20 BULLETIN" 220, U. S. DEPARTMENT OP AGRICULTURE. 



the curb; and section H illustrates the grouted brick surface ready 

 for travel. In addition, a properly constructed mixing box is shown, 

 in which the grout is prepared. 



CULVERTS AND BRIDGES. 



The first highway bridges in this country were constructed of wood 

 and required much attention for repairs and renewals. Later, 

 wrought and cast iron were combined with wood in what became 

 known as combination bridges. The present general demand for 

 structures composed of more permanent materials is due largely to 

 the expense and inconvenience of maintaining those of wood or com- 

 bined wood and iron. In many sections there is also need for stronger 

 bridges because of the increase in weight of traffic. Steel possesses 

 the requisite strength, but it is subject to rust and rapid deterioration 

 if not protected with a durable covering. Portland cement concrete 

 is quite free from objectionable qualities, and by skillfully combining 

 steel and concrete in highway bridges it is possible to utilize the good 

 features of each material, and at the same time overcome their dis- 

 advantages. Reinforced concrete bridges may be made strong and 

 durable. The concrete can be cast in any shape, and the surface 

 finished in many different ways to secure a pleasing appearance. 

 Such bridges cost more in the beginning, but practically nothing to 

 maintain, while those of wood, or steel not incased in concrete, 

 require frequent painting and renewal of parts. 



Culverts constructed of stone and concrete are generally superior 

 to those made of pipe and are less liable to injury by freezing. Hard- 

 burned bricks are suitable for culvert material in localities where 

 they are cheap and where stone or concrete materials are not easily 

 available. Brick culverts are not so durable as those made of stone 

 or concrete, especially in cold climates, where freezing is likely to 

 occur. 



Plate XII, figure 1, represents a bridge of the incased I-beam type, 

 having a span of 24 feet, height of 6 feet, and width of roadway of 20 

 feet. The steel I-beams are 18 inches in depth, weigh 55 pounds per 

 linear foot, and are spaced 3 feet 3 inches apart. The concrete is 

 held firmly to the bottoms of the beams by wires of No. 10 gauge in 

 the form of vertical loops spaced 8 inches apart. Lettered cards are 

 used to designate the parts, as follows: 



A, 6^-inch reinforced concrete floor slab; B, |-inch twisted steel 

 bars, spaced 8 inches apart and resting on the I-beams; C, steel I- 

 beams; E, wooden forms (framing 3 by 6 inches, 3 feet on centers; 

 boards 1J by 8 inches); F, parapet and railing; G, wing walls (top 

 thickness, 12 inches, bottom thickness varies with height of wall, 

 generally not less than four-tenths of height); H, footing (course 

 obtained by adding 1 foot to thickness of abutment wall at base, 



