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 1⅛th 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.