The Hartford herald. [volume] (Hartford, Ky.) 1875-1926, 1922-05-03
SOME FUNDAMENTAL
PROBLEMS OF PAVE
MENT CONSTRUCTION
fBy viii p. Blair)

way COnstTuCtlOu. ne WIN lNcS
on the street or road and pays the
large share OF the Improvement tax,
knows less about pavements than
he does about any other thing for
which he spends his money. This
is not we'll when we consider the
street and highway improvements.
1t may surprise that man when I,
say that pavements are damaged as
much by natural forces as they are
by wear and tear of heavy traffic.
These natural agencies of de-
struction are water and temperature
changes. Moisture swells the soil
much as 1t swells sponge. This
swelling frequently is sufficient to
lift the pavement slab. The slab
may then be resting on the soil at
weak point of heavy saturization
and, at point not far removed, may
be resting ON a drier soil of greater
bearing power. A load on the pave
ment under such conditions FREE
quently causes cracking and in some
types of pavement, cracking is fol-
lowed by rapid disintegration.
Moisture makes the sub-soil un-
stable. Therefore the pavement is
not always uniformly supported. 1t
must be realized that the function
of pavement is not to support the
entire weight of traffic of itself, but
to transmit the weight and impact
to the soil upon which it rests.
In paving road over soil that
does not easily drain itself-heavy
clay for instance-particular atten-
tion should be paid to artificial
drainage. Money spent in necessary
drainage, while adding a little to the
first cost, will save ten times as
much throughout the life of the
pavement by cutting down main
tenance costs.
Recent research has developed the
. fact that placing a layer of coarse
mategia l-crushed rock, gravel or
slag-between the pavement and
the sub-soil, will keep the moisture
from rising in the soil, thereby per-
hitting the sub-grade to remain
stable. Moisture cannot travel up
ward through a porous material
containing such voids as are present
in coarse material. There is moth
ing there. to suck it up like a sponge
and hold on to it.
The air in the vo.ds also doubt
less helps to obstruct the water
movement. In winter when soil
freezes these voids allow room for
expansion, thus preventing heaving
and cracking of the surface. Furth-
er, such coarse material provides
the best kind of flexible, yet dura-
BILE base. This small degree of fiexi-
bility is necessary to permit the base
to adjust itself to the movement OF
expansion and contraction caused by
heat, cold and moisture.
These newly discovered reasons
for the use of coarse base material
should interest the taxpayer be-
cause they mean that local material
can be used at a saving in cost.
Under certain conditions pave
ments are contracted and expanded
by cold and heat. Rigid pavement
slabs, unlike steel for instance, can
not Withstand such expansion and
contraction without cracking.
Heat of the sun during the day
will cause the top of pavement
surface to expand while the cool of
the earth on the bottom side pre-
vents expansion, q This causes the
pavement to warp. sometimes en-
tirely lifting away from the sub-soil
In the center and pressing down on
1t tightly at each edge. At night this
Is reversed. The cool night air al
lows the top of the slab to contract
while the heat stored in the earth
during the day causes the under
side to expand. This often lifts the
edges from the soil and leaves only
the center supported. Heavy loads
In the center Of the slab during the
day and on the edges at night,
where such conditions exist. may
easily crack the slab.
Because of these destructive
movements. the rigid type of pave
ment is losing popularity among
many highway engineers. The, rig
Id slab cannot adjust itself to these
movements and is ultimately ruined.
The need today is for a pavement
surface made up OF hard. tough
units, and the units bound together
with some bituminous material so
as to provide nexibility or "come