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ROTTERDAM Repaving near the Central Railroad Station, showing method of surfacing concrete, using longitudinal instead of transverse guides as is done elsewhere; hard wood blocks to be laid directly on concrete.
City and Westminster... 1
1 3 sand Liverpool.
1 6 crushed
1 31 sand Berlin and Charlottenburg.. 1 Hamburg. .
1 3 sand
6 Thames ballast, unscreened river gravel.
3 broken stone, 3-inch gauge.
Cement in each case, under the best specifications prevalent in the different cities, is artificial Portland bought under specifications very similar to those standard in New York. But it should be noted that, except in the best work, in the continental cities the contractor is usually permitted to use on streets of secondary importance a slag cement of inferior grade and in some cases a lime mortar, so that the concrete would be of much less strength than the proportions indicate. It is frequently constructed of bank gravel, no separate measurement of sand and gravel being made, but the proportion of cement added to the run of the bank. Often the old macadam road material is used, when a street is repaved with an impervious surface, to furnish the material for concrete, it being screened at the side and mixed with cement and sand.
Work of concrete mixing is done almost entirely by hand, although concrete mixing machinery has been introduced in Germany to a small extent. The subfoundation is generally rough graded with about the same accuracy prevalent in New York, but it is more usual to finish the concrete surface smooth. It is commonly laid between forms set transversely to the axis of the street at intervals of about 15 feet, materials being turned over on a mixing board between these forms directly into place. Final finish to the surface is then obtained by drawing a straight-edge across the width of the street, the concrete being mixed wet enough so that finer material flushes to the surface and produces a fairly smooth finish under the straight-edge. Where this method is not followed, the forms are placed longitudinally and the finished surface obtained by drawing along them a template formed to the proper transverse section. Where wood pavement is to be laid, particular attention is given to obtaining an accurate smooth surface and a certain amount of hand-smoothing is sometimes employed; but it was noted in London and in other cities that by employing the method of accurately setting transverse strips to the surface grade on narrow widths of concrete and then drawing a straight-edge or a screed along on these adjacent strips, between which the work of mixing and placing was being carried on, that a sufficiently smooth surface was obtained with very little subsequent surfacing. For asphalt and granite surfaces it is not attempted to obtain such smcoth results on the surface. but usually a more accurate surface results on all classes of work than with us, though it is at the sacrifice of speed. The quality of the concrete is in no case superior to that now being produced in Manhattan, and frequently inferior.
A review of the pavement surfaces of European cities, after their inspection and the consideration of the various figures and details noted in regard to them which have just been given, shows that in the cities of importance there is no extensive amount of any peculiar or superior class of material to those which we are accustomed. The principal types are, as with us, various kinds of stone block, wood blocks and sheet asphalt with minor amounts of various composition blocks and different types of water-bound and bituminous macadam. On account
of its being practically the first type employed in any city, the stone pavement will be considered first.
Stone Pavements. The earliest classes of this type of pavement and one which exists in considerable amounts at the present time, consist of round cobbles or rough irregularly broken stones laid as closely as possible on the original surface of the ground. Where the latter was hard and not broken by trenches, this furnished an unyielding surface as uniform as the nature of the faces of the blocks would permit; but, at best, rough, noisy, hard to clean and consequently dusty. The first improvement on this was the adoption of squared blocks having generally cubical dimensions, the type known in this country as “Belgian" block. This is, perhaps, the most frequently used block at the present time in the cities of Central Europe. Blocks are usually laid without foundation, but their regular shape permits of joints not exceeding one inch in width and a surface much smoother under wheel traffic than that of the cobble. Another variation, but of the same class, is the type of large slab block pavements employed in Italy and other of the more southern countries, composed of stones measuring from 18 inches to 24 inches on the top and 6 inches in depth, laid with comparatively close-fitting joints and on a level street furnishing fairly good foothold for horses, while having a smooth surface under wheel traffic. Variations from the cubical shape of the “Belgian" block were introduced in German and English cities approximating more nearly to the dimensions of our granite paving block of today, but having the same general quality and wide joints.
A large part of the stone paved streets in the continental cities (constituting generally a majority of the total area of carriageways) is paved with one or another of the above types of block, either a granite or a trap, the latter called whin stone in Great Britain and porphyry in France and Germany. The granite being a softer material and subject to more rapid wear has not been as largely employed in the past, except on grades where its lack of extreme hardness make it less slippery for horses' feet.
With the introduction of concrete foundation under stone pavements came a reduction in the depth of the blocks, and today where this foundation is used in France and Germany the blocks have a depth of from 5 to 6 inches, instead of 8 inches or more, as when laid without foundation. In the German cities two standard types of specifications are recognized for these two classes of stone, and where concrete foundation is employed stones of smaller surface area and more closely dressed are used,
As characteristic of the extent to which concrete foundation is employed under stone pavements on the continent, however, reference may be made to the city of Paris where only one-eleventh of the entire area of stone block streets is so con. structed. Liverpool has tried a great number of different types of block both with and without concrete foundation, although for the past twenty-five years foundations have been almost always used.
The principal classes and dimensions are noted in the Liverpool specifications in the Appendix. But the fact that certain peculiarly shaped blocks are seen giving satisfaction in the streets of Liverpool does not indicate that this is a standard dimension nor one to be recommended for employment at the present time, because much difficulty is now experienced in obtaining any but the standard sett from the Welsh and Scotch quarries which supply the English cities.
The question of the possible variations in the shape of stone paving blocks used at the present time is studied at length in some of the reports made to the International Road Congress held in London this year, which show a vast number of different shapes, the consideration of which is not of particular importance in a study of this character. The principal point of interest is, that from this great mass of various dimensions of stones there has been evolved a general uniform type of what is regarded as the best shape for use at the present time,