Sources:
BC-TOP-9A, Rev 2 -- Design of Structures for Missile Impact (1.5 MB PDF)
AIAA 2018-2019 Graduate Team Missile Systems Design Competition (3.89 MB PDF)

Where:

T = Thickness of concrete element to be just perforated (inches)
F_C = Compressive strength of concrete element (PSI)
W = Weight of Projectile (lbs)
D = Diameter of Projectile (inches)
V_S = Striking Velocity (ft/sec)

This formula will provide you with the thickness of a concrete element that "will just be perforated by a missile".

Spalling of concrete from the side opposite the contact surface of the element may occur even if the missile will not perforate the element. For an estimate of the thickness that will just start spalling, it is recommended that the following equation be used:

Where:

T_S = Concrete element thickness that will just start spalling (Inches)
T = Thickness of concrete element to be just perforated (inches) [NOTE: See previous equation above.]