Those fleas came from a host.
Do you keep a pet in the garage? If so, treat your pet with a monthly medication that kills eggs and larvae too.
Do you have rats? If so, put out rat poison, eliminate the food source, close off how they are entering the garage.
Raccoons? Squirrels? Find and seal off how they are entering the garage.
It doesn't matter what you do to kill fleas in the garage if you don't address how the fleas got there.
After you've treated your pet and eliminate ways for rodents to enter, you can sprinkle borax all over the concrete floor.
Here's how:
How to Rid My Garage of Fleas | eHow.com