Shrinkage is the decrease in volume of concrete in the absence of load with time. In the plastic stage concrete shrinks in volume due to absorption of water or loss of water.
Due to bleeding the surface water evaporates rapidly causing and this uneven shrinkage causes shrinkage cracks. It is depends upon the efficiency of the curing method as well as water cement ratio, cement lean mixes and amount of water added.
It can be reduced by using low water cement ratio, presaturated aggregate, watertight and non absorbent.
Factors Affecting:
- Water cement ratio
- Cement type and content
- Ambient humidity
- Type of aggregate and content
- Admixtur
1. Water cement ratio
It increases with the increase in water cement ratio.
2. Cement type and Content
Rapid hardening cement gains strength rapidly and hence shrinks more than ordinary portland cement. The shrinkage increases with the increase in cement content.
3. Ambient Humidity
The shrinkage increases with the decrease in humidity
4. Type of aggregate and content
Aggregates restrain the extent of shrinkage in concrete hence concrete with larger aggregate content produces less.
5. Admixture
Firstly shrinkage increases with addition of calcium chloride and reduces with lime replacement
Types of Shrinkage :
- Plastic
- Drying
- Carbonation
- Autogenous
1. Plastic
If the volume reduction in concrete occurs before it becomes hard or still in plastic stage it is called plastic .
2. Drying
The reduction in volume of concrete that occurs preliminary due to moisture loss after the concrete has hardened is known as drying .
3. Carbonation
Carbon dioxide reacts with the moisture present in the atmosphere forming carbonic acid which then react with to form calcium carbonate and some other cement compounds are also decomposed.
4. Autogenous
In the sealed concrete the macroscopic volume change occurring with no moisture movement to or from the paste is called autogenous.