=Medium mixture.= One that is between the wet and dry mixture. This consistency is recommended for general work. =Dry mixture.= One about like damp earth. If a handful is squeezed it will retain its shape. This consistency requires thorough ramming to eliminate voids and is used when forms are to be removed immediately, but should not be used where a water-tight job is expected. The porous structure of the concrete in Figure 2 is due to the fact that it was placed as a dry mixture. ESTIMATING. ESTIMATING CONCRETE. In estimating the amount of concrete in a given piece of work and the quantities of materials required, the unit of measurement is usually the cubic yard (27 cubic feet). The following examples will explain best the method of determining the quantities required: =Example 1.=--A wall 9 inches thick, 12 feet high, and 30 feet long has a door opening 3 feet wide and 6 feet high, also a footing 18 inches wide and 9 inches deep. The concrete is to be mixed in the proportions of 1:2:4. The volume of the footing is found by multiplying together the dimensions expressed in feet, thus, 1-1/2 × 3/4 × 30 = 33-3/4 cubic feet. Similarly, the volume in the wall is 3/4 × 12 × 30, less the door opening 3/4 × 3 × 6 = 256-1/2 cubic feet. The total volume in footing and wall is 290-1/4 cubic feet = 10-3/4 cubic yards. To find the quantity of cement, sand, and gravel, multiply the amounts for 1 cubic yard, indicated in line 7 of Table 1, by 10-3/4, and it will be found that 65 sacks of cement, 4.83 cubic yards of sand, and 9.56 cubic yards of gravel are necessary to build the wall. =Example 2.=--A pavement 27 feet long, 4 feet wide, and 6 inches thick has a 5-inch base mixed in the proportions of 1:3:5 and a 1-inch surface mixed in the proportions of 1:2. The volume in the base is 27 × 4 × 5/12 = 45 cubic feet = 1-2/3 cubic yards. 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 1-2/3 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. The volume in the floor is 10/2 × 10/2 × 22/7 × 1/2 = 39-2/7 Cubic feet. The area of the larger circle is 5 × 5 × 22/7 = 78-4/7 cubic feet. The area of the smaller circle is 4-1/2 × 4-1/2 × 22/7 = 63-4/7 cubic feet. The area of the wall, therefore, is 15 cubic feet and the volume is 15 × 4 = 60 cubic feet.