Fillers are employed to close the joints between blocks and bricks. They are often of some water proof cement, thus rendering the pavement im pervious, as well as binding it together. Where bituminous in character, like asphalt, coal tar, or the two combined, they also yield somewhat to contraction and expansion, without permanent rupture, as is the ease with the Portland cement and other burnt stone fillers. Portland cement filler is a grout composed of I part cement and 1 or perhaps 2 'farts tine sand, mixed with suffi cient water to cause it to run easily, thus tilling the interstices. Murph y grout is a patented filler composed chiefly of ground iron slag, car bonate of lime, and sand, mixed with water. Bit u m ino us cements are many and variable, lint may be classed as coal-tar pitch and asphalt. Residuum oil (obtained in the refining of petroleum) is generally, but not always. used with asphalt cements. (See ASPHALT.) Coal-tar pitch alone becomes brittle and cracks or breaks in winter, while in summer it becomes hot and flows. it is designated commercially by the de gree of hardness to which it has been distilled. In and near New York it is common to mix 100 pounds of No. 4 coal-tar pitch (commercially known as paving cement), 20 pounds asphalt, and 3 pounds residuum oil. This last combina tion js used with hot gravel to fill the joints in block stone pavements. Where asphalt is used, some 10 per cent. of its weight in residuum oil is sometimes added. All the bituminous cements are poured hot. Sand alone is also used as a filler, but not where imperviousness is desired.
Ramming is employed on stone, wood, and brick pavements to bring the blocks or bricks to a firm bed and even surface. Rolling with steam or other rollers is a necessity with sheet asphalt and is sometimes done with other pavements.
CI oteE OF PAVINO MATERIALS depends on a va riety of factors, such as serviceability, durability, cost of construction, repairs, and maintenance, case of cleaning, noiselessness, and other sanitary or related qualities. Cinder serviceability, the main factors are a minimum resistance to trac tion and a good foothold for horses, or absence of slipperiness. Asphalt fulfills the first of these
eondifions most admirably, and probably brick comes next. Asphalt is slippery. both in wet weather and on heavy grades. Brick is less slip pery. Newly laid wood gives a smooth pavement, but it is liable to wear unevenly and to he slippery in wet. weather. For durability, or long wearing powers, the best stone blocks cannot be excelled, and under heavy traffic they are very serviceable, although offering more resistance to traction than several other materials. In first cost. as phalt and the best stone blocks generally rank high; but this is largely offset by their durability and low cost of repairs. Brick pavements are very cheap in same localities, and if placed on gond foundations they are sufficiently durable. Wood may be cheap also, hut unless specially treated wood is used it. generally proves ex pensive to keep in repair and requires frequent renewals. For ease of cleaning nothing can sur pass asphalt, and it gives rise to less noise than either stone blocks or brick. Brick is also easily cleaned, and far quieter than stone blocks. The latter are not only noisy, but also hard to clean, and unpleasant to ride over, on account of their roughness. The smoothness of asphalt and brick, and the quickness with which their surfaces dry after a rain or a sprinkling, make them great dust-spreaders, but the ease and thoroughness with which they may be cleaned render an avoid ance of this nuisance comparatively easy. The large joints of stone blocks collect and retain dirt, but, as it is next to impossible to clean all of it from between the stones, it dries and spreads when subjected to the sun and wind. The dampness of wood pavements is one of the chief objections urged against them. They absorb un clean water readily and tend to keep the streets in a slimy condition, particularly in moist cli mates and on streets not freely exposed to the sun and wind.