MODEM KOAD CONSTRUCTION
A PRACTICAL TREATISE ON THE ENGINEERING PROBLEMS OF
ROAD BUILDING, WITH CAREFULLY COMPILED SPECI-
FICATIONS FOR MODERN HIGHWAYS, AND
CITY STREETS AND BOULEVARDS
AUSTIN T. BYRNE
CIVIL ENGINEER
AUTHOR OF
'HIGHWAY CONSTRUCTION," "MATERIALS AND WORKMANSHIP'
ILLUSTRATED
AMERICAN TECHNICAL SOCIETY
CHICAGO
1917
COPYRIGHT, 1917, BY
AMERICAN TECHNICAL SOCIETY
COPYRIGHTED IN GREAT BRITAIN
ALL RIGHTS RESERVED
INTRODUCTION
THE science of good road building is an old one as evidenced
by the many highways in Europe which have withstood the
wear of travel for centuries. Most of these famous roads were
cut from solid rock or built of crushed stone of such a character
as to be unaffected by weather conditions. Modern road building,
however, has been largely influenced within the past fifteen years
by the enormous increase in the amount of travel due to the auto-
mobile. This has not only been the means of developing new road
surface to meet the more severe requirements of this type of vehicle
but it has developed a country-wide interest in good roads, thus
making it possible for the enthusiastic travelers to take long tours
without meeting the formerly ever-present bugaboo of bad roads,
besides making the ordinary town-to-town travel more satisfactory.
_ T>
Lb. water absorbed = - X62.37 per cu. ft. of rock
in which A is the weight in air; B is the weight in water immediately
after immersion; C is the weight in water after immersion for 96
hours; and 62.37 is the normal weight in pounds of a cubic foot of
water.
The durability of a stone used for roads is affected to a certain
extent by its capability of absorbing water. In cold climates a low
absorptive capacity is essential to resist the disintegrating effects
of alternate freezing and thawing.
Specific Gravity. The specific gravity is determined either by
HIGHWAY CONSTRUCTION 91
weighing in a specific gravity balance or by weighing in air and
water, and applying the formula
Specific gravity = j^rr^r
in which W is the weight in air, and W\ is the weight in water.
Specific gravity and porosity are closely related. The specific
gravity varies with the density or compactness of the aggregation of
the mineral grains forming the stone. The closer the grains the more
compact the stone, and the less will be the amount of interstitial
space and hence the less the porosity.
From the specific gravity the weight per ton or per cubic yard
may be determined. A knowledge of the weight is useful in decid-
ing between two otherwise good stones; the heavier will be the more
expensive, due to increased cost of transportation. On a water-
bound macadam road it is an advantage to have a detritus with a
high specific gravity, as it will not be moved so easily by rain and
wind as one of low specific gravity.
Cementing Quality. The cementing quality of the stone dust is
determined by placing 500 grams of the rock, broken to pass a J-inch
mesh screen, in a ball mill, together with 90 cubic centimeters of
water and 2 steel balls weighing 20 pounds. The mill and its charge
are revolved for 2J hours at a rate of 2000 revolutions per hour.
The operation produces a stiff dough, of which 25 grams are placed
in a metal die 25 millimeters in diameter, and subjected to a pressure
of 132 kilograms per square centimeter, producing a cylindrical test
piece. The test piece is dried in the air for 20 hours, after which it
is heated in a hot-air oven for 4 hours at a temperature of 200
Fahrenheit and then cooled in a desiccator for 20 minutes. When
cool it is tested in the impact testing machine in the same manner
as the test for toughness, using a hammer weighing 1 kilogram and
a fixed height of fall of 1 centimeter. Blows are struck until the
test piece fails. The average of the number of blows on 5 test
pieces is taken as the result of the test. A result of 10 is considered
to indicate a low cementing quality; 10 to 25 is considered fair; 26
to 75 good; 76 to 100 very good; over 100 excellent.
Species of Stone. The rocks most extensively used for broken-
stone roads are trap, granite, limestone, sandstone, boulders, or field-
stone.
92 HIGHWAY CONSTRUCTION
Trap rock is hard and tough and has good wearing and binding
qualities. Granite is brittle and its cementing value is low. Lime-
stone is deficient in hardness and toughness but possesses good
binding qualities and for light traffic roads is eminently suitable.
Sandstones are rocks made up of grains of sand cemented together
by siliceous, ferruginous, calcareous, or argillaceous material; they
are usually deficient in binding quality and resistance to abrasion.
With a bituminous binder good results are obtained. Fieldstones
are a mixture of the hardest parts of the granites, sandstones, lime-
stones, etc., distributed by glacial action and which have resisted
the disintegrating effects of the weather. Owing to their variable
character and unequal hardness they wear irregularly and make a
very rough road.
Shape and Size of Stone. The shape and size of the fragments
of stone affect the enduring qualities of the road. The nearer the
fragments approach the cubical form with irregular jagged sides,
the more satisfactory will be the results.
The size of the stone must be such as will not fracture or crush
under the action of the roller during compaction nor become loosened
under traffic. For the harder rocks the size varies from 1J to 2|
inches and for the softer rocks from 2J to 4 inches. The sizes do
not refer to the actual dimensions of the stone, but to the size of the
hole in the screen; thus IJ-inch stone is that which is retained by a
1^-inch opening and passes through a 2- or 2J-inch opening.
Thickness of the Broken Stone. The thickness of the broken
stone is determined for the given conditions by the formula pre-
viously stated, and ranges from 4 to 16 inches. Where the thickness
exceeds 6 inches, the excess may be composed of sand, gravel, field-
stone, ledge rock, or broken stone, as previously stated in the dis-
cussion on foundations; the choice depending on availability and
cost. For use in the base all are equally effective.
Spreading the Stone. The method employed for laying the
covering varies with the thickness. When the finished thickness is
4 inches all the stone to be used is laid in one course. When the
finished thickness exceeds 4 inches the stone is laid in two or more
courses; the top or wearing course, being composed of the best and
most expensive stone, is made the least thickness compatible with
good construction and maintenance. To provide for the shrinkage
HIGHWAY CONSTRUCTION
93
of the stone under the roller, the depth of the courses of loose stone
should exceed the finished depth by from 25 to 30 per cent.
The stone is hauled upon the roadbed in vehicles of various
types provided with broad-tired wheels. In some types of vehicle
it is spread in layers as the vehicle is drawn along the roadbed ; with
others it is dumped in heaps and spread by hand with forks and
brought to an even surface by raking, Fig. 62.
Compacting the Broken Stone. The stone is compacted by
rolling with heavy rollers drawn by horses or propelled by steam or
other power, Fig. 63. The steam roller is more effective than horse-
Fig. 62. View Showing Spreading of Lower Course of Macadam Road
Courtesy of United States Department of Agriculture
drawn rollers. The usual weights of steam rollers are 5, 10, and 15
tons; the 10-ton being the one generally used, although the weight
of the roller should be selected in accordance with the bearing power
of the natural soil. A roller having excessive weight may cause
injury to the roadbed, by rolling it into undulations that will permit
water to collect and consequently cause damage. A roadbed which
will stand a heavy roller in dry weather may be injured by it during
wet weather. For a weak roadbed it is well to use two rollers, one of
light weight to form a crust and a narrow, heavy roller to compact it.
94
HIGHWAY CONSTRUCTION
The roller should commence at one edge or border of the road-
way, and move along that edge until within about 25 feet of one
end of the spread stone; it then should cross over to the other edge
and proceed along this edge to the beginning, crossing over and
overlapping the strips previously rolled until the center of the road
is reached. The rolling is continued in this manner until the stones
cease to creep in front or sink under the roller. If, during the first
passages of the roller, low spots appear, they should be filled to
grade with stone of the same size as is in the course being rolled.
Fig. 63. Compacting Broken Stone by Steam Roller
Courtesy of United States Department of Agriculture
After about two passages of the roller, the binder, consisting of
the screenings from the stone being used for the course, is spread in
a thin layer over the surface of the partly compacted stone and
sprinkled with water, which washes it into the voids in the stone;
the rolling then is continued, Fig. 64. The operation of applying
the binder, sprinkling, and rolling is repeated until a wave of water
and screenings rises in front of the roller. Each course is treated
and rolled in the same manner. If the screenings from the rock
that is being used are not suitable for binding, screenings from other
rock, clay, sand, or loam are substituted.
HIGHWAY CONSTRUCTION
95
An excess of binder and water will shorten the time required to
consolidate the stone and produce the appearance of a good piece of
work, but under traffic it will wear unevenly and go to pieces quickly.
Suppression of Dust on Macadam and Telford Roads. Since
the introduction of mechanically , propelled vehicles, broken-stone
roads constructed according to the principles of Telford and McAdam,
have proven inadequate to the demands of the changed traffic.
The adhesion between the particles of stone is insufficient to
react against the propulsive force exerted by the driving wheels,
Fig. 64. Rolling and Sprinkling Second Course of Macadam Road to Complete
Binding Process
Courtesy of United States Department of Agriculture
hence the stones are loosened, and although the rubber tires with
which the motor vehicles are equipped produce little dust by attri-
tion or wearing away, the vehicle moving at high speed creates a
partial vacuum. The current of air which then rushes in to re-estab-
lish the equilibrium picks up the small particles of stone displaced
and loosened by the thrust of the driving wheels and distributes
them in the form of dust, which is very disagreeable to other users
of the road and residents along it. The large stones that are loosened
96 HIGHWAY CONSTRUCTION
are thrown about and ground upon one another and thus increase
the amount of fine material ready to be scattered as dust.
The frequent repetition of these actions causes the pavement
to pit and. disintegrate. The destructive effect is intensified, the
greater the speed, and where the irregularities of the surfaces are
such as to cause the wheels to leave it, there is produced a bounding
motion that is continued for some distance and is particularly dis-
astrous. Shearing of the road fabric is very severe on steep grades
and curves due to the slipping of the driving wheels when the pro-
pulsive force is greater than the adhesion between the tire and the
road surface. The damage arising from this is more extensive
during wet weather and is intensified when the wheels are equipped
with bars, chains, studs, and other anti-skidding devices.
The formation bf dust and mud cannot be prevented absolutely,
because all materials, by attrition and the disintegrating action of
the elements, yield dust when dry and mud when wet. If the sur-
face of a water-bound macadam road could be maintained in a
moist condition, there would be no dust, but moistening with water
even in cities, towns, and villages is expensive, and in rural districts
the cost is prohibitive and the practice would be impossible, owing
to the absence of water available for the purpose. Hence in dealing
with existing road surfaces a remedy has been sought in more fre-
quent cleansing and in the use of some substitute for water which
would be cheap, effective, lasting, and easily applied. To meet this
demand several "dust-laying" compositions have been placed on
the market, and experiments have been made with some of these,
but it has been demonstrated clearly that, with but few exceptions,
they have a very temporary effect, and their application must be
frequent and thorough.
Under the head of exceptions, that is, of the more or less
permanent methods, are included the following: (1) the cementing
of the surface stone by a bituminous cement or binder. When
the binder is applied by the penetration method, the surface is
described by the general term " bituminous-macadam"; and when it is
desired to indicate the kind of binder, the descriptive names, "asphalt-
macadam", "tar-macadam", etc., are used. When the binder is
applied by the mixing method, the construction is called "bitumi-
nous-concrete", or specifically designated by the trade or patented
HIGHWAY CONSTRUCTION 97
name as, "bitulithic", "warrenite", "amiesite", "filbertine", "rock-
asphalt", etc.; (2) binding the stone with hydraulic cement, the
surface so formed being called "concrete-macadam", or "concrete
pavement". These will be discussed later under their respective
headings.
Turning to the details of the various temporary methods, we
find the following :
(1) Fresh Water. This is the simplest remedy, but not always
the most practicable nor the cheapest.
(2) Sea Water. This is a simple remedy but available only on
the seacoast. The salts contained in sea water are highly anti-
septic and deliquescent; a light sprinkling will suppress the dust for
several hours. Its use, however, is objected to for the reason that
it injures the varnish and running gear of vehicles, corrodes cast-
iron street fittings, and when the road surface on which it has been
used has dried the dust then produced, containing salt, injures food
and other goods exposed to it. Moreover, after a few weeks' use
the dust is converted into a pasty mud that adheres to the wheels
and causes the surface of the road to be "picked up".
(3) Deliquescent Salts. The chief advantage of these salts is
that their effect is more lasting than that of water. The salt used
most extensively is calcium chloride obtained as a by-product in the
manufacture of soda by the ammonia process. The salt may be
applied either in solution or in the dry form. It takes up water
rapidly and proves very efficient where the atmospheric moisture is
sufficient to feed the salt. Glutrin, the commercial name for the
waste sulphite liquor obtained in the manufacture of paper from
wood pulp by the sulphite process, reduces the formation of dust,
but the treatment must be repeated frequently. Waste molasses
or "black strap" from sugar refineries mixed with milk of lime
possesses good dust-suppressing qualities.
(4) Coal- Tar Coating. Refined coal tar applied either hot or
cold in the form of a spray minimizes the production of dust, renders
the surface waterproof, and reduces wear. The success attending
its use depends upon the quality of the tar, the state of the weather,
which must be clear and dry, the condition of the road surface,
which must be dry and free from dust and dirt, and, in the case of
hot application, that the tar is not overheated.
98 HIGHWAY CONSTRUCTION
(5) Solutions of Coal Tar and Petroleum. Several patented
preparations of coal tar are on the market. The principle of all is
practically the same, namely, the solution of the tar or oil in water
by a volatile agent, which on evaporation leaves a more or less
insoluble coating on the road surface. The more favorably known
of these preparations are "tarvia" and "westrumite".
(6) Crude Petroleum and Residuum Oil. Crude petroleum
containing a large percentage of asphalt gives the best results.
Petroleum having paraffin and naphtha as a base refuses to bind,
and produces a greasy slime. The residuum oils obtained in the
distillation of petroleum having asphaltum for a base have yielded
good results in many cases.
Two methods are followed in applying the oil: (a) The sur-
face of the road to be oiled is prepared by removing the dust with
hand or power brooms. The oil, in the cold method, is applied by
specially designed sprinkling wagons, at the rate of from one-third
to one-half gallon per square yard. After being applied the oil is
covered with sand or stone screenings and may or may not be
rolled. The oil is applied once or twice a year according to whether
the traffic is light or heavy. The surface of the road must be dry
when the oil is applied.
(b) The oil is sprinkled over the surface and mixed with the
dust. If the oil is merely sprinkled, the mixture of dust and oil
made by the action of the traffic will become very sticky and will be
removed in spots by adhering to the wheels. For the purpose of
facilitating the handling and of securing a deeper penetration
than is possible with cold oil, the oil is heated to a temperature of
about 140 Fahrenheit and applied in the same manner as the
cold oil.
(7) Oil Tar and Creosote. Oil tar is the residual liquid from
the manufacture of carbureted water gas and oil gas. The tar used
for road purposes is obtained by distilling the original tarry liquid to
remove the light oil, naphthalene, and creosote. Various grades of
tar are produced according to the temperature at which the dis-
tillation is stopped. The higher the temperature of distillation, the
harder and more brittle the tar.
The oil tar either alone or mixed with creosote is applied in the
same manner as coal tar.
HIGHWAY CONSTRUCTION 99
Bituminous=Macadam
Features of Bituminous=Macadam. A bituminous-macadam
wearing surface differs from the previously described water-bound
broken-stone surface only in the kind of binder and the quality
of the stone. The bituminous binder is prepared from asphalt,
asphaltic oils, refined water-gas tars, refined coal tars, and com-
binations of refined tars and asphalts.
The bituminous binders adhere to comparatively porous and
relatively soft stone, such as limestone, better than to the hard
stones, such as trap and granite. Consequently, the stone used
with the bituminous binders may be inferior in hardness and binding
quality to that required for water-bound macadam.
Methods of Construction. The essentials necessary to the
successful construction of a bituminous covering are: (1) the exclu-
sion of both subsoil and surface water from the foundation; (2) a
solid unyielding foundation; (3) a stone of suitable quality and
size; (4) that the stone shall be entirely free from dust, otherwise
the dust will interpose a thin film between the stone and the bitu-
minous binder and prevent the latter from adhering to the stone;
(5) if the stone is to be used hot, that it shall not be overheated;
and if is to be used cold, that it shall be dry, for if wet or damp,
the bituminous material will not adhere to it; (6) that the bituminous
cement shall be of suitable quality; free from water, for which the
stone has a greater affinity than for bitumen, and would thus pre-
vent adhesion; free from ammoniacal liquor, which is apt to saponify
some of the oily constitutents and thus render them capable of
combining with water and therefore apt to be washed out; free
from an excess of light oils and naphtha, which act as diluents
and volatilize on the surface of the road, forming a skin that is
not durable; free from an excess of free carbon, because it has no
binding value and is liable to be converted into dust and mud.
Two general methods with various modifications in the minor
details are employed for applying the bituminous binder to form
the wearing surface, viz, the penetration method, and the mixing
method.
Penetration Method. In this method, the stone is spread and
packed slightly by rolling. The bituminous binder is then applied
by one of the following ways: by hand from pouring pots; by a
100
HIGHWAY CONSTRUCTION
nozzle leading from a tank cart; or by a mechanical distributor using
air pressure to discharge the material through nozzles that spread
it in a finely divided stream or spray, Fig. 65. The binder is heated,
usually by steam from the roller, but when hand pots are used, it
is heated in kettles over fires. The quantity applied is about If
gallons per square yard. After the binder is distributed, it is covered
with a light coating of stone
dust, sand, or gravel, and the
rolling is continued. In some
cases, after the rolling is com-
pleted, another application of
the binder is made at the rate
of about one-half gallon per
square yard; this is called a
"paint coat" and is covered
with a light sprinkling of stone
screenings.
Mixing Method. In this
method the stone to be used
for the wearing surface, vary-
ing in size from J to 1J
inches, is cleaned and dried,
then mixed with a sufficient
quantity of the binder to coat
all the stones thoroughly.
The mixing is performed by
manual labor on a mixing
board, Fig. 66, or by raking
the stones through a bath of
liquid binder, or by passing
through a mechanical mixing
machine, Fig. 67. The coated stones are spread upon the foundation
in a layer having a thickness of about 3 inches and are covered
with a light coating of stone screenings free from dust; then are
compacted by rolling, Fig. 68. Wherever the binder flushes to
the surface it is covered with screenings and rolled. When the
rolling is completed, the surplus screenings are swept from the
surface. The cleaned surface then is covered with a coat of the
Fig. 65. Spreading Bituminous Binder by Pressure
Nozzle, Penetration Method
Courtesy of Barrett Manufacturing Company,
New York City
HIGHWAY CONSTRUCTION 101
Fig. 66. Hand Method of Mixing Stone and Binder
Courtesy of Barrett Manufacturing Company, New York City
Fig. 67. Machine Method of Mixing Stone and Binder
Courtesy of Barrett Manufacturing Company, New York City
Fig. 68. Rolling Bituminous Macadam Road Surface
Courtesy of Barrett Manufacturing Company, New 'York City
102
HIGHWAY CONSTRUCTION
binder called a "seal coat", for the purpose of insuring the water-
proofing and complete filling of the voids, Fig. 69. For this coat,
about one-half gallon of binder is used per square yard of surface.
Screenings again are spread and may or may not be rolled.
Advantages and Disadvantages of the Penetration Method. The
advantage of the penetration method is the ease and rapidity with
which it can be carried out, and the low cost for equipment and labor.
The disadvantages of the penetration method are: (1) the
difficulty of obtaining an absolutely uniform distribution of the binder,
thus producing "lean" and "fat" spots that will prove defective under
traffic; (2) it is wasteful, in that it is necessary to use more binder
than actually is required to coat the stones and bind them together;
7T\
Fig. 69. Spraying Seal Coat by Auto Truck, One-Half Gallon to the Yard
Courtesy of Barrett Manufacturing Company, New York City
(3) it is difficult and sometimes impossible to use a binder of suffi-
cient original consistency to produce a satisfactory bond, owing to
the bitumen setting too rapidly when applied to cold stone.
Advantages and Disadvantages of the Mixing Method. The
advantages of the mixing method are : (1) the producing of a uniform
fabric in which the cement is distributed uniformly and cements
each individual stone; (2) that construction can be carried on in
colder weather than is permissible with the penetration method.
If hot stone is used, a bitumen can be employed of such original
consistency as is required to sustain the traffic satisfactorily.
The disadvantage of the mixing method is the greater cost,
HIGHWAY CONSTRUCTION 103
due (1) to the increased labor, and (2) to the more elaborate equip-
ment and apparatus required.
Bitulithic. Bitulithic is composed of stone, ranging in size from
2 inches to ^iU of an inch, and dust, which are dried, heated,
and mixed in predetermined proportions, so as to reduce the voids
to about 10 per cent, and cemented by a hot bituminous cement
manufactured from either coal tar, asphalt, or a combination of
both. The cement is added in sufficient quantity not only to coat
every particle and to fill all of the remaining voids but with enough
surplus to result in a rubbery and slightly flexible condition of the
mixture after compression.
The mixture is spread, while hot, to such depth as will give
a thickness of 2 inches after compressing with a 10-ton roller.
After rolling, a composition coating called a "flush coat" is spread
over the surface; this being covered while sticky with hot stone
chips which are rolled until cool. The purpose of the stone chips
is to form a gritty surface to prevent slipping.
Amiesite. Amiesite is a patented preparation of crushed stone
or gravel, coated w r ith an asphaltic cement. It is laid in two courses
and a surface finish. The first course, -composed of stone ranging
from \ inch to \\ inches, is spread to a depth of 3 inches, blocks
or strips of wood being used to insure uniformity of depth, then
rolled once. The second course is composed of stone \ inch and
less, spread 1 inch deep, then rolled. The surface finish consists
of screenings or sand, used in sufficient quantity to fill the voids.
Rock Asphalt. The rock asphalt most used in the United
States is a sandstone containing from 7 to 10 per cent of asphalt.
It is prepared for use by pulverizing and is used either hot or cold.
It is spread upon the surface of the stone to a depth of about \\
inches and rolled with a steam roller; the rolling is repeated daily for
several days, or until the asphalt becomes hard.
Definitions of Bituminous Materials. The most recently
adopted definitions of the bituminous materials employed in road
construction are:
Native Bitumen. Native bitumen is a mixture of native or
pyrogenous hydrocarbons and their non-metallic derivatives, which
may be gases, liquids, viscous liquids, or solids and which are soluble
in carbon disulphide.
104 HIGHWAY CONSTRUCTION
Artificial Bitumen. Artificial bitumen is produced by the
destructive distillation of pyrobitumens and other substances of
an organic nature; the bitumens so produced are commonly known
as tars, the word tar being compounded with the name of the material
which has been subjected to the process of destructive distillation,
thus designating its origin, as, coal tar, oil tar, etc.
Bituminous. Bituminous refers to that which contains bitumen
or constitutes a source of bitumen.
Emulsions. Emulsions are oily substances made mixable with
water through the action of a saponifying agent or soap.
Fixed Carbon. Fixed carbon is the organic matter of the
residual coke obtained upon burning hydrocarbon products in a
covered vessel in the absence of free oxygen.
Fluxes. Fluxes are fluid oils and tars which are incorporated
with asphalt and semi-solid or solid oil and tar residuums for the
purpose of reducing or softening their consistency.
Residuums, Residual Petroleum, or Residual Oils. These are
heavy viscous residues produced by the evaporation or distillation
of crude petroleums until at least all of the burning oils have been
removed.
Bituminous Cement. The bituminous cements or binders
are prepared from (1) coal-, oil-, and water-gas tars; (2) asphaltic
petroleums; (3) asphalt; and (4) combinations of asphalt and the
residues of distillation from asphaltic petroleums.
Coal- Tar Binder. Coal-tar binder is the residue obtained by
the distillation of the crude tar produced in the manufacture of
illuminating gas and the manufacture of coke for metallurgical
purposes; the required consistency is obtained by removing part
or all of the contained oils. Owing to the difference in the tem-
peratures employed in the two producing processes, the constituents
of the tar, while identical in their characteristics, differ in their
amount; the most marked difference being in the free carbon con-
tent, of which coke-oven tar has the least.
Oil-Gas Tar. Oil-gas tar is produced in the manufacture of
gas from oil. The tarry residue is rather varied in character and
is prepared for use by distillation; it usually contains a large amount
of free carbon.
Water-Gas Tar. Water-gas tar is produced in the manufac-
HIGHWAY CONSTRUCTION 105
ture of carbureted water gas for illuminating purposes and results
from the petroleum product employed for carbureting. It is a
thin oily liquid containing a large percentage of water. It is pre-
pared for road use by mechanical dehydration and distillation.
It has a strong gassy odor when applied to the road, but this dis-
appears in a few days.
Asphaltic Petroleums. Asphaltic petroleums are native petro-
leums which yield asphalts upon reduction. They are used in the
crude state, or after the illuminating and other oil constituents
have been removed by cracking or blowing.
Asphalt. Asphalt is the name by which the native semi-solid
and solid bitumen is known. Asphalt is the most permanent type
of binder and has been used for many years in the construction of
sheet-asphalt pavements. The semi-solid or tarry varieties are
called "malthas" and are the ones generally employed as a binder
for broken-stone roads; the solid variety is used almost exclusively
for street pavements. Rock asphalt or bituminous rock is the name
given to a great variety of sandstone rocks more or less saturated
or cemented by maltha or hard asphalt. In referring to rock asphalts
it is customary to add the name of the locality where they are found,
as "Kentucky rock asphalt", "Italian rock asphalt", etc. The
semi-solid varieties are used in the natural state or after the water
and volatile hydrocarbons have been removed by heating. The
hard varieties are prepared by softening with a suitable flux.
Combinations of Asphalt and Distillation Residues. Com-
binations of asphalt and the residue from the distillation of tars
and petroleums are made by adding either a refined maltha or a
pulverized solid asphalt; the mixing being accomplished by the
injection of compressed air through suitably formed nozzles.
Tests for Bituminous Materials. The bituminous materials
are subjected to certain tests for the. purpose of ascertaining their
chemical and physical properties. The results of the tests are
used in specifications to secure the furnishing of the desired quality
of material and to control the processes of manufacture; also to
form a record by which the behavior of the materials under similar
and dissimilar service conditions can be compared. The complex
character of the materials requires a suitably equipped laboratory
for the application of the tests.
106 HIGHWAY CONSTRUCTION
The tests are determinations of (1) amount of water contained;
(2) materials soluble in water; (3) homogeneity at a temperature
of 77 Fahrenheit; (4) specific gravity; (5) consistency or vis-
cosity measured by a standard penetration machine; (6) ductility,
or the distance the material can be drawn out before breaking;
(7) toughness, or resistance to fracture under blows in an impact
machine ; (8) melting or softening point, measured by a thermometer
while the temperature is raised through a water or oil bath; (9)
distillation the products yielded at different temperatures during
continuous distillation in a suitable flask or retort are caught and
weighed; (10) amount of free carbon a sample is dissolved in carbon
bisulphide, the solution filtered, and the insoluble residue weighed;
(11) amount of fixed carbon a sample is placed in a platinum
crucible and heated until the emission of flame and smoke ceases,
then is allowed to cool and the residue is weighed; and the difference
between the weight of the sample and the residue is the fixed carbon;
(12) paraffine the presence of paraffine is determined by treating
a sample with absolute ether, freezing the mixture, filtering the
precipitate, evaporating and weighing the residue; (13) amount
of bitumen contained found by weighing a sample of the dried
material, by adding carbon bisulphide to dissolve the bitumen,
and by drying and weighing the residue after the extraction is
completed. The loss is the amount of bitumen soluble in
carbon bisulphide. A sample also is treated with naphtha and
the character of the residue is noted as to whether it is sticky
or oily.
Concrete Pavements
Construction Methods. Several methods with many varia-
tions are employed for the construction of concrete pavements.
The principal ones are: (1) grouting method, the construction
being commonly called "concrete macadam"; (2) mixing method;
(3) block or cube method.
Grouting Method. In this method the stone is spread upon
the foundation and lightly rolled, after which a mixture of one
part of cement and three parts of sand in the dry state is spread over
the stone and swept into the interstices by brooms, then sprinkled
with water and rolled; more cement and sand are spread, sprinkled,
and rolled; the operation is repeated until the interstices are filled.
HIGHWAY CONSTRUCTION 107
A variation of this method, known as the Hassam paving, is made "
by spreading the stone, ranging in size from 1| inches to 2J inches,
and rolling it to a thickness of 4 inches, then filling the interstices
with a grout composed of one part cement and three parts of sand
mixed with water in a mixing machine, from which it flows over
the surface, the machines being drawn along the roadway for this
purpose; rolling is proceeded with at the same time and sufficient
grout is applied to fill the interstices. On the foundation so prepared
a wearing surface is formed; the stone is spread in the same manner
as in the first course; the grout, composed of one part cement and
two parts sand, mixed with sufficient water to make it very fluid, is
applied by flowing over the surface of the compacted stone. The
surface is finished by applying a thick grout composed of one part
each cement, sand, and pea-sized trap rock.
Mixing Method. In this method the ingredients are com-
bined into concrete by either hand or machine mixing; the con-
crete is deposited in place in one or two courses, the former being
called "one-course" pavement and the latter "two-course" pave-
ment. In the one-course method, the concrete mixed in the pro-
portions of one part cement, one and one-half parts sand, and three
parts stone is spread upon the prepared natural soil foundation
and tamped to a thickness of about 6 inches.
In the two-course work the concrete mixed in the proportions
of one part cement, two and one-half parts sand, and five parts
stone is spread upon the prepared natural-soil foundation and com-
pacted by rolling or tamping to the required thickness. On its
surface, and before the cement has set, the wearing surface of about
2 inches in thickness is placed and tamped to the required contour.
The mixtures used for the wearing surface vary, being composed
of sand and cement, or of sand, cement, and small broken stone.
The wearing surface of the Blome pavement is composed of one
part cement and one and one-half parts of aggregate, which is
made up of 50 per cent J-inch, 30 per cent J-inch, and 20 per cent
-tV-inch granite screenings. The surface is formed into 4J-inch
by 9-inch blocks by cutting grooves \ inch wide and \ inch deep
by means of special tools.
Materials. The materials used in the construction of concrete
pavements should be selected with care. The stone should be a
108 HIGHWAY CONSTRUCTION
hard tough rock, free from dust and dirt, and graded so as to reduce
voids to the minimum. The sand should be free from loam, clay,
vegetable and organic matter, and should grade from coarse to
fine. The cement should be of a quality to meet the standard
tests. The water should be clean and free from organic matter,
alkalies, and acids. Rapid drying of the concrete should be pre-
vented by covering it with a canvas which is kept moistened with
water for several hours; after its removal the surface should be
covered with sand or earth which is to be kept moist for a period
of two weeks. Improperly mixed or constructed concrete pave-
ment will wear unevenly, crack, and rapidly become very defective.
Expansion Joints. To provide for the expansion and con-
traction of the concrete under changes of temperature, expansion
joints are formed at intervals ranging from 15 to 50 feet. The
edges of the joints are protected from injury by angle irons, and
the space between them, about J inch, is filled with a bituminous
cement which extends the full depth of the concrete. When the
concrete is laid between curbs longitudinal joints from f inch to
IJ inches wide, filled with bituminous cement, are formed along
the curb.
Reinforced-Concrete Pavement. Concrete pavements rein-
forced with steel in the form of woven-wire, Fig. 70, expanded metal,
and round bars are constructed in two courses, the reinforcement
being placed between the foundation course and the wearing surface.
Concrete with Bituminous Surface. In this type the surface
of the concrete pavement, constructed by either the grouting or
mixing method, is covered with a bituminous cement made from
either asphalt, coal tar, or a mixture of both.
Block or Cube Pavement. In' this type of pavement, blocks
or cubes of concrete are molded in a machine or cast in molds.
The blocks are stacked and allowed to season for three months,
during which time they are wet twice a day. They are laid by
hand on a sand cushion spread upon the foundation, then are brought
to a firm bearing and uniform surface by rolling with a light roller.
The surface is covered with a layer of sand or sandy loam which
is broomed and flushed by water into the joints and the rolling is
repeated; after which the surface is covered with a layer of sand,
and the traffic then admitted.
HIGHWAY CONSTRUCTION
109
A variation from the methods described is made in the patented
pavement "rocmac". This is composed of broken stone cemented
by silicate of lime, obtained by mixing powdered carbonate of lime
with a solution of silicate of soda and sugar. The silicate of lime
mortar is spread upon the foundation to a depth of about 2 inches,
over which the broken stone is distributed to such a depth as will
give, when compacted, a depth of about 4 inches. It is rolled and
sprinkled with water until the mortar flushes to the surface, and
Fig. 70. Laying Reinforced Concrete Road. Woven Wire Fabric in Foreground Ready
to Be Placed between Upper and Lower Coat
Courtesy of Municipal Engineering and Contracting Company, Chicago
then is covered with a layer of stone screenings and finally opened
to traffic.
MAINTENANCE AND IMPROVEMENT OF ROADS
Repair and Maintenance of Broken=Stone Roads. These
terms frequently but erroneously are used interchangeably. Repair
means the restoring of a surface so badly worn that it cannot be
maintained in good condition. A well-maintained road should not
require repairs for a considerable length of time. The maintenance
of a road is the keeping of it, as nearly as practicable, in the same
condition as it was when constructed.
Good maintenance comprises: (1) constant daily attention
to repair the ravages of traffic and the elements; (2) cleansing to
110 HIGHWAY CONSTRUCTION
remove the detritus caused by wear, the horse droppings, and
other refuse; and (3) application of water or other dust layer.
When the surface of a water-bound broken-stone road requires
to be renewed, it is loosened and broken up by scarifying, the new
stone spread, rolled, watered, and bound in the same manner as in
new construction; or the old surface is cleansed from dust and
other matter by sweeping and washing, and the new stone spread
upon it, compacted and finished as in new construction.
The resurfacing of water-bound roads with a bituminous con-
struction is becoming common. The methods employed are the
same as heretofore described under "bituminous macadam".
Systems of Maintenance. Several systems for maintaining
roads are in use, the one yielding the best results being that which
provides for the continuous employment of skilled workmen. The
men so employed become familiar with the peculiarities of the
sections in their charge and with the best way to deal with them.
Efficient maintenance requires that the surfaces be kept smooth
so that surface water may flow away rapidly" and that the injury
caused by traffic on uneven surfaces may be avoided; that incipient
ruts, hollows, and depressions be eliminated by cutting out the area
involved in the form of a square or rectangle and filling with new
material; that dust and horse droppings be removed; that loose
stones be removed; that gutters be clear so the rain water may be
removed quickly; that ditches and culverts be cleaned out in advance
of the spring and fall rains; that weeds and grass be removed from
the earth shoulders, and that these and the dust sweepings be not
left on the sides of the road to be redistributed, but be removed
immediately and disposed of in such manner as will not cause injury;
that bridges be examined and repaired at least twice a year.
Improvement of Existing Roads. The improvement of existing
roads may be divided into three branches: (1) rectification of align-
ment; (2) drainage; (3) improvement of the surface.
The first of these consists in the application of the principles
which have been laid down for the location, etc., of new roads and
will include straightening the course by eliminating unnecessary
curves and bends ; improving the grade either by avoiding or cutting
down hills and by embanking valleys; increasing the width where
necessary, and rendering it uniform throughout.
HIGHWAY CONSTRUCTION 111
The second, or drainage, consists in applying the principles laid
down for the drainage of new roads, and in constructing the works
necessary to give them effect.
The third, or improvement of the surface, consists in improving
the surface by any of the methods previously described and that
the funds available will permit.
Value of Improvement. The improvement of roads is chiefly an
economical question relating to the w T aste of effort and to the saving
of expenditure. Good roads reduce the resistance to locomotion,
and this means reduction of the effort required to move a given load.
Any effort costs something, and so the smallest effort costs the least,
and therefore the smoothest road saves the most money for every-
one who traverses it with a vehicle.
Cost of Improvement. Before undertaking any improvement
generally it is required to know the cost of the proposed improve-
ment and the benefits it will produce. In the improvement of roads
the amount of money that may be expended profitably for any
proposed improvement may be calculated with sufficient accuracy
by obtaining first the following data: (1) the quantity and quality
of the traffic using the road; (2) the cost of haulage; (3) plan and
profile of the road; and (4) character and cost of the proposed
improvements. From the data ascertain the total annual traffic and
the total annual cost of hauling it. Next, calculate the annual cost
of hauling the given tonnage over the road when improved. Then
the difference between the two costs will represent the annual inter-
est on the sum that may be expended in making the improvement.
For example, if the annual cost of haulage over the existing road is
$10,000 and the cost for hauling the same tonnage over the improved
road will be $7000, the difference, $3000, with money at 6 per cent
per annum, represents the sum of $50,000 that logically may be
appropriated to carry out the improvement.
Traffic Census. The direction, character, and amount of traffic
using a road is obtained by direct observation during different
seasons of the year. As a preliminary to observing the traffic it is
usual to determine the weight of the vehicles; this is done by weigh-
ing typical vehicles and by establishing an average weight for each
type. The traffic is classified according to the motive power as
horse-drawn vehicles and motor vehicles. Each of these classes is
112
HIGHWAY CONSTRUCTION
TABLE XI
Traffic Census
Average Hours per Day ; for
Taken at
By
.Days
EMPTY VEHICLES
LOADED VEHICLES
Nov. to
March
Aug. to
Oct.
Nov. to
March
Aug. to
Oct.
Pleasure
Horse < / T - , ,
[Light
[ Commercial < Medium
(Heavy
fMotorcycles
Pleasure j Runabouts
[Touring cars
Motor
(Light
Commercial j Medium
( Heavy
divided into pleasure and commercial traffic, the latter class being
subdivided into loaded and non-loaded vehicles. The number of
horses to a vehicle in horse-drawn traffic and the speed of motor
vehicles may be noted. A summary of data is suggested in Table XL
The observations are made from 6 a. m. to 6 p. m., during a period
of seven days each month, with occasional observations, from 6 p. m.
to 6 a. m. or for the entire 24 hours if the amount of traffic requires it.
The weight of the traffic is ascertained by multiplying the
number of each kind of vehicle by the average weight established
for that type.
HIGHWAY CONSTRUCTION
PART II
CITY STREETS AND HIGHWAYS
The first work requiring the skill of the engineer is the laying
out of town sites properly, especially with reference to the future
requirements of a large city, where any such possibility exists.
Few if any of our large cities were so planned. The same principles,
to a limited extent, are applicable to all towns or cities. The topog-
raphy of the site should be studied carefully, and the street lines
adapted to it. These lines should be laid out systematically, with a
view to convenience and comfort, and also with reference to econ-
omy of construction, future sanitary improvements, grades, and
drainage.
Arrangement of City Streets. Generally, the best method of
laying out streets is in straight lines, with frequent and regular inter-
secting streets, especially for the business parts of a city. When
there is some centrally located structure, such as a courthouse, city
hall, market, or other prominent building, it is very desirable to have
several diagonal streets leading thereto. In the residence portions
of cities, especially if on hilly ground, curves may replace straight
lines with advantage, by affording better grades at less cost of grad-
ing, and by improving property through avoiding heavy embank-
ments or cuttings.
Width of Streets. The width of streets should be proportioned
to the character of the traffic that will use them. No rule can be
laid down by which to determine the best width of streets; but it
may be said safely that a street which is likely to become a com-
mercial thoroughfare should have a width of not less than 120 feet
between the building lines the carriage-way 80 feet wide, and the
sidewalks each 20 feet wide.
In streets occupied entirely by residences a carriage-way 32 feet
wide will be ample, but the width between the building lines may be
114
HIGHWAY CONSTRUCTION
as great as desired. The sidewalks may be any amount over 10 feet
which fancy dictates. Whatever width is adopted for them, not
more of it than 8 feet need be paved, the remainder being occupied
with grass and trees.
Street Grades. The grades of city streets depend upon the
topography of the site. The necessity of avoiding deep cuttings or
high embankments which seriously would affect the value of adjoin-
ing property for building purposes, often demands steeper grades
than are permissible on country roads. Many cities have paved
streets on 20 per cent grades. In establishing grades through unim-
proved property, they usually may be laid with reference to securing
the most desirable percentage within a proper limit of cost. But
when improvements already
have been made and have been
located with reference to the
natural surface of the ground,
the matter of giving a desirable
grade without injury to adjoin-
ing property frequently is one
of extreme difficulty. In such
cases it becomes a question of
a
Fig. 71. Diagram Showing Arrangement of
Grades at Street Intersections
how far individual interests shall
be sacrificed to the general good.
There are, however, certain con-
ditions which it is important to
bear in mind: (1) That the
longitudinal crown level should be sustained uniformly from street
to street intersection, whenever practicable. (2) That the grade
should be sufficient to drain the surface. (3) That the crown levels
at all intersections should be extended transversely, to avoid forming
a depression at the junction.
A rrangements of Grades at Street Intersections . The best arrange-
ment for intersections of streets when either or both have much
inclination is a matter which requires careful consideration and
upon which much diversity of opinion exists. No hard or fast rule
can be laid down; each will require special adjustment. The best
and simplest method is to make level the rectangular space aaaaaaaa,
Fig. 71, with a rise of one-half inch in 10 feet from AAAA to B,
HIGHWAY CONSTRUCTION
115
placing gulleys at AAAA and the catch basins at ccc. When this
method is not practicable, adopt such a grade (but one not exceeding
2J per cent) that the rectangle AAAA shall appear to be nearly
level; but to secure this it must have actually a considerable dip in
the direction of the slope of the street. If steep grades are con-
tinued across intersections, they introduce side slopes in the streets
thus crossed, which are troublesome, if not dangerous, to vehicles
turning the corners, especially the upper ones. Such intersections
W* 1
IlklU
M{p<3./d kj
1 *^ 1
I 1
! 46 75^
I 1
^ ] (jO'
1 9^ ' !
( \50.3 95.96 |>
95.96 _/|
,5^'
1Z
JZ'
^' /"
s
^
1
N
^
t
^ 5ft?'
5\- e
\\ll.90 31. IK
;
50^ Vj
' Q^
1 |
M ~^
tfi !
t- A !
fa. 72
r
Fig. 72. Diagram Showing Arrangement of Intersections for Steep Grades in
Duluth, Minnesota
are especially objectionable in rainy weather. The storm water
will fall to the lowest point, concentrating a large quantity of water
at two receiving basins, which, with a broken grade, could be divided
among four or more basins.
Fig. 72 shows the arrangement of intersections in steep grades
adapted for the streets of Duluth, Minnesota. From this it will be
seen that at these intersections the grades are flattened to 3 per
cent for the width of the roadway of the intersecting streets, and that
the grade of the curbs is flattened to 8 per cent for the width of the
116
HIGHWAY CONSTRUCTION
intersecting sidewalks. Grades of less amount on roadway or side-
walk are continuous. The elevation of block corners is found by
adding together the curb elevations at the faces of the block corners,
and 2J per cent of the sum of the widths of the two sidewalks at the
corner, and dividing the whole by two. This gives an elevation
equal to the average elevation of the curbs at the corners, plus an
average rise of 2J per cent across the width of the sidewalk.
"Accommodation summits" have to be introduced between
street intersections in two general cases: (1) in hilly localities, to
avoid excessive excavation; and (2) when the intersecting streets
are level or nearly so, for the purpose of obtaining the fall necessary
for surface drainage.
The elevation and location of such a summit may be calculated
as follows: Let A, Fig. 73, be the elevation of the highest corner;
Fig. 73. Diagrams for Calculating "Accommodation Summits" between
Street Intersections
B, the elevation of the lowest corner; D, the distance from corner
to corner; and R, the rate of the accommodation grade. The
elevation of the summit is equal to
DXR+A+B
The distance from A or B is found by subtracting the elevation of
either A or B from this quotient, and dividing the result by the rate
of grade. Or the summit may be located mechanically by specially
prepared scales. Prepare two scales divided to correspond to the
rate of grade; that is, if the rate of grade be 1 foot per 100 feet, then
one division of the scale should equal 100 feet on the map scale.
HIGHWAY CONSTRUCTION 117
These divisions may be subdivided into tenths. One scale should
read from right to left, and one from left to right.
To use the scales, place them on the map so that their figures
correspond with the corner elevations; then, as the scales read in
opposite directions, there is of course some point at which the oppo-
site readings will be the same. This point is the location of the
summit, and the figures read off the scale give its elevation. If the
difference in elevation of the corners is such as not to require an
intermediate summit for drainage, it will be apparent as soon as
the scales are placed in position.
When an accommodation summit is employed, it should be
formed by joining the two straight grade lines by a vertical curve,
as described in Part I. The curve should be used both in the crown
of the street and in the curb and footpath.
Where the grade is level between intersections, sufficient fall for
surface drainage may be secured without the aid of accommodation
summits, by arranging the grades as shown in Fig. 74. The curb is
Curb Level
_. ~ ' Bottom of Gutter , .
Fig. 74. Diagram Showing Arrangement of Grades to Avoid
"Accommodation Summits"
set level between the corners; a summit is formed in the gutter; and
receiving basins are placed at each corner.
Transverse Grade. In its transverse grade the street should be
level ; that is, the curbs on opposite sides should be at the same level,
Fig. 75. Street with Unequal Transverse Grade but with Level Street
and the street crown rise equally from each side to the center. But
in hillside streets this condition cannot be fulfilled always, and
opposite sides of the street may differ as much as 5 feet. In such
cases the engineer will have to use his discretion as to whether he
shall adopt a straight slope inclining to the lower side, thus draining
118
HIGHWAY CONSTRUCTION
the whole street by the lower gutter, or adopt the three-curb method
and sod the slope of the higher side.
In the improvement of old streets with the sides at different
levels, much difficulty will be met, especially where shade trees have
1
Fig. 76. Street with Unequal Transverse Grade Inclined so as to Drain by Lower Gutter
to be spared. In such cases, recognized methods have to be aban-
doned, and the engineer will have to adopt methods of overcoming
the difficulties in accordance with the conditions and necessities of
each particular case. Figs. 75, 76, and 77 illustrate several typical
Fig. 77. Street with Unequal Transverse Grade with Three Curbs and Higher Slope Sodded
arrangements in the cases of streets where the opposite sides are at
different levels.
Transverse Contour or Crown. The reason for crowning a
pavement i.e., making the center higher than the sides is to
provide for the rapid drainage of the surface. The most suitable
form for the crown is the parabolic curve, which may be started at
the curb line, or at the edge of the gutter adjoining the carriage-way,
Fig. 78. Method of Obtaining Transverse Contour or Crown of a Road
about one foot from the curb. Fig. 78 shows this form, which is
obtained by dividing the abscissa, or width from the center of the
street to the gutter, into ten equal parts, and by dropping perpen-
diculars at each of these divisions, the lengths of which are deter-
mined by multiplying the rise at the center by the square of the
HIGHWAY CONSTRUCTION
119
TABLE XII
Rise of Pavement Center above Gutter for
Different Paving Materials
PAVING MATERIAL
PROPORTIONS OF RISE AT
CENTER TO WIDTH OF
CARRIAGE-WAY
Wood blocks
Stone blocks
Brick
Asphalt
1 : 100
1 :80
1 :80
1 :80
successive values of the abscissas. The amounts thus obtained can
be added to the rod readings; and the stakes, set at the proper dis-
tance across the street, with their tops at this level, will give the
required curve.
The amount of transverse rise, or the height of the center above
the gutters, varies with the different paving materials; smooth pave-
ments requiring the least, and rough ones and earth the greatest
rise. The rise is generally stated in a proportion of the width of the
carriage-way. The most suitable proportions are shown in Table XII.
Drainage of Streets. Sub-Foundation Drainage. The sub-
foundation drainage of streets cannot be effected by transverse
drains, because of their liability to disturbance by the introduction
of gas, water, and other pipes.
Longitudinal drains must be depended upon entirely; they may
be constructed of the same materials and in the same manner as road
drains. The number of these longitudinal drains must depend upon
the character of the soil. If the soil is moderately retentive, a single
row of tiles or a hollow invert placed under the sewer in the center of
the street generally will be sufficient; or two rows of tiles may be
employed, one placed outside each curb line. If, on the other hand,
the soil is exceedingly wet and the street very wide, four or more
lines may be employed. These drains may be permitted to dis-
charge into the sewers of the transverse streets.
Surface Drainage. The removal of water falling on the street
surface is provided for by collecting it in the gutters, from which it is
discharged into the sewers or other channels by means of catch basins
placed at all street intersections and dips in the street grades.
Gutters. The gutters must be of sufficient depth to retain all the
water which reaches them and prevent its overflowing on the foot-
120 HIGHWAY CONSTRUCTION
path. The depth should never be less than 6 inches, and very
rarely need be more than 10 inches.
Catch Basins. Catch basins are of various forms, usually circu-
lar or rectangular, built of brick masonry coated with a plaster of
Portland cement. Whichever form is adopted, they should fulfill
the following conditions :
(1) The inlet and outlet should have sufficient capacity to
receive and discharge all water reaching the basin.
(2) The basins should have sufficient capacity below the outlet
to retain all sand and road detritus, and prevent its being carried
into the sewer.
(3) They should be trapped so as to prevent the escape of
sewer gas. (This requirement frequently is omitted, to the detri-
ment of the health of the people.)
(4) They should be constructed so that the pit can be cleaned
out easily.
(5) The inlet should be constructed so as not to be choked
easily by leaves or debris.
(6) They must offer the least possible obstruction to traffic.
(7) The pipe connecting the basin to the sewer should be
freed easily of any obstruction.
The bottoms of the basins should be 6 or 8 feet below the street
level; and the water level in them should be from 3 to 4 feet lower
than the street surface, as a protection against freezing.
The capacity and number of basins will depend upon the area
of the surface which they drain.
In streets having level or light longitudinal grades, gullies may
be formed along the line of the gutter at such intervals as may be
found necessary.
Catch basins usually are placed at the curb line. In several
cities, the basin is placed in the center of the street, and connects to
inlets placed at the curb line. This reduces the cost of construction
and cleaning, and removes from the sidewalk the dirty operations of
cleaning the basins.
Catch basins and gully pits require cleaning out at frequent
intervals; otherwise the odor arising from the decomposing matter
contained in them will be very offensive. No rule can be laid down
for the intervals at which the cleaning should be done, but they must
HIGHWAY CONSTRUCTION 121
be cleaned often enough to prevent the matter in them from putre-
fying. There is no uniformity of practice observed by cities in this
matter; in some, the cleaning is done but once a year; in others, after
every rain-storm; in still others, at intervals of three or four months;
while in a few cities the basins are cleaned out once a month.
FOUNDATIONS
The stability, permanence, and maintenance of any pavement
depend upon its foundation. If the foundation is weak, the surface
soon will settle unequally, forming depressions and ruts. With a
good foundation, the condition of the surface will depend upon
the material employed for the pavement and upon the manner of
laying it.
The essentials necessary to the forming of a good foundation are :
(1) The entire removal of all vegetable, perishable, and yielding
matter. It is of no use to lay good material on a bad substratum.
(2) The drainage of the subsoil wherever necessary. A per-
manent foundation can be secured only by keeping the subsoil dry;
for, where water is allowed to pass into and through it, its weak spots
will be discovered quickly, and settlement will take place.
(3) The thorough compacting of the natural soil by rolling with
a roller of proper weight and shape until there is formed a uniform
and unyielding surface.
(4) The placing on the natural soil so compacted of a thickness
of an impervious and incompressible material sufficient to cut off all
communication between the soil and the bottom of the pavement.
The character of the natural soil over which the roadway is to be
built has an important bearing upon the kind of foundation and the
manner of forming it; each class of soil will require its own special
treatment. Whatever its character, it must be brought to a dry and
tolerably hard condition by draining and rolling. Sand and gravels
which do not hold water, present no difficulty in securing a solid and
secure foundation; clays and soils retentive of water are the most
difficult. Clay should be excavated to a depth of at least 8 inches
below the bottom of the finished covering; and the space so excavated
should be filled in with sand, furnace slag, ashes, coal dust, oyster
shells, broken brick, or other materials which are not absorbent of
water excessively. A clay soil or one retaining water may be cheaply
122 HIGHWAY CONSTRUCTION
and effectually improved by laying cross drains with open joints at
intervals of 50 or 100 feet. These drains should be not less than
18 inches below the surface, and the trenches should be filled with
gravel. They should be 4 inches in internal diameter, and should
empty into longitudinal drains.
Sand and planks, gravel and broken stone successively have
been used to form the foundation for pavements; but, although emi-
nently useful materials, their application to this purpose always has
been a failure. Being inherently weak and possessing no cohesion,
the main reliance for both strength and wear must be placed upon
the surface covering. This covering usually (except in case of sheet
asphalt) composed of small units, with joints between them varying
from | inch to 1| inches posesses no elements of cohesion; and
under the blows and vibrations of traffic the independent units or
blocks will settle and be jarred loose. On account of their porous
nature, the subsoil quickly becomes saturated with urine and sur-
face waters, which percolate through the joints; winter frosts upheave
them; and the surface of the street becomes blistered and broken up
in dozens of places.
Concrete. As a foundation for all classes of pavement (broken
stone excepted), hydraulic-cement concrete is superior to any other.
When properly constituted and laid, it becomes a solid, coherent mass,
capable of bearing great weight without crushing. If it fail at all, it
must fail altogether. The concrete foundation is the most costly,
but this is balanced by its permanence and by the saving in the cost
of repairs to the pavement which it supports. It admits of access to
subterranean pipes with less injury to the neighboring pavement
than any other, for the concrete may be broken through at any
point without unsettling the foundation for a considerable distance
around it, as is the case with sand or other incoherent material; and
when the concrete is replaced and set, the covering may be reset at
its proper level, without the uncertain allowance for settlement
which is necessary in other cases.
Thickness of Course. The thickness of the concrete bed must
be proportioned by the engineer; it should be sufficient to provide
against breaking under transverse strain caused by the settlement of
the subsoil. On a well-drained soil, 6 inches will be found sufficient;
but in moist and clayey soils, 12 inches will not be excessive. On
HIGHWAY CONSTRUCTION 123
such soils a layer of sand or gravel, spread and compacted before
placing the concrete, will be found very beneficial.
The proportions of the ingredients required for the manufacture
of concrete are ascertained by measuring the voids in each ingredient.
The strongest concrete will be produced when the volume of cement
is slightly in excess of that required to fill the voids in the sand, and
the volume of the combined cement and sand exceeds by about 10
per cent the volume of the voids in the stone or other material used
for the aggregate. Concrete frequently is mixed in the arbitrary
proportions of 1 part of cement, 3 parts of sand, and 6 parts of stone,
and although the results have been satisfactory, the proportions may
not be the most economical.
The ingredients of the concrete should be thoroughly mixed
with just sufficient water to produce a plastic mass, without any
surplus water running from it. After mixing, the concrete should
be deposited quickly in place, and brought to a uniform surface and
thickness by raking, then tamped until the mortar flushes to the
surface, then left undisturbed until set. The surface of concrete
laid during dry, warm, weather should be protected from the drying
action of the sun while the initial setting is in progress. This may
be accomplished by sprinkling with water as frequently as the rate
of evaporation demands or by covering it with a layer of damp
sand, straw, hay, or canvas. During freezing weather it is customary
to suspend the laying of concrete for the reason that alternate freez-
ing and thawing disintegrate it.
Measuring Voids in the Stone and Sand. The simplest method
for measuring the voids and one sufficiently accurate for the manu-
facture of concrete is the "pouring method' 5 in which a suitable
vessel of known capacity (usually one cubic foot) is filled with the
material, in which it is desired to ascertain the voids. Water then
is poured into the vessel until its surface is flush with the surface of
the material. The water is measured, and its amount is considered
to equal the total of the voids.
STONE=BLOCK PAVEMENTS
Stone blocks commonly are employed for pavements where
traffic is heavy. The material of which the blocks are made should
possess sufficient hardness to resist the abrasive action of traffic, and
124
HIGHWAY CONSTRUCTION
TABLE XIII
Specific Gravity, Weight, Resistance to Crushing, and Absorptive
Power of Stones
MATERIAL
SPECIFIC
GRAVITY
WEIGHT
(Ib. per
cu. ft.)
RESISTANCE
TO CRUSHING
(Ib. per sq. in.)
PERCENTAGE
OF WATER
ABSORBED
Min.
Max.
Min.
Max.
Min.
Max.
Min.
Max.
Granite
2.60
2.80
163
176
12,000
35,000
0.066
0.155
Trap
2.86
3.03
178
189
19,000
24,000
0.000
0.019
Sandstone
2.23
2.75
137
170
5,000
18,000
0.410
5.480
Limestone
1.90
2.75
118
175
7,000
20,000
0.200
5.000
Brick, paving
1.95
2.55
10,000
20,000
sufficient toughness to prevent them from being broken by the
impact of loaded wheels. The hardest stones will not give neces-
sarily the best results in the pavement, since a very hard stone
usually wears smooth and becomes slippery. The edges of the
block chip off, and the upper face becomes rounded, thus making the
pavement very rough.
The stone sometimes is tested to determine its strength, resist-
ance to abrasion, etc. ; but, as the conditions of use are quite different
from those under which it may be tested, such tests are seldom
satisfactory. However, examination of a stone as to its structure,
the closeness of its grain, its homogeneity, porosity, etc., may assist
in forming an idea of its value for use in a pavement. A low degree
of permeability usually indicates that the material will not be
greatly affected by frost. For data see Table XIII.
Materials. Granite. Granite is employed more extensively for
stone-block paving than is any other variety of stone; and because
of this fact, the term "granite paving" is generally used as being
synonymous with stone-block paving. The granite employed should
be of a tough, homogeneous nature. The hard, quartz granites
usually are brittle, and do not wear well under the blows of horses'
feet or the impact of vehicles; granite containing a high percentage
of feldspar will be injuriously affected by atmospheric changes; and
granite in which mica predominates will wear rapidly on account
of its laminated structure. Granite possesses the very important
property of splitting in three planes at right angles to one another,
HIGHWAY CONSTRUCTION 125
so that paving blocks may readily be formed with nearly plane faces
and square corners. This property is called the rift or cleavage.
Sandstones. Sandstones of a close-grained, compact nature
often give very satisfactory results under heavy traffic. They are
less hard than granite, and wear more rapidly, but do not become
smooth and slippery. Sandstones are generally known in the
market by the name of the quarry or place where produced as
"Medina", "Berea", etc.
Trap Rock. Trap rock, while answering well the requirements
as to durability and resistance to wear, is objectionable on account
of its tendency to wear smooth and become slippery; it is also diffi-
cult to break into regular shapes.
Limestone. Limestone usually has not been successfully em-
ployed in the construction of block pavements, on account of its
lack of durability against atmospheric influences. The action of
frost commonly splits the blocks; and traffic shivers them, owing
to the lamination being vertical.
Cobblestone Pavement. Cobblestones bedded in sand possess
the merit of cheapness, and afford an excellent foothold for horses;
but the roughness of such pavements requires the expenditure of a
large amount of tractive energy to move a load over them. Aside
from this, cobblestones are entirely wanting in the essential requisites
of a good pavement. The stones being of irregular size, it is almost
impossible to form a bond or to hold them in place. Under the
action of the traffic and frost, the roadway soon becomes a mass of
loose stones. Moreover, cobblestone pavements are difficult to
keep clean, and very unpleasant to travel over.
Belgian=Block Pavement. Cobblestones were displaced by
pavements formed of small cubical blocks of stone. This type of
pavement was laid first in Brussels, thence imported to Paris, and
from there taken to the United States, where it has been widely
known as the "Belgian-block" pavement. It has been largely used
in New York City, Brooklyn, and neighboring towns, the material
being trap rock obtained from the Palisades on the Hudson River.
The stones, being of regular shape, remain in place better than
cobblestones; but the cubical form (usually 5 inches in each dimen-
sion) is a mistake. The foothold is bad; the stones wear round; and
the number of joints is so great that ruts and hollows are quickly
126 HIGHWAY CONSTRUCTION
formed. This pavement offers less resistance to traction than cobble-
stones, but it is almost equally rough and noisy.
Granite=Block Pavement. The Belgian block gradually has
been displaced by the introduction of rectangular blocks of granite.
Blocks of comparatively large dimensions were employed at first.
They were from 6 to 8 inches in width on the surface, from 10 to 20
inches in length, with a depth of 9 inches. They merely were placed
in rows on the subsoil, perfunctorily rammed, the joints filled with
sand, and the street thrown open to traffic. The unequal settlement
of the blocks, the insufficiency of the foothold, and the difficulty of
cleansing the street, led to the gradual development of the latest
type of stone-block pavement, which consists of narrow, rectangular
blocks of -granite, properly proportioned, laid on an unyielding and
impervious foundation, with the joints between the blocks filled
with an impermeable cement.
Experience has proved beyond doubt that this latter type of
pavement is the most enduring and economical for roadways sub-
jected to heavy and constant traffic. Its advantages are many,
while its defects are few.
Advantages.
(1) Adapted to all grades.
(2) Suits all classes of traffic.
(3) Exceedingly durable.
(4) Foothold, fair.
(5) Requires but little repair.
(6) Yields but little dust or mud.
(7) Facility for cleansing, fair.
Defects.
(1) Under certain conditions of the atmosphere, the surface of
the pavement becomes greasy and slippery.
(2) The incessant din and clatter occasioned by the movement
of traffic is an intolerable nuisance; it is claimed by many physicians
that the noise injuriously affects the nerves and health of persons
who are obliged to live or do business in the vicinity of streets
so paved.
(3) Horses constantly employed upon it soon suffer from the
continual jarring produced in their legs and hoofs, and quickly
wear out.
HIGHWAY CONSTRUCTION 127
(4) The discomfort of persons riding over the pavement is very
great, because of the continual jolting to which they are subjected.
(5) If stones of an unsuitable quality are used for example,
those that polish the surface quickly becomes slippery and exceed-
ingly unsafe for travel.
Blocks. Size and Shape. The proper size of blocks for paving
purposes has been a subject of much discussion, and a great variety
of forms and dimensions are to be found in all cities.
For stability, a certain proportion must exist between the depth,
the length, and the breadth. The depth must be such that when the
wheel of a loaded vehicle passes over one edge of the upper surface
of a block, the block will not tend to tip up. The resultant direction
of the pressure of the load and adjoining blocks always should tend
to depress the whole block vertically ; where this does not happen, the
maintenance of a uniform surface is impossible. To fulfill this re-
quirement, it is not necessary to make the block more than 6 inches
deep.
Width. The maximum width of blocks is controlled by the
size of horses' hoofs. To afford good foothold to horses drawing
heavy loads, it is necessary that the width of each block, measured
along the street, shall be the least possible consistent with stability.
If the width be great, a horse drawing a heavy load, attempting to find
a joint, slips back, and requires an exceptionally wide joint to pull
him up. It is therefore desirable that the width of a block shall not
exceed 3 inches; or that four blocks, taken at random and placed
side by side, shall not measure more than 14 inches.
Length. The length, measured across the street, must be
sufficient to break joints properly, for two or more joints in line lead
to the formation of grooves. For this purpose the length of the
block should be not less than 9 inches nor more than 12 inches.
Form. The blocks should be well squared, and must not taper
in any direction; sides and ends should be free from irregular pro-
jections. Blocks that taper from the surface downwards (wedge-
shaped) should not be permitted in the work; but if any are allowed,
they should be set with the widest side down.
Manner of Laying Blocks. The blocks should be laid in parallel
courses, with their longest side at right angles to the axis of the
street, and the longitudinal joints broken by a lap of at least 2 inches,
128
HIGHWAY CONSTRUCTION
Quite 'r Formed of 3 Rows of
Blocks, Set Longitudinally
Figs. 79 and 80. The reason for this is to prevent the formation
of longitudinal ruts, which would happen if the blocks were laid
lengthwise. Laying blocks obliquely and "herringbone" fashion has
been tried in several
cities, with the idea that
the wear and formation
of ruts would be reduced
by having the vehicle
cross the blocks diago-
nally. The method has
failed to give satisfactory
Cross Section
Fig. 79. Section Showing Method of Laying Ston<
Block Pavement
results; the wear was
irregular and the foothold defective; the difficulty of construction
was increased by reason of labor required to form the triangular
joints; and the method was wasteful of material.
The gutters should
be formed by three or
more courses of block,
laid with their length
parallel to the curb.
At junctions or inter-
sections of streets, the
blocks should be laid
/.I- r 1 T v~s_y -I
-^
pr-S '.'.'.'
~
ii iii
~
i i i _
F"la ga ing
ii iii
'
-
i i i
i i i i i
^
i ' i ' , ' i ' , ' ,
~
3
LiiiJaJrid
.5
Plan
Fig. 80.
Plan of Stone-Block Pavement Showing Method
of Laying Blocks
diagonally from the cen-
ter, as shown in Fig. 81. The reasons for this are: (1) to prevent
the traffic crossing the intersection from following the longitudinal
joints and thus forming depressions and ruts; (2) laid in this manner,
the blocks afford a more secure foothold for horses turning the
corners. The ends of the diagonal blocks where they abut against
the straight blocks, must be cut to the required bevel.
The blocks forming each course must be of the same depth, and
no deviation greater than J inch should be permitted. The blocks
should be assorted as they are delivered, and only those correspond-
ing in depth and width should be used in the same course. The
better method would be to gage the blocks at the quarry. This
would lessen the cost considerably; it would avoid also the incon-
venience to the public due to the stopping of travel because of the
rejection of defective material on the ground. This method undoubt-
HIGHWAY CONSTRUCTION
'129
edly would be preferable to the contractor, who would be saved the
expense of handling unsatisfactory material; and it also would leave
the inspectors free to pay more attention to the manner in which the
work of paving is performed.
The accurate gaging of the blocks is a matter of much impor-
tance. If good work is to be executed, the blocks, when laid, must be
in parallel and even courses; and if the blocks are not gaged accurately
to one uniform size, the result will be a badly paved street, with the
courses running unevenly. The cost of assorting blocks into lots of
uniform width, after delivery on the street, is far in excess of any
Fig. 81. Diagram Showing Method of Laying Stone Blocks at
Intersection of Streets
additional price which would have to be paid for accurate gaging at
the quarry.
Foundation. The foundation of the blocks must be solid and
unyielding. A bed of hydraulic-cement concrete is the most suitable,
and its thickness must be regulated according to the traffic; the
thickness, however, should not be less than 4 inches, and need not be
more than 9 inches. A thickness of 6 inches will sustain traffic of
600 tons per foot of width.
Cushion Coat. Between the surface of the concrete and the base
of the blocks, there must be placed a cushion coat formed of an
incompressible but mobile material, the particles of which readily
will adjust themselves to the irregularities of the bases of the blocks
130 HIGHWAY CONSTRUCTION
and transfer the pressure of the traffic uniformly to the concrete
below. A layer of dry, clean sand 1 inch to 2 inches thick forms an
excellent cushion coat. Its particles must be of such fineness as to
pass through a No. 8 screen; if the sand is coarse and contains peb-
bles, it will not adapt itself to the irregularities of the bases of the
blocks; hence the blocks will be supported at a few points only, and
unequal settlement will take place when the pavement is subjected
to the action of traffic. The sand also must be perfectly free from
'moisture, and artificial heat must be used to dry it if necessary.
This requirement is an absolute necessity. There should be no
moisture below the blocks when laid; nor should water be allowed
to penetrate below the blocks; if such happens, the effect of frost
will be to upheave the pavement and crack the concrete.
Where the best is desired without regard to cost, a layer of
asphaltic cement \ inch thick may be substituted for the sand, with
superior and very satisfactory results.
Laying Blocks. The blocks should be laid stone to stone, so that
the joint may be of the least possible width; wide joints cause
increased wear and noise, and do not increase the foothold. The
courses should be commenced on each side and should be worked
toward the middle; and the last stone should fit tightly.
Ramming. After the blocks have been set, they should be well
rammed down; and the stones which sink below the general level
should be taken up and replaced with a deeper stone or brought to
level by increasing the sand bedding.
The practice of workmen invariably is to use the rammer so as
to secure a fair surface. This does not give the result intended to be
secured, but brings each block to an unyielding bearing. The result
of such a surfacing process is to produce an unsightly and uneven
roadway when the pressure of traffic is brought upon it. The
rammer used should weigh not less than 50 pounds and have a
diameter of not less than 3 inches.
Fillings for Joints. All stone-block pavements depend for their
waterproof qualities upon the character of the joint filling. Joints
filled with sand and gravel of course are pervious. A grout of lime
or cement mortar does not make a permanently waterproof joint; it
becomes disintegrated under the vibration of traffic. An impervious
joint can be made only by employing a filling made from bituminous
HIGHWAY CONSTRUCTION 131
or asphaltic material; this renders the pavement more impervious
to moisture, makes it less noisy, and adds considerably to its
strength.
Bituminous Cement for Joint Filling. The bituminous materials
employed are: (1) coal tar having a specific gravity between 1.23
and 1.33 at 60 degrees Fahrenheit, a melting point between 120 and
130 degrees Fahrenheit, and containing not over 30 per cent of free
carbon. (2) asphalt, either natural or artificial, entirely free from
coal tar or any product of coal-tar distillation, and containing not
less than 98 per cent of pure bitumen soluble in carbon bisulphide.
Of the total amount soluble in carbon bisulphide, 98.5 per cent must
be soluble in carbon tetrachloride. The penetration, when tested
by the Dow method, must be not greater than 110, at 115 degrees
Fahrenheit, and at 77 degrees Fahrenheit must range between 25
and 60. The specific gravity at 60 degrees Fahrenheit must not be
more than 1.00.
The mode of applying the coal-tar filler is as follows : After the
blocks are laid, gravel heated to about 250 degrees Fahrenheit
is spread over the surface and swept into the joints until they are
filled to a depth of about 2 inches. The blocks then are rammed.
The coal-tar filler heated to a temperature between 250 and 300
degrees Fahrenheit is poured into the joints until they are about
half filled, hot gravel is swept in until it reaches to within | inch
of the surface, and hot filler is then poured in until it is flush with
the surface of the blocks; after this sufficient hot gravel is applied
to the joints to conceal the filler.
In applying the coal-tar filler it is essential that both the gravel
and filler are heated sufficiently. Otherwise the filler will be chilled
and will not flow to the bottom of the joint, but will form a thin layer
near the surface, which under the action of frost and the vibration
of traffic, will be cracked and broken up quickly; the gravel will
settle, and the blocks will be jarred loose, causing the surface of
the pavement to become a series of ridges and hollows. The filler
should not be applied during a rainfall or while the blocks are wet
or damp, for such a condition would prevent the filler from adhering
to the blocks. The asphalt filler is heated to a temperature between
400 and 450 degrees Fahrenheit and poured into the joints until
they are entirely filled.
132
HIGHWAY CONSTRUCTION
Hydraulic-Cement Filler is composed of equal parts of Portland
cement and sharp sand mixed with clean fresh water to a suitable
consistency. The joints between the blocks are filled to a depth
of 2 inches with gravel, and the blocks are rammed, after which
the filler is poured into the joints until they are filled flush with the
surface of the blocks. In dry weather the blocks should be mois-
tened by sprinkling with water before applying the filler. After
the filler has taken its initial set, the whole surface of the pavement
is covered with a layer of sand about J inch thick and if the weather
is dry and warm it is sprinkled with water daily for three days.
Traffic is not permitted to use the pavement until at least seven
days after completion.
Stone Pavement on Steep Grades. Stone blocks may be
employed on all practicable grades, but on grades exceeding 10
per cent, cobblestones
afford a better foothold
than blocks. The cob-
blestones should be of uni-
form length, the length
being at least twice the
breadth say stones 6
inches long and 2| inches
to 3 inches in diameter.
These should be set on a concrete foundation, laid stone to stone,
and the interstices filled with cement grout or bituminous cement;
or a bituminous-concrete foundation may be employed and the
interstices between the
stones may be filled with
asphaltic paving cement.
Should stone blocks be
preferred, they must be
laid, when the grade ex-
ceeds 5 per cent, with a
serrated surface, by either
of the methods shown in
Figs. 82 and 83. The method shown in Fig. 82 consists in slightly
tilting the blocks on their bed so as to form a series of ledges or
steps, which will insure a good foothold for horses' hoofs. The
Fig. 82.
Laying Stone Pavement on Steep Grades by
Tilting Blocks
Fig. 83. Laying Stone Pavement on Steep Grades by
Separating Blocks and Filling with Grout
HIGHWAY CONSTRUCTION
133
method shown in Fig. 83 consists in placing between the rows of
stones a course of slate, or strips of creosoted wood, rather less
than 1 inch in thickness and about 1 inch less in depth than the
blocks; or the blocks may be spaced about 1 inch apart, and the
joints filled with a grout composed of gravel and cement. The
pebbles of the gravel should vary in size between f inch and f inch,
BRICK PAVEMENTS
A brick pavement consists of vitrified bricks laid on a suitable
concrete foundation, Fig. 84.
Qualifications of Brick. The qualities essential to a .good
paving brick are the same as for any other paving material, viz,
hardness, toughness, and ability to resist the disintegrating effects
Vitrified Brick-
Fig. 84. Section Showing Method of Laying Vitrified Brick Pavement
of water and frost. These qualities are imparted to the brick by
a process of annealing, through which the clay is brought to the
point of fusion, and the heat then gradually reduced until the kiln
is cold.
Composition. The material from which is made the majority
of the brick used for paving is a shale. Shales are indurated clays
with a laminated structure and the appearance of slate, and occur
in stratified beds. The average composition of the shales that
have proved satisfactory for the manufacture of paving brick is
shown in Table XIV.
An excess of silica causes brittleness; or an excess of alumina
causes shrinking, cracking, and warping. Iron renders the clay
fusible and makes the brick more homogeneous. Lime in the form
of silicate is valuable as a flux, but in the form of carbonate it will
134
HIGHWAY CONSTRUCTION
TABLE XIV
Average Composition of Shales for Paving-
Brick Manufacture
CONSTITUENTS
PROPORTIONAL
PART
(per cent)
(Non-Fluxing)
Silica .
Alumina
Water and loss on ignition
Moisture
56.0
22.0
7.0
2.0
(Fluxing)
Sesquioxide of iron
Lime
Magnesia
Alkalies
7.0
1.0
1.0
4.0
Total
100.0
decrease the strength of the brick; at a high temperature it is changed
into caustic lime, which, while rendering the clay more fusible, will
absorb moisture upon exposure to the weather and thus cause the
brick to disintegrate. Magnesia exerts but little influence on the
character of the brick. The alkalies in small quantities render
the clay fusible.
Color. The color of the clay is of no practical importance;
it is due to the presence of the metallic oxides and organic substances.
Iron produces bricks which are either red, yellow, or blue, according
to the quantity present and the degree of heat; some organic sub-
stances produce a blue, bluish-gray, or black color.
The color of the brick is governed partly by the color of the
clay, by the temperature of burning, by the kind of fuel used, and
by the sand that is used to prevent the brick from sticking to the
dies or to each other in the kiln.
Manufacture. In the manufacture of the brick, the shale is
crushed usually in dry paws and then passed through a 4-mesh or
an 8-mesh screen. The screened material is mixed with water in
a pug mill to the required consistency. The finer the material
is crushed and the more thoroughly it is worked or tempered in
the mill, the more uniform and better the brick is.
The plastic clay, in the "stiff-mud" process, as it leaves the pug
mill is forced by an auger through a die which forms a bar of stiff
HIGHWAY CONSTRUCTION 135
clay of the desired dimensions, and this is cut by an automatic cutter
into bricks of the size required. The bricks then, in some factories,
are repressed in a die, during which the edges of the brick are rounded
and the lugs, grooves, and trade-mark stamped on the sides. When
repressing is not practiced, the bar of clay as it comes from the pug
mill is cut by wires, the brick being called "wire-cut lug" brick.
The bricks, made by either method, are placed in a heated
chamber to dry, this requiring from 18 to 60 hours according to the
clay, temperature, and plant arrangement. When dry the bricks
are stacked in the kiln, which is usually of the down-draft type with
furnaces built in the outer w r alls. The bottom of the kiln is perfo-
rated to allow the gases to pass through to the flues placed below the
floor and connected to the chimney. The heat from the furnace
passes upward into the kiln, then downward through the bricks
into the flues and thence to the chimney. At the beginning of the
burning the heat is applied slowly to drive off the contained water
without cracking the bricks. When the dryness of the smoke
indicates the absence of moisture in the bricks, the fires are gradually
increased until the temperature throughout the kiln is from 1500
to 2000 degrees Fahrenheit, this temperature being maintained from
seven to ten days. The kiln then is closed, the fires are drawn, and
the bricks are allowed to cool. This part of the process is called
annealing, and to produce a tough brick requires from seven to ten
days. The cooled bricks are sorted into different lots; the No. 1
paving bricks are generally found in the upper layers in the kiln.
Sizes. Two sizes of bricks are made: one size measuring 8JX
2| X 4 inches weighing about 7 pounds and requiring 58 to the square
yard. The other, measuring 8 J X 3 J X 4 inches and frequently called
"blocks", weighs about 9J pounds and requires 45 to the square
yard.
Characteristics. The characteristics of brick suitable for paving
are : not to be acted upon by acids shale bricks not to absorb more
than 2 per cent nor less than ^ of 1 per cent of their weight of water,
and clay bricks not to absorb more than 6 per cent of their weight
of water (the absorption by a shale brick of less than | of 1 per cent
of its weight of water, indicates that it has been overburned) ; when
broken with a hammer, to show a dense close-grained structure? free
from lime, air holes, cracks, or marked laminations; not to scale,
136 HIGHWAY CONSTRUCTION
spall, or chip, when quickly struck on the edges; hard but not
brittle.
Tests for Paving Brick. To ascertain if brick possesses the
required qualities they are subjected to three tests: (1) abrasion
by impact (commonly called the "rattler" test); (2) absorption;
(3) cross breaking.
The Rattler Test. The rattler is a steel barrel 28 inches long and
28 inches in diameter, the sides formed of 14 staves fastened to two
cast-iron heads furnished with trunnions which rest in a cast-iron
frame. It is provided with gears and a belt pulley arranged to
revolve at a rate of from 29 J to 30 J revolutions per minute. The
material employed to abrade the brick is spherical balls of cast iron,
the composition of which is: combined carbon, not less than 2.50 per
cent; graphitic carbon, not more than 0.10 per cent; silicon, not more
than 1 per cent; manganese, not more than 0.50 per cent; phos-
phorus, not more than 0.25 per cent; sulphur, not more than 0.08
per cent. Two sizes of balls or shot are used, the larger being 3.75
inches in diameter when new and weighing about 7J pounds, the
smaller being 1.875 inches in diameter and weighing 0.95 pounds.
A charge consists of ten large shot with enough small shot to make
a weight of 300 pounds. The shot is used until the large size is worn
to a weight of 7 pounds and the small shot is worn to a size that will
pass through a circular hole If inches in diameter made in a cast-
iron plate J-inch thick.
The brick to be tested are subjected to a temperature of 100
degrees Fahrenheit for three hours. Ten bricks are weighed and
placed in the rattler with a charge of spherical shot, and the rattler
is revolved for 1800 revolutions. The bricks then are taken out,
pieces less than 1 pound in weight are removed and the balance
weighed. From the weights before and after rattling the percent-
age of loss is calculated. The loss ranges from 16 per cent to 40 per
cent. Brick to be used under heavy traffic should not lose more than
22 per cent, and for light traffic not more than 28 per cent.
Absorption Test. The absorption test is made on five bricks
that have been through the rattler test. They are weighed, and
are immersed in water for 48 hours, then are taken out and weighed,
with the surplus water wiped off. From the weights before and after
immersion the percentage of water absorbed is calculated.
HIGHWAY CONSTRUCTION 137
Cross-Breaking Test. This test is made by placing a brick edge
on supports 6 inches apart. The load is applied at the center of
the brick, and is increased uniformly until fracture occurs. The
average of the result on ten bricks is used in computing the modulus
OTJ77
of rupture, R=r^-', in which IF is the average breaking load in
pounds, L the length between supports in inches, b the breadth,
and d the depth in inches.
Brick=Pavement Qualifications. Advantages. The advantages
of brick pavement may be stated as follows:
(1) Easy traction.
(2) Good foothold for horses.
(3) Not disagreeably noisy.
(4) Yields but little dust and mud.
(5) Adapted to all grades.
(6) Easily repaired.
(7) Easily cleaned.
(8) But slightly absorbent.
(9) Pleasing to the eye.
(10) Expeditiously laid.
(11) Durable under moderate traffic.
Defects. The principal defects of brick pavements arise from
lack of uniformity in the quality of the bricks, and from the liability
of incorporating in the pavement bricks too soft or too porous a
structure, which crumbles under the action of traffic or frost.
Foundation. A brick pavement should have a firm foundation.
As the surface is made up of small, independent blocks, each one
must be supported adequately, or the load coming upon it may force
it downwards and cause unevenness, a condition which conduces
to the rapid destruction of the pavement. Several forms of founda-
tion have been used such as gravel, plank, sand, broken stone,
and concrete. The last mentioned is the best.
Sand Cushion. The sand cushion is a layer of sand placed
on top of the concrete to form a bed for the brick. Practice regard-
ing the depth of this layer of sand varies considerably. In some
cases it is only | inch deep, varying from this up to 3 inches. The
sand cushion is very desirable, as it not only forms a perfectly
true and even surface upon which to place brick, but also makes the
138 HIGHWAY CONSTRUCTION
pavement less hard and rigid than would be the case were the brick
laid directly on the concrete.
The sand is spread evenly, sprinkled with water, smoothed,
and brought to the proper contour by screeds or wooden templets,
properly trussed and mounted on wheels or shoes which bear upon
the upper surface of the curb. Moving the templet forward levels
and forms the sand to a uniform surface and proper shape.
The sand used for the cushion coat should be clean and free from
loam, moderately coarse, and free from pebbles exceeding J inch
in size.
Manner of Laying. The bricks should be laid on edge or on
one flat, as closely and compactly as possible, in straight courses
across the street, with the length of the bricks at right angles to
the axis of the street. Joints should be broken by at least 3 inches.
None but whole bricks should be used, except in starting a course
or making a closure. To provide for the expansion of the pavement,
both longitudinal and transverse expansion joints are used, the
former being made by placing a board templet J-inch thick
against the curb and abutting the brick thereto. The transverse
joints are formed at intervals varying between 25 and 50 feet, by
placing a templet or building lath f-inch thick between two or three
rows of brick. After the bricks are rammed and ready for grouting,
these templets are removed, and the spaces so left are filled with
coal-tar pitch or asphaltic paving cement. The amount of pitch
or cement required will vary between 1 and 1J pounds per square
yard of pavement, depending upon the width of the joints. After
25 or 30 feet of the pavement is laid, every part of it should be rammed
with a rammer weighing not less than 50 pounds and the bricks
which sink below the general level should be removed, sufficient
sand being added to raise the brick to the required level. After
all objectionable brick have been removed, the surface should be
swept clean, then rolled with a steam roller weighing from 3 to 6
tons. The object of rolling is to bring the bricks to an unyielding
bearing with a plane surface; if this is not done, the pavement
will be rough and noisy and will lack durability. The rolling should
be executed first longitudinally, beginning at the crown and working
toward the gutter, taking care that each return trip of the roller
covers exactly the same area as the preceding trip, so that the second
HIGHWAY CONSTRUCTION
139
passage may neutralize any careening of the brick. due to the first
passage.
The manner of laying brick at street intersections is shown in
Fig. 85.
Joint Fillings. The character of the material used in filling
the joints between the brick has considerable influence on the success
Fig. 85. Method of Laying Bricks at Street Intersections
and durability of the pavement. Various materials have been
used such as sand, coal-tar pitch, asphalt, mixtures of coal tar
and asphalt, and Portland cement, besides various patented fillers,
as "Murphy's grout", which is made from ground slag and cement.
Each material has its advocates, and there is much difference of
opinion as to which gives the best results.
The best results seem to be obtained by using a high grade of
Portland cement containing the smallest amount of lime in its
140 HIGHWAY CONSTRUCTION
composition; the presence of the lime increasing the tendency of
the filler to swell through absorption of moisture, causing the pave-
ment to rise or to be lifted away from its foundation, and thus
producing the roaring or rumbling noise so frequently complained of.
The Portland-cement grout, when uniformly mixed and care-
fully placed, resists the impact of traffic and wears well with brick.
When a failure occurs, repairs can be made quickly, and, if made
early, the pavement will be restored to a good condition. If, how-
ever, repairs are neglected, the brick soon loosen and the pave-
ment fails.
Fig. 86. Grout Box Used in Laying Brick Pavement
Courtesy of National Paving Brick Manufacturers Association, Cleveland, Ohio
The office of a filler is to prevent water from reaching the founda-
tion, and to protect the edges of the brick from spalling under
traffic. In order to meet both of these requirements, every joint
must be filled to the top, and must remain so, wearing down with
the brick. Sand does not meet these requirements. Although at
first making a good filler, being inexpensive and reducing the liability
of the pavement to be noisy, it soon washes out, leaving the edges
of the brick unprotected and consequently liable to be chipped.
Coal tar and the mixtures of coal tar and asphalt have an advantage
in rendering a pavement less noisy and in cementing together any
breaks that may occur through upheavals from frost or other causes;
HIGHWAY CONSTRUCTION 141
but, unless made very hard, they have the disadvantage of becoming
soft in hot weather and flowing to the gutters and low places in
the pavement, there forming a black and unsightly scale and leaving
the high parts unprotected. The joints, thus deprived of their filling,
become receptacles for water, mud, and ice in turn; and the edges
of the brick are broken down quickly. Some of these mixtures
become so brittle in winter that they crack and fly out of the joints
under the action of traffic.
The Portland-cement filler is prepared by mixing 2 parts of
cement and 1 part of fine sand with sufficient water to make a thin
grout. The most convenient arrangement for preparing and dis-
tributing the grout is a water-tight wooden box carried on four
wood wheels about 12 inches in diameter, Fig. 86. The box
may be about 4 feet wide, 7 feet long, and 12 inches deep, furnished
with a gate about 8 inches wide, in the rear end. The box should
be mounted on the wheels with an inclination, so that the rear end
is about 4 inches lower than the front end.
Following are the successive operations of placing the filler:
The cement and sand are placed in the box, and sufficient water is
added to make a thin grout. The grouting box is located about
12 feet from the gutter, the end gate opened, and about 2 cubic
feet of the grout allowed to flow out and run over the top of the
brick (care being taken to stir the grout while it is being dis-
charged), Fig. 87. If the brick are very dry, the entire surface of
the pavement should be wet thoroughly with a hose before applying
the grout; if not, absorption of the water from the grout by the
bricks will prevent adhesion between the bricks and the cement
grout. The grout is swept into the joints by ordinary bass brooms.
After a length of about 100 feet of the pavement has been covered
the box is returned to the starting point, and the operation is
repeated with a grout somewhat thicker than the first. If this second
application is not sufficient to fill the joints, the operation is repeated
as often as may be necessary to fill them. If the grout has been
made too thin, or the grade of the street is so great that the grout
will not remain long enough in place to set, dry cement may be
sprinkled over the joints and swept in. After the joints are filled
completely and inspected, allowing three or four hours to intervene,
the completed pavement should be covered with sand to a depth
7\
HIGHWAY CONSTRUCTION
143
of about J inch, and the roadway barricaded, and no traffic allowed
on it for at least ten days.
The object of covering the pavement with sand is to prevent
the grout from drying or settling too rapidly; hence, in dry and
windy weather, it should be sprinkled from time to time. If coarse
sand is employed in the grout, it will separate from the cement
during the operation of filling the joints, with the result that many
joints will be filled with sand and very little cement, while others
will be filled with cement and little or no sand; thus there will be
Fig. 88. Coal-Tar Heating Tank
Courtesy of Barber Asphalt Paving Company,
Philadelphia, Pennsylvania
many spots in the pavement in which no bond is formed between
the bricks, and under the action of traffic these portions quickly
will become defective.
The coal-tar filler is best applied by pouring the material from
buckets, and brooming it into the joints with wire brooms; and in
order to fill the joints effectually, it must be used only when very
hot. To secure this condition, a heating tank on wheels is necessary,
Fig. 88. It should have a capacity of at least 5 barrels, and be kept
at a uniform temperature all day. One man is necessary to feed
the fire and draw the material into the buckets; another, to carry
HIGHWAY CONSTRUCTION 145
the buckets from the heating tank to a third, who pours the material
over the street. The latter starts to pour in the center of the street,
working backward toward the curb, and pouring a strip about 2 feet
in width. A fourth man, with a wire broom, follows immediately
after him, sweeping the surplus material toward the pourer and in
the direction of the curb. This method leaves the entire surface
of the pavement covered with a thin coating of pitch, which
immediately should be covered with a light coating of sand, the
sand becoming imbedded in the pitch. Under the action of traffic,
this thin coating is worn away quickly, leaving the surface of the
bricks clean and smooth, Fig. 89.
Tools Used by Hand in the Construction of Block Pavements.
The principal tools required in constructing block pavements com-
prise hammers and rammers of varying sizes and shapes, depending
on the material and size of the blocks to be laid; also crcnvbars, sand
screens, and rattan and wire brooms. Cobblestones, square blocks,
and brick require different types both of hammer and rammers
for adjusting them to place and for forcing them to their seats.
A cobblestone rammer, for example, is usually made of wood (gener-
ally locust) in the shape of a long truncated cone, banded with
iron at top and bottom, weighing about 40 pounds, and having
two handles, one at the top and another on one side. A Belgian-
block rammer is slightly heavier, consisting of an upper part of
wood set in a steel base; while a rammer for granite blocks is still
heavier, comprising an iron base with cast-steel face, into which
is set a locust plug with hickory handles. For laying brick, a
wooden rammer shod with cast iron or steel and weighing about 27
pounds is used. A light rammer of about 20 pounds weight, consist-
ing of a metallic base attached to a long, slim, wooden handle, is
used for miscellaneous work, such as tamping in trenches, next to
curbs, etc.
Concrete=Mixing Machine. Where large quantities of concrete
are required, as in the foundations of improved pavements, concrete
can be prepared more expeditiously and economically by the use of
mechanical mixers, and the ingredients will be mixed more thoroughly
than by hand. Thorough incorporation of the ingredients is an
essential element in the quality of a concrete. When mixed by
hand, however, the incorporation is rarely complete, because it
146 HIGHWAY CONSTRUCTION
depends upon the proper manipulation of the hoe and shovel. The
manipulation, although extremely simple, is rarely performed by
the ordinary laborer as it should be unless he is watched constantly
by the overseer.
Several varieties of concrete-mixing machines are in the market,
all of which are efficient and of good design. A convenient portable
Fig. 90. Smith Concrete Mixer on Truck with Gasoline Engine,
Power Charger, and Water Tank
Courtesy of T. L. Smith Company, Milwaukee, Wisconsin
type is illustrated in Fig. 90. The capacity of the mixers ranges
from 5 to 20 cubic yards per hour, depending upon size, regularity
with which the materials are supplied, speed, etc.
Gravel Heaters. A special type of oven usually is employed
for heating the gravel used for joint filling in stone-block pavements.
These heaters are made in various sizes, a common size being 9 feet
long, 5 feet wide, and 3 feet 9 inches high.
HIGHWAY CONSTRUCTION 147
WOOD=BLOCK PAVEMENTS
Wood-block pavements, Fig. 91, are formed of rectangular blocks
measuring from 3J to 4 inches wide, 5 to 10 inches long, and 4 inches
deep, impregnated with creosote, or other preservative, laid in a
bed of Portland-cement mortar spread upon a concrete foundation,
with the joints between the blocks filled with either Portland-cement
grout, or a bituminous filler.
The wood used is obtained from the long-leaved yellow pine
(pinus palustrus), lob-lolly pine (pinus taeda), short-leaved pine
(pinus echinata), Cuba pine (pinus heterophylla), black gum (nyssa
sylvatica), red gum (liquidambar styrraciflua), Norway pine (pinus
resinosa), or tamarack (larix laricina).
The wood should be cut from sound trees, free from cracks,
snakes, and knots.
The great enemy of
wood pavement is decay
due to a low form of
plant life called fungi.
The fungi attack the
wood from the outside,
and if the wood is in the
right Condition for the Fig 91> Section Showing Foundation for Wood-Block
spores to grow, they ulti-
mately will penetrate the entire structure of the wood. There
are three classes of fungi: one which attacks all parts of the
wood structure; another which attacks the cellulose; and a third,
which is the most common, and attacks only the lignin the name
of the many organic substances that are incrusted around the cellu-
lose, and which with the latter constitute the essential part of woody
tissue here the fungi dissolve the lignin and the cellulose to make
food for their development. Heat, air, and moisture are necessary
to the existence of the fungi; without any one of these elements
they cannot live. To destroy the fungus life and preserve wood
from decay many processes have been devised; the one that seems
to meet the requirements better than any other is the process of
creosoting.
Creosoting. This process consists in impregnating the wood
with the dead oil of tar, called "creosote", from which the ammonia
148 HIGHWAY CONSTRUCTION
has been removed. Its effect on the wood is to coagulate the albu-
men and thereby prevent its decomposition, also to fill the pores of
the wood with a bituminous substance which excludes both air and
moisture, and which is obnoxious to the lower forms of animal and
vegetable life.
The coal-tar creosote oil is used without admixture or adultera-
tion with other oils or tars. Its characteristics are : specific gravity,
1.03 to 1.08, at a temperature of 100 degrees Fahrenheit; contain
not more than 5 per cent of tarry matter, nor more than 2 per cent
of water, and not more than 8 per cent of tar acids, 99 per cent to
be soluble in hot benzol ; when subjected to distillation at gradually
increasing temperatures up to 400 degrees Fahrenheit not more than
5 per cent of distillate shall pass over, at 450 degrees not more than
35 per cent, and up to 600 degrees Fahrenheit not more than 80 per
cent; after complete distillation not more than 2 per cent of coke
shall remain; upon sulphonating a sample of the total distillate, the
residue shall not exceed 1 per cent.
For applying the creosote to the wood, several methods are
followed. The one in most favor for paving blocks is the "pressure
process", which essentially consists in: (1) steaming the w T ood for
the purpose of liquefying the sap and other substances contained in
the interfibrous spaces; (2) creating a vacuum for the purpose of
removing the liquefied substances; (3) injecting the creosote under
pressure.
The operation is performed in metal cylinders called "retorts",
6 or more feet in diameter and of any desired length, usually about
100 feet. The load of blocks, called a "charge", is placed upon
metal cars called "buggies" and is run into the retort cylinder, the
ends of which then are hermetically closed w r ith "heads" or doors.
Steam, at a gage pressure varying from 15 pounds to 45 pounds per
square inch, is admitted to the retort (in some plants a vacuum is
first created) and the pressure maintained for several hours. When
the operator considers that the steaming has been continued a suffi-
cient length of time, the products of condensation are removed from
the retort through a blow-off cock in the bottom; when this is accom-
plished an air exhaust, or vacuum pump is put in operation, and a
vacuum of from 20 inches to 26 inches is created and maintained
for about one hour, at the end of which time the creosote is allowed
HIGHWAY CONSTRUCTION 149
to flow into the retort until it is filled. A pressure pump then is
started to force the creosote into the retort until the pressure reaches
100 pounds to 150 pounds per square inch. This pressure is main-
tained until the required amount of creosote has been injected in
the wood, then the surplus is drawn off, the heads opened, and the
charge withdrawn.
The amount of creosote injected into the wood varies from 10
pounds to 22 pounds per cubic foot of wood. The amount is deter-
mined primarily by measuring the tanks and is verified by testing
sample blocks. A sample block is bored entirely through in the
direction of the fiber with an auger 1 inch in diameter, the hole
being located midway between the sides and about J the length of
the block from one end. The borings are collected, thoroughly
mixed, and the quantity and ratio of creosote to wood in the borings
determined by extracting the creosote completely with carbon
bisulphide.
The condition of the wood at the time of the treatment, is prefer-
ably dry and free from an excess of water. After treatment, and
until used, the blocks during dry weather should be sprinkled fre-
quently with water to prevent drying and cracking. The treated
blocks are sometimes subjected to tests to determine the resistance
to wear when saturated with water, the resistance to compression
and impact, and to ascertain the amount of water the wood will
absorb.
Laying the Blocks. The surface of the concrete foundation is
cleansed from dust and dirt by sweeping, then sprinkled with water.
Upon the cleaned surface a cushion coat is formed, by spreading a
layer of sand 1 inch thick, Fig. 92, or a mortar composed of 1 part
Portland cement and 2 parts sand, mixed with sufficient water to
form a stiff paste (the practice of using a mixture of cement and
sand slightly moistened with water produces a defective pavement).
The blocks are set upon the cushion coat with the fiber vertical,
Fig. 93, in straight, parallel courses at right angles to the axis of the
street, except at intersections where they are set at an angle of 45
degrees with the axis of the street. They are laid so as to have the
least possible width of joint (wide joints hasten the destruction of
the wood by permitting the fibres to broom and wear under traffic) .
Blocks in adjoining courses break joint by at least 3 inches. At the
150
HIGHWAY CONSTRUCTION
Fig. 92. Spreading Sand Foundation for Wood Blocks in LaSalle Street, Chicago
Courtesy of Engineering News, New York City
Fig. 93. Laying Wood Blocks in LaSalle Street, Chicago
Courtesy of Engineering News, New York City
HIGHWAY CONSTRUCTION
151
Fig. 94. Wood-Block Pavement Being Hammered and Rolled, Preparatory to Putting in Filler
Courtesy of Engineering News, New York City
Fig. 95. Spreading Sand Filler on Wood-Block Pavement
Courtesy of Engineering News, New York City
152 HIGHWAY CONSTRUCTION
curb it is customary to place one or two rows of blocks with the
length parallel to the curb and f inch therefrom.
After the blocks are laid they are brought to a uniform surface
by ramming with hand rammers or rolling with a light steam roller,
Fig. 94. When laid upon a mortar cushion, the rolling or ramming
must be completed before the mortar sets.
In some cases the cushion coat is omitted, the surface of the
concrete freed from dust by dry sweeping is covered with a thin
coat of a bituminous cement and the blocks laid directly upon it.
Sometimes, the side and one end of each block, when it is about to
be set in place, are dipped in the same bituminous material that is
used to cover the concrete, the blocks are placed in contact and the
surface is covered with a thin coating of the bituminous material,
this being covered with a layer of sand or fine gravel.
After the blocks have been brought to a uniform surface, the
joints are filled with either fine sand, cement grout, or a bituminous
cement, Fig. 95. When sand is used, it should be fine and dry,
spread over the surface of the pavement, and swept about until the
joints are filled. Cement grout is made of equal parts of Portland
cement and fine sand mixed with water to the required consistency.
It is spread over the surface of the blocks and swept into the joints
until they are filled. The surface of the pavement then is covered
with sand, and the grout is allowed to set for about seven days
before traffic is admitted. The bituminous filler is composed of
coal-tar pitch, asphalt, or combinations of these, and other ingre-
dients. The filler is applied hot in the same manner as described
under brick pavement. To provide for the expansion of the blocks
the joint next the curb is filled with bituminous filler.
Qualifications of Wood Pavements. Advantages. The advan-
tages of wood pavement may be stated as follows:
(1) It affords good foothold for horses.
(2) It offers less resistance to traction than stone, and slightly
more than asphalt.
(3) It suits all classes of traffic.
(4) It may be used on grades up to 5 per cent.
(5) It is moderately durable.
(6) It yields no mud when laid upon an impervious foundation.
(7) It yields but little dust.
HIGHWAY CONSTRUCTION 153
(8) It is moderate in first cost.
(9) It is not disagreeably noisy.
Defects. The principal objections to wood pavement are :
(1) It is difficult to cleanse.
(2) Under certain conditions of the atmosphere it becomes
greasy and very unsafe for horses. This may be remedied by cover-
ing the surface with a thin layer of fine sand or gravel; a similar
treatment will absorb the oil which exudes during warm weather.
(3) It is not easy to open for the purpose of gaining access to
underground pipes, it being necessary to remove rather a large
surface for this purpose, which has to be left a little time after being
repaired before traffic again is allowed upon it.
ASPHALT PAVEMENTS
Sheet=Asphalt Pavement. Sheet asphalt is the name used to
describe a pavement having a wearing surface composed of sand
graded in predetermined proportions, of a fine material or filler,
and of asphalt cement, all incorporated by mixing in a mechanical
mixer, and laid upon a concrete foundation, the surface of the latter
being covered with a thin layer of bituminous concrete called a
"binder".
Asphalt Cement. This is prepared from solid bitumen, refined
and fluxed with (1) the residuum from paraffine petroleum; (2) the
residuum from asphaltic petroleum; (3) a mixture of paraffine and
asphaltic petroleum residuums; (4) natural malthas, or is prepared
from (5) solid residual bitumen produced in the distillation of
asphaltic petroleums, and fluxed with residuum oil produced from
the same material.
Refined asphalt is that freed from the combined water and
accompanying inorganic and organic matter. By comparatively
simple operations the several varieties of asphalt may be separated
from their impurities. Two methods are employed for refining; one
using steam and the other direct fire. In both methods the asphalt
is placed in tanks and slowly heated until thoroughly melted, and
during the melting the mass is agitated by a current of either air or dry
steam. The method of using steam is superior to the fire method.
In the latter method there always is danger of overheating, in addi-
tion to the formation of coke and the cracking of the hydrocarbons.
154 HIGHWAY CONSTRUCTION
The varieties of asphalt known as gilsonite and grahamite, which
are practically pure bitumen, do not require refining, but they are
used to a very small extent in paving.
The greater part of the solid bitumen used for paving in the
United States is obtained from the \Yest Indies and South America.
The more extensively used being that found at Trinidad, W. I., and
at Bermudez, Venezuela. The asphalts known by the trade names
' 'California'' and "texaco" are produced by refining asphaltic oils,
and may or may not require to be fluxed.
Fluxes are fluid oils and tars which are mixed with asphalt to
produce a desired consistency. The refined asphalt is melted and
the flux previously heated added to it, in the proportion required to
produce the desired consistency. The mixture of asphalt and flux
is agitated either by mechanical means or by a blast of air until the
materials are thoroughly incorporated and the desired consistency
is obtained.
Sand. The sand should be siliceous and so free from organic
matter, mica, soft grains, and other impurities, that these will not
amount to more than 2 per cent of its volume.
Fine Material or Filler. This consists of any sound stone,
usually limestone or sand, pulverized to such fineness that the whole
will pass the No. 50 sieve, and not more than 10 per cent will be
retained on the No. 100 sieve, and at least 70 per cent will pass the
No. 200 sieve. Portland cement sometimes is used instead of the
pulverized stone.
The paving composition is prepared by heating the ingredients
separately to a temperature between 300 and 350 degrees Fahren-
heit, then incorporating them by mixing in a mechanical mixer.
The hot sand is measured into the mixer, followed by the hot filler;
these two materials are thoroughly mixed together, and the hot
cement is added in such a way as to distribute it evenly over the
mixed sand and filler; the mixing then is continued until the materials
form a uniform and homogeneous mass, with the grains of sand
completely covered with cement. A typical mixture is: sand 100
pounds; filler 17.5 pounds; bitumen in asphalt cement 17.5 pounds.
The proportions of the ingredients in the paving mixture are
not constant, but vary with the climate of the place where the
pavement is to be used, the character of the sand, and the amount
HIGHWAY CONSTRUCTION 155
and character of the traffic that will use the pavement. The ranges
are indicated in the following data:
Data for Asphaltic Paving
Asphaltic Paving Mixture.
CONSTITUENTS
Asphalt cement
Sand
Stone dust
PER CENT
12 to 15
70 to 83
5 to 15
Weight of Material. A cubic yard of the prepared material weighs about
4500 pounds. One ton of refined asphaltum makes about 2300 pounds of
asphalt cement, equal to about 3.4 cubic yards of surface material.
Wearing Surface per Cubic Yard of Material.
THICKNESS AREA
(inches) (sq. yd.)
21 12
2 IS
11 27
Laying the Pavement. The hot paving mixture is hauled to
the street and dumped at a place outside of the space m which it
spholttin.
inder l in.
Concrete 5 in.
Fig. 9f>. Section of Aephalt Pavement Showing Layers
is to be laid. It then is thrown into place with hot shovels, and
spread with hot rakes uniformly to such a depth as will give the
required thickness when compacted. The finished thickness
varies between 1| inches and 2 inches. The reduction of thickness
by compression is about 40 per cent generally. Before the mixture
is spread, the surfaces of curbs and street fittings that will be in
contact with it are painted with hot asphalt cement.
The pavement is constructed in two forms: (1) The paving
mixture is laid directly upon the surface of the concrete foundation;
156 HIGHWAY CONSTRUCTION
(2) the surface of the concrete foundation is covered with a coat
of asphaltic concrete, Fig. 96, called the "binder course", the object
of which is to unite more securely the wearing surface to the foun-
dation. This it does by containing a larger percentage of cement,
which, if put in the surface mixture, would render it too soft. The
binder is composed of sound, hard stone broken to pass a IJ-inch
screen, sand, pulverized stone, and asphalt cement, mixed in the
desired proportions. A typical mixture is: stone 100 pounds;
sand 40 pounds; stone dust 8 pounds; bitumen in asphalt cement
8 pounds.
The paving composition is compressed by means of rollers
and tamping irons, the latter being heated in a fire contained in an
iron basket mounted on wheels. These irons are used for tamping
such portions as are inaccessible to the roller, namely, gutters,
around manhole heads, etc.
Two rollers are sometimes employed; one, weighing 5 to 6
tons and of narrow tread, is used to give the first compression;
and the other, weighing about 10 tons and of broad tread, is used
for finishing. The rate of rolling varies; the average is about 1
hour for 1000 square yards of surface. After the primary com-
pression, natural hydraulic cement, or any impalpable mineral
matter, is sprinkled over the surface, to prevent the adhesion of
the material to the roller and to give the surface a more pleasing
appearance. When the asphalt is laid up to the curb, the surface
of the portion forming the gutter is painted with a coat of hot
cement.
Although asphaltum is a poor conductor of heat, and the cement
retains its plasticity for several hours, occasions may and do arise
through which the composition before it is spread has cooled; its
condition when this happens is analogous to hydraulic cement which
has taken a "set", and the same rules which apply to hydraulic
cement in this condition should be respected in regard to asphaltic
cement.
If the temperature of the air at the time of hauling is below
70 degrees Fahrenheit the wagons carrying it are covered with
canvas or other material to prevent the loss of heat. The tem-
perature when delivered at the place where it is to be used must
not be less than 280 degrees Fahrenheit.
HIGHWAY CONSTRUCTION 157
Two methods are followed in laying an asphalt pavement
adjoining street railway tracks: (1) a course of granite-block or
brick paving is laid between the rail and the edge of the asphalt;
(2) the asphalt is laid directly against the rail, which, if its tem-
perature is below 50 degrees Fahrenheit, is heated by suitable
apparatus to a temperature of about 60 degrees Fahrenheit
immediately before the asphalt is laid.
Foundation. A solid, unyielding foundation is indispensable
with all asphaltic pavements, because asphalt of itself has no
power of offering resistance to the action of traffic, conse-
quently it nearly always is placed upon a bed of hydraulic-
cement concrete. The concrete must be set thoroughly and
its surface dry before the asphalt is laid upon it; if not, the
water will be sucked up and converted into steam, with the
result that coherence of the asphaltic mixture is prevented, and,
although its surface may be smooth, the mass is really honey-
combed, so that as soon as the pavement is subjected to the action
of traffic, the voids or fissures formed by the steam appear on
the surface, and the whole pavement is broken up quickly.
Qualifications of Asphalt Pavements. Advantages. These
may be summed up as follows :
(1) It gives easy traction.
(2) It is comparatively noiseless under traffic.
(3) It is impervious.
(4) It is easily cleansed.
(5) It produces neither mud nor dust.
(6) It is pleasing to the -eye.
(7) It suits all classes of traffic.
(8) There is neither vibration nor concussion in traveling
over it.
(9) It is laid expeditiously, thereby causing little inconvenience
to traffic.
(10) Openings to gain access to underground pipes are easily
made.
(11) It is durable.
(12) It is repaired easily.
Defects. These are as follows:
(1) It is slippery under certain conditions of the atmosphere.
158
HIGHWAY CONSTRUCTION
The American asphalts are much less so than the European, on
account of their granular texture derived from the sand. The
difference is very noticeable; the European are as smooth as glass,
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