In 2024, Michigan Tech mixed 6% treated waste glass into asphalt; dynamic stability rose 42.3%, and anti-skid performance improved 38.6% over standard mixes

Most glass bottles placed in U.S. recycling bins ultimately end up in landfills, largely because separating colors is costly and glass is heavy. As the journal Case Studies in Construction Materials reported, an international team of engineers, including researchers at Michigan Technological University, found a novel use for the glass: in roadways. Asphalt concrete mixtures containing 6 percent surface-treated waste glass had dynamic stability 42.3 percent higher and an anti-skid value 38.6 percent higher than traditional asphalt mixtures, after testing in a controlled lab environment. Since a handful of U.S. cities have already experimented with glass-infused asphalt in past decades, this research could give road builders a more reliable, better-performing method for putting old glass bottles to use in future paving projects.

US glass recycling rates lag behind other materials

Glass recycling in the United States has never matched the levels of paper, metals, and plastics recycling. Data from the United States Environmental Protection Agency (EPA) show that 12.3 million tons of glass were generated in the country in 2018, the most recent year for which the agency has published comprehensive figures, and that this constituted 4.2 percent of all garbage. Still, only 31.3 percent of glass containers were recycled in 2018. One reason for the lack of glass recycling is not demand, but logistics. Glass is heavy; it shatters during collection, and glass of different colors cannot be resold to the same manufacturers, which makes it challenging to recycle. Asphalt solves some of these issues. Roads do not distinguish between clear and green glass, and state roadways are resurfaced regularly regardless of local demand for glass products.


Inside the Michigan Tech asphalt glass study

The researchers did not simply crush glass and mix it into asphalt. They focused on surface treatment, coating the waste glass with a silane coupling agent before mixing it into the asphalt, since untreated glass doesn't bind well with asphalt binder and can create weak points in the pavement. This extra treatment step adds cost and effort compared to simply crushing glass, but it is still far simpler and cheaper than sorting bottles by color for traditional recycling, since asphalt production doesn't care what color the glass was to begin with. Using techniques such as energy-dispersive X-ray spectroscopy, boiling water resistance tests, and contact angle measurements, they found that silane-treated glass adhered to the asphalt much better than untreated glass. Mixing in 6% glass by weight resulted in a 42.3% increase in dynamic stability, a measure of a pavement's resistance to rutting under repeated traffic load, and a 38.6% improvement in anti-skid performance, a measurement of tire grip on the road surface. Day to day, that means less rutting from your daily commute and more consistent traction in the rain, the kinds of problems most drivers are not really aware of until they become major issues.

Federal research already leaves room for glass in asphalt

Federal Highway Administration research has already documented satisfactory performance in hot-mix asphalt pavements incorporating 10% to 15% crushed glass in the wearing surface mix, also known as glasphalt. The same FHWA guidance notes that surface-course pavements with more than 15% waste glass can deteriorate as the asphalt binder strips off the glass, and that most highway departments currently allow 5 to 10% glass in their asphalt mixes. The Michigan Tech study used a 6% ratio, within the federal range, but added silane surface treatment to improve performance at a lower glass content.

Some US cities already paved streets with glass

The use of waste glass asphalt is not only a laboratory experiment; it has also been tried on real roads. The same Federal Highway Administration guidance notes that the City of Baltimore paved at least 17 streets with glass aggregate between the mid-1970s and mid-1980s, and the reflective particles gave the streets a visible sparkle in sunlight and under streetlights. New York City went further: Between 1990 and 1995, the city's Department of Transportation used approximately 250 thousand tons of waste glass in street resurfacing, the largest municipal glass asphalt program at the time. These early examples suggest that waste glass asphalt has been tried and tested on real city streets, as well as in laboratories. What the Michigan Technological study adds is a method, silane surface treatment, that helps glass adhere better and perform better at lower, more practical glass content than those early programs used.

The road ahead for glass-treated asphalt in the US

None of which should be interpreted as suggesting that glass-treated asphalt will pave America's interstate highways next summer. These tests took place in laboratories, not on actual roadways, and pavement systems engineers typically want to see how a given mix performs under a wide variety of conditions before recommending that a state DOT change anything. And yet, the groundwork has been laid. State DOTs have a wealth of information on methods of incorporating recycled materials into asphalt concrete from years of federal research. Still, this study presents a verifiable technique for improving the performance characteristics of glass in pavement. For environmentally aware Americans, this could be a more practical use for those beer bottles than the municipal incinerator.