Concrete that can heal its own cracks

Concrete capable of automatically closing cracks that appear in it is no longer a sci-fi idea. Such materials already exist and have been undergoing laboratory and full-scale testing for several years. Scientists are trying to create concrete that will not require immediate human-led repair after damage.

One of the most famous directions is being developed by Delft University of Technology in the Netherlands. A group of researchers led by engineer Henk Jonkers has developed concrete with bacteria capable of producing calcium carbonate—essentially limestone, which gradually fills the damage—after a crack appears. Under laboratory conditions, this material made it possible to heal cracks up to 0.8 millimeters wide.

However, interest in self-healing concrete is not limited to a single technology. Researchers around the world are studying several ways to make building materials autonomously repair damage—from using bacteria and microcapsules to processes that concrete can perform without any additional components.

How concrete can "fix" itself

In fact, even ordinary concrete has a certain capacity for self-healing.

After hardening, particles remain inside the cement matrix that have not had time to fully react with water. If the concrete receives a new portion of moisture through a newly formed crack, hydration can continue. The products of this reaction gradually fill part of the empty space.

Another important mechanism is associated with the formation of calcium carbonate. Under suitable conditions, chemical reactions inside the hardened cement paste lead to the appearance of calcite crystals, which deposit on the walls of the crack and gradually reduce its size. It is the continuation of hydration and the formation of calcium carbonate that belong to the main mechanisms of so-called autogenous self-healing.

However, the natural capabilities of ordinary concrete are limited. Therefore, scientists went further and tried to embed the "repair" mechanism directly into the material.

Bacteria that turn a crack into stone

This is precisely what Jonkers' group in Delft is working on. Special bacterial spores and a nutrient source—lactate—are added to the concrete mixture. Inside undamaged concrete, the bacteria remain dormant. When a crack appears and water penetrates it, conditions are created for their activation.

The bacteria consume the available nutrient source and facilitate the formation of calcium carbonate. As a result, a mineral material gradually forms inside the crack, filling the space between its walls.

The result is an unusual design: microorganisms effectively become part of the building material and activate precisely when damage occurs.

The technology has already moved beyond small laboratory samples. Delft researchers used the bacterial self-healing material in several full-scale demonstration projects—including the construction of concrete structures and repair mortars. Tests showed that adding the bacterial component did not lead to negative consequences for the material's structural properties.

At the same time, the experiment also revealed an important challenge: the structures turned out to be so well protected against crack formation that it was difficult for researchers to test the self-healing mechanism itself in real-world conditions. Therefore, in subsequent demonstration projects, they planned to intentionally allow more noticeable cracks to form and reduce the amount of conventional reinforcement that limits their width.

This already works, but does not yet replace repairs

This is precisely where the boundary between scientific development and mass technology lies.

Self-healing concrete is indeed capable of closing certain cracks, but this does not mean that a damaged bridge or a multi-story building can completely restore itself without the involvement of engineers.

Efficiency depends on the width and geometry of the crack, moisture levels, temperature, concrete composition, and other conditions. Even in a laboratory, two cracks of the same width can behave quite differently because their internal three-dimensional structure differs.

Therefore, researchers are currently addressing not so much the question of "whether concrete can be made to heal cracks," but rather a much more practical one: how reliably this process works in real structures and whether it can be relied upon when designing buildings and infrastructure.

This is especially important for watertight structures and reinforced concrete. A crack is not just an aesthetic issue: through it, water and dissolved substances can penetrate deep into the structure and accelerate the corrosion of steel reinforcement. If self-healing can reliably block these pathways, the service life of structures could potentially be extended.