Reinforced concrete is concrete in which reinforcement bars
("rebars"), reinforcement grids, plates or fibers have been
incorporated to strengthen the concrete in tension. The term Ferro Concrete
refers only to concrete that is reinforced with iron or steel. Other materials
to reinforce concrete can be organic and inorganic fibres as well as composites
in different forms. Concrete is strong in compression, but weak in tension,
thus adding reinforcement increases the strength in tension. In addition the
failure strain of concrete in tension is such low that the reinforcement has to
hold the cracked sections together. For a strong, ductile and durable
construction the reinforcement shall have the following properties:
* High strength
* High tensile strain
* Good bond to the concrete
* Thermal compatibility
* Durability in the concrete environment
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.
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
In wet and cold climates, reinforced concrete for roads,
bridges, parking structures and other structures that may be exposed to dicing
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 Reinforcement
Another, cheaper way or protecting rebars is coating them
with zinc phosphate.[1] Zinc phosphate slowly reacts with the corroding agent
(e.g., alkali) forming a stable hydroxyapatite layer.
Penetrating sealants typically must be applied sometime
after curing. Sealants include paint, plastic foams, films and aluminium 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.
Steel reinforced concrete elements should normally be
designed to be under-reinforced so users of the structure will receive warning
of impending collapse.
Characteristic strength of a material where less than 5% of
the specimen show lower strength.
Design strength of a material including a material safety
factor.
Ultimate limit state theoretical failure point with a certain
probability. State under factored loads and factored resistances.
Nominal strength actual strength from the material
properties is called the nominal strength . Its relation with design strength
can be represented as:
Nominal x Ø = Design strength
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 and location of the cracks can be limited and
controlled by reinforcement, placement of control joints, the curing
methodology and the mix design of the concrete. 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.
Carbonation
Rebar for foundations and walls of sewage pump station.
Carbonation, or neutralisation, is a chemical reaction between
carbon dioxide in the air with calcium hydroxide and hydrated calcium silicate
in the concrete. The water in the pores of Portland cement concrete is normally
alkaline with a pH in the range of 12.5 to 13.5. This highly alkaline
environment is one in which the embedded steel is passivated and is protected
from corrosion. According to the Pourbaix diagram for iron, the metal is
passive when the pH is above 9.5.[3] The carbon dioxide in 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 carbonation process will start at the
surface, then slowly move deeper and deeper into the concrete. The rate of
carbonation is dependent on the relative humidity of the concrete - a 50%
relative humidity being optimal. If the object is cracked, the carbon dioxide
in the air will be better able to penetrate 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, early failure due to corrosion may occur. The concrete
cover depth can be measured with a cover meter. However, carbonated concrete
only becomes a durability problem when there is also sufficient moisture and
oxygen to cause electro-potential corrosion of the reinforcing steel.
One method of testing a structure for carbonation is to
drill a fresh hole in the surface and then treat the cut surface with
phenolphthalein indicator solution. This solution will turn [pink] when in
contact with alkaline concrete, making it possible to see the depth of
carbonation. An existing hole is no good because the exposed surface will
already be carbonated.
Chlorides
The Paulins Kill Viaduct, Hainesburg, New Jersey, is 115
feet (35 m) tall and 1,100 feet (335 m) long, and was heralded as the largest
reinforced concrete structure in the world when it was completed in 1910 as
part of the Lackawanna Cut-Off rail line project. The Lackawanna Railroad was a
pioneer in the use of reinforced concrete.
Chlorides, including sodium chloride, can promote the
corrosion of embedded steel rebar if present in sufficient concentration. For
this reason, only use fresh raw water or potable water for mixing concrete.
Insure that the coarse and fine aggregates do not contain chlorides. Do not use
admixtures that contain chlorides.
It was once common for calcium chloride to be used as an
admixture to promote rapid set-up of the concrete. It was also mistakenly
believed that it would prevent freezing. However, this practice has fallen into
disfavor once the deleterious effects of chlorides became known. It should be
avoided when ever possible.
The use of de-icing salts on roadways, used to reduce the
freezing point of water, is probably one of the primary causes of premature
failure of reinforced or prestressed concrete bridge decks, roadways, and
parking garages. The use of epoxy-coated reinforcing bars and the application
of cathodic protecton has mitigated this problem to some extent. Properly
designed concrete mixtures that have been allowed to cure properly are
effectively impervious to the effects of de-icers. (One common problem today is
that concrete is allowed to "dry" (dries out) in two to three days by
contractors before it cures and thus ultimately develops less than 10% of its
design strength).
Another important source of chloride ions is from sea water.
Sea water contains by weight approximately 3.5% salts. These salts include sodium
chloride, magnesium sulphate, calcium sulphate and bicarbonates. In water these
salts dissociate and migrate with the water into the capillaries of the
concrete. Chloride ions are particularly aggressive and make up about 50% of
these ions.
[edit] Alkali silica reaction
Main article: Alkali Silica Reaction
This a reaction of the amorphous silica sometimes present in
the aggregates with alkalies, for example from the cement pore solution. 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 availability of alkali ions, and (3)
sufficient moisture, above 75%RH within the concrete.[4][5] This phenomenon has
been popularly referred to as "concrete cancer". This reaction occurs
independently of the presence of rebars: massive concrete structures such as
dams can be affected.
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.
Sulfates
Sulfates (SO4) in the soil or in groundwater, in sufficient
concentration, can react with the Portland cement in concrete causing the
formation of expansive products, e.g. ettringite or thaumasite, which can lead
to early failure of the structure. The most typical attack of this type is on
concrete slabs and foundation walls at grade where the sulfate ion, via
alternate wetting and drying, can increase in concentration. As the concentration
increases, the attack on the Portland cement can begin. For buried structures
such as pipe, this type of attack is much rarer especially in the Eastern half
of the United States. The sulfate ion concentration increases much slower in
the soil mass and is especially dependent upon the initial amount of sulfates
in the native soil. The chemical analysis of soil borings should be done during
the design phase of any project involving concrete in contact with the native
soil to check for the presence of sulfates. If the concentrations are found to
be aggressive, various protective coatings can be used. Also, in the US ASTM
C150 Type 5 Portland cement can be used in the mix. This type of cement is
designed to be particularly resistant to a sulfate attack.
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