A team of researchers from U of T’s Department of Civil & Mineral Engineering (CivMin) is helping ensure the long-term sustainability of the new Gordie Howe International Bridge by studying its components at the microscopic level.
The new Gordie Howe International Bridge connects Windsor, Ont. with Detroit, Mich., and is expected to open later this year. The CivMin team, led by Professor Karl Peterson, has been evaluating every concrete pour for the project since 2021.
“Entrained air gives space for water during freeze-thaw cycles,” says Peterson.
“There are a lot of theories about how water moves around when concrete is at freezing temperatures, but it helps the water move around without causing any damage.”
Throughout this process, Peterson and his team have examined upwards of 500 specimens to ensure consistency and reliability.
As a matter of regular practice, concrete is tested on the construction site prior to being poured. Additionally, multiple samples are obtained during each concrete pour for strength testing at later dates.
The additional step of verifying the regular distribution of the air content and spacing of the air-entrained concrete is achieved using a polished section of the hardened material and examining it at the microscopic level.
“The spacing factor is a simplification, but it’s a number that describes the distance to the nearest entrained air bubble,” says Peterson.
“The entrained air bubbles are a tenth, to a few tenths, of a millimetre in diameter and, ideally for freeze-thaw durable concrete, there should be a bubble within two-tenths of a millimetre for 95% of the binder.”
The bridge is anchored on land and spans the Detroit River. Peterson says that for him, the most exciting parts are the bridge pylon towers.
“Those are really low water-to-cement ratios, with a lot of silica fume, ground granulated blast furnace slag,” he says.
“It’s really a dark-coloured binder. We get concrete from everything from fence post holes to anything else that’s poured on site, but not the pre-cast sections. The samples arrive as cores, or cast cylinders, about four inches in diameter, a bit larger around than a hockey puck, and eight inches tall.”
The team of undergraduate summer student research assistants and graduate students includes Ekaterina Ossetchkina (CivMin MASc 2T3), Oleksiy Chernoloz (MinE 2T4 + PEY), Lucas Herzog Bromerchenkel (CivMin MASc 2T3), Fiona Higham (Year 3 CivE), Mahzabin Karim (CivE 2T3 + PEY), Liam MacHale (CivE 2T4 + PEY), Amy Montgomery (CivMin PhD student), Yuqi Hu (CivE 2T3 + PEY), and Wisdom Okoh (CivMin MASc student).
The work has also resulted in the publication of a research paper, published in the Journal of Microscopy in 2024.
