Reinforced concrete
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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.
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[edit] History
The use of reinforced concrete is a relatively recent invention, usually attributed to Joseph-Louis Lambot in 1848. 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.
The first reinforced concrete building constructed in the United States was the Pacific Coast Borax Company's refinery in Alameda, California, built in 1893.
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 prestressed concrete, to make routine use of it.
[edit] Physics and statics
Concrete is a mixture of cement (usually Portland cement) and stone aggregate. 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 have extremely high resistance to downward compressive stresses (about 3,000 psi, 35 N/mm²); 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.
If a non-elastic material, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists compression, stretching, bending, and other direct tensile actions. A reinforced concrete section (illustration, left) where the concrete takes the compression and steel takes the tension is so efficient in carrying forces that it can be made into almost any shape and size for the construction industry.
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.)
Three physical characteristics give reinforced concrete its special properties. First, the coefficient of thermal expansion of concrete is similar to that of steel, eliminating internal stresses due to differences in thermal expansion or contraction. Second, when the cement paste within the concrete hardens this 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. In these cases 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 a small 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 (usually the 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). In conservative construction projects like roadways and bridges, a series of rebar curtains or matrices are embedded in the concrete. In the U.S. rebar comes in two grades of carbon content, Grade 60 and Grade 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. Galvanized, epoxy-coated, and stainless steel rebar are also available for use in corrosive environments.
Typically, concrete will reach its nominal design strength after 28 days.
Reinforced concrete structures sometimes have provisions such as ventilated hollow cores to control their moisture.
Corrosion and freezing/thawing may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the rust expands, cracking the concrete and unbonding the rebar from the concrete. Freeze/thaw damage occurs when water penetrates the surface and freezes. The expansion of freezing water, and subsequent thawing, in microscopic cracks widens the cracks, causing flaking, and eventual structural failure.
In wet and cold 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.
[edit] Common failure modes of steel reinforced concrete
Corrosion and freeze/thaw cycles may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the oxidation products (rust) takes more space than the original steel and tends to flake, cracking the concrete and unbonding the rebar from the concrete.
[edit] 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.<ref>http://www.corrosion-doctors.org/Thermo/ironE-pH.htm</ref> 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 concrete. If the object is cracked the carbon dioxide of the air will be more able to penetrate deep into the concrete. 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 design or building codes. If the reinforcement 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.
[edit] Chlorides
Chlorides, including sodium chloride, promote the corrosion of steel rebar. For this reason, in mixing concrete only fresh water may be used, and the use of salt for deicing concrete pavements is strongly discouraged.
[edit] 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.The Conditions for alkali silica reaction are: (1)aggregate containing an alkali reactive constituent, (2)sufficiently high alkalinity,(3)sufficient moisture, above 75%RH within the concrete. <ref>http://www.bbc.co.uk/dna/h2g2/A4014172</ref><ref>http://www.cementindustry.co.uk/main.asp?page=272</ref>
[edit] Conversion of high alumina cement
Resistant to weak acids and especially sulfates, this cement cures quickly and reaches very high durability and strength. It was greatly used after World War II for making precast concrete objects. However, it can lose strength with heat or time (conversion), 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.<ref>http://www.quest-tech.co.uk/hac.htm</ref>
[edit] Sulfate attack
Sulfates in soil or groundwater can react with Portland cement causing expansive products, e.g ettringite or thaumasite, which can lead to early failure.<ref>http://www.ach.com.au/qcl/pdf_files/Conc_sc.pdf</ref>
[edit] 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.
Concrete reinforced with fibers (which are usually steel or "plastic" fibers) is less expensive than hand-tied rebar, while still increasing the tensile strength many times. Shape, dimension and length of fiber 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 ductile 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.
[edit] Non-steel reinforcement
Some construction cannot tolerate the use of steel. For example, MRI machines have huge magnets, and require nonmagnetic 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 a structure's 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.
[edit] References
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[edit] See also
- Building construction
- Construction engineering
- Fibre reinforced concrete
- Structural engineering
- Tie rod
- Rebar
- Concrete
- Steel fixer
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