The word asphalte in its generally accepted sense implies a natural rock consisting of pure carbonate of lime, intimately combined and impregnated with mineral bitumen in very variable proportions; that used for roads or footpaths should not contain less than 7 or more than 12 per cent. of bitumen.

The rock when broken takes an irregular fracture without definite cleavage; it is principally derived from Val de Travers, Seyssel, Sicily, Chieti, Auvergne, Lobsann, and Limmer. Its grain should be regular and homogeneous, the finer the grain the better.[90]

When exposed to the atmosphere asphalte gradually assumes a grey tint, by reason of the bitumen evaporating from the surface leaving a thin film of limestone behind. The stone is usually taken from open quarries, but at Val de Travers shafts are sunk and the general treatment is similar to a coal mine.

Bitumen, it must be borne in mind, is itself a mineral product found in Trinidad and some other places; it is composed of carbon, hydrogen, and oxygen.

The weight of a cubic yard of natural asphalte is about 3874 lbs., its specific gravity is 2·114, but this of course varies with its percentage of bitumen.

The following is a test for asphalte given by Mr. Deland in a paper he read before the Institution of Civil Engineers in the year 1880.[91]

“A specimen of the rock freed from all extraneous matter, having been pulverised as finely as possible, should be dissolved in sulphurate of carbon, turpentine, ether or benzine, placed in a glass vessel and stirred with a glass rod. A dark solution will result, from which will be precipitated the pulverised limestone. The solution of bitumen should then be poured off. The dissolvent speedily evaporates, leaving the constituent parts of the asphalte, each of which should be weighed so as to determine the exact proportion. The bitumen should be heated in a lead bath and tested with a porcelain or BaumÉ thermometer to 428° Fahr. There will be little loss by evaporation if the bitumen is good, but if bituminous oil is present the loss will be considerable—gritted mastic should be heated to 450° Fahr. The limestone should next be examined. If the powder is white and soft to the touch it is a good component part of asphalte, but if rough and dirty on being tested with reagents it will be found to contain iron pyrites, silicates, clay, etc. Some asphaltes also are of a spongy or hygrometrical nature. Thus, as an analysis which merely gives so much bitumen and so much limestone may mislead, it is necessary to know the quality of the limestone and of the bitumen.

“For a good compressed roadway an asphalte composed of pure limestone and 9 to 10 per cent. of bitumen, non-evaporative at 428° Fahr., is the most suitable. Asphaltes containing much more than 10 per cent. of bitumen get soft in summer and wavy, those containing much less have not sufficient bind for heavy traffic, although asphalte containing 7 per cent. of bitumen properly heated does well for court yards, as it sets hard when cold.”

For roadways “compressed” asphalte should be used and not “mastic,” which is only fitted for footpaths, court-yards, etc. Compressed asphalte roadways are constructed as follows:

The asphalte rock is first crushed in a “Blake’s” or other suitable crusher, then pulverised in what is known as a “Carr’s disintegrator,” until it is reduced to a powder; this powder is then heated up to between 212° and 250° Fahr. in revolving cylinders and is laid about 2¹/2 inches in thickness upon a concrete foundation previously prepared for its reception, the powder is carefully raked to the required contour and then either rolled or punned with iron punners previously heated to prevent the adhesion of the powder to them.

A roadway thus prepared presents many advantages over macadam, granite setts or wood, the following passage amply describing one of them:[92]

“An indispensable feature of a weight-carrying pavement must be the absolute exclusion of water at the surface as nearly as it can be insured, and in this one respect it cannot be questioned that a surface like asphalte has no equal, the absorption being so gradual as to be inappreciable during any possible continuance of moisture.”

In addition to this indisputable fact the advantage of durability is claimed for asphalte, but this must vary considerably with the quality of the material and of the work. Mr. Hayward estimates the life of an asphaltic Val de Travers compressed roadway at 17 years, and it is claimed for it that it will wear until it becomes quite thin, very heavy traffic breaking it up when it is worn to about ³/4 of an inch thick. Another advantage claimed for asphalte is cleanliness, and this is evidently indisputable, as, being impervious, none but imported mud or dust can be formed upon it.

In addition to the foregoing the following advantages are also claimed:

Pedestrians can walk on asphaltic roadways as well as on the footways.

It is comparatively noiseless under traffic, though in this case wood is better, as the clatter of the iron-shod horses’ feet upon asphalte is very apparent.

It is expeditiously laid, and when repairs are necessary they can easily be effected; no pavement shows less signs of openings being made in it for gas and water-pipe repairs than asphalte.

The rapid laying causes less inconvenience to traffic in the streets.

Ease of traction; but here steps in the one great objection to asphalte as a roadway paving, viz. danger to horses by slipping and falling, of which I shall say more hereafter.

Cellars and vaults under the streets are kept dry, by reason of its impermeability to moisture.

Easily cleansed, especially by mechanical sweeping, and snow is easily removed.

It is very pleasing to the eye, being so uniformly regular and of good colour.

There is no vibration or concussion in travelling over it, and apart from the question of safety it is delightful to drive over it.

It is a cool pavement at night; it does not absorb heat during the day, and consequently none radiates from it after the sun has gone down.[93]

The great objection to asphalte as a material for roadways arises from the fact that it is extremely slippery when damp,[94] irrespective of temperature, and this in the climate of England is frequently the case. The result of this slipperiness is, that not only do horses frequently fall upon it, but it is also difficult to stop a horse when drawing a load, thus causing more risk to foot-passengers of being run over, and straining the horse considerably in its efforts. Again, in thoroughfares crowded with vehicular traffic, constant stoppages occur, and in starting again it is painful to witness the struggles of the horses to keep their footing and overcome the inertia of their load. When a horse falls he has very great difficulty in rising, but on the other hand, although he may be strained, a horse never breaks his knees upon this class of pavement. How far this might be altered if all the streets of a town were paved with asphalte, is a fair matter for argument, as it is asserted that horses are very nervous on going from one pavement to another, and accidents frequently happen in consequence.

The strewing of sand upon asphalte renders it less slippery, but in addition to the interference of the traffic whilst this is being done, there are the further objections, of the possible injury of the sand cutting into the asphalte, the expense of labour and materials, and the mud caused thereby which has afterwards to be removed. Another plan is to frequently wash the asphalte with water, but this is expensive and only of temporary benefit.

Another objection to asphaltic roadways is that they cannot with safety be constructed of greater gradient than 1 in 60, and it must also be borne in mind that fine weather is necessary both for the construction and repairs of a roadway of this description.

Very little smell, and that not of an unpleasant character, arises from the work when compressed asphalte is being used, the mastic is however temporarily unpleasant to those who dislike the odour.

With reference to the question of the cost of compressed asphalte for roadways: it is of course a matter depending upon local circumstances as to the first cost, but it must be remembered that the compressed asphalte hitherto laid has been nearly all that of the Val de Travers Company, who charge a fixed price per square yard for laying according to thickness required, the distance of the locality from London, and other local circumstances. With reference to maintenance, this is a question dependent mainly upon traffic, but here again the company will undertake to keep in repair at so much per square yard per annum for a certain number of years.

It would, however, perhaps be a better plan not to enter into such an agreement, but to arrange for repairs under a schedule of prices, but this must greatly depend upon the character of the work in the first place, and other local considerations.

Mr. Ellice Clarke gives the following as the cost of Val de Travers compressed asphalte.[95]

The cost is reduced to 100,000 tons per annum per yard of width.

Nothing is charged for renewal, as the annual sum for maintenance provides the asphalte in perpetuity.[96]

The following table[97] may here be of use:

Table showing the agreed Cost per Annum of certain
Asphalte Carriageway Pavements in the City of London.

The cost of foundations is included in this table, but their thickness is not mentioned; the excavation was done for the contractors.

With the one serious objection of slipperiness, compressed asphalte seems a most suitable material for the surface of a roadway, but that objection is of considerable weight when we reflect that the great object of roadways is that of “traffic,” and it is for that purpose they are constructed; still, in cities where a heavy business traffic is going on, this class of roadway has so many advantages that where cheap horses are driven it might be used; where, however, valuable horses are used for pleasure driving, as in the west end of London and the corresponding better parts of cities, some other description of roadway should be maintained.

Mastic asphalte will be described in the chapter on footpaths.

Specimen Specification for a Compressed Asphalte Roadway.

Excavation and Concrete.

—The excavation and concrete[98] foundation may be specified to be executed in a manner similar to that contained in the specimen specification for wood paving,[99] except of course that the excavation will be shallower in this case.

Asphalte.

—The asphalte to be used shall be the pure unadulterated natural rock known as the Val de Travers, and be unmixed with any foreign or other matter whatever. The rock after being properly broken, shall be ground in a Carr’s disintegrator to a powder of such fineness, that not more than per cent. shall be left on a sieve containing meshes to the square inch and decrepitation by heat will not be accepted. This powder shall be heated to 240° F. or such other temperature as shall be found desirable, so as to eliminate all moisture, and carefully transported to the street in covered iron carts, in order that not more than 20° F. of heat shall be lost in transit. The powder must be spread upon the concrete inches in thickness[100] and carefully raked so as to have regularity of depth and surface.

Ramming.

—The powder must then be rammed with iron punners of not less weight than 10 lb. heated so as to prevent the adhesion of the asphalte. The ramming must be done lightly at first, so as to ensure equality of thickness, and afterwards augmented to heavy blows. Where the rammers are not available a T tool must be employed.

To meet some of the objections to compressed asphalte as a material for roadways the “Imperishable Stone Paving Blocks” have been introduced in America; they consist of asphalte formed into rectangular blocks under pressure of about one ton to the square inch, these are laid close together without any grouting, and a pavement of this description is said to combine all the advantages of wood and asphalte, though sufficient time has not yet elapsed to prove this.

In Salford, Manchester, etc., I believe “Woodward’s Patent Molten Ironstone Blocks” are used with some success where there is not any very exceptionally heavy traffic.

One of the principal reasons of durability in asphalte pavement is its elasticity, and it should be remembered that compressed asphalte does not begin to “wear” until all compression has ceased; this is the case with no other system of pavement—stone and wood both begin “wearing” from the day the traffic commences. Under ordinarily heavy traffic it may be estimated that it will take two years to complete the compression of asphalte, and the weight of a square foot of this pavement will at the expiration of that time be nearly the same as on the day it was laid, though the thickness is reduced during the first two years as much as it will be in the following eight.

Much is said about the advisability of good and dry concrete, but it may be as well to explain the reasons that necessitate so much care in the foundation. First, it should be always borne in mind that asphalte pavement is nothing more than a tough “carpet,” and has no power of itself of offering resistance to heavy traffic; consequently, if the substratum or concrete is not thoroughly solid and resisting, the weight of traffic will crush it, and the asphalte will at once give way in all directions. The concrete should be made strong enough to resist the traffic, and the asphalte is a simple covering to protect the concrete from direct contact with the wear and friction caused by the traffic. So much for the strength, but the dryness is of even still greater importance; for the best asphalte, laid by skilled workmen, on thoroughly first-rate but damp concrete, will rapidly go to pieces—a phenomenon takes place, which, although quite natural, is little realised by most engineers. When the hot asphalte is laid, the water is immediately sucked up and turned into steam, which tries to escape through the heated powder, and the result is that although the surface of the asphalte is smooth, the mass is really disintegrated from underneath by its bitter enemy “water,” and as soon as the surface begins to wear, the fissures formed by the passing of the steam appear on the surface and the whole pavement falls to pieces: thus accounting for some of the failures this description of roadway has met with under unskilled treatment.

This completes the subject of roadways; I will turn to that of footpaths in the next chapter.

[90] In this respect the Seyssel is the best, being of a very fine grain.[91] Vide ‘Minutes of Proceedings of Institution of Civil Engineers,’ vol. lx.[92] Vide Mr. Howarth’s paper on ‘Wood as a Paving Material under Heavy Traffic,’ ‘Minutes of Proceedings of the Institution of Civil Engineers, vol. lviii. p. 35.[93] In Paris and other cities liable to civil war or internal commotions, it is contended as an advantage of asphalte that it cannot be used for the construction of barricades, breastworks, or rifle pits.[94] Vide Mr. Haywood’s Report upon Asphalte and Wood Pavements, also Report on the Application of Science and Art to Street Paving and Cleansing of the Metropolis,’ and numerous other pamphlets and reports by eminent authorities upon the subject.[95] Vide ‘Asphalte and its Application to Street Paving,’ by E. B. Ellice Clarke, ‘Proceedings of the Association of Municipal and Sanitary Engineers,’ vol. vi. p. 52.[96] The asphaltic roadways of Paris, of which there were 290,000 square yards in the year 1878, cost from 10s. to 12s. per square yard to lay, and about 8·83 pence per square yard per annum to maintain, including the charge for renewing ¹/15th part of the surface every year, which is the method adopted there. Vide ‘Annales Industrielles,’ 1878.[97] Vide Mr. Haywood’s report on asphalte and wood pavements, 1874.[98] “Lime concrete ruins compressed work.” Vide ‘Asphalte and its Application to Street Paving,’ by B. Ellice Clarke. ‘Proceedings of the Association of Municipal and Sanitary Engineers and Surveyors,’ vol. vi. p. 46.[99] See p. 94 ante.[100] It must be ²/5ths more in thickness than that specified as finished.