I am responding to your
April 1979 special issue on concrete finishing. The author of the articles often referred to applying a dry shake when the concrete has started to set but before the water sheen has disappeared. I always thought that the dry shake was a bad concrete practice and highly disapproved of in most areas. I am a little confused.
You are right that applying a dry shake is a bad practice if the dry shake is portland cement alone and if it is being used for the purpose of drying up overly wet concrete so that it can be finished sooner. This practice puts too much cement into the surface and leads to such troubles as crazing, scaling and dusting. A warning against this kind of use is given on page 258 of the April 1979 special issue. The practice and materials that are advocated in the special issue are slightly different. The dry shakes to be used consist of a mixture of cement with one or more other materials such as color pigment, powdered metal aggregate, hard mineral fine aggregate such as traprock or silica, or a slip-resistant fine aggregate such as emery or aluminum oxide. Dry shakes containing pigment are used to put color in the surface; such mixtures usually contain fine silica aggregate as well, to provide hardness and prevent crazing. Dry shakes with metallic or hard mineral aggregates are used to produce unusually wear resistant surfaces. Dry shakes with slip-resistant aggregates are used to avoid hazardous slippery surfaces. The proper procedures for producing wear-resistant or nonslip surfaces with dry shakes are given on page 233, and for producing colored surfaces with dry shakes on page 239.