Reinforced concrete
Reinforced 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 tensile will cause rupture and consequent failure. For this reason, plain concrete cannot be used for any structural member subject to bending or direct tensile action. However, if steel bars are incorporated in such a way as to carry the tensile stresses (illustration, left), then reinforced concrete can be used in these roles. The rule is: concrete takes the compression, steel takes the tension.
There are two physical characteristics which are responsible for the success of reinforced concrete. Firstly, the coefficient of 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 cohesion between the concrete and steel.
Although the ridges on rebar help, there is sometimes not enough length (actually surface area) available for the concrete to stick to the rebar. In these cases the rebar is "tied", bending it so the bar can't pull out, and the bars reinforce each other in tension.
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 galvanised rebar is available. Concrete will have reached its nominal design strength at most 28 days after the pour.
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, many building codes for public works require epoxy-coated rebar, and concrete that has been painted or sealed to keep water out.
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.