- This might seem like a regular sidewalk, but it's no ordinary concrete.

It's got an invisible ingredient that could have a big impact on sustainability, recycled glass.

Recycling is one way to reduce the amount of waste going into landfills.

But recycling glass presents some unique challenges.

- Every city has challenges with glass for a couple reasons.

It's heavy.

It breaks.

So a lot of municipalities have stopped even taking glass.

The other thing that's happened over the years is there's been a consolidation of recycling of glass.

So right now, Charlotte ships its glass to Atlanta.

It costs more to ship there than it's worth.

So we looked at what can we do with glass?

Well, glass is sand.

So if you crush glass back into sand, you can use that sand back into concrete as a replacement for natural sand.

- Envision Charlotte's Innovation Barn in Charlotte, North Carolina, hosts a slew of sustainability projects, including working to compost food waste, repurpose waste plastic, and dial in closed-loop systems to grow food.

And it's also home base for the Crush Truck.

- So in this compact box, we've got 15 hammers spinning violently, crushing the glass and turning it into the sand.

- This compact machine can process four to five tons of glass per day and is set up to run off of two three-kilowatt-hour batteries that are charged via solar panel, making it possible to run completely off-grid.

All right, let's crush some glass.

- I'll just show you the general like... - OK, do a demo.

- Ha ha!

- I'm going to need to get more glass.

She's going to be here for a while.

- This is very cool.

Ready?

- Excellently done.

- Oh, this is a nice, dark green.

[crushing glass] - Oh, yeah, that's pretty addictive.

[laughs] Empty wine, beer and liquor bottles are easy to collect because they're concentrated at bars and event venues.

But the crusher doesn't discriminate.

If it's glass, it crushes.

And the result, a mix of different sizes of crushed glass and paper label scraps.

- With this humble little trailer, we're creating a mini MRF, a micro MRF, if you will.

- MRF stands for Material Recovery Facility.

Industry speak for a place where recyclables from mixed waste streams are sorted by type and prepared for resale.

The resulting crushed glass can be sifted into varying sizes for reuse.

- This is one of the sizes, which is basically like sand you get at a beach.

- Yeah.

So this is actually crushed glass bottles crushed by the crushing machine.

And it's really even the color of this one looks like sand, which is kind of cool.

- It does.

- And it's not sharp.

It's like you can hold it and play with it and it's not scratching you.

That's so cool.

At the Innovation Barn, the crushed glass they've collected in partnership with the nearby Spectrum Center is stored in repurposed grain bags sourced from breweries, waiting for its full circle journey back to the Spectrum Center as a component of concrete needed for a construction project.

But how does all of this crushed glass become part of new concrete?

- Concrete is the most widely used human made material in the world.

Every concrete recipe is a different recipe depending on what it's being used for.

- There are four basic ingredients, rock or aggregate, sand.

- And then in the buckets, we have some cement, water.

In this recipe, we're going to actually replace some of the cement with some fly ash, which is in one of the buckets.

And then we're going to actually use a portion of the sand component of the mixed design and replace it with, in this recipe, 40 percent ground glass.

And that is glass that has been collected from the city of Charlotte, some from the Spectrum Center.

- This glass is the raw, unscreened crushed glass that comes straight out of the crusher.

So we get a wide range of particle sizes.

We get everything in there, the labels, the sugars, all the good and the bad.

- I can almost like smell the beer a little bit.

There's a little beer smell.

- Ideally, if we can figure out how to make it work in this state, it's the lowest impact, easiest to process.

And that's kind of what this initial proof of concept is, is to see if we can take the raw feed and make a quality concrete mix with it.

- So it's all about finding the right recipe.

- Correct.

Correct.

Whenever we're coming up with a new recipe, we start off in a controlled environment in small batches.

We call these trial mixes.

The analogy everybody uses, it's, you know, like making cookies.

Flour would be the same as the cement.

Maybe the chocolate chips and nuts is your coarse aggregate and the sugar is your sand.

And then your eggs and butter is kind of like your water.

And some of the admixtures we'll be using.

And certainly if you add too much or too little, you may not get the result you like.

And even if you bake it too long, once we mix the cement and water together, it's an exothermic reaction.

And we only have so much time to deliver that that concrete.

So just like you bake a cookie too long in the oven, if that concrete stays on the truck too long, you're not going to get what you want because it's going to start setting up and you got a bigger mess on your hands.

We certainly don't want to just guess and send concrete out that we know may or may not work.

So we go through the process in the lab.

- That process includes measuring the amount of air in the mix, performing something called a slump test to make sure that the concrete will be workable for different applications.

We're going to measure the slump against the original height, which is 12 inches.

I was looking for about a five to a six inch slump.

Can you see that?

Yeah.

Can I get a measurement for me?

- Looks like about closer to six.

- Six?

OK.

- And finally, pouring cylinders that will be cured for different amounts of time before being put to the test to see how much force they can handle.

- So this is a hydraulic press, basically what this is.

The machine will press down until we get an actual break.

[pop] [pop] [pop] - The machine measures the pressure at which the cylinder breaks, and that tells the lab how strong the concrete is, while the shape of how it broke tells them about the mix structure.

For the concrete industry, improving sustainability isn't a new concept.

- From the carbon footprint of concrete, a large portion of the CO2 associated with a yard of concrete comes from the production of Portland cement.

So if we can figure out how to make concrete with less cement, we can lower the overall carbon footprint of the concrete.

- One of the ways to reduce the amount of cement needed is to replace some of it with materials that act in the same way.

- And a number of these materials that we use are actually waste products from other industries.

So instead of putting them in landfills, we're actually able to reuse them and use them in concrete.

- Fly ash from coal-fired power plants can be used to replace a percentage of cement in the mix.

And preliminary testing is showing that recycled glass ground into a similarly fine powder can be used in the same way.

The glass does have to be ground really fine to replace cement, which adds more steps to processing.

And there are some additional challenges to sort out when ground glass bottles are used to replace sand in the recipe.

- A lot of it comes with the labels still mixed in with it.

You've got the natural sugars, which if you have too much sugar, it can actually delay the set of the concrete.

So that's something we've got to be conscious of.

- There's also a chemical reaction they look out for that can happen whether or not there's glass in the mix.

And it creates an expansive gel in poured concrete, which, as you can imagine, isn't ideal for strength or durability.

They found through lab tests that replacing some of the cement in the recipe with fly ash gets rid of this issue.

And smaller glass particles reduce this chemical reaction, while glass ground to a fine powder actually helps to mitigate the problem, just like fly ash.

- It's probably a matter of determining what type of concrete we want to put it in and what that material does to the concrete itself and what kind of risk/reward we have in terms of where we're going to use that going forward.

- One of the goals of all of this testing is to get to the point where concrete plants are able to mix up big batches of concrete containing recycled glass for different uses.

Incorporating a new material like ground glass into the workflow of a concrete plant like this one will require more experimenting with storage and flow through the plant to control for things like moisture.

- Being off by one smidge can mean a truck being rejected or pumped out.

So it's a lot more that goes into it than most people think.

- Amy is excited to see more crushers in use in Charlotte and beyond.

Rather than scaling up the size of a crushing facility, she says a network of smaller crushers actually makes more sense.

- If you start scaling up and you start getting the bigger crushers, you're going to need a huge facility.

You're going to have to start bringing in glass from all over the southeast to feed the beast and to make the economics work.

- By making the crusher mobile and working with event venues like the Spectrum Center and local festivals, Envision Charlotte is hoping to spread awareness of the possibilities of crushing and reusing glass waste more locally.

- One of the things with a circular economy is really keeping things local.

So being able to take our glass, turn it into sand and then replace that natural sand back into concrete.

I mean, we're one of the fastest growing cities in the country.

So we have a lot of concrete projects going on.

- Concrete's not going anywhere.

We build everything out of it.

So, you know, the fact that it's such a high volume material that we use means we have a lot of opportunity to do better.