TYPES OF WEARING SURFACE The final criterion of a pavement as far as the citizen and taxpaper is concerned is its surface. He takes for granted that it has some form of foundation, regrets its frequent torn-up condition, but judges it by the appearance of the surface, smoothness or irregularity, frequency of worn spots, noisiness under traffic, and slipperiness for horses. He considers that the terms cobble or Belgian block are names interchangeably descriptive of all stone pavements and has a hazy notion that macadam or asphalt are words defining all others, though occasionally he has heard of wood and regards brick as a kind of stone. He considers wear in a pavement surface an evidence of crookedness on the part of the administration that laid it, believes other cities to have more efficient departments and better streets than his own and knows that in Europe affairs are perfectly managed and mistakes never occur. Somewhere he believes can be found the perfect pavement that will never wear out and require no maintenance costs. The only method he has evolved to assist the search for it is a frequent shift of officials in the hope that from some other city can be obtained better men, and hence occurs the frequent "change partners all around" of the municipal and state road builders—a system that has its merits in diffusing and averaging knowledge. As a matter of fact pavements, like all other creations of man, must wear out in proportion as they are used. Assuming, as has been done, our problems to be those for the future in great cities, we can omit discussion of the many types of road surface suitable for use as a single layer or thickness directly over a subgrade of hard original surface or sometimes old telford or macadam. These include generally an aggregate of mineral fragments ranging in size from sand grains to 2 or 3 in. stone placed in a dense mass of uniform thickness from 2 to 6 in. to form a smooth surface with the voids filled and cemented together with some material of either mineral or vegetable origin. Under this head are ordinary water-bound macadam, bituminous macadam or bituminous concrete and the various cement grouted or mixed cement concrete roads. Good roads for suburban or light city traffic can be made of many varying types meeting this definition. They are not desirable for heavy traffic streets or where openings are to be made. The choice of a suitable city wearing surface is between two classes, one using solid rectangular blocks of hard material laid in close contact; the other employing a thin layer of mineral concrete aggregate, with a bituminous cementing material. The first class evolved from the old rough stone block pavement, first improving the blocks so as to have accurate square angles and plane faces and then introducing the use of a concrete foundation to allow of reduction in the thickness of the blocks and give uniform bearing. As the demand for smooth hard streets spread to regions distant from suitable stone quarries but possessing ample supplies of clay, there was naturally evolved the vitrified and hard burned paving brick which to-day forms one of the leading types in our country. The greater distribution of granite throughout Europe on the other hand has to a large extent forestalled any use of brick except in Holland and Belgium. Later a desire for elimination of noise brought about a demand for the use of wood blocks. As laid in England and also under the best conditions in our country, where the blocks are of pine treated with creosote oil to prevent decay and swelling, there is produced an excellent smooth surface having sufficient elasticity to absorb noise but hard enough for heavy traffic under all conditions of weather. The other class contains the pavement most popular in our cities at the present time on account of its smoothness, ease of repairs and comparative cheapness-sheet asphalt. It has the disadvantage of requiring an expensive construction and maintenance plant and hence is most economically used in the largest cities where extensive areas must be maintained. In view of the arguing in times past and still continuing on the relative merits of asphalt from various sources, it is well to bear in mind that it is essentially a sand or sand and fine stone surface with asphalt to the extent of 10% of the aggregate forming a filler and cementing material. For use in light traffic streets far distant from a suitable asphalt plant, on country roads or in smaller towns where it would not pay to install a plant, a form of ready made asphaltic concrete surface has been devised in the shape of compressed blocks. Although not in use to a great extent, such a type has advantages for the situation noted. Not much variety is possible in a selection among these classes. It is in the details governing the individual type and the manner of laying that the greatest differences have existed between various localities. Most of the discussions between paving experts concern these details. They usually fall into one of three classes: 1. Details as to the shape or composition of the block or aggregate; 2. Selection of the type of joint filler or cementing material; 3. Details of adjustment of contact between surface and foundation. CHOICE OF SURFACES. Only brief reference need be made to the solution of these problems. Beginning several years ago with an almost infinite variety of recommended details under each head, the faulty ones have gradually been self-eliminated, and in many respects we are back to the practices adopted some time ago by the slower working European minds, which apparently had not enough ingenuity to think out all the wrong ways for doing things that we devised before doing them right. Lack of attention to two fundamentals has caused many diversions from types that have finally evolved as most satisfactory for our great cities. These are that the surface must be easily removed and restored and that to give greatest success as a roadbed it must have some elasticity. This last is true even in our railroads, where for smooth running a rigid bed is not satisfactory. It is for these reasons that the weight of opinion seems to favor laying granite block surface on a thin sand cushion and filling the joints with a bituminous cement rather than to lay the blocks on a hard base directly or to grout the joints with cement mortar, either or both of which practices have been employed successfully as far as concerns the producing of a smooth surface, but which makes it noisy under hard tires and difficult to take up and relay. With wood blocks the surface itself is elastic and hence has been most successfully maintained on a hard smooth foundation, and it is the element of elasticity that is responsible for much of the favor accorded to asphalt. In any attempt to explain the choice of pavement surfaces in our cities, however, justice to our engineers demands recognition of the fact that they have not possessed the power, given to most European city engineers, to decide purely as an engineering question on the most suitable pavements as they find them. Two other decisive elements often supersede their choice. First, the opinions of superior elected officials based on popular ideas, personal idiosyncrasies or other motives. Second, influences brought to bear by those interested in the construction of special types. The claim is not made that any of these elements is necessarily improper, but that they should rightly come to the engineer to be decided by him as final authority, not as superior forces to which he must bend his judgment. Any consideration of the relative popularity of paving surfaces to-day must be taken up with the fact in mind that the prevalence of any particular type is not an absolute indication of its preference from an engineering standpoint. Some indication of the opinions of our municipal heads in New York on pavement surfaces was afforded by a recent series of addresses on our municipal government given under the auspices of the Academy of Political Science at Columbia University, in the remarks made by the heads of the three boroughs containing our heaviest traffic streets. The Presidents of Brooklyn and the Bronx praised granite block and sheet asphalt as most satisfactory to them. The President of Manhattan stated that in his borough three types were standard, that sheet asphalt while most prevalent because cheapest in first cost required frequent repairs under heavy traffic, and hence under such conditions wood was superior where a quiet pavement was desired, but that granite, although expensive, was in his opinion most desirable because of its durability and, under modern specifications and rubber tired traffic, was hardly more noisy than the others. EXISTING TYPES IN GREAT CITIES In an attempt to make some classification of the surfaces employed in our leading cities information was obtained recently from engineers throughout the country. A summary of results from eleven cities-Chicago, Cleveland, Cincinnati, Baltimore, Detroit, Philadelphia, St. Paul, Pittsburgh, Buffalo, Boston and Newarkshows many items of itnerest. Their total population is about 8 000 000. Here is a classified table of their paved roadways: |