Sewerage and Sewage Treatment

90. Materials.—The materials most commonly used for the manufacture of sewer pipe are vitrified clay and concrete. Cast iron, steel, and wood are also used, but only under special conditions. For pipes built in the trench, concrete, concrete blocks, brick, and vitrified clay blocks are used. Concrete is being used to-day more than bricks or blocks because it is cheaper. A decade or more ago all large sewers were built of bricks. Vitrified clay and concrete are used for manufactured pipe 42 inches and less in diameter. Concrete is used almost exclusively for larger sizes of pipe, particularly for pipe constructed in place, although a brick invert lining is advisable when high velocities of flow are expected.

The character of the external load, the velocity of flow and the quality of sewage are important factors in determining the material to be used in the construction of sewers. Reinforced concrete should be used for large sewers near the surface subjected to heavy moving loads. A high velocity of flow with erosive suspended matter demand a brick wearing surface on the invert. Many engineers consider concrete less suitable than vitrified clay or brick for conveying septic sewage or acid industrial wastes, as concrete deteriorates more rapidly under such conditions. Concrete should be used on soft yielding foundations, whereas a hard compact earth, which can be cut to the form of the sewer, is suitable to the use of brick or concrete.

Cast-iron pipe with lead joints is used for sewers flowing under pressure, or where movements of the soil are to be expected. If the sewage is not flowing under pressure, cement joints are sometimes used in the cast-iron pipe. Movements of the soil are to be expected on side hills, under railroad tracks, etc. Steel pipe is used on long outfalls or under other conditions where external loads are light and the cost is less than for other materials. Because of the thin plates used and the liability to corrosion steel is not frequently used. It should never be deeply buried nor externally loaded because of its weakness in resisting such forces. Like wood pipe, its lightness is favorable to use on bridges, but the greater heat conductivity of steel than wood necessitates protection against freezing in exposed positions. Wood is preferable only where the economy of its use is pronounced and the pipe is running full at all times. It is desirable that the wood pipe should be always submerged as the life of alternately wet and dry wood is short.

Corrugated galvanized iron and unglazed tile have been used for sewers, but usually only in emergencies or as a makeshift. Corrugated iron is not suitable on account of its roughness and liability to corrosion, and unglazed tile because of its lack of strength.

91. Vitrified Clay Pipe.—In general the physical and chemical qualities of clays before burning are not sufficient to cause their condemnation or approval by the engineer, as their behavior in the furnace is quite individual and depends greatly on the manner in which they are fired. The engineer is interested in the result and writes his specifications accordingly.

In the manufacture of clay pipe, the clay as excavated is taken to a mill and ground while dry, to as fine a condition as possible. It is then sent to storage bins from which it is taken for wet grinding and tempering. In this process the clay is mixed with water to the proper degree of plasticity. A variation of 1 to 1½ per cent in the moisture content will mean failure. Too wet a mixture will not have sufficient strength to maintain its shape in the kiln. Too dry a mixture will show laminations as it is pressed through the discs.

A press used in the manufacture of clay pipe is shown in cross-section in Fig. 71. With the piston heads in the steam and mud cylinders at their extreme upward positions, the mud cylinder is filled with clay of the proper consistency. Steam is then turned into the steam cylinder under pressure and the clay is squeezed into the space between the inner and outer shells of the die and mandrel to form the hub of the pipe. The pressure on the clay may be from 250 to 600 pounds per square inch. When clay appears at the holes, marked hh at the bottom of the mud cylinder, the bottom plate and the center portion of the die are removed and the remainder or straight portion of the pipe is formed by squeezing the clay between the mandrel and the outer wall of the die. A completely formed pipe can be seen issuing from the press in Fig. 72. Any sized pipe that is desired can be formed from the same press by changing the size of the dies and mandrel.

Curved pipes are made in two ways—by bending directly as they issue from the press, or by shaping by hand in plaster of paris molds. Junctions are made by cutting the branch pipe to the shape of the outside of the main pipe, fastening the branch in place with soft clay and then cutting out the wall of the main pipe the size of the branch. Special fittings are usually made by hand in plaster molds.

After being pressed into shape the pipes are taken to a steam-heated drying room where a constant temperature is maintained in order to prevent cracking of the pipes. They remain in the drying room from 3 to 10 days until dry, when they are taken to the kilns. If taken to the kilns when moist blisters will be produced.

The dried pipes are piled carefully in the kiln so that heat and weight may be as evenly distributed as possible, and the fire is then started in the kiln. The process of burning can be roughly divided into five stages:

1st. Water smoking, which lasts about 72 hours during which the temperature is raised gradually to 350 degrees Fahrenheit.

2nd. Heating, during which the temperature is raised to 800 degrees Fahrenheit in 24 hours.

3rd. Oxidation, during which the temperature is raised to 1,400 degrees Fahrenheit in 84 hours.

4th. Vitrification, in which the temperature is raised to 2,100 degrees Fahrenheit in 48 hours, and finally,

5th. Glazing, during which the temperature is unchanged but salt (NaCl) is thrown in and allowed to burn.

Oxidation must be complete before vitrification is started as otherwise blisters will be raised due to imprisoned carbon dioxide. The important points in vitrification are to make the required temperature within a reasonable time and to maintain a uniform distribution of heat throughout the kiln. When vitrification is complete as shown by a glassy fracture of a broken sample taken from the kiln, glazing is accomplished by throwing a shovelful of salt on the hottest part of the fire. About five to six applications of salt from two to three hours apart may be needed. The kiln is then allowed to cool and the manufacture of the pipe is complete. The completeness of vitrification is indicated by the amount of water that the finished pipe will absorb. Completely vitrified pipe will absorb no moisture. Soft-burned pipe may absorb as much as 15 per cent moisture.

Vitrified clay blocks are made of the same material and in the same manner as vitrified clay pipe.

The following data on vitrified pipe have been abstracted from the specifications for vitrified pipe adopted by the American Society for Testing Materials.

Pipes shall be subject to rejection on account of the following:

(a) Variation in any dimension exceeding the permissible variations given in Table 36.

(b) Fracture or cracks passing through the shell or hub, except that a single crack at either end of a pipe not exceeding 2 inches in length or a single fracture in the hub not exceeding 3 inches in width nor 2 inches in length will not be deemed cause for rejection unless these defects exist in more than 5 per cent of the entire shipment or delivery.

(c) Blisters or where the glazing is broken or which exceed 3 inches in diameter, or which project more than ⅛ inch above the surface.

(d) Laminations which indicate extended voids in the pipe material.

(e) Fire cracks or hair cracks sufficient to impair the strength, durability or serviceability of the pipe.

(f) Variations of more than ⅛ inch per linear foot in alignment of a pipe intended to be straight.

(g) Glaze which does not fully cover and protect all parts of the shell and ends except those exempted in Sect. 31. Also glaze which is not equal to best salt glaze.

(h) Failure to give a clear ringing sound when placed on end and dry tapped with a light hammer.

(i) Insecure attachment of branches or spurs.

Workmanship and Finish

(29) Pipes shall be substantially free from fractures, large or deep cracks and blisters, laminations and surface roughness.

(31) The glaze shall consist of a continuous layer of bright or semi-bright glass substantially free from coarse blisters and pimples.... Not more than 10 per cent of the inner surface of any pipe barrel shall be bare of glaze except the hub, where it may be entirely absent. Glazing will not be required on the outer surface of the barrel at the spigot end for a distance from the end equal to ⅔ the specified depth of the socket for the corresponding size of pipe. Where glazing is required there shall be absence of any well defined network of crazing lines or hair cracks.

(32) The ends of the pipe shall be square with their longitudinal axis.

(33) Special shapes shall have a plain spigot end and a hub end corresponding in all respects with the dimensions specified for pipes of the corresponding internal diameter.

(a) Slants shall have their spigot ends cut at an angle of approximately 45 degrees with the longitudinal axis.

(b) Curves shall be at angles of 90, 45, 22½, and 11¼ degrees as required. They shall conform substantially to the curvature specified.

(c) ... All branches shall terminate in sockets.

In Fig. 73 are shown the various forms of vitrified pipe and specials which are ordinarily available on the market.

The life of vitrified clay sewers and some observations on the results of the inspection of the sewers in Manhattan are discussed in Chapter XII. The strength of vitrified sewer pipes is shown in Table 37.

92. Cement and Concrete Pipe.—Although there is no general recognition of a difference between cement and concrete pipe, there is a tendency to term manufactured pipe of small diameter cement pipe, and large pipes or pipes constructed in place, concrete pipe. Cement, unlike clay, is used in the manufacture of pipe in the field or by more or less unskilled operators in “one man” plants. Great care should be used in the selection of cement, aggregate, and reinforcement for precast cement pipe since the shocks to which it is subjected in transit are more liable to rupture it than the heavier but steadier loads imposed on it in the trench.

The United States Government, various scientific and engineering societies, and other interested organizations have collaborated in the preparation of specifications for cement and cement tests. These specifications can be found in Trans. Am. Soc. Civil Engineers, Vol. 82, 1918, p. 166, and in other publications.

The following abstracts have been taken from the proposed tentative specifications for Concrete Aggregates, of the Am. Society for Testing Materials, issued June 21, 1921:

1. Fine aggregate shall consist of sand, stone screenings, or other inert materials with similar characteristics, or a combination thereof, having clean, hard, strong, durable uncoated grains, free from injurious amounts of dust, lumps, soft or flaky particles, shale, alkali, organic matter, loam or other deleterious substances.

2. Fine aggregates shall preferably be graded from fine to coarse, with the coarser particles predominating, within the following limits:

Sieves shall conform to the U. S. Bureau of Standards specifications for sieves.

3. The fine aggregate shall be tested in combination with the coarse aggregate and the cement with which it is to be used and in the proportions, including water, in which they are to be used on the work, in accordance with the requirements specified in Section 6....

7. Coarse aggregate shall consist of crushed stone, gravel or other approved inert materials with similar characteristics, or a combination thereof, having clean, hard, strong, durable, uncoated pieces free from injurious amounts of soft, friable, thin, elongated or laminated pieces, alkali, organic or other deleterious matter.

The following Table indicates desirable gradings, in percentages, for coarse aggregate for certain maximum sizes.

The manufacture of small size cement pipe requires relatively more skill than equipment. As a result great care must be observed in the inspection of cement pipe and in the enforcement of specifications. For large size concrete pipe and reinforced concrete pipe the difficulty of holding the pipe together during transportation and lowering into the trench aid in insuring a good product.

Cement pipe is made by ramming a mixture of cement, sand, and water into a cylindrical mold and allowing it to stand until set. The mold is then removed and the pipe stands for a further period of time to become cured. The selection and proportion of materials, the amount of water, the method of ramming, the period of setting, the length of time of curing, and the control of moisture and temperature during this period are of great importance in the resulting product. E. S. Hanson^[52] states that the most conservative engineers recommend a mixture of one sack of cement to 2½ cubic feet of aggregate measured as loosely thrown into the measuring box. In making up the aggregate, clean gravel or broken stone up to ¼ inch in size is used. The American Concrete Institute recommends that 100 per cent pass a ½-inch screen, 70 per cent a ¼-inch screen, 50 per cent a No. 10, 40 per cent a No. 20, 30 per cent a No. 30, and 20 per cent a No. 40. The materials should be carefully graded by experiment and not guessed at, as the behavior of all aggregates is not the same. Too coarse an aggregate is difficult to handle in manufacturing. It causes loss of pipe when the jacket or mold is removed and results in rough pipe, stone pockets, and pin holes through which water spurts when pressure tests are applied. Too fine an aggregate causes loss of strength and with ordinary mixtures tends to produce a pipe which will show seepage under internal pressure tests. The amount of water in the mixture will vary, from 15 to 20 per cent. The mixture should appear dry but should ball in the hand under some pressure.

The mixture can be rammed into the molds by hand or machine. A machine-made pipe is preferable as it produces a more even and stronger product. There are two types of machines for this purpose. One type consists of a number of tamping feet which deliver about 200 blows to the minute with a pressure of about 800 pounds per square inch of area exposed. In the other type a revolving core is drawn through the pipe, packing and polishing the concrete as it is pulled through, with special provision for packing the bell of the pipe. The tamping machines can make 1,500 feet of small size pipe to 300 feet of 24–inch pipe in a day. Machines of the second type can make 750 feet of 8–inch to 200 feet of 30–inch pipe in 30–inch lengths in 9 hours. The inside and outside forms for a 24–inch pipe are shown in Fig. 74 as used with the tamping machines. The forms are swabbed with oil before being filled in order to facilitate their removal. In making a Y-branch or other special, a hole is cut in the pipe or mold the size of the joining pipe which is then set in place and the joint wiped smooth with cement.

After the removal of the mold the pipe may be cured by the water or the steam process. Hanson states:

By the former the pipe are simply set on the floor of the plant and as soon as they are sufficiently strong so that they can be sprinkled with water without falling down; sprinkling is commenced and continued at such intervals for 6 or 7 days that the pipe will be moist at all times. This is a slower process than steam curing. It is also less uniform and less subject to control than where the product is cured by steam.

In the steam process the pipe is exposed to low-pressure steam with plenty of moisture in a closed receptacle for 24 hours, or until hardened. It has been found by tests that pipes sprinkled for 28 days are as strong as steam-cured pipes.

The dimensions of cement concrete sewer pipe as recommended by the Am. Society for Testing Materials are shown in Table 38.

The following has been abstracted from the description of the manufacture of one form of concrete pipe by G. C. Bartram.^[53] All pipe are manufactured in 4–foot lengths near the site at which they are to be installed because of their great weight, for example, 36–inch pipe weighs one ton. The plant for the manufacture of the pipe consists of cast-iron bottom and top rings for each size to be used on the job, and inside and outside steel casings. There are three bases for each steel casing as the pipes stand on the bases for 72 hours and the steel casing remains on for only 24 hours after the concrete has been poured. The pipes are then lifted off the bases and stored for aging. The pipes are cast with the spigot end up.

The concrete is ordinarily mixed in the proportions of 1 : 2 : 4. The materials are placed in the mixer in the following order: first, the stone, then the sand, then the cement, and finally the water. Sufficient water is added to make the concrete flow freely. In cold weather or for a hurry-up job the molds are covered with canvas and are steamed for 2 or 3 hours immediately after the concrete is poured. The molds are then removed but the pipe should be steamed before use. Otherwise they are allowed to stand 72 hours, as explained above. In cold weather the steam is used to prevent freezing and not to hasten the completion of the pipe.