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.

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