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Reinforced concrete

Reinforced concrete is plain concrete in which steel rods or bars ("rebars") have been incorporated to reinforce 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.

Tied Rebar

Plain concrete will carry extremely heavy compressive stresseses, 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, then reinforced concrete can be used in these roles.

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 differential 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, it's often crucial to "tie" the rebar, bending it so the bar can't pull out, and the bars reinforce each other in tension. Skillfully tied rebar forces the concrete into compression, where it has its greatest strength.

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 occurrs 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.

All concrete must be cured, by exposing it to water, to reach its best strength. Reinforced concrete structures sometimes have provisions (such as ventilated hollow cores) to control their moisture.

Corrosion and frost damage reinforced concrete. When rebar rusts, it 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,a nd 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.