Concrete is devastating to a piano.  Moisture wicks up into the
case and the glue joints suffer which in turn delaminates everything.
Even if the garage is heated you need to insulate the piano from the
floor (and not have it backed up to a concrete wall either).
Corrugated cardboard would be minimal, plywood and/or carpet would
be nice.  Putting the piano up on blocks may be a little precarious.

Considering the length of time, I'd overshoot the protection.

Regards,

Jon Page, piano technician
Harwich Port, Cape Cod, Massachusetts