The bacteria, once released into a damaged area, procreate and spread into the cracks—and then die. But don't be sad—in their wake, they leave behind calcium carbonate corpses as strong as the original concrete. And don't worry—the researchers were canny enough to design the bacteria to know when their work is done, so they don't run amuck and cover the world in concrete:
The bacteria also contains a self-destruct gene that keeps it from wildly proliferating away from its concrete target, because a runaway patch of bacterial concrete that continued to grow despite all efforts to stop it would be somewhat annoying
Yes, annoying is right—or like something out of a very dull horror movie.