Chicago covers alleys with special concrete that cleans polluted rain runoff . The city has been working to clean up the water since it was first built in the early 20th century and is now one of only a handful of cities around the world using this type of concrete. Chicago workers resurface alleys with permeable concrete that cleans rainwater before it runs into Lake Michigan . Solution avoids having to rebuild city 's massive alley system . Unique concrete also helps to keep city cooler on sunny days .
Chicago covers alleys with special concrete that cleans polluted rain runoff . The city has been working to clean up the water since it was first built in the early 20th century and is now one of only a handful of cities around the world using this type of concrete. Solution avoids having to rebuild city 's massive alley system . Unique concrete also helps to keep city cooler on sunny days .