The properties of hardened concrete are as follows:-
1. Strength
2. Shrinkage
3. Durability
4. Impermeability
1. Strength
The strength of concrete is basically called compressive strength
and depends on three factors.
a. Paste strength
b. Interfacial bonding
c. Aggregate strength
a. Paste Strength:
It is mainly due to the binding properties of cement that the
material is compacted together. If the paste has a high binding strength, it
will have a high concrete strength.
b. Interfacial Bonding:
Interfacial bonding is very essential with respect to strength.
Clay disrupts the bonding relationship between paste and aggregate. The aggregates
should be washed for better bonding between the paste and the aggregate.
c. Aggregate strength:
It is mainly aggregates that give strength to concrete, especially
coarse aggregates that act like bones in the body. Rough and angular aggregates
provide better bonding and higher strength.
Factors affecting the strength of concrete:
Following are the factors that affect the strength of concrete:
a. Water-cement ratio
b. Type of cementing material
c. Cement content
d. Type of aggregates
e. Air content
f. Admixtures
a. Water-cement ratio:
It is the water cement ratio that basically controls the strength
property. The lower the water cement ratio, the higher the strength.
b. Type of cement:
The type of cement affects the hydration process and therefore the
strength of concrete.
c. Amount of cementing material:
It is the paste that holds or binds all the materials. Thus the
greater the cement content the stronger it will be.
d. Aggregate Type:
Since they provide greater bonding, rough and angular aggregates
are better.
e. Admixtures:
Chemical mixtures such as plasticizers reduce the water to cement ratio and increase the strength of concrete at the same water cement ratio.
Mineral mixtures affect strength in later stages and increase strength by
increasing the amount of cement content.
2. Shrinkage
Concrete is subjected to either autogenous or induced volume
changes. Volume change is one of the most damaging properties of concrete,
affecting long-term strength and durability. For the practical engineer, the
aspect of volume change in concrete is important from the point of view that it
produces unsightly cracks in the concrete.
Hard concrete undergoes three types of shrinkage that are
important with respect to its dimensional stability:
1. Plastic shrinkage.
2. Drying Shrinkage.
3. Thermal shrinkage.
1. Plastic shrinkage:
It is the shrinkage that freshly placed concrete passes through until
it is completely set. It can also be called initial shrinkage.
There
is such a major volumetric change from evaporation, bleeding, seepage and
soaking by the formwork to water loss from fresh concrete.
Excessive shrinkage in the initial stages may develop extensive
cracking in the concrete at the setting. Therefore, all precautions should be
taken to avoid excessive loss of water due to evaporation.
2. Drying Shrinkage:
As the concrete is completely set and hardened, some further
shrinkage may result in moisture, or further loss of drying, due to contraction
of the gel-structure.
Such shrinkage is practically an essential and irreversible
property of concrete. Careful design of reinforcement has to be met to
avoid its side effects.
3. Thermal shrinkage:
This may be due to a drop in the temperature of the concrete being
held until it is fully set. Thus, when the concrete is placed at 30 ° C, cooled
to 15 ° –18 ° C, some shrinkage can be expected. This may be negligible in its
account. But when drying is added to shrinkage, it becomes necessary.
3. Durability
Environmental forces such as weathering, chemical attack, heat,
freezing and thawing seek to destroy concrete. The period of existence of concrete
without being adversely affected by these forces is known as durability.
Usually dense and strong concretes have better durability. Cube crushing
strength alone is not a reliable guide to durability. Concrete should have a
sufficient amount of cement content and a low water-to-cement ratio.
4. Impermeability
It is the resistance of concrete to the flow of water through its
pores. The excess water during concreting leaves a large number of continuous
pores leading to permeability.
Since permeability decreases the durability of concrete, it should
be kept very low by low water-cement ratios, dense and well graded aggregates,
good compaction and continuous curing in low temperature conditions. The
cement material used should be sufficient to provide adequate workability with
low water cement ratio and available compaction method.
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