Harold E. Babbitt

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.

Fig. 71.—Diagrammatic Section through Clay-pipe Press.

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.

Fig. 72.—Clay-pipe Press.
Courtesy, Blackmer and Post Manufacturing Co.

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 ? 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 ? 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 ? 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.

TABLE 36
Properties of Clay Sewer Pipe
Abstracts from Tentative Specifications of the American Society for Testing Materials
Internal Diameter, Inches	Minimum Crushing Strength, Pounds per Linear Foot. See Note 2	Maximum Absorption, Per Cent	Laying length, Feet	Diameter of Inside of Socket, Inches	Depth of Socket Inches	Taper of Socket	Minimum Thickness of Barrel. Inches	Permissible Variations						Number of Scorings on Spigot and Socket ? Inch Deep
								Length, Inches (-), per Foot	Internal Diameter, Inches		Length of Two Opposite Sides, Inches	Depth of Socket, Inches (-)	Thickness of Barrel, Inches (-)
									Spigot (±)	Socket (±)
6	1430	5	2, 2½, 3	8¼	2	1 : 20	?	¼	3 16	¼	?	¼	1 16	2
8	1430	5	2, 2½, 3	10¾	2¼	1 : 20	¾	¼	¼	5 16	?	¼	1 16	2
10	1570	5	2, 2½, 3	13	2½	1 : 20	?	¼	¼	5 16	?	¼	1 16	2
12	1710	5	2, 2½, 3	15¼	2½	1 : 20	1	¼	5 16	?	?	¼	1 16	2
15	1960	5	2, 2½, 3	18¾	2½	1 : 20	1¼	¼	5 16	?	?	¼	3 32	3
18	2200	5	2, 2½, 3	22¼	3	1 : 20	1½	¼	?	7 16	3 16	¼	3 32	3
21	2590	5	2, 2½, 3	26	3	1 : 20	1¾	¼	7 16	½	3 16	¼	?	3
24	3070	5	2, 2½, 3	29½	3	1 : 20	2	?	½	9 16	¼	¼	?	4
27	3370	5	3	33¼	3½	1 : 20	2¼	?	?	11 16	¼	¼	?	4
30	3690	5	3	37	3½	1 : 20	2½	?	?	11 16	¼	¼	?	4
33	3930	5	3	40¼	4	1 : 20	2?	?	¾	13 16	¼	¼	3 16	5
36	4400	5	3	44	4	1 : 20	2¾	?	¾	13 16	?	¼	3 16	5
39	4710	5	3	47¼	4	1 : 20	2?	?	¾	13 16	?	¼	3 16	5
42	5030	5	3	51	4	1 : 20	3	?	¾	13 16	?	¼	3 16	5

Note 1. For methods of making tests see Proc. Am. Soc. for Testing Materials.

Note 2. Concentrated load at end of vertical 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.

Fig. 73.—Standard Clay Pipe Specials.
Courtesy, Blackmer and Post Manufacturing Co.

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.

TABLE 37
Strength of Sewer Pipe
Strength in pounds per linear foot to carry loads from ditch filling material such as ordinary sand and thoroughly wet clay, with the under side of the pipe bedded 60° to 90° by ordinary good methods. From Proc. Am. Society for Testing Materials, Vol. 20, 1920, page 604.
Height of Fill Above Top of Pipe, Feet	Breadth of the Ditch a Little Below the Top of the Pipe
	1 Foot		2 Feet		3 Feet		4 Feet		5 Feet
	Ditch Filling Material
	sand	clay	sand	clay	sand	clay	sand	clay	sand	clay
2	265	280	615	635	970	990	1330	1,350	1,690	1,710
4	400	450	1055	1125	1745	1825	2455	2,535	3,165	3,250
6	470	545	1370	1500	2370	2525	3405	3,575	4,460	4,740
8	505	605	1600	1790	2875	3115	4215	4,495	5,595	5,890
10	525	640	1765	2015	3275	3610	4900	5,295	6,590	7,020
12	535	660	1880	2185	3600	4030	5485	6,000	7,460	8,035
14	540	675	1965	2320	3855	4380	5975	6,620	8,225	8,950
16	545	680	2025	2425	4065	4675	6395	7,165	8,890	9,775
18	545	685	2070	2505	4230	4920	6750	7,630	9,480	10,520
20	545	690	2100	2565	4365	5130	7050	8,060	9,995	11,190
22	545	690	2125	2610	4470	5305	7305	8,425	10,445	11,795
24	545	690	2140	2645	4560	5445	7525	8,750	10,840	12,340
26	545	690	2150	2675	4630	5575	7705	9,035	11,185	12,830
28	545	690	2160	2695	4685	5680	7860	9,280	11,490	13,270
30	545	690	2165	2715	4725	5765	7990	9,500	11,755	13,670
Very great	545	690	2180	2770	4910	6230	8725	11,075	13,635	17,305

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:

Passing No. 4 sieve	100 per cent
Passing No. 50 sieve, not more than	50 per cent
Weight removed by elutriation test, not more than	3 per cent

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.

Gradings of Coarse Aggregates
Maximum Size of Aggregate Inches	Circular Openings, Inches								Passing Screen Having Circular Openings ¼ Inch in diameter, not more than
	3	2½	2	1½	1¼	1	¾	½
3	100			40–75					15 per cent
2½		100			40–75				15 per cent
2			100			40–75			15 per cent
1½				100			40–75		15 per cent
1¼					100			35–70	15 per cent
1						100		40–75	15 per cent
¾							100		15 per cent

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.

Fig. 74.—Details of 24–Inch Concrete Pipe Form.

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.

TABLE 38
Properties of Cement Concrete Sewer Pipe
1917 Specifications of American Society for Testing Materials, with Subsequent Revisions
Internal Diameter, Inches	Laying Length, Feet	Diameter at Inside of Socket, Inches	Normal Annular Space, Inches	Depth of Socket, Inches	Taper of Socket	Minimum Thickness of Barrel, Inches	Limits of Permissible Variations					Minimum Crushing Strength, Pounds per Linear Foot at End of Diameter	Maximum Absorption, Per Cent
							Length, Inch per Foot (-)	Internal Diameter, Inches		Depth of Hub (-) Inches	Thickness of Barrel (-) Inches
								Spigot (±)	Socket (±)
6	2, 2½, 3	8¼	½	2	1 : 20	?	¼	3 16	3 16	¼	1 16	1430	8
8	2, 2½, 3	11	?	2¼	1 : 20	¾	¼	¼	¼	¼	1 16	1430	8
10	2, 2½, 3	13¼	?	2½	1 : 20	?	¼	¼	¼	¼	1 16	1570	8
12	2, 2½, 3	15?	?	2½	1 : 20	1	¼	¼	¼	¼	1 16	1910	8
15	2, 2½, 3	19¼	?	2½	1 : 20	1¼	¼	¼	¼	¼	3 32	1960	8
18	2, 2½, 3	22¾	?	2¾	1 : 20	1½	¼	¼	¼	¼	3 32	2200	8
21	2, 2½, 3	26½	¾	2¾	1 : 20	1¾	¼	5 16	5 16	¼	?	2590	8
24	2, 2½, 3	30¼	¾	3	1 : 20	2?	?	5 16	5 16	¼	?	3070	8
27	3	34	?	3¼	1 : 20	2¼	?	5 16	?	¼	?	3370	8
30	3	38	1	3½	1 : 20	2½	?	?	?	¼	?	3690	8
33	3	41½	1	4	1 : 20	2¾	?	?	?	¼	3 16	3930	8
36	3	45½	1¼	4	1 : 20	3	?	½	½	¼	3 16	4400	8
39	3	49	1¼	4	1 : 20	3¼	?	½	½	¼	3 16	4710	8
42	3	53	1½	4	1 : 20	3½	?	½	½	¼	3 16	5030	8

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.

Fig. 75.—Triangle Mesh Reinforced Concrete Pipe.
As made by the Am. Concrete Pipe and Pile Co., Chicago.

One layer or ring of reinforcement is used for sizes from 24 to 48 inches and two layers or rings for larger pipe. A type of reinforcement sometimes used is the American Steel and Wire Company’s Triangular Mesh, an illustration of which is shown in Fig. 75. The wire mesh is cut to fit and is placed in a slot in the cast-iron base. The slot is then filled with sand so that the concrete cannot enter, thus leaving a portion of the reinforcement exposed. The inside reinforcement extends through and out of the spigot of the completed pipe. In the trench the two reinforcements overlap in the key-shaped space left on the inside of the pipe by the design of the bell and spigot. This space is shown in Fig. 76 A. When the pipe is placed in the trench the key-shaped space is plastered with mortar and a piece is knocked out of the bell to receive the grout with which the joint is closed. A spring steel band is then put on the outside of the joint and grout poured into the hole at the top. The band is removed as soon as the joint materials have set.

The rules for the reinforcement of concrete pipe recommended in Volume XV, 1919, of the Transactions of the Concrete Institute are as follows:

No reinforcement is approved for pipe between 30 and 60 inches in diameter or in rock or hard soils. For pipe 36 inches in diameter or less the minimum thickness of shell shall be 5 inches. For 60–inch pipe the minimum thickness shall be 7 inches with intermediate sizes in proportion. Reinforcement for circular pipe shall consist of one or two rings of circular wire fabric or rods of the areas shown in Table 39. All sewers near the surface and subject to vibration should be reinforced. For sewers 6 feet or less in diameter the reinforcement should consist of at least ½ of 1 per cent of the area of the concrete. It should be placed near the inside at the crown and near the outside at the haunches. If large horizontal pressures are expected the pipe should be reinforced for these reverse stresses, which involves placing the reinforcement near the outside at the crown and near the inside at the haunches. The minimum thickness of the walls of sewers greater than 6 feet in diameter with flat bottom and arch, with or without side walls, should be 8 inches.

Three methods for the reinforcement of concrete sewers are shown in Fig. 76 B.

93. Proportioning of Concrete.—In the proportioning of concrete questions of strength, of permeability, and of workability^[54] may need consideration. All of these qualities are affected by the amount of cement, the nature and gradation and relative proportions of the fine and the coarse aggregate, and the amount of mixing water used.

Other things being equal the strength varies with the amount of cement put into the concrete. For the same amount of cement and the same consistency of the mixture, the strength increases with increased density of concrete (that is, with decreased voids), and the effort should be made so to proportion the fine and coarse aggregates as to produce the densest concrete (least voids) with the aggregates available. For the same consistency, the strength then will vary with the ratio of the amount of cement to the amount of the voids.

So far as the mixing water is concerned, the greatest strength in the concrete will be attained at a rather dry mix; that which produces the least volume of concrete. The addition of more water results in a concrete of less strength; 40 per cent more water may give a concrete of less than half the normal strength. The reduction in strength is then very marked for the wetter mixes, and the water content used is a feature of considerable importance in the design of concrete mixtures.

Permeability is affected by the same elements as strength, but the size and discontinuity of the pores have a greater influence.

Workability is an important quality; in some respects it will have to be obtained at the expense of strength. Increasing the amount of mixing water increases the workability of the mixtures, with a resulting decrease in strength which may have to be accepted or else overcome by increasing the cement in the mix.

An excess of water is often used unnecessarily through ignorance of the injurious results. A high proportion of coarse aggregate, up to a certain limit, will give concrete of high strength, but the mixture will be harsh-working and not easy to place. Lower proportions of coarse aggregate will give greater workability and better uniformity of product, the latter being an important matter. It is apparent that the degree of workability of the mixture needed will depend upon the nature of the construction—for a pavement where the concrete will receive substantial tamping or working the water content may be much less than that which may need to be used in placing concrete around reinforcement in narrow members, or where little tamping or spading can be done. The nature of the work will affect the standard of consistency to be specified.

The proportioning of the concrete should then be dependent upon the needs of the structure and the manner of placing the concrete. The proportions selected should be carefully adhered to and especially should care be taken to see that the right quantity of mixing water is used.

The materials are commonly measured volumetrically (by bulk). Because of the variations which are introduced by volumetric measurement of the materials by the presence of varying degrees of moisture, measurements by weight would be more accurate, but these would also be affected by differences in the specific gravity of the materials. The methods of measuring, the allowance for moisture, as well as the proportions of the materials, should be specified.

The methods for proportioning concrete are:

(1) Arbitrarily selected proportions.

(2) Proportions based on minimum voids.

(3) Proportions based on trial mixtures.

(4) Proportions based on a sieve analysis curve.

(5) Proportions based on the surface area of the aggregates.

(6) Proportions based on the water-cement ratio and the fineness modulus.

(7) Proportions based on mortar-voids and cement-voids ratio.

Arbitrarily selected proportions are in quite general use; they are intended to apply to the materials most commonly used in the vicinity of the work. The most common practice is to use twice as great a volume of coarse aggregate as fine aggregate, as for instance 1 part cement, 2 parts fine aggregate, and 4 parts coarse aggregate. Decreasing the ratio of coarse aggregate to fine aggregate may give a more easily worked mix or require relatively less water for a given workability, and in some cases it will be proper to increase this ratio and thus secure an increase of strength. Judgment and experience with given materials may warrant changes from a stated ratio. The proportions are now frequently given as one part cement to a certain number of parts of the mixed aggregate, leaving the proportions of the fine to coarse to be determined otherwise, since small variations in the relation of these will not greatly affect the strength. Proportions in common use are:^[55]

The usual method of proportioning based on minimum voids is to assume that the particles of fine aggregate should fill the voids in the coarse aggregate and that the particles of the cement will fill the voids in the fine aggregate. About 5 to 10 per cent additional fine aggregate is generally added to push the particles of the coarse aggregate apart and thus give a more easily worked concrete and one freer from void spaces. This method is inaccurate, principally because of the effect of the moisture on the volume of the voids, and because the effect on the volume by the addition of water is unknown.

Trial mixtures may be made by carefully weighing each of the ingredients and then combining them to give a workable concrete. Using a given amount of cement, the proportion of ingredients, of the same total weight, which will give the least volume and therefore the densest concrete is adopted. When making the comparison the consistency of the mixes must be maintained constant.

Proportioning may be based on an ideal sieve analysis curve of the mixed cement and aggregates. The sieve analysis of the aggregates is made by screening a predetermined weight of the sample through a series of 5 to 8 sieves graded in size from slightly below the size of the largest particle to slightly above the smallest particle of the aggregate. The analysis is then expressed in the form of a curve. The ideal curve, according to Fuller,^[56] is shown in Fig. 77.

The method of proportioning concrete by surface areas is based on the theory that the strength of a concrete depends on the amount of cement used in proportion to the surface area of the aggregates.^[57]

The proportioning of concrete on the basis of a water-cement ratio and a fineness modulus was introduced by Prof. D. A. Abrams.^[58] It is based on the theory that with fixed conditions of aggregate, moisture, etc., the ratio of water to cement determines the strength of the concrete.

A method of proportioning concrete by determining experimentally the voids in mortars made up with a given amount of sand and definite proportions of cement, and then calculating the voids in the concrete made up by adding a definite amount of coarse aggregate to the mixture, has been developed.^[59] The method is based on the theory that the strength of the concrete is a known function of the ratio of the volume of cement to the volume of the voids in the concrete. The effect of varying the proportion of the ingredients, including an increase in the amount of mixing water beyond that required to give the densest mixture, may be found by the method, and a comparison may be made of results obtainable with different classes of fine and coarse aggregates.

Arbitrarily selected proportions, proportions based on voids, and proportions based on trial mixtures are usually satisfactory for small jobs where the amount of materials involved is not large. Where the saving in materials will permit, more accurate methods should be used. The methods can be studied more fully by reference to the original articles quoted in the footnotes, or to the following texts:

94. Waterproofing Concrete.—The waterproofing of concrete is most satisfactorily done by making dense mixtures. In practice such substances as hydrated lime, clay, alum and soap, and proprietary compounds such as Ceresit, Medusa, etc., are frequently mixed with the concrete under the theory that these very fine substances will fill any remaining voids and render the concrete impervious. The specifications of the Joint Committee issued on June 4, 1921, are much briefer and contain less detailed instruction than those issued earlier.^[60] The earlier instructions follow.

Many expedients have been resorted to for making concrete impervious to water. Experience shows, however, that when mortar or concrete is proportioned to obtain the greatest practicable density and is mixed to the proper consistency, the resulting mortar or concrete is impervious under moderate pressure.

On the other hand concrete of dry consistency is more or less pervious to water, and, though compounds of various kinds have been mixed with the concrete or applied as a wash to the surface, in an effort to offset this defect, these expedients have generally been disappointing, for the reason that many of these compounds have at best but temporary value, and in time lose their power of imparting impermeability to the concrete.

In the case of subways, long retaining walls, and reservoirs, provided the concrete itself is impervious, cracks may be so reduced, by horizontal and vertical reinforcement properly proportioned and located, that they will be too minute to permit leakage, or will be closed by infiltration of silt.

Asphaltic or coal tar preparations applied either as a mastic or as a coating on felt cloth or fabric, are used for waterproofing, and should be proof against injury by liquids or gases.

For retaining and similar walls in direct contact with the earth, the application of one or two coatings of hot coal tar pitch, following a painting with a thin wash of coal tar dissolved in benzol, to the thoroughly dried surface of concrete is an efficient method of preventing the penetration of moisture from the earth.

Tar paper and asphaltic compounds are not often used in sewer work as absolute imperviousness is seldom necessary.

95. Mixing and Placing Concrete.—Careful workmanship is desirable in the mixing and placing of concrete in sewers since water-tight construction is desired. Because of the difficulty of inspecting concrete in wet, dark and crowded excavations, and the careless habits of workmen experienced in concrete sewer construction, the highest class of concrete work cannot be expected. The situation is met by designing thick walls as shown in the sections illustrated in Fig. 22 and 23.

In the report of the Joint Committee on Concrete and Reinforced Concrete in Transactions of the American Society of Civil Engineers for 1917, on page 1101 the recommendation is made concerning the mixing and placing of concrete as follows:^[61]

The mixing of concrete should be thorough and should continue until the mass is uniform in color and is homogeneous. As the maximum density and greatest strength of a given mixture depends largely on thorough and complete mixing, it is essential that this part of the work should receive special attention and care.

Inasmuch as it is difficult to determine by visual inspection whether the concrete is uniformly mixed, especially where aggregates having the color of cement are used, it is essential that the mixing should occupy a definite period of time. The minimum time will depend on whether the mixing is done by machine or hand.

(a) Measuring Ingredients: Methods of measurement of the various ingredients should be used which will secure at all times separate and uniform measurements of cement, fine aggregate, coarse aggregate and water.

(b) Machine Mixing: The mixing should be done in a batch machine mixer of a type which will insure the uniform distribution of the materials throughout the mass, and should continue for the minimum time of 1½ minutes after all the ingredients are assembled in the mixer. For mixers of 2 or more cubic yards capacity, the minimum time of mixing should be 2 minutes. Since the strength of the concrete is dependent on thorough mixing, a longer time than this minimum is preferable. It is desirable to have the mixer equipped with an attachment for automatically locking the discharging device so as to prevent the emptying of the mixer until all the materials have been mixed together for the minimum time required after they are assembled in the mixer. Means should be provided to prevent aggregates being added after the mixing has commenced. The mixer should also be equipped with water storage, and an automatic measuring device which can be locked if desired. It is also desirable to equip the mixer with a device recording the revolutions of the drum. The number of revolutions should be so regulated as to give at the periphery of the drum a uniform speed. About 200 feet per minute seems to be the best speed in the present state of the art.

(c) Hand Mixing: Hand mixing should be done on a water-tight platform and especial precautions taken after the water has been added, to turn all the ingredients together at least 6 times, and until the mass is homogeneous in appearance and color.

(d) Consistency: The materials should be mixed wet enough to produce a concrete of such a consistency as will flow sluggishly into the forms and about the metal reinforcement when used, and which at the same time can be conveyed from the mixer to the forms without separation of the coarse aggregate from the mortar. The quantity of water is of the greatest importance in securing concrete of maximum strength and density; too much water is as objectionable as too little.

(e) Retempering: The remixing of concrete and mortar that has partly reset should not be permitted.

Placing Concrete

(a) Methods: Concrete after the completion of the mixing should be conveyed rapidly to the place of final deposit; under no circumstances should concrete be used that has partly set.

Concrete should be deposited in such a manner as will permit the most thorough compacting such as can be obtained by working with a straight shovel or slicing tool kept moving up and down until all the ingredients are in their proper place. Special care should be exercised to prevent the formation of laitance; where laitance has formed it should be removed, since it lacks strength and prevents a proper bond in the concrete.

Care should be taken that the forms are substantial and thoroughly wetted (except in freezing weather) or oiled, and that the space to be occupied by the concrete is free from all debris. When the placing of concrete is suspended, all necessary grooves for joining future work should be made before the concrete has set.

When work is resumed concrete previously placed should be roughened, cleansed of foreign material and laitance, thoroughly wetted and then slushed with a mortar consisting of one part Portland cement and not more than 2 parts of fine aggregate.

The surfaces of concrete exposed to premature drying should be kept covered and wet for at least 7 days.

Where concrete is conveyed by spouting, the plant should be of such a size and design as to insure a practically continuous stream in the spout. The angle of the spout with the horizontal should be such as to allow the concrete to flow without separation of the ingredients; in general an angle of about 27 degrees or 1 vertical to 2 horizontal is good practice. The spout should be thoroughly flushed with water before and after each run. The delivery from the spout should be as close as possible from the point of deposit. Where the discharge must be intermittent, a hopper should be provided at the bottom. Spouting through a vertical pipe is satisfactory when the flow is continuous; when it is checked and discontinuous it is highly objectionable unless the flow is checked by baffle plates.

(b) Freezing Weather: Concrete should not be mixed or deposited at a freezing temperature, unless special precautions are taken to prevent the use of materials covered with ice crystals or containing frost, and to prevent the concrete from freezing before it has set and sufficiently hardened.

As the coarse aggregate forms the greater portion of the concrete, it is particularly important that this material be warmed to well above the freezing point.

The enclosing of a structure and the warming of a space inside the enclosure is recommended, but the use of salt to lower the freezing point is not recommended.

(c) Rubble Concrete: Where the concrete is to be deposited in massive work, its value may be improved and its cost materially reduced by the use of clean stones saturated with water, thoroughly embedded in and completely surrounded by concrete.

(d) Under Water: In placing concrete under water, it is essential to maintain still water at the place of deposit. With careful inspection the use of tremies, properly designed and operated, is a satisfactory method of placing concrete through water. The concrete should be mixed very wet (more so than is ordinarily permissible) so that it will flow readily through the tremie and into place with practically a level surface.

The coarse aggregate should be smaller than ordinarily used and never more than one inch in diameter. The use of gravel facilitates the mixing and assists the flow. The mouth of the tremie should be buried in the concrete so that it is at all times entirely sealed and the surrounding water prevented from forcing itself into the tremie. The concrete will then discharge without coming in contact with the water. The tremie should be suspended so that it can be lowered quickly when it is necessary either to choke off or to prevent too rapid flow. The lateral flow preferably should not be over 15 feet.

The flow should be continuous in order to produce a monolithic mass and to prevent the formation of laitance in the interior.

In case the flow is interrupted it is important that all laitance be removed before proceeding with the work.

In large structures it may be necessary to divide the mass of concrete into several small compartments or units to permit the continuous filling of each one. With proper care it is possible in this manner to obtain as good results under water as in the air.

A less desirable method is the use of the drop bottom bucket. Where this method is used the bottom of the bucket should be released when in contact with the surface of the place of deposit.

Concrete sewers should be constructed in longitudinal sections in a continuous operation without interruption for the entire invert, side walls, or arch. In pouring the concrete it should be kept level in the forms and should rise evenly on each side of the sewer. All rough places in the concrete should be finished smooth by brushing with a grout of neat cement and water and honeycombs should be filled with neat cement or a one-to-one mortar.

96. Sewer Brick.—The quality of brick used in sewers is seldom specified with the minute care that is taken in the specifications for concrete, iron, and certain other materials of construction, as inferior materials in brick are more easily detected. The specifications of the Baltimore Sewerage Commission for sewer brick are:

Sewer brick shall be whole, new bricks of the best quality, of uniform standard size, with straight and parallel edges and square corners: they shall be of compact texture, burned hard and entirely through, free from injurious cracks and flaws, tough and strong, and shall have a clear ring when struck together. The sides, ends and faces of all bricks shall be plane surfaces at right angles and parallel to each other. Bricks of any one make shall not vary more than 1
16th of an inch in thickness, nor more than 1?th of an inch in width or length, from the average of the samples submitted for approval.

The truest bricks shall be used in the face of the masonry and the exposed surfaces shall be true and smooth planes.

All bricks delivered for use shall be culled by the Contractor when required. No brick thrown out in the culling shall be used in any work done under any contract of the Sewerage Commission, except that the best of the culls may be used in manholes, above the level of the top of the sewer, if permitted by the Engineer.

The average amount of water absorbed by the bricks, after being thoroughly dried and then immersed for 24 hours, shall not exceed 6 per cent. All bricks shall be uniform in quality and percentage of absorption.

Whenever vitrified bricks are required in the invert of the sewer, they shall be smooth, hard, tough, and of such durability as will fit them for this use. They shall be of standard size, well and uniformly burned, thoroughly vitrified throughout, and free from warps, cracks, and other defects. The surfaces and edges shall be true and straight and the corners sharp and square. They shall be in every respect satisfactory to the Engineer, and in all respects equal to the sample in the office of the Engineer.

The remaining paragraphs of the specifications deal with the manner in which samples shall be submitted and the necessity for conformity between the samples submitted and the bricks used.

A common size of brick in use for sewers is 2¼ × 4 × 8¼ inches, but the variations in size are many. The bricks in use on any one job should be as near the same size as possible as the extra mortar filling necessary to make up for small brick detracts from the strength of the sewer. Small brick are undesirable as the cost of laying small and large bricks is the same, but the thickness of the finished sewer is less. Sewer brick should not absorb more than 10 to 20 per cent moisture by volume, in 24 hours; except the special paving brick used to prevent erosion at the invert which should absorb less than 5 per cent moisture.

97. Vitrified Sewer Block.—Blocks and bricks are manufactured in a manner similar to the manufacture of vitrified sewer pipe described in Art. 91. J. M. Egan describes two types of sewer blocks^[62] as follows:

There are on the market two designs of blocks, one being a single-ring block and the other a double-ring block. The former has a ship-lap joint on the ends and a tongue-and-groove joint on the sides. In the double block the laps and joints are made in the construction of the sewer and the blocks are placed one on top of the other as in a two ring brick sewer. The blocks are hollow longitudinally with web braces. They are made for sewers from 30 inches to 108 inches in diameter and weigh from 40 to 120 pounds. They are 18 inches to 24 inches long, 9 to 15 inches wide, and 5 to 10 inches thick. Short lengths are made for convenience in construction and for use on sharp curves. Special blocks are made for connections and junctions.

A special block is also made for inverts, which has occasionally been used with brick sewers to avoid the difficulty of constructing with brick at this point. Such blocks are objectionable, as they leave a line of weakness along the longitudinal joint so formed. They are not used frequently in present day practice.

Vitrified blocks are generally cheaper than bricks, but they do not make so strong a structure. In some cases it is possible to lay vitrified block without the expense of high-priced bricklayers, thus saving on the cost of the sewer and obtaining a conduit with a smoother interior finish.

98. Cast Iron, Steel, and Wood.—Cast iron, steel, and wood pipe belong more to the field of waterworks than of sewerage, as they are not extensively used in the construction of sewers. There are, however, some special conditions under which these materials may be serviceable.

The iron used in cast-iron pipe for sewers, and in castings for manhole covers, inlet frames, etc., is seldom carefully or definitely specified. The standard specifications of the American Water Works Association with regard to the quality of iron for water pipe are:

All pipe and special castings shall be made of cast iron of good quality and of such character as shall make the metal of the castings strong, tough, and of even grain and soft enough to satisfactorily admit of drilling and cutting. The metal shall be made without the admixture of cinder iron or other inferior metal, and shall be remelted in a cupola or air furnace.

The specifications of the Sanitary District of Chicago for the quality of iron to be used in manhole covers, etc., are given on page 101.

Although sewer pipes are not ordinarily subjected to internal pressure, cast-iron pipe for sewers should be as heavy or heavier than water pipe to resist the corrosive action of the sewage and the external stresses that are to be imposed upon it. The sizes and details of standard cast-iron pipe used for both water works and sewerage can be found in specification of the American and New England Water Works Associations.

The quality of steel used for reinforcing concrete should be carefully specified because of the possibility of the substitution of inferior material. The specifications for “Billet Steel Concrete Reinforcement Bars,” of the American Society for Testing Materials^[63] are the standard for engineering practice, or the following specifications may be used:

All reinforcement shall be free from excessive rust, scale, paint, or coatings of any character which will tend to destroy the bond. The bars shall be rolled from new billets. No rerolled material will be accepted. All reinforcement bars shall develop an ultimate tensile strength of not less than 70,000 pounds per square inch. The test specimen shall bend cold around a pin, whose diameter is two times the thickness of the bar, 180 degrees without cracking on the outside portion. The reinforcing bars shall in all respects fulfill the requirements of the standard specifications of the American Society for Testing Materials for Billet Steel Concrete Reinforcing Bars serial designation A 15–14.

The steel used in pipe should be a soft, open-hearth steel with an ultimate tensile strength of 60,000 pounds per square inch, an elastic limit of 30,000 pounds per square inch, an elongation in 8 inches before fracture between 22 and 25 per cent, and a reduction in area before fracture of 50 per cent. The working strength of the steel is taken at 16,000 to 20,000 pounds per square inch in tension, 10,000 to 12,000 pounds per square inch in shear, and 20,000 to 24,000 pounds per square inch in bearing. A liberal allowance should be made for corrosion. The standard specifications for Open-Hearth Boiler Plate and Rivet Steel of the American Society for Testing Materials, Aug. 16, 1919, include “flange steel,” which is suitable for the manufacture of plates, and extra soft steel which is suitable for rivets.

Steel pipe should be coated both inside and out to protect it against corrosion. The various proprietary coatings are mainly coal tar pitches, or mixtures of coal tar pitch and asphalt. A coal tar pitch is a distillate of coal tar from which the naphtha has been removed and to which about one per cent of heavy linseed oil has been added. The coating is applied to the pipe at a temperature of about 300 degrees Fahrenheit, by dipping hot pipe in the heated coating material. The pipe should be carefully cleaned and all rust and scale removed before it is dipped. In some cases the steel is pickled before dipping. This consists in rolling the cold plates to a short radius to loosen the scale, heating them to about 125 degrees, and dipping them in a warm 5 per cent acid solution for about 3 minutes, and finally rinsing in a weakly basic wash water.

The woods commonly used for the manufacture of wood pipe are spruce, Oregon fir, Douglas fir, and California redwood. Wood pipe lines have been constructed of other kinds of lumber but only in more or less unusual conditions. The following has been abstracted from the specifications for California redwood given by J. F. Partridge.^[64]

The staves shall be of clear, air-dried, California redwood, seasoned at least one year in the open air, and shall be free from knots (except small knots appearing on one face only), sap, dry rot, wind shakes, pitch, pitch seams, pitch pockets, or other defects which would materially impair their strength or durability. The sides of the staves shall be milled to conform to the inside and outside radii of the pipe; and the edges shall be beveled to true radial planes. The staves shall be milled from stock sizes of lumber, the net finished thickness of the stave, for the various diameters of pipe, shall be as given in Table 40. The ends shall be cut square and slotted to receive the metallic tongues which form the butt joints. The slots shall appear in the same position on each stave, and shall be cut to make a tight fit with the tongues in all directions. The staves shall have an average length of at least 15 ft. 6 in. and not more than one per cent shall have a length of less than 9 ft. 6 in. Staves shorter than 8 ft. will not be accepted.

The bands shall be spaced on the pipe with a factor of safety of at least four, and shall consist of round, mild steel rods, connected with malleable iron shoes. Either open-hearth or Bessemer steel may be used.... The ultimate strength shall be from 55,000 to 65,000 lb. per sq. in.

The original reference should be consulted for complete details and for specifications for various kinds of wood and classes of pipe. The discussion following the specifications is of value.

Machine-made wood pipe is superior to stave pipe put together in the field. It is seldom manufactured in sizes large enough for use in sewers, which results in the almost exclusive use of field constructed stave pipe. The steel bands used to hold the staves together should be coated similarly to steel plates. All lumber, except California redwood should receive a preservative coating of creosote^[65] or other material. One of the best methods of preserving the wood is to keep it submerged and to maintain the pipe under internal pressure.