Concrete
Durability
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.
Concrete will remain durable if:
- The cement paste structure is dense and of low
permeability
- Under extreme condition, it has entrained 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 alkalis, Chlorides, sulphates 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 workability will be reduced leading to inadequate
compaction. However, if water is added to improve workability, 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 equipments used, type of formworks, and density of the steelwork
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
There are many types but the major
ones 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
- Sulphate Attack
- Chloride Ingress
- Delay Ettringite Formation
- Corrosion of reinforcement
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