Researchers in Sweden have discovered why concrete in underwater road tunnels can break down much faster than expected—and the surprising culprit is bacteria carried in by seawater.
The findings, from Chalmers University of Technology, shed new light on a growing problem in tunnels around the world and may help prevent costly repairs and dangerous structural damage in the future.
When road tunnels are built through rock, the walls and ceilings are sprayed with concrete to create a smooth surface and stop rocks from falling.
But in places like the Oslofjord tunnel in Norway, seawater can seep into the tunnel through the surrounding rock.
This brings in bacteria, which settle on the concrete and form slimy colonies called biofilms. These bacteria then begin feeding on substances in the concrete, breaking it down and making it porous and weak.
The research team has been monitoring the Oslofjord tunnel since 2014 and found that the bacteria can erode up to one centimeter of concrete each year.
Over time, this can lead to pieces of concrete falling from the tunnel roof and increase the risk of damage.
According to Associate Professor Frank Persson, this biofilm grows wherever seawater gets into the tunnel, and the concrete beneath it slowly dissolves.
What makes this discovery even more concerning is that similar bacteria-related damage is likely happening in other tunnels worldwide, especially those in marine environments.
While tunnels remain generally safe and are carefully monitored by authorities, the researchers stress the importance of early detection and regular inspections.
When new concrete is first sprayed, it has a very high pH level that helps protect it.
But over time, natural wear and tear lower the pH, making the environment more friendly to bacteria.
These bacteria use the concrete’s iron, sulfur, manganese, and nitrogen to survive, and in doing so, speed up the breakdown of the concrete and its steel reinforcements.
In some areas of the Oslofjord tunnel, the researchers saw the damage reach as deep as 10 centimeters in just five years.
Professor Britt-Marie Wilén explains that keeping an eye on biofilm is important because it acts as a warning sign.
Tunnel maintenance teams should track how water flows into tunnels and how the biofilm spreads. Lower water flow, especially with seawater, creates more acidic conditions, which leads to faster damage.
As ocean temperatures rise due to climate change, the researchers worry that the problem could get worse. Warmer water supports more bacterial growth and lowers pH levels, both of which make concrete degrade even faster.
By analyzing the microbes living in the tunnel using new DNA sequencing methods, the Chalmers team has also discovered previously unknown bacteria, offering new insights into how to prevent this kind of decay in the future.
