CHAPTER V.

ASPHALTUM AND COAL-TAR PAVEMENTS.

In

217. Asphalt was first employed for street-paving in Paris in 1838, but it was not employed to any great extent until 1854. 1869 it was introduced into London, and since then has been extensively used throughout Europe.

The success which attended this pavement led to its introduction into America. The great cost of importing the materials from Europe made the pavement so expensive as to induce American inventors to seek to manufacture a material which should have similar qualities. The result was the introduction of many substitutes and imitations, the majority of which proved defective.

The great cost of the imported material and the failure of the substitutes directed attention to the deposits of natural bitumen on the island of Trinidad, which could be brought here very cheaply. Experiments were made which demonstrated the possibility of making a mastic with Trinidad bitumen as its cementing material, as strong, elastic, and durable as that imported from Europe; but it was only after some years that this process was introduced and made a commercial success.

218. The difference between the asphalt pavements of Europe and those of America is due to the character of the materials. The former are composed of limestone rock naturally impregnated with bitumen, while the latter are composed of an artificial mixture of bitumen, limestone, and sand. The limestone in the European pavements becomes hard, smooth, and slippery under traffic, and is thus objectionable for general use in frosty latitudes. The granular nature of the sand used in preparing the Trinidad asphaltum diminishes the tendency to wear smooth and materially lessens the slipping of horses.

219. Although many deposits of bituminous rock are found in the United States, they have been used only to a limited extent,

and the island of Trinidad continues to be the main source of supply for the United States. This is due entirely to its advantage in cost of transportation. The railroad freight rates from the place of the deposits practically shut out the bituminous rock of California and Kentucky from competition in the Eastern States, and a similar condition may be said to affect the sale of Trinidad asphaltum in the cities of Europe, since the bituminous limestones of Val de Travers and Seyssel, having the advantage in freights, control the markets.

220. The cost of preparing the different varieties of asphaltum for street pavement is nearly the same; and as all appear to be about equally durable, the exclusive use of any one of them is due merely to the advantage in freights.

WEARING SURFACE
CUSHION COAT

CONCRETE

CRAVEL

FIG. 14.-SECTION OF TRINIDAD ASPHALT PAVEMENT.

221. The Advantages of Asphalt may be summed up as follows:

(1) Ease of traction.

(2) It is comparatively noiseless under traffic.

(3) It is impervious.

(4) It is easily cleansed.

(5) It produces neither mud nor dust.

(6) It is pleasing to the eye.

(7) It suits all classes of traffic.

(8) There is neither vibration nor concussion in travelling over it. (9) It is expeditiously laid, thereby causing little inconvenience. to traffic.

(10) Openings to gain access to underground pipes are easily made.

(11) It is durable.

(12) It is easily rapaired.

222. Defects of Asphalt Pavement.

(1) It is slippery under certain conditions of the atmosphere. The American asphalts are much less so than the European on account of their granular texture, derived from the sand. The difference is very noticeable: the European are as smooth as glass, while the American resemble fine sand-paper.

(2) It will not stand constant moisture, and will disintegrate if excessively sprinkled.

(3) Under extreme heat it is liable to become so soft that it will roll or creep under traffic and present a wavy surface, and under extreme cold there is a danger that the surface will crack and become friable. (In Washington, D. C., with a range of temperature from 5 to 150 degrees Fahr., no serious trouble has been experienced with the Trinidad asphalts.)

(4) It is not adapted to grades steeper than 24 per cent.

(5) Repairs must be quickly made, for the material has little hardness, and if, from irregular settlement of the foundation or local violence, a break occurs, the passing wheels rapidly shear off the sides of the hole, and it soon assumes formidable dimensions. In London this is prevented by constant watchfulness. Workmen are employed to traverse the street with a light repairing outfit, and whenever a defect is observed it is patched at once, and so effectually that the spot cannot be distinguished.

223. The strewing of sand upon asphalt renders it less slippery; but in addition to the interference of the traffic whilst this is being done, there are further objections, viz., the possible injury by the sand cutting into the asphalt, the expense of labor and materials, and the mud caused thereby which has afterwards to be removed.

224. Although pure asphaltum is absolutely impervious and insoluble in either fresh or salt water, yet asphalt pavements in the continued presence of water are quickly disintegrated. Ordinary rain or daily sprinkling does not injure them when they are allowed to become perfectly dry again. The damage is most apparent in the gutters and adjacent to overflowing drinking-fountains. This defect has long been recognized, and various measures have been taken to overcome it, or at least to reduce it to the minimum. In some cities ordinances have been passed seeking to regulate the sprinkling of the streets, and in many places the gutters are laid with stone, while in others the asphalt is laid to the curb and a space of 12 to 15 inches along the curb is covered with a thin coating of asphalt cement. This latter mode is followed in Washington, D. C. It is said that the pavements formed of asphalt cement in which "Maltha" or liquid asphalt is used, instead of the residuum of petroleum, as the fluxing agent, are not affected by moisture.

225. Asphalt laid adjoining centre-bearing street-car rails is quickly broken down and destroyed. This defect is not peculiar to asphalt. All other materials when placed in similar positions are quickly worn. Granite blocks laid along such tracks have been cut into at a rate of more than half an inch a year. The frequent entering and turning off of vehicles from car-tracks is one of the severest tests that can be applied to any paving material; moreover, the gauge of trucks and vehicles is frequently greater than that of the rails, so one wheel runs on the rail and the other outside. The number of wheels thus travelling in one line must quickly wear a rut in any material adjoining the centre-bearing rail.

To obviate the destruction of asphalt in such situations it is usual to lay a strip of granite-block paving alongside of the rail. These blocks are laid alternately as headers and stretchers, so as to form a toothing into the asphalt. This pavement should be of sufficient width to support the wheels of the widest gauge using the street.

226. Asphalt Pavement Injured by Illuminating-gas.—The asphalt pavements on some of the streets of Frankfort, Germany, became friable and porous. City Engineer Dehnhardt attributed this to the escape of illuminating-gas. This view was ridiculed by several German authorities on this material. The pavements were

taken up, aud it was found that the gas-pipes had several leaks under the worst parts of the street. Some of the injured pavement and pieces of sound pavement were tested. The sound fragments were placed in a tube through which gas was allowed to flow. After a week the samples were reduced to the same friable condition in which parts of the pavement had been found. The samples after several weeks' exposure to the atmosphere regained their original good condition. The explanation offered is that a portion of the carburetted hydrogen of the gas is absorbed by the asphalt, thus destroying its cohesion.

227. Durability.-The systems adopted for the maintenance of asphalt pavements renders it difficult to ascertain their actual life. under traffic. They are repaired immediately they need it, and as each repair is so much new material laid, the whole surface is really relaid in the course of years. Col. Haywood states that in his opinion asphalt will last without extensive repairs from four to six years, and that in the course of ten years the entire surface will have been renewed.

228. That asphalt successfully sustains an enormous traffic is shown by the following figures: From London, Cheapside has a traffic of 13,772 vehicles in 24 hours; Mansion House Street, 23,332 vehicles in 24 hours. Cornhill, Holborn Viaduct, and many others have a daily traffic of upwards of 12,000 vehicles. These streets are paved with asphalt.

229. There are no streets in America or elsewhere in the world that have so much traffic as the above-mentioned London streets. Among the vehicles that travel on them are omnibuses loaded with passengers inside and out, light vehicles of all descriptions, carts, carriages, and brewery trucks loaded with tons of ale and porter.

Cheapside was paved in 1870, and the pavement remained in constant use for 19 years, with of course extensive repairs; but up to 1889 the carriageway was never closed entirely for a general relaying of the pavement. In 1889, the contract for maintenance of the asphalt having expired, a new contract was made and a new surface of asphalt was laid.

230. St. Louis, Mo.-"The asphalt laid on Pine Street in 1883 is now in good condition after a test of eight years under a mixed traffic of 3000 vehicles in 12 hours from 7 A.M. to 7 P.M. The work was carefully executed, and consists of a 6-inch hydraulic-cement