Engineers in Arizona have found an unusual way to support a major highway expansion — by using thousands of tonnes of recycled glass converted into lightweight foam.

During the widening of a 23-mile section of Interstate 17 north of Phoenix, construction crews used foam glass aggregate at 19 locations. The material, made entirely from recycled glass, helped support additional traffic lanes without placing excessive weight on old drainage structures beneath the highway.

Recycled glass solves a difficult road problem

The I-17 section between Anthem Way and Sunset Point carries more than one million motorists each year. The expansion involved adding travel lanes and dynamic flex lanes to improve traffic movement.

However, decades-old concrete box culverts beneath the road had originally been designed to support only two lanes. Using conventional soil, stone or concrete fill could have placed excessive pressure on these structures.

Instead of replacing the culverts, engineers used ultra-lightweight foam glass to reduce the additional load.

How glass becomes lightweight road material

The recycled glass is crushed into a powder and mixed with a foaming agent before being heated in a kiln. The process creates bubbles within the softened glass, producing lightweight, porous pieces.

According to the Arizona Department of Transportation, the resulting material is strong, waterproof, rot-resistant, non-flammable and durable. Its closed-cell structure also prevents it from absorbing water and becoming heavier during rainfall.

Turning waste into infrastructure

The project also offers an environmental benefit by giving recycled glass another use instead of sending it to landfills.

The $522-million I-17 improvement project has combined conventional construction with recycled materials. Rock removed during blasting was also crushed and reused in road construction.

The use of foam glass demonstrates how materials normally treated as waste can become useful components of modern infrastructure while helping engineers address difficult construction challenges.