1In this final lecture of the materials series, we turn to one of the more counterintuitive innovations of recent decades: concrete that repairs itself.
2Concrete is the most widely consumed substance on Earth after water, yet its tendency to crack imposes staggering maintenance costs worldwide.
3Conventional repair involves closing roads, injecting epoxy, and accepting repeated disruption, a cycle that inspired researchers to seek biological alternatives.
4The breakthrough came from an unexpected quarter: bacteria capable of precipitating calcium carbonate, embedded dormant within the concrete mix.
5When a crack admits water, these microbes awaken, metabolise added nutrients, and deposit limestone that seals the fissure.
6Early laboratory trials suggested healing of cracks up to nearly a millimetre wide, though field performance proved considerably more modest.
7What I want to emphasize here is that the mechanism mimics biomineralisation, the same process by which corals construct their skeletons.
8It was not the strength of the healed material but the longevity of the bacteria that initially sceptical engineers questioned.
9Spores had to survive decades of dormancy in an alkaline environment so hostile that most organisms perish within days.
10Encapsulating the spores in clay pellets, rather than mixing them freely, extended their viable lifespan beyond initial expectations.
11Cost, predictably, remains the obstacle: the bacterial additive still raises production expenses well above what contractors routinely tolerate.
12Nevertheless, pilot projects in tunnels and marine structures, where access for repair is especially hazardous, have already demonstrated genuine commercial promise.