Hoover Dam is concrete; and without concrete, this dam could not have been built. 3,250,000 cu.yd. were placed in the dam itself, and another million plus yards in the appurtenant works. Concrete consists of four ingredients-sand and crushed rock aggregate, water and Portland cement. These must be mixed in the proper proportions to yield strong concrete. Aggregate is perhaps the most important of the materials in the concrete because it makes up as much as three quarters of the dam's mass. The aggregate must be clean and free of clays, salts and organic matter. A source of aggregate near the dam was needed so that it would not have to be transported too
