Popular Science Monthly/Volume 2/March 1873/Asphalt Pavements
WITHIN the past few years several streets in the city of New York have been overlaid with a compound called by its inventors "asphalt," but better characterized by the public as "poultice pavement." When first laid down, this material appeared to fulfil all the requirements of a serviceable pavement, being smooth, hard, and apparently durable. Actual experience, however, showed the material to be so friable that in a short time the ordinary travel on the streets reduced it to a fine, black powder, which, in dry weather, was thrown up in murky clouds by every passing vehicle and every gust of wind, finding entry into the adjacent houses through the minutest crevices, and in wet weather was kneaded into a nasty, viscous mud. Soon the streets thus paved were impassable, and the commissioners were forced to order the removal of what was left of the "poultice" pavement.
This experience was not well calculated to conciliate public opinion in favor of asphalt as a material for paving; and, though travellers returning home from Paris were loud in praise of the asphalted roadways of that capital, contrasting the roar of our granite-paved, filthy thoroughfares, with the smooth, noiseless, cleanly streets of the French capital, they were listened to with incredulity. Their "odious comparisons," instead of causing us to envy the Parisians for their happy solution of the great question of pavements, had only the effect of exciting compassion for the poor outside barbarians who put their trust in asphalt. For, had we not tried asphalt here, and found it wanting?
Yet it was not the asphalt pavement at all which proved a failure in these experiments. The material used was a spurious compound; a mixture of sand and gas-house refuse, which had this only in common with asphalt, that the two substances went by the same name. Here the adage, "Give a dog a bad name," was reversed. The genuine asphalt pavement was thus involved in the ill-fortune of its base counterfeit; only for a little while, it is to be hoped. So soon as we discover within convenient distance from our centres of population native deposits of asphalt, we shall avail ourselves of the improvements introduced into the art of road-making in France, and the "coming man" can go about his affairs without having his ears stunned by the clatter and roar of vehicles; the horse of the future perform his service without constantly risking life and limb; and the carriage of the future roll along without being jolted to pieces. In France the asphalt pavement is as much a success as the railway, and, as we are still seekers, still experimenters in this matter, it is perhaps well that we learn the processes followed by French engineers.
Dr. L. Meyn, of Halle, has published a pamphlet on "The Asphalts," which sums up all the information that is accessible regarding this material. In the present paper we propose to give, mostly in his own words, the history of asphalt as a material for paving foot-paths and roadways.
Natural asphalt, or asphalt-stone, is a porous, calcareous rock, saturated with bitumen, or natural tar, and capable of being worked into a tough, hard mastic. It is not unlike mortar in general appearance, and its color is usually chocolate, giving a fracture of lighter color. Its grain is fine, and each molecule of limestone is coated with the bitumen. The proportion of the latter in the mass varies from 7 to 15 per cent. At a temperature of between 338° and 356° Fahr., asphalt-stone falls to powder, and may then be dissolved in hot melted bitumen.
This substance does not occur very abundantly in Nature, the only deposits of any importance, so far as yet known, being found in the Val de Travers and at Seyssel, in Switzerland; Seefeld, in the Tyrol; Lobsan, in Alsace; Hölle, in Holstein; and Limmer, in Hanover. The deposit at Hölle is by far the most extensive in Europe; but, though here the particles of the limestone are thoroughly permeated by the bitumen, the material is not considered by the Paris Asphalt Company to be suitable for pavements, because it contains an excess of pure petroleum. Yet Dr. Meyn thinks that by exposure to air the greater part of this surplus may be dissipated, and the remainder oxidized.
Ancient authors state that Babylon was partly built with asphalt, and that an asphalt cement was used for the walls of Nineveh. A Greek physician, Eirinus, endeavored to bring this substance into use as a building material in 1712. He was the first to discover a method of reducing the asphalt of the Val de Travers to the liquid state, and obtained the monopoly from the King of Prussia of all the asphalt beds he might discover in the principality of Neufchâtel. He published in 1721 a "Dissertation on Asphalt," in which he gives as follows the process of making asphalt cement: "The preparation of this cement is very easy. The stone must be slightly warmed till it can be coarsely powdered. A small quantity of pitch is added, to make it thinner and more soluble, and then the whole is melted over a slow charcoal-fire." This cement was to be used instead of mortar, and also to protect wood and stone-work against decay. There is still to be seen at Couvet, a little village in the Val de Travers, a flight of stone steps, dating from the time of Eirinus; the lower steps are coated with asphalt, and are almost entirely unimpaired, while the upper ones, which were not so protected, are worn into holes. But the material did not continue long in use for building purposes, for in 1802, after the discovery of asphalt-stone at Seyssel, near Geneva, we find the preparation of a mastic, from bituminous limestone and tar, heralded as a new invention.
In 1832, the Seyssel quarries fell into new hands, and from that period we date the progress made in the matter of asphalt pavements. The new proprietor of the quarries, Count Sassenay, devoted himself exclusively to producing a continuous and homogeneous material, and carefully instructed his workmen in the best manner of laying this pavement. The celebrated foot-path of the Pont Royal, the fine pavement of the Place de la Concorde, Paris, as well as many pavements at Lyons, belong to that period. Sassenay's process is still employed in preparing asphalt for paving foot-paths, and the mode of laying down the pavement is as follows: The foundation must be even, for any inequality of its surface will cause the pavement to wear out more rapidly in some parts than in others. The mastic is broken into pieces, and melted in caldrons with bitumen at a temperature between 150° and 170° Cent. Then from 60 to 70 per cent, of sand is added, and the mixture is ready for application to the surface to be covered.
But still a good pavement for roadways was a desideratum. First the experiment was tried of laying down a bed of broken freestone, and pouring over this melted asphalt mastic. This method, however, proved unsuccessful, for, when cold, the mastic was too brittle, and if one of the corners of the stones was struck and broken by carriage-wheels, a hole was made, which gradually widened, and was difficult to mend. In Lyons, a layer of asphalt mastic, two inches in thickness, was spread over four inches of concrete. This was found to be an excellent pavement, but a fatal objection to it was its costliness.
It was at last observed that the roads and paths leading to the quarries of the Val de Travers were always in good condition, firm, solid, and elastic. Here was an asphalt pavement, formed of the small pieces of the mineral which fell from the carts, and which were pressed down and flattened by the wheels. The first highway asphalted on this principle was that between Bordeaux and Rouen. The road was first macadamized, and then covered to the depth of an inch and a half with asphalt, broken up in small pieces. As the plan appeared to be a complete success at first, several other roads were asphalted in the same way. Soon, however, the crushed granite of the macadam began to cut its way through the asphalt, and broke its continuity, thus allowing it to be permeated with dirt and water. Finally the problem was solved by a Swiss engineer, A. Merian, of Basle, who proposed to lay down powdered asphalt in a warmed state on the street, and, by applying strong pressure, to form at once an impermeable, elastic surface. The French engineers readily adopted the suggestion, and the first trial of the new method was made in the Rue Bergère. The engineers cried "Eureka!" and well they might, for experience in Paris shows that—1. The asphalt costs, in the first instance, one-third less than stone pavement; and 2. That the annual cost of maintenance is three-quarters less than that of a macadamized road.
The process of preparing the asphalt pavement is thus described by M. Léon Malo: The asphalt-stone is brought direct from the quarries, and broken up into small pieces, about the size of those used for macadamized roads; it is then heated over a stove in a drum-shaped iron vessel with feet, till it crumbles into powder. In order that the powder may not lose its heat, the whole apparatus is conveyed on to the street where it is to be applied. Then a foundation of béton is laid, about four inches deep, which may, however, be thicker or thinner, according to the nature of the soil. On a macadamized road the concrete may be omitted; but on loose soil it should be laid as thick as six inches. The arch of the roadway should only be just sufficient to drain off the rain-water. The powdered asphalt is then spread over the surface, to a depth of 16 to 20 inches (according to the amount of traffic), and stamped down. Then a heavy roller is drawn over it.
With regard to the danger of horses slipping and falling on the smooth surface of the asphalt pavement, the following facts are of interest:
At Lyons, which has long had mastic roads, a number of cavalry-horses fell on a street of compressed asphalt. This accident arose from the circumstance that the asphalt had been laid on an old macadamized road, and had therefore that considerable arch which is unnecessary and dangerous for the asphalt road. The cavalry riding in a long line, those horses near the side of the road slipped on the steep incline.
At Marseilles, where the asphalt roads from the harbor to the town were made with a very gentle curve, there has been no increase in the number of accidents, though the traffic is enormous. A very slight arch of the road is quite sufficient to allow the rain to run off from such a smooth surface.
It is also not advisable to lay the asphalt on any street with a gradient greater than 1 in 60, though in London some streets having a gradient of 1 in 57, and even to 1 in 46, have been covered with asphalt without any apparent danger. It is of course extremely difficult to get any accurate information about the number of accidents on the streets, general vague impressions being worthless; but in Paris the number of horses which were observed to fall in the Rue Neuve des Capucines, during two months, was as follows: In the former, which was paved with sandstone, in blocks, from Fontainebleau, one horse in 1,308 fell; on the latter, which was covered with asphalt, one in 1,409, so that the balance was in favor of the asphalt.
In snow or frost asphalt is not so slippery as granite, being in itself warmer, and also more easily warmed by the slightest rays of the sun; hence, the ice is more slow in forming, and quicker in melting, than on granite.
It has been proved that the greatest number of accidents to horses happen when the asphalt is not cleaned, for the surface is never muddy or greasy, except with foreign matter, and this ought to be constantly washed off with water, which is plentifully laid on in Paris, and to some degree in London. At any points where this cannot be done, a slight sprinkling of coarse sand will prevent the horses slipping. This is only a temporary remedy, but valuable in case of emergency. It is one of the great advantages of asphalt, however, that it is so easily and cheaply cleaned.
In case of a conflagration, the asphalt pavement will not help to spread it. In London heaps of wood were set fire to on asphalt pavements, but, when the embers were raked away, there were only a few weak flames seen issuing from the asphalt, and they went out of their own accord in a few moments.
In conclusion, we give the results of experience with these pavements in London, and these are: 1. The first cost of the asphalt road is the same as that of a granite pavement. 2. The annual cost of maintenance is a trifle less. 3. Where a granite pavement is worn out in from seven to ten years, an asphalt pavement is still in perfect condition. 4. Asphalt, when taken up, may be used again and again.