breadth. Many pavements thus laid with stones of considerable
breadth still remain, but experience proved that it was
a mistake to suppose that broad stones having a larger base
would support better the weight and shocks of heavy traffic;
on the contrary, a wide stone has a tendency to rock on its
bed, and also to wear round on the top and become slippery.
To obtain an evener surface and a better foothold for the
horses the stones were reduced in width, and in 1840 a granite
pavement was laid by Walker on Blackfriars Bridge, which
may be considered the first of modern set pavements. The
stones were 3 in. broad and 9 deep; they were laid on a bed
of concrete 1 ft. thick and were jointed with mortar. The
reduction of breadth to about 3 in. was generally followed,
but it was some time before a concrete foundation was
employed to any great extent, the frequent breaking up to
which streets are subject having prevented it. In London a
foundation of broken stone has been continued in some thoroughfares,
the sets being evenly bedded in gravel upon it and rammed
with a heavy wooden rammer. Hard core-a mixture of broken
stone, clinker, brick rubbish and old building materials-has
also been largely used to form a foundation. In the northern
towns of England cinders have been employed, and where the
traffic is exceptionally heavy a pitched foundation of stones
on edge has been laid when the sets were not paved upon an
old macadamized surface. The concrete for a foundation to
a paved street should be made with the best Portland cement,
thoroughly mixed in proper proportions with the sand and
gravel or other materials used, water being added as sparingly
as possible. A thickness of 6 in. of well-made cement concrete
is sufficient for the heaviest traffic, and it can be cut out in
slabs for pipe-laying or repairs and can be ire laid and cemented
in its place. To obtain the best result a new foundation should
not be paved upon for a week. A foundation of bituminous
concrete is sometimes used where only a thin bed can be laid,
in consequence of there being an old foundation which it is
undesirable to disturb. It is made by pouring a composition
of coal-tar, pitch and creasote oil while hot over broken stone
levelled and rolled to the proper form, and then spreading a thin
layer of smaller broken stone over the surface and rolling it in.
It has the advantage that it can be paved upon a few hours
after it has been laid.
The best materials for pavement sets are the hard igneous and metamorphic rocks, though millstone grit and other hard sedimentary rocks of the same nature are used when the traffic is comparatively light. Excessively hard stone which wears smooth and slippery is objectionable in spite of its durability. Joints simply nlled in with gravel are of course pervious to water, and a grout of lime or cement does not make a permanently watertight' joint, as it becomes disintegrated under the vibration of the traffic. Grouted joints, however, make a good pavement when there is a foundation of concrete or broken stone or hard core. Where there is not a regular foundation imperiousness in the joints is of great importance. In some of the Lancashire towns the joints have for many years past been made by first filling them with clean gravel, well shaken in by ramming, and then pouring in a composition of coal-tar, pitch and creasote oil, which is allowed to percolate and fill up 'the interstices, the pavement being finished by covering it with small gravel. Joints so formed are impervious to wet and have a certain amount of elasticity; the foundation is kept dry; and the pavement with bituminous grout of this kind keeps its form well for many years. The objection is made that in hot weather the composition runs from the joints and makes the streets unpleasant for foot-passengers. A pavement consisting of broad, smooth, well-jointed blocks of granite for the wheel tracks, and pitching between for the horse track, was laid by Walker in Commercial Road (London) for the heavy traffic to the West India Docks in 1825, and similar pavements have been successfully used elsewhere, principally for heavy traffic, in streets only wide enough for one vehicle. In Milan, Turin and other towns of northern Italy tramways of the same sort are extensively used for the ordinary street traffic. The tractive force required is small, while the foothold on the horse track is good; but the tram stones are slippery for horses to pass over. The rigidity of the roadway renders it more suitable for slow heavy traffic than for light quick vehicles, and the improvement in other pavements has limited the application of this one in ordinary streets.
Brick Paving.-Since about 1885 brick as a paving for carriage-ways has been adopted to a considerable extent, chiefly in the form of shale bricks, in American cities. The clay is a hydrated silicate of alumina, containing about 24% of alumina with 15% of iron, lime, soda, potash and magnesia. Lime is injurious, but alkalis to the extent of 3% are needed to ensure a slight degree of vitrification. Various tests are used to determine their liability to absorb moisture and to be abraded. That for abrasion is made by rolling half-bricks in an iron barrel or rattler in company with pieces of castiron for a given time, and noting the effect on the surfaces, but particularly on the angles, which should be tough enough to resist chipping. Comparisons are also made with test pieces of granite that are mixed with the bricks. To guard against chipping, the best-made bricks are pressed over again, and the upper angles rounded to a radius of three-eighths of an inch. Upon a foundation of concrete or well-rolled ballast a cushion or bed of coarse sand from half an inch to 3 in. thick is laid, and on this the bricks are set. They are then rolled till level, or are heavily rammed, a plank being interposed between the bricks and the rammer. No channel courses are used. Pitch is poured in at the joints, but by no means on the surface, as that would make them slippery. Brick roadways have stood well under hard wear for fourteen years. Although in the United Kingdom bricks are produced unequalled for hardness and finish, no serious attempt has been made to introduce a tough brick for roadways that will neither chip nor wear smoothly. In various experiments with bricks that seemed most suitable they stood hard traffic for about a year. Clay of absolutely uniform character, and kilns that will ensure perfect equality in firing, are requisite. Slag bricks, made to interlock in the form of a double hexagon, the surface being grooved to a small pattern, have stood good tests for wear and foothold on a perfectly level surface. Many attempts have been made to use compositions, into which asphalt or cement usually enters, for making blocks or slabs, square or hexagonal, that can be laid down on a concrete foundation. A mosaic of macadam set in an iron frame is fixed by running molten slag into the back of the block. Small square pieces of oak are formed into blocks, end-grain upwards. Staffordshire blue bricks, made with holes to hold wooden plugs, have been used with some success. Broad blocks not firmly fixed down usually become loose and tilt when subjected to traffic.
Asphalt Paving.-Asphalt was first used for street paving in Paris in 1854. It was introduced in London in 1869, when Threadneedle Street was paved by the Val de Travers Asphalt Company, and since then it has been extensively used for paving both streets and footways. The material is a hard limestone impregnated with bitumen in the proportion of from 6 to 8% in the Seyssel rock, and from ro to 12 in that from Val de Travers. Asphalts containing less than the former proportion have not sufficient coherence for street pavements, and those containing more than the latter proportion soften from heat in the summer. Asphalt is employed either as a mastic or compressed. The mastic is previously prepared in cakes and is melted for use in caldrons with a small quantity of bitumen, and for a street pavement is thoroughly mixed with sand or grit. It is spread in one thickness on a concre te foundation, covered with sand, and beaten to an even surface. This material has not proved so successful for street surfaces as compressed asphalt. To produce this, the rock asphalt, previously reduced to a fine powder by mechanical means, is heated in revolving ovens to from about 220° to 250° F., spread while still hot, and compressed into a solid mass by hot
