Monitoring the health of bridges requires either drilling out a core sample or embedding boxy sensors that monitor for the intrusion of rust-inducing salt. The first method is labor-intensive; the second relies on wires that can break or degrade. Researchers at SRI International in Menlo Park, CA, are developing an alternative they call a smart pebble. The wireless device, the size of a piece of gravel, can be poured right along with the concrete. The SRI team built the pebble by linking a new type of chloride sensor to an off-the-shelf wireless-communications chip and tucking both into a housing made of cementlike materials. The sensor develops a voltage whose strength is determined by salt concentration. To collect the sensors’ readings, a device mounted on the underside of a van could emit a blast of radio energy as the vehicle drives over a bridge. Each radio query identifies an individual pebble; the code number that bounces back indicates whether salt levels are above or below a certain threshold. SRI is preparing prototype pebbles for the California Department of Transportation, which plans to test them this year.