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Reinforced concrete, also called ferroconcrete in some countries, is concrete in which reinforcement bars ("rebars") or fibers have been incorporated to strengthen the material that would otherwise be brittle.
History
The use of reinforced concrete is a relatively recent invention, usually dated to 1848 when Jean-Louis Lambot became the first to use it. Joseph Monier, a French gardener, patented a design for reinforced garden tubs in 1868, and later patented reinforced concrete beams and posts for railway and road guardrails. Most reinforcement is made of steel, but fiber-reinforced plastic materials are available.
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.
Physics and statics
Concrete is a mixture of portland cement and stone aggregate. Stone aggregate can be made up from small grains of sand, fist-sized rocks, or a combination of sizes. When mixed with a small amount of water, portland cement hydrates to form a microscopic opaque crystal lattice structure encapsulating and locking the aggregate into its rigid structure. Typical concrete mixes employ a washed-gravel aggregate and have extremely high resistance to downward compressive stresses (about 3,000 lb/sq in); however, any appreciable stretching or bending (tension) will break the microscopic rigid lattice resulting in cracking and separation of the concrete. For this reason, typical concrete must be supported or placed on an earth footing to prevent cracking.
The significance of this "additive resistance to forces" cannot be overstated. Concrete and steel are very inexpensive everyday building materials which allow the creation of very expensive structures. Depending on the type of concrete mix and steel employed, reinforced concrete structures can support 300 to 500 times their combined weight and behave, according to general mechanics, as a single structural entity. (For comparison, consider that typical student-constructed balsa wood bridges only support 20 to 80 times their weight.)
Happily, three physical characteristics come together to give reinforced concrete its special properties. First, the coefficient of thermal expansion of concrete is very nearly identical to that of steel, eliminating internal stresses due to differences in thermal expansion or contraction. Second, when cement cures, the microscopic lattice conforms to the surface details of the steel, permitting any stress to be transmitted efficiently among the different materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel. Third, the alkaline chemical environment provided by calcium carbonate (lime) causes a passivating film to form on the surface of steel, making it much more resistant to corrosion than it would be in neutral or acidic conditions.
Although the ridges on rebar offer increased surface area to resist tension forces, sometimes there is not enough embedment of reinforcing steel in the concrete to fully transfer tensile forces between the concrete and rebar. For example, it is possible to pull a rebar rod from cured concrete. In cases where there would be extreme tension forces, the rebar may be bent into a 180 degree hook, which itself will transfer half of the capacity of the rebar to resist tension forces to the 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 (bottom) side. In the case of continuous girders where the tensile stress alternates between top and bottom of the member, multiple runs (layers) of steel may be used or the steel may be bent into a zig-zag shape within the beam.
The relative amount of steel required for typical strengthing is usually quite small and varies from 1% for most beams and slabs to 6% for some columns (based on the area of a cross section of the member). Reinforcing bars are round and vary by eighths of an inch from 0.25 in to 1 in diameter (in Europe from 8 to 30 mm in steps of 2 mm). Interestingly, there does not appear to be specialized rules for the placement, pattern, or joining of rebar (other than it should be placed towards the side where the tension forces will be the strongest). In conservative construction projects like roadways and bridges, a series of rebar curtains or matrixes are embedded in the concrete. Rebar comes in two grades of carbon content, 60 and 40, which typically sell for the same price. Grade 60 has a higher carbon content and, therefore, a higher tensile strength, but its stiffness can make it difficult to bend and cut. Construction workers always prefer to use grade 40 rebar. 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 may require epoxy-coated rebar or a well composited concrete well planes structure. 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.
Common failure modes of steel reinforced concrete
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.Carbonation
The water in the pores of the cement is normally alkaline, this alkaine environment is one in which the steel is passive and does not corrode. According to the pourbaix diagram for iron the metal is passive when pH is above 9.5.* The carbon dioxide from the air reacts with the alkali in the cement and makes the pore water more acidic, thus lowering the pH. Carbon dioxide will start to carbonate the cement in the concrete from the moment the object is made, this process will start at the surface and slowly move deeper and deeper into the object. If the object is cracked the carbon dioxide of the air will be more able to penitrate deep into the object. When designing a concrete structure it is normal to state the concrete cover for the rebar (the depth within the object that the rebar will be). The minimum concrete cover is normally regulated by standards or design codes. If the rebar is too close to the surface then an early failure due to corrosion may occur.One method of testing a structure for carbonation is to drill a fresh hole in the surface and then treat the surface with Phenolphthalein, this will turn pink when in contact with alkaline cement. It is then possible to see the depth of carbonation. An existing hole is no good as the surface will already be carbonated.
Chlorides
Chlorides such as salt which is used for deicing roads is able to promote the corrosion of steel rebar. For this reason, in mixing concrete only fresh water may be used.Concrete cancer
Also called concrete rot, is a rather ill-defined term which means different things to different experts.*. It is however, unrelated to the disease cancer in humans or animals. Concrete Cancer was a term originally used by Professor Harold Roper of Sydney University in an interview with a Sydney Morning Herald journalist in the 1980's when he was describing the effect of corroding reinforcement spalling pieces of concrete from concrete surfaces. The term has since been spread almost world wide by journalists and other non-technical people.=Alkali silica reaction
= This is found when the cement is too alkaline, it is due to a reaction of the silica with the alkali. The silica (SiO2) reacts with the alkali to form a silicate in the Alkali silica reaction (ASR), this causes localised swelling which causes cracking.See and [http://www.cementindustry.co.uk/main.asp?page=272 for details.
=High alumina cement
= Resistant to weak acids and especially sulphates, this cement cures quickly and reaches very high durability and strength. It was greatly used after World War Two for making precast concrete objects. However, it can lose strength with heat or time, especially when not properly cured. With the collapse of three roofs made of prestressed concrete beams using high alumina cement, this cement was banned in the UK in 1976. Subsequent inquiries into the matter showed that the beams were improperly manufactured, but the ban remained. *.=Sulphate attack
= Sulfates can attack cement which can lead to an early failure.*Fiber-reinforced concrete
Fiber-reinforcement is mainly used in shotcrete, but can also be used in normal concrete. Fiber-reinforced normal concrete are mostly used for on-ground floors and pavements, but can be considered for a wide range of construction parts (beams, pilars, foundations etc) either alone or with hand-tied rebars.
Fiber (steel or "plastic" fibers) reinforced concrete is less expensive than hand-tied rebar, while still increasing the tensile strength many times. Shape, dimension and length of fibre is important. A thin and short fiber, for example short hair-shaped glass fiber, will only be effective the first hours after pouring the concrete (reduces cracking while the concrete is stiffening) but will not increase the concrete tensile strength. A normal size fibre for European shotcrete (1 mm diameter, 45 mm length—steel or "plastic") will increase the concrete tensile strength.
Steel is the strongest commonly-available fiber, and come in different lengths (30 to 80 mm in Europe) and shapes (end-hooks). Steel fibres can only be used on surfaces that can tolerate or avoid corrosion and rust stains. In some cases, a steel-fiber surface is faced with other materials.
Glass fiber is inexpensive and corrosion-proof, but not as strong as steel. Recently, spun basalt fiber, long available in Eastern Europe, has become available in the U.S. and Western Europe. Basalt fibre is stronger and less expensive than glass, but historically, has not resisted the alkaline environment of portland cement well enough to be used as direct reinforcement. New materials use plastic binders to isolate the basalt fiber from the cement.
The premium fibers are graphite reinforced plastic fibers, which are nearly as strong as steel, lighter-weight and corrosion-proof. Some experimeters have had promising early results with carbon nanotubes, but the material is still far too expensive for any building.
Non-steel reinforcement
Some construction cannot tolerate the use of steel. For example, MRI machines have huge magnets, and require nonmetallic buildings. Another example are toll-booths that read radio tags, and need reinforced concrete that is transparent to radio.
In some instances, the lifetime of the concrete structure is more important than its strength. Since corrosion is the main cause of failure of reinforced concrete, a corrosion proof reinforcement can extend the life substantially.
For these purposes some structures have been constructed using fiber-reinforced plastic rebar, grids or fibers. The "plastic" reinforcement can be as strong as steel. Because it resists corrosion, it does not need a protective concrete cover of 30 to 50 mm or more as steel reinforcement does. This means that FRP-reinforced structures can be lighter, have longer lifetime and for some applications be price-competitive to steel-reinforced concrete.
The main barrier to use of FRP reinforcement is the fact that it is neither ductile nor fire resistant. Structures employing FRP rebars may therefore exhibit a less ductile structural response, and decreased fire resistance.
See also
Civil engineering | Composite materials | Concrete | Construction | Steel | Structural engineering | Building engineering
Stahlbeton | Hormigón armado | בטון מזוין | Cemento armato | 鉄筋コンクリート | Gewapend beton | Żelbet | Concreto armado | Железобетон | คอนกรีตเสริมแรง
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