664. There seems to be no very good reason why concrete is not more generally employed in the construction of all large sewers. With sizes less than two or two and one-half feet in diameter the difficulty of removing the centers prohibits the use of concrete in the ordinary way, and although some appliances have been devised for building these small sewers as monoliths by a mold that advances as fast as the concrete is tamped in place, they have not proved popular. The difficulty of obtaining a perfect grade, and the undesirable feature of leaving the green concrete unsupported, are probably reasons sufficient for this lack of popularity.
For the larger size sewers concrete has several advantages over brick. First may be mentioned the very smooth finish that may be obtained on the invert, appreciably increasing the velocity of flow over that usually obtained with brick inverts. Cheaper labor may be employed in concrete work with less danger of annoyances from strikes. The Cost is from one-third to one-half less than for brick.
The City of Washington was one of the first to use concrete extensively in sewer construction1. For sizes up to twenty-four inches internal diameter the concrete is used only as a foundation and bedding for the ordinary sewer pipe. For a twenty-four inch sewer the pipe rests in a bed of concrete twenty-seven inches wide at the bottom, flaring to forty inches wide at the level of the center of the pipe, and then carried up with plumb sides for six inches, and finally finished by planes tangent to the upper curve of the pipe. At the joints there are bands of concrete extending over the top, so that at these places the pipe is entirely inclosed. Similar forms are used for the smaller sizes with corresponding decreased dimensions. For all sewers between ten inches and twenty-four inches the sub-grade is six inches below the exterior of the pipe, and in all cases the band about the joint is four inches thick at the top.
1 Described by Capt. Lansing H. Beach, Corps of Engrs., U. S. A. Report Operations District of Columbia, 1895.
666. The method of laying these sewers is as follows: The trenches are 2 1/2 to 3 feet in width, with " headers" about 2 feet wide, left at intervals of 10 to 16 feet, which are tunneled through. The grade and line pegs are placed in the headers at the ground surface, and a cord is stretched on the sewer line over at least four stakes, at a convenient height above the grade, and thus parallel to the bottom of the sewer.
When the trench is to the required grade, a six inch layer of concrete, made with one barrel natural cement, two barrels sand and four barrels gravel, is placed. This concrete is rammed with iron rammers weighing sixteen pounds, and having eighteen square inches ramming surface. The pipe is then laid upon this bed and each section is tested for line and grade. For the former, a plumb bob is used with its cord held against the grade cord already mentioned, and for testing the grade a graduated pole is used, with a projection at the bottom which sets on the interior of the pipe, just within the open end.
Concrete is then lowered in buckets, deposited on top of the pipe and allowed to fall down on the sides so as not to disturb the alinement. When enough concrete to secure the pipe has been thus placed, it is rammed and the concreting continued until the required form is obtained, as already described. The concrete in the bands carried over the joints is not rammed but is beaten with wooden paddles and heavy trowels to compact it and bring it to the desired form, four inches thick and four inches wide at the top, and flaring to twelve inches wide (in the direction of the sewer) at the top of the pipe.
The quantities of concrete materials required to lay one hundred linear feet of pipe sewers as described above are given as follows: —
size of sewer...... 8 inch 12 inch 18 inch 24 inch.
Cement, bbls....... 6.76 10.58 14.77 19.14
Sand, cu. yd....... 2.07 3.23 4.52 5.85
Gravel, cu. yd...... 4.16 6.47 9.04 11.70
With natural cement costing $0.79 per barrel in sacks, sand $0.47 per cu. yd., gravel $0.75 per cu. yd., and laborers $1.50 to $1.75 per day, foremen, masons and inspectors $4.00 per day, the average Cost of laying pipe sewers in this manner was approximately as follows, exclusive of the Cost of the pipe: 8-inch, $1.11; 12-inch, $1.14; 15-inch, $1.46; 18-inch, $1.60; 21-inch, $1.67; 24-inch, $2.32 per foot.
In the construction of some 17,000 feet of sewers for the Chicago Transfer and Clearing Yards,1 concrete was used for all sewers of thirty-six inches diameter and over. The excavation was mostly in blue clay and done by steam shovel to a depth of twenty feet, the remainder being removed by hand shovels and swing derrick. The material was such that in general the bottom of the trench could be trimmed to the form of the exterior of the sewer. The thickness of the ring of concrete was 8 inches for 36 and 42-inch sewers, 10 inches for 48-inch, and 12 inches for 84 and 90-inch sewers.
The concrete was composed of one part "Steel Pozzolana" (slag) cement, three parts sand and five parts broken stone. The cement was of course very finely ground and showed high seven-day tests. The Cost was $1.30 per barrel delivered. The sand was the Chicago "torpedo" sand, coarse and of good quality, and Cost about ninety cents per cubic yard delivered. The stone was a limestone from Summit, 111., crushed in two sizes, namely, 1 to 2 1/2 inches and 1/2 to 1 1/2 inches. These two sizes of stone were mixed in proportions one part of the coarser to two of the finer. The Cost of stone was about $0.80 per cubic yard delivered.
The concrete was mixed by a rotary mixer of the continuous type provided with radial blades. The mixer was mounted on a flat car, with engine and upright boiler. Three cars of stone, the mixer car, two cars of sand and one of cement made up the concrete train, which ran on a track laid close to the trench and was kept near the work by a small locomotive. The mixer was supplied by wheelbarrows running from the material cars on plank runways attached to the cars. The concrete was also transported in wheelbarrows from the mixer to the trench.