What if concrete could heal its own cracks?

Concrete is in roads, bridges and buildings. It is very hard.

But concrete can crack. Your skin heals when you scrape it. What if concrete could heal too?

Look around a town and you will see concrete everywhere: sidewalks, bridges, tunnels, the walls and floors of tall buildings. It is hard and strong, but over the years it gets cracks. Some are tiny, some are big, and fixing them takes time and money.

Your body heals a small cut all by itself. So what if concrete could heal its own cracks? Scientists are already working on it. In this lab we follow three clues: why concrete cracks, bacteria that make stone, and a very old Roman recipe.

Concrete is one of the most widely used materials on Earth, and much of the work of keeping bridges, tunnels and buildings safe is finding and repairing cracks. Engineers have long accepted cracking as something to manage rather than prevent.

Self-healing concrete aims to change that: a material that seals small cracks on its own when water reaches them. This thought experiment examines three threads: why concrete cracks and why cracks matter, the bacteria-based concrete developed at TU Delft, and recent MIT research on how ancient Roman concrete healed itself. Established findings and reasoned guesses are labeled separately.

Concrete is great at being squeezed. But it is not good at being stretched.

When concrete is pulled or bent, it can crack. So builders put steel bars inside.

Concrete is very strong when it is squeezed, but weak when it is stretched. The American Concrete Institute says its strength against pulling is only about 10 percent of its strength against squeezing. That is why most concrete has steel bars inside, which take the stretching.

Cracks are a problem because they let water in. A spin-off company from TU Delft, a Dutch university, explains what happens next: water reaches the steel bars and they rust. Rust takes up more space than steel, so the rusting bars push outward and can burst the concrete apart.

The American Concrete Institute notes that unreinforced concrete is weak in tension, with a tensile strength of about 10 percent of its compressive strength, so steel reinforcement is used wherever tensile stresses would exceed that.

Cracks matter mostly because of what gets in through them. A 2023 TU Delft article, an interview with the head of the university's spin-off company Green Basilisk, describes what is sometimes called 'concrete rot': water penetrates the structure, the steel reinforcement corrodes, and the expanding rust bursts the concrete. Concrete that has to withstand salt water is especially susceptible, he says. Sealing cracks early is therefore the target of every self-healing approach.

Scientists put tiny sleeping bacteria into concrete. When water gets into a crack, the bacteria wake up.

They make limestone, a kind of stone, and fill the crack!

At TU Delft, scientists Henk Jonkers and Erik Schlangen came up with concrete that has bacterial spores mixed in: tiny, sleeping forms of bacteria. As long as the concrete is dry, they rest. When water seeps into a crack, it wakes the spores, and they begin to make limestone, which fills the crack.

If all concrete could do this, small cracks might seal themselves before water reached the steel. Bridges and buildings might need fewer repairs. That is an idea, not a promise: it depends on how big the cracks are and how well the healing works in real structures.

In the bacteria-based approach invented by Henk Jonkers and Erik Schlangen at TU Delft, spores of limestone-producing bacteria are cast into the concrete. Water entering a crack activates them and they produce limestone that fills the crack. The spin-off Green Basilisk also makes a repair mortar with the same self-healing property for existing structures.

If concrete healed generally, small cracks might be sealed before water reached the reinforcement, potentially extending service life and cutting repair work. How large a crack can be healed, how long the agents stay active, and what it costs are the practical questions; the size of any benefit is a reasoned expectation, not a measured result.

Healing concrete needs water and something that makes new stone.

The new stone grows in the crack and plugs it up.

Both kinds of healing concrete use the same trick: when water gets into a crack, it starts a reaction that makes new stony mineral right where it is needed. In the bacteria concrete, living cells make limestone.

Ancient Roman concrete did it without bacteria. MIT researchers found small white lumps of lime in it. When a crack forms, it tends to run through these lumps. Water dissolves some of the lime, and the calcium crystallizes again as calcium carbonate, the mineral in limestone, sealing the crack.

Both strategies are triggered by water and deposit calcium carbonate in the crack. In bioconcrete the bacteria do the chemistry. In ancient Roman concrete, an MIT-led team reported in Science Advances in 2023 that millimeter-scale lime clasts, long dismissed as poor mixing, act as calcium reservoirs. Cracks preferentially travel through the brittle, high-surface-area clasts; water then forms a calcium-saturated solution that recrystallizes as calcium carbonate or reacts with pozzolanic material.

The team made hot-mixed concrete with quicklime, cracked it and ran water through: within two weeks the cracks had healed and water stopped flowing. Identical samples made without quicklime never healed.

Long ago, the Romans built with concrete. Some of their buildings are still standing after about 2,000 years!

Scientists are learning from their recipe.

Roman builders made roads, harbors, aqueducts and great buildings from concrete. The Pantheon in Rome, dedicated in 128 CE, still has the world's largest dome made of concrete without steel bars.

In 2025, MIT researchers studied a Roman building site at Pompeii that had been buried in volcanic ash in 79 CE. They found dry quicklime already mixed with other ingredients, ready to use. This was the clearest evidence yet that Romans 'hot-mixed' their concrete, the method that makes its self-healing lime lumps.

MIT News notes that the Pantheon, dedicated in 128 CE, still has the world's largest unreinforced concrete dome, and some Roman aqueducts still deliver water to Rome. In December 2025, a Nature Communications study by Admir Masic's MIT team analyzed an exceptionally preserved construction site at Pompeii, buried in the eruption of 79 CE. Intact quicklime fragments pre-mixed with dry ingredients, plus lime clasts in the walls, gave the clearest evidence yet of hot mixing, unlike the slaked-lime method described by the Roman architect Vitruvius.

In Reuters coverage summarized by MIT News, Masic is quoted as saying that the ancient process is not a direct replacement for modern standards, but that its principles can inform the design of durable, low-carbon concretes.