Popular Science Monthly/Volume 56/March 1900/Modern City Roadways
|MODERN CITY ROADWAYS.|
ENGINEER OF HIGHWAYS, BOROUGH OF BROOKLYN.
ONE of the conspicuous results of cheapened transportation and the facility with which the products of field, forest, mine, and factory can be transferred to the consumer has been the rapid increase in population of all our cities. In 1890 over forty-five per cent of the population of New York State (nearly six millions) was concentrated in four cities, while it is estimated that the greater city of New York contains at present not less and probably more than fifty per cent of the State's population. Nor is this tendency characteristic only of American cities, though the general impression seems to be that it is more conspicuous with us. In fact, many European cities (notably those of Germany) have outstripped ours in growth. In 1870 Berlin had about 150,000 less people than New York; in 1890 it had over 73,000 more. In 1875 Hamburg exceeded Boston in population by but 6,000, while in 1890 the German city was more than 121,000 ahead.
Meanwhile the rural population the world over has increased very slowly, or has positively decreased. The massing together of large numbers of people, without proper regard to sanitary conditions, has always resulted in great mortality, as witness the terrible plagues which have swept over the old cities of Europe, and the disastrous results during the summer of 1898 of concentrating large numbers of our volunteers in camps not subjected to rigid sanitary regulations.
It has been amply demonstrated, however, that our cities can be made at least as healthful as the country districts, and an increasingly large number of engineers are engaged in such city building.
One branch of this municipal work will be considered in this paper—that of street improvement. The first impression gained by a stranger entering a city is undoubtedly that produced by the appearance of its streets. If they are poorly paved, irregular,
dirty, and generally unkempt, he will consult his time table to see how soon he can get away. If they are broad, smooth, clean, well shaded and lighted, he will stay as long as possible.
In spite of the pride of the American people in the development of our cities, and notwithstanding the fact that their wealth enables them to have only the best, they have been slow to appreciate the value of thoroughly well-paved streets. As stated by Mr. Albert Shaw, European cities have been ahead of us in accepting the doctrine that "smooth and clean highways are a wise investment from every point of view, and that so long as the work is done in a thorough and scientific manner the result is worth having, regardless of cost. No city should think itself rich enough to prosper without them, and no city is so poor that it can not afford them if it has any reason whatever for continued existence. Good roadways are cheap at any cost, and bad ones are so disastrously expensive that only a very rich country, like the United States, can afford them."
Space will not permit even a brief history of street paving, or an attempt to sketch its development, but reference will be made to the different kinds in general use, A Street in Pompeii, showing Old Roman Pavement. and the kind most in favor in various cities. Probably no one has introduced the subject of pavements without reference to the Roman roads.
While Carthage was probably the first city to boast of paved streets, the Romans soon followed its example, and all over Europe, Asia, and Africa, as far as the domain of their emperors extended, they built with the greatest care and at enormous expense that magnificent system of roads which were often supposed, in the middle ages, to be of supernatural origin, and remain the wonder of our modern civilization. These roads were generally from four to six metres in width, and were constructed in this way: The roadbed was excavated; in it was placed a layer of stones, which were sometimes united with mortar. These stones were such as were most available, sometimes rounded stones similar to the cobblestones with which we are familiar, and in some cases in the Alps the foundation was a compact mass of angular stones, two feet or more in their longest dimension, carefully fitted together.
On this foundation was placed a layer of plaster made of stone or brick pounded with mortar; then a course of sand and lime or sand and clay, leveled and pounded until very hard. The top or wearing surface was made of irregular flat stones, fitted together with nicety and united with cement. The total depth of these roads, or pavements, as they can properly be called, was from three to (in some cases) seven feet. It is said that in the province of Hispania alone (Spain and Portugal) twenty thousand miles of roads were built.
The first stone pavements to be laid in modern city streets were those formed of stones in their natural condition, variously known as bowlders, pebbles, or cobblestones.
The first attempt at a street pavement in this country was doubtless that referred to by Mrs. John King Van Rensselaer, in the Goede Vrouw of Mana-na-ta, where she says, in speaking of what was once called Brower Street, because it passed by the great brewery built by one of the first of the Van Cortlandts: "This street lies between Whitehall and Broad, and was one of the first lanes laid out by the settlers, and was commonly known as 'The Road.' In 1657 it was paved with small round cobblestones, and the circumstance created such a sensation that the country people visited it as a curiosity, and it was one of the sights of the little dorp. The burghers laughingly nicknamed it Stone Street, which name it still retains. The improvement was effected by Madame Van Cortlandt, as she could not endure the dust that filled her tidy house, caused by the heavy brewers' wains that were constantly passing her door."
This cobblestone pavement, laid on Stone Street nearly two centuries and a half ago, has been a persistent type, and, on account of their availability and cheapness, such stones continued to be used in many cities Street in Naples, showing Large Paving Stones. until within a very few years. When they were well shaped and uniform in size they made quite a durable pavement, and, though rough and noisy, were capable, when well laid, of sustaining a considerable traffic. Fortunately, the better class of these stones are-now so scarce and the poorer ones are so execrable that this type of pavement is becoming obsolete, though there are many miles for which more civilized pavements are yet to be substituted, two hundred and thirty-eight miles of which are unfortunately in the Borough of Brooklyn. The next step in advance was the use of stone shaped to uniform size, or approximately so, and with a more or less smooth surface. This is the pavement in most general use to-day, and for permanency and, consequently, cheapness can not be surpassed. When first used, these blocks were quite large, and the size has been decreased until the best stone pavements laid at the present
time in Great Britain are six-inch cubes, or still smaller, with a surface four inches square and a depth of seven inches.
But stone pavement when most carefully laid and maintained is noisy and unpleasant to ride over, and in these days we can never reconcile such a pavement with a handsome residence street. The writer experienced a distinct shock when on riding over Euclid Avenue, in Cleveland, last year, he found it still paved with Medina sandstone blocks, and it seemed that this famous street was still living on the reputation which Bayard Taylor gave it years ago as the handsomest street in the world.
In looking about for something more quiet and smooth than stone, the first material tried was wood. In London the first wood pavement was laid in the Old Bailey in 1839, and was soon followed by many others. None of these pavements lasted more than seven years, and, as they cost more than granite and were so short-lived, a prejudice arose against them, and as they wore out they were mostly replaced with granite. Since that time wood pavement has become popular again, and a large area is now covered with it. The material most generally in use is Baltic fir, though there is quite a large amount of Australian hard wood which is more durable. The people of London seem willing to bear the greater expense and submit to the annoyance of more frequent renewals for the sake of the quiet, and wood is certainly the least noisy of all known pavements.
Paris had at the close of 1893 more wood than asphalt, the areas of pavements of different kinds being as follows:
|Stone||7,541,258||sq. yds.,||71.5||per cent.|
|Gravel or macadam||1,724,632||"||16.3||"|
Berlin also has some wood pavements, but asphalt seems more popular, though by far the greatest area is still of stone pavements.
The most durable wood pavements are those made of the hard woods of Australia, which are especially adapted to this purpose.
They are mostly of the eucalyptus family, the red gum, blue gum, black butt, tallow-wood, and mahogany, Mr. George W. Bell, in a pamphlet published in 1895, gives some remarkable statistics as to the durability of these pavements. He cites the case of George Street, in Sydney, which sustains a very heavy traffic, and on which a wooden-block pavement had been in constant use for over ten years, without repair of any kind. The only piece of wood pavement of this class which has been laid in this country, to the writer's knowledge, is on Twentieth Street, between Broadway and Fifth Avenue, in the Borough of Manhattan, where, in 1896, the Australian "kari" wood was laid. The work was done with the greatest care, and the resulting pavement has proved quite satisfactory. When Fifth Avenue was lately repaved the use of this material was considered, but, on account of the popular prejudice against all wood pavements and the delay which would be involved in obtaining the blocks, the idea was abandoned.
When wood pavements are spoken of in most of our cities, the taxpayer pictures to himself the round cedar block so generally
in use in Western cities. These arc used on account of their cheapness. They are usually laid on one or two courses of plank. The blocks are round, from four to eight inches in diameter and six inches in depth, are set as closely as possible to each other, and the joints are filled with gravel, after which they are usually poured full of pitch. Such a pavement, when new, is quite agreeable to ride over. It soon, however, becomes uneven; the defective blocks quickly decay; the surface not being impervious to water, the wet foundation under a pavement with so little rigidity becomes soft, and the mud or slime works its way up between the blocks, and the process of decomposition is expedited. We hear sometimes of the floating pavements of Chicago. These are such cedar-block pavements which are said to rise with the floods of water filling the roadways after heavy rainfalls, and from specimens of the ment which may be seen in that city considerable sections must have floated away. The round block has nothing to recommend it but its cheapness, and this usually proves to be expensive economy. In Galveston, Texas, creosoted yellow pine blocks have been laid for some years with general satisfaction. They are laid directly on the fine sand, which is water-rammed so as to be very compact. The surface is formed with great care by a template to the exact
grade and crown, and the joints are filled with similar fine sand. In Indianapolis creosoted blocks have been laid for several years, sixty thousand square yards having been put down during the past season. They are laid as closely as possible on a concrete foundation, with a sand cushion of one inch, and the joints filled with paving cement, composed of ten per cent of refined Trinidad asphalt and ninety per cent of coal-tar distillate, after which the surface is covered with half an inch of clean coarse sand or granite screenings.
Improved wood pavements are a luxury. They have many points of superiority over asphalt. They are so considered in London, where their use is continued, although they require renewal
oftener than asphalt, and much more often than granite. They will undoubtedly be used more frequently in this country when the people are willing to pay the additional cost for the quiet and freedom from dust and from the somewhat disagreeable glare of asphalt.
For a dozen years or more brick has been used for street pavements in the cities of the middle West, The use of this material is by no means new. It began in Holland in the thirteenth century, and in the seventeenth century the highway from The Hague to Scheveningen was paved with brick. In Amsterdam such pavements are said to last from ten to twenty years, or an average of fourteen years. After about ten years they are commonly turned over and relaid, after which they will last about four years more. The size in common use is about the same as that made in this country.
A good paving brick should be tough enough to withstand the wear to which a street surface is subjected without chipping or cracking, and should not absorb more than from two to four per cent of its weight of water after submersion for forty-eight hours. It has not the wearing qualities of granite, although there is one block on Ninth Avenue, in the Borough of Manhattan, which has been subjected to very heavy traffic for eight years, has had no repairs to speak of, and its condition to-day compares very favorably with almost any street pavement of equal age which has been subjected to similar traffic.
Another kind of street improvement which must be considered is macadam. In small towns, and some quite large cities, most of the streets are improved in this way. When well maintained and kept smooth, but not too hard, it forms a most agreeable surface for driving. It should not, in the writer's judgment, be classed as a pavement at all, certainly not as a permanent one, and its use should be restricted to park drives and boulevards (for maintaining which liberal appropriations can be secured), and to suburban roads, where sewers and subsurface pipes have not yet been laid, and where temporary roads are required to furnish convenient communication between centers of population, and to assist in developing these districts.
Macadam has no place in a city street, nor is it wise to lay it on the entire width of a roadway. It best serves its purpose when laid in a comparatively narrow strip, leaving the sides of the road unimproved, except for the formation of earth gutters, so that the surface
water can readily soak away where the soil is sufficiently porous.
Macadam is the most expensive of all street surfaces to keep in thoroughly good condition, and in this country it is rarely, if ever, so maintained, except in some of our park roads.
The pavement which is to-day, more generally than any other, superseding stone on all streets where the traffic is not excessive nor the grades extreme, is asphalt. It is scarcely necessary to attempt to give a history of the use of this material, how its adaptability to paving-purposes was first discovered by the improved condition of the roads over which it was hauled from the French mines for use in reservoir and tank linings, etc. The drippings from the carts were observed to have been compacted by travel until a smooth, hard roadway resulted. The first street to be paved with it was Rue Bergera, in Paris, in 1854, and it was so successful that in 1858 Rue St. Honore was similarly treated. An asphalt pavement was laid in Threadneedle Street, London, in May, 1869, and in Cheapside and Poultry in the fall of 1870, while in Berlin its use began in 1873.
The laying of bituminous pavements in this country began in 1861), and they were first made of tar concrete, or Scrimshaw. Asphalt began to be used within the next year or two, and its popularity has been astonishing, as will be seen from the fact that on January 1, 1898, the area of this kind of pavement laid in the United States was, as nearly as could be ascertained, thirty million square yards.
There is a notable difference between the European and American asphalts. The former may be called natural and the latter artificial pavements. In the former the material, as it comes from the mine, is ground to a powder, heated, placed upon the foundation prepared for it, and tamped into approximately the same condition as before it was disturbed, though usually the product of several mines is mixed in order to obtain the best percentage of bitumen, but nothing is added to or taken from the bituminous rock. In the pavement usually laid in America, on the other hand, only a small proportion of the material is brought from the asphalt deposits, the principal part of it (sand) being obtained near at hand. In the one case the cost of long ocean or rail transportation has to be paid on the entire mass forming the pavement, while in the other this expense attaches to but from twelve to fifteen per cent of the material. This, of course, is a great advantage, and at recent prices it is scarcely possible for the European rock asphalts to compete with the artificial ones.
The making of a pavement from one of the standard asphalts may be briefly described as follows: The material as found in Nature has this composition:
|Organic matter, not bitumen||7.63||"|
This is cooked until the water has been driven off, and some of the mineral matter has settled.
The above analysis is of Trinidad Pitch Lake asphalt, and is a particularly favorable result. This material is too hard for use in making a pavement, and it has to be softened or fluxed by the addition of something which will accomplish this purpose. In order to do this there is usually added to each one hundred pounds of refined asphalt about eighteen pounds of heavy petroleum oil. After this addition we have the asphaltic cement ready to combine with mineral matter, which is so selected that when asphaltic cement is added at the rate of about seventeen pounds of the cement to eighty-three pounds of the other all the particles will be coated, and more could not be added without making the pavement too soft. What is found to accomplish this best is fine stone dust and sand.
The asphaltic cement and sand are heated separately to about 300° F. The stone dust is then added to and mixed with the hot sand in the proportion of from five to eighty in the case of fine, well graduated sand, to fifteen to sixty-seven for coarse sands, having less variation in size. The asphaltic cement is then added, and the materials are mixed to a homogeneous mass, which is ready to be taken to the street. It should reach there at a temperature not less than 250°, and is spread with hot iron rakes so as to give usually a thickness of two inches after consolidation. After spreading, it is rolled with a hand roller, after which a small amount of hydraulic cement is swept over the surface, and it is thoroughly rolled with a steam roller of not less than ten tons, the rolling to be continued as long as the roller makes any impression on the surface.
The foundation is usually of cement concrete about six inches thick, though asphalt pavements are often laid over old stone pavements. Between the foundation and the wearing surface there is generally laid what is called a binder course, one inch thick and formed of small broken stone, to which has been added asphaltic cement, the same as is used in making the wearing surface. Five or six pints of this cement are used to each cubic foot of stone.
The pavement just described is made from Trinidad asphalt, the material from which nearly all the earlier artificial asphalt
pavements in this country were made, and which was used almost exclusively until within the last half dozen years.
Within that time, however, it has been discovered that there are a number of other deposits of asphalt well adapted to use for street pavements. A very large deposit containing a high percentage of bitumen and very little mineral matter is located near the coast in the State of Bermudez, in Venezuela. Large deposits have been found in several places in California, and in Utah, Kentucky, and Texas, and a number of other places. The Kentucky product is classed as a natural rock asphalt, as it is a sandstone impregnated with bitumen. It has been mixed with about an equal portion of German rock asphalt and used with very satisfactory results in Buffalo. These asphalts are quite different in their composition, and each requires somewhat different treatment. The Bermudez, being richer in bitumen and softer, requires the addition of very little flux. The California deposits furnish their own flux in a liquid asphalt or maltha, which is almost absolutely pure bitumen, and the use of petroleum residuum is thereby avoided altogether.
It has been recognized since 1836 that the bitumen which forms the greater part of natural asphalts can be separated into two substances, which have been commonly known as petrolene and asphaltene,
the former of which possesses the cementitious qualities essential to the making of a successful pavement. Instead of the arbitrary names—petrolene and asphaltene—these substances are sometimes more aptly designated as active and inert bitumen. It has been found that of the bitumen extracted from asphalts which have given the most satisfactory results in making street pavements, sixty-nine per cent or more is soluble in petroleum naphtha having a specific gravity of 72° Beaumé.
An asphalt pavement can not be economically kept in good condition unless every defect which may develop is immediately repaired. When the smooth, hard surface is once broken, disintegration proceeds very rapidly, and a large hole is soon formed. The more general distribution of smooth pavements, however, will tend to distribute the traffic more evenly, and the increasing use of rubber tires and rubber shoes for horses, to say nothing of the probably quite general use of motor vehicles, within the next decade will result in the elimination of the forces at present most destructive to pavements.
Much regret is often expressed that asphalt pavements should be so frequently opened for the purpose of laying or obtaining access to subsurface pipes and conduits, and thereby mutilated. As a matter of fact, there is no pavement at present in use which can be so effectively andrestored as asphalt. When skillfully done, almost no trace of such an opening can be found.
The first question to arise, when it has been determined to pave a street, will be the selection of material, or the kind of pavement to be laid. In determining this, the governing considerations will be the traffic to be sustained, its density and character, the rate of grade, and the presence or absence of railroad tracks.
If the traffic be very heavy and the street given up wholly to business, ease of traction, durability, and economy of maintenance are of first importance, while quiet, comfortable riding, and beauty can be sacrificed to them. Many efforts have been made to determine the relative force required to draw a load over different kinds of surface under similar conditions. The following is from a table compiled by Mr. Rudolph Hering, from different authorities, the force being that necessary to move one ton on a level grade at a speed of three miles an hour:
|Kind of Road.||Pounds.|
|Ordinary dirt road||224|
|Very hard, smooth macadam||46|
|Good stone block||45|
|Best stone block (London)||36|
|Granite tramway||121 to 131|
|Iron railway||8 to 111|
The question of durability occurs next, and the different kinds of pavement which may be considered for city streets may be rated as follows, it being assumed that the traffic is not excessively heavy:
|Kind of Pavement.||Life in years.|
|Best granite block on concrete.||30|
|Granite block laid on sand.||20|
|Best wood—rectangular block.||10|
|Cedar block—round—on sand.||5|
No class of municipal work comes so near to the daily life of an urban population—both the business and the home life—as the surface improvement of city streets, and no expenditure is too great (provided the work is skillfully and honestly done) to make them smooth, clean, sanitary, and beautiful.