The Highway Research Board of the National Academy of Sciences - National Research Council has the responsibility for administration and direction of the AASHO Road Test near Ottawa, Ill. This 25 million dollar cooperative highway research project involved the construction, starting in 1956, of about 15 miles of dual lane highway. To make the components of the pavement structures uniform and in accordance with rigid specifications, a large construction materials testing program was carried out by the Road Test Materials Branch. This program led to an examination of modern techniques for materials testing, among which was the nuclear system for in-place density determination utilizing the so-called flatiron surface gage principles. Since commercial equipment available at the time did not appear to have reached a satisfactory state of development, the Materials Branch undertook a development program founded primarily on work by Paul F. Carlton, B. W. Pocock, and engineers and physicists of the Nuclear Chicago Corp. This early Road Test work was reported in 1958 in Highway Research Board Special Report 38.