Introduction
Reinforced
concrete (RC) (also called reinforced cement
concrete or RCC) is a composite
material in which concrete's
relatively low tensile strengthand 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. Reinforced concrete may also be permanently
stressed (concrete in compression, reinforcement 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 post-tensioning.
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 factors
·
Thermal compatibility, not causing unacceptable
stresses (such as expansion or contraction) in response to changing temperatures.
·
Durability in the concrete environment,
irrespective of corrosion or sustained stress for example.
·
François Coignet was
the first to use iron-reinforced concrete as a technique for constructing
building structures.[1] In
1853, Coignet built the first iron reinforced concrete structure, a four-story
house at 72 rue Charles Michels in
the suburbs of Paris.[1] 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.[2] 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.[3][4][5]
·
Joseph
Monier was a French gardener of the nineteenth century, a
pioneer in the development of structural, prefabricated and reinforced concrete
when dissatified with existing materials available for making durable
flowerpots.[6] He
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.[7]
·
Before 1877 the use of concrete construction,
though dating back to the Roman Empire, and having been reintroduced in the
early 1800s, was not yet a proven scientific technology. American New
Yorker Thaddeus
Hyatt published a report titled An Account of
Some Experiments with Portland-Cement-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 reported 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.[2][8]
·
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.[9] Gaining
increasing fame from his concrete constructed buildings, Ransome was able to
build two of the first reinforced concrete bridges in North America.[10] 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.
·
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 scientific technology.[7]
·
In April 1904, Julia
Morgan, an American architect and engineer who pioneered the
aesthetic use of reinforced concrete, completed her first reinforced concrete
structure, the 72-foot bell tower at Mills College, El Campanil,[11] which
is located across the bay from San Francisco. Two years later, El Campanil
survived the 1906 San Francisco earthquake without
any damage,[12] which helped build
her reputation and launch her prolific career.[13]
·
One of the first skyscrapers made with
reinforced concrete was the 16-story Ingalls
Building in Cincinnati, constructed in 1904.[5]
·
The first reinforced concrete building in
Southern California was the Laughlin Annex in
Downtown Los Angeles, constructed in 1905.[14][15] 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.[16][17]
·
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.[18]
·
The National Association of Cement Users
(NACU) published in 1906 “Standard No. 1”,[19] and in 1910 the
“Standard Building Regulations for the Use of Reinforced Concrete”.[20]
·
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
concrete.
·
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.
·
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
industr
Three
physical characteristics give reinforced concrete its special properties:
1.
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.
2.
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.
3.
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 passivated.
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. Carbonation
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 barsare
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 in -homogeneous.
Mechanism of composite action of reinforcement and concrete -
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 steel-concrete
interface.
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.
Anti-corrosion measures -
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, Steel Bars for
Concrete Reinforcement, 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 of protecting rebars is coating them with zinc phosphate.[22] 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 bentoniteclay, 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.
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 the design.
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 instantaneous.
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.[23]
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 resistances.
Reinforced concrete structures are normally
designed according to rules and regulations or recommendation of a code such as
ACI-318, CEB, 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 references:[24][25]
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.
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.[26]
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.
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.
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.[27][28] 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.
Resistant to weak acids and especially sulfates, this
cement cures quickly and has very high durability and strength. It was
frequently used after 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 on the 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.
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.
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, plastic fibers) or
Cellulose polymer fibre 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,[citation
needed] and comes in different lengths (30 to
80 mm in Europe) and shapes (end-hooks). 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.[citation needed] 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 and elasticity.[30][31]
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-magneticbuildings. 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.[32] (The
improvement is thought to be due to the formation of pathways out of the bulk
of the concrete, allowing steam pressure to dissipate.[32])
Darby, A., The Airside Road Tunnel, Heathrow
Airport, England,
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 stresses. Special design techniques are
necessary to deal with such problems.
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.
Reinforced Cement Concrete | Advantages, Uses, Types,
& Purpose.
Reinforced cement concrete (R.C.C) is the
combination of ordinary concrete with the reinforcement to increase its
compressive and tensile strength to a great extent.
Concrete
is a versatile material for modern construction which is prepared by mixing
well-proportioned quantities of cement (even
lime in some cases), sand,
crushed rock or gravel, and water.
It has been used from foundations to the
rooftops of buildings, in the construction of highways roads traffic, and
hydro-power tunnels, irrigation canals, drains, and all other conceivable
structures.
Purpose of Reinforcement in Concrete.
As you
know that, Concrete has
a very high compressive strength, but it is low in tensile strength.
Thus, when only the compressive loads are
acting on the concrete surface, then there is no need of using reinforcement in
it.
But where tensile forces are also involved,
as in, beams and slabs, there is a very high risk of its failure when plain
concrete is used.
Steel, however, as we know, has a very high
tensile strength (and also have good compressive strength).
Hence, when these two (concrete and steel)
are combined together,
a material of construction is obtained that
is capable of withstanding all the three types of forces likely to act upon a
structure, i.e., compressive loads, tensile stresses, and shear forces.
Such a
material is known as Reinforced
Cement Concrete.
It has proved extremely useful and reliable
in engineering construction.
Nature of Reinforced Cement Concrete:
The main principle in the preparation of the
reinforced cement concrete is to make a structural material in which
(i) Steel
serves the purpose of bearing the main tensile stresses;
(ii) concrete
bears the main compressive forces, both acting in complete unison;
Concrete
and steel are compatible in following aspects:
(i) Concrete is
basically alkaline in nature, (the principal component being Calcium hydroxide)
and this prevents rusting of the steel reinforcement used within it;
(ii) The
bond or ‘grip’ between the steel and concrete is established easily;
(iii) The
coefficient of thermal expansion of concrete is almost identical with that of
steel.
This prevents the risk of cracking due to
expansion at different rates.
Types of Reinforcement used in R.C.C:
Reinforcement
used in concrete is principally made of steel of different types.
Further, it may be made in required shape and
volume.
Some common types of reinforcement are:
(i) Mild Steel Bars:
These come in various diameters and are
required to possess a characteristic strength in tension which is specified in
relevant codes.
This steel bar used as reinforcement can be commonly
bent easily without cracking at the bends.
(ii) Hot Rolled Bars and Cold Worked Bars:
They are specially prepared reinforcements.
The first type has a characteristic strength
in tension which is almost double than that of mild steel bars.
Further, as these come commonly in thick
sections.
They can be bent by heating (up to 100°C)
without developing any defects.
This is not possible with the ordinary mild
steel bars.
Similarly, the cold worked steel bars come in
twisted or stretched forms having elongated ribs or such structures along their
length.
They
also have a much higher characteristic strength of the order of 425 N/mm2 against 250 N/mm2 for mild
steel bars.
Such bars may not be heated for bending and
re-bending.
(iii) Steel Fabric:
This is made from a variety of bars and
wires.
These may include plain round wires, indented
and deformed wires, deformed steel bars of cold-worked type.
The mesh from such wires is made by welding
together straightened lengths very carefully and strictly in accordance with
the specifications.
Otherwise, the mechanical properties of
reinforcement may be affected adversely.
Placement of Reinforcement:
It requires very complex and careful design
considerations for each member of reinforcement concrete.
Thus, the size, shape, spacing, and location
of reinforcement will be entirely different in a slab or beam or a column.
In
beams, for example, steel bars may be required more in the
lower sections and in fixed beams, in the end, sections as well where the
tensile stresses are most effective.
The top section of the beam may need no
reinforcement.
The horizontal reinforcements are often tied
up with square stirrups at suitable intervals.
These stirrups also provide additional
strength to the Reinforced Cement Concrete against shearing stresses.
The reinforcement requires the minimum
prescribed covering of concrete.
The covering is essential to protect the
reinforcement from deterioration under attack from weathering agencies and also
from casual fires.
The concrete covering
varies from 25 mm to 80 mm depending on the environment in which the RCC member
has been placed.
It is also important that the reinforcement
must be clear of rust, dust, and grease at the time of placement.
This will ensure a better bond between
concrete and reinforcement.
Advantages of Reinforced Concrete (RCC) -
There are 100s of advantages of Reinforced
Concrete, but here we will discuss some important advantages of Reinforced
Concrete.
(i) Structures
made from Reinforced Concrete are durable.
(ii) It
has a high compressive strength (due to concrete).
(iii) It
has a high tensile strength (due to reinforcement).
(iv) It
is resistant to fire and other climate changes.
(v) Easily
available almost anywhere in the world.
(vi) Too
much expertise is not required for working on it, normal skilled labor can also
do it.
(vii) It
can be molded in any form, shape.
(viii) It
can be used in any part of the structure i.e., from foundation to the top
roofing.
(ix) Repairing
cost is almost nil.
(x) It
is more economical compared to other materials.
When steel reinforcement is used in structure
along with cement concrete, it is called reiforced cement concrete structure.
concrete is strong in compression and weak in
tension hence, tension is taken by steel reinforcement.
Steel reinforcement is used to resist
primarily tension,shear ,torsion.sometimes designed to resist compression.
Reinforcement is set in place first depending
on design, which gives details about place,spacing between steel rods, diameter
of rods,type steel rod etc.
It is a composite material that help to
reinforced having higher ductility. It is also known as precast.Designing and
implementing is the optimal structure of any key of design.There are some
important properties of reinforced cement concrete:
1 High Strength
2 high toleration power
3 Good bond
4 long durability
5 Best compatibility
There
are many structure are build with using Reinforced cement concrete such as
walls , frames, fountain, beams, slabs etc.
Cement concrete and reinforcement steel are
two different materials’ Cement concrete is capable of taking compressive
strength whereas steel takes the tensile strength. So as a composite material,
the Reinforced cement concrete is made and well watered for gain in strength
for cementitious material and it becomes a single material. The floor slabs,
roof slabs and beams of RCC CAN TAKE ANY LOADS which may result in bending
compression and tension. Rcc columns will mainly take axial compression
Rock (the temples and old monumental
buildings are constructed using rocks cut into shape) can take load if you
apply over it. But it cannot withstand if you load more than its capacity. You
could see many temples have mantapas (big halls) constructed placing too many
pillars (1000 pillars mantapa) to avoid breakage in the slabs provided for the
roof. It is basically to withstand the stone within its characters. The pillars
are too close the reason is the rocks do not have the bending characters of it
can bend for the over load. The over load causes breakages in the stones.
Concrete made from cement and having
something which provides more strength to it(means reinforcing it), is called
Reinforced cement concrete.
Concrete
is a mixture of binding material (cement) + fine aggregate + coarse aggregate +
admixture (if required) + water. This mixture after hardening, changes into a
solid material, which has very high compressive strength but very low tensile
strength. So, to provide it tensile strength, we provide steel bars in it
because steel bars have high tensile strength. So that the overall combination,
i.e. concrete made from cement and having something which provides more
strength to it (steel bars for tensile strength), is called Reinforced cement
concrete.
Concrete is weak in tension and in general if
the concrete is subjected to only compression we use PCC ( Plain Cement
Concrete). If tension is also to be taken up by the concrete steel is provided
in the tension zone and this is RCC ( Reinforce Cement Concrete).
It is steel bars used alongwith
cement concrete as a material (Embedded, with a good bonding
strength between them).
As the concrete
is strong in compression but weak when subjected
to tension force, these steel bars help take
in the tension force, hence “reinforcing” the concrete.
Reinforced concrete in
terms of threaded bar are also termed as reinforced
cement concrete in short RCC in which steel is embedded in such a
manner that the two materials act together in resisting forces. It is a
composite material in which concrete possess low
tensile strength and ductility possess reinforcement having higher
tensile strength.
Plain cement concrete when castes in situ can
take only compression load.
For taking tensile load, steel bars are
embaded in the concrete. Such used bars are called reinforcement,
Therefore
cement concrete with steel bars is called or known as reinforced cement
concrete.
(RCC) stands for Reinforced Cement Concrete.
Cement concrete reinforced with steel bars, steel plates etc to increase the
tension.
Because, cement concrete good resist in compression but weak in tension and
steel is good in withstanding both tension and compression.
Concrete is a hard mass produced by mixing
together cement coarse aggregates, and fine aggregates and then placed in the
desired place and compacted thoroughly .
But the problem with concrete is that it is
weak in tension that is it cannot take tensile force even though it is good in
taking compressive forces.
so to
cater to the limitation of concrete we use steel in tension zone of concrete so
that steel can take up tensile forces easily
Reinforced cement concrete is the combination
of ordinary concrete with the reinforcement to increase its compressive and
tensile strength to a great extent.
You
know Concrete has a very high compressive strength, but it is low in tensile
strength.
Reinforced Cement Concrete (R.C.C.) -
Concrete
is good in resisting compression but is very weak in resisting tension. Hence
reinforcement is provided in the concrete wherever tensile stress is
expected. The best reinforcement is steel, since tensile strength of steel
is quite high and the bond between steel and concrete is good. As the
elastic modulus of steel is high, for the same extension the force
resisted by steel is high compared to concrete.
However in tensile zone, hair cracks in concrete are unavoidable.
Reinforcements are usually in the form of mild steel or ribbed steel bars
of 6 mm to 32 mm diameter. A cage of reinforcements is prepared as per the
design requirements, kept in a form work and then green concrete is poured.
After the concrete hardens, the form work is removed. The composite
material of steel and concrete now called R.C.C. acts as a structural
member and can resist tensile as well as compressive stresses very well.
- Concrete is good in resisting compression but
is very weak in resisting tension. Hence reinforcement is provided in the
concrete wherever tensile stress is expected. The best reinforcement is
steel, since tensile strength of steel is quite high and the bond between
steel and concrete is good. As the elastic modulus of steel is high, for
the same extension the force resisted by steel is high compared to
concrete.
- Cement - A cement is a
binder, a finely powdered mixtures of inorganic compounds that sets and
hardens and can bind other materials together when combined with water.
Cements used in construction can be characterized as being either
hydraulic or non-hydraulic, depending upon the ability of the cement to be
used in the presence of water. Non-hydraulic cement, referred to as mortar,
is a lime based paste that will not set in wet conditions or underwater,
it sets as the cement dries and reacts with carbon dioxide in the air.
- Hydraulic cement is made by
replacing some of the cement in a mix with activated aluminium silicates,
pozzolanas, such as fly ash, Blast-furnace slag etc. This allows setting
in wet condition or underwater. All Portland cements are hydraulic
cements. Hydraulic cements are sometimes referred to as being “water
resistant” because they can cure in wet or submerged environments and do
not deteriorate with contact with water.
- 5. Portland Cement - is the
most common type of cement in general use around the world, used as a
basic ingredient of concrete, mortar, stucco, and most non-specialty
grout. Typical constituents of Portland clinker plus gypsum: Clinker
Notation Mass % Tricalcium silicate (CaO)3 · SiO2 C3S 45–75% Dicalcium
silicate (CaO)2 · SiO2 C2S 7–32% Tricalcium aluminate (CaO)3 · Al2O3 C3A
0–13% Tetracalcium alumino ferrite (CaO)4 · Al2O3 · Fe2O3 C4AF 0–18%
Gypsum CaSO4 · 2 H2O 2–10%
- Assignment Lab and Field
Test of Concrete - Assignment
- Advantage and Disadvantages
of R.C.C. - Assignment
- TWA Flight Centre, New York Philips Pavilion, Brussels Notre-Dame, France Sydney Opera House Reinforced cement concrete is a combination of concrete and steel bars(reinforcement bars) where they carry the compressive force and tension of a structure simultaneously.
PCC
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