Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/611

ROADS 587 rock, and from 10 to 12 in that from Val de Travers. Asphalts con- taining less than the former proportion have not sufficient coher- ence for street pavements, and those containing more than the latter proportion soften from heat in the summer. Asphalt is employed either as a mastic or compressed. The mastic is pre- viously prepared in cakes and is melted for use in caldrons with a small quantity of bitumen, and for a street pavement is thoroughly mixed with sand or grit. It is spread in one thickness on a concrete foundation, covered with sand, and beaten to an even surface. This material has not proved so successful for street surfaces as com- pressed asphalt. To produce this the rock asphalt, previously reduced to a fine powder by mechanical means, is heated in revolv- ing ovens to from '220" to 250, spread while still hot, and compressed into a solid mass by hot disk -shaped rammers, and afterwards smoothed with irons heated to a dull redness. The original rock is thus as it were reconstructed by taking advantage of the power of coherence of the molecules under pressure when hot. In heating the powder the moisture combined in the limestone must be driven o!f without reducing the proportion of the bitumen more than is unavoidable. The powder cools very slowly and may be conveyed long distances from the ovens ; it may even be kept till the next d:iy before use. When laid it should still retain a temperature of from 150' to 200. It is spread evenly with a rake by skilled workmen for the whole width of the street to a thickness ajjout two -fifths greater than the finished coating is intended to be. Ramming is commenced with light blows to ensure equality of compression throughout and is continued with increased force until the whole is solidified. The ramming follows up the spreading, so that a joint i.s required only when the work is interrupted at the end of a day, or from some other cause. In a few hours after it has beun laid an asphalt pavement may be used for traffic. When finished, its thickness may be from 1 to 1 inches, according to the traffic ; a greater thickness than the latter cannot be evenly compressed with certainty. The asphalt loses thickness by com- pression under the traffic for a long time and to the extent, it is said, of one-fifth or one-fourth, but the wear appears to be very small. A pavement in Paris which had lost more than one-fourth of its thickness was found to have lost only 5 per cent, of its weight after sixteen years' wear. The pavement in Cheapside, after four- teen years under exceptionally heavy traffic, has been reduced, where not repaired, from its original thickness of 2J to about 1| inches. The wear-resisting power of the asphalt is due to its elasticity ; tracks are made by the wheels at first, but when thoroughly compressed by the traffic the surface retains little or no trace of the heaviest loads. Repairs are easily and quickly made by cut- ting out defective places and ramming in fresh heated powder, which can be done in the early morning without stopping the traffic. An unyielding foundation is indispensable ; it should be of the best Portland cement concrete, 6 inches in thickness, which must be well set and perfectly dry throughout before the asphalt is laid, or the steam generated on the application of the hot powder will prevent coherence and lead to cracks and holes in the asphalt, which quickly enlarge under the traffic. For the same reason the asphalt should be laid in dry weather. The concrete foundation must ba carefully formed to the proper profile, with an inclination towards the sides of not more than 1 in 50, which is sufficient with so smooth a surface. About 1 in 50 is the steepest gradient at which an asphalt pavement can be safely laid. When either dry or wet it affords good foothold for horses, but when beginning to get wet, or drying, it is often extremely slippery. This is said to be due to dirt on the surface, and not to the nature of the material. Sand is strewed over the surface to remedy the slip- periness ; it tends, however, to wear out the asphalt, and great cleanliness is the best preventive. An asphalt pavement can be kept cleaner than any other, is impervious to moisture, and dries quickly. It is noiseless, except from the clatter of horses' feet on it ; it is the pleasantest pavement to travel upon, but it has the drawback of imperfect foothold and slipperiness at times. The cost of a compressed asphalt pavement 2 to 2 inches thick on a Portland cement concrete foundation 6 inches thick is from 13s. to 16s. a square yard, and the maintenance is usually undertaken for a period of seventeen years by the company laying the pave- ment, the first two years free and at 3d. to Is. 6d. per square yard, according to the traffic, in succeeding years. ,'om- Comparison of Street Surfaces. The comparative cost of various wrative street surfaces in Liverpool, including interest on first cost, sinking ost. fund, maintenance, and scavenging, when reduced to a uniform standard traffic of 100,000 tons per annum for each yard in width of the carriage-way, is given by Mr Deacon as follows : Per square yard per year. Set pavement of hard granites Hid. ,, ,, softer granites Is. 2fd. Bituminous concrete Is. lOJd. Wood pavement 2s. 2Jd. Macadam, on hand-pitched foundation 2s. HJd. Taking the standard of traffic at 40,000 tons per annum for each yard in width, the cost for the last three pavements is : Per square yard per year. Bituminous concrete Is. IJd. Wcx >d pavement Is. 8id. Macadam Is. 11 Jd. Asphalt paving may be placed between wood and bituminous con- crete in the above order. These comparisons show the high cost of a macadamized surface in a street where the traffic is great. However well it may be maintained, a macadamized street must be dirtier and dustier than any pavement, though it is superior to them all in safety and to set pavements in the matter of noise. Bituminous concrete or asphalt macadam is cheaper, cleaner, and quieter than ordinary macadam and is sufficiently durable when the traffic is not heavy. For heavy traffic no pavement is so econo- mical as granite sets ; but for the sake of quiet and cleanliness a wood or asphalt pavement is often preferable. Asphalt can be kept cleaner than any other pavement and is the pleasantest to travel over ; wood, on the other hand, is quieter for the residents, less slippery, and can be laid on steeper gradients. The comparative ease of draught on various surfaces is largely Draught, influenced by the amount of foothold afforded, and it may be doubted if dynamometer experiments, however carefully made, are altogether conclusive. The tractive force is influenced by the gradient, the diameter of the wheels, the friction of the wheel axles, and the speed, as well as by the resistance of the road surface, and these must be all taken into account to obtain accurate results. Some recent experiments made, under the direction of Sir J. W. Bazalgette, with Easton and Anderson's horse dynamometer on London street surfaces gave the following mean results : Tractive force on the level. Macadamized surface 40-7 to 44-29 lb per ton. Asphalt 39-0 39'32 Wood 33-62 36-63 Granite sets 26-2 27'0 ,, The gross load was 4 tons, drawn at a speed of from 2 to 6 miles an hour. It is remarkable that the tractive force on asphalt is so high ; but the other results are consistent with former experiments by Morin, Macneill and others. The comparative safety of granite, wood, and asphalt pavements Safety, in the City of London was the subject of careful observations, which were fully reported on by Mr W. Hay wood in 1873. The pavements selected were granite sets 3 inches wide, ligno-mineral pavement of beech blocks 3 inches wide, improved wood pavement of fir blocks 3 inches wide, and Val de Travers compressed asphalt pavement. On known lengths of these the traffic, the accidents to horses, the weather, and other circumstances were observed for fifty days, and when the distance traversed was taken into account it was found that as a mean result a horse might be expected to travel 1 32 miles on granite without falling, 191 on asphalt, and 446 on the im- proved wood pavement. The condition of the weather had consider- able effect : on the granite when dry a horse might be expected to travel 78 miles without falling, when damp 168, and when wet 537 ; on wood when damp 193 miles, when wet 432, and when dry 646 ; on asphalt when damp 125 miles, when wet 192, and when dry 223. It thus appeared that wood pavement was less slippery than either granite or asphalt in a marked degree, it being only more slippery than granite when both pavements were wet. About 85 per cent, of the falls on the wood pavement were falls on the knees, which are less likely to injure the horses and are less incon- venient to the traffic than other falls. On the granite the falls were falls on the knees or complete falls in about equal proportions, with about 7 per cent, of falls on the haunches. On the asphalt 43 per cent, were complete falls and 24 per cent, falls on the haunches. Watering. On macadamized roads in Great Britain watering is Water- only good for the road itself when the materials are of a very sili- ing. cious nature and in diy weather. With other materials the effect is to soften the road and increase wear. In and near towns water- ing is required for the comfort of the inhabitants, but it should not be more than enough to lay the dust without softening the road, and the amount required for this may be greatly reduced by keeping the surface free from mud, and by sweeping off the dust when slightly wetted. Pavements are watered to cleanse them as well as to lay the dust, but it must be remembered that both wood and asphalt are more slippery when wet, and that therefore watering should be obviated as far as possible by thorough cleans- ing. Hydrostatic vans, by improvements in the distributing pipes and regulating valves, water a wide track uniformly with an amount of water which can be regulated at pleasure. Where hydrants exist in connexion with a water supply at high pressure, street watering can be effected by a movable hose and jet, a method much more effective in cleansing the surface, but using a much larger quantity of water. Another method which has been tried, but not much used, is to lay perforated pipes at the back of the kerb on each side of the road, from which jets are thrown upon the surface. The first cost is considerable, and the openings for the jets are liable to choke and get out of order. Deliquescent salts have been used for street watering, by which the surface is kept moist, but at the expense of the moisture in the air. Sea water has the same effect in a less degree.