Durability Of Concrete - Pro Civil Engineer

The durability of concrete can be defined as its ability to resist weathering action, chemical attack, abrasion or another process of deterioration and hence retains its original shape, dimension, quality and serviceability.

What is durability of concrete?

Durability is the ability to last a long time without significant deterioration. A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement. The production of replacement building materials depletes natural resources and can produce air and water pollution.

Durability of concrete Definition : the ability of a material to resist weathering action, chemical attack, abrasion, and other conditions of service.

What is meant by durability of concrete? Durability of concrete is determined by its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration, and will retain its original form, quality, and serviceability when exposed to its environment. Durable concrete is a result of proper design, proportioning, placement, finishing, testing, inspection, and curing.

Factors Affecting Durability of Concrete

Following are the factors affecting durability of concrete.

1. Cement Content
2. Aggregate Quality
3. Water Quality

1. Concrete Compaction

Curing Period
Permeability
Moisture
Temperature
Abrasion
Carbonation
Wetting and Drying Cycles
Freezing and Thawing
Alkali-Aggregate Reaction
Sulfate Attack
Organic Acids

#1. Cement Content

How Cement Content Affecting Durability of Concrete? Quantity of cement used in concrete mix will also be a factor affecting durability of concrete. If cement content used is lower than the required, then water cement ratio becomes reduced and workability also reduced.

Adding more water to this mix results in formation capillary voids which will make concrete as permeable material.
If excess cement content is used, problems like drying shrinkage, alkali-silica reaction may occur which finally effects the durability of concrete.

#2. Aggregate Quality

How Aggregate Quality or Quantity Affecting Durability of Concrete?

Aggregates generally make up 70% to 85% of the mass of a concrete mixture. Their grading, size, mineralogical composition, porosity, surface texture, and shape greatly influence the properties of unhardened and hardened concrete. Effects on workability are described by Tattersall (1991).

Obviously, any lack of durability of aggregates has a direct and undesirable consequence on the durability of concrete.

Use of good quality aggregates in concrete mix will surely increase the durability of hardened concrete. The shape of aggregate particles should be smooth and round. Flaky and elongated aggregates effects the workability of fresh concrete.

For better bond development between ingredients rough textured angular aggregates are recommended but they require more cement content.
Aggregate should be well graded to achieve dense concrete mix.
Aggregates should be tested for its moisture content before using. Excess moisture in aggregate may lead to highly workable mix.

#3. Water Quality

How Water Quality or Quantity Affecting Durability of Concrete? Quality of water used in concrete mixing also effects the durability of concrete. In general, potable water is recommended for making concrete.

pH of water used shall be in the range of 6 to 8.
Water should be clean and free from oils, acids, alkalies, salts, sugar, organic materials etc.
Presence of these impurities will lead to corrosion of steel or deterioration of concrete by different chemical reactions

#4. CHEMICAL ADMIXTURES

The durability of concrete and reinforced concrete can be significantly enhanced with the use of various chemical admixtures. These include air-entraining admixtures for freezing and thawing resistance. Water-reducing and high-range water-reducing admixtures (super plasticizers) reduce the water content, the water–cement ratio or w/cm, all of which result in lower permeability to aggressive elements. The corrosion inhibitors improve corrosion resistance in the presence of chloride ions or reduced pH, and ASR inhibitors control alkali–silica reactivity. Shrinkage reducing admixtures (SRAs) reduce drying shrinkage cracking and ultimately lower the permeability.

#5. ASR Inhibitors

Several compounds have been investigated for use as admixtures in concrete to control ASR damage. Lithium compounds are the best-known ASR inhibitors. When ASR occurs, in the presence of lithium ions, a minimally expansive lithium-bearing ASR gel is formed. This is generally not damaging to the concrete (Farny and Kerkhoff, 2007). While many lithium compounds could be used for this purpose, lithium nitrate (LiNO3) is considered more effective while being safer to handle and is therefore the more desirable additive.

Deleterious actions are basically due to physical and chemical causes.

Physical Reasons or Physical durability of concrete

Abrasion:

Abrasion takes place between two bodies especially when one is at rest and other is moving. Abrasion causes physical deterioration of concrete

Erosion:

A portion of concrete especially coarse aggregate phase may get eroded resulting in physical damge.

Cavitation:

Cavitation is a kind of pitting in case of concrete especially when it is subjected to negative atmospheric pressure.

Chemical Reasons or Chemical durability of concrete

Internal And External Sulfate Reaction

External Sulfate reactions are common in marine environment.

In concrete constituents,some internal source may contain sulfates especially during mixing stage. sulfate rich aggregates can be one of the reasons, excess of added gypsum may also be cause of internal sulfate reaction.

Delayed Ettringite Formation (DEF) can be another cause for internal sulfate reaction.

The reason for external and internal sulfate reaction is same and three things are necessary

Presence of sufficient water
High permeability
Sulfates from internal or external sources

2. Alkali Aggregate Reaction

This type of reaction occurs between alkalis in the cement paste which has very large PH (around 13.5) and some typical reactive aggregates with lower PH value. Such aggregates include chert, flint, christobalite, chalcedony and other volcanic glasses.

The reaction prducts absorb water and expands causing cracking. AAR can be diagnosed very easily by observing map type cracking pattern on the surface of concrete.

Map type cracking easily visible indicating alkali-aggregate reaction.

Observing at a microscopic level, cracking in general and that due to AAR, in particular, starts within the ITZ or the paste zone.

AAR Types

AAR can be further divided into ASR and ACR i-e Alkali silicate reaction and Alcali carbonate reaction. Obvious from their names ACR is a reaction between cement alkali and carbonate aggregates (CaCo3, MgCo3), On the other hand, ASR is a reaction between silicate aggregates and alkali cement paste.