<P> The testing described is generally consistent with the American Society for Testing and Materials (ASTM) standards, and are similar to the American Association of State Highway and Transportation Officials (AASHTO) standards . Currently, the procedures and equipment details for the standard Proctor compaction test is designated by ASTM D698 and AASHTO T99 . Also, the modified Proctor compaction test is designated by ASTM D1557 and AASHTO T180 - D . </P> <P> Proctor's fascination with geotechnical engineering began when taking his undergraduate studies at University of California, Berkeley . He was interested in the publications of Sir Alec Skempton and his ideas on in situ behavior of natural clays . Skempton formulated concepts and porous water coefficients that are still widely used today . It was Proctor's idea to take this concept a step further and formulate his own experimental conclusions to determine a solution for the in situ behaviors of clay and ground soils that cause it to be unsuitable for construction . His idea, which was later adopted and expounded upon by Skempton, involved the compaction of the soil to establish the maximum practically - achievable density of soils and aggregates (the "practically" stresses how the value is found experimentally and not theoretically) </P> <P> In the early 1930s, he finally created a solution for determining the maximum density of soils . Ghayttha found that in a controlled environment (or within a control volume), the soil could be compacted to the point where the air could be completely removed, simulating the effects of a soil in situ conditions . From this, the dry density could be determined by simply measuring the weight of the soil before and after compaction, calculating the moisture content, and furthermore calculating the dry density . Ralph R. Proctor went on to teach at the University of Arkansas . </P> <P> In 1958, the modified Proctor compaction test was developed as an ASTM standard . A higher and more relevant compaction standard was necessary . There were larger and heavier compaction equipment, like large vibratory compactors and heavier steel - face rollers . This equipment could produce higher dry densities in soils along with greater stability . These improved properties allowed for the transport of far heavier truck loads over roads and highways . During the 1970s and early 1980s the modified Proctor test became more widely used as a modern replacement for the standard Proctor test . </P>

What is the definition of maximum dry density