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The functions of aggregate in bitumen-aggregate systems and typical gradings for the principal types of bituminous construction for road base and surface courses, and for irrigation and flood control, are presented. There are three types of aggregate structures. A keyed type of structure is used in penetration macadam and surface treatments. Gap-gradings are used in certain open-graded construction and in stone-filled sheet asphalt. Uniformly graded aggregates are generally used in bituminous concrete and mat construction. Aggregates are classified either as coarse or fine. The definitions of these classifications vary with types of aggregate and construction and with the specifying agency. Mineral fillers are necessary in some construction to obtain density and impermeability, physical stability, and to extend the life of the bitumen. Standard gradings for coarse aggregate are given in A.S.T.M. Specifications D 448 - 42 T. Two methods are used for specifying aggregate gradings. The separate percentage method is commonly used for sheet asphalt and to some extent for bituminous concrete. The cumulative percentage method is used for all other types of construction. Typical gradings, obtained from the standard specifications of state highway departments, Federal agencies, and other bodies, are given for a number of road base and surface courses, linings and facings for canals, flood channels, and revetments.
Benson, Jewell R.
Bituminous Engineer, U. S. Bureau of Reclamation, Denver, Colo.