Concrete may not seem like a particularly hospitable environment for life, but bacteria do actually live inside it. A new study led by the University of Delaware has examined this concrete microbiome to find out how they get there, how they change over time, and how we might use them in future to monitor or even repair defects.
Bacteria can be hardy little creatures, constantly turning up in places that were previously assumed to be too harsh for life, like polar ice, toxic environments, in subglacial lakes, in extremely dry hot and cold deserts, and even in space. So in a way it’s not too surprising that some of them would thrive in concrete, despite it being a dry, salty, alkaline and nutrient-poor environment.
But little is known about the species that call concrete home, and what implications they might have for buildings and structures. So the new study set out to characterize them.
The team made 40 concrete cylinders in two groups – a standard mix that’s prone to an alkali-silica reaction (ASR) that degrades the concrete, and one made with fly ash that’s resistant to it. These were then left on a rooftop for about two years, with DNA samples taken every six weeks.
The researchers found that after contaminants from the lab were removed, between 50 and 60 percent of the bacteria in the samples hitched a ride on the precursor materials like gravel and sand. Others may have been delivered there later from the environment, seeping in through cracks. The most common types were Proteobacteria, Firmicutes, and Actinobacteria.
Generally, the diversity of the communities decreased over time, which isn’t too surprising given the environment they have to endure. Interestingly though, there were some slight rebounds in diversity each summer, and seasonal variation in composition as well. There are still a lot of questions raised by the study, however.
“What are they eating?” says Julie Maresca, lead author of the study. “It’s possible they’re eating the dead bodies of other microbes. If there’s nothing to eat, some of them can form spores or form a dormant cell type and do nothing until it rains, then eat as much as they can and go dormant again.”
While there’s still plenty of work to do to better understand this tiny world, the potential implications could be huge. The team identified several types of bacteria, including Arcobacter, Modestobacter, Salinicoccus, Rheinheimera, Lawsonella, and Bryobacter, which seemed to be associated with the degradation reaction. Hypothetically, monitoring these bacteria in concrete structures like buildings and bridges could one day be used as an early warning system for those at risk of collapse.
Longer term, the bugs might even be able to take an active role in repairing damage. Some can produce calcium carbonate that can patch up cracks, but unfortunately they currently don’t seem to live very long. Previous studies have tried encasing them in microcapsules that release the bacteria only when they get wet, which is usually when the damage first starts.
More study is needed, and the team says the next steps are to analyze the DNA of samples taken from real-world concrete structures.
The research was published in the journal mSystems. The team describes the work in the video below.
Source: University of Delaware