Reinforced concrete (Ferro concrete) is plain concrete in which steel reinforcement rods or bars ("rebars") have been incorporated to strengthen the naturally brittle concrete. The use of reinforced concrete is a relatively recent invention, usually being considered as covering the last 150 years, and its accidental discovery is commonly ascribed to a Parisian gardener named Monier in about the year 1860.

The major developments of reinforced concrete have taken place since the year 1900; and from the late 20th Century, engineers have developed sufficient confidence in a new method of reinforcing concrete, called post-tensioned concrete, to make routine use of it.

Plain concrete will carry extremely high compressive stresses, but any appreciable tension will cause rupture and consequent failure. For this reason, plain concrete is limited in its use as a structural member subject to bending or direct tensile action. When steel bars are incorporated into concrete (illustration, left), a reinforced concrete section is created. This reinforced concrete section is much more efficient in carrying tensile forces due to bending or direct tension than a plain concrete section with the same dimensions.

The general rule is: concrete takes the compression, steel takes the tension. However, initially a newly-formed concrete member will behave according to general mechanics, until the concete cracks in tension.

There are two physical characteristics which are responsible for the success of reinforced concrete. Firstly, the coefficient of thermal expansion of concrete is very nearly identical to that of steel, preventing internal stresses due to differences in thermal expansion or contraction. Secondly, when concrete hardens it grips the steel bars very firmly, permitting stress to be transmitted efficiently between both materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel.

Although the ridges on rebar help, there is sometimes not enough length (actually surface area) of embedment of reinforcing steel in the concrete to fully bond or transfer force between concrete and rebar. In these cases the rebar may be bent into a 180 degree hook, which itself will transfer half of the capacity of the rebar between the rebar and concrete.

In some structural members where minimum cross-section is desired, steel may be used to carry some of the compressive load as well as tensile load. This occurs in columns. Continuous beams in buildings generally require some compressive steel at the columns, but beams and slabs usually have reinforcing steel only on the tension side. In the case of continuous girders where the tensile stress alternates between top and bottom of the member, the steel is bent accordingly into a zig-zag shape within the beam.

The amount of steel required for adequate reinforcement is usually quite small, varying from 1% for most beams and slabs to 6% for some columns. The percentage is usually based on the area in a right cross section of the member. Reinforcing bars are round and vary by eighths of an inch from 0.25" to 1" in diameter (in Europe from 8 to 30 mm in steps of 2 mm). Also galvanized rebar is available. Typically, concrete will have reached its nominal design strength at most 28 days after the water was mixed into the cement mix.

Reinforced concrete structures sometimes have provisions (such as ventilated hollow cores) to control their moisture.

Corrosion and frost may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the rust expands, cracking the concrete and unbonding the rebar from the concrete. Frost damage occurs when water penetrates the surface and freezes. The expansion of freezing water in microscopic cracks widens the cracks, causing flaking, and eventual structural failure.

In wet and freezing climates, reinforced concrete for roads, bridges, parking structures and other structures that may be exposed to deicing salt require epoxy-coated rebar. Epoxy coated rebar can easily be identified by the light green color of its epoxy coating.

Penetrating sealants must be applied some time after curing, when the concrete has dried to at least several inches of depth. One especially exotic process is to surround the cured concrete member with a vacuum bag filled with resin monomer, and then after the monomer has penetrated several inches into the concrete, the monomer is cured with a gamma ray source. This produces a very hard, attractive surface that can be dyed through the material, so chips and scratches are less visible.

Less expensive sealants include paint, plastic foams, films and aluminum foil, felts or fabric mats sealed with tar, and layers of bentonite clay, sometimes used to seal roadbeds.