VITRIFIED BRICK ROADS
Vitrified brick roads consist of a foundation course of Portland
cement concrete, broken stone or slag macadam, or of brick laid flat,
the first named being by far the most generally used, and a wearing
course of vitrified brick.
=Vitrified Brick.=--Vitrified brick are made from clay of such a
character that when heated to the required temperature they will fuse
into a glassy texture. Brick roads are constructed on roads carrying
the severest of traffic and the brick must therefore be tough and of
high resistance to wear.
Not all of the clays from which brick may be manufactured will produce
a product suitable for road construction, and paving brick, even
though truly vitrified, are of different degrees of durability,
depending upon the nature of the clay and the care exercised in the
manufacture.
Paving brick are manufactured by the stiff mud process, which means
that the clay is molded into form in a relatively dry condition. To
accomplish this, considerable pressure is exerted in forcing the
column of clay through the dies, which form the prism from which the
brick are cut. If the clay is unsuitable in character or is not
properly ground and mixed, the brick will possess planes of weakness
between the various layers of clay which have been pressed together,
and these planes, called laminations, are a source of weakness if too
marked. It is usual to specify that the brick used for road surfaces
shall be free from marked laminations.
If the brick is not properly burned it will be only partly vitrified
and therefore not of maximum durability. It is customary to specify
that the brick shall show a glassy fracture indicating complete
vitrification.
Various defects of a minor nature occasionally develop in the brick
during the successive steps in the manufacturing process. Check cracks
resulting from the burning or from too rapid cooling are often
encountered, but unless these are deep, that is 3/16 inch or more,
they do not impair the wearing quality of the brick, nor indicate
structural weakness. Kiln marks are formed on some of the brick due to
the weight of the brick above in the kiln. These depressions are not
objectionable unless the brick are so distorted that they will not lie
evenly in the pavement.
Spacing lugs or raised letters are formed on one face of the brick to
insure sufficient space between the brick for the filler. These lugs
or letters are not less than 1/8 inch nor more than 1/4 inch high and
of such design that they will not obstruct the free flow of filler
into the joints between the brick.
Several varieties of paving brick are to be had, the difference being
principally in the design or size.
=Repressed Brick.=--In this type of brick the spacing lugs are formed
by pressing the green brick, after it has been cut to size, into a
mold on one face of which are recessed letters or other devices into
which the clay is pressed, thus forming the spacing lugs.
=Vertical Fiber Brick.=--These brick are designed to be laid with one
wire-cut face up and spacing is provided by two or more beads on the
side of the brick. Sometimes the vertical fiber brick has no spacing
lug, it being contended that the irregularities of the brick are such
as to provide all of the space required. In practice this does not
always work out, as the brick are so regular in shape that when laid
there is too little space between the brick to permit the introduction
of a suitable filler. The use of brick without spacing lugs is just
beginning and is not yet a generally accepted practice.
=Wire-cut-lug Brick.=--This is a type of non-repressed brick which has
spacing lugs provided by cutting one face in a special manner which
provided lugs for spacing. In this type the wire cut face is the one
between the brick as they are laid in the pavement.
=Tests for Quality.=--The standard test for quality of paving brick is
the rattler test. The brick rattler consists of a barrel of 14 sides
24 inches long, mounted so as to rotate at a speed between 29.5 and
30.5 revolutions per minute. The duration of a test is 1800
revolutions. Ten brick constitute a charge and these are placed in the
rattler along with 300 lbs. of cast iron spheres. The spheres are of
two sizes, the smaller being 1-7/8 inch in diameter when new, and the
larger 3-3/4 inches in diameter when new. Ten of the larger spheres
are used and the balance of the charge is made up of the small size.
When tested in the standard manner the loss allowable for the several
classes of service are as follows:
------------+---------------+----------------
| | Maximum Loss
Traffic | Average Loss | for any Brick
------------+---------------+----------------
Heavy | 20 per cent | 24 per cent
Medium | 22 per cent | 26 per cent
Light | 25 per cent | 28 per cent
------------+---------------+----------------
=Other Tests.=--Sometimes the absorption test is specified for paving
brick, but it is rarely a vitrified brick that will pass the rattler
tests which fails to pass a reasonable absorption test. Absorption of
water in an amount exceeding 4 per cent indicates incomplete
vitrification and failure of such brick is almost certain during the
rattler tests.
The cross breaking test is also sometimes employed, but generally
only to check the general quality of the brick. Failure in service
more frequently occurs from excessive wear than from any other cause
and the cross breaking test has little significance, except for brick
less than 3 inches thick, which are to be laid on a sand bedding
course.
=Foundation.=--The foundation for brick roads is usually of Portland
cement concrete, the thickness varying with the nature of the traffic
and the kind of soil upon which the pavement is built. For well
drained soils and normal highway traffic, 5 inches is the ordinary
thickness of foundation. Under favorable conditions such as locations
with sandy soils or in semi-arid or arid regions where the soil is
always stable, the foundation may be four inches thick, and a
considerable mileage of brick road has been built with concrete
foundations less than four inches thick.
In other locations the soil and traffic conditions require a base six
inches or more in thickness, and the proper thickness can be
determined only after all of the factors involved are known and have
been analyzed. It is impractical to adopt a standard thickness of
foundation that will be equally economical for all locations and all
kinds of traffic. As the brick pavement is essentially a heavy traffic
type of surface, the design cannot be varied greatly with similar
foundation conditions because the weight of individual loads is the
significant factor and this does not vary so much as the volume of
traffic. A variation in volume of traffic may be compensated for by a
variation in the quality of the brick as already set forth.
The mixtures for the concrete foundation vary widely because of the
variation in the aggregates employed. If the fine and coarse aggregate
for the concrete are of good quality a mixture of one part cement, two
and one-half parts sand and five parts of coarse aggregate would
insure concrete of adequate strength. A somewhat leaner mixture is
sometimes employed and would be satisfactory if the aggregates were of
exceptional concrete making quality. Mixtures of sand and pebbles
(unscreened gravel) may also be used if care is exercised to secure a
mixture of adequate strength. The proportion will of necessity vary
with each particular material and the discussion of the various
considerations involved may be obtained from various standard works on
concrete and concrete materials.
Broken stone macadam is sometimes utilized for the foundation course
of the brick pavement and such foundations are constructed as
water-bound, which is described in a previous chapter. The thickness,
like that of the concrete foundation, varies with the soil conditions
and the weight of the loads that are expected to use the road. The
macadam is placed in a single layer and is rolled and bonded with
screenings as described in the chapter dealing with water-bound
macadam. Six inches is a common thickness for the macadam base. This
type of foundation should be employed only where the soil is quite
stable and where material costs are such as to insure that the macadam
base is materially cheaper than one of concrete. This would usually be
in locations where the cost of cement is high because of long hauls
and where suitable macadam materials may be obtained close at hand.
Old macadam roads are sometimes utilized for the foundation for the
brick surface, but the instances where this is permissible are
comparatively few in number. When an old macadam is to be used it is
reshaped to the proper cross section and re-rolled and bonded so as to
afford a stable foundation of the proper cross slope.
BEDDING COURSE FOR BRICK SURFACES
In order to equalize the variations in size and shape of the brick,
they are laid on a bedding course composed of material into which the
brick may be forced by rolling. In this way the upper surfaces of all
brick can be brought to the proper elevation to insure smoothness and
easy riding qualities. Several kinds of bedding course are now
employed.
=Sand Bedding Course.=--The sand bedding course has been referred to
as a sand cushion, but as a matter of experience the cushion effect is
slight, although sometimes pavements have become uneven because the
brick have pushed down into the sand after the pavement was used for a
time. The sand for the bedding course should preferably be fine
grained, all particles passing the eight mesh sieve, but ordinary
concrete sand is satisfactory. The sand need not be clean, as a
comparatively large percentage of silt or clay does not impair the
usefulness of the material.
[Illustration: Fig. 19.--Cross Sections for Brick Highways]
=Sand Mortar Bedding Course.=--In order to eliminate the tendency for
the straight sand bedding course to shift because of the impact of
traffic on the brick, a lean cement mortar is sometimes employed
rather than the straight sand. Sand and cement in the ratio of one
part cement to four or five parts of sand are mixed dry, and after the
brick have been rolled, is moistened to furnish water to hydrate the
cement. The sand employed is ordinary clean concrete sand.
=Green Concrete Bedding Course.=--In the monolithic type of brick
road construction, the brick are laid directly on the green concrete
base before the concrete has taken a set and the irregularities of the
brick are taken up by rolling them until bedded in concrete.
FILLERS FOR BRICK SURFACES
The spaces between the brick are filled with some material that will
prevent the brick from being displaced and prevent water getting to
the bedding course. A suitable filler must adhere to the brick and
fill completely the spaces between them. It must withstand traffic so
as to remain intact in the joints and when in place it must be rigid
enough to prevent displacement of the brick.
=Cement Grout Filler.=--One of the most commonly used fillers for
brick pavements consists of a grout composed of Portland cement and
fine sand. When properly mixed and applied the grout filler meets all
requirements for a filler except that it is non-elastic and some means
must be adopted for caring for pavement expansion.
=Bituminous Fillers.=--Asphaltic materials and tars are widely used as
fillers for brick pavements. Such fillers are of high melting point
and consequently solid at ordinary temperature. They are poured into
the joints hot and when they cool are firm enough to comply with the
requirements for a filler. In addition, they have enough ductility to
accommodate the expansion of the pavement due to temperature changes.
=Mastic Fillers.=--Mastic consists of a mixture of about equal volumes
of fine sand and a solid bituminous material. The mixture is prepared
at high temperature and is worked into the joints between the brick
while hot. When cool it resembles the straight bituminous filler
except that the mastic is somewhat more resistant to wear than the
straight bituminous filler.
EXPANSION JOINTS
It is recognized that brick will expand and contract with changes in
temperature. When a bituminous or mastic filler is employed there is
sufficient yield to the filler to accommodate the change in dimension
in the brick, but when the grout filler is used either the expansion
joint must be provided or the pavement must be designed to withstand
the compression due to expansion of the brick. Expansion joints may
consist of a sheet of bituminous mastic prepared for the purpose and
set in place in the pavement. The sheet of joint material is simply
inserted between courses of brick at the proper place.
Another method of forming an expansion joint consists in placing a
strip of wood between courses of brick at the place where a joint is
required. After the pavement has been grouted, the wooden strip is
pulled out and the joint is filled with a suitable bituminous filler.
=Marginal Curb.=--If the sand bedding course is employed, it is
necessary to provide curbing along the sides of the brick to hold the
bedding course in place. The curb is usually constructed integral with
the base and of concrete of the same mixture as the base. The width of
the curb is usually six inches and the top of the curb is at the same
elevation as the edge of brick surface.
CONSTRUCTION OF THE SURFACE
Before the construction of a brick surface should be undertaken on a
road, the drainage should be provided for even more completely than
for a less costly type of surface since it does not pay to jeopardize
the stability of the pavement by failure to provide adequately for the
stability of the supporting soil. Grades should also be reduced to the
economical limit.
The earth subgrade is brought to the proper elevation and cross
section and is thoroughly rolled. If there are places where the soil
will not compact properly under rolling, these places are corrected by
taking out the material and back filling with new material that will
properly compact under the roller.
The aggregates for the concrete may be distributed along on the
prepared subgrade or may be stored in stock piles or bins at
convenient points. If stored on the subgrade, a traction mixer is
employed which is drawn along the road as the work progresses, the
materials being placed directly in the mixer. If stored at a central
point, they may be transported to the mixer on the road and dumped
directly into the mixer, or the mixer may be set up at the storage
piles and the concrete hauled in trucks to the road where it is
deposited and shaped.
The concrete is spread to the proper thickness and tamped either by
hand or by machinery. If the marginal curb is to be employed, it is
constructed immediately after the concrete for the base has been
finished but before the cement begins to set.
After the foundation concrete has set, the bedding course is spread
and struck off to the proper thickness. When the bedding course
consists of sand-cement mortar, the sand and cement are mixed dry and
spread to prescribed thickness. It is considered to be desirable to
roll the sand bedding course with a light hand roller before the brick
are placed, but the sand-cement bedding course is not rolled. The
bedding course must be carefully shaped by means of a templet or
strike board before the brick are placed.
The brick are laid in straight courses across the pavement, with the
spacing lugs all in the same direction if brick with spacing lugs are
employed, and with the lugs in contact with the brick of adjoining
courses. If brick without spacing lugs are used they are laid loosely
so that there will be room for the filler between the brick of
adjoining courses.
After the brick have been laid they are rolled to bed them in the sand
or sand-mortar bedding course and thus secure a smooth surface. For
this purpose a light, power driven, tandem roller is used and the
rolling is continued until the brick are thoroughly bedded. Any
defective brick that are noted are removed and replaced with good
brick and after this culling has been completed the surface is once
more thoroughly rolled. If a cement-sand bedding course is employed,
the surface is sprinkled just after the final rolling so that water
will flow down between the brick and moisten the bedding course
sufficiently to cause the cement to set. In some cases, the
sand-cement bedding course is sprinkled just before the brick are laid
but in warm weather the setting would take place before the brick
could be rolled if that were done. In cool weather the setting is
sufficiently slow to permit rolling before the bedding course hardens.
The filler is applied to the surface after the rolling. If the
bituminous type of filler is employed, the hot filler is poured onto
the surface and worked into the joints by means of squeegees, with
comparatively little material left on the surface. In some instances
cone-shaped pouring pots are employed and the material is poured
directly into the joints.
The cement grout filler is applied in the same general manner as the
bituminous filler. The grout, consisting of equal parts of sand and
cement, is mixed to a thin consistency and poured onto the surface and
is then worked into the joints with squeegees. Two or more
applications are usually required to effect a complete filling of the
joints. The surface should be covered with sand and be kept moist
until the cement grout has set.