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Glassphalt: Have Roads Made with Recycled Glass Changed Pavement?

By July 19, 2011March 10th, 2016No Comments

When the City of New York repaved a section of Fifth Avenue twenty years ago along the front of the Plaza Hotel with something called glassphalt, the pavement sparkled from tiny flecks of recycled glass in the aggregate mix. But it was neither the recycled nature of the glass, nor the resilience with which the material can stand up to the traffic and temperature swings of the Big Apple, that caught the attention of the hotel owner, Donald Trump. The famous real estate magnate just liked how the street glistened.

And he wanted more of this glassphalt, according to a 1991 Knight-Ridder wire story. True to form, Trump demanded that the other streets ringing the hotel get the same shimmering, glass-infused pavement.

Glassphalt has been used in other parts of the U.S. and across the world in the twenty years since. In fact, it first arose in the 1970s as a potential way to recycle the considerable volume of amber, blue, yellow, green and clear glass bottles in the waste stream. Annually, approximately seven million tons of glass are disposed of in landfills.

The problem with recycling glass is that mixed colors in the recycling stream mean that a specific color cannot be achieved – clear bottles cannot use amber material, for example. Which is what led, 40 years ago, to exploration of other uses of glass where color would not be an issue. Researchers found that by mixing it into pavement, where color was irrelevant, it could work functionally and possibly in a cost-effective way.

The economics of turning that tonnage of waste glass into pavement is enticing. Using it could reduce municipal costs associated with disposal. And on a pound-for-pound basis, crushed glass processing is sometimes less expensive than gravel and sand. And while extensive data on the durability of asphalt roads seems lacking, we know anecdotally that some of those roads from the 1970s on forward have held up pretty well. It just might be better pavement in certain applications.

So why is glassphalt not used routinely in road building? There are several problems:

Variable rock economics. At the most, the glass content would comprise between 10 and 20 percent of the pavement. So standard asphalt-making equipment and procedures would always be used, but this would reduce the costs from aggregate (crushed rock) that makes up the bulk of asphalt. Aggregate pricing varies from location to location, a function of raw sourcing and the distance required for transport. So in some areas, glass provides a cost-effective dilution, but in other areas not.

Municipal variables (recycling and asphalt manufacture). According to the Glassphalt Paving Handbook (Asphalt Institute Manual Series No. 4, 1989, University of Missouri-Rolla), “The best possibility for sustained production of glassphalt is in communities with municipal asphalt plants, because the community can make a direct correlation between the extra costs incurred in glassphalt installation and the savings from diverted solid waste tip fees.” The handbook goes on to say this is not the case in most of the U.S.

Not a simple substitution. The Glassphalt Paving Handbook also makes a point of how you cannot simply feed broken or crushed glass into the aggregate. After collecting the glass (by whichever method the municipality uses), it needs to be processed to a specific size (aggregate), after which batch modifiers must be altered along with the asphalt manufacturing operation.

Limited applications. While the presence of glass in the pavement presents no danger to humans or damage to vehicle tires, the skid resistance of glassphalt pavement is slightly less than that of standard asphalt. This then limits its use to lower-speed roadways – and prevents it from being used to build 65 mph highways.

In summary, the Clean Washington Center, a Seattle non-profit group that serves as a clearinghouse on recyclable materials of all kinds, says that “a great number of glassphalt demonstration projects have been performed in cities around the country. Most of these projects have not progressed past the pilot state because of economics.”

Separately, a 1999 study conducted (by the NYC Bureau of Management Audit) of glass recycling programs in eight major cities – New York, Los Angeles, Chicago, Houston, San Diego, Philadelphia, Baltimore and San Francisco – found that only New York was sending its glass to glassphalt plants, and by that time it was not actually used by the city but sold to contractors working elsewhere. The program has apparently been discontinued since then because, according to Adrienne Esposito, executive director of the Citizens Campaign for the Environment, “it made the roads too slippery in the rain and too shiny.”

Donald Trump might take issue with Esposito on the latter problem. Since when is there something that sparkles too much?

Durability of glassphalt

But even if used only for slow-zone streets, or as pothole fill, might the economics of glassphalt change if considered over the life of the roads?

Donald Schaefer, once the mayor of Baltimore, later the governor of Maryland and after that the state comptroller, believes it was a high-quality, long-lasting material. In 2000, he told Baltimore City Paper writer Charles Cohen, “If I had to do it all over again, I’d do most of the streets in the city of Baltimore with glassphalt. It worked out well.” Only one Baltimore area contractor was using the material as of ten years ago, and only as a pavement undercoating.

An article (January 2005) in Resource Recycling bemoans the fact that recycled glass is subject to commodity market pricing fluctuations and thus most uses for the material make for unreliable customers. But that’s not always the case. The article cites Adams County, Wisconsin, where the price of cullet (crushed glass) is lower than hauled-in stone aggregate. The county claims their parking lots made with glassphalt “feature no potholes and require much less maintenance compared to projects constructed with gravel, saving even more money in the long term. Glass is also much easier to move around than gravel, according to contractors.”

So while glassphalt can be cost effective and perhaps superior to standard asphalt in some applications, it is hard to imagine it becoming a widespread substitution for crushed stone in making durable roads. The relative economics simply do not work.

And there’s no one that Donald Trump can fire to change that.

If not glass, how about polypropylene fabric?

Paving fabrics are another “unusual” road building technique that is used on 100 million square yards of road every year, according to Ray Myers, the executive director of the Asphalt Interlayer Association.

While there are some variations between the handful of manufacturers of paving fabric, the make up of waterproofing membranes (also known as paving fabrics or reinforcing fabrics) as originally developed by Phillips Petroleum Company in the 1970s are a non-woven polypropylene sheet, non-woven but needlepunched, that is heat-bonded on one side, along with a liquid asphalt tack coat. It is typically laid onto old pavement, especially where there are cracks, and then overlaid with new asphalt. An important feature of this material is its flexibility.

Pavement fabrics have one, very important job with regard to pavement preservation: to prevent moisture from entering. Studies show that use of paving fabrics extend the life of the pavement by 100% to 300%. But they cannot be placed on any existing pavement: this system can only work when placed on stable, even if cracked (“alligatored”), roads. It is not a pothole repair method.

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