Concrete Pumping
There are different sizes of pumps with different capabilities, so it is important for the pumper to know exactly what kind and how much concrete needs to be pumped.
A concrete mix basically consists of sand and rock of various sizes glued together with a mixture of cement and water. Rock displace a significantly large portion of the volume of concrete.
For example, in a typical six sack mix, the fine and coarse rocks in that mix will make up approximately 67 percent of the total volume. The gradation and overall fineness of the rock used will affect the physical properties and the cost of the mix.
-- Tips courtesy of HomeAdvisor.com