Concrete - a Roman invention - was made from a changing recipe of lime mortar, volcanic sand, water, and small stones ("caementa," from which the English word "cement" is derived). The mixture was placed in wooden frames and left to dry and bond with a facing of brick or stone in a procedure somewhat like the casting of statues in bronze or other metals. When the concrete was completely dry, the wooden molds were removed, leaving behind a solid mass of great strength, though rough in appearance, which was often covered afterward with stucco or even sheathed with marble revetment.
Despite this, concrete walls were much less costly to construct than walls built of imported Greek marble or even local Italian tufa and travertine. The advantages of concrete, however, go well beyond economy of construction, for it is possible to fashion shapes out of concrete that cannot be achieved by masonry construction, especially the huge vaulted and domed ceilings (without internal supports) that the Romans came to prefer over the post-and-lintel structures of the Greeks and Etruscans.
The use of concrete enabled the Roman architect to think of architecture in terms radically different from those used by earlier builders. Roman architecture became an architecture of space rather than of sheer mass.
Buffalo Pavement Markers
The Romans used a cement made only of lime to manufacture a concrete with aggregates of broken bricks and stones. This cement slowly dissolves in water, but it becomes almost as strong as modern concrete when mixed "with pozzolana," a volcanic ash found at Pozzuoli near Naples. The Romans did not invent concrete, but a combination of pozzolanic concrete and outer surfaces of excellent stone, or good brick of burnt clay, allowed them to erect the majestic and massive structures which survive to this day.
Ideal as it is for construction, concrete too has some unfortunate properties. If not properly wetted, or cured, while it hardens, it shrinks and cracks, allowing humidity to rust the reinforcing bars. Moreover it continues to stretch or shorten, creeps, under constant tension or compression loads, up to three or more years after hardening.
Modern Concrete
 The Frank Lloyd Wright-designed Alice Millard House (La Miniatura) uses precast concrete block, as do his other buildings in southern California |
Concrete is a mixture of cement, sand,
crushed stone or pebbles, and water. The water and cement paste fills the
voids between the grains of sand and these fill the voids between the
stones. After a few days the cement paste starts to harden or set and at
the end of four weeks it gives concrete its nominal ultimate strength,
which is as good as that of some of the strongest stones. Concrete
mixtures are "designed" by specialized laboratories and mixed in
strictly controlled proportions in concrete plants from which they are
carried to the site in the revolving drums of large trucks, that keep
mixing them en route. Concrete samples in the shape of cylinders or cubes
are taken from each truckload and tested for compressive strength after
seven and twenty-eight days. The strength of concrete depends on the
ratio of water to cement, and of cement to sand and stone. The finer and
harder the aggregates ( sand and stone) , the stronger the concrete. The
greater the amount of water the weaker the concrete.
Portland Cement
In 1824, Englishmen, Joseph Aspdin patented Portland cement..
Portland cement, as modern cement is called, is a mixture of
limestone and clay, burned in a furnace and then pulverized. Impervious to
water, it actually becomes stronger if submerged after it hardens. Samples
of concrete taken thirty years after a concrete boat sank during World War
I showed that the concrete had doubled its compressive strength.
Reinforced Concrete
Unfortunately, even the best concrete has a tensile strength barely one
tenth of its compressive strength, a property it has in common with all
stones. The invention of reinforced concrete remedied this deficiency and
produced a structural material that, pound per pound, is the most
economical.
In reinforced concrete, bars of steel are embedded in the concrete in those areas where pulls will develop under loads, so that the steel takes the tension and concrete the compression. For example, the bottom of a beam supported at its ends is always in tension, while its top is in compression. Steel bars set near the bottom of the beam prevent the concrete from cracking under tension and make the beam work as if it were made of a material, like steel or wood, capable of resisting both kinds of stress.
Concrete Block
'Fireproof' Houses
Reinforced concrete was originally invented in France towards the middle of the 1800s and its use spread
very rapidly all over the world. It is today the most commonly used structural material.
Combining the compressive strength of concrete and the tensile strength of steel, reinforced concrete can be poured into forms and given any shape suitable to the channeling of loads. It can be sculpted to the wishes of the architect rather than assembled in prefabricated shapes. It is economical, available almost everywhere, fire-resistant, and can be designed to be lightweight to reduce the dead load or to have a whole gamut of strengths to satisfy structural needs.
Definitions:
- CONCRETE: a hard strong building material made by mixing a cementing material (commonly Portland cement) and a mineral aggregate (washed sand and gravel or broken rock) with sufficient water to cause the cement to set and bind.
- CEMENT: a powder made from alumina, silica, lime, iron oxide, and magnesia burned together in a kiln and finely pulverized--which when mixed with water to form a plastic mass hardens by chemical combination and by gelation and crystallization, and is used as an ingredient in mortar and concrete.
- NATURAL CEMENT: a naturally occurring clayey limestone which, when calcined and finely pulverized, produces a hydraulic cement (cement capable of setting and hardening by a reaction with water).
- FLINT: gray, brown, or black quartz.
- LITHIC: of or relating to stone.
- GRANOLITH: artificial stone of crushed granite and cement.
- MARBLE: limestone crystallized in varying degrees by metamorphism ranging from granular to compact in texture, usually white or veined or tinted or mottled.
- PETROLITHIC: of, relating to, or constituting a road surface consolidated to a rocklike firmness.
- PORTLAND CEMENT: (developed c. 1824).Portland cement, as modern cement is called, is a mixture of limestone and clay, burned in a furnace and then pulverized. Impervious to water, it actually becomes stronger if submerged after it hardens.
- SYENITE: a visibly crystalline plutonic rock with granular texture composed largely of alkali feldspar, with subordinate plagioclase and mafic (ferromagnesian) minerals, the intrusive equivalent of trachite.
- VULCANITE: a hard vulcanized rubber.
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