Many of Europe's most famous buildings are more than a thousand years old. Though this is a testament to some of the incredibly sturdy materials used by the likes of the Ancient Romans, historic building preservation is an issue that's becoming increasingly challenging and costly.
A solution may be found in several recent advances in self-healing concrete, including a sticky self-healing bacterial paste, that has the potential to regenerate ancient stone structures, European Commission-backed Horizon Magazine reports.
The promise of self-healing concrete
In the European Union alone, bridge maintenance costs €4 to 6 billion every year, while replacing those bridges would cost a staggering €400 billion at least, estimates show.
Such massive sums inspired the scientists behind the recent study to look for a method that would see the bridges, and other buildings, heal themselves — their investigations led them to develop a sticky bacterial paste.
Research — such as this study from Bath University — has already shown that concrete infused with bacterial spores eventually harden into calcite, creating a self-healing concrete that heals cracks in old buildings.
Can stone buildings also regenerate?
While that is a great advance for concrete structures, what of historic buildings made from stone?
A team from a European project called Geoheal recently developed a technique that lets users spray or brush stonework with a bacteria paste that starts healing damage as it occurs.
"Stone and geological materials are by nature bioreceptive," said Geoheal’s Michael Harbottle, who is also a professor of geoenvironmental engineering at Cardiff University, told Horizon Magazine.
"The bacteria we have used can happily live in such environments and lead to new mineral formation, as long as they have access to water, oxygen and nutrients," he continued.
Tests show improved masonry microstructure
The researchers from Geoheal tested their bacterial paste at Tintern Abbey in Monmouthshire, Wales, a historic landmark founded in 1131.
Using two types of bacteria, Sporosarcina pasteurii and Sporosarcina ureae, they found that their paste did indeed improve the microstructure of the masonry without changing the outward appearance of the stone.
What's more, the paste did not affect the breathability of the stone, a recurring issue in many stone protection treatments.
Self-healing bacteria treatments have the potential to greatly improve the safety of critical infrastructure and prevent catastrophic events such as the 2018 collapse of the Morandi Bridge in Genoa, Italy, which resulted in 43 people losing their lives.
However, large parts of these structures are typically underground, making inspection a challenging task.
Self-healing properties in underground conditions
Another Euopean project, called the Geobacticon project, which ended in December 2020, investigated the efficiency of these bacterial self-healing techniques in underground concrete.
The Geobacticon project highlighted the fact that different soil types with varying moisture and acidity can affect concrete in a variety of different ways.
As well as gaining vital clues as to how self-healing concrete works in underground settings, the researchers on the Geobacticon project also discovered that the self-healing bacterial concoction can also protect the steel bars encased within the concrete.
Moisture and chemicals from the soil can gradually weaken steel over many years, leading to weakened structures and potential disasters — so a method for preventing this corrosion is incredibly valuable.
Enabling 'bolder, more sustainable designs'
Magdalini Theodoridou, an engineer at Newcastle University who worked on the Geoheal project, says: "in new construction, the possibility of incorporating self-healing materials and structural elements would enable bolder and more sustainable designs."
In the future, the researchers say that self-healing stonework and concrete might even lead to exciting new building methods as well as forms of architecture that could remain sturdy for centuries.