36 tubes that are each 36" (one yard) long is the same as one tube that is 36 yards long. Now you are looking for the volume of concrete (presumably in cubic yards), so we need to find the volume of a tube that is 18" (half a yard) across and 36 yards long.
The volume of a tube (cylinder) is the length times the radius squared times pi (about 3.1416). The radius is half the distance across (half a yard), so the radius in your case is a quarter yard. So to calculate the cubic yards of concrete, we compute:
36 x (1/4)2 x 3.1416 = 35 (cubic) yards of concrete
(each tube holds just a little less than a cubic yard).
Hope this helps,
Stephen La Rocque.