The idea is not new, but its use has just taken a new step: used in the manufacture of sidewalk slabs for a decade, concrete with ground glass powder will soon be used for the first time. in the construction of a bridge. And it is in Montreal that it will be established.
Posted September 8, 2020 at 6:00 AM
With its concrete cast in curved lines, the new bridge that the team of engineer Étienne Cantin Bellemarre will soon be building will not only be beautiful, but also green. Or rather, it will be “glass”.
The project manager of the city of Montreal is not a bit proud of the site currently occupying him. By the end of autumn, the first section of the Darwin bridges, allowing Boulevard L’Île-des-Sœurs to cross a cycle path, will be completed. Users will not notice much of a difference, but concrete specialists will certainly take a closer look. Because this bridge is the first in the world to test a concrete recipe to which broken glass powder has been added.
“Mixed glass is what is recovered at the sorting center and broken into pieces that are too small to be separated by color,” explains William Wilson, professor of civil engineering at the University of Sherbrooke and associate in the SAQ chair in glass repair. In order to be able to produce new foundry glass from the recovered glass, it must be free from contamination and sorted by color. Due to the inability of sorting centers to deliver good quality material to foundries, most of the recovered glass is currently being sent to landfill.
The New Darwin Bridges
Precisely to find an outlet for all this mixed glass, Sherbrooke researchers have developed recipes to integrate it into concrete production. This basically consists of four ingredients: water, sand, gravel and cement. The glass powder in the recipe created at the University of Sherbrooke aims to replace some of the cement. “By using glass instead of cement,” says Wilson, “you reduce the greenhouse gas production associated with cement production.”
Since 2011, tens of miles of concrete sidewalks with glass powder have been rolled out in Montreal. “Dosages vary between 5% and 25%,” says Étienne Cantin Bellemarre. After almost 10 years of observation, the results are astonishing: concrete with glass powder is more resistant because it is less permeable to chloride ions (the famous de-icing salts used in winter that corrode metal reinforcements).
And all this for the same costs as traditional concrete.
Sometimes life does things right. The use of glass powder has only advantages. We value a material to make concrete more durable and resistant.
Engineer Étienne Cantin Bellemarre
Last April, the replacement of cement with ground glass powder in concrete production was finally recognized by US standards, which often serve as international standards.
It is therefore time to test the new concrete in a more complex structure. When Montreal’s engineers began planning to replace the Darwin Bridges, built in the 1960s, they saw it as the ideal project to test what new concrete could do in reinforced structures.
In this case 10% of the cement was replaced with glass powder, an estimated saving of 40,000 kg of cement. Ultimately, the concrete of the two bridges will contain an amount of glass equal to 70,000 bottles of wine. That’s… about 0.04% of the number of bottles sold annually by the SAQ. This is small? “Not at all,” William Wilson replies. Darwin Bridges: “It is a relatively small structure. And structures, we make dozens, hundreds of them in Quebec every year. Not to mention the sidewalks! ”
Can we expect that glass powder will now be incorporated into all concrete? “It remains a pilot project,” recalls Étienne Cantin Bellemarre. If we realize that it doesn’t meet our requirements, we will stop using it. But everything suggests that it is responding very well. We have experience with sidewalks, the elements that receive the most salts in our infrastructure, so we are confident that there will be no problems. ”
The sand problem
The glass powder in the recipe created at the University of Sherbrooke aims to replace some of the cement, the production of which is highly polluting – the McInnis Cement plant in Gaspésie is presented as the largest emitter of greenhouse gases in Quebec . But this new concrete does not yet offer an alternative to the use of sand in the production of concrete, which also poses an environmental problem. Sand is the third most exploited natural resource in the world after air and water, and most of this sand is used in the manufacture of concrete. In some countries, sand extraction has resulted in the destruction of river beds and beaches. Because sand is not a renewable resource, several experts predict that a sand shortage may arise.