Fleas prefer to burrow in fabrics such as carpet or upholstery, but they can live on concrete and other solid surfaces for a while as well. Killing fleas on concrete is typically easier than killing them on carpet. It's easier to get the flea killing product in contact with the fleas; they have nowhere to hide.
Using Flea Spray
Vacuum the floor, using a vacuum cleaner with good suction. Vacuuming sucks up fleas and doing this alone will remove a significant amount of fleas. When you are finished, remove the vacuum bag and discard in your outdoor trash bin. If you are using a bagless vacuum, empty the receptacle into a trash bag and discard the bag.
Ventilate the area by opening windows and doors, if possible, as suggested by Quality Cat Care. Extinguish open flames, such as candles or stove burners. Many types of flea sprays are flammable.
Spray the concrete with flea spray, starting at the corner farthest away from the room's exit. Continue spraying until you've covered the entire room.
Close the door or otherwise block off access to the room until the spray is fully dry and fumes have dissipated.
Using Flea Shampoo
Mix 1 capful of flea shampoo per gallon of water in a mop bucket. Fleas will drown in soapy water anyway, but the added flea killing ingredients in these shampoos will quicken the process.
Mop the floor, spreading the solution over the entire area.
Leave the room, and do not permit pets or people to enter the room until the floor is dry.
Mop the concrete floor the following day, with clean water, to rinse off the flea shampoo residue.