Reinforced concrete

Last updated: September 24, 2007.

Steel and concrete seem the most modern of materials, yet they can be traced back through thousands of years of human civilization. Although steel itself has been used only since the middle of the 19th century, iron, the major constituent of steel, has been found in tools dating back to 3000 BC. Concrete, the quintessential component of the modern urban landscape, was discovered in the floor of a Stone-Age Yugoslavian village built around 5600 BC. Although a type of concrete called pozzolanic cement was widely used in bridges and aqueducts built by the Romans from 200 bc onward, the technique of making concrete was largely forgotten until reinvented in the 18th century. Today's modern cityscape is built from reinforced concrete, concrete strengthened by steel bars and stronger than either material alone.

Photo: Pouring concrete from a mixing truck. Picture by James F. Cline III courtesy of U.S. Army and Defense Visual Information Center.

What is steel?

Although steel is mostly iron, the other materials present in it, mainly carbon, give it its unique strength and other properties. Over 90 percent of all steels are general-purpose carbon steels, which contain up to 2 percent carbon. The other main varieties are extremely tough tool steels (containing tungsten and molybdenum for added strength, and typically used for tools and cutting machinery), alloy steels (with other elements added to give them specific properties for specific jobs), and stainless steels (containing around 12 percent chromium to give them a decorative appearance and prevent rusting).

Steel was first produced around 1860 by William Kelly (1811-1888) in the United States and Henry Bessemer (1813-1898) in England. The Bessemer process by which most steel has been produced ever since involves blasting drafts of air through a furnace containing impure molten iron. The oxygen in the air bonds chemically with the impurities and removes them, leaving behind steel. Modern variations include the basic oxygen process, in which pure oxygen is used instead of air, and the electric furnace, in which massive electrodes create high temperatures by passing enormous electric sparks through iron.

Find out more about steel in our main article on iron and steel.

Photo: You can't properly join steel together with adhesives; you have to heat it and melt it together (a process known as welding). Picture by Ssgt. Martin Wright, courtesy of U.S. Navy and Defense Visual Information Center.

What is concrete?

Concrete is a type of artificial stone made by mixing dry aggregate (sand or gravel) and cement, then adding water. This makes a soft mix that can be molded easily or transported in a rotating concrete mixer. Different types of concrete can be made by varying the basic ingredients. Stronger concrete can be made by increasing its density, which involves increasing the cement and reducing the aggregate and water. Concrete typically gets stronger as it gets older. It takes several days for wet concrete to set properly, but it will continue to gain in strength for at least five years after that. The properties of concrete can be varied in other ways. It can be made either waterproof, to resist rain, or porous. It can be smoothed off or textured to look like wooden paneling.

What makes steel and concrete so strong?

Steel is a mixture of ferrite (ordinary, soft iron), cementite (or iron carbide, a hard and brittle iron-carbon alloy), and pearlite (a strong iron-carbon alloy with properties somewhere between ferrite and cementite). Steel can be made harder by increasing the carbon content, which increases the amount of pearlite and cementite relative to the amount of ferrite and so makes the steel more brittle. Steel can be made very much harder by heating it to around 14,500°F (8000°C). This produces a moderately strong steel called austenite. If the steel is cooled slowly, the austenite turns back into ferrite and pearlite. However, quenching the steel (cooling it rapidly in water) changes the austenite into a hard and brittle steel called martensite, which is toughened by tempering (raising and holding the temperature for a time).

Steel is not always as strong as it looks. Metallurgists believe the ocean liner Titanic was sunk because the steel from which its hull was made contained too much nitrogen. This made it very brittle at low temperatures and particularly susceptible to damage from the fatal iceberg in the sub-zero temperatures of the Atlantic.

Concrete owes its strength to the way in which water and cement combine chemically and bind together the particles of aggregate into a very dense mass. Allowing concrete to dry slowly means the concrete stays moist for longer so the chemical reaction is more effective. This explains why concrete is harder if it dries more slowly and why it becomes harder over time as the chemical reaction continues to occur.

Reinforced concrete in the modern world

Steel and concrete have been partners in construction since at least the middle of the 19th century. Reinforced concrete was pioneered by French engineer François Hennebique (1842-1921) and French architects were among the first to appreciate its remarkable qualities. Other notable advocates included Finnish architect Eero Saarinen (1910-1961), who designed New York's first reinforced concrete skyscraper, the CBS building, in 1965 and used the material in the famous swooping roof of the Trans World Airlines (TWA) building at New York's John F. Kennedy International Airport.

Geodesic domes

Photo: Lowering a geodesic roof into place on a fuel tank. Picture by Deborah Kermgard, courtesy of U.S. Airforce and Defense Visual Information Center.

Invented by American architect Richard Buckminster Fuller (1895-1993), the geodesic dome does away with bulky beams and girders by distributing the weight of a building through an external steel skeleton. This is made up of thousands of interconnected triangles or hexagons, each strut of which carries an equal but relatively small part of the load. Fuller's biggest dome, some 384 ft (117 m) in diameter, was constructed in 1958 in Baton Rouge, Florida.

Strengthening concrete

Steel is good at withstanding tensile stress (bending forces), whereas concrete is good at bearing compressive stress (squeezing forces) but can crack under tensile stress. The idea of reinforced concrete—concrete strengthened with steel or glass fibers—is to combine these two qualities to produce a material that is stronger than either material alone.

Reinforced concrete can be made by forming the concrete inside a metal or timber framework or by casting the concrete around steel bars. Another variation called stressed or prestressed concrete involves molding wet concrete around pretensioned steel wires. The wires compress the concrete as it sets, making it much harder.

Photo: Making reinforced concrete. These construction workers from the US Navy are spreading wet concrete from a truck onto a grid of steel reinforcing bars. When the concrete sets, the steel bars will give it added strength. Picture by Lt. Edward Miller, courtesy of U.S. Navy and Defense Visual Information Center.