Building Materials - Table of Contents................ Illustrated Architecture Dictionary ..

Concrete

On this page, below:

Romans

Modern Concrete

Portland Cement

Reinforced Concrete

Definitions

Former area quarries

See also:

Reinforced Concrete - Table of Contents

Concrete Block 'Fireproof' Houses in Buffalo, NY

The Transition From 19th to 20th Century in Industrial Design


Romans

Excerpts from
Rome
By Robert Hughes
Vintage Books, 2011, pp. 110-111





Romans

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.


Buffalo Pavement Markers
Includes photos and explanation of some history that most people are unaware of

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.

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.



Concrete Block
'Fireproof' Houses

Photos and history

Modern Concrete
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


Reinforced concrete with rebar being used in 2012 bridge constructuon

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.

Reinforced concrete 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.

Rebar: a steel rod with ridges for use in reinforced concrete.  First Known Use: 1953


Definitions:


Former area quarries:


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