The daily banner. (Cambridge, Md.) 1897-2008, 1922-04-25

DieInS VI llFir tnStfNCtAoit Nlh'
Care.
Washington April Pit is high
time that the iaymen be made &C.
guainted with some Of the funda-
mental problems OF street and high
way construction. He who lives on
the street OF road and pays the large
share OF the Improvement tax, knows
less about paraments than lie does
abou, any other thing for which he
spends his money. This is not well
when we consider the r,iiiions being
spent annually for street and nigh
va improvement
1t may surprise that man when l
say that pavemcnIs ar" danaged as
much by natural forces as they are
by wear and tear Of heavy traffic.
These natural agencies OF destruc-
ton are water and temperature
changes. Moisture swe'i5 the soil
much as it 3weils n sponge. This
swelling Frequently IS sufficient to
lift the pavement slab. The slab
may be resting on the SO at a weak
point OF heavy SaturiZation and, al a
rOint not far removed. may be rest
ing on drier soil of greater bearing
power A load on the pavement I'M
der such conditions freQu entity causes
cracking and in some types of pave
ments, cracking is followed by rapid
disintegratioa..
Moisture makes the Sub-SOil UNI
stable Therefore the pavement is
not always lnif 5rln', supported. Ii
must be reaiIzeri hat the function OF
q Daven4ent IS not ~ support the en-
tire weight OF traffic of itself, but -
transmit the weight and impact 70
the soil upon which it rests.
In paving a road over soil that
does not easily drain itself-heavy
clay for instance-particula atter-
ton should be paid to artificial dar n
age Money spent in necessary
drainage, while adding 4 little TO the
first cost, will save ten times as much
throughout the life of the pavement
by cutting down maintenance costs.
Recent research has developed the
fact that placing a layer of coarse
material-crushed rock, gravel o."
siag-between 6he pavement and thu
sub-soil will keep the moisture from
rising in the soil, thereby permitting
the sub grade to remain stable.
moisture canno. uayed upward
through q porois material contain
ing such voldS IS are present lin
coarse material. There is >:nothfng
there to suck il up like q sponge and
hold on to it,
The air in the vOids also doubtless
helps to obstruCr the water move
ment. In winter when soil freezes
these. Voids ALLOW room for expansion,
thus preventing heaving and crack
ing OF the surface. Further, such
coarse material provides the, bes,
kind Of a flexible, yet durable base.
This small degree of flexibiIity is
necessary to permit the base to ad-
just itself TO the movement of eX-
pansion and contraction caused by
heat, cold and moisture
These newly discovered reasons for
the use of coarse base material
should interest the taxpayers because
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
S41pS. olSSN 1O3 IhSLdnCS,
not Withstand such expansion anu
! contraction without cracking.
Heat. of the sun during the day
will cause the top of a pavement sur
face to expand while the cool of the
earth on the bottom side prevents eN-
pansiOn. This causes the pavement
to warp. sometimes entirely lifting
away from the Sub-soil in the center
, and pressing down on it tightly at
each edge. 4. At right this is reverse q
The cool night ar allows the top of
the slab to contract while the heat
stored in the earth during the day
causes the unde- side To expand.
This often lifts the edges from the
soil and leaves only the center sup
ported Heavy loads in the center
of the slab during the day and on the
edges at night, where such cond:-
tons exist, may easily crack the slab.
Because Of these destructive move
ments. the rigid typo of pavement is
losing popularity among many high
way engineers. The rigid slab can
not adjust Itself to thesa 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 BIZ
luminous material SO as to provide
hexibility or "come and CO. This
gives hard, tough surface that will
resist wear, tear and impact and al
the same time provides surface sur
ticiently flexible to move up and
down with the eub-soil's heaving and
expand and contract under tempera
tlre changes, all without cracking.
These are a few of the basic prin
cipies that the laymen must under
stand and study further, if he is to
intelligently select pavement type,
and spend his taxes economically.
Economy in buying shoes or a suit OF