Q#5:
a-
Briefly
explain how various environmental factors affect the durability of concrete?
b-
What
is the mechanism of sulphate attack on concrete; how it can be controlled?
Solution#
Answer A
Durability
A DURABLE CONCRETE IS ONE THAT PERFORM satisfactorily in workig enviromet during its anticipated exposure conditions service.
Durability is
the ability of a physical product to remain functional, without
requiring excessive maintenance or repair, when faced
with the challenges of normal operation over its design
lifetime. There are several measures of durability in use, including
years of life, hours of use, and number of operational cycles. In economics,
goods with a long usable life are referred to as durable
goods.
A DURABLE
CONCRETE IS ONE THAT PERFORM satisfactorily in workig enviromet during its
anticipated exposure conditions service.
dURABILITY Of
concrete is its ability to resist weathering action, chemical attack, abrasion
or any other process deterioration
Environmental
factors affect the durability of concrete;
Physical Durability
Physical
durability is against the following actions
1.
Freezing
and thawing action
2.
Percolation
/ Permeability of water
3.
Temperature
stresses i.e. high heat of hydration
Chemical Durability
Chemical
durability is against the following actions
1.
Alkali
Aggregate Reaction
2.
Sulphate
Attack
3.
Chloride
Ingress
4.
Delay
Ettringite Formation
5.
Corrosion
of reinforcement
TEMPERATURE
Durability of concrete may be defined as the
ability of concrete to
resist weathering action, chemical attack, and abrasion while maintaining its
desired engineering properties. ... When the temperature increases the volume of the concrete increases and when
the temperature falls
the concrete contracts
MOISTURE
Higher water-to-cement ratios
result in greater spacing between the aggregates in cement, which affects
compaction. Similarly, increased moisture levels reduce the concrete’s
compressive strength and durability. As concrete’s surface area increases, particularly
with the addition of fine aggregates, so does the demand for water. The
increased water leads to a higher water-to-cement ratio.
When excess water creates greater spaces between aggregate materials, the voids
fill with air after the moisture evaporates. The resulting inadequate
compaction reduces the concrete’s strength. Concrete with trapped air levels as
little as 10 percent experience reductions in strength of up to 40 percent.
Quality
of water
if the water has pollutant i it,
it will result in decrease in strength of concrete and the workability is goig
to decrease
Freezing
and Thawing:
Deterioration of concrete from
freeze thaw actions may occur when the concrete is critically saturated, which
is when approximately 91% of its pores are filled with water. When water
freezes to ice it occupies 9% more volume than that of water. As the seasons
pass, concrete goes through the process of freezing and thawing resulting in
repeated loss of concrete surface With the addition of an air entrainment admixture,
concrete is highly resistant to freezing and thawing.
Alkali aggregate reaction:
Use of non-reactive aggregate from alternate
sources . use of low alkali ordinary Portland cement having total alkali
content not more than 0.6 percent (as Na20 equivalent).Measures to reduce the
degree of saturation of the concrete during service such as use of impermeable
membranes.
Answer B
sulfate attack
sulfate attack of concrete is a complex
process, which includes physical salt attack due to salt crystallization and
chemical sulfate attack by sulfates from soil, groundwater, or seawater.
Sulfate attack can lead to expansion, cracking, strength loss, and
disintegration of the concrete
Sulfate attack can be 'external' or “internal”. “External”
EXTERNAL:
Due to
penetration of sulphates into the concrete from outside For example :
high-sulphate soils and ground waters, or atmospheric or industrial water
pollution.
INTERNAL:
Due to a soluble
source being incorporated into the concrete at the time of mixing, gypsum in
the aggregate.
MECHANISM:
Sulfate attack has occurred at various
locations throughout the world. Some common sulfate environments are soils,
groundwater, transport fluids, contained soils or fluids, and seawater. Many concrete
structures are exposed to these environments and accumulation of sulfates at an
exposed face increases the potential for deterioration.
The
mechanism of distress for sulfate attack is an expansive pressure caused by the
transformation of monosulfoaluminate to ettringite. This mechanism can be
described as a sequence of processes. First, the external sulfate reacts with
calcium hydroxide to saturate the pore solution and precipitate gypsum (CaSO4 •
2H2O). The increased concentration of SO4 promotes the transformation of
monosulfoaluminate to ettringite (3CaO• Al2O3• 3CaSO4 • 32H2O). This
transformation causes an increase in solid volume, which results in the
deterioration of concrete by inducing cracking, softening, and spalling. The
expansion mechanism is caused by pressure from ettringite crystal growth or
swelling due to absorption of water. Another measurable concrete property
identified with the sulfate attack mechanism is a decline in compressive
strength.
Different sulfates can effect the concrete differently. Magnesium sulfate is the most severe because of the presence of magnesium ions. These ions can cause additional corrosive reactions through the formation of Mg(OH)2 and ettringite. This also decomposes the C-S-H
PROTECTION:
The methods given
below can be adopted to protect concrete from sulphate attack.
USE OF SULPHATE
RESISTING CEMENT:
1.QUALITY CONCRETE:-
A well designed,
placed and compacted concrete which is dense and impermeable exhibits a higher
resistance to sulphate attack. Similarly, a concrete with low water/cement
ratio also demonstrates a higher resistance to sulphate attack.
2.USE OF AIR-ENTRAINED ADMIXTURE:
Use of
air-entrainment to the extent of about 6% has beneficial effect on the sulphate
resisting qualities of concrete. The beneficial effect is possibly due to
reduction of segregation, improvement in workability, reduction in bleeding and
in general better impermeability of concrete.
3.USE OF POZZOLANA:
Incorporation of
or replacing a part of cement by a pozzolanic material reduces the sulphate
attack. Admixing of Pozzolana converts the leachable calcium hydroxide into
non-leachable cementitious product. This pozzolanic action is responsible for
impermeability of concrete. Secondly the removal of calcium hydroxide reduces
the susceptibility of concrete to attack by magnesium sulphate.
4.HIGH PRESSURE STEAM CURING:
High pressure steam curing improves the resistance of concrete to sulphate attack. This improvement is due to the change of C3AH6 into a less reactive phase and also to the removal or reduction of calcium hydroxide by the reaction of silica which is invariably mixed when high pressure steam curing method is adopted.
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