INSTRUCTIONS TO INSPECTORS ON STREET PAVING WORK
INTRODUCTORY NOTE
Any general code of instructions for inspectors on street paving work
must necessarily be little more than tentative or suggestive, as applied
to any one city.
That here presented is intended for use in connection with the
specifications in Part I of this book. Different specifications might
make necessary many changes in the instructions that follow. The local
or special conditions in any city might also make them inapplicable
without material modifications and additions.
The author believes that in every city where considerable street
pavement work is prosecuted and a number of inspectors employed on such
work, some such code of instructions to inspectors should be formulated
and used. Even where the inspectors are of high character and fully
competent, the adoption of a system of rules and regulations relating to
their work will tend to unify procedure, prevent misunderstandings and
promote good discipline.
The preparation of such a code of instructions requires no little time
and thought, especially where there are no precedents to suggest what is
needed, or to be used as rough patterns. It is with the hope that the
code here presented may be found useful in this way, even though it may
be not appropriate for adoption in any given city, that it is offered to
municipal engineers.
GENERAL REMARKS ABOUT INSPECTION AND INSPECTORS
The necessity of having competent and honest inspectors upon any public
work during its construction need not be here argued. It is generally
understood and recognized in the case of work carried out by contract.
Where the contractor is incompetent, careless or unreliable, inspection
is absolutely necessary if good work is to be secured. Even where the
contractor thoroughly understands his business and intends to faithfully
comply with the specifications and to do all his work in a proper
manner, the inspector cannot safely be dispensed with. The contractor
cannot at all times be on the work; whatever may be his intention and
instructions, foremen and laborers cannot be depended upon to exercise
the proper care and judgment, even where they have no inclination,
motive or interest to do otherwise, which is not always the case. It
seems to be deeply and almost irradicably fixed in the mind of the
average foreman on municipal work that he is expected to show his
ability and skill by evading or circumventing the strict requirements of
the specifications and the watchfulness of the municipal agents,
whatever his employer may _say_ to the contrary. Even when fairly
careful, honest and conscientious he may need friendly oversight. He is
naturally and properly anxious to save all the money he can for his
employer, whether from a desire to loyally serve that employer’s
interest, or to enhance his reputation for doing work cheaply. He may
not appreciate the importance of minor requirements of the
specifications and may believe they can be ignored without real
detriment to the utility of the completed work. Like other men he may
sometimes be careless or forgetful, however good his intentions.
These human qualities are not confined to foremen on _contract_ work.
They are liable if not likely to be found in the foremen on work done
for the municipality by the direct, or day’s work system. They may be
encouraged by the impression or belief that they will not be held by the
municipal authorities to as strict an observance or accountability as
they would be if the work were being done by a contractor—which is too
often true. The absence of some of the motives of foremen employed by
contractors is, therefore, not a sufficient reason for dispensing with
inspectors on work done by the city direct.
It should be trite to say that inspectors should be chosen with some
regard to their qualifications for the work they are expected to do. One
cannot, of course, expect to secure experts for such positions at the
rate of compensation usually paid; but it is reasonable to require that
an appointee to such positions shall possess the following
qualifications:
That he shall be honest, trustworthy and loyal.
That he shall be a man of at least average intelligence and common
sense.
That he shall have some practical knowledge of or experience in the work
he is employed to inspect.
That he shall be a man of good personal habits.
That he shall be habitually punctual, industrious, and alert, and shall
possess a fair share of that quality commonly called “backbone.”
It will not, I am sure, be denied that a very large number of the
inspectors found upon municipal public work fall short in one or more of
these qualifications.
Inspectors who habitually expect or accept compensation or gratuities
from the contractor violate the first requirement, and their number is
astonishingly large. These are useless, or worse than useless, to the
municipality. There are not a few contractors who will take advantage of
the presence on the work of such inspectors to do things that they would
not do if thrown wholly on their own honor and responsibility.
Contractors can hardly be blamed for expecting some compensation for the
money thus exacted or accepted by the inspector from them.
The requirement that the appointee shall have some practical knowledge
of and experience in the work he is expected to do, while often
disregarded, is of real importance. Good workmanship embraces many small
details that are essential, and inferable from the specifications, but
cannot be stated at length therein, and the inspector should have a good
working knowledge of these.
Unfortunately, large numbers of inspectors are employed for personal or
political reasons, or because more competent men are not available. They
know little or nothing from practical knowledge and experience about the
work they are assigned to inspect. Efficient service cannot be expected
from such, even if they possess the other necessary qualifications. The
obvious remedy, where a sufficient number of qualified inspectors cannot
be obtained at call, would be to train up a permanent force by having
the promising tyros serve a sufficient apprenticeship under older and
more experienced inspectors; but this is seldom done.
Inspectors, whether appointed by the engineer in charge or not, should
be absolutely under his control, including the power of dismissal
without unnecessary formalities. Where they are under civil service
regulations and can be dismissed only upon charges and after a hearing
before some third person, the obstacles in the way of promptly getting
rid of an incompetent or unfaithful inspector are often so great as to
be discouraging if not deterrent. He may be clearly below a reasonable
standard of honesty or efficiency but it may be very difficult to
formulate distinct charges and support them with positive evidence.
Thus, one may be morally certain that an inspector is accepting
gratuities from a contractor, but unable to prove it by legal evidence.
If in such cases the judgment and integrity of the responsible engineer
cannot be trusted, he is not fit, himself, for the position he occupies.
The question of how much authority and power shall be given to the
inspector is always a troublesome one. If he be too much limited or
restricted, he cannot properly discharge his duties or command the
necessary respect from the contractor and his employees; if he be given
too much authority and is disposed to be arbitrary or unreasonable, the
contractor may be unjustly treated. So far as the definite requirements
of the specifications are concerned there can usually be no room for
going astray, but in the numberless little details not specifically
covered in the specifications, or where their language and intent is not
clear, differences and disputes may result which require a degree of
knowledge and judgment that the inspector may not possess, and where his
decisions might be against the interests of the municipality or unjust
to the contractor. In such cases the matter should always be referred to
the engineer or his assistant.
Where inspectors are employed upon work done directly by the
municipality the relation between them and the superintendent or foreman
should be clearly defined and understood. Briefly stated, this relation
should be the same as that between the inspector and the contractor on
contract work.
INSTRUCTIONS FOR THE GUIDANCE OF INSPECTORS ON STREET PAVING AND
INCIDENTAL WORK
GENERAL
=1.= The chief duty of the inspector is to see that the work to which he
is assigned shall be carried out in full and strict accordance with the
plans and specifications therefor, and with such additional instructions
as may from time to time be given by the engineer.
=2.= These instructions are intended to supplement but in no case to
take the place of the specifications for the work, which must be fully
and strictly complied with, unless they are changed or modified by the
engineer.
=3.= Wherever the words “_the engineer_” are used in these instructions
they refer to the chief engineer in charge of the work for the
municipality, or his authorized deputies or agents.
Wherever the words “_the contractor_” are used they refer to the person,
or firm, or corporation under contract to perform the work to which you
are assigned, or his agents or employees engaged upon the work.
=4.= You will report to and be under the exclusive direction and control
of the engineer.
=5.= Upon being assigned to any work you will obtain copies of the plans
and specifications under which it is to be done, and will carefully read
and acquaint yourself with all their provisions and requirements. If any
part of these plans and specifications are not clearly understood by
you, you will ask the engineer for explanations and instructions, and
will be governed thereby. Failure to understand or to be familiar with
the plans and specifications will in no case be accepted as an excuse
for not complying with and enforcing them.
You will also obtain, study and familiarize yourself with all the blank
forms, reports, etc., that are to be used by you on the work, so that
they may be correctly utilized; you will also obtain a proper supply of
such apparatus, tools, stationery, etc., as you will need upon the work.
=6.= In inspecting any work done under contract you are to assume that
the contractor is bound to carry out in good faith the plans and
specifications attached to his contract, and that he may be, and must
be, held to a strict compliance with them unless you receive
instructions to the contrary from the engineer. You will be held
responsible for the enforcement of these plans and specifications so far
as they relate to the doing of the work.
=7.= In case the contractor or his agents shall neglect or refuse to
comply with the plans and specifications or to perform the work in a
proper manner, or in accordance with the instructions of the engineer,
you will direct him to suspend the particular work in fault and will
report all the facts promptly to the engineer and await his
instructions, which, when received, you will proceed to enforce.
=8.= You will be expected to be upon the work at all times when work is
in progress, whether during ordinary working or office hours, or not.
But in case the contractor shall work continuous day and night forces,
or otherwise so conduct the work as to require the presence of an
inspector for unusual or unreasonable hours of service, you will notify
the engineer and ask instructions or assistance. If for any reason you
are to be unable to be present whenever work is in progress you will
promptly notify the engineer in time to enable him to supply a
substitute.
=9.= All reports and records required of you by the engineer must be
promptly made up and forwarded to him at the time and in the manner he
may direct. You will promptly acknowledge in person or writing any
instructions received from the engineer and in case such instructions
are not clearly understood by you, you will ask for explanation or
advice.
You will keep a notebook or diary in which all occurrences or matters of
interest relating to the work will be recorded each day, which book will
be delivered to the engineer on the completion of the work, or whenever
your connection therewith may end. All diaries, notebooks and records
kept on the work are the property of the city.
=10.= While your first duty is to see that the work is done in
accordance with the plans and specifications, you will, so far as
consistent with this duty, be considerate of the interests of the
contractor and facilitate his work as much as practicable. Materials
should be inspected promptly upon delivery and in such a manner as not
to delay or discommode the contractor more than is necessary. Errors,
defects or delinquencies in the work should be detected and called
attention to promptly, so as to avoid tearing up and re-construction as
far as practicable.
=11.= Orders and instructions should be given to the contractor or his
foremen or representatives on the work and not to the laborers, unless
in urgent cases.
=12.= You shall not at any time, before, during, or after the completion
of the work, ask or accept from the contractor or any of his agents or
employees, or from any other person whom you may have reason to believe
is acting for the contractor, any money, gifts, gratuities or other
considerations of value, whether as compensation for services rendered
him or otherwise. The penalty for violation of this rule will be
dismissal from the service followed by such legal prosecution as the
engineer may direct. This rule does not apply to the ordinary small
courtesies common between gentlemen associated in business.
=13.= It is a condition of your employment that your services shall be
loyal to the interests of the municipality, efficient, and satisfactory
to the engineer, and that whenever in his opinion your service falls
short of this standard he may dismiss you at any time, without previous
notice.
STREET PAVING WORK
=14. Sub-foundation.=—The usual procedure in preparing the
sub-foundation for a pavement is:
The removal of the old pavement if one exists on the street.
The setting of new or the resetting of the old curbing. (If concrete
combined curb and gutter is used it is not constructed until after the
rough grading is done.)
The rough grading of the street.
The fine grading of the street.
In this work the main things for the inspector to look after are the
following:
If an old pavement is to be taken up and any part of it is to be
reserved for use by the city, see that the material is not injured in
handling and is disposed of as the specifications or the instructions of
the engineer require.
=15. Curbing.=—New curb stone should be inspected as soon as practicable
after it is delivered on the street so that the contractor may have time
to replace any that may be rejected. Rejected stones should be plainly
marked on their tops with red paint, and the contractor required to
remove them from the street. Stones that are defective in dressing
should be given some distinctive mark and the attention of the
contractor called to them so that the necessary redressing may be done
well in advance of the setting, and reinspected.
If the old curb stone is to be redressed and reset, it should be
inspected as soon as possible after it is taken out and all stones that
are not suitable for re-use rejected and removed from the work. The
redressing of the stones should be looked after carefully. The tendency
in this work is toward the use of old stones that do not, or cannot be
made to comply with the specifications, and toward laxity in the
dressing.
=Setting Curbing.=—In the setting of curbing the points to be looked
after are:
To see that the trenches are excavated to the full dimensions so as to
provide space for the use of the full quantity required of concrete,
gravel or broken stone in which the curb is to be embedded.
To see that the stones are set to true grade and alignment.
That the concrete, gravel or crushed stone used is of the proper
quality, the proper quantity used, and that it is placed and compacted
so as to fill the spaces designated under and around the stone. The
tendency of the workmen is to treat these details as unimportant and to
slight the work.
After the curbing is set any spaces between it and the edge of the
sidewalk are to be cleaned out, filled and trimmed with Portland cement
mortar.
=16. Rough Grading.=—If plows and scrapers are used in the rough
grading, see that they are not allowed to penetrate or to disturb the
natural soil below the true grade, and to prevent this it is better to
require that the plowing and scraping shall not approach nearer than one
or two inches to the true grade.
If embankment or filling is necessary, the filling material must be
deposited in regular layers not more than six inches deep and each layer
must be well compacted by rolling or ramming before the next layer is
applied.
=17. Fine Grading.=—The fine grading (with pick or mattock and shovel),
to bring the sub-grade to the true grade and contour required, should be
done with reasonable accuracy and care. Allowance must be made for
compression under the roller, so that when completed the surface shall
not vary more than half an inch below nor more than a quarter of an inch
above the true grade, in order that room for the full thickness of
concrete foundation may be provided, and no more. Humps, depressions and
irregularities of the surface are to be avoided. The fine grading will
be completed by thoroughly rolling or tamping the surface to make the
soil firm and dense. On some soft or wet, clayey soils, excessive
rolling may produce a plastic or “liver-like” condition and the rolling
should be discontinued where such a tendency appears.
=18. Work Incidental to Grading.=—Where it is a part of the contractor’s
work to reconstruct or readjust catch basins, manholes, covers, and
other structures, he should be required to keep this work done well in
advance of the laying of the concrete foundation. If this work is
delayed until after the main body of the concrete is in place, leaving
openings to be filled later, hurriedly and perhaps carelessly, it may
result in weak patches in the concrete around these structures, where,
in fact, the concrete should be strongest.
CONCRETE FOUNDATIONS
=19.= The concrete foundation of a pavement is a most important part of
the work, and must not be slighted in any way.
=20. Quality of Materials.=—The quality of the materials used for making
the concrete are fully described in the specifications, and they must be
substantially and uniformly adhered to.
The Portland cement delivered on the street will presumably have been
previously inspected and accepted by the engineer, but any change in the
kind, or brand, or apparent quality of the cement delivered after the
beginning of the work should not be permitted without the approval of
the engineer. Barrels or bags of cement not plainly marked with the
brand, or name of the manufacturer must be rejected. You will frequently
make balls of stiff mortar from the cement and sand used (1 part cement,
2 parts sand) and set them aside for observation. If in eight hours
these balls do not set up hard the fact should be promptly reported to
the engineer.
The sand and stone will naturally vary slightly from time to time, but
should not differ materially from the requirements of the specifications
or the standards adopted by the engineer. Stone or gravel that contains
any considerable quantity, say more than 5%, of foreign matter, such as
soil or refuse, should be rejected. Stone containing an excessive
quantity of fine fragments or screenings bunched together in the pile,
should be mixed with the mass of larger stone before it is used, so as
to produce a mass of fairly even composition. Stone that is coated with
mud, or that is not sound and hard should be thrown out.
Sand that is moderately coarse and does not contain more than 7½% of
clay or soil, and is free from vegetable refuse, may be used.
=21. Storing Materials.=—Cement stored on the street must be stacked up
on dry lumber floors at least four inches from the ground and
effectually covered to protect it from rain or snow. Packages of cement
which, when turned out for use contain hard lumps should be rejected.
Sand and stone must be stored on tight lumber floors to prevent their
admixture, when being shoveled up, with street soil or other impurities.
This requirement must be enforced.
=22. Proportioning Concrete.=—The proportions of cement, sand and stone
or gravel named in the specifications must be strictly adhered to.
A barrel of Portland cement will be considered as four cubic feet and a
standard bag of cement as one cubic foot.
It is usual for the contractor to measure the sand and stone by
wheelbarrow loads. In that case you will measure and determine the
capacity, in cubic feet, of the convenient or average wheelbarrow load,
and determine the size and number of loads each of sand and stone
required for one batch of concrete (if made by hand) and thereafter see
that the quantities thus determined are used, and the wheelbarrow
loading uniform. To measure the contents of a wheelbarrow load, use a
square box holding one cubic foot. It cannot be accurately measured on
the barrow.
As a check upon the proportioning of materials, you should occasionally
keep a record of the quantities of cement, sand and stone used for, say,
one or two hours, and mark and measure up the quantity of concrete made
with them, and from this data compute the ratios of the materials used.
(In ordinary street concrete, proportioned 1:3:6, the volume of rammed
concrete made will just about equal the volume of the stone used.)
Where the concrete is mixed by a machine with automatic feeding devices,
reliance must not be placed upon the machine for proper proportioning.
Frequent tests of the materials used and the volume of concrete turned
out should be made as described above, and any necessary corrections
made in the feeding devices of the machine. This test should be made
daily if practicable. The eye will usually detect any material change in
the composition of the concrete, and whenever such changes are observed
the composition ought to be at once checked up by measurement.
=23. Mixing Concrete.=—See that the specifications are followed. Use
sufficient water to make what is commonly called a wet concrete but not
so wet that free water will drain from the mixed batch before ramming.
The test for proper mixing will be that all the fragments of stone are
completely covered with mortar.
=24. Placing Concrete on Street.=—All concrete must be placed and rammed
before it begins to stiffen or set. Concrete should be so handled that
the mortar will segregate or separate from the stone as little as
possible. When in place on the street it should make a mass of uniform
consistency and appearance. The concrete should, as soon as placed on
the street, be graded to a proper thickness and surface, and then well
compacted by ramming. Insist on good ramming; this part of the work is
not often properly done. The surface of the concrete should be brought
to the true grade by proper grading and tamping. Where necessary to
raise the surface, additional concrete, made of small stone, may be
applied during the tamping. Neat mortar should not be used for this
purpose. Sweeping mortar over the surface with street brooms should not
be permitted—it is useful only to cover up defects and to produce an
extra smooth surface (which is not usually desirable). To measure the
thickness of the bed of concrete, lay a straight-edge upon the surface
with the end of the straight-edge projecting a foot or two beyond the
edge of the concrete, and measure the space from the lower edge of the
straight-edge down to the sub-foundation.
Care must be taken to finish the surface of the completed concrete at
the proper distance below and truly parallel to the grade and contour of
the finished pavement. This is more important in the case of sheet
asphalt pavements, since irregularities in the concrete will cause
variation in the thickness of the finished asphalt surface, which will
affect the durability of the pavement. The grade stakes for the concrete
should therefore be carefully set and worked to, and humps or
depressions between the stakes should be avoided.
=25.= In hot, dry, weather the concrete must be kept damp by watering it
until it is at least five days old, and horse-travel over it must be
prohibited until it has set up hard enough not to be injured thereby.
=26. Old Paving Stone Foundations.=—If a foundation is to be made of old
paving stone blocks reset, the important points to be looked after are,
The preparation of the sub-foundation.
The sand bed.
The setting of the blocks.
The grouting of the blocks.
The sub-foundation should be prepared, where necessary, as stipulated in
Sects. 16 and 17 of these instructions. This part of the work is likely
to be carelessly done unless proper work is insisted upon.
The sand used as a cushion course must be of good quality and reasonably
clean. The use of mixed sand and soil taken from the street excavation
should not be permitted.
The blocks, after being cleaned from adhering dirt, should be set with
reasonable care, well bedded in the sand cushion, and laid at such an
elevation below the completed pavement surface, that after being well
rammed, the surface will be at the proper elevation below the pavement
datum.
The grouting of the joints is the most important part of the work. See
that the mortar is made and used in accordance with the specifications,
and that the joints are completely filled.
=27. Broken Stone Foundations.=—The specifications for broken stone
foundations are sufficiently explicit, and compliance should be insisted
upon. Thorough rolling is especially important.
SHEET ASPHALT PAVEMENTS
=28.= The manufacture of the mixture for sheet asphalt pavement, and the
laying of it on the street, involve a number of separate elements or
operations. The quality and durability of the pavement will depend
largely upon the skill, intelligence and accuracy with which each and
all parts of the work are carried out. The inspector, both at the mixing
plant and on the street, must give constant and intelligent attention to
the work.
=29. Inspecting at Paving Plant.=—Inspectors at the plant where the
asphalt paving mixture is made will be provided with the following
apparatus:
1 nest of standard sieves, Nos. 10, 30, 50, 80, 100 and 200
2 thermometers for sand
1 tank thermometer
2 standard flow-plates and one dozen cartridges for same
1 balance for weighing sand
6 dishes, glass or porcelain, 4 inches in diameter.
=30.= The more important things for the Plant Inspector to watch during
the manufacture of asphalt paving mixture are:
_a._ Quality and grading of the sand.
_b._ Temperature of heated sand.
_c._ Uniform quality of the refined asphalt and flux.
_d._ Temperature of melted bitumens.
_e._ Uniform consistency of asphaltic cement.
_f._ Correct ratios of sand and asphaltic cement.
_g._ Thorough mixing of materials.
=31. Sand.=—The general quality and grain-size of the sand, required by
the specifications, must be closely watched and adhered to.
It is hardly ever possible to secure a natural sand that will conform
strictly to the theoretical composition, but _substantial_ compliance
should be required unless the Chief Engineer directs otherwise. After
any sand has been approved for a given contract or street, all the sand
thereafter used for that street should conform closely to the standard
so approved; for any material change in the quality of the sand will
change the quality of the pavement produced, unless the proportions of
the other materials are changed accordingly. Any material change in the
sand should therefore be promptly reported to the Chief Engineer.
=32.= Sand for the sieve tests should be taken from the hot-sand box and
allowed to become cold. From the sample thus taken, weigh out 50 grams.
Arrange the nest of sieves with the No. 200 at the bottom followed by
Nos. 100, No. 80, No. 50, No. 30 and No. 10 at the top.
Place the weighed sample upon the No. 10 sieve and thoroughly shake the
nest until all the sand that will, has passed through the various
sieves; then weigh the various portions, beginning with that which has
passed the No. 200 (deducting in each case the weight of the dish). The
portion _remaining_ on the No. 200, is the quantity _passing_ the No.
100, etc. The sum of all these portions should be 50 grams, though it
will usually be slightly less because of unavoidable waste. In use, keep
the sieves clean and as free as possible from adhering grains by using a
bristle brush.
=33. Temperature of the Sand.=—The temperature of the heated sand should
be frequently noted by plunging the thermometer into the mass. It should
not fall below 315° F. nor exceed 375° F. It is very important to avoid
over-hot sand, because, in the mixer, the very thin films of bitumen
covering the hot-sand grains are readily and quickly scorched by the
excessive heat in the sand.
=34. Refined Asphalt and Fluxing Oil.=—Unless otherwise notified by the
Chief Engineer you will assume that the stock of refined asphalt on hand
at the beginning of the work has been inspected and is satisfactory. You
will, however, note the appearance of the material and the character and
marking of the barrels or packages, and will not permit the use of any
new shipments of materials of different appearance or brand without
authority from the Chief Engineer. The above instructions apply also to
the oil used for fluxing.
=35. Ratio of Asphalt and Flux.=—You will frequently note and record the
weight of asphalt and of flux used in charging the melting tanks.
=36. Temperature in Melting Tanks.=—The melting kettles are usually
equipped with fixed thermometers, but their reading should be frequently
checked with your detached tank thermometer. You will see that the
temperatures in the kettles are kept within the limits named in the
specifications. See that the melted asphaltic cement is kept constantly
agitated as required by the specifications.
=37. Uniformity of Asphaltic Cement.=—It is very important that the
asphaltic cement shall be of uniform composition and consistency, or
hardness.
When the composition and consistency of the asphaltic cement to be used
for the surface-course of any street has been determined upon and
approved by the engineer, you will take out and preserve a sufficiently
large quantity to be used thereafter as a standard, and each subsequent
tank of cement, before it is used, should be tested by comparing it with
this standard in the following manner, using the flow-plate apparatus:
Melt and fill two cartridges with the standard cement, and two with the
cement to be tested, marking them for identification. When cold,
carefully remove with a wetted dull knife any surplus cement from the
ends of the cartridges so as to leave them exactly full and the metal
ends clean. Immerse them in cold water until they are all of the same
temperature. Then place and fasten these filled cartridges on the
grooved plate with their lower ends exactly to the zero line on the
plate, the standard samples alternating with the new. Then place the
plate, at an inclination of about 45°, in front of a source of uniformly
distributed heat, as, for instance, near a steam boiler or large steam
pipe, or (when the weather is favorable) in the sunshine, facing the
sun. Leave the plate in this position until the bitumen in the
cartridges shall have melted and flowed down the grooves an average of
from four to five inches. Then measure the lengths of flow of each from
the zero line on the plate. If the average flow of the new samples
differs more than 5 per cent. from that of the standards, the new cement
should be regarded as unsatisfactory and should be corrected by adding
more asphalt or flux, as may be required.
(If a _penetration apparatus_ is supplied or is available, the
penetration method may be used instead of this flow-method for
determining the consistency of the cements. Proper instructions for its
use will accompany the apparatus.)
=38. Ratio of Sand and Cement.=—When the proper ratio of sand and
asphaltic cement has been determined for any street you will see that
this ratio is carefully adhered to. The correctness of the weighing
scales used should be frequently tested.
=39. Mixing.=—When the sand and asphaltic cement are properly mixed the
appearance of the mass will be perfectly uniform throughout, with every
grain of sand entirely and evenly coated with cement. The mixing should,
however, be continued somewhat beyond the attainment of this condition,
so as to insure thoroughness. After the proper result has been arrived
at, either the _time_ in the mixer, or the _number of revolutions_ of
the mixer shaft should be noted and the standard thus determined
substantially adhered to thereafter. With the standard mixers commonly
used, the hot sand and the filler (dust) should first be put into the
mixer and mixed for about fifteen seconds; then the asphaltic cement
added and the mixing continued as long as necessary (in the usual mixer
not less than sixty seconds).
=40. Records.=—You will keep complete records of the results of your
observations and tests in a book which will be provided for the purpose,
and will make such reports to the Engineer as he may direct.
=41. Inspection on the Street.=—The leading things to be kept in mind by
the street inspector, in the laying of asphalt pavement on the street,
are the following:
_a._ Temperature of mixtures when applied to the street.
_b._ Proper preparation of the street surface before material is laid.
_c._ Laying and rolling of the binder-course.
_d._ Laying of the surface-course.
_e._ Rolling the surface-course.
=42. Inspector’s Equipment.=—The inspector will be supplied with the
following apparatus:
1 armored thermometer reading from 200° F. to 500° F.
1 graduated spatula.
1 twelve-foot straight-edge.
1 fifty-foot tape line.
=43. Temperature of Mixture.=—If proper care has been taken at the
mixing plant the mixture will not arrive on the street at too high a
temperature, and your principal care will be to see that it is not laid
at too low a temperature. The minimum temperature named in the
specifications should be adhered to. While the main or interior mass of
a wagon load may be well above that temperature, the top and outer part
of the load may be, particularly in cold weather, too cold to be safely
used. Some of this colder portion may usually be sufficiently reheated
by mixing it with the hotter material, if properly handled in unloading;
but any material that is so cold as to be lumpy when unloaded, or, more
particularly, when being raked out, should be discarded. This applies to
both surface and binder mixtures.
It must not, however, be overlooked that the different kinds of asphalts
have different appropriate working temperatures, and for some asphalts
the minimum temperature named in the specifications may be too low. The
best practical guide is the manner in which the mixture behaves in
raking. It must always be so hot that it will, under the rake, break up
into a uniform, crumbling or powdery mass. If it does not do this it is
(unless the mixture is excessively rich in bitumen or improperly
compounded) too cold.
=44. Preparation of Street Surface.=—Before the binder-course is laid,
all loose material, rubbish, street dirt and other matter foreign to the
concrete surface must be removed and the concrete surface swept, if
necessary, to properly clean it, with street brooms. Neither binder nor
surface mixture shall be laid upon wet surfaces. Before the spreading of
the surface mixture on the binder the latter must be cleaned of all
foreign matter and, if necessary, swept. If the binder is covered with
mud from wagons or other travel, the surface must be scrubbed clean. Any
part of the binder-course that may have become broken or loosened before
the surface-course is applied must be taken up and new material laid in
its place with the same care as the original.
=45. Laying Binder-course.=—The binder mixture must be carefully spread
and raked to such thickness that after being rolled it will present an
even, true surface not varying more than one-fourth inch from the
intended finished surface of the binder. The tendency is to lay the
binder-course carelessly and to roll it insufficiently. Nearly the same
care in raking and rolling as for the surface-course, should be insisted
upon. If the binder is not thoroughly compressed before it becomes cold
it is likely, in future hot weather, to soften and yield under heavy
travel and thus to start depressions in the pavement which will increase
with time.
=46. Laying the Surface-course.=—The requirements of the specifications
should be rigidly enforced in the laying of the surface-course. Joints
against a cold edge of previously laid surface must be cut back until
solid, fully compressed material of full thickness is reached, and the
raw edge completely but thinly painted with liquid paving cement or
pitch. No masses or fragments of cold mixture, whether of binder or
surface, must be allowed to remain on the surface of the binder in
advance of the placing of the surface-course, to be covered up by the
latter. Such cold masses will not be compressed by the roller, but will
later, under a hot sun and heavy travel, yield and start depressions in
the pavement. The raking out requires to be properly and skilfully done.
The tines of the rakes must penetrate to the binder, so that the raked
material will be a uniform mass from top to bottom.
=47. Rolling.=—You will insist upon the rollers being placed upon the
freshly raked surface just as soon as the material will bear them
without being squeezed out or displaced laterally. The tendency is to
keep the rollers off too long, thus permitting the chilling of the
surface and preventing its proper compression. Do not take the
contractor’s word as to how soon the rolling may be begun, but have
trials made until you are able yourself to judge. The rolling by the
heavy roller should be very thorough: keep the roller at work constantly
until the surface is too cold to be impressed. In operating the roller
lengthwise of the street begin at the gutters and work toward the center
of the street. Cross rolling and diagonal rolling must be insisted upon
wherever the width of the street will permit it.
=48. Proper Thickness and Surface.=—While the completed surface is still
warm enough to permit it, measure the thickness of the surface-course by
forcing the graduated spatula through it to the binder and noting the
depth of penetration. Also test the trueness of the finished surface
with the straight-edge, and if found defective in either thickness of
pavement or trueness of surface, insist on more careful grading and
raking on the further work. Try the completed gutters with water to see
that they are so truly laid that puddles of water will not stand in
them.
=49.= Asphalt surface must not be laid when rain or snow is falling, or
so long as the street surfaces are wet. Surface mixture raked out and
caught in a shower before it is well enough rolled to exclude water must
be taken up and discarded.
Unless unavoidable, neither binder nor surface should be laid when the
street surface or the air is below 40° F. As a rule, the lower the
temperature the of air the greater is the care necessary to properly lay
asphalt pavement.
=50.= Measure each day the area of pavement laid and record that, and
the quantity (number of batches) of surface mixture used, and note any
lack of uniformity in the area laid per batch.
ASPHALT BLOCK PAVEMENT
=51.= You may assume, unless otherwise instructed, that the Engineer has
inspected and approved the general character of the blocks already
delivered for the work, unless their inferiority be obvious, but you
will be expected to observe them carefully as laid and to reject blocks
that vary in dimensions from those permitted by the specifications, and
those that are broken, disfigured or injured. But during the progress of
the work you will note the character of subsequent deliveries and report
to the Engineer any apparent change in the quality of the blocks.
The more important points to look after in the laying of the asphalt
blocks are: the quality and grading of the mortar bed in which the
blocks are set, and the care and accuracy of the block setting. The
mortar must be of cement and sand in the ratios specified, and
sufficient water used to make a medium stiff mortar; the use of merely
moistened powder must not be permitted; and any mortar that has begun to
set up before the blocks are laid and rammed must be discarded.
=52.= In setting the blocks the principal requisites are: that they be
well bedded in the mortar; that they be set so that after ramming their
tops will be accurately in the pavement datum; that they be set and
pressed as closely together as possible, levers or mauls being used to
force them into close contact, both at ends and sides.
GRANITE BLOCK PAVING
=53.= Assuming that the foundation has been properly constructed, the
more important things for the inspector to look after are:
_a._ The quality and shape of the blocks.
_b._ The sand cushion.
_c._ The setting of the blocks.
_d._ The ramming of the blocks.
_e._ The filling of the joints.
=54. The Blocks.=—Assuming that the general _quality_ of the granite has
been approved, you will need only to observe and reject blocks made from
soft or weathered or otherwise defective stone. Any material divergence
of the blocks from the correct form or from the sizes specified as
permissible will be readily caught by the eye as they are brought to the
street, and they can then be thrown out. The proper dressing of the
blocks is important and should be watched carefully. While you are not
expected to examine each individual block, close observation of the
blocks as they are handled and laid will enable you to detect and reject
those that are materially defective in shape or dressing; or excessively
wide joints will call attention to these defects as the blocks are set.
=55. Sand Cushion.=—The sand used for the cushion bed should be
moderately coarse and must be fairly clean and pure. The tendency with
contractors is to use any dirty sand or sandy loam available on the
street. Such material, especially if it becomes filled with water, will
yield under the blocks and will not support them properly. The sand bed
should not vary materially in thickness. It should be laid and graded
not more than fifty nor less than twenty feet in advance of the setting
of the blocks.
=56. Setting of the Blocks.=—The blocks should be delivered in front of
the block setters on the blocks already set. The blocks must be set in
straight courses from curb to curb, the courses running at right angles
to the street (except at street intersections). All the blocks in any
one course must be of the same width. Each block should be so set that
the whole of its bed will be seated upon sand. The usual practice of
block setters is to make a bed by drawing the sand into a small ridge
upon which the outer edge of the block is rested, its weight canting it
against the course already laid. This usually leaves a cavity under the
back edge of the block which neither the subsequent joint filling or
ramming closes up, so that the block may rest permanently on only a part
of its bed. The practice is so fixed with block setters that it will
require firmness and persistency to prevent it, but you should insist
that it be so modified as to give a solid bearing for each block over
its whole base.
The blocks must be set solidly against each other so as to make the
joints of the least possible width. Unless close attention is given to
this the joints are likely to greatly exceed the width permitted by the
specifications. Care should be taken that the blocks are set truly
vertical so that their upper face shall be parallel to the plane of the
street. In setting the blocks their straightest and truest face should
be placed upward so as to make the surface of the pavement as smooth and
even as possible. Block setters are often careless about this.
Ordinarily no grade stakes are set for the surface of the pavement and
templates are seldom used, the block setters using only their eyes to
make the surface conform to the intended contour of the street. This
usually results in the completed street being more or less wavy and
irregular, and in a lack of regular and uniform cross-section in
different parts of the street, often very perceptible to the eye. Care
should be taken to avoid this as much as possible. Straight-edges and
templates, cut to the proper curve for the surface, should be provided
and used.
=57. Ramming the Blocks.=—This part of the work is likely to be slighted
unless carefully watched. The important thing is to have each block
equally and sufficiently rammed to bring it to a firm bearing on the
sand, as well as to bring its top to the proper grade. The rammers will
be disposed to touch lightly blocks or areas which if thoroughly rammed
would be driven below the true surface of the pavement, and subsequent
travel may force these down, making depressions in the surface. It is a
very common practice for the men working the rammers to shirk on this
work by allowing the rammers to fall with little more force than that
due to their own weight. This should be detected and remedied.
=58. Filling the Joints.=—The specifications are quite full and clear
regarding this part of the work and little further need be said.
Care is necessary, whether the joints be filled with gravel and
bituminous cement or with grout, to secure the complete filling of all
the joints. If the weather is cold at the time the work is done, the
gravel filling may become so chilled before the bituminous cement is
poured that it will not flow to the bottom of the joints. For this
reason it is important that the cement be worked at the highest
temperature that it will bear without injury.
If grout filling is used care is necessary to make the mortar of the
right consistency. If too thick it will not flow freely to the bottom of
the joint; if too thin its strength will be materially reduced.
Experiments should be made to determine the proper consistency, and when
this is found, the quantity of water used in making the mortar should be
determined and thereafter measured out for each batch.
The mortar must be kept constantly stirred until used, otherwise the
sand is likely to settle to the bottom. Whatever filling is used, a
second and sometimes a third pouring will be necessary to completely
fill the joints.
BRICK PAVEMENT
=59.= The matters requiring most attention in the construction of brick
pavement (assuming that the foundation is in place) are the character
and quality of the brick, the sand cushion, the setting of the brick,
and the filling of the joints between the brick.
=60. Inspecting the Brick.=—The examination, testing and acceptance, in
a general way, of the brick to be used for the work will be made in the
engineer’s office, and your duty will be confined in this matter to
seeing that the brick delivered and used are in accordance with the
standard established and the general requirements of the specifications.
You can best inspect the brick as they are being laid or after they have
been laid, and before the surface has been rammed or rolled. Presumably
the brick-setters will discard most of the unsuitable brick during the
progress of their work, particularly if they are given to understand
distinctly the defects that will cause rejection.
As the setting is completed, go carefully over the surface of the
pavement and mark plainly all the bricks that are to be removed for
defects and see that all such are taken out and replaced with good
brick. The principal defects that should be observed are: soft or
underburned bricks; those that are warped or deformed so that they will
not fit closely to their neighbors, or have warped or ill-shaped tops;
those that are broken or chipped or show injurious cracks, and those
that are perceptibly wider or narrower than their neighbors in the same
course.
Soft or underburned brick may be detected with most certainty by having
the contractor wet the surface of the pavement by sprinkling. The soft
brick will absorb more water and remain damp longer than the hard brick
and can thus be readily detected. This test can be applied, however,
only to pavements the joints of which are to be filled with grout, as
bituminous filling should not be applied to brick with wet surfaces, and
to wait until they become dry after the wetting would delay the work
unwarrantably.
=61. Sand Cushion.=—See that the sand used is free from foreign matter
and pebbles, as required by the specifications, and that the sand bed is
of uniform thickness, correctly gaged, and rolled to the true plane of
the pavement, additional sand being added or surplus removed where
necessary to secure this result. See that the cushion is not thereafter
disturbed or, if disturbed, that it be restored to its original
condition.
=62. Setting the Brick.=—The bricks are to be set as closely together as
possible both at sides and ends, so that the joints will not be wider
than permitted by the specifications. Where the bricks have lugs or
letters on one side, the lug-sides shall in no case be laid together.
The courses must be reasonably straight entirely across the street. See
that the bricks in adjoining courses break joint not less than three
inches. Brick-setters are likely to be careless about this. Do not
permit bats to be used except where necessary at the ends of courses or
to fit the pavement against street railroad rails or around manholes or
other structures in the street, and insist that this filling in shall
follow immediately after the brick-setting, and that close fits shall be
made.
=63. Rolling the Pavement Surface.=—This should be carefully and
uniformly done so as to insure the even surface and contour of the
pavement. Do not allow the roller to make sharp turns on the freshly
laid brick surface as it will disturb and cant the loose brick. The
roller should run back onto the fully completed pavement to make any
necessary turns or considerable changes of direction. Quick starts or
stops of the roller on the fresh-laid brick should be avoided for the
same reason.
=64. Filling the Joints.=—Whether grout or bituminous filling be used
the important thing is to get all the joints completely filled. Where
bituminous cement is used, if the work is done in very cool weather the
brick is likely to chill the cement before it reaches the bottom of the
joints unless the cement be applied as hot as possible without injuring
it, and the pouring be rapidly and carefully done.
If grout filling be used, see that the mortar is made of the proper
ratio of sand and cement and that it is of the proper consistency to
just flow into the joints, and that after mixing it is kept stirred
until used. Repour all joints where necessary to completely fill them.
=65. Gutters and Expansion Joints.=—See that the gutters are laid with
longitudinal courses of brick and their joints filled with bituminous
cement as provided in the specifications.
WOOD-BLOCK PAVEMENT
=66. Foundation.=—The instructions relating to the preparation of the
sub-foundation and the concrete foundation already given apply to
wood-block pavement.
=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.
[Illustration: 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.
-----
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:
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.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 65:
The importance of securing high quality and great uniformity in the
surface course cannot be urged too strongly.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 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.
Footnote 72:
Upon the general subject of time guarantees of municipal work, see