Chapter X: Part II: Instructions to Inspectors on Street Paving Work (2)

=67. Inspection of Blocks.=—However carefully the lumber may have been inspected before its manufacture into blocks, the subsequent seasoning, treatment, handling and exposure to the weather will develop many defects and you will need to observe the delivered blocks closely, either before or after they are set, and reject those that do not comply with the specifications. The principal defects that you will be able to detect after the blocks are treated are the following: Southern pine blocks made of second-growth or inferior wood, partly shown by the number of annual growth rings per inch. Blocks having a less number of these annual growth rings per inch than is permitted by the specifications should be rejected. The treated blocks, especially when they are exposed to the weather for some time, are likely to develop seasoning cracks, “shakes” and loose knots, not observable in the lumber, and blocks showing these to an injurious extent should be rejected, though in many cases the defective part may be split off and the remainder of the block used for closers. If the blocks have become very dry and thoroughly seasoned out, they should be well sprinkled with water about twenty-four hours before they are to be set, to forestall excessive swelling and “humping” when a rain storm comes upon the new pavement.

=68. Mortar Bed.=—It is the common practice in preparing the mortar for the mortar bed under the blocks to simply dampen the mixed sand and cement so that it may still be spread and handled like damp sand. See that sufficient water is used to make a moderately stiff mortar, suitable for being worked with a trowel. This mortar should be prepared in sufficiently small quantities at a time to insure that it will not begin to set before the blocks be placed in it and rammed.

=69. Setting the Blocks.=—The setting of the wooden blocks is a comparatively simple operation and the specifications are sufficiently full and clear for your guidance. See that the blocks are so set that their tops, after the ramming is completed, shall be truly in the pavement datum, without humps or depressions.

=70. Filling the Joints.=—See that the sand used is perfectly dry, fine and clean, and that it is swept about on the pavement until the joints are completely filled.

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.

=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.

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Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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).

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 47:

The object of this is to make a gutter offering less obstruction to
the flow of water.

Footnote 48:

This is the simplest and most effective way to detect soft and
underburned brick.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 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?

Footnote 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.

Footnote 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.

Footnote 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.

Footnote 57: