Carbon nanotubes are prized for both their strength and their piezoresistance—they change their electrical resistance as they're stressed. Xun Yu, a mechanical-engineering professor at the University of Minnesota–Duluth, is cooking up a concrete mix that contains 0.1 percent carbon nanotubes, making it harder to crack than traditional concrete, and smart too. By embedding electrodes into it as it sets, Yu can measure changes in electrical resistance to detect compression from passing cars. Future versions will better calculate speed and vehicle weight on the go for a real-time view of the road's stress. Meanwhile, a new concrete mix developed by Victor Li, a professor of civil and environmental engineering at the University of Michigan, contains unhydrated cement grains that are activated when exposed to carbon dioxide in air and water from rain—exactly what you'd find in a small crack in the road. The reaction produces a calcium carbonate seal, restoring the slab to its normal load-bearing capacity.