Concrete Decay and Bacteria Eating Away Tunnel Infrastructure
Deep beneath the mountains and seas, where road tunnels weave their way through rocky terrain and under seabeds, a slow but relentless attack is underway.
Not from earthquakes or wear-and-tear from traffic, but from a much smaller foe: microbes. Yes, the concrete linings of subsea road tunnels are being eaten alive by bacteria, and the consequences could be far more serious than previously thought.
A new and ground-breaking study led by Chalmers University of Technology in Sweden has revealed how saltwater intrusion and microbial colonisation are teaming up to degrade concrete at an alarming pace. In particular, the Oslofjord tunnel in Norway has become a living laboratory for observing the unexpected speed at which this decay occurs.
Microbial Mischief
Sprayed concrete, or shotcrete, has been widely used in tunnel construction since the 1990s. It provides a smooth surface and helps prevent rocks from falling. However, in subsea tunnels like the Oslofjord, where seawater seeps through cracks and pores, things start to get murky. Quite literally.
Saltwater intrusion not only compromises the physical structure of the concrete but also brings in an uninvited microbial guest list. These bacteria settle on the concrete surface, forming sticky colonies known as biofilms. Once established, the biofilms begin a metabolic feast, feeding on minerals and compounds in the concrete.
Frank Persson, Associate Professor of Molecular Biology and Microbial Ecology at Chalmers, highlights the scale of the issue: “We have been taking measurements in the Oslofjord tunnel since 2014, and we can see that the bacteria eat their way into the concrete surface up to one centimetre a year. Where there is saltwater intrusion, a biofilm will form, and the concrete covered by the biofilm will gradually dissolve.”
A Global Problem in the Making
While this study zeroes in on Norway, the implications reach much further afield. Similar environmental conditions exist in tunnels across the world, from Japan to the United States. Wherever road tunnels meet the sea or other saline environments, the potential for this kind of biocorrosion looms.
The study’s co-author Britt-Marie Wilén, Professor of Environmental and Wastewater Engineering at Chalmers, offers a clear warning: “This type of biofilm is a pretty clear warning signal. You need to monitor the water flow and the spread of the biofilm and locate loose and damaged concrete to spray again, if necessary.”
To make matters worse, new concrete starts out with a high pH level, creating a hostile environment for microbial life. But as the material ages and chemically degrades over time, the pH drops, creating ideal conditions for bacteria to thrive. These bacteria don’t just sit idly by; they metabolise key elements such as iron, manganese, sulphur, and nitrogen, accelerating the corrosion of reinforcing steel and further weakening the concrete.
Five Years, Ten Centimetres
It may sound unbelievable, but under extreme conditions, bacteria have been observed to penetrate as deep as 10 cm into the concrete in just five years. That’s not just a threat to the surface layer – it’s an all-out assault on the tunnel’s structural integrity.
To combat this, the researchers recommend a comprehensive monitoring strategy:
- Track pH Levels: Regular pH assessments of concrete surfaces help identify areas becoming microbe-friendly.
- Monitor Groundwater Flow: Slower flows mean less dilution and more acidic biofilms.
- Map Biofilm Spread: Use imaging and sensors to detect where biofilms are forming and growing.
- Reinforce Damaged Zones: Apply new layers of sprayed concrete as a preventive measure.
Interestingly, Wilén also points out that this is not just a saltwater problem: “Similar degradation of the concrete is likely to also occur in similar tunnels where freshwater is able to leach into the concrete. However, the problem is probably greater in environments where seawater penetrates, partly because seawater is favourable for bacterial growth but also because the salt accelerates corrosion in the reinforcement.”
Warmer Waters, Faster Decay
If climate change felt like a distant concern for the infrastructure world, here’s a reason to pay closer attention. Warmer oceans reduce the pH level of seawater, enhancing the ideal conditions for bacteria to flourish. So, as global temperatures rise, so too could the rate of tunnel degradation.
This paints a troubling picture for coastal and underwater infrastructure. In an age when resilience and sustainability are paramount, the industry must now account for microbial corrosion as part of long-term maintenance and asset management strategies.
New Species, New Discoveries
There’s a silver lining to all this microbial mayhem. The Chalmers team, using advanced DNA sequencing technologies, has uncovered new and previously unknown bacteria within the Oslofjord tunnel. One of the most significant discoveries was the identification of the Anammoxibacteraceae family – a group of bacteria that metabolise nitrogen and may play a larger role in natural nutrient cycles than previously understood.
These findings don’t just deepen our understanding of biocorrosion but could also inform broader microbiological and environmental research. The study, “Microbial acidification by N, S, Fe and Mn oxidation as a key mechanism for deterioration of subsea tunnel sprayed concrete,” was published in Nature Scientific Reports and authored by Sabina Karačić, Carolina Suarez, Per Hagelia (retired), Frank Persson, Oskar Modin, Paula Dalcin Martins, and Britt-Marie Wilén.
Forward Thinking for Tunnel Longevity
Despite the ominous findings, there’s hope. Norway’s tunnel systems remain safe thanks to stringent monitoring and maintenance protocols. However, the study is a wake-up call to engineers, designers, and policymakers everywhere. Infrastructure needs not just to be built strong but also smart – designed with microbial resilience in mind.
Mitigation strategies could include:
- Developing new concrete mixes resistant to microbial colonisation
- Using protective coatings that slow or prevent biofilm formation
- Implementing real-time sensor networks for early detection of pH changes and biofilm growth
As the researchers note, these microbes aren’t going anywhere, but with the right data and proactive management, we can stay one step ahead.
Building Smarter, Building for the Future
Infrastructure has long been about brute strength – steel, stone, and concrete. But as this research shows, it’s time to think smaller. Microscopic organisms, often ignored in the planning phase, can determine whether a tunnel lasts 100 years or needs major repairs in 20.
The Chalmers University of Technology, founded in 1829 and still pushing boundaries today, has once again demonstrated that innovation is the best defence against unforeseen challenges. Their work underscores a critical truth: to build sustainably, we must first understand the unseen.
