Bacteria that survive in extreme conditions without water are mixed in to the concrete and come alive when it breaks and gets wet. They then produce limestone which seals the cracks.

Billions of tiny insects from the species known as Concretians have been trained to colonize and repair broken concrete.

Genetically modified ants are fed a diet of limestone and natural glue that they spit onto cracks occurring in concrete in order to heal it and save their colonies.

None of the above. Nothing survives without food or water for that long. Self-healing concrete is a conspiracy perpetuated by a secret government group that, makes up stories like this, when not promoting standardized testing.