The cement grout filler is applied in the same general manner as the bituminous filler. The grout, consisting of equal parts of sand and cement, is mixed to a thin consistency and poured onto the surface and is then worked into the joints with squeegees. Two or more applications are usually required to effect a complete filling of the joints. The surface should be covered with sand and be kept moist until the cement grout has set. CHAPTER X BITUMINOUS ROAD MATERIALS AND THEIR USE Tars and asphaltic materials of various kinds are widely used for road construction and maintenance, especially for road surfaces subjected to motor traffic. Materials of this character that are employed in highway work possess varying degrees of adhesiveness, and while they may be semi-solid or viscous liquids at air temperature, they melt on the application of heat and can be made sufficiently fluid to mix with the mineral aggregates that may be used in the road surface. Upon cooling, the bituminous materials return to the previous state and impart a certain amount of plasticity to the mixture, at the same time serving as a binding or cementing agent, which is sufficiently stable for many classes of road construction. =Classes of Bituminous Materials.=--Bituminous materials may be classified, according to the source from which they are obtained, as coal tars, water gas tars, native or natural asphalts and oil or petroleum asphalts. =Coal Tar.=--Coal tar is obtained as a by-product in the manufacture of illuminating gas from coal. It is also obtained in the manufacture of coke from coal. The tar thus obtained is manufactured into products that are used for dust layers on gravel or macadam roads, binders for macadam and gravel surfaces, fillers for brick, wood block and stone block pavements and for expansion joints. These various materials differ mainly in their consistency at air temperature. (They may differ widely in chemical composition, but that need not be considered herein.) =Water Gas Tar.=--Water gas tar is obtained as a by-product in the manufacture of illuminating gas from crude petroleum. It is used for the same kinds of construction as coal tar, and the products utilized for the several purposes, like the coal tars, differ mainly in consistency. =Natural Asphalt.=--Natural asphalt is found in deposits at many places in the world, existing in beds or pools where it has exuded from the earth or as veins in cavities in the rocks. It is of varying composition and consistency, but those kinds in most general use are solid or very viscous liquids at air temperature. Of the deposits that have been developed on a commercial scale, the Trinidad lake in the British West Indies and Bermudez deposit in Venezuela are best known. Both of these materials are too hard in the natural state to be used for road construction, and are softened, or fluxed as it is called, with fluid petroleum oil before being used.