Study finds concrete eating bacteria on street tunnels

A biofilm forms on the interior of concrete, while sea water penetrates into the concrete of a street tunnel. A study by researchers at Chalmers University of Technology (in Sweden) points to the mechanisms that are behind the dismantling and their unexpectedly fast progress.

Allow me to get together with some specific science messages for a moment.

Salva water invasion

When building a vehicle tunnel by rock, the roof and walls are sprayed with concrete in order to create an even surface layer and prevent stones on the street loosening and falling. When they are surrounded by sea water, the phenomenon known as salt water intrusion occurs. The penetration of salt water is a process that bears bacteria that then form biofilms (or colonies) on the surface of the concrete.

That bacteria feed on the materials in concrete, weaken the surface and make them porous, which leads to high costs and increases the risk of damage if the parts of the concrete fall from the roof of the tunnel. Researchers at Chalmers University claim that they have discovered new knowledge of the processes for this breakdown.

Frank Persson – Chalmers University of TechnologyChalmers University of Technology | Catharina BjörkThe study, “Microbial acidification by N, S, FE, FE- and MN oxidation as a key mechanism for the deterioration of the underground tunnel sprayed concrete“Was released in Nature Scientific Reports. The authors are Sabina Karačić, Carolina Suarez, Per Hagelia*, Frank Persson, Oskar Modin, Paula Dalcin Martins and Wilén. First author Karačić was a doctoral student at Chalmers, but no longer in the department.

Frank Persson, Associate Professor of Molecular Biology and Microbial Ecology at the Department of Architecture and Civil Engineering of Chalmers, says that the researchers have made measurements in the Oslofjord tunnel since 2014. The tunnel runs about 23,970 feet long and reaches about 440 feet below the middle sea level.

Persson states that you have observed bacteria that eat up to 1 cm a year in the concrete surface. However, the biofilm forms at the point of salt water impression, but all concrete in contact with water can be preserved.

Not unique in Norwegian tunnels

As reported in the official announcement, a spray concrete has been used on a larger scale in street tunnels, and since then the researchers have been able to see this pollution of biofilm. Nevertheless, there are currently very few similar studies on biocorrosion in underwater tunnels in marine environments.

Britt -Marie Wilen – Technology of Chalmers University of TechnologyChalmers University of Technology | Anna-Lena LundqvistAlthough research was carried out in Norway, the phenomenon is likely to occur in similar environments around the world, according to Persson and his colleagues. According to Britt-Marie Wilén, professor of environmental and wastewater technology at the department for architecture and civil engineering, professor of environmental and sewage technology in the Chalmers Department of Architecture and Civile Engineering, is probably also in similar tunnels in which fresh water appears in the concrete, probably also in similar tunnels in which fresh water can be exhausted into the concrete.

“The problem is probably greater in environments in which sea water penetrates, partly because the sea water is cheap for bacterial growth, but also because the salt accelerates the corrosion in the reinforcement. Climate change also makes the oceans warmer, and with warmer water the pH value continues to drop,” says Wilén.

New concrete sprayed on walls and ceilings in tunnels has a high pH value, but since the concrete age occurs a natural chemical degradation, which drops the pH of the concrete and makes the environment more hospitable for bacteria.

The bacteria accelerate the corrosion of concrete reinforcement and in turn the breakdown of the concrete itself if they metabolize iron, manganese, sulfur and nitrogen in concrete. The researchers have seen that this interacting breakdown can be relatively fast locally. Under extreme conditions, the bacteria can penetrate up to 10 cm in five years. Chalmers University of Technology | Britt-Marie Wilén

Wilén explains that cast concrete is more compact. The strength helps to resist against deterioration – that is, it takes a long time for the concrete to deteriorate. On the other hand, sprayed concrete is more porous and more susceptible to breakdown by bacteria.

Wilén adds:

[Sprayed concrete] Often contains steel fibers for increased strength. If the iron is oxidized due to corrosion, it is also [can] We act as a substrate for some bacteria (iron -oxidizing bacteria) that can further accelerate biocorrosion. Bridges in water and especially in salt water can achieve biofilm growth and thus biocorrosion.

Wilén sees the biofilm as a clear warning signal. The progress of progress is helpful: measure the pH of the water that is transported on the concrete, monitor the spread of the biofilm and locate every loose/damaged concrete and remove the biofilm.

“Another option may be to add material that seals the cracks in the rock/concrete,” says Wilén.

Beton originally has a very high pH value and bacteria cannot survive. As soon as the PH value goes down, you can colonize the surface and start forming biofilm under which the pH value can drop, and then the biokorrosion begins. As soon as it has started, it can continue to be accelerated if substrate is available. The CA begins dissolved and the concrete becomes more porous and weak.

Security

The researchers emphasize that Straßentunnel are generally safe despite this biofilm and are monitored by the Norwegian authorities. It could be a dangerous situation if the concrete becomes too thin so that concrete pieces can fall. As far as the researchers know, this has not happened.

Your recommendation based on the studies is the measurement of the PH value of the concrete continuously, the groundwater flow through the rock and the monitoring of the spread of the biofilm. The groundwater flow influences the growth of biofilm, especially at lower currents, whereby the pH value of the biofilm is lower, which contributes to a faster breakdown of the concrete than with a higher groundwater flow that neutralizes the acidity in the biofilm.