A viral story about a 1970s street paved with recycled glass has put a forgotten piece of engineering back in the spotlight. Glasphalt, a blend of asphalt and crushed waste glass, has been used as an alternative road surface since the early 1970s, and the record shows it can perform well when it is mixed correctly.
A real experiment, in many places
According to US Federal Highway Administration guidelines on recycled materials, studies and field trials in the late 1960s and early 1970s placed glass test strips at about 33 locations across the United States and Canada.
Some cities went further. From the mid-1970s to the mid-1980s, Baltimore paved at least 17 streets with glass, partly for the sparkle it gave the road. Between 1990 and 1995, New York City’s transport department used about 250,000 tons of glass in resurfacing work.
Why it works
Crushed glass has sharp, angular edges, which help a road mix resist sideways movement. The FHWA guidance reports stability comparable to, or better than, conventional asphalt. Surface layers containing 10 to 15 percent crushed glass have performed satisfactorily, and lower layers could take up to about 25 percent.
Why it did not take over
Glass has drawbacks that limit how much of it can go into a road:
- Weak bonding. Glass does not absorb the asphalt binder, so mixes with too much glass, or with coarse pieces, tended to “strip” and break up. Engineers added about 2 percent hydrated lime to help.
- Glare. More than 15 percent glass noticeably increases reflectivity, especially on wet roads.
- Grip and tyres. Early researchers worried about flat or elongated shards and unusually high tyre wear, and large particles could turn slick in the wet, although measured skid resistance stayed within recommended limits.
As a result, most highway departments cap glass at 5 to 10 percent of a mix and crush it to fine sand-sized grains for road surfaces. Glasphalt never vanished; it just settled into a supporting role.
Further reading
Image: Verre pilé Crushed glass on urban road Lamiot 09 by Lamiot, BY-SA 4.0 via Wikimedia Commons
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