The volume in the top is 27 × 4 ×1/12 = 9 cubic feet =1/3 cubic yard.
Multiplying the quantities in line 9 of Table 1 by 12/3 and those in line 2 by it is found that the base requires 7.74 sacks cement; 0.86 cubic yard sand; 1.43 cubic yards gravel; and the top requires 4.49 sacks cement; 0.33 cubic yard sand.
Example 3.[1]—A tank 9 feet inside diameter has walls 6 inches thick and 4 feet high (above the floor). The floor is 6 inches thick, the concrete is to be 1:2:4.
[1] A practical rule in finding the area of a circle is to multiply one-half the diameter (radius) by itself and the product by22/7. In finding the volume in the wall of a circular structure, such as a silo or tank, the area of the circle formed by the inside circumference is deducted from the area of the circle formed by the outside circumference and the remainder is multiplied by the height.
The volume in the floor is10/2 ×10/2 ×22/7 ×1/2 = 392/7 Cubic feet.
The area of the larger circle is 5 × 5 ×22/7 = 784/7 cubic feet.
The area of the smaller circle is 41/2 × 41/2 ×22/7 = 634/7 cubic feet.
The area of the wall, therefore, is 15 cubic feet and the volume is 15 × 4 = 60 cubic feet.
The total volume in the structure is 992/7 cubic feet or 32/3 cubic yards. Multiplying the quantities in line 7 of Table 1 by 32/3, it is found that the following material is needed: 22.14 sacks of cement; 1.65 cubic yards of sand; 3.27 cubic yards of gravel.