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CHAPTER IV.

CONCRETE SEWERS.

THE use of concrete in sewer construction is growing constantly, both in connection with brickwork, either for backing or as an integral part of the sewer ring, and also separately in cases where brick or pipe is not easily available. For building small sewers the Chenoweth process is convenient, allowing, as it does, a continuous mixing and placing of the concrete without stopping to make or move the necessary forms. This process was used in 1894 for building 900 feet of 24-inch pipe and a mile of 10-inch pipe at Scarborough-on-the-Hudson. The concrete was composed of 5 parts broken stone, 2 parts sand, and I part cement, and was reported to have cost for the larger size, 95 cents, and for the smaller, 30 cents, per foot, for the conduit alone in place, as compared with 97 cents and 23 cents for the corresponding sizes of vitrified tile. The process, invented by Mr. Alexander Chenoweth, of New York City, is described as follows: A collapsible mandrel, held apart by wedges, is placed on grade, and a thin galvanized ribbon is wound spirally around the mandrel. The concrete is tamped around the mandrel to the proper thickness. The mandrels are then loosened and drawn forward, while the ribbon is left in place supporting the green concrete. A new piece of ribbon is attached to that in place, and wound around the mandrels. The trench is filled with the ribbon of steel in place, and the ribbon is not moved for about 10 days, when it is withdrawn from the rear through a manhole. The inventor claims that a length of several hundred feet of pipe can be freed 1 Eng. News, Vol. 26, p. 369; Vol. 31, p. 81; Vol. 33, p. 223; Vol. 51,

p. 164.

from the ribbon in this way. An experimental piece of sewer built after this patent in 1891 at High Bridge 1, is still in good condition.

Another invention for making concrete pipe continuously is that of Mr. W. L. Ransome, of Chicago, which has the advantage that all the concrete, even that of the invert, can be tamped in place. The essential part of the invention is the mold, which is cylindrical but cut off obliquely at the front. When placed in position in the trench, which is trimmed properly to grade, the prow of the mold is located at the beginning of the sewer. A cover box or outside mold is laid on the trench bottom at such a depth below the core mold as will give the proper thickness to the pipe. The cover box, drawn ahead, slowly smooths, and by its weight compresses the earth bottom. The core mold following, with its long, oblique prow, gives the thickness to the concrete, which is tamped from the front end. A cover mold, also cut off obliquely, gives the thickness to the top of the pipe. The three molds are drawn ahead at a rate corresponding to the rate of placing the concrete, and the green concrete is found to be self-sustaining in the smaller sizes of pipe. When the pipe is larger than 24-inch diameter a modification has to be made. The top part of the core mold is made with a projecting horn, on which are strung half-rings of iron. These rings, supported by small iron struts, are left behind at intervals as the mold moves ahead. The struts are placed vertically and horizontally by a boy who stays inside for this purpose.

This form of mold has been used at Oakland, Cal., where 400 feet of cable conduit were laid per day, and at Denver, Col., where it was employed for making 7000 feet of 38-inch water pipe. In this latter city, with a gang of 30 men, performing all their various duties systematically, the machine was capable of making about 600 feet of pipe daily, although on account of stoppages and delays the average daily rate did not exceed 300 feet. The proportions used were three and three

1 Eng. News, Vol. 26, p. 369.

2 Eng. Rec., Vol. 53, p. 349.

and one-half parts of river gravel to one part of cement. The cost of the pipe was $1.35 to $1.50 per foot, with cement at $3.75 per barrel, gravel $1.25 per yard, and wages $1.75 to $2.00 per day. The cost of the same size vitrified pipe, if it could be bought, would be about $3.00 per foot in place.

There has been some attempt in the past to make and use cement pipe in the same way that sewer pipe are made, viz., singly in molds, afterwards to be jointed together in the trench. Brooklyn for many years had the distinction of being the one city which demanded cement pipe for all its sewer extensions. Washington, D.C., uses cement in the form of concrete largely, making the pipe in place, and generally of larger sizes than vitrified pipe are made.

There has been a prevalent opinion that a cement pipe was likely to be more porous and more brittle than vitrified pipe, and therefore to be shunned. Of late years, however, several cement sewer pipe machines have been devised and put on the market, which will probably result in the increasing use of cement pipe. Formerly, the high price of cement prevented competition with clay pipe, but in the past few years this does not hold. There seems to be no reason why well-constructed cement sewer pipe should not last as long as vitrified pipe, unless, indeed, subjected to acids which attack the concrete matrix. There are good and bad grades of cement pipe, and the pipe must be properly made and used, or the results will prove unsatisfactory. The possibility of weak and porous spots in cement or concrete pipe is probably the greatest fault. One shovelful of gravel deficient in or poorly mixed with cement makes a defect in the pipe line which cannot be remedied. Where the cement layer is as thin as it must be in a cement pipe to compare with a vitrified clay pipe, the danger is, of course, greater than with concrete in thicker layers. Not for a moment even must the vigilance of the inspector or the faithfulness of the workmen be relaxed if good pipe are to be obtained. Even under these conditions, some imperfections are likely to be found in the pipe.

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During 1904, the United States Geological Survey conducted a series of experiments on concrete pipe, reinforced with steel rods. Seven pipes were made, each 5 feet in diameter, 20 feet long, the concrete being 6 inches thick in all pipes. The tests were made with the hope that the pipes would show themselves capable of withstanding an interior pressure of at least 100 feet without excessive leakage. The materials were carefully mixed and placed, and every precaution taken to secure good pipes. The results of testing the first two pipes were such that the engineer in charge concluded that it was practically impossible to make a concrete pipe non-porous without some water-proof plaster on the inside. Without the plaster, the pipes, though six inches thick, leaked so much that it was not possible to get any pressure in them. The water leaked away faster than the pumps could supply it. He found the greatest leakage where tamping seams occurred, places where different batches of concrete met, and where the tamping was not sufficient for thorough incorporation. He found it difficult to get water-tight joints with short lengths, and insists that concrete pipe must have imperfections, many of which cannot be easily avoided. Altogether the experiments were not favorable to concrete or cement pipe, proving without question the supreme importance of eternal vigilance, and, even with it, the impossibility of obtaining pipe good enough to withstand any internal pressure without good plaster coating on the inside.

Cement pipes are made by tamping a dry mixture of sand and cement, either 1: 3 or 1: 4, into a vertical mold. The molds can be removed at once and are ready for a second pipe. Three men can mold and set aside about 4 twenty-four inch pipes and 9 twelve-inch pipes per hour.

The following table shows the thickness of cement pipe as made by the Miracle Company, with their estimate of quantities and cost. 2

These figures were computed for a 1 : 3 mixture, the pipe made 1 Water Supply Papers, No. 143.

2 Catalogue Cement Machinery Manufacturing Company, Columbus, Ohio.

in two-foot lengths, the sand costing 75 cents per cubic yard, and the cement $2.00 per barrel. Twenty-four-inch pipe in threefoot lengths, made by the author for testing purposes, a few at a time, cost at the rate of about 50 cents per foot.

At Coldwater, Mich.,' in the summer of 1901, use was made of molded blocks for the construction of the arch of a 31-foot circular sewer. The invert up to the horizontal diameter was of gravel concrete, the roughly shaped trench bottom serving as the outside form. The blocks were molded in advance of field construction, each block being solid, 24 inches long along the line of the sewer, 5 inches on the intrados, 8 inches on the extrados, and 8 inches through, or thick. The gravel cost but little, the molds were of wood lined with tin, and the cement cost $1.35 per barrel. The blocks cost about 12 cents each under those conditions, or at the rate of about $4.20 per cubic yard for the concrete in the form of blocks. This is, of course, a low price for concrete in such small forms, and it is possible that under other conditions the use of brick might be cheaper. The only advantage of the blocks over the concrete placed in mass in the arch is that the forms can be made somewhat cheaper, and can be moved ahead as soon as the key block is placed. Otherwise, this method has no advantage over other methods.

The greatest use of concrete in sewer construction, however, is not in the form of molded pipe, nor yet of blocks, both made

1 Eng. Rec., Vol. 48, p. 101.