Nailing a joist hanger to ledger with concrete behind
Joist hangers resist loads in shear, not withdrawal. While a 16d is more common, full resistance is developed in a 2x.

Simpson Strong-Tie makes a short nail that is 1 1/2” long x 9 ga. diameter.

Oh, and be sure to order the nails treated (galvanized) or stainless steel based on the type of pressure treated ledger.