and it becomes a sustainable material. Cities can be literally broken down and reformed again and again.

[Why Not Recycled Concrete?]

Image: MO Stevens

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DISCUSSION

alwillia
renniSaint

As our infrastructure crumbles basically simultaneously all over the country we are taking a LOT of lessons learned from the last cycle of major work and applying it to new projects. There are advances in materials and also advances in how the projects are being bid so as to include products that are much longer lasting by having the builder be responsible for long periods of maintenance.

Unfortunately, as many have pointed out, the process of turning cement and aggregate into concrete is basically a chemically driven one way road. You can’t easily turn old concrete back into its constituent parts and reuse it. You can however use it as aggregate either as a gravel replacement or in new concrete. As noted in the article though you realistically want to use it in things like slab on grade and the like. Now, you might think that this is a bad thing but you would be shocked by how much is actually needed. Trust me, you won’t run out of people wanting to buy aggregate any time soon. Which leads us nicely to another interesting thing. Aggregate is incredibly expensive in cities. Why does aggregate cost so much in cities? Well, basically sprawl and NIMBYs. If a gravel pit is close to a major metro area the land is often worth so much to developers that it gets sold and developed. Or, even if the owners want to continue operating the pit all the land around them gets developed and they get forced out by zoning and ever more stringent operating conditions. So, the only place to get gravel is from far away which raises transport costs. If you can get the aggregate from within the metro area by just chewing up old concrete then you can save some money.

So now lets change gears a bit and think about why we are having to tear down these concrete structures in the first place. Well, generally the concrete isn’t the problem. Usually if you see a failed concrete structure it is either a design issue or the inevitable failure of the reinforcing steel. One of the basic tenets of designing a concrete structure is “It will crack, always.” So, all engineers can really do is try to determine where and how much it will crack. Since we’ve accepted it will crack you can almost guarantee moisture will get in and now we get to the inevitable failure of the steel part. See, reinforcing steel rusts and when it does it causes what is called oxide jacking. Basically iron oxide takes up more space than regular old iron so it pops away the concrete. This lets in more moisture and the problem accelerates.

People have been working for decades on this problem. Epoxy coated bar, electro-galvanized bar, even pure stainless steel! Often though it is simply more economical to plan on rebuilding the thing rather than use any of these expensive options.

Well, eventually someone got the bright idea to not use steel at all. Rebar made from fiber materials (glass, carbon, etc.) has come on the market. This stuff is awesome because it never rusts, never wrecks your concrete, is super lightweight so it can be moved way easier and in a lot of cases is really good for the environment because making steel takes an insane amount of energy. Unfortunately it is still crazy expensive. Or was, until about two months ago. A company just came on the market with a basalt-fiber rebar that is actually cheaper than regular old steel and it’s primarily made from the waste material of mining iron! I could talk about that stuff all day because it really is just the coolest. But I’m probably the only one on this site that gets really pumped for major technological advances in concrete reinforcement (fairly niche fetish, I know).

I believe that making structures that actually last a long time is the truly smart and environmentally sound path. Thankfully a lot of the projects we will see in the near future are finally going to be built this way for a reason nobody can argue with. It will save money.