PAVEMENT (ante). Although the term pavement is really only applicable to a single kind of covering for a roadway, it is better here to regard it as including all the various materials used in constructing durable roads, and especially the streets of cities. The oldest. the best, and the most durable of all roads, the Roman, was built under wholly different conditions from any modern road. The raised center-part on which was the pavement was first excavated and a double layer of flags was set, on this a kind of rough concrete some 18 in. thick, and on this the large thick paving-stones, random-jointed, but cut and butted to match with perfect accuracy, the whole in mortar and cement. This is a flat wall, not a roadway, and, costing more than any modern race would put into such an improvement, remained just as good as at first when, as at Pompeii, the chariot-wheels had worn ruts 4 in. deep in the stones.
Modern road-making dates only from the French superintendent of 1775, Tresaguet. He nude a foundation of large flat flags, double thick, if desired; on this laid broken stone; and over all sand and fine stones. The system was good, but the administration of the time was without means of timely repair. Macadam's system, of about 1820, rests on two points; a dry road is a good road, therefore the compact earth itself will serve as a foundation,and there is no need of heavy flags beneath the surface proper; again, large stones never amalgamate, and dirt, sand, and soft stories, only turn to dust or mud. On a rammed and rounded surface, dry and in good condition, are successively spread two layers of medium cracked stone, and on these one or two more of fine cracked stone. All Materials must be clean, and all stone must be hard.' As soon as the materials are amalgamated by traffic, the road must be carefully surfaced and continually repaired. His whole thickness was of about 91- in., and the crown of 1 in. to the yard. Telford made certain changes in construction, and Polonceau invented the heavy roller, to avoid the wear and tear of vehicles in consolidating the surface. At present the " metaled" road is laid on a foundation of clear screened gravel, on rammed cobble stones or on a compacted clay; two layers of stone will be sufficient, and these are amalgamated by the roller. It is a mistake to suppose that the surface needs repicking and re-rolling at intervals; the surface is lightly picked when new materials are added, but 1111 further disturbance should be avoided. Paved roads are usually made by ramming to a crowned surface, throwing on this to in. screened sand, laying the pavings on this, and ramming to an even surface with a 'savior's rammer. All old work, both in this and other countries, was done with cobble-stones; and although some modern work has been made of small flags with cuts upon them, the best and only satisfactory pavement is a block. These are either square or oblong, and should be about 1-/ to 2, as deep as wide, and not tapered, as they must be turned and relaid when worn smooth. Joints should always break, nor should they ever be allowed to get in line with the street. since wheels immediately start a rut at the point. At the crossing of two streets, we usually set the blocks in circles. but abroad they are set at right angles to the X joining the corners. The best material for our common square-block pavement is the blue gneiss or the trap, but for long blocks, exposed to more fracture, any of the granites. For bridge-stones at crossings, greywack, quartzoze gneiss, the ohlite limestones, or the compact granites. No gutter stones are needed, nor is there, as once supposed, an enormous pressure outwards. as if the thrust of an arch. The rounded surface, or crown, has been much reduced; originally Thr is now enough. Metaled roads should
he kept rather flat in the center, with a quicker slope at the sides; paved roads, the opposite. Friction is greater on paved roads, but traction is easier; in other words, carriages take the macadam; wagons, the pavement. The life of a pavement depends on the stone, and on the traffic, but in a city street of average use it may be called about 14 years. Some of the London pavements laid in mortar exceed this, and the Paris pavements, constantly repaired, do better yet. A road must be capable of drainage, since no road-bed is waterproof, the surface water being thrown off; but, infiltration taking place continually if on a sand, no impervious bed must be interposed, and if on a clay, and it be found impossible to sand-fill below frost level, a crowned surface and a layer of cobble-stones must act as sub-soil drains. The surface of the road must not be broken up. In the newer streets of Paris, sewers and pipes are beneath the sidewalks, but in the older, as el,s'ewhere, the streets are continually torn up by the With us the system is lax. To relay a long line of pavement over an excavation by throwing in dirt, and setting the pavement to one longjoint, is to violate every known Principle of road-building. Over a pipe the earth should lit settled by water, then more put on and rammed; finally, for heavy traffic, 2 in. concrete at least under the stones. In laying block pavements everything depends upon careful hand-work. If a block drives too much or too little, it should be replaced, the joints shiluld be carefully hand-filled with fine screened sand, and, a light covering of sand sprinkled over when finished, Should be thoroughly flushed down before opening for traffic. No inch layer of sand need be left; it only chokes the sewers. Above all, every defect must be immediately remedied by competent workmen, with portable tool-boxes and equipments, under a responsible superintendent.
A bitumen covering to a sidewalk and, for easy traction and lessening of noise, to a road-bed, was the subject of experiment at Paris from before 1840 to 1851, when the first real success was effected. Many experiments with bitumen and wood were made in America from 1802 to 1874. A wood pavement must expose 'the cross-grain of the wood; it should be kyanized or otherwise treated against rot, and it should begraveled so that the sand, imbedding itself in the wood, offers a harder and, in wet weather, a rougher surface. The mixture of sand and bitumen does not set, and sand, lime, and bitumen show the effect of hot weather. The addition of asphalt, a calcareous earth soaked with bitumen almost supplies the needed element, but the success of the process has never been attained without compression. Our wood pavements, and all the asphalt or bitumen, surfaces laid on them or on sand have failed. They soaked, rotted, burst, or sunk, because of inattention to the foundation. An asphalt pavement must be prepared as in France, by a concrete foundation. or by a mixed foundation ending in rolled gravel, as for a Telford metaled road; or it limy be laid on a well-rammed, repaired block pavement. The asphalt, whatever mixture may he selected, is applied hot and rolled slightly and gradually, then heavily. It need be only thick enough to make a surface; it is easily cut out and easily repaired; and in fact, a street made piece by piece, is, if rightly done, after sonic traffic, homogeneous. The asphalt must, however, be per fectly free from damp, and in wet weather repairs can only lie made by heat and under cover. The real objection is its slipperiness in wet weather, and the disagreeable and muddy condition it always shows in wet weather, unless continually swept and watered.