Rocks of more than ordinary size (about 6-inch-diameter) are to be exposed in a panel. What is the best method to use?
The best method is to precast the concrete using the sand embedment method to expose the aggregate. A layer of fine sand is put in the form to a depth of one-third the diameter of the aggregate to be used. Aggregate is then placed in the sand and pressed to the bottom of the form, keeping the aggregate pieces as close together as possible. The sand and aggregate are then sprayed with enough water to moisten them and settle the sand around the aggregate sufficiently to hold it securely. Steel is set on chairs supported on the aggregate, and the concrete is placed carefully so that the aggregate and chairs are not disturbed. Usually the concrete is placed in two layers, with the second layer being placed immediately after the first. Usually the surface is vibrated with a vibrating screed, though an immersion vibrator may be used. The panels are removed from the mold after about five days and sand is cleaned off the panel with compressed air, waterblasting or brushes.