Ferroconcrete

Reinforced concrete is concrete in which steel reinforcement bars ("rebars"), plates or fibers have been incorporated to strengthen a material that would otherwise be brittle.
History

Building contractor François Coignet was the first to use iron-reinforced concrete as a technique for constructing building structures when he built Parisian houses in the 1850s. His own house at 72 rue Charles-Michels in Paris, built in 1862 still stands. The German company Wayss & Freitag was formed in 1875, with A.G. Wayss publishing a book on reinforced concrete in 1887. Their major competitor in Europe was the firm of François Hennebique, set up in 1892.

The first recorded residence built of reinforced concrete in Scotland was Glencripesdale House, built in 1874, which was the 28 bedroom castle which was the main feature of the 35,000 acre Glencripesdale Estate.

The first entirely reinforced concrete fully-framed building in Britain was the flour mill built for Weaver and Co in Swansea in 1897. Designed by Hennebique, the local agent was Louis Gustave Mouchel, the French Consul in Briton Ferry.

A reinforced concrete system was patented in the United States by Thaddeus Hyatt in 1878. The first reinforced concrete building constructed in the United States was the Pacific Coast Borax Company's refinery in Alameda, California, built in 1893.
Use in construction

Concrete is reinforced to give it extra tensile strength; without reinforcement, many concrete buildings would not have been possible.

Reinforced concrete can encompass many types of structures and components, including slabs, walls, beams, columns, foundations, frames and more.

Reinforced concrete can be classified as precast or cast in-situ concrete.

Much of the focus on reinforcing concrete is placed on floor systems. Designing and implementing the most efficient floor system is key to creating optimal building structures. Small changes in the design of a floor system can have significant impact on material costs, construction schedule, ultimate strength, operating costs, occupancy levels and end use of a building.



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Behavior of reinforced concrete
Materials

Concrete is a mixture of cement (usually Portland cement) and stone aggregate. When mixed with a small amount of water, the cement hydrates form microscopic opaque crystal lattices encapsulating and locking the aggregate into a rigid structure. Typical concrete mixes have high resistance to compressive stresses (about 4,000 psi (28 MPa)); however, any appreciable tension (e.g. due to bending) will break the microscopic rigid lattice resulting in cracking and separation of the concrete. For this reason, typical non-reinforced concrete must be well supported to prevent the development of tension.

If a material with high strength in tension, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists compression but also bending, and other direct tensile actions. A reinforced concrete section where the concrete resists the compression and steel resists the tension can be made into almost any shape and size for the construction industry.
Key characteristics

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 between 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 the steel, making it much more resistant to corrosion than it would be in neutral or acidic conditions.

The relative cross-sectional area of steel required for typical reinforced concrete is usually quite small and varies from 1% for most beams and slabs to 6% for some columns. Reinforcing bars are normally round in cross-section and vary in diameter. Reinforced concrete structures sometimes have provisions such as ventilated hollow cores to control their moisture & humidity.
Anti-corrosion measures

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In wet and cold climates, reinforced concrete for roads, bridges, parking structures and other structures that may be exposed to deicing salt may benefit from use of epoxy-coated, hot dip galvanised or stainless steel rebar, although good design and a well-chosen cement mix may provide sufficient protection for many applications. Epoxy coated rebar can easily be identified by the light green colour of its epoxy coating. Hot dip galvanized rebar may be bright or dull grey depending on length of exposure, and stainless rebar exhibits a typical white metallic sheen that is readily distinguishable from carbon steel reinforcing bar. Reference ASTM standard specifications A767 Standard Specification for Hot Dip Galvanised Reinforcing Bars, A775 Standard Specification for Epoxy Coated Steel Reinforcing Bars and A955 Standard Specification for Deformed and Plain Stainless Bars for Concrete Reinforcment

Another, cheaper way or protecting rebars is coating them with zinc phosphate. Zinc phosphate slowly reacts with the corroding agent (e.g., alkali) forming a stable hydroxyapatite layer.

Penetrating sealants typically must be applied some time after curing. 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.

Reinforcement and terminology

A beam bends under bending moment resulting in a small curvature. At the outer face (tensile face) of the curvature the concrete experiences tensile stress while at the inner face (compressive face) it experiences compressive stress.

A "singly-reinforced" concrete means that the concrete element is only reinforced near the tensile face and the reinforcement, called tension steel, is designed to resist the tension.

A "doubly-reinforced" concrete means that beside the tensile reinforcement the concrete element is also reinforced near the compressive face for assisting the concrete to resist compression. The latter reinforcement is called compression steel.

An "under-reinforced" concrete means that the tension capacity of the tensile reinforcement is smaller than the combined compression capacity of the concrete and the compression steel (under-reinforced at tensile face). When the reinforced concrete element is subject to increasing bending moment, the tension steel yields while the concrete does not reach its ultimate failure condition. As the tension steel yields and stretches, an "under-reinforced" concrete also yields in a ductile manner, exhibiting a large deformation and warning before its ultimate failure.

An "over-reinforced" concrete means that the tension capacity of the tension steel is greater than the combined compression capacity of the concrete and the compression steel (over-reinforced at tensile face). An "over-reinforced" element will fail suddenly, when the concrete fails brittle and crashes before yielding of the tension steel. It is however possible to push the design of an "over-reinforced" concrete element to "under-reinforced" concrete element by adding sufficient compression steel. There is however a limit in the quantity of both tension and compression steel for practicality of placement of reinforcement.

Reinforced concrete elements should be designed to be under-reinforced so users of the structure will receive warning of impending collapse.
[edit] Common failure modes of steel reinforced concrete

Reinforced concrete can fail due to inadequate strength, leading to mechanical failure, or due to a reduction in its durability. Corrosion and freeze/thaw cycles may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the oxidation products (rust) expand and tends to flake, cracking the concrete and unbonding the rebar from the concrete. Typical mechanisms leading to durability problems are discussed below.
Mechanical failure

Reinforced concrete can be considered to have failed when significant cracks occur. Cracking of the concrete section can not be prevented however the size of the cracks can be limited and controlled by reinforcement. Cracking defects can allow moisture to penetrate and corrode the reinforcement. This is a serviceability failure in limit state design. Cracking is normally the result of an inadequate quantity of rebar, or rebar spaced at too great a distance. The concrete then cracks either under excess loading, or due to internal effects such as early thermal shrinkage when it cures.

Ultimate failure leading to collapse can be caused by crushing of the concrete matrix, when stresses exceed its strength; by yielding of the rebar; or by bond failure between the concrete and the rebar.

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