This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
133. The two methods used in mixing concrete are by hand and by machinery. Good concrete may be made by either method Concrete mixed by either method should be carefully watched by a good foreman. If a large quantity of concrete is required, it is cheaper to mix it by machinery. On small jobs where the cost of erecting the plant, together with the interest and depreciation, divided by the number of cubic yards to be made, constitute a large item, or if frequent moving is required, it is very often cheaper to mix the concrete by hand. The relative cost of the two methods usually depends upon circumstances, and must be worked out in each individual case.