The selection of dean, hard, well-graded aggregates is of the utmost importance. Prac tice in supplying reinforcement to minimize cracks, or in inserting transverse joints, is not yet settled. The function of the small amount of reinforcement supplied is to keep together two portions of the concrete which have in evitably cracked due to settlement or shrinkage, and so prevent failure of the slab, rather than to prevent cracks. The latter remain small and soon fill up. Reinforcement is so formed of reticulated metal, as wire mesh, expanded metal, in amount about 28 pounds per 100 square feet of surface. Transversejoints 34 inch thick spaced 36 feet apart are filled with fibre matrix or bitumen and are protected by metal plates which, however, may be omitted if the filler is of a character that runs out under traffic.
A single track road should be 10 feet wide, and a double track road 18 feet wide. On each side is a macadam or gravel strip for turning out, the total width of roadway is 20 and 26 feet, respectively. The thickness of the road way slab is 6 inches at the edges and increases to 7 or 8 inches at the middle, depending upon loads and other circumstances. The slab is built either with a wearing surface on a con crete base as in sidewalks, or preferably as a one course slab floated on the surface. In 1914 there were 14,200,000 square yards built, equiva lent to 1,500 miles of 15-foot roadway, and in 1915, some 15,000,000 square yards.
The cost of constructing concrete roads in 1915 in the United States, excluding grading, drainage or roadbed, varied from 98 cents to $1.28 per square yard, depending upon locality and other conditions. In 13 States out of 17 the cost was less than $1.12 a square yard, or $6,570 a mile for one mile of single track road 10 feet wide.
Maintenance of concrete roads is taken to mean, not the repair of obviously defective con struction, resulting from careless workmanship and poor materials, but such work has been usually done in filling cracks or joints or small holes, with tar and sand. The cost per mile for such work in Ohio in 1914-15 was $29 per mile for 122 miles. Extensive data of cost of maintenance are lacking. It may be estimated that the maintenance cost just cited is not likely to be exceeded in pavements one to five years old.
Concrete walls for foundations and cellars may be made 1:3:6. A 10-inch thick wall will stand when hardened a pressure of 6 feet of earth, and if supported laterally by joists may be 6 inches thick Building walls are single, or double with an open space. A single wall 6 inches thick is equivalent to a 12-inch brick wall. Each face of a hollow wall is 3 or 4 inches thick, and the air space may vary in middle, 9 inches being a convenient size. Con crete should be wet to flow readily in the forms. Concrete building blocks with air spaces of various designs are machine made of rather dry or porous concrete. These are apt
to be of inferior quality of and un sightly in place. With special machinery and aggregates a satisfactory wall can be secured. Plain concrete is used for retaining walls, piers, dams, in about the same sections as the stone masonry it displaces. The sections of brick or stone arches may be reduced from 10 to 2.0 per cent. Some reinforcement in arches. is always advisable. Reservoirs and tanks, chu.n neys, silos, etc., should be reinforced. Especial extension of uses of concrete on the farm is to be noted.
Reinforced Concrete.— Reinforced concrete is concrete with metal, usually steel embedded to assist in carrying tensile stress. Good con crete carries tensile stress in the earlier .stages of loading up an extension corresponding -to about 5,000 pounds per square inch in. the steel, after which the steel does increasing. duty. Computations for design omit this tension in the concrete. Steel is also used to. assist in carrying compression in concrete in columns, arches and the compression flange of beams. Steel is also inserted in concrete to provide for shrinicage when setting and expansion and contraction under change of temperature and to minimize effects of settlemenL A simple beam of plain concvete breaks on the lower or tension face, when the strength of the concrete in compression on the upper face has reached only about 1/10th its capacity. When steel rods whose cross section equals about 1/100th of the cross section of the beam are introduced in the tension side, the beam becomes more nearly of equal strength in both faces of 1:2:4 concrete. Its capacity is very largely increased. The composite action of the steel and concrete depends upon the bond between the steel and concrete. The possibility of carrying tensile stress through the concrete by steel leads to great flexibility in design, and allows new forms and constricted sections. The advantage of such construction in difficult situations is marked. Reinforced concrete has thus replaced stone or brick masonry in many fields of construction, such as retaining walls, arches, foundations, and has opened up new fields in reservoirs, tanks, buildings, conduits, chimneys. The combination possesses in a large degree the advantages of both steel and concrete. Steel is protected from corrosion and fire, and is used in its most economic form. Expensive con nections, use of skilled labor and scarcity of product in rush times are obviated. The artistic possibilities are apparent. As in plain concrete construction, the necessity of care in inspection of materials and building operations, the effect of freezing weather or of rapid dry ing, the dead weight on foundations and the problems of waterproofing need consideration.