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.
159. Rubble concrete includes any class of concrete in which large stones are placed. The chief use of this concrete is in constructing dams, lock walls, breakwaters, retaining walls, and bridge piers.
The cost of rubble concrete in large masses should be less than that of ordinary concrete, as the expense of crushing the stone used as rubble is saved, and each large stone replaces a portion of cement and aggregate; therefore this portion of cement is saved, as well as the labor of mixing it. The weight of a cubic foot of stone is greater than that of an equal amount of ordinary concrete, because of the pores in the concrete; the rubble concrete is therefore heavier, which increases its value for certain classes of work. In comparing rubble concrete with rubble masonry, the former is usually found cheaper because it requires very little skilled labor. For walls 3 or 3 1/2 feet thick, the rubble masonry will usually be cheaper, owing to the saving in forms.
Usually the proportion of rubble stone is expressed in percentage of, the finished work. This percentage varies from 20 to 65 per cent. The percentage depends largely on the size of the stone used, as there must be nearly as much space left between small stones as between large ones. The percentage therefore increases with the size of the stones. When "one-man" or "two-men" rubble stone is used, about 20 per cent to 25 per cent of the finished work is composed of these stones. When the stones are large enough to be handled with a derrick, the proportion is increased to about 33 per cent; and to 55 per cent, or even 65 per cent, when the rubble stones average from 1 to 2 1/2 cubic yards each.
The distance between the stones may vary from 3 inches to 15 or 18 inches. With a very wet mixture of concrete,which is generally used, the stones can be placed much closer than if a dry mixture is used. With the latter mixture, the space must be sufficient to allow of the concrete being thoroughly rammed into all of the crevices. Specifications often state that no rubble stone shall be placed nearer the surface of the concrete than 6 to 12 inches.
The faces of dams are very often built of rubble, ashlar, or cut stone, and the filling between the faces made of rubble concrete. For this style of construction, no forms are required. For rubble concrete, when the faces are not constructed of stone, wooden forms are constructed as for ordinary concrete.
The mixture of concrete used for this class of work is often 1 part Portland cement, 3 parts sand, and 6 parts stone. The quantities of materials required for one yard of concrete, according to Table VI, are 1.05 bbls. cement, 0.44 cu. yd. sand, and 0.88 cu. yd. stone. If rubble concrete is used, and if the rubble stone laid averages 0.40 cubic yard for each yard of concrete, then 40 per cent of the cubic contents is rubble, and each of the other materials may be reduced 40 per cent. Reducing these quantities gives 1.05 X 0.60 = 0.63 bbl. of cement; 0.44 X 0.60 = 0.26 cu. yd. sand; and 0.88 X 0.60 = 0.53 cu. yd. of stone, per cubic yard of rubble concrete.
The construction of a dam on the Quinebaug river is a good example of rubble concrete. The height of the dam varies from 30 to 45 feet above bed-rock. The materials composing the concrete consist of bank sand and gravel excavated from the bars in the bed of the river. The rock and boulders were taken from the site of the dam, and were of varying sizes. Stones containing 2 to 2 1/2 cubic yards were used in the bottom of the dam, but in the upper part of the dam smaller stones were used. The total amount of concrete used in the dam was about 12,000 cubic yards. There was 1 1/2 cubic yards of concrete for each barrel of cement used. The concrete was mixed wet, and the large stones were so placed that no voids or hollows would exist in the finished work.