How Does Reinforced Concrete Work?

Reinforced concrete is concrete cast around embedded steel, usually in the form of bars, rods or mesh, so that the two materials act together as a single structural material. People often ask how does reinforced concrete work. The short answer is that concrete resists squeezing well but cracks easily when stretched, and the steel carries the stretching forces that the concrete cannot.^[1][2] The combination is used in buildings, bridges, dams, pavements and foundations, and the Federal Highway Administration (FHWA) reports that reinforced concrete makes up over half of the U.S. bridge inventory.^[3]

The modern technique is usually dated to 1867, when the French gardener Joseph Monier patented iron-reinforced concrete.^[4][5] The pairing succeeds for three practical reasons: the steel bonds firmly to the concrete, both materials expand and contract by similar amounts as temperatures change, and the alkaline chemistry of concrete protects the steel from rust.^[6][3] The main weakness is that this protection can fail over time, so corrosion of the embedded steel is the leading durability problem.

The basic mechanism

Compression and tension

Concrete is a mixture of cement, water, sand and gravel that hardens into a stone-like solid. It is strong under compression but weak under tension. FHWA material on corrosion gives concrete’s tensile strength as typically only 1 to 2 megapascals, a small fraction of what the same material can withstand when squeezed.^[2] A university course text describes unreinforced concrete as “virtually useless” for resisting tensile forces.^[1] Steel, by contrast, is strong in both tension and compression.

Where the steel goes

A simple beam supported at both ends shows why placement matters. Under load the beam bends. The upper part is squeezed and the lower part is stretched. Concrete handles the squeezing. Steel bars are therefore placed in the lower, stretched zone, where cracks would otherwise open. According to the course text, the American builder William E. Ward was the first, in 1873, to take full advantage of steel’s tensile strength by placing bars below the neutral axis of a beam.^[1]

Concrete can still crack under tension, and designers expect it to. The steel does not prevent small cracks. It holds the two faces together so the crack stays narrow and the member keeps carrying load. Reinforcement is also bent into stirrups, loops that wrap the main bars and resist diagonal shear forces near supports. François Hennebique’s early system already combined bent-up bars with stirrup hoops.^[1]

Bond and thermal compatibility

For the two materials to share load, force must pass between them. This transfer is called bond. Ward noted that cement adhered to iron so firmly that a break occurred in the cement rather than at the interface.^[1] Modern bars are roughened or ribbed to improve grip, and the American engineer Ernest L. Ransome patented twisted steel bars in 1884 for the same purpose.^[6][7] Because bond stress varies along a bar, design codes specify a development length, the distance a bar must be embedded to anchor fully, and bars that are too short need hooks or end plates.^[6]

Temperature changes would tear the materials apart if they moved at different rates. According to one encyclopedic summary, concrete expands by about 10 to 15 millionths per degree and steel by about 12 millionths, a close match that avoids damaging thermal stress at the interface.^[6]

History

Early experiments

Several inventors tried iron in cement before Monier. Joseph-Louis Lambot built a boat from iron mesh coated in mortar in about 1849 and displayed it at the 1855 Paris fair; he is credited with inventing ferrocement.^[8][9] François Coignet, a French industrialist, built a four-story iron-reinforced concrete building at Saint-Denis in 1853 and patented his approach in 1855.^[5] In 1854 the Englishman William Boutland Wilkinson reinforced the roof and floors of a house with steel rods and rope, and patented the solution.^[5]

Monier and the patents

Monier, a gardener at the Tuileries, wanted stronger flowerpots, because plain concrete pots cracked. He received a patent on 16 July 1867 for iron-reinforced troughs and showed the invention at the Paris Exposition that year.^[4][5] Further patents followed for pipes and basins (1868), building panels (1869), bridges (1873) and beams (1878).^[8] In 1875 he designed a shallow arch bridge of 16.5 meters at the Chazelet estate near Bourges, France, which Grace’s Guide calls the first iron-reinforced concrete bridge and which still exists.^[8] Monier sold rights to German and Austrian builders, and Gustav Wayss obtained them all by 1887.^[4]

Spread and standardization

François Hennebique, who had seen Monier’s work at the 1867 exposition, patented his own system in 1892.^[4] He franchised it internationally, and more than 7,000 structures had been built under his license by 1902.^[9] Fires in Chicago, Seattle and Baltimore in 1871, 1889 and 1904 pushed American interest toward concrete frames, which survived fire better than exposed iron or steel.^[9]

The Ingalls Building in Cincinnati, completed in 1903, is recognized by the American Society of Civil Engineers (ASCE) as the world’s first reinforced concrete skyscraper, at 16 stories and 210 feet.^[10] It was designed as a monolithic box in which slabs, beams, columns and stairs were cast as one unit using Ransome’s system.^[7] The Ingalls Building helped win acceptance for the technique in a modern skyscraper.

Materials and construction

Reinforcing steel

Most reinforcement is rebar, short for reinforcing bar, though welded wire mesh and fibers are also used. The distance from the outer surface to the nearest bar is the cover. A university text gives typical cover as roughly 25 to 75 millimeters, which shields the steel from water.^[1] FHWA notes that bridge decks have often been built with epoxy-coated bars and covers of 65 millimeters or more.^[3]

Prestressed and precast concrete

In prestressed concrete, steel tendons are tensioned so that the concrete is permanently compressed before it carries any load. In pretensioning, concrete is cast around tendons already under tension; in post-tensioning, tendons run through ducts and are tightened with hydraulic jacks after the concrete hardens.^[11] The French engineer Eugène Freyssinet patented his first prestressing method in 1928, and his 1939 anchorage application marked the birth of post-tensioning.^[12] The Precast/Prestressed Concrete Institute notes that pretensioned members are typically made in a factory, which links the method to precast concrete.^[11] Invention & Technology magazine reports that prestressed construction accounts for about half of new U.S. bridges each year.^[9]

Failure modes and durability

How reinforced concrete fails

An encyclopedic summary lists several failure modes. In serviceability terms, cracking lets moisture reach the steel. At the ultimate limit, the concrete may crush when compressive stress exceeds its strength, the bars may yield or rupture, or bond between bar and concrete may fail.^[6] How much steel goes where is the task of structural engineering, and minimum requirements are set in a building code.

Corrosion of the steel

Fresh concrete pore water is highly alkaline, with a pH of about 13.0 to 13.8, and in that environment steel forms a thin protective oxide film about 10 nanometers thick.^[3][2] Two processes can destroy it. Carbonation occurs when carbon dioxide from the air reacts with the concrete and lowers its pH to roughly 8 to 9. In dense, high-quality concrete it advances about 1 millimeter per decade or less.^[2] Chloride ions, from deicing salts or seawater, can also break down the film. FHWA reports that active corrosion can begin with chloride at the steel as low as 0.6 kilograms per cubic meter of concrete, although no single threshold has been agreed.^[3][2]

The corrosion product, rust, occupies two to four times the volume of the original steel according to the National Ready Mixed Concrete Association (NRMCA), which pushes against the surrounding concrete.^[13] Because concrete is weak in tension, the pressure causes cracking, delamination and spalling of the cover. FHWA estimates the direct annual cost of corrosion on bridges at $5.9 billion to $9.7 billion, and says indirect costs could raise the total as much as tenfold.^[3]

Common defenses include thicker cover, a water-to-cementitious-materials ratio of about 0.40 to 0.50 to reduce permeability, supplementary materials such as fly ash, slag or silica fume, corrosion-inhibiting admixtures, and coated, galvanized or stainless bars.^[13] Their long-term value is debated: FHWA found that epoxy-coated bars suffered premature corrosion in some Florida marine bridge substructures and called their performance over 75 to 100 year design lives uncertain.^[3]

Context and limitations

Because the steel is hidden, damage can be hard to see. FHWA guidelines call for visual inspection of bridges every two years.^[3] Concrete frames also compete with masonry and exposed structural steel, which differ in cost, fire resistance and speed of construction. Resistance to fire was one reason for the adoption of concrete frames after the urban fires of the late 1800s.^[9]

FAQs

Why is steel put inside concrete? Concrete is weak in tension and steel is strong in tension. The steel carries the stretching forces so that beams, slabs and bridges can span greater distances without breaking.

Why do steel and concrete not separate when it gets hot? They expand and contract by similar amounts. One summary gives about 10 to 15 millionths per degree for concrete and about 12 millionths for steel.

Who invented reinforced concrete? No single person did. Lambot, Coignet and Wilkinson experimented in the 1840s and 1850s, but Joseph Monier’s patent of 16 July 1867 is the one most often cited as the start of the modern technique.

Why does reinforced concrete crack and spall? Rust takes up two to four times the volume of the steel it replaces. The expanding rust pushes against the cover concrete, which is weak in tension, and breaks it.

What is prestressed concrete? It is concrete squeezed in advance by tensioned steel tendons so that it stays in compression under load. Eugène Freyssinet patented the first method in 1928.

What was the first reinforced concrete skyscraper? ASCE recognizes the 16-story, 210-foot Ingalls Building in Cincinnati, completed in 1903.