Popular Science Monthly/Volume 61/July 1902/A Modern Street
|A MODERN STREET.|
By Dr. S. F. PECKHAM.
NEW YORK CITY.
A MODERN street is laid on a concrete foundation, the surface of which may consist of brick, asphalt block or sheet asphalt.
It is of the greatest importance that the foundation should be properly constructed. It is true that in cities where a large area of Belgian block pavement has been already laid, sheet asphalt is often laid upon these blocks; but while streets can be made in this way, they are, when so constructed, more or less liable to criticism in several respects; but chiefly from the fact that the Belgian block, as compared with concrete, is an unstable foundation, liable to yield under unusual or excessive strain and always sure to carry the surface with it. While this cause of the criticism is usually absent, especially upon residence streets, there have been conspicuous examples of its presence in almost every large city, where the custom of laying asphalt surfaces on Belgian block prevails. We therefore consider the normal modern street as laid on a concrete foundation.
The French engineers determined the elements of this problem, nearly half a century ago, by very costly experiments for which the city of Paris was mainly responsible. As all the roadways of France are more or less completely under governmental supervision, the results of these experiments have been described in several masterly memoirs by some very able French engineers, in the Annales des Ponts et Chaussées, which is the official organ of the French governmental supervision of bridges and roads. Space forbids mention here of more than a summary of these results. It was found that of all the materials that were available, only a concrete made of Portland cement of good quality mixed with clean sharp sand and broken stones was always to be depended upon. If a concrete foundation could always be laid on well drained earth properly rolled or rammed, that was never invaded by frost and never disturbed by openings for all the multitudinous purposes that disturb modern streets, concrete that will not stand such rigid tests might be used instead of that made of Portland cement. But, streets laid under such conditions and subjected to such use are an exception, indeed they are rare, and for that reason should not be made the rule. The French engineers determined that under any circumstances, nine inches of concrete would hold any traffic; that six inches would hold up all ordinary traffic and that four inches would hold in many special cases. They adopted a thickness of six inches of Portland cement concrete as a general rule, and that rule has been followed in England and the United States with very general satisfaction.
It is imperatively necessary that this foundation should present to the material laid upon it an absolutely unyielding surface, for the reason that the bricks or blocks being jointed and the thin sheet of asphalt, while continuous, possessing very little strength in itself, either of them is bound to follow a yielding foundation with disastrous results. As before stated, an unyielding foundation is therefore, within reasonable limits, an absolute necessity; for, while the average street in our northern cities is liable to the vicissitudes of frosts, and still more liable to the vicissitudes of frequent openings, the evil effects of these vicissitudes can be for the most part avoided by constructing a concrete foundation sufficiently strong to form a bridge over any such weak spots of limited extent, and thus hold up the surface. This very obvious requirement will never be found in a concrete that is neither thick enough nor strong enough.
The action of the Commissioners of Accounts of the City of New York in insisting that only Portland cement of good quality should be used in the street foundations of Greater New York only confirmed and was confirmed by the conclusions reached by the French engineers more than a generation ago. Why such a question should have been raised at this time by engineers presumably familiar with the literature of their profession is not apparent.
In constructing this foundation the contractor is required to proceed as follows:
It is very necessary that the concrete should be thoroughly set and dried out before the surface should be placed upon it. It is therefore advisable that a modern street should be constructed during dry and warm weather.
If the surface is to be of brick, a brick made especially for the purpose is used that is uniformly burned until vitrified. The size should be as uniform as possible, they should be hard enough to resist abrasion and of a texture that renders them impervious to water. When they are laid the dry surface of concrete is swept clean and strewn with dry, clean sand, upon which the bricks are laid and pounded to the proper contour. The joints are filled either with sand or bituminous grout. In the latter case the pavement is impervious to water and the surface makes a very clean and excellent street.
The asphalt blocks are made of asphaltic cement and pulverized rock in the form of large bricks. The materials are thoroughly incorporated together and pressed out while hot in a sort of brick-machine of great power, which renders the resulting block as solid as the materials can be made. The Mocks are required by the specifications to be 4 x 5 x 12 inches and to vary not more than one quarter of an inch in dimensions. They are laid on their edges on a cushion of two inches of sand, so that the surface presented to wear shall be 4 x 12 inches, either in parallel rows or in a diagonal across the street. The surface, after the wear of a summer, during the heat of which it softens slightly, becomes nearly coherent and as continuous as sheet asphalt. On grades, the slight irregularities due to jointing offer a better foothold to the hoofs of horses than sheet asphalt and in this respect the asphalt blocks are the best. It is found in practice that on streets subjected to very heavy traffic, asphalt blocks are more easily broken than a good quality of sheet asphalt, but for streets that are subjected only to light traffic, asphalt block pavements are durable, clean and sightly, and cost little or nothing for repairs over long periods of time.
The larger number of so-called asphalt streets are laid with sheet asphalt, in imitation of the surface first laid about fifty years ago in France from the natural bituminous rock occurring at Pyrimont and Seyssel, near the border of France and Switzerland and near the headwaters of the river Rhone. This material consists of chalk saturated with bitumen, which, when extracted, is found to be very permanent in the air, impervious to water and very tenacious. When the bituminous rock is heated to a moderate temperature it falls into powder that can be screened to remove the flints that are in the chalk. It is then spread, while hot, with rakes, and rolled into a sheet that lasts until it wears out. This bituminous rock was called 'asphalt' by M. Leon Malo, the distinguished French engineer, through whose efforts and inventive skill the laying of these streets became an established industry in France.
M. Malo did not identify 'asphalte' as thus named by him, with asphaltum, which is the solid variety of bitumen, and had been known for an immemorial period before any one thought of using bituminous limestone in the construction of streets.
M. Malo says in his paper, published in 1861:
To this end he proposes a nomenclature of which he gives the following summary:
First, bitumen or pitch, the materials which impregnate asphalte.Second, asphalt, the calcareous rock, impregnated naturally by bitumen or pitch.
This definition gave this peculiar bituminous mineral a name that passed to the streets made from it, and these streets became known in England and the United States as "asphalt streets.'
The great popularity of these streets in Europe led to attempts to construct similar streets in the United States. This was at first a difficult problem as at that time no bituminous limestone was known in America, and those who attempted to introduce an imitation of an
asphalt-surfaced street were forced to use such materials as were available. A number of streets were laid in Washington, D. C, of a sort of bituminous concrete, made of coal tar pitch and broken stone that was laid very thick. Some of these streets are now in existence, more than 20 years old. and in fairly good condition. A better imitation, or what was supposed to be a better one, was made of pitch from the celebrated pitch lake of Trinidad and the liquid residuum of the petroleum stills, which was then a drug on the market.' This mixture was tempered with sufficient sand to make about 90 per cent. of the mineral matter and laid, spread and rolled like the European asphalt. The experiment proving a success, the name 'asphalt street' was applied to these artificial imitations of asphalt. No harm came from this wrong application of M. Malo's word for many years, as the use of it was confined to street surfaces of which natural bitumen was the principal constituent.
About 1890, the late Joseph D. Weeks, of Pittsburg, visited the Pacific Coast and found that the petroleum refiners of California were making a solid residuum from the distillation of petroleum and calling it 'asphalt.' Mr. Weeks immediately conceived the idea that California petroleum contained 'an asphaltic base,' or in other words, it might be considered to be asphaltum dissolved in petroleum from which it could be separated by distilling off the petroleum. This very erroneous conclusion led to a second one, viz., that the residuum of the distillation of California petroleum is practically the same thing as natural asphaltum. On the contrary, these residuums of petroleum, no matter in what manner they may be made, are the product of destructive distillation and should be made the subject of prolonged and careful experiment before they are used in any considerable quantity as an equivalent for natural asphaltum.
The difficulty which M. Malo feared would follow the careless use of terms to designate the different forms of bitumen has overtaken us, inasmuch as the word 'asphalt' is now applied to a large number of the most heterogeneous substances, quite unlike in many respects, but having other properties, denominated bituminous, in common.
An enumeration of these materials will illustrate my meaning. The natural solid bitumens are M. Malo's asphalts, called in the United States rock asphalt, and including bituminous sandstones, as veil as limestones, of which there are large deposits in California, the Indian Territory and Kentucky; Trinidad pitch; Bermudez, Cuban, California, Mexican and other asphaltums; Gilsonite, the bituminous coquina or shell limestone of Uvalde County, Texas, with the extracted bitumens of California and the Indian Territory. All these materials have been used successfully in making asphalt surfaced streets.
The so-called artificial asphalts, called asphalts by local application of the word, are the solid residuums of the so-called asphaltic petroleum, found in California and Texas; Pittsburg Flux, which is the residuum made by burning out the hydrogen from petroleum with sulphur; residuums made by several other patented methods and sold under various trade names, with the solid residuum s from the ordinary distillation of petroleum, all of which are very unlike natural asphalts or any imitation of them in which natural bitumen is used and among which there is great diversity of properties and qualities.
To construct a street of any of these natural or artificial materials, all passing under the name of 'asphalt' the same general method of procedure should be followed.
If the crude asphalt is to be refined, it is put into an enormous kettle, or still, where it is slowly melted in order to drive oft' the water and light oils. Such oils make the work on the street dangerous from liability of fire. The refined pitch or asphaltum, from which an excess of mineral matter has settled or any sticks or other organic matter has been skimmed off, is drawn into barrels.
A fluid residuum of heavy petroleum or mineral tar. which has also been deprived of water and light oils, is mixed with the asphalt in the proportion of 20 parts by weight of oil to 100 parts of asphalt. It is of the highest importance that this mixture should be very complete. If the blending is imperfectly done, the oil will wash out from the asphalt after the street is laid and the surface will dry out, crack and disintegrate. This asphaltic cement should be plastic and very tenacious and should preserve these properties through a wide range of temperature. If it becomes brittle at zero or nearly fluid at or about 100° Fahr., it, will not answer, as it will be brittle and break up in winter and will soften and flow in summer.
To prepare the surface mixture, an asphaltic cement possessing the proper qualities is mixed with sand and pulverized rock in such proportions that when finished it will contain about eleven per cent, of bitumen. The proportions depend upon the kind and quality of the crude bitumen, the locality in which the street is to be laid and the traffic to which it will be subjected. The sand should be clean and sharp and should consist of both fine and coarse particles in such proportions that the fine particles will fill the voids between the coarse particles, leaving the bitumen to hold all of the particles together.
Every element in the construction of an asphalt-surfaced street is important if the result is to be in every respect a durable and satisfactory street. No part can be slighted or neglected in either materials or workmanship. If the street is to be first class each and all of these must be first class of its kind.
The material and drainage of the sub-soil is of the highest consequence. Water is a great enemy to asphalt streets, particularly to those constructed of Trinidad pitch. It is therefore of the greatest imports nee that the subsoil on which the concrete foundation is laid should be as solid and dry as possible. All excavations made in the subsoil should be puddled and rammed, in order that the rolling may result in a perfectly uniform and unyielding surface.
Of equal importance is a proper concrete foundation. It should not only be sufficiently strong, but it should be as impervious to water as possible. Soft, spongy foundations of natural cement or Portland cement of inferior quality are not only an inadequate support for the yielding surface, but they are easily penetrated by water from below and act as conveyors of water, while a sound and firm foundation of the best Portland cement concrete not only presents an unyielding support to the surface, but also, when properly set and dried before the surface is laid, keeps the surface as free as is possible under the circumstances from ground water, and thus contributes to the longevity of the surface.
The concrete surface, having been laid as nearly as possible of the proper contour, and having been thoroughly set, dried out and swept clean, is covered with a binder course to fill the inequalities of the concrete and form a bond between the concrete and surface that will not only attach the surface to the concrete, but will bold the surface in place and keep it from sliding from the center towards the gutters. The binder consists of small broken stone, which should be sound and clean. Each piece should be completely covered with a soft asphaltic cement containing more residuum oil than the surface mixture. The stone and cement should be thoroughly mixed by machinery at a temperature that will render the bitumen perfectly fluid, but not sufficiently high to burn or otherwise injure it. It is then dumped into carts and carried to the streets, where it is spread with hot rakes, and rolled to the proper contour.
The binder is an important part of an asphalt street surface, and should be carefully compounded and laid. If it is deficient in bitumen, it will absorb bitumen from the surface, causing the surface to crack and disintegrate. If it is well supplied with bitumen that is not too soft, the surface is preserved from becoming too dry by absorbing bitumen from the binder, or, in the technical phrase of the art, 'is nourished from the binder.'
The binder being laid and rolled presents a surface that should be exactly parallel to the surface of the finished street. Upon this surface the surface mixture of asphaltic cement and sand is brought in carts, while still hot, and is spread also with hot rakes and rolled, first with hand rollers and finally with heavy steam rollers, until cold. The rollers are prevented from sticking by strewing the surface with hydraulic cement or fine sand. The rolling is an.important element of a good asphalt-surfaced street, as upon that depends the complete solidity of the surface mixture.
As both good and bad streets have been made of about every variety of natural bitumen in the form of asphalt or asphaltum that can be had, the quality of a street appears to depend quite as much upon the technical skill of those who lay the street as upon the kind of material of which it is constructed. Of the various substitutes now being offered for natural bitumens too little has been demonstrated by use to warrant any conclusions concerning them.
- I wish herewith to express my obligations to the Warren-Scharf Asphalt Paving Company for the illustrations accompanying this article.