I will try to confirm this, but I believe each load of airfield supplies will incrementally repair strats by a small percent.
EDIT:
I was having trouble getting this to work.
Hamhawk was able to successfully resupply strats by dropping them on a concrete pad from 1k.
I thought they would repair within a mile wherever they were dropped.
Apparently, they need to land on concrete, but yes, the strats will incrementally rebuild with field supplies.
Maybe he will pop in and give us a resupply lesson.