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.
The dry mortar method consists of a dry, rich mixture, with finely crushed stone. The concrete is usually composed of 1 part cement, 3 parts sand, and 3 parts crushed stone known at the 1/4-inch size, and mixed dry so that no mortar will flush to the surface when well rammed in the forms. When placed, the concrete is not spaded next to the forms; and being dry, there is no smooth mortar surface, but there should be an even-grained, rough surface. With the dry mixture, the imprint of joints of the forms is hardly noticed, and the grain of the wood is not seen at all. This style of finish has been extensively used in the South Park system of Chicago, and there has been no efflorescence apparent on the surface, which is explained by "the dryness of the mix and the porosity of the surface."