The Making of City Streets

In this category fall the processes known as bituminous mac adam, grouted macadam, single-coat bituminous carpet, bitumin ous sand-carpet on a bituminous sub-crust, bituminous clinker carpet on a bituminous sub-crust. In many cases these surfacing materials are laid upon the old macadam road-crust suitably regu lated, but where this does not afford sufficient strength a concrete foundation is formed. In all these bituminous surfacing methods, the need for securing the greatest possible compactness by the elimination of voids is achieved by elaborate precautions in grad ing the materials. It is necessary to determine experimentally the exact proportions of ingredients of varying grades needed to attain the highest degree of density in the resultant mixture.

Cement-concrete, after gaining a wide vogue in Canada and the United States, is now extending its sphere of use in Great Britain as a running surface for carriageways. Its use as a foundation for asphalt or wood blocks has long been an established practice. Special interest attaches to the application of concrete in view of the wide range of materials utilisable as aggregate, as compared with the very uneven geographical distribution of good road-stone supplies. On the other hand, concrete is somewhat handicapped by the lengthy period of setting and "curing," which is requisite before the new surface can be subjected to the strain of traffic. This difficulty may be mitigated in future by the use of quick setting cements, several types of which (ciment fondu, ferrocrete and other aluminous cements) are now available.

Standardisation.

Standard specifications and practice have not yet been evolved for concrete road construction. There is considerable variety of method. The construction may take the form of one-course work or two-course work, the former con sisting of one grade of concrete for the full thickness, while the latter provides two layers of concrete mixed in varying propor tions. One-course work is simpler, and avoids the risk of cleavage between two layers of differing consistency. For two-course work a poorer concrete can be used for the lower layer.

Methods of Laying.

In the actual process of laying the con crete, there is much variety of practice, some engineers preferring to proceed continuously, while others adopt the "alternate bay" system, in which the carriageway is djvided into slabs of uniform length, each extending the full width of the carriageway. The odd numbers would be laid first, separated by gaps representing the even numbers ; these gaps would not be filled until the odd num bers had completely set, thus counteracting the effects of expansion and contraction. The transverse joints separating the slabs are sometimes disposed at right angles across the road ; in other in stances they lie at an angle of 60° or so to the kerb-line, so that the impact of the wheels of the vehicle does not strike any slab simultaneously.

The aggregate, consisting of crushed stone or gravel, should be clean and well graded in size, so as to reduce the percentage of voids to a minimum. Risks of failure in the concrete are greatly reduced by washing the aggregate. The proportions commonly adopted vary from I cement, II sand and 3 gravel to I cement, 2 sand and 4 gravel. Only a bare sufficiency of water should be added—a very wet mixture encourages voids. Mechanical mixing plant is generally adopted.

The precautions to be taken during the setting of the concrete depend largely on the climatic conditions. In moist weather no harm will arise from leaving the green concrete uncovered for a few hours, but in hot sunny weather exposure will lead to the formation of hair cracks. Heavy canvas is often used for pro tecting the concrete during the first 24 hours of setting. The canvas is then removed, and a layer of earth spread over the surface about 2 in. thick and left there for a fortnight or so. For the purpose of indurating the surface of the cement concrete, recourse is sometimes had to two or three applications of a weak solution of silicate of soda.

In London several interesting experiments are being tried (1929) in the use of rubber as a paving material for carriageways, but the high cost appears likely to hinder any general adoption of this method.

Reinforcement.

Where anxiety is felt as to the soundness of the subsoil on which concrete is laid, and it is accordingly desired to form a slab qualified to serve as a raft or bridge, one or two layers of metal reinforcement are of ten introduced into the concrete. In other cases reinforcement is adopted owing to the fact that the thickness of the concrete has had for various reasons to be reduced.

There are large numbers of fabrics in use, expanded metal of 6 in. and other meshes, woven wire, mesh fabrics in which the main tension wires of drawn mild steel run longitudinally with cross-wires welded thereto at regular intervals; other systems make use of twisted or vertical bars; double reinforcement is frequently used in "mattress" form, the upper and lower layers being connected by diagonal members. The weight of reinforce ment varies in ordinary practice from 5 lb. to 25 lb. per yard superficial of concrete foundation.

Traffic problems (see TRAFFIC AND TRAFFIC REGULATIONS and various articles under the heading of TRANSPORT) have created many new problems in road construction. These developments have necessitated the elimination to a great extent of narrow bridges and under passes, sharp curves, blind intersections and crossings. (C. H. BR.)