With the usual type of gravity dams, the up-stream face is vertical or nearly so. The pressure of the water is thus exerted horizontally, tending to overturn the dam, which must therefore be made heavy enough to prevent same from occurring.

In the reinforced-concrete dam, the slope of the water face may be so fixed that the pressure on the foundation is controlled by the designer, and the safety factor is made at least five.

The usual type of reinforced-concrete dam consists of an inclined slab of reinforced concrete extending from the heel to the crest, and spanning between and supported by transverse buttresses of concrete, resting upon the foundation. Another inclined slab may or may not be used to form an apron or spill-way. The deck

is usually increased in thickness from the crest to the heel on account of the increase in pressure as the water deepens.

The principles governing the design of reinforced-concrete dams are the same as those used for the design of masonry dams as far as the external pressures are concerned. However, as reinforced-concrete dams are usually of triangular cross-section, they have a much wider base than masonry structures, which greatly increases their resistance to overturning. This resistance is further increased by the weight of the water above the face or deck, which usually has an inclination of from 30° to 45° with the horizontal.

An increase in the height of the water flowing over a masonry or solid dam increases the pressure thereon and causes the line of press

ure to rise, thereby greatly increasing the overturning moment on the dam without in any way increasing the resisting moment to the same.

FIG. 102.-Ashokan Dam of the New Water Supply System for the City of New York. One of the Largest Dams in the World.

In a triangular dam, however, with a broad base, as in the hollow reinforced-concrete dam, when the head of water flowing over the dam is increased, the lines of pressure become more nearly vertical,

the overturning moment is actually reduced, and the stability is in no way endangered. Owing to the reduction in weight it may be necessary sometimes to fill hollow dams with sand, earth, or gravel to increase its resistance to sliding.

Reinforced-concrete dams are particularly fitted to poor foundation conditions on account of the broad base and consequent low unit pressures. This will often enable a large saving in cost.

Concrete Reservoirs.-The construction of reservoirs of concrete present but few features not already discussed in the sections on walls and dams. The principal difficulty encountered is in obtaining a watertight bottom, as extensive areas of shallow concrete are subject to cracking on account of settlement, shrinkage, and expansion. The best means to avoid this cracking is by having a double lining. The under lining is laid in a continuous sheet and covered with a sheet of a good asphaltic material, and over this is placed concrete, in sections ten feet square, the joints between the sections being filled with an asphaltic material.

CHAPTER XXVII

CONCRETE SIDEWALKS, CURBS, AND PAVEMENTS

Advantages of Concrete Sidewalks.-Materials, Equipment, and Forms.-Construction of the Sidewalk.-Coloring and Protection.-Tables of Dimensions and Materials Required. Concrete Curbs and Gutters.-Concrete Roads and Pavements.Table of Offsets for Crowning Roads.

THE class of work in which the value and adaptability of concrete has been brought most intimately to the attention of laymen and municipal authorities is cement sidewalk construction. Being one of the oldest forms in which cement has been employed, its use in this connection has grown so rapidly that no important community is without its miles of well-paved walks; and what had formerly been a luxury employed only by large towns and cities, has now become an every-day necessity in all progressive communities. In fact, it is due to the introduction of the cement walk, that many hundreds of communities have been enabled to provide themselves with walks at all; for the cement walk possesses all the merits of the older forms of wood, brick, and stone, and few of their defects, and the low cost and maintenance charges place it within the reach of almost any up-to-date home.

The beauty, convenience, noiselessness, durability, and economy of well-constructed walks have always had a highly beneficial influence on property values wherever they have been constructed.

Concrete as a material for curbs and gutters is just as advantageous as for sidewalks, and its adaptability for the roadway of streets is now becoming quite generally recognized and will continue to be more so in the future. The question of its use as a paving material is taken up later in this chapter.

Materials for Sidewalks.-Sidewalks should be constructed only of Portland cement, as the natural and slag varieties are unfitted. for constant exposure to the elements. A good sand or screenings and a clean, hard, and durable stone should be employed and the same well graded. Five per cent of clay may be allowed. The

same principles should be followed in selecting these materials as have been outlined in Chapter V.

Tools and Equipment.-The tools and equipment employed by the sidewalk builder are shown in the accompanying illustration and their use is there indicated.

Forms.-Forms for the sides are made of sound wood, at least two inches thick by five inches wide, while the cross forms and protection strips may be of metal. The forms must be firmly secured by stakes driven at frequent intervals (about 2 feet) and placed upon proper lines and grades. Specially designed metal forms are economical and very desirable on any large work.

Construction.-The foundation of the sidewalk is, as in all other structures, the most important element upon which the lasting qualities of the structure depend. To assure a good foundation, the soil underneath it should be well drained by means of broken-stone trenches, tile pipes, or other suitable means depending upon the character of the soil and local drainage facilities.

The soil should be brought to the required elevation of the subgrade either by excavation or fill as the original surface may require. If in excavation, all spongy and bad spots must be removed and replaced by sand or other good material.

If in fill, the material should be wetted and tamped in layers not more than 6 or 8 inches thick. The fill should extend far enough (at least 2 feet) on either side to allow the material to assume its natural slope without danger of running from under the walk.

Preparing the Sub-Base.-The subgrade should be at least 12 inches below the finished surface and slope toward the curb at least 1/2 inch per foot for drainage. The foundation or sub-base should be at least 8 inches thick, made of hard cinders, slag, gravel, crushed stone, or broken brick, grading from 1/4 to 4 inches in size.

Concrete Base.-In placing, the under bed of concrete should be well tamped until its upper surface is at the required height, about 3/4 inch below the finished surface of the sidewalks.

At all driveway crossings, increase the thickness of sub-base 2 to 3 inches, and of the top 1/2 inch.

Do not remove cross forms until concrete is well compacted and set, and in laying adjoining sections, see that old surface is clean and free from loose mortar.