CEMENT CONCRETE ROADS
The cement concrete road is one of the later developments in highway
construction, but the type has had sufficient use to show that it is
one of the satisfactory types for heavy mixed traffic, and, where the
proper materials are available, it is one of the economical types of
construction.
=Destructive Agencies.=--It is well to have clearly in mind at the
outset that the concrete in a road surface is subjected to certain
destructive agencies not usually significant in connection with the
use of concrete, and these are so often disregarded that the average
serviceability of the concrete road surface is sometimes much lower
than it would be if built with due regard for the effect of traffic on
concrete surfaces. In most structural uses of concrete, its strength
in compression only is utilized, and the factor of safety is such as
to eliminate to some extent failures due to inferior materials or
workmanship.
The concrete road surface is subjected to compression under wheel
loads, to bending, causing tension in the concrete, to abrasion from
wheels, and to tension and compression due to effect of temperature.
The weight of the wheel loads may cause sufficient distortion of the
road slab to produce rupture. The aggregates may be crushed under
wheel loads if the material is too soft. Abrasion from steel tired
vehicles wears away the concrete unless it is hard and durable.
Changes in dimension due to the effect of change in temperature
introduce tension or compression into the road slab and may result in
cracks. Freezing and thawing in the subgrade subjects the slab to
vertical movement and discontinuous support with the result that
longitudinal and transverse cracks occur.
The foregoing indicates the importance of securing good concrete for
road surfacing, and that is accomplished by using suitable aggregates,
by proper design of the road surface and by following established
construction methods.
=Design.=--The widths usually adopted for concrete roads are: for
single track roads, 9 or 10 feet, and for double track roads, 18 or 20
feet. The thickness is 6 to 8 inches at the middle, varying with
climatic conditions and with the kind of soil upon which the concrete
is laid. The thickness at the edge is 1 inch less than at the middle
except that 6-inch surfaces are usually of uniform thickness, the
total crown being 2 inches. The thickness of the two course pavement
is the same as would be used for a single course pavement in the same
location. The surface of either width has a total crown of one or two
inches to insure water running off the surface. The earth foundation
is often flat, the crown being obtained by making the slab thicker at
the middle than at the edge. Fig. 18 shows cross section for concrete
roads.
[Illustration: Fig. 18.--Cross Section for Concrete Highway]
In the state of California, concrete roads four or five inches thick
and surfaced with a bituminous carpet mat have been successfully
constructed. Similar designs have been used in a few other places, but
for general practice it is unsafe to depend upon such a thin slab.
Climatic and soil conditions probably account for the success of the
thin roads in California.
=Concrete Materials.=--The coarse aggregate for the concrete may be
broken stone or pebbles screened from natural gravel. Durability is
necessary, but it is also important to have uniformity in the concrete
so that the road surface will wear uniformly and consequently keep
smooth. Supplies of broken stone are likely to contain a small
percentage of soft pieces and such of these as are at the surface when
the concrete is finished will crush under traffic, leaving a pit in
the surface. Pebbles screened from gravel are also likely to be
variable in durability and should be carefully inspected if they are
to be used as aggregate for concrete roads. The harder limestones,
some sandstones, pebbles from many of the gravel deposits and
practically all of the igneous rocks make satisfactory aggregates for
the concrete road.
Sometimes none of the coarse aggregates readily available are
sufficiently durable or uniform for the wearing surface of the
concrete road, but a suitable aggregate may be obtained at relatively
high price by shipping considerable distances. In such cases what is
known as the two course type of concrete road is employed. The wearing
course usually is about 2 inches thick and is constructed with
selected aggregates of good quality shipped in for the purpose. The
lower course is constructed of aggregates which do not possess the
desired qualities for a wearing course, but which are satisfactory for
concrete not subjected to abrasion. The aggregates for the wearing
course will be selected with the same regard for uniformity and
durability that would be the case if they were for the one course
pavement.
Bank run gravel, or run of the crusher stone, is generally not
sufficiently uniform as regards proportion of fine and coarse material
to produce uniformity in the concrete, and the use of aggregates of
that character is not permissible for the wearing course, but under
proper inspection they may be used for the lower course of two course
pavements.
=Fine Aggregate.=--The fine aggregate is generally natural sand, but a
mixture of natural sand and stone screenings is sometimes employed.
The fine aggregate of whatever character must be clean, free from
organic matter and sand, must contain no appreciable amount of mica,
feldspar, alkali, shale or similar deleterious substances and not
exceed two and one-half per cent of clay and silt. The sand is of such
a range of sizes that all will pass the one-fourth-inch sieve and that
not exceeding about five per cent will pass the 100-mesh sieve.
=Proportions.=--Various mixtures for the concrete are employed because
these may properly vary to some extent with the exact character and
grading of the aggregates. Experience seems to have shown that the
concrete used for the wearing surface should have a crushing strength
of at least 2500 pounds per square inch, and the mixture adopted is
based on the requirements that will give the desired crushing
strength. The common mixture for the one course pavement is one part
cement, two parts sand and three and one-half parts coarse aggregate.
For the wearing course of the two-course type of pavement, a mixture
of the same kind is very often specified.
While these are perhaps the most widely adopted proportions, many
others have been used, especially where the aggregates exhibit
peculiarities or the traffic conditions are unusual. It is desired to
emphasize that the purpose is to obtain concrete of the desired
strength and there can be no such thing as "standard" proportions.
=Measuring Materials.=--In considering the methods employed for
measuring aggregates, emphasis should be placed on the futility of
rigid requirements for the aggregates, both as regards quality and
range of sizes, if the materials are carelessly proportioned at the
mixer. If even reasonably near uniform wearing qualities are to be
secured throughout the entire area of the concrete road surface,
successive batches of concrete must be alike, and to insure that, the
aggregates including the water in each batch of concrete must be mixed
in exactly the same proportions. The aggregates are measured in
various ways, all essentially alike in that the intent is to insure
exactly the same amount of each ingredient for each batch of concrete.
One method is to place bottomless boxes in wheelbarrows, fill the
boxes level full and then lift off the box. Another is to use a
wheelbarrow with a bed of such shape that the contents will be a
multiple of 1 cubic foot when level full. For the larger jobs, the
aggregates are hauled in industrial cars, each having sufficient
capacity for a batch of concrete. The car body is provided with a
partition so as to separate the fine and coarse material.
The water is measured in a tank which automatically refills to the
same level each time it is emptied and when adjusted for a mixture
will introduce the proper amount of water for each batch. It is highly
important to use the least amount of water that will produce workable
concrete.
=Preparation of the Earth Foundation.=--The concrete road is generally
placed directly on the natural soil which has been brought to the
proper cross section. Some engineers advocate that in preparing the
subgrade, the earth be thoroughly rolled; others prefer not to roll
the subgrade. If fills of considerable depth are constructed, they
should either be rolled as built or else should be allowed to settle
for some months before the concrete road is placed, preferably the
latter.
=Placing the Concrete.=--The concrete is placed between substantial
side forms of a height equal to the thickness of the concrete road
slab at the edge, and is shaped roughly by means of shovels.
Various methods have been developed for striking the surface to the
exact shape desired and smoothing it. If hand finishing methods are
employed, a plank template is cut to the prescribed cross section and
the concrete is shaped by drawing the template along the side forms.
Sometimes the template is used as a tamper, being moved along very
slowly accompanied by an up and down motion that tends to tamp the
concrete. The template is then drawn along a second time to smooth the
surface finally.
After the surface has been struck off by hand, it is finally smoothed,
first by rolling crosswise with a slight hand roller about 8 inches in
diameter and 30 inches long. The final finish is effected by dragging
a piece of web belting back and forth across the surface.
Machines designed to tamp the concrete and strike it off to the
required cross section are also employed for finishing. The machine is
power operated and is carried on wheels that run on the side forms,
and the machine moves slowly along as the tamping progresses. The
concrete is tamped, struck off to shape and smoothed with the belt at
one operation. This method of finishing produces denser and stronger
concrete than can be produced by hand finishing methods.
=Placing Concrete for Two-course Road.=--The methods employed for the
two-course concrete road are much the same as for the one-course road.
The concrete for the lower course is placed and struck off by means of
hand tools, and after that course has progressed a few feet, the upper
course is placed and finished as has been described for the one-course
road.
=Curing the Concrete.=--The setting action of cement is a chemical
process, not merely a drying out of the water introduced in mixing the
concrete. The chemical action is progressive for a long time, but is
more rapid during the first few hours than during the later periods,
and the concrete reaches about three-fourths of its maximum strength
at the end of seven days. During the setting period and particularly
during the first few days, plenty of water must be available to the
cement.
To prevent too rapid loss of water from the concrete during the
setting period, the surface must be protected from the wind and sun.
This is accomplished by first covering with canvas as soon as the
concrete has hardened sufficiently and by later covering with earth,
to a depth of two inches. The earth covering is kept wet for about ten
days and is left in place for about one month.
In some places the ponding method of curing is adopted. The surface is
divided into sections by earthen dikes and the space inside the dikes
filled with water to a depth of two or three inches. The water
covering is maintained for two weeks or longer.
No traffic is permitted on the surface for one month, and in cold
weather traffic may be kept off the surface for a longer period.
=Expansion Joints.=--To permit the concrete slab to accommodate itself
to changes in dimension due to temperature changes, expansion joints
1/2 inch wide are placed about every thirty feet. These consist of a
sheet of some prepared bituminous material placed in position as the
concrete is poured.
Experience seems to indicate that in spite of the expansion joints,
the concrete will crack more or less and many engineers think it
advisable to omit expansion joints in constructing the pavement and
when cracks develop to pour bituminous material into them, thus
forming expansion joints.
The prevailing practice in rural highway construction is to omit the
expansion joints, but they are commonly adopted in city pavements.
=Reinforcing.=--To minimize the cracking, either bar or wire mesh
reinforcing is used in the concrete. If bars are used they are placed
in the concrete as it is poured so as to form a belt around each
section about 15 feet square. If the mesh type is employed, a part of
the layer of concrete is placed and smoothed off and a strip of the
mesh laid in place. Additional concrete is then poured on top of the
mesh to bring the slab to the required thickness.
=Bituminous Coatings on Concrete Surfaces.=--The concrete road surface
is sometimes coated with a layer of bituminous material and stone
chips or gravel pebbles. This is particularly advisable where no
really satisfactory aggregates are available and the concrete surface
would not possess sufficient durability. The bituminous material is
applied hot to the surface and is then covered with stone chips or
gravel pebbles, ranging in size from 3/4 inch down to 1/4 inch, the
resulting coating being about 3/4 inch thick. Many failures of this
type of surface have been recorded due to the difficulty of securing
adhesion to the concrete. This seems to be due in part to inability to
get the proper bituminous materials and in part to climatic effects.
Considerable progress has been made in developing this type of surface
and it may eventually become a satisfactory maintenance method.
=Characteristics.=--The concrete road is of a granular texture and is
not slippery. It is of course rigid and noisy for steel tired
vehicles. It is an excellent automobile road and its low tractive
resistance makes it a desirable surface for horse drawn vehicles. It
possesses a high degree of durability if properly constructed. It is
likely to crack indiscriminately but as a general rule the cracks are
not a serious defect.
=Maintenance.=--The cracks that appear in the concrete surface are
filled once or twice a year, tar or asphalt being employed. The dust
and detritus is cleaned out of the cracks and the hot filler poured
in, with enough excess overflowing to protect the edges.