Definition
•
Strength of
concrete is its resistance to rupture
•
Strength of
concrete are-
•
Strength in
compression
•
Strength in
tension
•
Strength in shear
•
Flexural strength
Concrete is
stronger in compression
•
Concrete is
stronger in compression but weak in tension
•
Concrete consists
of two phase : aggregate and cement paste
•
At microscopic
level, there is an interfacial zone between aggregate and hardened cement past
called transition zone.
•
The transition
zone contains more water less cement so it is of poor quality and is weak.
•
This zone reduces
bond between aggregate and paste
•
Now when
compressive force is applied, the function of transition zone (the weak cement
paste) is only to transfer force between aggregates
•
But when tension
force (Pulling force) is applied, the
weaker zone (transition zone formed around the aggregate) has to resist the
force
•
It means the
transition zone of weak cement paste has to bind the aggregate, which it fails
to do as it is weak
•
And hence crack
appears.
•
That’s why
concrete is strong in compression and weak in tension
Setting vs hardening of concrete
•
Setting
•
Setting means the
concrete becomes stiff.
•
It means the
concrete is no longer fluid but still concrete may be weak
•
Setting takes
place within short period of time
•
Concrete should
be placed within the initial setting time
•
Hardening
•
Hardening is the
process of gaining strength
•
It is due to
hydration i.e. formation of calcium silicate hydrate
•
It may continue
for long time
Factor affecting strength of concrete
•
Porosity in concrete: Porosity is due to void (air) in concrete. The more
porous the concrete is, the weaker it is.
•
Water-cement ratio: As the water-cement ratio increases, the strength of
concrete decreases but workability increases. So optimum water-cement ratio is
chosen considering both workability and strength of concrete
•
Quality of aggregate: The stronger the aggregate, the higher the strength of
concrete
•
Relative humidity: As the relative humidity decreases, the strength also
decreases because without moisture, hydration reaction cannot proceed.
•
Compaction of concrete: If
concrete is well compacted, the air will be removed and the void decreases.
Hence the strength of concrete increases.
•
Age of concrete: As the age of concrete increases, the strength of
concrete increases in a decreasing rate.
•
Majority of
strength is gained during first 28 days.
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