Durability

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

Deleterious actions are basically due to physical and chemical causes.

Physical Reasons

Abrasion:

Abrasion takes place between two bodies specially 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 cocnrete especially when it is subjected to negative atmospheric pressure.

Chemical Reasons:

  1. Internal And External Sulfate Reaction

Corrosion
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.

Alkali Aggregate Reaction- Durability 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 with in 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 reaction between cement alkali and carbonate aggregates (CaCo3, MgCo3), On the other hand ASR is reaction between silicate aggregates and alkali cement paste.

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