The Making of City Streets

THE MAKING OF CITY STREETS In the principal streets of the world's great cities where highly durable paving materials on a concrete foundation have long been in use—mastic asphalt, compressed asphalt, wood blocks, granite setts, etc.—recent changes of method are mainly confined to the strengthening of the concrete foundation by an increase in thick ness or the introduction of metal reinforcement. In some areas in London it is now the practice to lay as a foundation for wood blocks a bed of cement-concrete as much as 12 in. thick, contain ing a steel reinforcing fabric. Intolerance of traffic interruptions encourages the introduction of time-saving devices. Growing use is made in the busiest thoroughfares of quick-setting cements, such as ciment f ondu and ferrocrete, which reduce by a fortnight or more the period which must elapse before the road can be opened to traffic. Various experiments have been tried in the use of rubber as a paving material ; in some instances it has taken the form of a wearing cap applied to a block of wood or other mate rial; in other instances the entire block is composed of rubber. The elasticity of the material renders it somewhat liable to dis placement under the stress of traffic.

Foundations.

The choice of materials for foundations is usually dictated by considerations of transport, though in certain favoured districts the hard, compact subsoil of chalk or rock needs but little improvement. On a firm stratum of chalk a well compressed layer of clinker will often serve to carry the surfacing coat. Equally serviceable as a natural foundation is a well-drained bed of coarse gravel; finer gravels, and even so fine a material as sand, are not to be despised, provided that the latter has no oppor tunity of escaping laterally under pressure. In France a thick bed of river sand is not infrequently used as the foundation for sett paving. The bearing capacity of clay, loam and other similar subsoils is so diminished by the presence of water that great care must be taken to ensure adequate drainage before road-making operations are begun. The want of uniformity and reliability in

the chance supplies of materials formerly used as "hardcore" foundation for new roads in areas devoid of quarries has led to a notable extension in the use of cement concrete as a road foundation. Hardcore derived from demolished buildings con sists of ingredients possessing every degree of durability from broken fragments of plaster to the hardest vitrified bricks. These heterogeneous materials thrown down pell-mell in a layer 12 in. thick and subsequently crushed under a 10-ton steam roller form the foundation of most of the roads on outskirts of great cities, and it is not surprising that carriageways so constructed show obvious signs of weakness under the stress of modern traffic.

Surfacing Methods.

Water-bound macadam (composed of broken granite, trap or hard limestone) still remains in use on lightly trafficked roads, but as it does not possess impervious qualities it is usually treated with hot surface dressings such as tar or various bituminous mixtures, applied in warm, sunny weather to a thoroughly swept, dry carriageway, the surface of which is then "gritted" with chippings or shingle. As surface dressings, various asphaltic or bituminous emulsions are applied cold, regardless of weather. Tar is used as a surface dressing and as a "binder" for mixing with materials prior to their application so as to give cohesive qualities to the entire thickness of the road crust. By this means such materials as limestone and slag can be utilised, which are not so well adapted to waterbound methods.

Natural asphalts after a 7o years' test still remain in high favour for the paving of streets in wealthy cities. During the twelve years 1916-1928, however, much research has been devoted to the problem of utilising the valuable properties of asphalts and bitumens in some cheaper form of pavement suitable for wider adoption. Asphaltic and bituminous mixtures are now utilised as a matrix to bind together an aggregate of broken stone or clinker, applied in one or two coats as a road-surfacing material.