The Washington herald., 1907-03-24

The possibilities of cement construction
in various forms began to attract the
attention Of engineers about forty years
ago. but It Is only recently that the gen
eral public has become interested in this
work. The attention OF the public has
probably been attracted by the surprising
things that have already been accom-
plished with this new form OF construe-
ton, says the Springfield Republican. The
steady rise in the prices of the Older
constructural materials has forced build
ers to seek a newer and cheaper form,
and this in turn has stimulated the en-
gineers and afforded them practical op-
portunities to actually test the material
which they had been studying. The dis
eovery Of Portland cement In the early
part OF the last century afforded a far
better building material than had been
known before. At first IL was used only
as cement in the proper sense Of the
word, and It was not until the engineers
had evolved concrete that the period OF
cement construction can be said to have
begun.
The properties Of concrete are such as
to commend It TO builders and engineers
from the outset. II has all OF the
strength of stone, combined with plas-
ITCHY in the first stages. II Is ireproOf.
even more than granite. and can be used
for permanent construction either in the
tropics or the arctic regions. It's expan-
sion and contraction in response 10 the
changing seasons is SO nearly the same
as that Of iron and steel that il can be
safely used in conjunction with that
metal Added to all of these qualities.
and from the public viewpoint more Inn
portant, is the fact that concrete IS an
extremely cheap material under average
conditions.
How Concrete Is Made.
Concrete is an artificial stone, produced
by thorough mixture OF cement sand,
crushed rock or gravel, and water. The
cement unites chemically with the wafer
and binds the aggregate so firmly TO-
gether that the crushing strength of con
crete is equal to that of the most durable
stone. The use OF concrete is as old as
civilization itself. 1t is sNfound in the
buildings Of Egypt Central America,
Greece. and India. The largest dome in
existence, that OF the Pantheon at Rome.
i42 fet in diameter IS q solid mass OF
concrete. and has stood for over 2o00
years. During the Dark Ages its use was
lost and not revived until the discovery
of Portland cement In the early part of
the last century. The use Of plain con-
crete was. however. confined to the con
structiou OF foundations. walls, and sim
liar structures on account Of its low
tensile strength. making it expensive In
cases where cross-bending stresses were
to be overcome This deficiency may be
remedied by imbedding in the concrete
steel rods Of the proper sizes and in the
proper places, taking care OF all tensile
stresses from bending. change OF temper
ature, and initial set of concrete. This is
called re-enforced concrete
Although concrete has come as a sub
stitute to many Of the building lines. in
bridge construction 1t has apparently
filed long felt need Up TO a very re-
cent period, a time well within the
memory OF many persons now living,
there was but one alternative. that Of the
stone arch. and this form Of construction
was not only very costly bat also failed
70 lend itself to many locations. The cost
of stone masonry has been. and Is yet.
in many instances. prohibitive, and
makeshift has in the majority of cases
taken its place, But even with its use
there has ever been the desire to make IL
as permanent as possible, and 20 this end
the present age Of iron began TO make
itself felt in bridge construction. About
sixty years ago iron was first used by the
introduction of wrought bars into wooden
trusses, these bars being in tension, while
eF. \
although It was merely common sense
which placed them so. AT that period
engineers were just beginning TO study
trusses and the strength and loading Of
the various members. and even as late as
1919 the most OF the companies which
were then building iron bridges did so by
guesswOrk and main strength. In the
evolution Of the iron industry the bridges
built Of part wood and part iron gave
way to those in which the tension mem
bers were made of wrought iron and the
compression of cast iron. Owing to the
bTittleness Of the latter metal, il was
soon discarded. and bridges were gener-
ally built of wrought iron throughout
The discovery of the Bessemer and OPEN
hearth processes for the manufacture Of
steel produced a metal with so much
more strength than wrought iron that
engineers generally adopted it, believing
that the question Of permanent bridge
construction has been solved.
Defects of Steel Bridges.
There were q few engineers. however,
who shook their heads and said that this,
too. was makeshift like the rest. Of a
little longer life, perhaps. and good while
il was new, but-how long would it last?
The steel enthusiast replied that It was
good for just so long as it was kept prop
erly painted to prevent cOrrosion and con
sequent loss of material 1t was found
that this was not the true answer. Steel
may last in its entirety so long as it is
kept perfectly protected from corrosion;
but when It Is subjected to great num-
Der of vibrations, even though they be
well within its calculated strength, 1t be-
comes 'fatigued." Under continual VI
bration the molecular form of the metal
becomes altered in Il manner resembling
capitalization. and it has often been no
ticed that rod which has been so
changed ia character is more brittle than
cast iron. So the engineers who shook
their heads have been to degree iusti-
ned by the results. and steelWork cannot
be considered as permanent unless it is
protected from erosion and is free from
vibration.
The protesting engineers did not content
themselves with shaking their heads; they
began to look for some material or combi- |
nation of materials which might meet the
necessary requirements. It must be plas /
tic, strong, economical, and permanent
and, in REINFORCED or armored concrete. /
they claim a good result The design and
construction Of bridges built OF this ma
teTial has. as matter Of course. been
governed by the same laws of evolution /
that controlled the progress of the iron 1