The Hennebique Construction Company and several others have patented methods of building bulkhead walls of reinforced concrete by constructing portions on shore, floating them to place and sinking by depositing concrete in prepared chambers. While the method has been employed in Europe, it has as yet been little tried in this country.

Appearance of Retaining Walls.-While the object of the retaining walls mainly is utility, pleasing and æsthetic appearance may

FIG. 86.-Concrete Blocks in New York City Bulkheads.

be obtained at but slight additional cost, and often at no additional cost whatever. The maintenance of a pleasing surface once obtained depends upon the construction and materials in and about the walls. We frequently see long stretches of carefully shaped and built walls disfigured by rust and smoke stains, efflorescence, checks, and cracks, and other disagreeable causes. In most cases these may be avoided by the use of a rough instead of smooth surface finish, and the avoidance of ironwork above the wall, the rust from which, carried down by water, is the cause of the rust stains. Efflorescence may largely be avoided by adding a small

percentage of water-repellant compound to the cement in the concrete placed against the exposed surface forms. This will render the surface water-repellant and prevent absorption during rainstorms, which bring out the stain.

The prevention of percolation of water through walls and arches is frequently desirable and is readily effected by enveloping the structure in a 2 or more ply bituminous shield, using the membrane method as described in the chapter on waterproofing.

With this on the earth side, and the surface, water-repellant, the wall will maintain its fresh appearance if iron work above it is avoided, or proper drainage from same provided, and if a crackfree surface has been obtained.

The disfigurement by smoke and locomotive gases can be avoided only in one way: by giving the face of the wall a dark color during construction so that such staining will not be noticeable. Considering the large amount of money spent by the railroads to obtain pleasing effects in concrete work, it seems wrong to construct surfaces which particularly invite such disfigurement from the very

start.

THICKNESSES OF WALLS AND QUANTITIES OF MATERIALS FOR DIFFERENT HEIGHTS OF BASEMENTS.

1 Part Portland Cement to 2 Parts of Sand to 5 Parts of Gravel or Stone.

TABLE XXIV.-DIMENSIONS OF GRAVITY RETAINING WALLS AND QUANTITY OF MATERIALS FOR DIFFERENT HEIGHTS OF WALLS.*

Proportions: 1 Part Portland Cement to 2 Parts Sand to 5 Parts Gravel or Stone.

Note.-A large single load of sand or gravel is about 20 cu. ft.

A large double load of sand or gravel is about 40 cu. ft.

*From "Concrete Construction about the Home and on the Farm," published by the Atlas Portland Cement Co.

CHAPTER XXIII

CONCRETE ARCHES AND ARCHED BRIDGES

Definitions.-Parts of an Arch.-Methods of Failure.-Design of an Arch.—Abutments and Piers.-Reinforced-Concrete Arches.-Arch Bridges.-Arch Centres. -Concreting the Arch.

THE value of the arch as a structure of great beauty and economy has been known for many thousands of years, and while many elaborate arches have been constructed of the finest stones, it has remained for the present generation to see arches of masonry of such light sections and such beautiful lines as to challenge the admiration of observers. This combination of beauty, lightness, and consequent economy has been rendered possible only by combining in the arch the resisting power of steel in tension and of concrete in compression, as described in this chapter.

DEFINITIONS-PARTS OF AN ARCH

Soffit.-The inner or concave surface of the arch.

Intrados.-The line of intersection between the soffit and a vertical plane normal to the axis of the arch.

Extrados.-The line of intersection between the outer surface of the arch and a vertical plane normal to the axis of the arch. Crown. The highest point of the arch.

Skewback. The inclined surface on which the end of the arch.

rests.

Abutment.-A skewback and the masonry which supports it.
Springing Line. -The inner edge of the skewback.

Haunch. That part of the arch between the crown and the skewback.

Spandrel. The space between the extrados and the roadway. Spandrel Filling.-Material placed on top of arch between spandrel walls. It may be either earth or masonry, or a combination of both, or a system of relieving arches which carry the roadway.