REINFORCED CONCRETE
Reinforced
concrete
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
and/or ductility.
Materials
Concrete is a mixture of coarse
and fine aggregates with a binder material. When mixed with a small amount of
water, the cement hydrates to 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 not
only 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:
- 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 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 physic-chemical conditions when concrete becomes carbonated. Carbonization of
concrete
along with chloride ingress are
amongst the chief reasons for the failure of reinforcement bars in concrete.[3]
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.
Pre-stressed
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.
