Glass in Asphalt Gets Another Look
BY AsphaltPro Staff
Researchers in Canada give new hope to the use of recycled glass in asphalt pavements with the work of a large partnership. During a Dec. 1, 2015, press conference, Robert Poëti, Quebec’s Minister of Transport, lauded the integration of glass as a component of road manufacturing in the industry’s overall efforts to limit green house gas (GHG) emissions, etc.
Also during the press conference, the École de technologie supérieure (ÉTS), City of Montréal, Société des alcools du Québec (SAQ), Éco Entreprises Québec and RECYC-QUÉBEC announced its partnership on a $450,000 (Canadian) three-year pilot program to test the use of crushed glass in asphalt pavement.
According to a press release, the ÉTS research team expects the integration of glass in base courses to improve drainage and insulation to extend the life of asphalt pavements.
The concept of using crushed glass as an aggregate isn’t new. In fact, the industry has been using the term “glasphalt” to describe this process for decades. Studies to test the performance of glasphalt began in the 1960s and ’70s, when test pavements were laid in 33 locations in North America. The studies showed that the high angularity of crushed glass, compared to rounded sand, enhanced asphalt’s stability, when crushed to a proper size and used in proper proportions. Other benefits include low absorption, low thermal conductivity offering enhanced heat retention, and specific gravity, meaning more volume of asphalt cement (AC) per ton.
However, when used in high percentages or with large glass particle sizes, test pavements showed stripping and raveling problems, in addition to high tire wear and excessive glare.
“Since we have better equipment and better knowledge now, it should work better,” Alan Carter, research team collaborator and engineering professor at ÉTS, said.
“The new techniques consider fundamental characterization of material and aggregates used in our laboratory,” Carter said. “The mix design used in our rheological approach is based on elastic and viscoelastic properties measured with the complex modulus, which gives the real asphalt mixture behavior.
With a better understanding of the behavior of glass aggregates, it’s possible to take better advantage of the glass. We also developed a new kind of asphalt mix that includes glass. This new mix gives us the opportunity to recycle more glass than in a conventional mix.”
Additionally, the team will have a doctoral student researching improvements in glass’s ability to absorb binding agents, as well as minimize stripping. The team estimates it will be testing pavements with a 15 to 25 percent glass aggregate makeup, all under 5 mm in size.
A 2012 report from the U.S. Federal Highway Administration has found satisfactory results using between 10 and 15 percent crushed glass, with higher blends—of up to 25 percent—for use in base or binder course mixes. The report states that hot mix asphalt pavements with more than 15 percent waste glass “may experience deterioration due to stripping of the asphalt cement binder from the waste glass.” The FHWA report said most highway departments allow use of 5 to 10 percent glass in their asphalt mixes. And although some areas allow 6.4 mm to 12.7 mm gradations, many have specified the use of 9.5 mm gradations. The report also states that optimum performance is achieved when glass is used as a sand or fine aggregate substitute, less than 4.75 mm.
The City of Montréal has provided test slabs to help evaluate the mix performance beginning in 2017. If the pilot is successful, Montréal officials hope to use the new combination on their city streets within a few years.
“The big dream is to use recycled glass for new innovations rather than for waste,” Alain Brunet, SAQ President and CEO said.