Definition
The ability of concrete to withstand the
conditions for which it is designed without deterioration for a long period
of years is known as durability.
OR
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.
Durability is defined as the capability of
concrete to resist weathering action, chemical attack and abrasion while
maintaining its desired engineering properties. It normally refers to the
duration or life span of trouble-free performance. Different concretes
require different degrees of durability depending on the exposure environment
and properties desired. For example, concrete exposed to tidal seawater will
have different requirements than indoor concrete.
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Concrete will remain durable if:
- The
cement paste structure is dense and of low permeability
- Under
extreme condition, it has en-trained air to resist freeze-thaw cycle.
- It
is made with graded aggregate that are strong and inert
- The
ingredients in the mix contain minimum impurities such as alkalies,
Chlorides, sulfates and silt
Factors Affecting Durability of
Concrete
Durability of Concrete depends upon the following
factors:
Cement content
Mix must be designed to ensure cohesion and prevent
segregation and bleeding. If cement is reduced, then at fixed w/c ratio the
work ability will be reduced leading to inadequate compaction. However, if water
is added to improve work ability, water / cement ratio increases and resulting
in highly permeable material.
Compaction
The concrete as a whole contain voids can be caused
by inadequate compaction. Usually it is being governed by the compaction
equipment's used, type of form works, and density of the steelworks
Curing
It is very important to permit proper strength
development aid moisture retention and to ensure hydration process occur
completely
Cover
Thickness of concrete cover must follow the limits
set in codes
Permeability
It is considered the most important factor for
durability. It can be noticed that higher permeability is usually caused by
higher porosity .Therefore, a proper curing, sufficient cement, proper
compaction and suitable concrete cover could provide a low permeability
concrete
Types of Durability of Concrete
There are many types but the major Concrete
Durability types are:
- Physical
durability
- Chemical
durability
Physical Durability
Physical durability is against the following
actions
- Freezing
and thawing action
- Percolation
/ Permeability of water
- Temperature
stresses i.e. high heat of hydration
Chemical Durability
Chemical durability is against the following
actions
- Alkali
Aggregate Reaction
- Sulfate
Attack
- Chloride
Ingress
- Delay
Ettringite Formation
- Corrosion
of reinforcement
Causes for the Lack of Durability
in Concrete
1. External Causes:
- Extreme
Weathering Conditions
- Extreme
Temperature
- Extreme
Humidity
- Abrasion
- Electrolytic
Action
- Attack by a natural or industrial liquids or gases
2. Internal Causes
a) Physical
- Volume
change due to difference in thermal properties of aggregates and cement
paste
- Frost
Action
b) Chemical
- Alkali
Aggregate Reactions
i. Alkali Silica Reaction
ii. Alkali Silicate Reaction
iii. Alkali Carbonate Reaction
- Corrosion
of Steel
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