The total shrinkage strain in concrete is composed of the following:
- Autogenous Shrinkage, which occurs during the hardening of concrete.
- Drying Shrinkage, which is a function of the migration of water through hardened concrete.
Drying shrinkage often referred to simply as shrinkage is caused by the evaporation of water from the concrete. Both Shrinkage and creep introduce time-dependent strains. However, shrinkage strains are independent of stress conditions of concrete.Shrinkage cracks in RC are due to different shrinkage between the cement paste, the aggregate and the reinforcement. Its effects can be reduced by prolonged curing..
The most important factors that influence shrinkage are:
- type and content of aggregates.
- w/c ratio.
- effective age at transfer of stress.
- degree of compaction.
- effective section thickness.
- ambient relative humidity
- presence of reinforcement.
In clause 18.104.22.168 of IS 456, it recommends the value for total shrinkage strain for design as 0.0003.
Concrete expands with rise in temperature and contracts with fall in temperature. To limit the development of temperature stresses, expansion joints are to be provided, especially when the length of the building exceeds 45m as per clause 27 of IS 456.
Temperature stresses may be critical in the design of concrete chimneys and cooling towers. In large and exposed surfaces of concrete such as slabs, nominal reinforcements are usually placed near the exposed surfaces to take care of temperature and shrinkage stresses.SP 24:1983 recommends a value of 11×10-6 mm/mm per degree Celcius for the design of liquid storage structures, bins and chimneys.
When exposed to fire, both concrete and steel reinforcement of RC members loose about 60% of their characteristic strength at a temperature of 500ºc.