Where the waterway required is too large to permit the use of vitrified pipe, concrete culverts are, in most instances, the most economical to use, and in many locations they may be placed more cheaply than the larger sizes of pipe culverts. Concrete, made of good materials, and properly mixed and placed, is a very durable material and will last indefinitely. A well designed concrete culvert should therefore require very little in the way of maintenance.
These culverts are built either with arched or flat tops. For small spans, the rectangular box form is usually the most economical. The arched culvert for small spans is usually built of solid concrete without reinforcement, and is heavier than the box form, unless the culvert be very small. For longer spans the rein forced arch is desirable.
Fig. to shows the section of a concrete culvert in which the sides and bottom, as well as the top, are reinforced with steel rods, for the purpose of taking the tension due to the tendency to bend under the loads which come upon it. Concrete is a good material for resistance to compression, but offers slight resistance to tension; the introduction of steel rods to take the tensions, therefore, make it possible to construct the walls and top of the culvert much lighter than they could otherwise be built.
Structures of this kind must be carefully designed and constructed in order to secure good results. The steel should consist of small rods well bedded in the concrete. They should be placed with the center of the rods about two inches from the inner surface of the concrete.
The area of steel re quired in the top of such a culvert is usu ally about one per cent of the area of the concrete above it.
Concrete culverts, like pipe culverts, must be protected by a covering of earth from the shocks of the traffic. This covering should be 18 inches in thickness, and in no case should be less than 12 inches. The ends of the culvert must be protected. by walls, which should extend at least two feet below the bottom of the culvert.
The thickness of the top and sides must depend upon the loads which may come upon the culvert and upon the character of the concrete. They should usually
be designed to safely carry a heavy road roller. The concrete should be made of the best grade of Portland cement mixed with good quality sand and gravel, or broken stone, so as to produce a very dense, homo geneous concrete, the proportions for the top being about I part cement, 2 parts sand, 4 parts broken stone; for the sides and bottom, i part cement, 21 parts sand, 5 parts broken stone or I part cement, 3 parts sand, 6 parts broken stone. The following tables give approximate dimensions for culverts suitable for country roads under these conditions: It may sometimes be desirable to leave out the con crete bottom, and extend the side walls deeper, as with a stone box culvert. Where this is done, the side walls should extend at least 18 inches below the bottom of the culvert, and should widen at the bottom into a footing which will give a firm foundation to the structure.
For small culverts on country roads the side walls may be of plain concrete, with a thickness of about one-third of the height. For the smaller sizes this may in many instances be cheaper than the reinforced sides. For openings not more than i8 inches to 2 feet square a semicircular arched opening, without rein forcement, with a thickness of arch and side wall of about one-third the diameter, may be cheaper than the rectangular form of opening.
In work of this kind, great care must be taken to secure good materials; the broken stone should be of good hard material, not too uniform in size, varying from about finch to iI or i inches; sand should preferably be coarse and not uniform in size, it should be clean, hard sand; cement should meet the specifi cations of the American Society for Testing Materials for Portland cement. The mixing and placing of the concrete must be carefully done so as to secure a thorough and uniform mixture of the ingredients, and a dense, compact mass of concrete in the culvert.