[New York author of articles in Applied Science.]
The life of properly made concrete is not known. However, specimens
from the times of the Romans are yet in good condition. This material
has very considerable ability to resist compression; it is practically
fireproof and teredo proof; when properly protected it is probably but
little deteriorated by weather changes; and, if properly made, it is
probably inappreciably subject to chemical disintegration when
submerged. But there is one palpable fault--it is weak when subjected
to tensile stresses. On the other hand--steel has great tensile
resistivity and is strong under transverse stress. These two
materials--concrete and steel--supplement each other in valuable
qualities. The possibility of using them in combination depends
largely upon the fact that their co-efficients of expansion are
practically the same for moderate thermal fluctuations. Steel is
easily corroded. Nor is it strictly fireproof, as temperatures which
are not excessive will induce bending and buckling. When it is
surrounded by concrete, steel is protected against both fire and
corrosion.
In many situations, steel would not alone supply the best material of
construction. And the same remark applies to concrete. A striking
instance is the case of the six new docks in Baltimore. Three of these
had already been constructed of wood and stone, when it became evident
that the building of the remaining three and of the long bulkhead
which was part of the scheme along the same lines would entail a
larger expense than the use of reinforced concrete construction. Steel
by itself would have been impossible of consideration because of its
susceptibility to corrosion. Concrete alone could not be used because 323
of the excessive cost of the increased amount of Portland cement. It
is said, that a retaining wall of all concrete would have cost about
$600 per linear foot. Reinforced concrete costs about $58 per foot.
Steel is used, not merely as a reinforcement, but as the material of
forms. Used thus, it may, at times, not only retain the concrete in
position but also prevent the action of the surrounding soil or water.
The possibility of using steel for forms depends largely upon the fact
that many applications of concrete are becoming standardized rapidly.
Thus is permitted a re-use of the steel form that justifies the
expense. But the employment of steel forms sometimes involves the use
of steel in the handling of them. A further use, although perhaps more
remote, is in connection with the arrangements for the handling of the
mixed concrete and of the raw materials. Still more remote, but still
a necessary application, is the use of steel and iron in the crushing
mills and the like. When we look at the question and inform ourselves
of the ramifications, it is not difficult to see that concrete and
steel are materials whose engineering applications are mutually
involved. Concrete is certainly replacing steel in some applications.
But, notwithstanding this, these two are to be regarded as unopposed
to each other on the whole.
When concrete is cast about steel, an adhesive bond ensues. But this
is scarcely to be regarded as sufficient to enable the two to act as
one under tensile stresses. A mechanical bond should be employed. This
is the explanation of the somewhat complicated forms of standard
reinforcement bars.
Concrete properly reinforced is an admirable material for factory
construction. It permits of rapid erection, is fireproof, has a long
life, is adapted to weather conditions, and is economical. The floors
of concrete buildings are easily cleaned and do not develop splinters.
One of the large automobile factories--that of the Geo. N. Pierce 324
Company at Buffalo, N. Y.--is a good instance of the rapidity with
which reinforced concrete buildings may be erected. Within seven
months of the date of signing the contract with the Trussed Concrete
Steel Company, Detroit, Mich., which employs the Kahn system of
reinforcement, certain large structures were ready for use. The floor
space here is 325,000 square feet. It was necessary to provide a
number of large areas unbroken by supports. It was found possible to
use girders having spans of 55 and 61 feet. When subjected to a load,
a girder develops compressive strains above and tensile ones below.
The concrete is well adapted to withstand the one, but not the other.
In an ordinary bridge truss, there may be diagonals that are also
under tensile stress. In the Kahn system of reinforcement, a
horizontal bar from which rigidly attached diagonals extend upward and
outward is provided with a view of enabling the girder to withstand
the tensile stress. In accordance with this design the long girders
were constructed. Girders providing runways for 3-ton cranes were also
constructed. A load of 14 tons placed upon one of the reinforced
concrete girders having a span of 25 feet induced a deflection of only
1/16 inch. This girder is 12 inches wide and 22 inches deep and its
reinforcement consists of three 1 x 3 in. Kahn bars. Hollow tile was
largely employed here in connection with the concrete.
What is known in the trade as the corrugated bar, supplied by the
Corrugated Steel Bar Company is a steel reinforcing rod which provides
shoulders by means of which the concrete is mechanically engaged. This
general type of reinforcement is, however, not confined to this
concern. By means of this style of bar, the engineer is able to secure
the desired mechanical interlock. As the concrete and steel expand and
contract they do so together--unless the temperature change is
excessive--and so the relation between the two is maintained. Such
standard types of reinforcing bars are applicable to multitudes of 325
construction. An interesting example is the railroad bridge over the
Vermilion River near Danville, Illinois. There are three arches, the
central one of which has a span of 100 feet. About 130 tons of
corrugated bars were employed in the construction of this beautiful
bridge.
Another good example of bridge construction is the bridge over the
Maumee River near Waterville, Ohio. This structure follows the designs
of the National Bridge Company. It has a width of 16 feet between
copings and crosses the river at a point where it is 1,000 feet wide.
It is said that this reinforced bridge will carry a load of 5 tons per
linear foot. The arches are 12 in number, the longest having a span of
90 feet, and the shortest, one of 75 feet. The loading of a bridge
arch produces a lateral thrust upon the piers. If the next arch is not
loaded, then this thrust is unbalanced and must be cared for. This was
done in this case by employing part of the 100 tons of reinforcement
in a vertical position. This bridge having a very long expectation of
life was built at a cost of $77,000. The total amount of concrete was
about 9,200 cubic yards.
The city of Philadelphia has gone into the construction of city
bridges of concrete in rather an extensive way. Among a total of 30 or
more is the reinforced bridge across Poquessing Creek, having a span
of 71 feet. This bridge is rather flat, having a rise of but 9½ feet.
The reinforcement employed here consisted in part of angle bars placed
in pairs to form a kind of T-bar. The principal reinforcement here was
the arch ribs. These were each composed of two of the T-bars arranged
one above the other in such manner that their points of nearest
approach were at the crown. These were latticed together. Such ribs
were placed 4 feet apart. Transversely disposed steel rods held the
whole together. The mechanical interlock here depended upon was due,
no doubt, to the mutual disposition of the various rods, etc.
A railway viaduct, one-half mile or more in length is another example 326
of the Kahn methods. This structure belongs to the Richmond &
Chesapeake Railway and is located at Richmond, Va. There is a span of
70 feet which has girders nearly 6 feet deep. At another span the
girders, probably of about the same depth, sagged but ⅛ inch upon
removal of the falsework.
A style of reinforcement much used consists of a net-like fabric of
metal. This is employed largely in floors to bind the whole mass
together. In the manufacture of this netting, a Canadian company has
found it desirable to repair the inevitable breakages of strands in
manufacture by the use of the Davis-Bournonville Company’s
oxyacetylene torch. It is said that welds can be made on the average
of one in two minutes in the case of an ordinary weight of the fabric.
This netting is made by expanding sheets of perforated metal from a
narrow to a considerable width. It is during this expansion that the
strands sometimes break.
Another style of floor reinforcement is the fabric made from wire.
That floors properly reinforced are quite substantial may be judged
from the case of the United States Fidelity & Guarantee Company. Their
building in Baltimore was exposed to intense heat in the great fire of
1904. In fact, a considerable part of the side walls and the front
fell, leaving floors of concrete. A load of brick giving a pressure of
300 pounds per square foot was arranged on one of the floors to a
distance of 5 feet to each side of one of the girders. The deflection
amounted to ⅛ inch. This was about 1/20 of 1 per cent of the span.
This is an example of Hennibique construction.
Reference has already been made to the Kahn truss reinforcement. With
the same general object in view, the Hennibique truss has been
designed. There are two horizontal bars, one above the other. The
upper is, however, not perfectly horizontal except near the center.
Towards either side, this bar rises as it recedes from the center. 327
These two bars are enveloped by loose stirrups arranged vertically and
at intervals. These are open at the top and closed below.
There are two varieties of piles--the bearing pile and the sheet pile.
Their duties are quite different. One sustains a compressive load, the
other withstands a transverse thrust. But concrete has been used for
both kinds. In the case of the bearing pile, its own intrinsic
qualities are eminently suitable. It has good compressive resistance;
it is teredo proof, and has the prospect of long life whether
conditions are wet, dry or a mixture of the two. Wood makes an
admirable bearing pile, if constantly submerged, but it is a prey to
the teredo. The necessity for constant submergence limits the
usefulness of the wood pile. It must be cut off below the hydraulic
level, and this necessitates carrying the foundation footings to a
lower level than would otherwise ordinarily be the case. With the
concrete bearing pile, on the contrary, the footings may be
constructed at any level desired as the pile itself may be partly
submerged and partly in the dry. However, the concrete pile may be
subjected to other than compressive stresses, especially during its
placement. And so, some reinforce it. Some, no doubt, have in view a
possible buckling when in the ground, particularly if the surrounding
soil is yielding. Reinforcement both longitudinal and transverse is
employed. Longitudinal bars are arranged at intervals around and
within the periphery. These may be bound together by separate hoops
disposed along the length or by wire wound about the longitudinals in
spirals. In the case of concrete sheet piling, the concrete supplies a
surface and forms a protective covering to the imbedded reinforcement
which is here a vital matter and consequently indispensable. In the
dock improvements at Baltimore, to which reference has already been
made, reinforced concrete sheet piling was largely used. The steel
sheet pile could not well have been used here because of its 328
susceptibility to corrosion. The concrete slabs, 12 × 18 inches in
cross-section perform the duty of retaining masses of earth in place
both above and below the water line. There were certain other concrete
constructional elements of an auxiliary character. The total
reinforcement amounted to about 1,200 tons.
With regard to its fireproof qualities, an eloquent testimonial arises
from the fact that the immense Marlborough-Blenheim Hotel at Atlantic
City, a concrete and tile structure, is said to enjoy a saving of
$18,000 per year in fire insurance premiums. The insurance is based on
$600,000. This structure is 560 feet in length and has a width varying
from 60 to 200 feet.
Reference has been made to the close identity of the co-efficients of
expansion for steel and concrete for moderate intervals of
temperature. While this is so, if the thermal range is considerable,
the concrete and the steel cannot be expected to expand and contract
together. In most engineering construction, the range is small, say
150 degrees F., but there are exceptions. One of these relates to the
material used in tall chimneys. The hot gases which pass up these give
rise to rather high temperatures. In fact, it is well-recognized
practice to build a large part of such chimneys double, one shell
enveloping another, with an air space between. Some four or five years
ago what is, perhaps, the very tallest concrete chimney in the United
States was built for the Colusa Parrot Mining & Smelting Company,
Butte, Montana. It is 352½ feet high and has a flue 18 feet in
diameter. A solid wall 1½ feet thick constitutes the base of 21 feet
in height. Above this level, an air space 4 inches wide radially is
arranged between two shells of 5 and 9 inches thick. The inner one is
the thinner. The steel reinforcements used were T-bars. The footing is
of reinforced concrete and rests upon a fill 18 feet deep. A further
important factor which has to be considered is the serious effect 329
of repeated stresses. Partly because of this, it is recommended that a
large factor of safety be adopted. Further, the best practice would
seem to be in the direction of a complete divorce between the inner
and outer shells all the way up and of a uniformity in wall thickness
from bottom to top. Vertical cracks have been noted in some chimneys.
This would indicate the advisability of strong circular reinforcement.
It is thought that a tone concrete following the formula 1:2:2 is
better for the outer shell than a cement mortar. It is said to be
stronger, denser and more impervious to water than a mortar following
the formula 1:3. In order to secure adhesion between layers, the fresh
concrete should be applied wet and the old should perhaps be
resurfaced by tooling.
The compressive resistivity of the usual concretes is considerable.
However, in certain bridge construction in New York City, a need was
felt for a concrete which should have a very high compressive
resistance. And so experiments were made with a concrete formed by
substituting wire nails for the crushed stone. About 60 tests were
made with concrete following the formula 1:2:2⅔. The resulting
material was quite heavy. A cubic foot weighed 196 pounds as compared
with 130 to 160 pounds for ordinary concretes. Eighty-eight pounds of
nails were used in one cubic foot bringing the cost to about $2.30.
This was certainly very expensive material. But where extraordinary
qualities are desired, we have to spend money. Cubes were cast
measuring 6 inches on a side. These were tested to destruction at
different stages of maturity. After the lapse of one week, the lowest
crushing resistance obtained was 2,770 pounds per square inch and the
highest 3,330 pounds. After one month, the minimum crushing strength
was 3,050 pounds, the maximum 8,340 pounds, while the average was
5,645 pounds. When a year had gone by, it was found that four cubes
gave an average of 10,410 pounds. However, the average resistance of 330
17,235 pounds was obtained in the case of cubes 15 months old.
Since concrete is but little affected by water and by fluctuations
between wet and dry conditions, it is not at all remarkable that it
has been employed for sewer and water tower construction. In the
United States a high standpipe has been constructed at Attleboro,
Massachusetts. This is 118 feet high and has an internal diameter of
50 feet. The wall varies from 18 inches in thickness at the bottom to
8 inches at the top. The concrete was made according to the formula
1:2:4. There is another tower 110 feet high and having an external
diameter of about 35 feet. At Anaheim, California, a large tank
together with its substructure has been constructed entirely of
reinforced concrete. The floor of the tank is about 60 feet above the
surface. The tank itself is 38 feet in height and 30 feet in diameter
and has a wall varying in thickness from 5 to 3 inches. The
reinforcement employed was the twisted steel bar.
In order to prevent corrosion of the reinforcement, it is thought
necessary to guard against water entering and dissolving away the
caustic lime contacting with the steel. One way would be to give the
concrete itself a very dense character. Another is to fill the
external pores with a bituminous or oleo-resinous paint. Or, an
insoluble substance suited to fill the pores may be one of the
ingredients when the concrete is mixed. Finally a flexible waterproof
coating may be employed where conditions permit. As to the steel
itself--it is desirable to have it uniform, as then reliance may be
placed upon calculations. For this reason, one of the great concrete
construction companies recommends mild steel as opposed to high carbon
steel.
One of the great recommendations of concrete is that it permits
wonderful rapidity of construction. We had an example of this in the 331
case of the Geo. N. Pierce automobile factory. Another was in
connection with the construction of junction caissons for certain
subsurface tubes of the tunnel of the Hudson Companies. These caissons
were three in number and were located on the Jersey shore opposite New
York City. These structures were quite large, being about 100 feet in
length and having a width of about 45 feet. These caissons, one or two
of which were put under air pressure, were constructed of concrete
with steel reinforcement. The use of concrete in the tunnel system and
in the Terminal Building has been very extensive. To complete the
concrete construction, about half a million barrels of Portland
cement, so it is thought, must be consumed. The Gatun Locks at Panama
will require only about four times this amount. The twisted steel bars
of the reinforcement have been used in large quantity.
The work on the water front at Baltimore to which reference has
already been made involved a considerable variety of reinforced
concrete construction. For retaining walls sheet piles were employed.
These ordinarily had a face of 18 inches and a thickness of 12 inches
and a length of 27 feet. As it was not necessary to retain the soil by
an impervious bulkhead, these piles did not interlock. However, they
had to resist a horizontal thrust, and so wales were strung along the
outside at the top. These wales were themselves of concrete reinforced
by means of imbedded lattice girders of steel. In position, the
girders lay flat and thus gave their chief strength to the horizontal
thrust. The wales were supported, in part, by concrete piers. These
were placed by means of steel caissons. These cofferdams were of sheet
steel 27 feet deep and were sunk by open air methods. When in place,
the concrete was put in and the pier thus formed. An upward surface of
the pier provides a means of absorbing the horizontal thrust of the
wales. The piers themselves are, some of them, mutually tied together
across the dock; others are tied to reinforced concrete piles sunk in 332
the body of the dock. The ties are themselves of reinforced concrete.
The steel of the caissons served only as a mold. It is now a matter
apparently of but little importance how soon it corrodes. The
extensive concrete work at Baltimore was done by the Raymond Concrete
Pile Company.
While the question of the teredo seems to have been a factor at
Baltimore because of the probability of its presence in the harbor
when certain sewerage improvements are carried out, this matter was
really an insistent thing in connection with a wharf constructed by
the United Fruit Company at Bocas del Toro in the Republic of Panama.
This wharf is itself of reinforced concrete. But the bearing piles are
what interest us. The native wooden piling, so it seems, would at this
general location become seriously damaged by the teredo within a year.
Some kinds of timber might be expected to have a longer life. The
service of creosoted piles has been estimated as about 15 years.
Besides, piles 70 feet in length were desired. This requirement put
the ordinary reinforced concrete piles out of consideration. What was
actually done was to use an untreated timber pile and then to encase
it where it passed through the water in a reinforced concrete shell.
This shell was made of such size as to allow a space between it and
the enclosed wooden pile. A rich concrete was put in this space at the
bottom and thus excluded the external water. Upon pumping out the
retained water, the major portion of the space was filled with a lean
concrete and a top layer of rich concrete then added in which the
column reinforcement was placed. The steel used for reinforcement was
in the main round bars of mild steel. The piles averaged 58 feet in
length; the shells, 18.4 feet. The cost of these shells was $1.78 per
linear foot. It is said that the cost of the untreated wooden pile
together with its protective coating was not greater than what would
have been the expense for a creosoted pile.
At both the Baltimore docks and the wharf in the tropics, concrete 333
is exposed to the action of sea water. But there is no violence in
this action. However, a very large application of concrete
construction has been recently carried out in a very much exposed
maritime situation off the coast of Florida. It is 156 miles from the
mainland to the island of Key West. Scattered along this interval are
a number of islands, so that in reality the total linear amount of
intervening land is about one-half the distance. Some of the water
passages are only a few hundred feet in width; one is about 2½ miles
wide. The greater portion of the aqueous route is of a shallow depth.
But for about 6 miles the water reached depths up to 30 feet; and this
in connection with an exposed situation. Reinforced concrete viaducts
have been built to accommodate trains and resist the storms. A quarter
million barrels of cement and about 5,700 tons of steel went into
these works.
The viaduct from Long Key is 2 miles long and passed through water
having a depth ranging from 13 to 20 feet. The floor of the Gulf is of
coral. To construct a pier, about 30 piles would be driven in with
their tops projecting up from the floor. A cofferdam would be sunk to
include them and a seal of concrete 1 yard thick be placed. The water
could now be pumped out and the form concreted. The reinforcement
would, of course, be put in place before depositing this concrete. The
pier would then be allowed 3 weeks to mature. The concrete was mixed
with fresh water to avoid the effect of sea water on the steel.
Corrugated bars were used in reinforcing the walls and the 184 arches.
High water is 31 feet below the top of this structure, so that the
track is well protected from the waves.
It may surprise some, but concrete has actually been used as the chief
material in the construction of boats. A reinforced concrete boat was
built thirteen years ago for use on the River Tiber in Italy. Not only
the hull but posts and roof of the structure above deck were of 334
concrete. This house boat was 67 by 21 feet. Another Italian boat is
the Liguria, a barge in actual service. It is 57 by 18 feet and is
rated at 150 tons. The Gretchen is an American example of the stone
boat. She has sailed over long distances on the Atlantic and was
reputed as comparatively a rapid sailer in a heavy sea. Her
reinforcement was a multitude of small rods. This boat drew 14 feet of
water and was 65 feet long and had a beam measurement of 16 feet.
Concrete is an obvious material for coal pockets, especially because
of its fireproof character. A further advantage is the avoidance of a
large maintenance charge. At Charlestown (Boston), the Lehigh &
Wilkes-Barre Coal Company had been expending about $1,000 yearly on
repairs upon a coal pocket. This has now been replaced by a concrete
structure having a capacity of 10,000 tons. It has a depth of 24 feet,
and has a length of 182 feet and a width of 92 feet. It is founded
upon 750 Simplex concrete piles. If wooden piling had been used, the
amount of excavation thus necessitated would have been very
considerable because it would have been necessary to cut them off 10
feet below the surface in conformity with the building laws. Moreover,
about 2,000 wooden piles would have been required because of the limit
of ten tons’ bearing capacity per pile. With the concrete piles,
however, the footings for the columns were constructed with but little
excavation. The columns, side walls, girders, beams, floors--pretty
much everything except the roof--were of reinforced concrete. When a
full load of coal is filled in on the floor, the weight per square
yard is 18 tons.
A similar application is to the construction of grain elevators.
Reinforced concrete has been used at Baltimore in two important
buildings of this kind and also in the case of a third at Buffalo. The
question of fire is here very important. The grain elevator of the
Pennsylvania Railroad at Baltimore is the largest of the three and is 335
constructed to hold 1,000,000 bushels. There are 53 cylindrical bins
having a common height of 79 feet. There are four rows of eight each.
The remaining twenty-one bins occupy spaces in between, three rows
seven in a row. The set of 32 have the larger size and measure 24.2
feet in internal diameter. The walls are 8 inches thick and have both
vertical and circumferential reinforcement. The vertical reinforcement
is round bars of 1⅜-inch diameter. The circumferential reinforcement
consists of interlaced flat bars. By a patented device the bins were
cast in sections. This mold would be attached to the heavier vertical
reinforcement and jacked up as needed.
It is unnecessary to emphasize the fact that concrete while economical
is not cheap. So that when large masses are used, it is advisable to
reduce the expense by using what may be called “pudding stones.” At
McCalls Ferry a large dam and adjoining power house span the
Susquehanna River. This is a tremendous application of concrete.
However, pudding stones were very properly employed in the
construction of the great dam. Here steel was employed not so much to
reinforce but to supply frames for the molding surfaces. Great pelican
cranes of steel were also employed to handle the concrete, etc. The
face of the dam is a double curve and thus required a precise mold.
Sections of the dam, 40 feet in length, would be constructed to
alternate with open spaces of the same length. When it was desired to
close such open spaces, a great steel apron would be let down on the
upstream face. Concrete could then be laid in the open space.
In all the applications of reinforced concrete with which our
attention has so far been occupied, the case has either been one of
well-recognized practice or closely related to such practice--with the
possible exception of concrete barges. There are two other lines of
engineering application in which it is very desirable to employ 336
concrete, but where we are scarcely entitled to regard its use as
anything more than experimental. Reference is made to telegraph poles
and cross-ties. If a concrete pole really proves adapted to its
service, then we may expect a great reduction in maintenance expense.
It is estimated that renewals of wooden poles in the United States
cost yearly $13,000,000. The prospect of getting a pole which will not
need renewal for a long period is certainly attractive. But the actual
service is severe. This is due not so much to the load which must be
carried as to the horizontal movements under wind pressure. But by
using proper reinforcement, it is thought by some, the pole may be
made to withstand the horizontal thrusts. Some experiments have been
made of a type of pole recommended by the American Concrete Pole
Company, Richmond, Indiana. Four vertical rods bound together by wire
constitute the reinforcement. Such a pole 7 x 7 inches at the top and
12 x 12 inches at the bottom was tested to destruction. This pole was
30 feet long and had its butt end sunk 5 feet into the ground. The
vertical rods were ⅝ inch in diameter and were bound with No. 9
wire. A horizontal thrust or pull at the top of 840 pounds
accomplished a deflection of 6 inches. When this was increased to
1,780 pounds, the deflection amounted to 17 inches. When 2,800 pounds
pressure was employed, the deflection was 30 inches accompanied by a
slight cracking. A deflection of a full yard together with cracking at
the ground line resulted from a pressure of 3,640 pounds. When 7,200
pounds pressure was employed, the cracking became bad and the
deflection amounted to 60 inches. A cedar pole of the same size was
deflected 11 inches by a pull of 840 pounds. With 1,780 pounds, the
deflection was nearly a yard (33 inches); and with 2,200 pounds the
pole broke about 3 feet from the ground. The problem of the telegraph
pole will probably be solved, if this has not already been done. 337
With regard to the cross-tie the case is more difficult. Plain
concrete slabs or beams cannot be used after the manner of the wooden
tie because of their want of elasticity. What is called “center
binding” would be disastrous to plain concrete. The rocking action of
the passing load is also a factor which enters. One method of dealing
with center binding is to divide the tie into two parts, connecting
them with steel rods. The Corell tie is an example of this. In the
Percival tie, the under part of the concrete block is given a
sharpened edge. Beneath the rail itself, the cross-section is a kind
of oval. There is longitudinal reinforcement in the form of four rods,
three arranged at the top and one near the bottom. Three rods are
bound with wire. There is a cushion block of wood which absorbs and
distributes the shocks from the bottom of the rail. Screw spikes and
metallic sockets are employed. Some three or more years ago a hundred
such ties were put in service in a Texas railway. In June, 1909, seven
only were found to have received serious injury. It is thought that
this damage was scarcely chargeable to the ties themselves as when in
position they were between wooden ones whose deterioration might
easily have been the cause of undue disturbance being thrown on the
concrete ties.
We have considered to a slight extent the use of steel as the material
of concrete forms. This line of application, however, promises to
become a very large one. Two notable constructions are now under way
in which the steel form plays a large part. These are the great Gatun
Locks of the Panama Canal and the Catskill Aqueduct. The three double
locks at Gatun will require about 2,000,000 cubic yards of concrete.
Each pair of locks is on a separate level and has three longitudinal
walls. One separates the lock chambers. This central wall is 60 feet
in width. It is not solid as so much concrete would not be required as 338
the water level is approached. Consequently, there is a kind of
V-section which traverses it longitudinally. This is filled in except
for three galleries--one for drainage, one for the electric wires and
one for the men. There is a longitudinal culvert arranged below the
fill in the body of the concrete wall. In the side walls of the lock
chambers are other longitudinal culverts. From the central supply
culvert transverse distributing culverts run off beneath the floors of
the adjacent lock chambers. These have vertical outlets into the lock
chambers themselves. Similarly, but for purposes of emptying the
locks, the longitudinal culverts arranged along the outside are
connected by transverse culverts and vertical openings with the lock
chambers. The members of the two systems of transverse culverts
alternate with each other. The main supply culvert has a diameter of
22 feet part of the way and of 18 feet part of the way. Now these many
culverts, various in form and size, are to be molded in the mass
concrete by means of steel forms. As originally announced, there would
be 12 forms of open hearth boiler steel for the main supply culvert.
Each of these weighs 177,000 pounds. One hundred forms were to be
required. The two main outlet culverts of similar dimensions to the
main supply culvert were thought to require 21 forms, each 12 feet in
length and having a weight of 300,000 pounds. The transverse culverts
were to require 100 forms, each having a length of 10 feet and a
weight of 217,000 pounds. There were thus to be 133 forms having an
aggregate weight of 15,000 tons. It is possible that there may be some
modifications of this plan in minor particulars. The side walls of the
lock chambers are to be mainly vertical planes having a height of,
say, 81 feet. To retain the fresh concrete in place, 12 face plates,
constructed of sheet steel are to be used. These are 7½ inches in
thickness, having face dimensions 78 x 36 feet. Steel towers running on
suitable tracks control these face plates. It is estimated that 339
towers and plates will have an aggregate weight of 26,000 tons. So
that, quite apart from any possible reinforcement application, steel
to the total of about 41,000 tons is to be used for forms and
immediate accessories. But this 41,000 tons is not all. The concrete
is to be cast in great monoliths and to retain the ends of these while
the concrete is fresh, steel girders 6 feet high are to be employed.
If these locks were to be of stone then steel would have played a
rather subordinate part.
The Blaw Collapsible Steel Centering Company are engaged at Panama,
but they are also applying their systems of molding concrete to the
great aqueduct which is to supply New York City with water from the
Catskill Mountain region on the other side of the Hudson River. A
steel centering is used to give form to the interior. Steel forms are
also employed to shape the upper part of the external surface. At
Baltimore, more than three miles of sewer construction was carried out
in accordance with the system of the same company. The centering used
for one portion where the height was 11 feet and the width 12¼ feet
(inside) was employed in 50-foot lengths. In 2 hours, 6 men could
remove such a 50-foot section together with its falsework and have it
in readiness for a repetition of its service. A typical half-round
Blaw center consists of one or more steel plates bent to conform to a
cross-section of a semi-circle. Turnbuckles retain this shell in
position. If we are going to employ this form in sewer construction,
we first dig out our trench to such dimensions and form as to furnish
the mold for the outside surface of the lower part of the concrete
sewer. We then lay concrete in a longitudinal strip along the bottom,
giving the upper surface the form of a shallow gutter. When this is
sufficiently hardened, the semi-circular center may be slid along it
to suitable position. The center has its concavity opening upwards.
The concrete of the invert of the sewer is now placed. The same or a 340
duplicate center may now be used to mold the interior of the upper
part of the sewer.
Portland cement has been in use for a long time. But reinforced
concrete is so modern that in some important lines of engineering
application the fundamental data underlying practice are not fully
determined. In what may be regarded as the first decade (1870-1880) of
the considerable manufacture of Portland cement in the United States,
the total amount produced was only 42,000 barrels. Fifty years and
more would be required for the production of enough cement to
construct the Gatun Locks. Over a decade would be necessary to yield
enough cement for the operations of the Hudson Companies. The price at
this period was about $3.00 per barrel. In 1908 it was 85 cents. But
the production in this year was more than 1,200 times that in 1880.
The value per year of the present output is about $50,000,000.