I assume that the tubes are 12 in in diameter and 48" long.
The volume of a cylinder is
where r is the radius and h is the height. You have the dimensions in inches so first you need to convert them to yards. There are 36 inches in a yard so
r = 12/2 = 6 inches = 6/36 yards and h = 48/36 yards
Thus the volume in cubic yards is
( 6/36 )2 ( 48/36 ) = 0.116
But you have 15 such cylinders so you need
15 0.116 = 1.75 cubic yards of concrete.