Arizona engineers used ultra lightweight foam glass made from recycled containers at 19 locations along a 23 mile stretch of Interstate 17, allowing the highway to be widened without placing excessive weight on ageing drainage structures.
When engineers began widening Interstate 17 north of Phoenix, Arizona they faced a problem beneath the road. Several concrete box culverts carrying stormwater had been built decades ago when the highway was narrower and were not designed to withstand the additional load that conventional soil and rock fill would have created.
Rather than demolishing and rebuilding the structures, engineers adopted a lightweight alternative made from recycled glass.
The material known as ultra lightweight foam glass aggregate was installed at 19 locations along the 23-mile section between Anthem Way and Sunset Point. The project marked the first use of the material by the Arizona Department of Transportation on a state infrastructure project.
Turning discarded glass into lightweight fill
The foam glass begins as discarded glass containers. The material is crushed into a fine powder and mixed with a foaming agent before being heated in a kiln. As the glass softens, the foaming agent creates tiny bubbles throughout it, producing a porous, closed cell material.
The resulting aggregate is considerably lighter than conventional soil or crushed rock while remaining strong enough for use as structural fill. Its closed cell structure also limits water absorption, helping prevent substantial increases in weight when exposed to rain or groundwater.
For the I-17 project, this allowed engineers to raise and widen the highway while reducing the additional load placed on the existing culverts.
Instead of reinforcing the old structures to carry heavier conventional fill, engineers reduced the weight of the new material placed above them.
Recycled material provides an alternative to landfill waste
The project also demonstrates another potential use for glass that is difficult to recycle into new containers. Differences in colour, composition and contamination can make some discarded glass unsuitable for closed loop bottle production.
Converting such material into construction aggregate provides another route for keeping it out of landfills.
The foam glass used on I-17 was manufactured in Florida and transported to Arizona in bulk bags before being installed beneath the highway.
The material is also described as inert, non-leaching, rot-resistant and non-flammable, characteristics that can make foam glass useful in other construction applications.
Excavated rock was reused as well
Recycled glass was not the only material reused during the highway project.
The mountainous terrain required extensive excavation and blasting, producing large quantities of rock. Contractors crushed much of the material on site and reused it as aggregate base, embankment support and paving material.
This meant the project was able to reuse two different material streams discarded glass from outside the region and rock generated directly by highway construction.
Reusing the excavated rock also reduced the need to transport material away from the site and lowered demand for newly quarried aggregate.
Potential applications beyond highways
Lightweight foam glass can be particularly useful where engineers need to construct over weak ground, existing structures or locations where additional weight could create stability problems.
Its potential applications include bridge approaches, retaining walls, embankments and construction over soft soils.
The I-17 project demonstrates how recycled materials can serve a specific engineering purpose rather than simply replacing conventional materials for environmental reasons.
The approximately $522 million I-17 improvement project was designed to increase capacity through a mountainous section by adding third travel lanes and dynamic flex lanes.
The Arizona experience shows how infrastructure projects can combine waste reuse with engineering solutions: instead of strengthening ageing structures to carry heavier loads, engineers can sometimes reduce the weight of the materials placed above them.