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.
On large contracts the concrete is generally mixed by machinery. The economy is not only in the mixing itself but in the appliances introduced in handling the raw materials and the mixed concrete. If all materials are delivered to the mixer in wheel-barrows, and if the concrete is conveyed away in wheel-barrows, the cost of making concrete is high, even if machine mixers are used. If the materials are fed from bins by gravity into the mixer, and if the concrete is dumped from the mixer into cars and hauled away, the cost of making the concrete should be very low. On small jobs the cost of maintaining and operating the mixer will usually exceed the saving in hand labor and will render the expense with the machine greater than without it.
It has already been stated that good concrete may be produced by either machine or hand mixing, if it is thoroughly mixed.
Tests made by the U. S. Government engineers at Duluth, Minn., to determine the relative strength of concrete mixed by hand and mixed by machine (a cube mixer), showed that at 7 days, hand-mixed concrete possessed only 53 per cent of the strength of the machine-mixed concrete; at 28 days, 77 per cent; at 6 months, 84 per cent; and at one year, 88 per cent. Details of these tests are given in Table X.
(From "Concrete and Reinforced Concrete Construction," by H. A. Reid)
Age, and Method of Mixing
Age 7 Days
Age 28 Days
Age 6 Months
Age One Year
The mixture tested was composed of 1 part cement and 10.18 parts aggregate.