BITUMINOUS CONCRETE PAVEMENT
71. Preparation of Foundation.—Where a bituminous concrete wearing surface is to be placed upon an old pavement or Macadam road the specifications must be closely followed. A good foundation is as necessary, if good results are to be attained, as in the case of other pavements. The dressing down of high points and the filling up of depressions in the old pavement should be carefully done, so that the bituminous wearing surface shall be of uniform thickness, and shall be rigidly supported at all points.
72. Bituminous Concrete Surface.—The instructions given for the manufacture and laying of sheet asphalt pavement should be followed here insofar as they are applicable.
HYDRAULIC CONCRETE ROADWAY PAVEMENT
73. Like other structures made of hydraulic concrete, the utility and durability of concrete roadway pavement depends largely upon the good quality of the materials used and the skill and fidelity with which the work is done. It is especially important that the second, or surface, course of the concrete shall be made and placed in strict accordance with the specifications and that a high degree of uniformity shall be secured in the composition, consistency and workmanship of that part of the work.
74. Sub-foundation and Foundation.—The preparation of the sub-foundation will be the same as for other pavements, and the first, or foundation course, of concrete, will be constructed as in the case of the concrete foundation for other pavements.
75. Material for Surface-course.—It may be assumed that, in general, the materials intended to be used on the work have been inspected and approved by the Engineer, but this should not prevent you from calling his attention to any defective or inferior materials that may be delivered on the street when the work is begun, or thereafter, and preventing the use of any materials that are not fully up to the requirements of the specifications.
76. Mixing the Surface Course.—See that the cement sand and stone are proportioned accurately and that the quantity of water used with each batch is measured, so as to make the concrete of uniform consistency. Do not permit any defective batches of concrete to be used in the surface-course—if suitable for the bottom course they may be used in it. Care in this respect is particularly necessary with machine-mixed concrete where the machine is not working normally or the men operating it are careless or unskillful.
77. Placing Surface Course Concrete.—See that the surface course is placed, graded and rammed before the bottom course concrete begins to set. This is imperative. Also that the concrete is distributed and graded in such a way as will not separate or segregate the mortar from the stone; that the grading of the surface is so accurate that it will not be necessary later to add additional concrete to that already graded and rammed; that the ramming is thorough and uniform over the whole surface, and that the rolling is well done.
78. Expansion Joints.—Care should be taken to have the expansion joints made as the specifications require. See that they extend entirely through both courses of concrete; that the corners are properly compacted and troweled; that the joints are kept clean until they are filled with the bituminous cement, and are completely filled with the cement.
79. Care of Finished Work.—Do not permit the completed work to be disturbed by travel over it, and see that the concrete is kept moist, until it shall be set up hard, and that the street is not opened for travel until the engineer so directs.
CONCRETE COMBINED CURB AND GUTTER
80. General.—The specifications for the construction of Hydraulic Concrete Combined Curb and Gutter are quite full and clear and the duty of the inspector will consist mainly in seeing that they are faithfully carried out.
81. Concrete.—As in other concrete construction, the utility of the work depends largely on the care and skill with which the concrete is made and placed. See that the prescribed quality and ratio of materials are used and that the concrete is thoroughly mixed and properly placed in the forms and well tamped. Especial care is necessary to secure a continuous and satisfactory exposed surface by forking and working the mortar into contact with the forms, which must be placed and maintained in true line and surface.
Concrete Combined Curb and Gutter
82. Removing Forms.—Good judgment is required as to the proper time to remove the forms. They must remain until the concrete has set hard enough to be fully self-sustaining, but before it has set so hard that the wire brush will have no effect on the surface.
83. Corner Protection.—See that the metallic corner or nose piece is correctly placed and that it is solidly anchored by and fully embedded in the body of the concrete.
84. Patching.—The practice of patching up cavities or irregularities in the exposed face of the curb and gutter with neat mortar, or dressing the surface with dry cement, must not be permitted. If a section of the curb is found, when the forms are removed, to be imperfect, the whole section must be removed and replaced.
HYDRAULIC CEMENT SIDEWALKS
85. General.—While the work of constructing concrete sidewalk is comparatively simple it is often carelessly and unskillfully done. The things that need most attention by the inspector are the following:
86. Materials.—The materials used in the work must be fully up to the quality called for by the specifications. This applies more particularly to the cement and sand. These are not always properly tested by the engineer and you should frequently make samples of stiff mortar (1 cement, 2 sand) and set them aside for observation. If they do not, in summer weather, become very hard at the end of nine hours, the fact should be reported to the engineer.
87. Drainage.—See that the necessary grading is properly done and that the drainage course is made of suitable material properly compacted. See that drain tiles are properly laid and connected as designed by the engineer. If cinders are used for the drainage course, see that they are screened to remove ashes and fine material, and that they are thoroughly drenched with water at least five days before they are placed in the walk.
88. Two-course Work.—If the sidewalk is laid in two courses, see that the surface-course is put on and tamped before the bottom-course concrete has begun to set. This requirement must be strictly enforced.
89. Finishing the Surface.—Dry or pure cement must not be used for trimming up or smoothing off the surface of the walk. After the surface has been properly completed by straight-edge and trowel, see that the wire broom is used as directed to remove the glaze and to slightly roughen the surface.
See that the expansion joints are made as specified, and that they extend entirely through both courses of concrete.
90. See that the walk, after completion is properly protected from injury and from frost, and that the concrete is kept moist until it becomes well set.
[1]. In specifications to be used in any particular city the official name of the city government, as the City Council, the Commissioners of Public Works, etc., should be used instead of this general designation.
[2]. Such a proviso as this seems proper in justice to both the city engineer and the contractor; the former should not be held responsible for the acts of his assistants when they transcend the authority conferred upon them, and the latter should be put upon his guard with reference to requirements which he is not satisfied are sanctioned or approved by the city engineer.
[3]. It may be objected that this requirement is unusual and unnecessary, since such practices are recognized as wrong, and as presumptive of fraud and malpractice on the part both of the contractor and the inspector. It cannot, however, be denied that in many cities such means are employed by contractors to unduly influence the action of inspectors and that not infrequently the latter not only accept, but persistently demand, valuable considerations from the contractor. Silence of the specifications on this point cannot, of course, be construed into consent, but there is no good reason for the silence. There should be left no excuse for misconception of the position of the city or of the engineer upon this point.
[4]. This section is intended as much for the control and limitation of city officials as of contractors. The practice of carelessly or purposely allowing municipal contracts to be expanded greatly beyond the stated limits or the original intended volume and cost without formal authorization by the proper municipal body in which the power to make contracts is lodged, is dangerous and wrong and should be prohibited. In one instance coming to the notice of the author a contract originally intended to cover $50,000 worth of work was expanded by the department head without any authority from the city council until the final estimate reached the enormous sum of over $400,000.
[5]. As outlined in the introduction, these specifications are designed to secure the construction of the pavement in a proper manner, the city assuming responsibility for the character and utility of the work. The guarantee here proposed is therefore intended to cover only a proper compliance with the specifications, for which the contractor may properly be held responsible, and not the sufficiency or utility of the work, if constructed according to the specifications. The period of guaranty should therefore be short, not exceeding two years.
[6]. In the great majority of cases the most satisfactory and, in the end, the most economical foundation for a pavement is hydraulic cement concrete. Old paving-block foundation, if constructed as specified in Sect. 36, will give results equally as satisfactory, but if a fair market exists for the blocks taken up from the street, it will usually be found more economical to sell them and construct a concrete foundation for the new pavement. Broken stone or gravel foundations may serve the purpose fairly well upon a street of light travel, but it should never be used on streets of considerable or heavy travel. Its lower first cost is the only thing in its favor, but this will, in nearly every case, be more than offset by the better service and greater durability of the pavement, even on streets of light travel, if laid upon an adequate hydraulic concrete foundation. Proper repairs to pavement surfaces cut into for pipe work, etc., are difficult to make and hardly ever satisfactory over broken stone foundation for the reason that the lack of cohesion in the material allows it to loosen or crumble away from under the edge of the pavement surface, and it is difficult to restore it to its original solidity and strength. The first cost saved by its use is usually not great; for whenever its use would be permissible at all, a comparatively thin and lean concrete would give better results, at a very slight increase in cost. To illustrate: On a suburban street with light travel a concrete foundation four inches in thickness, the concrete made with Portland cement in the ratio of 1 cement, 4 sand, and 8 stone, would be stronger and in every way better than a foundation eight inches in depth of broken stone. At the usual prices of materials and labor, the former may cost about $0.46 per sq. yd., and the latter about $0.40 per sq. yd.; but for the latter there would be required ⅑ cu. yd. more sub-foundation excavation, worth about four cents, so that the equivalent cost would be $0.44 per sq. yd. The difference, two cents per sq. yd., is insignificant when compared with the greater value, better service, and greater durability of a pavement on the concrete foundation. It is sometimes held that the broken stone foundation provides necessary sub-drainage. But all the standard pavements are, or soon become, impermeable to water from the surface, and seepage from the sub-foundation can be better taken care of by the sub-drainage specified in Sect. 26, which should usually cost not more than five cents per square yard of the pavement; and if drainage be required, these sub-drains should be used even with the broken stone foundation.
The practice of laying pavement surfaces, particularly those of asphalt, upon a foundation of old stone blocks, carelessly reset, with the joints unfilled with mortar, is all wrong and should never be resorted to. The integrity and durability of an asphalt pavement depends largely upon the strength and rigidity of its foundation; to lay an asphalt surface, however good, over such an old block foundation, is an inexcusable waste of money.
Old stone block and cobble-stone pavements, that have become solidified in place by long travel over them, make a good foundation for asphalt or other pavements, provided they can be utilized without taking up or disturbing the old pavement; but such cases occur so rarely that they have not been considered in these specifications.
A thoroughly consolidated old McAdam pavement, if not worn too thin, also makes a very satisfactory pavement foundation if it can be used undisturbed, or by simply trimming off the high points.
Low places in old pavements, that are otherwise satisfactory for a foundation, may be brought to the proper elevation with hydraulic concrete. “Binder” material is usually specified for this purpose in foundations for asphalt pavements, but hydraulic concrete is both better and cheaper.
[7]. The specifications for Portland cement here given are practically those adopted by the “American Society for Testing Materials.”
Natural Cement. While these specifications uniformly refer to the use of Portland cement, it is not intended to convey the idea that natural cement concrete is not suitable for pavement foundations; on the contrary, it may be used with entire confidence, as the experience in a large number of cities has proven beyond question. Whether Portland or natural cement shall be used is usually a question of relative cost. At the present very low prices of Portland cement in most cities, more strength in pavement foundations can usually be obtained per dollar expended for cement, from Portland than from natural cement. The specifications for natural cement, as adopted by the American Society for Testing Materials differ from those for Portland cement in the following particulars:
The specific gravity shall not be less than 2.8.
Fineness. The residue left on a No. 100 sieve shall not exceed 10 per cent., and on a No. 200 sieve shall not exceed 30 per cent.
Setting. It shall not begin to set in less than ten minutes, nor set hard in less than thirty minutes; but shall set hard within three hours.
Tensile Strength (per sq. in.).
| Neat. | 24 | hours in moist air | 50 to 100 lbs. |
| 7 | days (1 day in air, 6 days in water) | 100 to 200 lbs. | |
| 28 | days (1 day in air, 27 days in water) | 200 to 300 lbs. | |
| 1 part cement, 3 parts sand. | |||
| 7 | days. (1 day in air, 6 days in water) | 25 to 75 lbs. | |
| 28 | days. (1 day in air, 27 days in water) | 75 to 150 lbs. | |
Soundness. Standard pats kept in air and in water should remain firm and hard and show no signs of cracking or disintegration.
[8]. The frequent requirement that the fine material shall be screened out, is not necessary or advisable. Experiments and experience have shown conclusively that unless an unusual amount of fine material and “dust” be present, or unless this fine material be allowed to separate and aggregate in masses by itself, the resulting concrete is improved rather than deteriorated by its presence.
Where there is an unusual excess of “dust” in the crushed stone, the quantity of sand used in the concrete should be decreased accordingly.
[9]. Many specifications do not require this and in a number of cities where the specifications do require it, contractors habitually neglect to comply. When stone and sand are deposited directly upon the earth, it is very difficult to avoid taking up earth and mud with the materials, particularly when the street is wet and muddy. Lumps of soil and débris unquestionably injure the concrete. The cost of providing a lumber floor is comparatively small, as the plank may be used over and over again. Specifications should, therefore, contain this requirement and it should be enforced.
[10]. The ratios of the materials may appropriately be varied with the strength and soundness of the sub-foundation, the amount of travel on the street, and with the thickness of foundation it is proposed to use. Where good materials are used and the work is properly done, a 1:3:5 concrete six inches thick is sufficient for streets of the heaviest travel. For streets of light travel a 1:5:9 concrete will usually give entirely satisfactory results. The most economical thickness for a concrete foundation is an important consideration. The strength of concrete may be said to increase, within usual limits of practice, with the ratio of cement in it. The strength of concrete beams or slabs increases in the ratio of the square of their depth. To secure a required amount of strength in a pavement foundation, we may therefore vary the richness of the concrete and the depth of the foundation so as to secure the requisite strength at the least total cost of materials and labor. This will be influenced by the cost of materials and labor in each particular locality.
Within certain workable limits there is no reason why the same principles of proportioning the strength of a pavement foundation to the work required of it should not be applied as are employed in designing other engineering structures.
The practice, usual in many cities, of adopting general specifications requiring a standard thickness of foundation and composition of concrete, and applying these to all streets, regardless of the quantity and character of travel which the pavement is expected to carry, is illogical and often very wasteful. If such a standard foundation is sufficient for the streets of heaviest travel, it is obviously a sheer waste of money to use it on the suburban streets carrying the lightest travel. It is therefore better in preparing standard specifications for pavement in any city to leave blanks for the ratios of the concrete and for the thickness of the foundation, to be filled in, in each individual case, as the judgment of the engineer may dictate.
While it is important that the foundation of any pavement shall be adequate, it is inexcusable to waste money in providing superfluous strength. For the great majority of suburban streets, carrying but little except the local travel, a foundation four inches thick made of good Portland cement concrete in the ratios of 1:4:8 will prove entirely satisfactory. Hundreds of such streets paved over a foundation of that thickness, made of natural cement concrete in the ratios of 1:2:4 can be cited where the foundation has proved entirely satisfactory.
The character and firmness of the sub-foundation must, of course, be taken into consideration in designing the foundation.
[11]. The routine here described produces better concrete with less expenditure of labor, than the one often followed of putting all the dry materials on the concrete board before any mixing is begun. The writer has proved this from actual records covering a large quantity of work.
[12]. The objections to using mortar for plastering over the concrete are: that it is more costly than concrete; that the two materials may, under certain conditions, separate and the thin mortar surface break up under travel; that, if permitted, the mortar may be used to cover up defective concrete, and that in the case of asphalt pavements the pavement surface is more likely to “shift” on the smooth surface of the mortar than on the rough surface of the concrete. The practice of going over the fresh concrete with street brooms should not be permitted. The only argument in favor of it is that it may be used to conceal defective patches in the concrete.
[13]. See foot-note[[6]], p. [23]. The cost of filling the joints of old block pavement with mortar or grout is considerable. It will hardly ever be less than 20 cents and may exceed 35 cents per square yard, depending on the volume of joints and the local cost of material and labor. The cost of resetting and ramming the blocks with proper care will usually be from 10 cents to 12 cents per square yard, so that the cost of the foundation, exclusive of the value of the blocks, may vary from 30 cents to 47 cents per square yard.
As a good concrete foundation 6 inches in depth can be laid for from 70 to 90 cents per square yard, it is obvious that if the old blocks can be sold for as much as the difference between the cost of the old block and the concrete foundation, nothing will be saved by using the old block foundation. In at least one city, asphalt pavement has been extensively laid over old stone block foundation relaid in a very careless manner, the joints being filled usually with the old sand or loam found in the street. This practice cannot be too strongly condemned. Asphalt pavement surfaces resting on such a foundation are necessarily short-lived and unsatisfactory. The practice of opening the street to travel for a period after the blocks are relaid and before the asphalt surface is applied, helps, under favorable conditions, to consolidate the foundation, but does not remove the objections to it. If heavy rains intervene, the sub-foundation becomes saturated with water, and its resistance so reduced that the stone blocks settle out of shape, particularly in soft spots, and they are usually hastily raised and reset just before the asphalt surface is applied. The result is an insecure foundation fatal to the durability and usefulness of the pavement.
[14]. Where there is a possibility that gravel may be used, the contractor should be asked to name prices for the gravel foundation as well as the stone foundation, since, unless this be done, the change from the one material to the other might be held to be illegal.
[15]. There has been much discussion as to the suitableness of these oil asphalts, called “residual pitches,” for use in making asphalt pavements. When properly prepared from suitable asphaltic oils, so as to comply with the specifications here given, there can be no doubt that good pavements can be made with them. But as they appear on the market, being usually produced at different localities and refineries from crude oils of differing qualities, distilled by somewhat differing methods, and usually at temperatures of from 900 degrees to 1200 degrees, they are likely to vary so greatly in quality as to make their use inadvisable without careful technical inspection. Unless, therefore, the engineer is prepared to make, or to have such inspection made, it is hardly wise or safe to permit their use. They stand, in this respect, upon a footing different from the better-known natural asphalts obtained from large deposits of practically uniform character and quality, where the simpler process of refining is less likely to effect injuriously the chemical quality of the material.
It may be confidently predicted that any of these “residual pitches” which comply with these specifications will, if properly handled, make a good pavement.
[16]. The possibility that some of these compounds or artificial asphalts, may be suitable for use in pavements is not denied. But in the absence of a fuller knowledge of them than we now have, and in the light of present experience, the only safe course is to reject them.
[17]. It is not intended here to enter into an extended discussion of the fact that some asphalts are injuriously affected by water, and the bearing which this fact should have upon the selection of an asphalt for pavement purposes. There can be no doubt that modern treatment and methods of construction have tended to diminish but not wholly to prevent the disintegrating effect of water upon pavements made with such asphalt, and if the engineer could be certain that his pavements would be constructed by contractors guided by long experience and the best expert advice, he might perhaps safely disregard this provision. Since in practice he can have no such assurance, the provision is a wise one and it does not involve any serious or material hardship to the contractor.
[18]. It is now well recognized that the character and quality of the sand used is one of the most important elements in determining the utility and durability of an asphalt pavement. A satisfactory sand should be insisted on, even if it involves a very considerable increase in the first cost of the work.
While our knowledge of the subject is not complete, experience seems to indicate pretty clearly that a sand of the quality and size-grading here specified as typical, may be depended upon to produce a good pavement.
[19]. The use of Portland cement in surface mixtures to be laid on streets of heavy travel, or those exposed to damp foundations, is very strongly recommended. On streets of the heaviest travel, or where the pavement will be exposed to unfavorable conditions of dampness, particularly if the pulverized stone is not very finely ground, the Portland cement may constitute twenty per cent. of the pulverized material, or “dust,” as it is commonly called. Ordinarily from five to ten per cent. may be used to advantage on all streets of moderately heavy travel.
[20]. For determining the consistency of individual batches of asphalt paving cement with a standard sample, the flow-plate method is most convenient and sufficiently accurate. For a description of the latest improved form of this apparatus see Engineering News of Aug. 22, 1912, p. 347. It can only be used, however, where the cements to be compared are of the same general composition—the same refined asphalt and tempering agent—as the standard.
[21]. The practical value of the exact determination of the ductility of asphalt paving cements is regarded by the author as not well established. It is customary, however, in most recent specifications to require it. Some asphaltic cements that have been quite successfully used for pavements have not complied with such requirements, while good coal-tar pitches will greatly exceed them. Where this test is used the usual specification requires that a briquette of asphaltic cement having a cross-section of one square centimeter, at penetration 50, shall elongate to the extent of not less than 20 nor more than 85 centimeters at 77° F. If the asphaltic cement varies from 50 penetration an increase or decrease of at least 2 centimeters will be required for each five points above or below 50 penetration (Dow apparatus to be used).
[22]. It should be noted, in a general way, that there is quite a difference in the temperature to which different asphalts may be safely subjected.
Trinidad asphalt, and the California residual pitches, will not be injured by the higher range of temperatures named in these specifications, while Bermudez and some other asphalts should be worked at as nearly as possible to the lower range of temperatures named. The relative amount of loss of the different asphalts when exposed for seven hours to the temperature of 325 degrees (Sect. 44) will supply a rough practical guide as to the temperature permissible—the greater the loss, the closer should the lower range of temperatures be adhered to.
[23]. The open base-course, or “binder,” composed only of crushed stone, coated with pitch or asphaltic cement, extensively and almost exclusively used up to a recent date, is no longer advocated by the best authorities on asphalt pavements. The aim is now to make the base-course as dense and strong as the surface-course.
[24]. The importance of proper and thorough rolling is not usually fully appreciated, and this part of the work is often shirked by the contractor. Not only should the heavy roller be at work as soon as the material will bear it, but the roller should, when work is progressing regularly, be kept at work all the time. It is a safe motto that the final rolling cannot be overdone.
[25]. There is still a wide difference of opinion as to the advisability of laying the asphalt surface directly against the rail, many engineers preferring to set one or more courses of paving brick, or stone paving blocks between the rail and the edge of the asphalt. The writer’s experience is to the effect that, if the work is properly done, the first-named form of construction is preferable. If the asphalt be laid against a rail so cold that the asphalt material in contact with or near the rail is chilled before it can be compressed, the work will necessarily be unsatisfactory. As to durability, wheels following the line of the rail or of the paving blocks will sooner or later form a rut in the asphalt which will require repair—and there is not much difference in the results. The attempts to prevent the formation of ruts by setting blocks alternately as headers and stretchers is not always successful, even when the work is well done, and the difficulty of properly compressing the tongues of asphalt between the headers is so great that it is usually not well done. The asphalt settles under travel or is gouged out, leaving a streak of rough pavement, and the difficulty and cost of repairs is considerably increased. When paving blocks or bricks are used, they should be firmly and carefully set in the concrete foundation. Probably the most satisfactory construction of this kind consists of two or three lines of the best paving brick set with their length parallel to the rail. It is somewhat easier to get at and repair rail joints with this construction.
[26]. The practice of laying asphalt block pavement upon crushed stone, or a sand foundation, on streets carrying a considerable travel, is inadvisable. See foot-note[[6]], page [23].
[27]. Hard limestone may be used where trap is not procurable except at a prohibitive cost; but unless the difference in cost is very great the trap will be the most economical in the end.
[28]. The pressure commonly specified is 5000 lbs per. square inch, but recent investigations make it doubtful if that pressure is ever attained, or is, indeed, practicable with any presses so far constructed.
[29]. Asphalt blocks are now very commonly laid upon their sides, even where they are made as thin as two inches, on streets carrying very considerable travel. The practice is not to be recommended unless the blocks are at least four inches thick. Blocks two inches thick will give good service on private driveways and streets of quite light travel.
[30]. Granite block pavement is, as a rule, used upon, and appropriate for streets of the heaviest class of travel, and should, therefore, be provided with the best and strongest foundation. It is nearly always poor economy to lay granite blocks upon a broken stone, gravel, or sand foundation.
[31]. Like other stones, granite from various localities differs widely in strength, hardness and brittleness. Great hardness, accompanied with comparative brittleness, is not desirable in granite for paving blocks. Such material usually polishes by travel and becomes quite slippery, and it is likely to become “turtle-backed,” that is, the corners are likely to be chipped off or worn off, making the pavement very rough and uneven. The quality of the granite to be used in any one city is generally determined by the available supply, and specifications must be drawn with reference thereto.
[32]. Except on streets of excessively heavy travel there seems to be no good reason for making the blocks more than five inches deep. Blocks of this depth are quite sure to become deformed by irregular wear before the pavement will need to be renewed.
[33]. The widths of joints here specified as allowable are based upon the assumption that they will be filled with Portland cement grout as specified in Sect. 75. If this grout filling is used narrower joints are not necessary, as the grout has sufficient strength to support the corners of the blocks, and sufficient hardness to resist the wear of travel (largely protected as it is by the blocks themselves) and to cause the blocks to wear down quite evenly.
In many European cities the specifications require much closer joints. The granite there available appears to break out naturally to truer lines and better surfaces than that used in the Eastern states, at least, so that the cost of dressing the blocks abroad is not as great as here.
The City of New York has recently adopted specifications for “Special Improved” Granite Block pavement intended to approximate the Liverpool standard. These require that the blocks shall be not less than 6 nor more than ten inches long, not less than 3½ nor more than 4½ inches wide and five inches in depth. “The blocks are to be rectangular with tops and sides uniform in thickness, to lay closely, and with fair and true surface, free from bunches and so cut or dressed that when laid stone to stone the joints shall not exceed ⅜ of one inch. The head of the block shall be so cut that it shall not have more than one-quarter of an inch depression from a straight-edge laid in any direction across the head and held parallel to the general surface of the block.” The joints are filled with bituminous cement.
The above specification is very difficult to meet from the granite available to New York without excessive and expensive cutting, and examination of the pavements laid under these specifications shows that the joints greatly exceed the width specified.
It is believed that equally good results may be secured by permitting somewhat wider joints filled with grout, and the cost would be materially reduced.
[34]. The most satisfactory arrangement of courses at street intersections is that shown by Fig. 14, page 208, Tillson’s Street Pavements and Paving Materials, 2d edition.
[35]. The practice of filling the joints with gravel to a depth of one inch or more before the blocks are rammed is of doubtful utility. If the blocks are set closely against each other they will be well held in place while the ramming proceeds. In practice it is hardly possible to closely gage the depth of such preliminary gravel filling, and the top or final filling whether of grout or of gravel and bituminous cement, is likely to vary greatly in depth, and the lateral support of the blocks is thus likely to vary in strength and rigidity.
[36]. It has been the almost universal custom, in this country at least, to fill the joints in granite paving either with gravel alone or with gravel and bituminous cement. But the reasons that have led engineers to prefer grout filling for brick pavements apply with equal force to granite pavement. It makes a stronger and harder filling than the gravel and bituminous cement, and gives a better support to the edges of the paving blocks, thus tending to prevent chipping and “turtle-backing” in the pavement. It is also a materially cheaper filling than the gravel and bituminous cement. But to provide for the expansion and contraction of the pavement by changes of temperature, it is desirable that a strip in the gutters, and an occasional strip across the whole street, shall be filled with the more yielding material, as specified.
[37]. The object of adding asphalt and Portland cement is to make the cement stronger and less susceptible to changes of temperature. Pure coal-tar pitch is very brittle at low temperatures, and is liable to flow from the crown of the street to the gutters in hot summer weather. A cement made as here specified is not only much stronger and less brittle in cold weather, but requires a materially higher temperature to cause it to flow than does pure pitch.
[38]. As the wheels of vehicles frequently follow along the lines of the rails, thus concentrating their effect on a narrow strip near the rails, and as the continuous joint against the rail makes the pavement weaker there, the mortar bed, and the greater care in setting the blocks along and near the rail are advisable.
[39]. If the joints are parallel to the direction of travel on the street the wheels of vehicles are more likely to abrade or break off the corners of the stone and form incipient ruts.
[40]. These specifications conform in most particulars to those adopted by the “Association for Standardizing Paving Specifications,” and are substantially the same as those recommended by the “National Paving Brick Manufacturers’ Association” though they differ in some details from each. The latest specifications adopted by the Association for Standardizing Paving Specifications are very full and satisfactory and are to be highly commended.
[41]. Here again a good concrete foundation is recommended, as being in the end the most satisfactory and economical. See foot-note[[6]], page [23].
[42]. The Association for Standardizing Paving Specifications (New Orleans meeting) adopted a standard size for paving bricks and blocks as follows: Paving brick, 8½ inches long, 2½ inches wide and 4 inches in depth. Paving blocks 8½ inches long, 3½ inches wide and 4 inches deep. There seems to be no sufficient reason for confining the brick to these dimensions.
[43]. The specifications of the Association for Standardizing Paving Specifications require that all paving brick shall have lugs on one side, and allow a projection of ¼ inch from the face of the brick. The object is to provide a wider joint between the bricks in order to facilitate the filling of the joints. The author does not believe these lugs necessary, nor that brick without lugs, but otherwise acceptable, should be excluded. It is certain that many of the best brick pavements ever constructed have been built of bricks without lugs. If lugs are required they should preferably not project more than one-eighth inch. The same reasons that make narrow joints desirable in other block pavements apply equally to brick pavements.
[44]. The A. S. P. S. Specifications permit a loss of 22% with the block size, but do not name a permissible loss for “brick” size.
[45]. Absorption of less than one-half of one per cent., usually indicates that a shale brick has been over-burned, resulting in increased brittleness.
[46]. The absorption test is falling into disfavor, particularly with the manufacturers. The author believes that it possesses a distinct value and should be retained.
[47]. The object of this is to make a gutter offering less obstruction to the flow of water.
[48]. This is the simplest and most effective way to detect soft and underburned brick.
[49]. If the joints are to be filled with bituminous cement, substitute for sections 90 and 91 the following:
Directly after the completion of the rolling and ramming, all the joints in the brick pavement and between it and the curbing, manholes or other structures, shall be filled with a bituminous cement in the following manner:
The bituminous cement shall be composed, by weight, of one hundred (100) parts of straight-run coal-tar pitch commercially known as number four and twenty (20) parts of refined Trinidad asphalt melted and thoroughly mixed together at a temperature of about 350° F., to which shall be added twenty (20) parts of dry Portland cement, which shall be thoroughly incorporated with the hot bitumen until a homogeneous mass is produced, and kept agitated so as to prevent settlement or separation until the cement is used. If another asphalt is used instead of Trinidad the quantity added to the pitch must be sufficient so that the cement will not flow at a temperature lower than one hundred and twenty-five degrees Fahrenheit (125° F.). This cement while at a temperature of about 325 degrees F. shall then be poured from a spouted vessel into all joints and vacancies in the pavement until they are completely filled, repouring being resorted to if necessary to accomplish the complete filling of the joints. After the joints are thus filled a layer of sand one-half inch thick will be spread over the whole surface of the pavement and allowed to remain until the engineer shall direct its removal.
[50]. The bituminous-cement joints are principally for the purpose of providing for the expansion of the pavement in very hot weather.
Experience seems to have proved that cement grout is, everything considered, the best and cheapest filling for the joints in brick pavement. If the filling is properly done, the edges of the brick are supported and the corners do not chip off. With the expansion joints provided at intervals by the bituminous-filled joints, the curbs will not be forced out of line, nor will the pavement be raised from its sand bed by expansion, causing the rumbling sound sometimes noticed.
Where grout filling is used there seems to be no necessity for covering the surface of the pavement with sand, as is usually done, provided the grout is kept damp.
[51]. Since immunity from early natural decay is secured by preservative treatment, the important requisite for wood paving blocks is capacity to withstand the wear and tear of the travel on the street. We have as yet no very satisfactory data as to the ability of the various species of wood to endure the somewhat peculiar and special duty to which paving blocks are subjected.
The test which seems to most nearly approach to what is wanted is that of crushing strength, when the force is applied to the end of the sample, parallel to its fibers; but this does not embrace the effect of impact to which paving blocks are subjected under street travel. Whether this may be considered a function of the end-crushing strength or not is an open question, though there seems good reason to believe that it will prove to be so; and if so, there is no good reason why woods of substantially equal strength under the end-crushing test should not show about the same endurance under street travel, independent of the element of natural durability, which is practically eliminated by preservative treatment.
The end-crushing strength per square inch of some of the kinds of timber named as acceptable is about as follows:
| Southern long-leaved yellow pine | 6900 lbs. |
| Lob-lolly pine | 6500 lbs. |
| Short-leaved pine | 5900 lbs. |
| Cuban pine | 7900 lbs. |
| Norway pine | 6700 lbs. |
| Red gum | 7100 lbs. |
It was formerly very customary to specify that only Southern long-leaved yellow pine might be used for paving blocks, though this requirement was seldom strictly enforced. The fact is, that with the exception of the test based upon the number of growth rings per inch, it requires an expert knowledge, acquired only by long experience, to distinguish with certainty the species of Southern pine from the appearance of the lumber alone. It is now almost impossible to obtain in the market shipments of strictly long-leaved yellow pine, and while that wood is undoubtedly superior to the other pine timbers for paving blocks it seems useless to specify its exclusive use, or to propose specifications designed to exclude lumber made from other species of pine. It was doubtless the recognition of this situation that influenced the Association for Standardizing Paving Specifications, at its last (New Orleans) convention to adopt a specification which practically admits everything known in the market as “Southern yellow pine” having annual growth rings averaging less than eight to the inch and excluding all timber having less than six rings to the inch. Within these limits these specifications practically admit all pine lumber shipped from Southern mills.
The specifications here proposed, by limiting the number of growth rings to nine per inch, would not confine the lumber to true long-leaved yellow pine, but would secure a more mature and solid quality of lumber. It is true, however, that both these specifications and those adopted by the A. S. P. C. exclude most “Cuban pine” lumber which is very rapid growing, the growth rings often numbering but three or four to the inch, though the strength of the wood from this species indicates that it may safely be used for pavement.
[52]. The question of the most economical depth for wood paving blocks is as yet unsettled. In New York City, blocks 3½ inches in depth are adopted as the standard and are being used on streets of the heaviest travel, the practice of Berlin, Paris and other foreign cities being thus followed. The arguments in favor of these short blocks are lower first cost, and that, with much deeper blocks, the usual uneven wear of a wood pavement will make it so rough as to require removal before the blocks are worn down so as to be split up and dislodged from their places. While reliable data on these points are wanting, it seems to the writer very unwise to use such short blocks on streets of heavy travel, and he would recommend that the minimum length for use on such streets be 4½ inches, and he would prefer 5 inches.
On streets of light travel a length of 3½ inches should be satisfactory.
Recent observations on heavy travelled streets in New York indicate that when long-leaved yellow pine blocks become worn down to a remaining depth of about 2⅛ inches they split up into fine slivers and the pavement goes to pieces.
[53]. Most of the more recent specifications require the use of a heavy oil, said to be composed of creosote oil with an admixture of refined tar, on the ground that the tar is necessary as a water-proofing agent to prevent the creosote oil from being dissolved out by water or evaporated into the air. It is claimed that if moisture can thus be excluded from entering the wood, decay will be prevented, even in the absence of the antiseptic elements of creosote oil. It is not intended to discuss this matter at length here. We know from long experience that genuine creosote oil is the best preservative of wood so far found; also that creosoted piles have stood in tidal waters, alternately exposed to water and air, for twenty-five years and still retain sufficient creosote oil to resist the Teredo—a very severe test. Why experiment with a comparatively untried material, particularly when it costs as much as the genuine creosote oil, is rather more difficult to force into the wood, and has some admitted objectionable qualities?
[54]. It is a common practice of contractors in some cities, in the laying of both wood-block and asphalt block pavement, where a mortar bed is called for, to substitute a bed of mixed sand and cement, dampened only to such a degree as will make the mass pulverulent like damp sand, the claim being made that ordinary mortar cannot be spread and gaged properly. This claim is unfounded. The objection to the practice is that the dampened mixture does not contain sufficient water to cause the cement to set, and with the practically water-tight paving surface, does not receive, even in rainy weather, the necessary amount of water. If the weather be dry, the small quantity of moisture in the mixture quickly evaporates, leaving the so-called mortar bed not much better than a layer of sand alone. The writer has found such alleged mortar dry and unset two weeks after the pavement had been completed. If real mortar is not to be used, a layer of sand might almost as well be substituted at first.
[55]. In a number of cities the specifications require the joints in wood-block pavement to be filled with Portland cement grout. If the blocks are set as closely together as they should be, the joints will be so narrow that no grout, thick enough in consistency to be of value, will enter them, except for a short distance down from the top, the remaining depth of the joints remaining unfilled. An examination of any well-laid wood-block pavement soon after it has been attempted to fill the joints with grout will verify this statement. Furthermore, the oil which exudes from the blocks, acting on the thin films of grout, seems to deteriorate the mortar and to render it practically inert. On the contrary, fine dry sand will readily run into and completely fill the joints, and under travel the joints will soon become impervious to water. The sand filling is therefore regarded as better, and it costs less than the grout filling.
[56]. Wherever an old pavement or macadam road can be utilized it makes an excellent foundation for a pavement of this kind, provided it is not in too dilapidated a condition, extends from curb to curb, and its surface conforms near enough to the desired street surface so that the necessary changes and repairs will not be too expensive. Where the new pavement is expected to carry quite a heavy travel it is not advisable to use plain crushed stone for filling depressions and leveling up the surface. It is difficult, even where proper care is used, to make such patches of broken stone as firm and strong as the adjoining old pavement, which is a necessary condition to secure satisfactory results; for if the masses of broken stone yield under travel, slight depressions will form over them in the bituminous surface, which will in time become holes requiring repairs. The 1:4:9 concrete specified for this work is not very much more expensive than plain broken stone, it will not shift or break up under travel, and will in the end prove a better investment.
[57]. Where a new foundation is required broken stone or macadam is most frequently used for bituminous concrete pavements. Unless such foundations are constructed in the same way and with about the same care as is necessary for a macadam road it is liable to prove unsatisfactory. Under the very heavy wheel loads that may occasionally pass over the streets, imperfectly compacted broken stone is likely to shift sufficiently to start incipient ruts which will enlarge and in time necessitate expensive repairs. Such conditions are frequently seen on bituminous concrete pavements subjected to heavy travel. These pavements, like sheet asphalt pavements, require a foundation that will be absolutely unyielding under travel. For this reason a concrete foundation will generally be found more economical in the long run than a broken stone foundation. The increased first cost per square yard is not very great and this additional money will in most cases prove a good investment. At the usual prices of material and labor a square yard of 4 inch concrete should cost about 50 cents, while a properly constructed broken stone foundation 6 inches thick (which would not nearly equal in strength and rigidity 4 inches of concrete) would cost about 45 cents per square yard. Considering the much greater durability and lower cost of repairs of the pavement on the concrete foundation, this small additional cost is not worth consideration. While the specifications are made to cover the three kinds of foundation, it is assumed that the kind of foundation to be used will be decided in advance, and that the part of these specifications relating to the others kinds of foundation will, in actual use be omitted.
[58]. A bituminous cement composed largely of coal-tar pitch has heretofore been most used in pavements of this character. It is not denied that very good pavements have been, and can be built with this material, but the superiority of the asphaltic cement here specified is so great that it is true economy to use it. The difference in cost at prevailing prices of material will be ten to twelve cents per square yard. The greater durability and serviceability of the pavement made with the asphaltic cement will, particularly on streets of comparatively heavy travel, far more than justify this additional cost.
[59]. Hydraulic concrete pavement is to be recommended only for country roadways and for city streets of very moderate travel. While our experience with this kind of pavement is yet limited there is reason to believe from the nature of the material that it will not prove to be a satisfactory or economical pavement for streets of heavy travel. But in all cities and towns there are many residence streets where the travel is very light, and yet where a permanent pavement is wanted and warranted. For these, it is believed that a properly constructed concrete pavement will prove very satisfactory and durable, and the low cost at which it can be constructed should make it very attractive to city officials and property owners. The author has advocated its use under such conditions for many years (see Engineering News, July 21st, 1904). Like other composite pavements its utility and durability will depend largely upon the good quality of the materials used and the skill and thoroughness with which the work is done.
The specifications here offered are the result of the observation and experience of the author, and it is believed that pavements laid in accordance with them will give very satisfactory results.
[60]. A number of engineers advocate the construction of concrete pavement in one homogeneous course, and quite a number of pavements have been constructed in this way.
Like any other composite pavement, it is called upon to perform two functions; to safely sustain the weight of loads passing over it, and to resist wear and abrasion of its surface. A material and form of construction that meets the first requirement may not meet the second. Experience has proved that ordinary 1:3:6 concrete makes an entirely satisfactory foundation for any pavement, but it lacks the hardness and strength to successfully resist the surface abrasion of travel. To secure this quality a richer and harder concrete is called for, but it is unnecessary that the foundation should be equally hard. To construct the pavement in two courses as here specified would seem to be the logical way, especially as it decreases the total cost, and should make a more durable pavement.
[61]. Some engineers advocate a greater total thickness of the pavement than is here specified (6 inches). Considering that this pavement should never be used on heavy traveled streets, a total thickness of concrete of six inches will have ample strength to carry the loads to which it will be subjected. If so, it is a useless waste of money to increase the thickness of the concrete.
[62]. The use of limestone for the top course (unless it is of very superior quality) is not advisable or economical unless the cost of trap rock is so high as to be prohibitive, which, considering its superior durability under the wear of travel, will not often be the case.
[63]. It is advisable to remove the screenings from this surface mixture for two reasons: first, to secure greater uniformity of composition. If the screenings are allowed to remain in the aggregate, there is danger of segregation into patches of different sized aggregate and different ratios of materials, which it is very important to avoid, and second, the small fragments of stone are more likely to be crushed under the concentrated weight of wheels than the larger masses, and to thus start disintegration. Lack of uniformity in the composition and homogeneity in this surface-course concrete is especially to be guarded against, otherwise the surface of the pavement will wear unevenly and depressions and ruts are likely to result.
[64]. The ideal composition of this surface-course concrete is one where the stone forms the largest possible part of the mass consistent with sufficient mortar to fill the voids and thoroughly bind the fragments of stone together.
[65]. The importance of securing high quality and great uniformity in the surface course cannot be urged too strongly.
[66]. This requirement must be strictly enforced. Otherwise there will be danger that the two courses may not properly adhere to each other. It is the writer’s experience that if this rule is observed there will be no danger of the two courses separating.
[67]. The purpose of this rolling is mainly to evenly compress the mass and thus secure its uniform density. It also produces a truer surface than can usually be secured by ramming alone.
[68]. Among engineers there is quite a wide difference of opinion as to the proper spacing of expansion joints, and, in fact, as to the necessity or advisability of providing them at all. It has been suggested that it might be better to omit them entirely, allowing the pavement to form its own expansion joints by cracking along lines where natural forces dictate. Such cracks by their irregularity give a bad appearance to the surface, but observation seems to indicate that the edges of these natural joints wear as well as those made by expansion joints. Further observation and experience is needed in the matter. In most concrete it is known that some contraction takes place during the setting of the cement, regardless of temperature changes, and cracking is probably due as much to this permanent contraction as to that caused by low temperature. The coefficient of expansion of concrete by heat is variable but so small that expansion joints ⅛ inch wide every fifty feet along the street should provide for temperature changes.
[69]. If the expansion joints are not thus filled with bituminous cement they will become filled and packed with incompressible stone, sand, etc., that will not permit expansion.
[70]. The practical value of oiling concrete pavements has not yet been determined by sufficient experience. There is reason, however, to believe that the slight coating of bitumen will materially preserve the surface from abrasion and that its benefit will thus be greater than its cost. It will also tend to prevent the very slight dust that might otherwise exist on the pavement.
[71]. In this class of pavements the contractor or promoter may properly be required to assume responsibility for the character and utility of the work produced, and the municipal authorities should assume no part of such responsibility.
[72]. Upon the general subject of time guarantees of municipal work, see Chapter XI, “Municipal Public Works,” by the author.
[73]. Concrete combined curb and gutter is suitable for use on the great majority of residence streets, and others where the travel is not excessive, or where it will not be subjected to specially severe use, as on business streets where heavy vehicles are likely to be often backed against the curb. If properly constructed it will have sufficient hardness and strength to withstand all ordinary usage; it makes a better appearance, particularly on residence streets, than any other kind of curbing, is durable, and is usually less expensive than any other suitable, equally durable and equally well-finished curbing of natural stone, since the gutter displaces an equal area of pavement.
[74]. The sketch here presented conforms pretty closely to usual practice except in the width of the gutter. It is not uncommon to make the gutter from two to three feet wide. This is not necessary or desirable. A width of 15 to 18 inches forms a sufficient gutter to carry away all drainage except during very heavy rainfalls. Where the gutter projects out into the street sufficiently far to be exposed to large numbers of heavily loaded wheels the outer corner is likely to become broken off or unduly abraded.
[75]. Curbing of all kinds is more likely to be injured by freezing and the heaving of frost under and around it than from any other cause. Good drainage is the best protection against such injury. It is important that these drains shall be connected with sewers, drains or other outlets, so that water will not stand in them.
[76]. The most notable departure of these specifications from usual practice is the use of a solid body of rich, homogeneous concrete for the whole section of the structure, thus avoiding the use of two courses and qualities of concrete—the core concrete and the facing. The most common cause of failure of concrete curbs and gutters is the separation, more or less, of the facing from the core concrete. Without doubt this can be prevented by the use of proper materials, careful work, and the strict observance of the rule that the facing course must be applied before the core concrete has begun to set. But it is difficult to always secure these favorable conditions. Computation will show that the difference in cost of materials, between the usual two-course construction and a single body of rich concrete throughout, is not very great, while the saving in cost of labor is so considerable as to make the actual difference in cost of the two types very small. There can be no doubt that the simpler construction and the consequent greater certainty of securing a durable and satisfactory job is greatly in favor of the construction here recommended.
[77]. The appearance of “hair cracks” on the surface of rich concrete, finished by troweling, and the blotched appearance of the surface of concrete curbing, are usually caused by improper finishing. The glazed surface produced by troweling, particularly where pure, dry cement or neat mortar is applied is almost sure to develop hair cracks, and the varying texture of the surface is likely to absorb water unevenly and thus produce, in time, the unevenly colored or blotched surface so often seen.
[78]. The utility and durability of hydraulic concrete sidewalks depends largely on the quality of materials and workmanship employed in the work. Too frequently, specifications for this work are not sufficiently full, or not prepared with the requisite care, or the work is not properly supervised or inspected while in progress. The aggregate importance and cost of this sidewalk work in our cities warrants more care and attention than it generally receives.
[79]. Determined by the thickness of the drainage course adopted.
[80]. It is customary in many cities to require that the drainage course under the concrete shall have a depth of twelve or more inches. This deep-drainage is designed to prevent the heaving of the sidewalk by freezing. Experience seems to prove that this is not necessary, particularly if tile drains are provided to carry off the water from the drainage course, as specified. Comparatively dry material, even earth, does not heave with freezing; on the other hand, if the material and the trench in which it is placed is wholly or partly filled with water, heaving is liable to occur in severe freezing weather, whatever the depth of the drainage course. Experience has proven, however, that four inches of drainage material is sufficient if the water is drained out of it, while if allowed to stand saturated with water, deeper drainage will give little if any better results. Where the soil is sandy or the natural drainage is otherwise good, no drainage course is necessary.
[81]. Except in the matter of low first cost cinders are not desirable for the drainage course. In time, this material is likely to slack, or decompose, and shrink in volume more or less and to allow the sidewalk to settle. The hollow sound one often notices when walking over a sidewalk and the cracks that frequently appear, are usually caused by the irregular settlement of the drainage course. The object of wetting down the cinders several days before they are used is to cause as much as possible of this slacking to take place before the cinders are used in the drainage course.
[82]. There is a good deal of diversity of practice in the thickness of the concrete to be used. For all ordinary sidewalks three inches of bottom course and one inch of surface-course are ample, and in many cases the thickness of the surface course has been reduced to one-half inch with satisfactory results. Three-fourths inch of surface-course, if fairly uniform in thickness and of good quality, will generally be ample for ordinary sidewalks.
[83]. It is customary to make the surface-course concrete much richer than this, but it is not necessary if the materials are good and the work well done.
[84]. This requirement is very important and should be strictly enforced, otherwise there is danger that by the action of water, frost, and time, the two courses may separate and the surface-course break up—a condition not infrequently observed.
[85]. The troweling not only helps to secure a true surface, but tends to produce a dense surface on the concrete; but it is not desirable that this surface shall be smooth and glassy, hence the slight roughening of the surface with a wire brush.
[86]. Care must be taken to make and leave these expansion joints open to their full width entirely through the concrete. The practice of forming these expansion joints by partitions of iron plate, against which the blocks of sidewalk are built is not advised, for the reason that they are likely to prevent the thorough compression of the concrete surface against or near the plates.
TRANSCRIBER’S NOTES
- Silently corrected obvious typographical errors and variations in spelling.
- Retained archaic, non-standard, and uncertain spellings as printed.
- Re-indexed footnotes using numbers and collected together at the end of the last chapter.