(RC) is a composite material in which concrete's
relatively low tensile strength and ductility are counteracted by the
inclusion of reinforcement having higher tensile strength or
ductility. The reinforcement is usually, though not necessarily, steel
reinforcing bars (rebar) and is usually embedded passively in the
concrete before the concrete sets. Reinforcing schemes are generally
designed to resist tensile stresses in particular regions of the
concrete that might cause unacceptable cracking and/or structural
failure. Modern reinforced concrete can contain varied reinforcing
materials made of steel, polymers or alternate composite material in
conjunction with rebar or not.
may also be
permanently stressed (in tension), so as to improve the behaviour of
the final structure under working loads. In the United States, the
most common methods of doing this are known as pre-tensioning and
For a strong, ductile and durable construction the reinforcement needs
to have the following properties at least:
High relative strength
High toleration of tensile strain
Good bond to the concrete, irrespective of pH, moisture, and similar
Thermal compatibility, not causing unacceptable stresses in response
to changing temperatures.
Durability in the concrete environment, irrespective of corrosion or
sustained stress for example.
2 Use in construction
3 Behavior of reinforced concrete
3.2 Key characteristics
3.3 Mechanism of composite action of reinforcement and concrete
3.4 Anchorage (bond) in concrete: Codes of specifications
3.5 Anti-corrosion measures
4 Reinforcement and terminology of beams
5 Prestressed concrete
6 Common failure modes of steel reinforced concrete
6.1 Mechanical failure
Alkali silica reaction
6.5 Conversion of high alumina cement
Steel plate construction
8 Fiber-reinforced concrete
9 Non-steel reinforcement
10 See also
11.1 Further reading
12 External links
François Coignet was a French industrialist of the nineteenth
century, a pioneer in the development of structural, prefabricated and
reinforced concrete. Coignet was the first to use iron-reinforced
concrete as a technique for constructing building structures. In
1853, Coignet built the first iron reinforced concrete structure, a
four-story house at 72 rue Charles Michels in the suburbs of Paris.
Coignet's descriptions of reinforcing concrete suggests that he did
not do it for means of adding strength to the concrete but for keeping
walls in monolithic construction from overturning. In 1854, English
builder William B. Wilkinson reinforced the concrete roof and floors
in the two-storey house he was constructing. His positioning of the
reinforcement demonstrated that, unlike his predecessors, he had
knowledge of tensile stresses.
Joseph Monier, a French gardener and known to be one of the principal
inventors of reinforced concrete, was granted a patent for reinforced
flowerpots by means of mixing a wire mesh to a mortar shell. In 1877,
Monier was granted another patent for a more advanced technique of
reinforcing concrete columns and girders with iron rods placed in a
grid pattern. Though Monier undoubtedly knew reinforcing concrete
would improve its inner cohesion, it is less known if he even knew how
much reinforcing actually improved concrete's tensile strength.
Before 1877 the use of concrete construction, though dating back to
the Roman Empire and reintroduced in the mid to late 1800s, was not
yet a proven scientific technology. American New Yorker Thaddeus Hyatt
published a report titled An Account of Some Experiments with
Concrete Combined with Iron as a Building Material,
with Reference to Economy of Metal in Construction and for Security
against Fire in the Making of Roofs, Floors, and Walking Surfaces
where he stated his experiments on the behavior of reinforced
concrete. His work played a major role in the evolution of concrete
construction as a proven and studied science. Without Hyatt's work,
more dangerous trial and error methods would have largely been
depended on for the advancement in the technology.
G. A. Wayss was a German civil engineer and a pioneer of the iron and
steel concrete construction. In 1879, Wayss bought the German rights
to Monier's patents and in 1884, he started the first commercial use
for reinforced concrete in his firm Wayss & Freytag. Up until the
1890s, Wayss and his firm greatly contributed to the advancement of
Monier's system of reinforcing and established it as a well-developed
Ernest L. Ransome
Ernest L. Ransome was an English-born engineer and early innovator of
the reinforced concrete techniques in the end of the 19th century.
With the knowledge of reinforced concrete developed during the
previous 50 years, Ransome innovated nearly all styles and techniques
of the previous known inventors of reinforced concrete. Ransome's key
innovation was to twist the reinforcing steel bar improving bonding
with the concrete. Gaining increasing fame from his concrete
constructed buildings, Ransome was able to build two of the first
reinforced concrete bridges in North America. One of the first
concrete buildings constructed in the United States, was a private
home, designed by William Ward in 1871. The home was designed to be
fireproof for his wife.
One of the first skyscrapers made with reinforced concrete was the
Ingalls Building in Cincinnati, constructed in 1904.
The first reinforced concrete building in Southern California was the
Laughlin Annex in Downtown Los Angeles, constructed in 1905.
In 1906, 16 building permits were reportedly issued for reinforced
concrete buildings in the City of Los Angeles, including the Temple
Auditorium and 8-story Hayward Hotel.
On April 18, 1906 a magnitude 7.8 earthquake struck San Francisco. The
strong ground shaking and subsequent fire destroyed much of the city
and killed thousands. The use of reinforced concrete after the
earthquake was highly promoted within the U.S. construction industry
due to its non-combustibility and perceived superior seismic
performance relative to masonry.
In 1906, a partial collapse of the Bixby Hotel in Long Beach killed 10
workers during construction when shoring was removed prematurely. This
event spurred a scrutiny of concrete erection practices and building
inspections. The structure was constructed of reinforced concrete
frames with hollow clay tile ribbed flooring and hollow clay tile
infill walls. This practice was strongly questioned by experts and
recommendations for “pure” concrete construction using reinforced
concrete for the floors and walls as well as the frames were made.
The National Association of
Cement Users (NACU) published in 1906
“Standard No. 1”, and in 1910 the “Standard Building
Regulations for the Use of Reinforced Concrete”.
Use in construction
Rebars of Sagrada Família's roof in construction (2009)
Many different types of structures and components of structures can be
built using reinforced concrete including slabs, walls, beams,
columns, foundations, frames and more.
Reinforced concrete can be classified as precast or cast-in-place
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.
Without reinforcement, constructing modern structures with concrete
material would not be possible.
Behavior of reinforced concrete
See also: Concrete, Cement, Construction aggregate, and Rebar
Concrete is a mixture of coarse (stone or brick chips) and fine
(generally sand or crushed stone) aggregates with a paste of binder
material (usually Portland cement) and water. When cement is mixed
with a small amount of water, it hydrates to form microscopic opaque
crystal lattices encapsulating and locking the aggregate into a rigid
structure. The aggregates used for making concrete should be free from
harmful substances like organic impurities, silt, clay, lignite etc.
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
not only compression but also bending and other direct tensile
actions. A composite section where the concrete resists compression
and reinforcement "rebar" resists tension can be made into almost any
shape and size for the construction industry.
Three physical characteristics give reinforced concrete its special
The coefficient of thermal expansion of concrete is similar to that of
steel, eliminating large internal stresses due to differences in
thermal expansion or contraction.
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.
The alkaline chemical environment provided by the alkali reserve (KOH,
NaOH) and the portlandite (calcium hydroxide) contained in the
hardened cement paste 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. When the cement paste is exposed
to the air and meteoric water reacts with the atmospheric CO2,
portlandite and the calcium silicate hydrate (CSH) of the hardened
cement paste become progressively carbonated and the high pH gradually
decreases from 13.5 – 12.5 to 8.5, the pH of water in equilibrium
with calcite (calcium carbonate) and the steel is no longer
As a rule of thumb, only to give an idea on orders of magnitude, steel
is protected at pH above ~11 but starts to corrode below ~10 depending
on steel characteristics and local physico-chemical conditions when
concrete becomes carbonated. carbonatation of concrete along with
chloride ingress are amongst the chief reasons for the failure of
reinforcement bars in concrete.
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.
Distribution of concrete (in spite of reinforcement) strength
characteristics along the cross-section of vertical reinforced
concrete elements is inhomogeneous.
Mechanism of composite action of reinforcement and concrete
A heavy reinforced concrete column, seen before and after the concrete
has been cast in place around the rebar cage.
The reinforcement in a RC structure, such as a steel bar, has to
undergo the same strain or deformation as the surrounding concrete in
order to prevent discontinuity, slip or separation of the two
materials under load. Maintaining composite action requires transfer
of load between the concrete and steel. The direct stress is
transferred from the concrete to the bar interface so as to change the
tensile stress in the reinforcing bar along its length, this load
transfer is achieved by means of bond (anchorage) and is idealized as
a continuous stress field that develops in the vicinity of the
Anchorage (bond) in concrete: Codes of specifications
Because the actual bond stress varies along the length of a bar
anchored in a zone of tension, current international codes of
specifications use the concept of development length rather than bond
stress. The main requirement for safety against bond failure is to
provide a sufficient extension of the length of the bar beyond the
point where the steel is required to develop its yield stress and this
length must be at least equal to its development length. However, if
the actual available length is inadequate for full development,
special anchorages must be provided, such as cogs or hooks or
mechanical end plates. The same concept applies to lap splice length
mentioned in the codes where splices (overlapping) provided between
two adjacent bars in order to maintain the required continuity of
stress in the splice zone.
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 corrosion-resistant reinforcement such as
uncoated, low carbon/chromium (micro composite), epoxy-coated, hot dip
galvanised or stainless steel rebar. Good design and a well-chosen
concrete mix will provide additional protection for many applications.
Uncoated, low carbon/chromium rebar looks similar to standard carbon
steel rebar due to its lack of a coating; its highly
corrosion-resistant features are inherent in the steel microstructure.
It can be identified by the unique ASTM specified mill marking on its
smooth, dark charcoal finish.
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 A1035/A1035M Standard
Specification for Deformed and Plain Low-carbon, Chromium,
Concrete Reinforcement, A767 Standard Specification for Hot Dip
Galvanised Reinforcing Bars, A775 Standard Specification for Epoxy
Steel Reinforcing Bars and A955 Standard Specification for
Deformed and Plain Stainless Bars for
Another, cheaper way of protecting rebars is coating them with zinc
Zinc phosphate slowly reacts with calcium cations and
the hydroxyl anions present in the cement pore water and forms 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.
Corrosion inhibitors, such as calcium nitrite [Ca(NO2)2], can also be
added to the water mix before pouring concrete. Generally,
1–2 wt. % of [Ca(NO2)2] with respect to cement weight is
needed to prevent corrosion of the rebars. The nitrite anion is a mild
oxidizer that oxidizes the soluble and mobile ferrous ions (Fe2+)
present at the surface of the corroding steel and causes them to
precipitate as an insoluble ferric hydroxide (Fe(OH)3). This causes
the passivation of steel at the anodic oxidation sites.
Nitrite is a
much more active corrosion inhibitor than nitrate, which is a less
powerful oxidizer of the divalent iron.
Reinforcement and terminology of beams
Two intersecting beams integral to parking garage slab that will
contain both reinforcing steel and the wiring, junction boxes and
other electrical components necessary to install the overhead lighting
for the garage level beneath it.
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 beam is one in which 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 beam is one in which besides the tensile
reinforcement the concrete element is also reinforced near the
compressive face to help the concrete resist compression. The latter
reinforcement is called compression steel. When the compression zone
of a concrete is inadequate to resist the compressive moment (positive
moment), extra reinforcement has to be provided if the architect
limits the dimensions of the section.
An under-reinforced beam is one in which 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. In this case the yield stress of the steel governs
An over-reinforced beam is one in which 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).
So the "over-reinforced concrete" beam fails by crushing of the
compressive-zone concrete and before the tension zone steel yields,
which does not provide any warning before failure as the failure is
A balanced-reinforced beam is one in which both the compressive and
tensile zones reach yielding at the same imposed load on the beam, and
the concrete will crush and the tensile steel will yield at the same
time. This design criterion is however as risky as over-reinforced
concrete, because failure is sudden as the concrete crushes at the
same time of the tensile steel yields, which gives a very little
warning of distress in tension failure.
Steel-reinforced concrete moment-carrying elements should normally be
designed to be under-reinforced so that users of the structure will
receive warning of impending collapse.
The characteristic strength is the strength of a material where less
than 5% of the specimen shows lower strength.
The design strength or nominal strength is the strength of a material,
including a material-safety factor. The value of the safety factor
generally ranges from 0.75 to 0.85 in Permissible stress design.
The ultimate limit state is the theoretical failure point with a
certain probability. It is stated under factored loads and factored
Reinforced concrete structures are normally designed according to
rules and regulations or recommendation of a code such as ACI-318,
Eurocode 2 or the like. WSD, USD or LRFD methods are used in
design of RC structural members. Analysis and design of RC members can
be carried out by using linear or non-linear approaches. When applying
safety factors, building codes normally propose linear approaches, but
for some cases non-linear approaches. To see the examples of a
non-linear numerical simulation and calculation visit the
Main article: Prestressed concrete
Prestressing concrete is a technique that greatly increases the
load-bearing strength of concrete beams. The reinforcing steel in the
bottom part of the beam, which will be subjected to tensile forces
when in service, is placed in tension before the concrete is poured
around it. Once the concrete has hardened, the tension on the
reinforcing steel is released, placing a built-in compressive force on
the concrete. When loads are applied, the reinforcing steel takes on
more stress and the compressive force in the concrete is reduced, but
does not become a tensile force. Since the concrete is always under
compression, it is less subject to cracking and failure.
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.
Cracking of the concrete section is nearly impossible to prevent;
however, the size and location of cracks can be limited and controlled
by appropriate reinforcement, control joints, curing methodology and
concrete mix design. Cracking 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 while it cures.
Ultimate failure leading to collapse can be caused by crushing the
concrete, which occurs when compressive stresses exceed its strength,
by yielding or failure of the rebar when bending or shear stresses
exceed the strength of the reinforcement, or by bond failure between
the concrete and the rebar.
Concrete wall cracking as steel reinforcing corrodes and swells. Rust
has a lower density than metal, so it expands as it forms, cracking
the decorative cladding off the wall as well as damaging the
structural concrete. The breakage of material from a surface is called
Detailed view of spalling probably caused by a too thin layer of
concrete between the steel and the surface, accompanied by corrosion
from external exposure
Main article: carbonation
Carbonation, or neutralisation, is a chemical reaction between carbon
dioxide in the air and calcium hydroxide and hydrated calcium silicate
in the concrete.
When a concrete structure is designed, it is usual to specify the
concrete cover for the rebar (the depth of the rebar within the
object). 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 incurs a
durability problem only when there is also sufficient moisture and
oxygen to cause electropotential corrosion of the reinforcing steel.
One method of testing a structure for carbonatation is to drill a
fresh hole in the surface and then treat the cut surface with
phenolphthalein indicator solution. This solution turns pink when in
contact with alkaline concrete, making it possible to see the depth of
carbonation. Using an existing hole does not suffice because the
exposed surface will already be carbonated.
Chlorides, including sodium chloride, can promote the corrosion of
embedded steel rebar if present in sufficiently high concentration.
Chloride anions induce both localized corrosion (pitting corrosion)
and generalized corrosion of steel reinforcements. For this reason,
one should only use fresh raw water or potable water for mixing
concrete, ensure that the coarse and fine aggregates do not contain
chlorides, rather than admixtures which might contain chlorides.
Rebar for foundations and walls of a sewage pump station.
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.
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 fell into
disfavor once the deleterious effects of chlorides became known. It
should be avoided whenever possible.
The use of de-icing salts on roadways, used to lower 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 protection has mitigated this problem to some
extent. Also FRP (fiber-reinforced polymer) rebars are known to be
less susceptible to chlorides. Properly designed concrete mixtures
that have been allowed to cure properly are effectively impervious to
the effects of de-icers.
Another important source of chloride ions is sea water. Sea water
contains by weight approximately 3.5 wt.% salts. These salts
include sodium chloride, magnesium sulfate, calcium sulfate, and
bicarbonates. In water these salts dissociate in free ions (Na+, Mg2+,
Cl−, SO42−, HCO3−) and migrate with the water into the
capillaries of the concrete.
Chloride ions, which make up about 50% of
these ions, are particularly aggressive as a cause of corrosion of
carbon steel reinforcement bars.
In the 1960s and 1970s it was also relatively common for magnesite, a
chloride rich carbonate mineral, to be used as a floor-topping
material. This was done principally as a levelling and sound
attenuating layer. However it is now known that when these materials
come into contact with moisture they produce a weak solution of
hydrochloric acid due to the presence of chlorides in the magnesite.
Over a period of time (typically decades), the solution causes
corrosion of the embedded steel rebars. This was most commonly found
in wet areas or areas repeatedly exposed to moisture.
Alkali silica reaction
Main article: Alkali–silica reaction
This a reaction of amorphous silica (chalcedony, chert, siliceous
limestone) sometimes present in the aggregates with the hydroxyl ions
(OH−) from the cement pore solution. Poorly crystallized silica
(SiO2) dissolves and dissociates at high pH (12.5 - 13.5) in alkaline
water. The soluble dissociated silicic acid reacts in the porewater
with the calcium hydroxide (portlandite) present in the cement paste
to form an expansive calcium silicate hydrate (CSH). The
alkali–silica reaction (ASR) causes localised swelling responsible
for tensile stress and cracking. The conditions required for alkali
silica reaction are threefold: (1) aggregate containing an
alkali-reactive constituent (amorphous silica), (2) sufficient
availability of hydroxyl ions (OH−), and (3) sufficient moisture,
above 75% relative humidity (RH) within the concrete. This
phenomenon is sometimes 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 has very high durability and strength. It was frequently
World War II
World War II to make precast concrete objects. However, it
can lose strength with heat or time (conversion), especially when not
properly cured. After 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 (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
grades where the sulfate ion, via alternate wetting and drying, can
increase in concentration. As the concentration increases, the attack
Portland cement can begin. For buried structures such as pipe,
this type of attack is much rarer, especially in the eastern 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. A chemical analysis of soil borings to
check for the presence of sulfates should be undertaken during the
design phase of any project involving concrete in contact with the
native soil. If the concentrations are found to be aggressive, various
protective coatings can be applied. 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.
Steel plate construction
Steel plate construction
In steel plate construction, stringers join parallel steel plates. The
plate assemblies are fabricated off site, and welded together on-site
to form steel walls connected by stringers. The walls become the form
into which concrete is poured.
Steel plate construction speeds
reinforced concrete construction by cutting out the time-consuming
on-site manual steps of tying rebar and building forms. The method
results in excellent strength because the steel is on the outside,
where tensile forces are often greatest.
Main article: Fiber reinforced concrete
Fiber reinforcement is mainly used in shotcrete, but can also be used
in normal concrete. Fiber-reinforced normal concrete is mostly used
for on-ground floors and pavements, but can also be considered for a
wide range of construction parts (beams, pillars, foundations, etc.),
either alone or with hand-tied rebars.
Concrete reinforced with fibers (which are usually steel, glass, or
plastic fibers) is less expensive than hand-tied rebar[citation
needed]. The shape, dimension, and length of the fiber are important.
A thin and short fiber, for example short, hair-shaped glass fiber, is
only effective during the first hours after pouring the concrete (its
function is to reduce cracking while the concrete is stiffening), but
it will not increase the concrete tensile strength. A normal-size
fiber for European shotcrete (1 mm diameter, 45 mm
length—steel or plastic) will increase the concrete's tensile
strength. Fiber reinforcement is most often used to supplement or
partially replace primary rebar, and in some cases it can be designed
to fully replace rebar.
Steel is the strongest commonly available fiber, and
comes in different lengths (30 to 80 mm in Europe) and shapes
Steel fibers 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 in weight, and
corrosion-proof. Some experiments have had promising
early results with carbon nanotubes, but the material is still far too
expensive for any building.
There is considerable overlap between the subjects of non-steel
reinforcement and fiber-reinforcement of concrete. The introduction of
non-steel reinforcement of concrete is relatively recent; it takes two
major forms: non-metallic rebar rods, and non-steel (usually also
non-metallic) fibres incorporated into the cement matrix. For example,
there is increasing interest in glass fiber reinforced concrete (GFRC)
and in various applications of polymer fibres incorporated into
concrete. Although currently there is not much suggestion that such
materials will replace metal rebar, some of them have major advantages
in specific applications, and there also are new applications in which
metal rebar simply is not an option. However, the design and
application of non-steel reinforcing is fraught with challenges. For
one thing, concrete is a highly alkaline environment, in which many
materials, including most kinds of glass, have a poor service life.
Also, the behaviour of such reinforcing materials differs from the
behaviour of metals, for instance in terms of shear strength, creep
Fibre-reinforced plastic/polymer (FRP) and glass-reinforced plastic
(GRP) consist of fibres of polymer, glass, carbon, aramid or other
polymers or high-strength fibres set in a resin matrix to form a rebar
rod, or grid, or fibres. These rebars are installed in much the same
manner as steel rebars. The cost is higher but, suitably applied, the
structures have advantages, in particular a dramatic reduction in
problems related to corrosion, either by intrinsic concrete alkalinity
or by external corrosive fluids that might penetrate the concrete.
These structures can be significantly lighter and usually have a
longer service life. The cost of these materials has dropped
dramatically since their widespread adoption in the aerospace industry
and by the military.
In particular, FRP rods are useful for structures where the presence
of steel would not be acceptable. For example,
MRI machines have huge
magnets, and accordingly require non-magnetic buildings. Again, toll
booths that read radio tags need reinforced concrete that is
transparent to radio waves. Also, where the design life of the
concrete structure is more important than its initial costs, non-steel
reinforcing often has its advantages where corrosion of reinforcing
steel is a major cause of failure. In such situations corrosion-proof
reinforcing can extend a structure's life substantially, for example
in the intertidal zone. FRP rods may also be useful in situations
where it is likely that the concrete structure may be compromised in
future years, for example the edges of balconies when balustrades are
replaced, and bathroom floors in multi-story construction where the
service life of the floor structure is likely to be many times the
service life of the waterproofing building membrane.
Plastic reinforcement often is stronger, or at least has a better
strength to weight ratio than reinforcing steels. Also, because it
resists corrosion, it does not need a protective concrete cover as
thick as steel reinforcement does (typically 30 to 50 mm or
more). FRP-reinforced structures therefore can be lighter and last
longer. Accordingly, for some applications the whole-life cost will be
price-competitive with steel-reinforced concrete.
The material properties of FRP or GRP bars differ markedly from steel,
so there are differences in the design considerations. FRP or GRP bars
have relatively higher tensile strength but lower stiffness, so that
deflections are likely to be higher than for equivalent
steel-reinforced units. Structures with internal FRP reinforcement
typically have an elastic deformability comparable to the plastic
deformability (ductility) of steel reinforced structures. Failure in
either case is more likely to occur by compression of the concrete
than by rupture of the reinforcement. Deflection is always a major
design consideration for reinforced concrete. Deflection limits are
set to ensure that crack widths in steel-reinforced concrete are
controlled to prevent water, air or other aggressive substances
reaching the steel and causing corrosion. For FRP-reinforced concrete,
aesthetics and possibly water-tightness will be the limiting criteria
for crack width control. FRP rods also have relatively lower
compressive strengths than steel rebar, and accordingly require
different design approaches for reinforced concrete columns.
One drawback to the use of FRP reinforcement is their limited fire
resistance. Where fire safety is a consideration, structures employing
FRP have to maintain their strength and the anchoring of the forces at
temperatures to be expected in the event of fire. For purposes of
fireproofing, an adequate thickness of cement concrete cover or
protective cladding is necessary. The addition of 1 kg/m3 of
polypropylene fibers to concrete has been shown to reduce spalling
during a simulated fire. (The improvement is thought to be due to
the formation of pathways out of the bulk of the concrete, allowing
steam pressure to dissipate.)
Another problem is the effectiveness of shear reinforcement. FRP rebar
stirrups formed by bending before hardening generally perform
relatively poorly in comparison to steel stirrups or to structures
with straight fibres. When strained, the zone between the straight and
curved regions are subject to strong bending, shear, and longitudinal
Special design techniques are necessary to deal with such
There is growing interest in applying external reinforcement to
existing structures using advanced materials such as composite
(fiberglass, basalt, carbon) rebar, which can impart exceptional
strength. Worldwide, there are a number of brands of composite rebar
recognized by different countries, such as Aslan, DACOT, V-rod, and
ComBar. The number of projects using composite rebar increases day by
day around the world, in countries ranging from USA, Russia, and South
Korea to Germany.
Types of concrete
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Concrete Inhomogeneity of Vertical Cast-In-Situ Elements In
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reinforced concrete" Sci. Technol. Adv. Mater. 9 (2008) 045009 (free
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index based on nonlinear numerical simulation of structures subjected
to oriented lateral cyclic loading". 9 (3). Retrieved 23 December
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(2016). "Remote sensing and photogrammetry techniques in diagnostics
of concrete structures". Computers and Concrete. 18 (3): 405–420.
doi:10.12989/cac.2016.18.3.405. Retrieved 2016-12-14.
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Concrete frame structures.".
Steel Institute (CRSI) is a national trade
association that stands as the authoritative resource for information
related to steel reinforced concrete construction.
Concrete Research: http://www.concreteresearch.org
Timeline of concrete
Ancient Roman architecture
Ground granulated blast furnace slag
Reversing drum mixer
Flow table test
Segregation in concrete
Energetically modified cement
Rosendale cement (natural cement)
Voided biaxial slab
Institution of Structural Engineers
International Federation for Structural Concrete