1 Engineering News, Sept. 21, 1899.
The practice which has been followed to some extent of facing the spandrel walls with cut stone masonry, is considered questionable. The Cost of ashlar facing is likely to be so great as to discourage the use of headers of sufficient length to give a good bond with the concrete, and it is next to impossible to make this equal to monolithic concrete construction. Again, since concrete is frequently used to protect ashlar masonry that has started to disintegrate, it is rather a reversal of what has been found good practice to face concrete with a thin layer of cut stone. No criticism is intended of the method of building a pier of large dimension stone with concrete hearting, as this is a different matter. But a thin parapet or spandrel wall faced with a mere shell of cut stone, however beautiful it may be when built, is likely to take on a somewhat dilapidated appearance after ten years' service, especially if it is called upon to pass through one or two floods of unusual violence.
As already intimated, the Cost of a concrete arch, especially where reinforcement is used, is, under ordinary circumstances, considerably less than a masonry arch of equal appearance and strength. The only exceptions to this rule are where the facilities for obtaining stone suitable for masonry are exceptional, and where the work is far removed from cement-producing regions and from the coast. The ability to employ common labor for much of the construction work in a concrete arch is an advantage only partially offset by the necessity of having somewhat more careful work done upon the arch centers and more careful supervision of construction.
The concrete of the arch ring should be of the best quality, especially if steel reinforcement is not used. For this purpose, the stone, broken to a size not exceeding two inches in any dimension, should be mixed with a quantity of mortar a little more than sufficient to fill the voids, and composed of one part Portland cement to two parts sand. Interiors of piers and abutments may be made of a poorer mixture, such as one Portland cement to three of sand and six of broken stone, or even in some cases where abutments are massive, one to four to eight concrete may be employed.
Substantial centers must be provided for concrete arches, and the lagging should be sized, dressed on the upper side, and laid with radial joints parallel to the arch axis. Two inch plank sized to one and three-quarters inches is usually employed for lagging, and the supporting ribs should be from three to four feet centers. For spans up to forty feet a braced wooden rib with one center support and two end supports is used, but for longer spans a trussed center with supports ten to eighteen feet apart is employed. The centers should be made rigid and the camber need be very slight, say from 1/1200 to 1/600 of the radius at the crown. Not less than twenty-eight days should be allowed to elapse after building the arch before striking the centers.
A method that has been largely employed in building the arch ring is to divide the arch into longitudinal rings by planes at right angles to the arch axis. It is believed to be better practice, however, to build the arch as a series of voussoir courses beginning with the spring course, but not necessarily proceeding in order from the springing to the crown. The advantages of this method of building the arch, in transverse courses parallel to the axis of the intrados, are that the planes of weakness may be made at right angles to the line of pressure; the unequal loading, and consequent settlement of the centers, has less tendency to crack the sections or to separate one section from another. In cases of failure of concrete arches under excessive floods, the tendency of the arch to separate along a longitudinal joint forming a plane of weakness has been clearly shown.
746. The tendency of the center to rise at the crown as the arch ring is built up on the haunches is sometimes overcome by temporarily loading the crown. In constructing the ring in voussoir courses, the order of the work may be so arranged as to distribute the loading on the centers in any manner desired. Such an expedient was adopted in the construction of the Illinois Central R. R. arch across Big Muddy River, where the arch ring was divided into nineteen voussoirs. The two springers were built first, then the fifth row of voussoirs towards the crown on each side, followed by the ninth row, the third and seventh. The intermediate blocks were then built in order toward the crown, the second, fourth, sixth and eighth, and finally the keystone. In this way the weight was well distributed on the centers, and the load on the two sides of the crown was kept symmetrical. The monolithic blocks forming the voussoirs that were built in molds had recesses on either side, which were made by securing planks to the interior of the mold. When the intermediate blocks were built, the concrete thus keyed into the blocks first made.
The division of the work into voussoir courses will usually admit of such size molds or blocks that two, one on either side of the center, may be completed in a day. If it becomes necessary to interrupt the laying of a block, however, a vertical bulkhead should be constructed in the mold, with key or dowel pins if desired, to assist in making a bond when the block is completed.
To provide a smooth face, a thin facing mortar of one part Portland to two parts sand is desirable, laid at the time of building the concrete in accordance with methods already described. A thicker layer of granolithic may be used on the soffit and will somewhat more effectually prevent the broken stone of the concrete settling on the lagging, which is always likely to occur to the detriment of the appearance of the finished work.