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.

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:

1) The cement paste structure is dense and of low permeability

2) Under extreme condition, it has entrained air to resist freeze-thaw cycle.

3) It is made with graded aggregate that are strong and inert

4) The ingredients in the mix contain minimum impurities such as alkali's, Chlorides, sulphate and silt.

Factors affecting durability of concrete

Durability of Concrete depends upon the following factors

1) 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.

2) Compaction

The concrete as a whole contain voids can be caused by inadequate compaction. Usually it is being governed by the compaction equipment used, type of form works, and density of the steelworks

3) Curing

It is very important to permit proper strength development aid moisture retention and to ensure hydration process occur completely

4) Cover

Thickness of concrete cover must follow the limits set in codes

5) 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:

1) Physical durability of concrete

2) Chemical 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

Causes for the Lack of Durability in Concrete

1. External Causes:

A) Extreme Weathering Conditions

B) Extreme Temperature

C) Extreme Humidity

D) Abrasion

E) Electrolytic Action

F) 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