When seawater enters road tunnels, bacteria form biofilms on the concrete. These biofilms feed on the concrete, making it porous and prone to damage. This process, known as ‘saltwater intrusion,’ can lead to costly repairs and safety risks, as concrete and stones might fall from the tunnel roof.
A recent study by researchers at Chalmers University of Technology in Sweden has uncovered the microbiological mechanisms behind this rapid concrete breakdown. Their findings show how the degradation progresses unexpectedly fast in tunnels surrounded by seawater, like those in the Oslofjord.
Since 2014, researchers studying the Oslofjord tunnel have observed that bacteria can erode up to one centimeter of concrete annually. When a saltwater intrusion occurs, biofilms develop on the concrete, gradually dissolving its surface.
This phenomenon has been explored in projects connected to the Norwegian Public Roads Administration in collaboration with Sweden’s Transport Administration. Researchers believe this issue is likely present in similar saltwater environments globally.
As concrete in tunnels ages, its pH decreases due to natural chemical processes, creating a favorable environment for bacteria. These bacteria metabolize elements like iron, manganese, sulfur, and nitrogen from the concrete, accelerating the corrosion of its reinforcement and the concrete’s breakdown. Researchers have observed that bacterial penetration can reach up to 10 cm within five years under extreme conditions.
Britt-Marie Wilén, a professor at Chalmers University, emphasized that biofilms serve as a warning sign. She recommended monitoring water flow and the spread of biofilms and repairing damaged concrete by respraying when needed.
Although road tunnels with biofilms remain safe under Norwegian authorities’ monitoring, researchers recommend ongoing measures to ensure stability. These include regularly measuring the concrete’s pH, studying groundwater flow, and observing biofilm spread. Slow groundwater flow lowers the biofilm’s pH, accelerating concrete degradation, while higher flow helps neutralize the acid in the biofilm.
While this research focused on Norway’s Oslofjord tunnel, similar concrete degradation might occur in other tunnels where freshwater infiltrates the concrete.
Chalmers researchers have uncovered previously unknown microorganisms by studying the microbial communities in the Oslofjord tunnel. With advancements in DNA sequencing and data processing techniques, they now have groundbreaking ways to analyze and interpret these findings, opening new doors to understanding microbial activity in such environments.
Journal Reference:
- Sabina Karačić, Carolina Suarez, Per Hagelia*, Frank Persson, Oskar Modin, Paula Dalcin Martins and Britt-Marie Wilén. Microbial acidification by N, S, Fe, and Mn oxidation as a key mechanism for deterioration of subsea tunnel sprayed concrete. Scientific Reports. DOI: 10.1038/s41598-024-73911-w
Source: Tech Explorist
