🕑 Reading time: 1 minuteShrinkage is an inherent property of concrete. The shrinkage in concrete can be defined as the volume changes observed in concrete due to the loss of moisture at different stages due to different reasons.
Types of Shrinkage in ConcreteThe shrinkages can be classified into the following:
- Plastic Shrinkage
- Drying Shrinkage
- Autogenous Shrinkage
- Carbonation Shrinkage
Plastic Shrinkage in ConcreteIt is observed after the casting of concrete that water required for concrete strength gain is escaped into the atmosphere due to the process of evaporation, from the surface of the structure. This will create cracks on the surface of the structure. Another reason for shrinkage cracks under plastic shrinkage type is due to the water absorption from the concrete by the aggregate. The aggregate particles or the reinforcement will come in the way of subsidence due to which the cracks may appear on the surface of the structure or internally around the aggregates. In the case of floors and the pavements, where the surface area is exposed to drying in a large extent compared to the depth when are subjected to the sun and the drying wind, the surface dries very quickly causing plastic shrinkage. In the case of a mix design where the water cement ratio is high, there is the possibility of excess water pathways causing bleeding. This excess water due to bleeding will accumulate at the surface of the slabs. When these are exposed to dry weather conditions, the surface dries up and collapses creating cracks.
Prevention of Plastic ShrinkageThe following remedies can be applied to prevent the cause of plastic shrinkage:
- The escape of water from the surface can be prevented by covering the surface with the help of polyethylene sheeting. Prevention of water evaporation will prevent plastic shrinkage.
- Proper vibration of the concrete can prevent plastic shrinkage.
- Plastic shrinkage in concrete structures can be reduced by use of aluminum powder.
- The use of expansive cement can also help in the control of plastic shrinkage.
Drying Shrinkage in ConcreteDrying shrinkage is caused by the loss of surface -absorbed water from the calcium silicate hydrate ( C-S-H) gel and also due to the loss of hydrostatic tension in the small pores. Swelling is the opposite phenomenon of shrinkage. This shrinkage is mainly due to the deformation of the paste, though the aggregate stiffness also influences it. It takes place once the concrete has set is called as the drying shrinkage. Most of the kinds of drying shrinkage take place in the first few months of the concrete structure life. The withdrawal of the water from the concrete, that is stored within the unsaturated air voids causes drying shrinkage. A part of this shrinkage caused can be recovered by immersing the concrete in water for a specified time. This is termed as the moisture movement. This can be calculated by means of Schorer's Formula:
Es = 0.00125 (0.90 -h)Where Es is the shrinkage strain and 'h' is the relative humidity as a fraction. The rate of this shrinkage will decrease with time. Almost 14 to 34 % of the shrinkage occurs in 2 weeks and 40 to 70 percent of the shrinkage occurs within 3 months. Almost 80 % shrinkage would have caused within one year.