in a garage? wouldn't the car crush them?
I think they are filled underneath and sealed on top.
The pressure a car tire exerts on the floor equals the pressure inside the tire - usually about 32 lbs per square inch. Since a bottlecap is about 1 square inch, each one only has to hold up about 32 lbs. I'd still think they're filled with grout underneath, both for support and to keep them in place. To me, the bigger worry with car tires would be torque when you turn the wheels.