AMOUNT OF WEAR BROKEN-STONE The amount of wear will vary greatly with the climate, the amount and character of the traffic, the nature of the stone, and the method of construction.
It has been estimated * that in Great Britain 20 per cent of the wear was due to atmospheric causes and 80 per cent to the traffic and that with fast stage coaches three fourths of the wear by traffic was due to horses' feet and one fourth to the wheels, while with or dinary vehicular traffic about six tenths was due to the horses' feet and four tenths to the wheels. The relative wear from horses' feet and wagon wheels was deduced from a comparison of the wear of iron in the horses' shoes and in the tires of the wheels; but is not stated upon what the estimates of the wear from pheric causes were based. Any such general estimate must not be considered as even approximately true for any particular case, since the relative wear varies greatly with the exposure to sun and wind, the drainage, the strength of the road, the character of the maintenance, etc.
In France, where the relation between the wear and the traffic has been carefully studied, some engineers maintain that the wear increases in the same proportion as the traffic, while others contend that the wear increases in a greater ratio than the traffic. Table 25, page 246, is frequently cited to establish the latter view. These data were obtained in the following manner: Owing to the falling in of a tunnel, the traffic on a particular road was suddenly in creased from its usual amount of 1,378 tons per day to 2,264 tons, 3,150 tons, and 5,315 tons on different sections, at which rates it continued for 74 days, after which it fell on all portions of the road to 1,772 tons per day. The consumption of material was determined by measuring the amount of detritus removed from the road and also by comparing the amount of material less than 2 centimeters in diameter in the body of the road at the beginning and the end of the period. According to Table 25, increasing the
traffic four-fold increased the wear thirteen-fold; but the case was an extreme one, since the increased traffic was unusually heavy, and since the road material, schist, is unusually friable and defi cient in cementing power. Under the 1,378 tons per day, the wear was at the rate of 11 inches per annum; and under the 5,315 tons per day, it was at the rate of more than 2 feet a year. Appar ently a majority of road engineers hold that an increase of traffic increases the wear per ton, but in a much less ratio than in Table 25. It is certainly true that heavy loads cause more wear than light ones, even when the total weight transported is the same.
In France careful observations are made by the govern ment engineers to determine the amount of wear in terms of the traffic. The traffic is expressed in "units" representing a horse harnes, l to a loaded wagon; and to reduce the other traffic to this unit, the following values are used: In 1876 the average amount of material required for the Routes Nationales was 53 cubic yards per mile per 100 "units" of daily traffic, the range for the different departments being 15 to 265, but usually from 27 to 102.t In 1893 the average was 49 cubic yards per mile per 100 "units" of travel, the stone having an average c()-efficient of wear (§ 282) of 10.854 374. In Austria the traffic has not decreased, but the material employed in repairs has continually decreased since 1856. From 1865 to 1872 the average consumption of material was 84.9 cubic yards per mile per annum per 100 vehicles.
No observations seem to have been made to determine the relation between wear and traffic for English or American roads.
Professor Shaler says: "If the wear on macadam is more than inch per year the presumption is that true economy demands a more enduring form of pavement." *