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Durability
is gaining credence of late among Civil Engineers. It has been
estimated that about 40% of construction budget worth several billions
is being spent on repair & maintenance of structures. Thus
escalation of replacement costs and the growing emphasis of life-cycle
cost have driven engineers to be durability conscious. Durability is
also the emphasis in the new Indian code for concrete, 456-2000. |
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Durability
of a concrete structure may be defined as its ability to resist
weathering action, chemical attack, abrasion or any other form of
deterioration so that it lasts for its intended service life. In other
words, a well constituted and properly consolidated, cured and
water-tight
concrete is said to be durable as long as capillary pores and micro
cracks in the interior do not become interconnected pathways leading to
the surface of the concrete. |
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Loading
effects as well as heating/cooling or wetting/drying cycles in service
are responsible for the propagation of micro cracks that normally exists
in reinforced concrete structures at the transition zone between cement
matrix and aggregates or at the interface between the cement mortar and
reinforcing bars. This mechanism in reinforced concrete structures (RCS)
in service causes a gradual loss of water tightness. The initiation and
propagation of damages of RCS can occur due to penetration of
environmental aggressive agents like water, air and ions of sulphate,
chloride and sodium through the interconnected pores. |
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In
general, permeability factor has great influence on durability of
concrete. Quite a few elements contribute to this permeability factor.
Sufficient cover, cement content (subject to max & min consideration
as per IS: 456/2000), low W/C ratio, proper compaction and curing of
fresh concrete to name a few.
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Similarly
proper attention to aggregate size and grading, thermal and drying
shrinkage strain and avoiding premature or excessive loading are
necessary steps to reduce the incidence of micro-cracking in the
transition zone, which appears to be a major cause of high permeability
of concrete in practice.
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Water
is generally involved in every form of deterioration since it is the
primary vehicle for the diffusion of other aggressive ions, such as
chlorides & sulphates into the concrete mass. Hence the need for
using less water just required for hydration and curing. |
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Sulphates are found in ground water, subsoil and seawater. Sulphates in
solution react with hydrated C3A in the hardened concrete to form a
chemical call ettringite and monosulfoaluminate. This leads to expansion
cracking and loss of strength. |
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Corrosion of steel is an electro-chemical mechanism. This mechanism in
the presence of moisture results in the expansion of metal (steel) to
about 6-8 times its original volume. This expansion results in spalling
of concrete. Adequate concrete cover, impermeable concrete and
protective coating of steel will protect against corrosion. Presence of
chlorides in concrete/steel is extremely dangerous as they cause
leaching/corrosion in concrete. |
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A reaction occurs between certain types of silica
present in
aggregates and alkalis contained in cement. Due to this reaction, a gel
made up of alkalis and alkaline-earth silicates is formed. This gel has
tendency to absorb water and swell. Usage of blended cements is a
solution. |
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Cracks, one of the deterrent factors cannot be prevented but controlled.
Adequate precaution during design and construction stage will help to
control the formation of cracks. |
The essentials to ensure durability are:
- Grade of concrete based on exposure condition. -
View
- Cover to the embedded steel
- Detailing of the Reinforcement steel
- Low water-cement ratio
- Minimum cement content
- Workmanship, to obtain full compaction and efficient curing
- The shape and size of the member
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