=Petroleum Asphalt.=--Petroleum asphalt is a residue remaining after the fluid products have been distilled from petroleum. Residues of this sort are not always suitable for road construction, but a number of brands of road material are obtained from this source. Oil asphalts are used for dust layers, for binders for macadam roads, for asphalt cements for sheet pavement surfaces, and for fillers for block pavements and expansion joints. =Mixtures.=--Water gas tars and asphalts are sometimes mixed to produce road materials, and likewise native asphalts and residues obtained from petroleum are sometimes mixed to produce asphalt cements for paving mixtures. =Classification according to Consistency.=--The various bituminous materials may be classified according to consistency in discussing the various uses to which they may be put. =Road Oils.=--Road oils are fluid petroleum oils of such consistency that they may be applied cold or by heating slightly. They are used as dust layers on earth, gravel and macadam surfaces. Their efficacy depends upon the binding properties of the small amount of asphaltic material that is contained in the oil. =Liquid Asphalts.=--These are somewhat less fluid than the road oils, and must always be heated before application, but are viscous liquids at ordinary temperature. These materials are obtained from crude petroleum or semi-solid native bitumens, in which case they are usually called malthas. Both coal tars and water gas tars of semi-solid consistency are also employed for the same class of construction as the liquid asphalts. These materials are used for carpeting mediums on macadam roads and as cementing agents in the construction of hot-mixed macadam. =Asphalt Cements.=--The solid asphaltic materials used for hot-mixed types of construction are called asphalt cements. They may be petroleum residues or native asphalts fluxed with petroleum oils. They are solids at ordinary temperature and must be heated to a temperature in excess of two hundred and fifty degrees before they are sufficiently fluid to use. Asphalt cements are used for sheet asphalt and asphaltic concrete construction and for hot-mixed bituminous macadam. =Fillers.=--Fillers are solid asphalts or tars that are used for filling expansion joints in rigid pavements and for filling the spaces between the blocks in brick, wood block and stone block pavements. =Bitumen.=--Bituminous materials are all soluble to a greater or lesser extent in carbon disulphide and the soluble portion is called bitumen. It is the bitumen that gives to the materials the cementing properties utilized in road construction. Mixtures of mineral aggregates and bituminous materials for various purposes are proportioned with bitumen as a basis. Therefore, less of an asphalt containing one hundred per cent bitumen will be used than of one containing less than one hundred per cent of bitumen. TABLE 8 PROPERTIES OF ASPHALTIC ROAD MATERIALS