Concrete is the most widely used structural material used on the planet today, but it has an annoying habit of breaking down over time, leading to cracks and pit holes that require repair or replacement. Until now. Now we have bio-concrete. Bio-concrete is a living, breathing, stone capable of self repair. Born out of an unholy collaboration between microbiologist Henk Jonkers and concrete technologist Eric Schlangen, this self-healing bio-concrete is a living mixture of concrete, bacterial spores, and microbe-sustaining nourishment in the form of calcium lactate.
One of the many things known to contribute to the degradation of concrete is exposure to water. However, when bio-concrete is exposed to water the bacteria impregnated in it set to work converting calcium lactate into calcite, which fills in any cracks and holes. The bacteria is capable of repairing cracks in the concrete up to half a millimetre wide. This new concrete abomination is now out of the lab undergoing real-world testing and could be on the commercial market in 2-3 years.
From The Sarah Jane Adventures, episodes 1 & 2, The nightmare man (a two part story):
At some point in the early 21st century, a sentient concrete disguised itself as a flyover in Chiswick, attempting to control the minds of the populace, before being defeated by Sarah Jane Smith. In 2010, Sarah Jane used K9 and a piece of the concrete to contact Luke Smith while he was trapped in the Nightmare Man's realm.
Now think about how much concrete is in your city, then consider the consequences of all that being alive. The question is, how long before it attains enough critical mass for the bacteria to form sentience? Do you think it will appreciate thousands of people walking all over it every day? Your foot path could be plotting revenge against you! Not to mention the statues in the park. What's to stop them creeping up behind you in the blink of an eye? Our downfall won't be by terminator robots it will be insidious masonry blocks. Mark my words.