In 2024, Arizona put 100% recycled glass beneath a 23-mile interstate expansion; the unusual material helped protect 19 sets of existing highway culverts

A highway expansion in Arizona is using an unusual material beneath the road: foam made from 100 percent recycled glass. The lightweight fill is being placed at 19 locations along the I-17 Improvement Project between Anthem Way and Sunset Point, where additional lanes are being built along a 23-mile stretch of interstate.

The material may look like ordinary white gravel from a distance, but its purpose is far more specific. Arizona transportation officials are using Ultra-Lightweight Foam Glass Aggregate to help protect existing concrete box culverts from the added weight of the expanded roadway. It is a construction solution that turns discarded glass into structural fill while addressing a problem created by widening an older highway.

Why did the I-17 project need a lightweight material?

The existing culverts beneath I-17 were originally designed to support the weight of a two-lane roadway. Expanding the interstate means placing additional pavement and road infrastructure above structures that were not designed for the same load.


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Instead of simply adding conventional fill, engineers are using foam glass around the affected locations. Its very low weight reduces the additional load placed on the existing culverts while still providing the support needed for the new roadway.

That matters because the culverts are not decorative features buried beneath the highway. They are concrete structures that allow water to pass beneath the road, helping the transportation system function safely during drainage events. Strengthening the area around them is therefore part of making the expanded interstate work with infrastructure that is already in place.

What exactly is foam glass?

Foam glass is produced by taking recycled glass and processing it into a lightweight, porous aggregate. The glass is crushed into a fine powder and mixed with a foaming agent before the mixture is heated in a kiln.

As the glass softens, the foaming agent creates bubbles throughout the material. Those tiny air-filled spaces give the finished product its unusual combination of low weight and structural usefulness.

The resulting material is described as inert, non-leaching, rot-resistant, non-flammable and durable. It is also waterproof, which is particularly useful in a project involving drainage structures beneath a major interstate.

Why use recycled glass beneath a highway?

The environmental angle is built into the material itself. Rather than sending used glass toward disposal, the process turns it into a construction product that can replace heavier conventional fill in a specific engineering application.

The foam glass being used on I-17 is made entirely from recycled glass. That does not mean the entire highway is made from recycled material, but it does give one component of the expansion a second life.

The choice also shows how recycled materials can serve a practical engineering purpose rather than being used simply because they are considered environmentally preferable. Here, the lightweight characteristic of foam glass is central to the reason engineers selected it.

How is the material being used along I-17?

Large white bags containing the foam glass can be seen within the construction corridor. The material is being installed at 19 locations where existing box culverts need additional support as the interstate is widened.

The I-17 Improvement Project covers the corridor between Anthem Way and Sunset Point and includes both highway widening and flex lanes. The expansion is intended to increase capacity along a heavily used section of interstate while working around infrastructure that was built for an earlier roadway configuration.

Using lightweight fill allows the project team to deal with one of the less visible challenges of highway expansion. From the driver's perspective, the finished road may look like a conventional widening project. Beneath the pavement, though, engineers must account for the condition and design limits of structures that have been there much longer.

Why is this material unusual for Arizona?

The foam glass used for the project is manufactured in Florida, and its application on I-17 represents its first use in Arizona, according to the project information.

That makes the interstate expansion a local test of a construction material already capable of meeting a very particular need. Its role is not simply to replace ordinary aggregate everywhere on the project. Instead, it is being used where its lightweight and water-resistant properties provide a clear engineering advantage.

The approach also reflects a broader shift in infrastructure construction. Roads require enormous quantities of aggregate and fill, so even a relatively specialized material can become significant when it is used repeatedly across a major transportation project.

The I-17 application brings those two priorities together: an existing highway structure needs protection from additional weight, and recycled glass can be processed into a material light enough to help solve that problem.

For drivers, the foam glass will eventually disappear beneath the completed roadway. Yet its role illustrates how much of modern highway construction happens outside the view of the finished pavement. In this case, recycled glass is being transformed into lightweight infrastructure support, helping an older drainage system accommodate a larger interstate above it.