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1. What is durability? How
significant is it in
construction?
The
environment has a vital impact
on the life of every structure.
While foundations have to
withstand the action of water
and chemicals in the soil,
superstructures are subjected to
weathering action and attack by
harmful chemicals present in the
atmosphere.
In
order for a construction to
perform its structural function,
it is required not only to be
strong but also to be durable.
It is important that the
structure retains its original
form, quality and serviceability
throughout its lifetime. This is
an aspect of durability that is
desired in construction.
Durability assumes even greater
significance for concrete
structures in coastal regions
where it is subjected to harsh
and frequent onslaughts of
weather.
The
microstructure and properties of
any material change with
environmental interaction. When
the properties of the material
deteriorate to such an extent
that it is no longer safe to use
the material, it is considered
to have reached the end of its
service life.
The
problems of deterioration and
mishaps due to collapsing
buildings, in the past few
years, have underscored an
important point in the design
and construction of buildings,
whatever their nature. Builders,
engineers and architects need to
give equal importance to
strength parameters and
durability characteristics.
2. What are the socio-economic
implications of durability?
Durability has economic
implications. It has been found
that about 40 percent of the
total resources of the
construction industry are
applied to repairs and
maintenance. Replacement costs
of parts, or of the entire
structure increases the
lifecycle costs, rather than the
initial costs. With no controls
exercised on the various repair
materials and practices, it
often leads to repeated repairs
and uncontrolled costs.
Maintenance and lifecycle costs
escalate. This is another reason
why building-in strength and
durability features into
construction is of primary
importance.
Ecologically, also, there is a
need to conserve natural
resources by making materials
last longer. Premature failures
of buildings make the human
costs also very high.
3. What is meant by
deterioration of concrete?
Cement concrete is an
alkaline/basic material. Being
porous in nature it is
vulnerable to both physical and
chemical attacks. Deterioration
starts on the surface, gradually
extends inside and then gets
aggravated, leading to its final
destruction.
Precise detailing, good
materials, correct structural
design, proper mix proportions,
standard construction practices
with regard to mixing, placing,
compacting and curing, are
equally important in making good
concrete. Less importance given
during any of these steps means
an invitation to deterioration
process.
4. What are the types of
physical deterioration of
concrete?
Abrasion
is the wearing-off of the
concrete surface, e.g. as in
pavements, industrial floors,
etc.
Erosion
of concrete takes place in canal
linings, spillways, pipes, etc.,
through the action of solid
particles in suspension. Dense
and strong concrete is safe
against such damages.
Cracks
develop inside concrete due to
pressures exerted by the process
of crystallization of salts,
like sulphates and chlorides,
entering into the concrete from
the surface in the form of
solutions. Cracks also develop
due to heating and cooling, as
well as wetting and drying.
Cracks are, initially,
discontinuous micro-cracks that
later get interconnected and
start allowing the circulation
of air, carbon dioxide and water
along with injurious chemicals,
which cause further
deterioration. Dense, strong,
and impermeable concrete can
prevent the development of these
micro-cracks.
Fire
is another agent of
deterioration. It allows the
building to maintain its
structural integrity for some
hours, which often facilitates
rescue operations. But
temperatures above 500oC
cause surface spilling and loss
of strength due to the
decomposition of hydrates.
5. What are the deterioration
processes in concrete due to
chemical reactions?
The
main causes of chemical
deterioration in concrete are
the aggressive agents in the
environment carried by air and
water.
Concrete is normally alkaline,
with a pH value above 14. If,
due to chemical reactions, the
alkalinity drops below 12.5, the
environment becomes aggressive.
Rain water, ground water, sea
water, sewage, industrial
effluents, etc., carry carbon
dioxide, sulphates and chlorides
in combination with sodium,
calcium, magnesium, etc.
Magnesium salts are found to be
most aggressive.
6. What property of concrete
allows and results in
deterioration processes?
Permeability of concrete is what
allows maximum damage.
Deterioration in concrete begins
with loss of water-tightness.
When air or water is allowed to
penetrate the concrete, carrying
chemicals dissolved or dispersed
in it, the concrete is said to
be permeable. Permeability is
mainly the hydraulic
conductivity of water. Pore
size, continuity of pores, micro
cracks from weathering, loading
effects and their connectivity
determine the permeability of
concrete.
Cement pastes, as well as
aggregates have pores and
influence permeability. Low
cement content, high water
cement ratio, poor
proportioning, inadequate
mixing, improper compaction, and
insufficient curing affect the
process of hydration,
development of hydration
products, pore size, pore
distribution and, consequently,
the permeability of concrete.
The aggregate paste bond becomes
weak and allows micro crack
formation.
Durability is a factor that is
also closely linked with the
permeability of concrete. Thus,
any activity that arrests
permeability ensures durability.
7. What is sulfatic attack?
Concrete is vulnerable to
sulfatic attack, because the
tricalcium aluminate (C3A)
constituent of Portland cement
reacts with sulphate ions in the
presence of free lime which is
liberated during the hydration
of the cement in order to form
calcium sulfoaluminate hydrates
(Ettringite).
The ettringite occupies a larger
volume than the original
reaction compounds of the
hydrated cement, giving rise to
internal stresses. This leads to
cracking of the concrete and,
subsequently, to its
disintegration.
Concrete made with SRPC is less
vulnerable to sulphate attack
because its C3A
content is much lower than that
of Ordinary Portland Cement,
thereby reducing the formation
of ettringite.
Sources of sulphates
Sulphates are distributed widely
in nature, and are present
virtually in all the soils. They
are also found in seawater,
marshy areas and in some arid
regions the aggregates are also
contaminated with sulphates.
Concrete placed in soil or water
containing sulphates is
vulnerable to chemical attack.
The degree of attack will depend
on the concentration and type of
sulphate present. The most
common sulphates are calcium
sulphate (Gypsum) sodium
sulphate and magnesium sulphate
(Epsom Salt). Calcium sulphate
is the least soluble as compared
to magnesium and sodium
sulphates. Ammonium sulphate and
magnesium sulphate are most
aggressive to cement and
concrete.
8. How do sulphates attack a
concrete construction?
The
most common occurrence of
sulphate attack can take the
form of progressive loss of
strength. Up to 0.1 percent
sulphates in soils and 1500 PPM
in water can be tolerated. If
present in greater quantities,
sulphates cause the
deterioration of concrete.
Sulphates change the chemistry
of concrete, combining with the
calcium aluminates present in
it, to cause an expansion that
results in cracks. Deterioration
due to seawater is not stop
limited to buildings in close
proximity to the sea alone.
Moisture from the sea is carried
on to the surrounding land by
the winds, and thus the
chemicals also affect the
buildings along coastal regions.
9. How can both strength and
durability be ensured in a
construction?
Strength and durability go
together. One cannot be ensured
at the cost of the other. It is
not that the initial cost alone
has to be minimum. Durability
brings down the lifetime cost of
a building.
Ensuring the selection of proper
materials, not only cement but
also aggregates, testing them
before use, making a proper mix
design, a good structural
design, proper batching, mixing,
placing, compacting, and
curing make a structure
strong and durable.
10. How do your obtain the best
concrete?
Begin with the right type of
cement and good and clean
aggregates.
For
the desired grade of concrete,
workability, exposure
conditions, minimum cement
content, level of quality
assurance, the maximum water
cement ratio is decided and the
mix proportions should be
designed using standard methods.
Materials should be batched by
mass and water by measured
volume.
Materials should be mixed in a
mechanical mixer, for at least
two minutes.
Use
rigid formwork that prevents
loss of concrete slurry.
Concrete should be placed
without segregation, compacted
by vibration and allowed
adequate moist curing.
11. With early deshuttering will
I get my dream house faster? How
much time is needed for curing
and removal of formwork?
Early stripping of formwork -
propagated as an added 'economy'
due to the high early strength
attained by concrete can lead to
the elimination/evaporation of
the mixing water and, if the
curing is inadequate, it can
have further adverse effect on
the permeability and hence the
durability of concrete. The
result, cracks in the concrete,
pealing paint and plaster, and
heavy costs of repairs.
It
is recommended that curing
should be done for 14 days with
high levels of supervision.
Curing ideally should be
continued for 21 days. For
concrete slabs and columns
curing should be started after
12 hours of placing the concrete
and the surface should be kept
moist and protected from direct
sunlight and hot air/wind.
The
Bureau of Indian Standards (BIS)
has set the following timings
for
Removal of shutterings.
Walls columns and vertical faces
of all structural members : 24
to 48 hours.
Slabs (props left
under)
:
3 days.
Beam soffits (props left under)
: 7 days.
Removal of props under slabs
I. Spanning up to 4.5m
: 7 days
II. Spanning over 4.5m
: 14
days.
Removal of props under beams and
arches
I. Spanning up to 6m
: 14 days
II. Spanning up to 6m
: 21 days
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