CONCRETE AND ITS REINFORCEMENT
Pouring and ramming are easier and cheaper than cutting and carving
. . . Concrete for dwellings ensures comfort and safety from fire
. . . Strengthened with steel it builds warehouses, factories and
bridges of new excellence.
Stone and wood in the builder’s hands require skill and severe labor for
their shaping; vastly simpler and easier is the task of molding a wall
from wet clay, or other semi-plastic material. It was long ago
discovered that certain mixtures of clay and sand, duly mingled and
burned, became as hard as stone. To this discovery we owe, among other
arts, that of brick-making. In joining brick to brick, or stone to
stone, a mortar of uncommon strength was used by the Romans. All by
itself, when laid a little at a time, it formed a strong and lasting
structure. Then it occurred to some inventive builder, Why not save
mortar by throwing into it gravel and bits of broken stone? He
accordingly reared a wall of what we should now call rude concrete,
whose lineal descendant to-day is a semi-plastic mass of Portland
cement, sand, and gravel or broken stone, together with the necessary
water. Its use allows the ease and freedom of pouring, while affording
structures with all the strength of stone or brick.
For much of the early work lime and sand were mixed to make a mortar of
the usual kind, in which stone or gravel was embedded. Afterward it was
found that volcanic ashes, such as those of Puzzuoli near Naples, formed
with lime a compound which resisted water and was therefore suitable for
structures exposed to damp or wet. In the middle ages concrete was
employed throughout Europe, after the Roman fashion, for both
foundations and walls. In walls it was usually laid as a core faced with
stone masonry, large stones often being embedded in the mass. About
1750, while building the third Eddystone Lighthouse, John Smeaton
discovered that a limestone which contained clay, when duly burnt,
cooled, ground, and wetted, hardened under water, was indeed a natural
cement, by which name it is still known. Deposits suitable for the
direct manufacture of natural cement were in 1818 discovered in Madison
and Onondaga Counties, New York, by Canvass White, an engineer who used
this cement largely in building the Erie Canal. Natural cement has a
powerful rival in Portland cement, due to Joseph Aspdin, of Leeds, who
in 1824 mixed slaked lime and clay, highly calcined. The resulting
clinker when ground, and only when ground, unites with water, the
strength of the union increasing with the fineness of the grinding.
Because this product looks like Portland stone, much used in England, it
was given the name of Portland cement. The raw materials suitable for
making it are widely distributed throughout North America, much more
widely than those from which natural cement may be had. This is the
principal reason why Portland cement is now produced in the United
States in about six-fold the quantity of natural cement.
So rapidly has concrete grown in public favor with American builders
that in 1905 they used seven-fold as much as in 1890. It has been widely
adopted for pavements, as at Bellefontaine, Ohio; for breakwaters, as at
Galveston and Chicago; for tunnels, as in more than four miles of the
New York Subway. The foundations beneath the power-house of the
Interborough Rapid Transit Company, New York, required 80,000 cubic
yards; for the new station of the Pennsylvania Railroad Company, New
York, a much greater quantity is being employed; in their turn these
figures will be far exceeded by the needs of the new Croton Dam for the
water supply of New York, and the Wachusett Dam for the water supply of
Boston.
[Illustration: Concrete silo foundation, Bricelyn, Minn.]
[Illustration: Concrete silo, Gedney Farms, White Plains, N. Y.]
Concrete has long been adopted for a variety of less ambitious purposes.
At St. Denis, near Paris, it was many years ago molded into a bridge of
modest span. It has formed thousands of dwellings in factory and mining
villages and towns, as well as many villas of handsome design. It is
particularly well adapted for silos, as here illustrated.[36] All this
expansion of an old art has been stimulated by a steady reduction in the
price of Portland cement, and by constant improvement in its quality. As
the manufacture has expanded, its standards have risen, its machinery
has become more economical and trustworthy in results. While the cost of
concrete has thus been lowered by a fall in the price of cement, the
wages of bricklayers and stone-masons have advanced, adding a new reason
for building in concrete, since it requires in execution but little
skilled labor. The good points of concrete are manifold; it forms a
strong, fire-resisting, and damp-proof structure. For mills and
factories another item of gain is that it forms a unit such as might be
hewn out of a single huge rock, vibrating machinery therefore affects it
much less than it does an ordinary building. At the same time its walls
and floors obstruct sound, conducing to quiet. Concrete must be honestly
made and used, otherwise, just as in the case of rubbishy bricks, ill
laid, it may tumble down from its own weight. And furthermore it is
necessary to recognize how widely concretes of diverse composition vary
in strength and durability. There should be a careful adaptation in each
case of quality to requirement. Concrete walls, as first produced, had a
forbidding ugliness; this is being remedied by surfacings of pleasant
neutral tones. A well designed residence executed in concrete at Fort
Thomas, Kentucky, is shown opposite this page.
[36] The illustration of a silo and its foundation are taken by
permission from “Concrete Construction about the Home and on the
Farm,” copyright 1905 by the Atlas Portland Cement Co., 30 Broad
St., New York. This book of 127 pages, fully illustrated, with
instructions and specifications, is sent _gratis_ on request.
In Mr. Edison’s judgment a vast field awaits the concrete industry in
building small, cheap dwellings. He once said to me, as he spoke of
his cement mill,--“What I want to see is an architect of the stamp of
Mr. Stanford White of New York take up this material. Let him design
half a dozen good dwellings for working people, all different. Each set
of molds, executed in metal, would cost perhaps $20,000. Such dwellings
could be poured in three hours, and be dry enough for occupancy in ten
days. A decent house of six rooms, as far as the shell would go, might
cost only three hundred dollars or so. It would be stereotypy over again
and the expense for the models would disappear in the duplications
repeated all over the country.”
[Illustration: MANSION IN CONCRETE, FORT THOMAS, KENTUCKY.
FERRO-CONSTRUCTION CO., CINCINNATI.]
Concrete is now supplied to builders in blocks, usually hollow and much
larger than bricks. When cast in sand they look like stone. Of course,
subjected as they are to more than ordinary stresses, their production
demands special care. The methods, therefore, which are adopted in
manufacturing these blocks may be taken as the best practice in the
industry broadly considered. Says Mr. H. H. Rice, of Denver:--“The sand
employed should be sharp, silicious and clean. The gravel used should
contain a fair proportion of as large sizes as can be advantageously
employed in the particular machine used. Where gravel is not available,
crushed stone takes its place. Care should be exercised to obtain stone
as strong as the mortar. What proportions of sand, gravel and broken
stone should be mixed together is a question determined by the extent of
their voids: these may vary from one third to one half the whole volume.
Assuming that we have to deal with the larger fraction, a mixture of 1
cement, 2 sand, 4 gravel, should be employed; this is classified as the
lowest grade of fat mixture. At times a lean mixture, 1 cement, 3 sand,
5 gravel, might be advantageously adopted. Where gravel or broken stone
is not used, the proportion of cement to sand should be as 1 to 4. A fat
mixture has greater tensile strength than a lean mixture, but resistance
to compression depends upon a thorough filling of voids. A lean mixture
thoroughly worked, proves more satisfactory than a fat mixture with
hasty and indifferent handling. With any mixture success is attained
only by completely coating every grain of sand with cement, and every
piece of stone or gravel with the sand-cement mortar. (See Mr. Umstead’s
results, page 240.)
[Illustration: Wall of two-piece concrete blocks.
American Hydraulic Stone Co., Denver.]
In producing concrete blocks there are three different methods, tamping,
pressing, and pouring, each adapted to a particular mixture for a
special kind of work. Two-piece walls, devised in 1902, deserve a word
of description. The pressed blocks of which they are built show the new
freedom conferred by concrete as a building material. Each block has a
long right-angle arm extending inward from the middle, and a short arm
extending from each end. In laying the blocks in a wall no portion of a
block extends through the wall. By leaving the exterior vertical joints
open to afford a free circulation of air, no part of a block on one side
of the wall touches any block from the opposite side; this prevents the
passage of moisture and produces in effect two walls, tied by the
overlapping arms or webs in alternate courses, and affording in its
bond a great resistance to lateral stresses. Blocks in other forms
equally useful are steadily gaining popularity.[37]
[37] Mr. H. H. Rice’s first-prize paper on the manufacture of
concrete blocks and their use in building construction appeared in
the Cement Age, New York, October, 1905. Permission to use his paper
and the illustration here presented, both copyrighted, has been
courteously extended by the publishers.
Concrete, although widely available to the builder, is in many cases a
material he cannot employ. For a store-house, thickness of wall,
ensuring an equable temperature, is an advantage; for an
office-building, reared on costly ground, this thickness is out of the
question. Beams, too, cannot have much length in a material which is
only one tenth as strong in tensile as in compressive resistance.
Clearly the scope for concrete by itself was to be limited unless it
could find a partner able to confer strength while adding but slight
bulk. An experiment of the simplest was to be the turning point in a
great industry.
Concrete Reinforced by a Backbone of Steel. Joseph Monier, the Pioneer.
Concrete, as one of its minor uses, had often been molded into tubs for
young trees and shrubs. In 1867, Joseph Monier, a French gardener, in
using tubs of this kind found them heavy and clumsy. By way of
improvement he built others in which he embedded iron rods vertically in
the concrete, securing thus a strong frame-work which permitted him to
use but little concrete, and make tubs comparatively light and thin.
Monier was not a man to rest satisfied with a single step in a path of
so much promise. Before his day builders had joined concrete and metal,
but without recognizing the immense value of the alliance. He proceeded
to build tanks, ponds, and floors of his united materials, at length
rearing bridges of modest proportions. His work attracted attention in
Germany and Austria, as well as at home in France, so that soon
reinforced concrete, as it was called, became a serious rival to brick
and stone. For two thousand years and more, concrete had been a familiar
resource of the builder; to-day with a backbone of steel it fills an
important place between masonry and skeleton steel construction, boldly
invading the territory of both.
[Illustration: Ransome bar.]
[Illustration: Corrugated steel bar. St. Louis Expanded Metal Fire
Proofing Co.]
[Illustration: Thacher bar.]
Disposal of Steel in Reinforced Concrete.
Reinforced concrete has been thoroughly studied with regard to its
properties and the forms in which it may be best disposed. Since the
strength of concrete is usually ten-fold greater in compression than in
tension, designs should be compressive whenever possible, all tensile
strains being carefully committed to the steel. In arched bridges the
strains are chiefly compressive, hence the success with which they are
executed in reinforced concrete. Mr. Edwin Thacher of New York, eminent
in this branch of engineering, sees no reason why spans of 500 feet
should not be feasible and safe. Some remarkable discoveries have
followed upon experiments with reinforcement diverse in form and
variously placed within a mass. To increase the strength of a square
steel bar Mr. E. L. Ransome twists it into spiral form; on square steel
bars Mr. A. L. Johnson places projections; Mr. Edwin Thacher rolls his
steel into sections alternately flat and round. All these contours have
large surfaces at which metal and concrete adhere. Reinforcing bars
designed by Mr. Julius Kahn and by the Hennibique Construction Company
are smooth, and slightly bent from straightness at intervals. In every
case the question is, Where will the tensile strength of the steel do
most good, because most needed? M. Considere has found that concrete
hooped with steel wire has more than twice the resistance of concrete in
which an equal amount of steel is centrally placed. In his floor
constructions M. Matrai gives steel wires the curves they would take
under a load. Keeping to its original lines the Monier reinforcement of
to-day consists in a rectangular netting of rods or wires. Somewhat
similar is the expanded metal backing invented by Mr. J. F. Golding; it
is sheet steel pierced with parallel rows of slits which are expanded
until the metal assumes the form shown in an accompanying illustration.
A lock woven-wire fabric of galvanized steel wire is made by W. N. Wight
& Company, New York, in any desired size of mesh, with an ultimate
strength of 116,000 pounds per square inch of metal.
[Illustration: Kahn bar.]
[Illustration: Hennebique armored concrete girder.]
[Illustration: Monier netting.]
For piling, reinforced concrete is extensively used. Its independence of
moisture, its exemption from the ravages of the teredo, render it much
preferable to timber for marine work.
[Illustration: Expanded metal diamond lath.]
[Illustration: Tree box in expanded steel.]
[Illustration: ROYAL BANK OF CANADA, HAVANA.
Built of concrete. Entrance.]
Molds for Reinforced Concrete.
Reinforced concrete, like every other new building material, has called
forth ingenuity in many ways. When, for instance, a factory is to be
reared much inventive carpentry is required to plan and construct the
forms, or molds, into which the liquid concrete is to be poured around
the steel skeletons. The footings, outside and inside columns, walls,
girders, beams, floor-plates, roofs, and stairs all require separate
forms, intelligently devised with a view to economy. For the Ingalls
Building, Cincinnati, the forms cost $5.85 per cubic yard of concrete in
place. White pine is the best wood for the purpose; it is readily worked
and keeps its shape when exposed to wind and weather. For common
buildings a cheaper wood, spruce or fir, may be chosen; even hemlock
will serve if a rough finish suffices. In most cases green lumber is
preferable to dry as less affected by water in the concrete. In fine
work the boards of which the molds are made are oiled, and may be used
over and over again. In all tasks a strict rule is that the reinforcing
metal be properly placed and remain undisturbed as work proceeds.
[Illustration: Lock-woven wire-fabric.
W. N. Wight & Co., New York.]
Buildings of Reinforced Concrete.
The Pugh Power Building, erected for manufacturing purposes in
Cincinnati, is a capital example of what can be done with reinforced
concrete. It is 68 feet wide, 335 long, and 159 high; its columns are
spaced fourteen to seventeen feet longitudinally, twenty to twenty-three
feet transversely; the floors are figured to bear a load of 230 pounds
per square foot. In the same city is the Ingalls Building, for offices,
100 by 50 feet, and 210 feet high, designed by Mr. E. L. Ransome of New
York. Among other structures of his design, executed in the same
material, is the St. James Episcopal Church, Brooklyn, New York;
buildings for the United Shoe Machinery Company, Beverly, Massachusetts,
and piano factories for the Foster-Armstrong Company, Despatch, New
York. The inspection shops of the Interborough Rapid Transit Company,
West 59th Street, New York, are also of reinforced concrete: no wood is
used in wall or roof.
Reinforced concrete forms nine bins in one of the grain elevators of the
Canadian Pacific Railway at Port Arthur, Ontario, on the shore of Lake
Superior. The walls are nine inches thick, reinforced horizontally and
vertically to a height of ninety feet and a diameter of thirty feet.
There are also four intermediate bins, the whole thirteen holding
443,000 bushels. At South Chicago the Illinois Steel Company has built
four similar bins for the storage of cement, each twenty-five feet in
diameter and fifty feet high, with walls five to seven inches thick.
Many chimneys have been built of the new material; notably the chimney
for the Pacific Coast Borax Company, Bayonne, New Jersey, 150 feet high,
with an interior diameter of seven feet. These dimensions are exceeded
at Los Angeles, California, where a chimney for the Pacific Electric
Company rises 174 feet above its foundations, with an inside diameter of
eleven feet. Both structures have hollow walls of the Ransome type
reinforced horizontally and vertically.
That reinforced concrete serves to build chimneys and flues is proof of
its fire-resisting quality. Concrete is a slow conductor of heat, and
both it and steel have almost the same slight expansibility as
temperatures rise, so that they remain together in a fire. Terra cotta,
which expands much more than steel when heated, cracks off from the
metal it was intended to protect, leaving it to bend or fuse in a blaze.
Concrete, furthermore, behaves well when its temperature is suddenly
lowered, as when a fireman dashes a stream of water upon it at a fire.
No wonder, then, that the reinforced concrete is more and more in
request in cities as the material for buildings rising higher and
standing more thickly on the ground than did buildings of old. In the
great fire in San Francisco, April, 1906, reinforced concrete withstood
extreme temperatures much better than any other material. It will be
largely used in rebuilding the city.
[Illustration: Column form, Ingalls Building, Cincinnati. A, A, yokes.
B, B, spacing pieces. From “Reinforced Concrete.” A. W. Buel and C. S.
Hill. Copyright, Engineering News Publishing Co., New York, 1904.]
Resistance to Fire and Rust.
Frequently the question is asked, Is the steel in reinforced concrete
liable to corrosion, so that its walls are likely to become weak and
insecure after a few years? With careful planning and faithful
workmanship the results prove to be worthy of confidence. Professor
Charles L. Norton of Boston has taken steel, clean and in all stages of
corrosion, and embedded it in stone and cinder concrete, wet and dry
mixtures, in carbon dioxide and sulphurous gases; other specimens were
intermittently exposed to steam, hot water, and moist air for one to
three months. Duly protected by an inch or more of sound concrete the
steel was absolutely unchanged while naked steel vanished into streaks
of rust. Mr. Ransome says that in tearing up a stretch of sidewalk in
Bowling Green Park, New York, in use twenty years, some embedded steel
rods were found in perfect condition. The Turner Construction Company,
of New York, exposed concrete blocks in which steel bars were embedded,
and laid them on a beach at low tide where they were covered by salt
water three or four hours every day; after nine months’ exposure the
blocks were broken disclosing the bars free from rust. Professor Spencer
B. Newberry records that a water main at Grenoble, France, built on the
Monier system, twelve inches in diameter, eighteen inches thick,
containing a framework of 1/16 and 1/4 inch steel rods, was found
perfectly free from rust after fifteen years’ service in damp ground. He
also states that a retaining wall of reinforced concrete in Berlin was
examined after eleven years’ use and the metal found uncorroded, except
in some cases where the rods were only 0.3 or 0.4 inch from the surface.
[Illustration: Section of chimney at Los Angeles, Cal.]
[Illustration: Coignet netting and hook.]
[Illustration: Cross-section of conduit, Newark, N. J. Expanded metal
reinforcement.]
Tanks, Standpipes, Reservoirs.
This waterproof quality of reinforced concrete recommends it as a
material for tanks and reservoirs. In 1903 a water tower was built at
Fort Revere, Massachusetts, for the United States Government,
ninety-three feet in height, octagonal in section, enclosing a tank
twenty feet wide, fifty feet high, with walls six inches thick at the
bottom, three at the top, coated inside with an inch of Portland cement.
At Louisville, Kentucky, a reservoir has been built 394 by 460 feet, and
about twenty-five feet high. Its walls and columns are concrete, its
roof is in reinforced concrete disposed as groined arches, each of
nineteen feet clear span. A reservoir wholly of reinforced concrete at
East Orange, New Jersey, is 139 by 240 feet, with a height of 22-1/3
feet. In the early days reinforced concrete was used for water-pipes:
more than a hundred miles of such pipes are now in service in Paris.
Water-pipes on the Coignet system employ thin steel rods hooked at both
ends and curved into encircling hoops. Other rods laid lengthwise run
through the hooks, so as to hold each part of the framework securely in
place. At Newark, New Jersey, 4,000 feet of single and 1,500 feet of
double 60-inch conduits, reinforced with 3-inch expanded steel, have
been recently laid.
The material thus available for systems of water supply is also
impressed into tasks of sewerage. In Harrisburg, Pennsylvania, a sewer
of this kind three miles long intercepts all other sewers, carrying the
whole stream below the city to an outfall in the Susquehanna River. A
water culvert, for somewhat similar duty, may on occasion be so heavily
reinforced as to carry railroad tracks with safety, as in a culvert for
a Western railroad shown in an accompanying figure.
[Illustration: Water culvert.]
New York Subway.
Part of the New York Subway is of reinforced concrete. Steel rods, about
1-1/4 inches square were laid at varying distances according to the
different roof loads, from six to ten inches apart. Rods 1-1/8 inches in
diameter tie the side walls, passing through angle columns in the walls
and the bulb-angle columns in the centre. Layers of concrete were laid
over the roof rods to a thickness of from eighteen to thirty inches, and
carried two inches below the rods, imbedding them. For the sides similar
square rods and concrete were used and angle columns five feet apart.
The concrete of the side walls is from fifteen to eighteen inches thick.
[Illustration: River des Pêres Bridge, Forest Park, St. Louis.]
[Illustration: Memorial Bridge, Washington, D. C.]
Bridges.
At first, properly enough, reinforced concrete was adopted with much
caution in bridge-building. To-day hundreds of bridges in this material
are doing service throughout the world. A good example of a small bridge
is that in Forest Park, St. Louis, spanning the River des Pêres. A
noteworthy design on a large scale, by Professor William H. Burr, of
Columbia University, New York, has been accepted for the Memorial
Bridge to cross the Potomac River at Washington. A centre-draw span of
159 feet in steel is to be flanked on each side by three spans of
reinforced concrete, each of 192 feet. These spans are ribbed arches,
having a rise of twenty-nine feet, with their exteriors in granite
masonry. In arguing for bridges in reinforced concrete, Mr. Edwin
Thacher points out that under normal circumstances their steel is not
strained to much more than one quarter of its elastic limit, so that a
large reserved strength is available for emergencies, while the
structure is more durable than a steel bridge and ultimately more
economical, comparatively free from vibration and noise, proof against
tornadoes and fire, and against floods also if the foundations are
protected from scour.