Concrete buildings that heal themselves

A living concrete that self-repairs any cracks could change the way we maintain our homes and cities. Hendrik Jonkers, a microbiologist at Delft University in the Netherlands is set to revolutionise the building industry by placing spores of mineral-excreting bacteria in the concrete mix.

“I think it’s a good solution,” says Rackel San Nicolas, materials and civil engineer at the University of Melbourne. “This process is more ecological than what we currently use.”

In modern cities concrete is everywhere, from high-rise housing to tunnels and bridges. In ancient Rome it was used to construct the Pantheon and Colosseum, massive structures that are still standing today. But both are less than 50 metres tall, whereas modern skyscrapers can be hundreds of metres high. This is possible because in modern buildings, liquid concrete is poured around a steel scaffold. The steel increases a structure’s tensile strength, making it less prone to breaking. Eventually, however, the steel can become a weakness.

Concrete is slightly porous and after 20 or 30 years water can begin to seep into it, carrying corrosives such as chloride, CO₂ or sea salt that can react with the steel inside the concrete. As steel starts to rust, it expands and pushes on the concrete, eventually forming cracks. Fixing the damage with plaster or steel plates is costly: about $6.5 billion is spent on concrete renovations in the European Union each year.

So Jonkers wondered if bacteria could be used to make a self-healing concrete. One obstacle is that concrete is too alkaline for most bacteria to survive in it. But Jonkers spent his early career researching extremophiles – organisms that thrive in normally inhospitable places – and had “a good hunch” that Bacillus pseudofirmus and B. cohnii might do the trick. They’re usually found in volcanic rocks or swimming in alkaline soda lakes. Their spores can survive at a pH of 12-13, which is exactly the pH of concrete.

To create his now patented “bio-concrete” Jonkers spikes normal wet concrete with an equal ratio of his bacterial spores and their food, calcium lactate. This mixture makes up 1% of the final concrete mix. As the concrete dries, the spores are entombed in the slab.

The spores can lie dormant for at least 200 years until they receive a wake-up call in the form of air and water entering a crack in the material. The activated spores then start munching on the food around them and excrete carbonate ions, which react with calcium in the concrete to produce limestone mortar. Once the crack is filled the bacteria are stifled by their own waste and die.

Jonkers’ bacteria take three weeks to seal cracks in concrete. The bacteria can only fill fine cracks – up to 0.8 millimetres wide – but an early repair stops them from getting bigger. The bacteria can heal existing concrete structures too, by way of a spray-on formulation. The sprayed-on microbes also eat, excrete and seal shut any fine cracks in the structure.

The bacterial spray will be commercially available late this year, with the bio-concrete due to be rolled out in 2016. Bio-concrete is 50% more expensive than conventional concrete, but Jonkers is confident the long-term savings will outweigh the initial cost. For underground structures such as basements, which are more exposed to moisture, he has calculated the bio-concrete would pay for itself within two years of the first cracks appearing.

San Nicolas agrees the technology could significantly extend a building’s life, but adds “it’s still a non-permanent solution – it will fix the problem for a while but not forever”.

The discovery has made Jonkers a finalist for the 2015 European Inventor Award. He’s partnering with Dutch company Verdygo which will use his bio-concrete to build a next-generation wastewater treatment plant in Limburg.

Should spore-laden buildings rouse our safety concerns? Not according to San Nicolas: “We’ve got bacteria everywhere – it wouldn’t be more dangerous than the concrete itself.”