In 2024, Arizona engineers used 100% recycled glass to widen 23 miles of Interstate 17 at 19 sites, protecting concrete culverts built for a narrower highway

Highway widening usually means adding more material: more soil, more rock, more concrete and, ultimately, more weight.

But on a busy stretch of Interstate 17 north of Phoenix, Arizona engineers faced a problem that made conventional construction surprisingly complicated. The highway was being widened over concrete drainage culverts that had been designed decades earlier to support a much narrower road.

Rather than demolishing and rebuilding those underground structures, engineers found another solution — replace much of the conventional fill with thousands of tonnes of recycled glass transformed into lightweight foam.


The unusual material, known as ultra-lightweight foam glass aggregate, was installed at 19 locations along a 23-mile section of I-17 between Anthem Way and Sunset Point.

It was the first time the material had been used on an Arizona Department of Transportation state infrastructure project.

Why old culverts created a major engineering challenge

The I-17 corridor is an important connection north of Phoenix, carrying substantial commuter, freight and recreational traffic. The improvement project was designed to increase capacity by adding third travel lanes and dynamic flex lanes through a mountainous section of the highway.

But widening a highway isn't simply a matter of paving additional asphalt.

At several locations, existing concrete box culverts run underneath the road, carrying stormwater beneath the highway. These structures were originally designed when I-17 had only two lanes.

Adding conventional fill such as soil, crushed rock or concrete above the culverts would have increased the load on structures that were never designed to carry it.

Rebuilding the culverts would have been possible, but it would also have meant considerably more excavation, expense and construction time — particularly challenging on a heavily traveled highway.

That's where foam glass came in.

The recycled glass material is remarkably light

Foam glass aggregate starts with something much more ordinary: discarded glass bottles and containers.

The recycled glass is crushed into a fine powder and combined with a foaming agent. The mixture is then heated inside a kiln until the glass softens.

As it heats, the foaming agent produces bubbles throughout the softened glass.

The result is a porous, closed-cell material that looks more like lightweight gravel than conventional glass.

And that's precisely what makes it useful for infrastructure.

Foam glass is far lighter than traditional stone aggregate while still providing enough compressive strength to function as structural fill. Its closed-cell structure also makes it resistant to water absorption, meaning it doesn't become substantially heavier when exposed to rain.

For material placed directly above underground drainage structures, that combination is particularly useful.

Instead of piling thousands of tonnes of ordinary earth and rock over the culverts, engineers could build up the highway using a material that imposed dramatically less weight on the existing structures.

Why recycled glass was a practical choice

There's another interesting part of the story: the glass wasn't simply being recycled into another bottle.

Municipal recycling streams can contain glass of different colors and types, making it difficult to turn all collected glass back into new containers. Using otherwise difficult-to-recycle glass as a construction material provides another route for keeping it out of landfills.

For the I-17 project, the foam glass was manufactured in Florida before being transported to Arizona in large bulk bags.

For drivers passing through the construction area, those bags became an unusual sight — but underneath the new highway lanes, they represented a carefully engineered solution to a structural problem.

The material is also inert, non-leaching, rot-resistant and non-flammable, giving it properties that can be useful well beyond this particular project.

Arizona wasn't just recycling glass — it was recycling its own rock, too

The foam glass wasn't the only example of material reuse on the I-17 project.

Because the highway passes through mountainous terrain, major blasting operations produced enormous quantities of rock. Rather than treating that material as construction waste, contractors crushed much of it on site and reused it for aggregate base, embankment support and paving.

That approach reduced the amount of material that needed to be hauled away while also reducing the demand for newly quarried aggregate.

The result is an unusual combination: mountain rock and recycled household glass were both turned into infrastructure materials for the same highway project.

A lightweight idea with wider potential

Foam glass aggregate isn't limited to highways.

Transportation agencies have increasingly investigated lightweight fill materials for situations where conventional soil and rock could place excessive loads on weak ground or existing structures.

Potential applications include retaining walls, bridge approaches, embankments and construction over soft soils.

The concept has also been used in infrastructure projects in Europe, while adoption in North America has been growing as agencies look for ways to control construction costs, reduce material waste and solve difficult geotechnical problems.

The I-17 project demonstrates why lightweight materials can sometimes be more valuable than simply adding more conventional material.

What does this mean for us?

The most interesting part of Arizona's highway project isn't necessarily the fact that recycled glass was used.

It's why it was used.

Engineers weren't looking for a novelty. They had an existing piece of infrastructure that was still useful but wasn't designed for the heavier, wider highway being built above it. Replacing that infrastructure would have required substantial work.

So they changed the equation by changing the material.

Instead of asking an old concrete culvert to carry more weight, they built the additional highway capacity with a much lighter fill.

The roughly $522 million I-17 improvement project therefore offers a glimpse of how future infrastructure could increasingly combine conventional construction with materials designed around very specific engineering problems.

Sometimes the smartest way to build a bigger road isn't to make the foundation stronger.

It's to make the road above it lighter.