TAR MACADAM. Broken stone with a tar binder has been used for road purposes in a comparatively small way in England for twenty or thirty years past; and the experience of Hamilton, Ontario, Canada, with this form of pavement has lately attracted considerable attention in this country. In a general way, two methods have been employed in using tar as a binder for broken stone, viz.: (1) the broken stone is mixed with sufficient tar more or less nearly to fill the voids, and then the mixture is deposited and compacted, the process being very much the same as that em ployed in laying hydraulic cement concrete; or (2) the broken stone is laid and rolled, and then a layer of tar is added and rolled, the intention being to force the tar into the interstices of the broken stone much as the stone-dust binder is worked into a broken-stone road. The product in the first case could appropriately be called tar concrete, and in the second tar macadam; and they will be so designated in this discussion. The former seems to be the more common in England, and the latter in Canada. Notice that these two methods are substantially the same as Warren's and Whinery's method for making asphalt macadam—see § 696 and § 697, re spectively.
In making tar concrete, care must be taken thoroughly to mix the tar and the stone, the former being hot and the latter dry. The mixing is done with shovels on a board platform, the tar being poured over the stone. Each fragment of stone should be thoroughly covered with tar; but more tar than enough to fill the voids is objectionable, since it increases the cost and decreases the durability of the road. Usually 10 or 12 gallons are required for a cubic yard of unscreened stone. The mixture is then placed in the road, and rolled while hot with the usual road roller, sand or dust being sprinkled over the surface to prevent the tar from sticking to the roller. Only a comparatively small amount of rolling is required to consolidate the mass. Not infrequently a wearing coat, consisting of a half inch to 1 inch of tar and screenings, is added on the top of the tar concrete; and herein the two methods referred to above merge one into the other.
In laying tar macadam, the broken stone is rolled until the fragments do not move under the foot in walking over the surface, and then a layer of hot tar is poured upon the surface and is evenly spread over it with brooms or shovels, after which it is rolled. If honeycombed spots appear while the rolling is in progress, more tar is added. After the surface of the layer of broken stone has been thoroughly filled with tar, the surface is flushed with moder ately soft tar, and over this is strewn a thin layer of stone chips about k to inch in longest dimension; and then the surface is again rolled, after which the road is thrown open to traffic.
It is hardly probable that tar macadam will come into anything like general use, either for country roads or for city streets. The cost of tar macadam is nearly or quite as much as that of the ordinary broken-stone road (compare § 712 with § 359-62); and the con struction of the former is neither so simple nor so certain as the latter. Tar macadam is of doubtful durability, since coal tar is subject to oxidation by the atmosphere, which renders it brittle and devoid of cementing power. Tar is affected by changes of temperature, and becomes friable in cold weather and soft in warm. There is less chance of success with tar macadam in the future than in the past, since in the present stage of manufacture, water gas is being substituted largely for coal gas, the quantity of gas tar produced is diminishing, its price is increasing, and its quality is deteriorating.
It is probable that the most favorable field for tar macadam is as a substitute for ordinary crushed stone on grades so steep as to wash seriously, and as a substitute for cobble gutters on crushed- . stone streets.