Of multiple-arch structures in concrete the Tunkhannock Railway Viaduct holds first place. It is 2,375 feet long and carries the Lackawanna Railroad across the valley of Tunkhannock Creek. The arches are semi-circular, 10 of them being of 180 feet span and two of 100 feet. They are supported on piers 36.5 feet by 43.5 feet. Each of the main arches is com posed of two ribs on which rest the spandrel columns which carry the deck at a level of 242 feet above the bed of the creek. The concrete in this viaduct measures 167,000 cubic yards, making it the largest concrete bridge structure in the world (1916). The second largest as to cubical content of concrete is the viaduct at Cleveland, Ohio, spanning the Cuyahoga River. This has 12 quadruple arches, averaging 140 feet in span, and contains 106,900 cubic yards of concrete. The river channel is spanned by a double-deck steel arch truss 591 feet long, with a clear headway of 96 feet above the river. The Martin's Creek Viaduct, carrying three tracks of the Lackawanna Railroad, is 1,611 feet long, supported by seven three-cen tred arches of 150 feet span with spandrel arches above. The deck is 48 feet wide and is at a level 150 feet above the creek. The Saskatoon Viaduct crosses the Saskatchewan River with 10 arches, four of which are of 150-foot span. The bridge is constructed of two 16-foot arch rings, 15 feet apart, and the floor of girders and beams is carried on span drel walls and columns. A very high arch viaduct carries a highway over the Cuyahoga River gorge near Akron, Ohio, at an elevation of 190 feet. There are five main arches, each of 127 feet. The three high piers are hollow.
The bow-string type of bridge has been very successfully constructed of concrete, re placing worn-out timber and iron bridges on highways.
Other concrete constructions have been used in special cases. The concrete bridge over the Miles River at Easton, Md., 1,075 feet long, was cast in sections at Baltimore, 60 miles dis tant. Piles were driven in sets of three across
the river, and upon each set was cast a cap of concrete. On these were laid the floor slabs, four abreast, and they were united into a mono lith by pouring cement. These slabs were 20 feet long, 5 feet wide and 18 inches thick. The side railings and ornamental facings were cast in special molds. Eleven bridges carrying trains in Kansas City were recently erected of con crete blocks, some of them weighing 48 tons, and molded seven miles from where they were placed. One of the latest forms of concrete construction is the arch built up of separate precasted voussoirs which are placed on a centre from suspension cables swung from construction towers on the river banks.
A great advance has been made within a few years in the ornamental phase of concrete as used in bridges. Besides an improvement in the general design, success has been attained in the surface appearances, both as to texture and color. Surfaces are tooled or bush-hammered, and in some instances the green concrete has been scrubbed with wire brushes to expose the aggregate, and by a judicious selection of the aggregate for such finishing, highly satis factory results are secured. Variation in color is obtained by using colored sand, broken tile and bricks and black oxide in the concrete mixture. Cast ornamental slabs have been used with effect on park bridges, where also, the railings offer especial opportunities for ar tistic finishing. Some excellent examples of this work are the Q Street Bridge over Rock Creek at Washington, the Sixty-sixth Avenue Arch in Philadelphia and the Long Island Rail road Viaduct at Forest Hills, L. I. In the An derson Memorial Bridge at Harvard, over the Charles River, a most pleasing result has been attained by the use of brick trimmings with the concrete. For additional information on concrete bridge construction see CONCRETE