Shrinkage 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 Concrete
The shrinkages can be classified into the following:
- Plastic Shrinkage
- Drying Shrinkage
- Autogenous Shrinkage
- Carbonation Shrinkage
Plastic Shrinkage in Concrete
It 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 Shrinkage
The 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 Concrete
Drying 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.
Factors Affecting Drying Shrinkage
The following are the main factors that affect the drying shrinkage:
1. Material Selection
Ingredients that are chosen for the concrete mix must be good quality to ensure chances of drying shrinkage. The properties of the ingredients in terms of quality and specification must be as per the standard codes of that region.
2. Water cement Ratio
Higher the water /cement ratio of the concrete mix, more is the chances for drying shrinkage. As the water/cement ratio increases, the strength of the paste and the whole stiffness will decrease. Hence shrinkage increase with the increase of water.
3. Environmental Conditions
The relative humidity of the site plays an important role on the drying shrinkage of the concrete structure. With the increase in humidity in the environment, there is a decrease in the decrease of shrinkage.
4. Cement Content
The rate of shrinkage will increase with the increase in the cement content
Type used in the Mix: Size of aggregate will influence the cause of drying shrinkage. The increase in the maximum size of aggregates will decrease the shrinkage.
The aggregate grading and shape have no appreciable influence on the shrinkage characteristics of concrete. Those aggregates that have high rough surface will resist the shrinkage.
5. Type of Cement used in the mix
As depending on the purpose, a wide variety of cement can be used, the shrinkage also varied accordingly. The rapid hardening cement will harden fastly, hence the shrinkage is more in the same compared to the ordinary Portland cement.
The rapid hardening cement will ask for higher water and high fines that will result in more shrinkage. The use of shrinkage compensating cement will help in either reduction or the elimination of the shrinkage cracks.
6. Admixture in Concrete
The addition of calcium chloride as an admixture into the concrete mix will increase the shrinkage. But this rate of shrinkage will decrease when it is replaced with the help of lime.
7. Size and Shape of the Specimens
The surface to the volume ratio is a factor that can influence the rate of shrinkage. The rate and the magnitude of shrinkage will decrease with the increase of surface to volume ratio.
8. Other Factors
The method of steam curing has little effect on the shrinkage. But it shows an effect when the carried out at high pressures.
Autogenous Shrinkage in Concrete
There is volume change happening even after the setting of the concrete structure. This volume may be in the form of shrinkage or in the form of swelling. When water is available it facilitates continued hydration.
This can create in the expansion of the concrete structure. But when there is no moisture content present to carry out this hydration, there is swelling in the concrete.
This type of shrinkage is hence a result of the withdrawal of water from the capillary pores that are present within the concrete. The hydration process is responsible for this water withdrawal.
The water is necessary for the hydration of hydrated cement. This process of water withdrawal from the capillary pores to carry out the hydration of hydrated cement is called as self-desiccation.
The shrinkage dealt with such conservative system can be named as autogenous shrinkage or autogenous volume change. This shrinkage is happening within the interior of the concrete member. The order of autogenous shrinkage is in the range of 100 x 10-6.
Factors Affecting Autogenous Shrinkage
With the increase in temperature the rate of autogenous shrinkage the autogenous shrinkage is related to the hydration process, the volume change will be higher.
2. Cement Content
More the cement content in the mix, higher will be the autogenous shrinkage. Irrespective of the amount of water that is present in the mix, the hydration process will be carried out to hydrate the unhydrated cement present.
More the cement content more will be the hydration. So, the shrinkage creating volume change will increase.
3. The composition of cement
Certain cement that has a high content of tricalcium aluminate) C3A and tetra calcium alumino ferrite (C4AF) will increase the autogenous shrinkage. These special compositions facilitate an increase of hydration product formation.
4. Mineral Admixtures
The addition of mineral admixtures will increase the hydration process and hence the autogenous shrinkage. For example, the mineral admixture like fly ash gain higher surface is to support more reaction and good quality hydration products.
This will demand for more water from the capillary pores. Hence the volume change in the concrete due to shrinkage is increased.
Carbonation Shrinkage in Concrete
The concrete cast has higher chances to react with the atmospheric gasses like carbon dioxide. This reaction is carried out in the presence of moisture. This will result in the formation of carbonates.
The calcium hydroxide in the concrete, which is a by product of hydration reaction will be found enormous in the concrete. This calcium hydroxide will react with the atmospheric carbon dioxide to give calcium carbonates. This will lead to the conversion of the concrete surface to get carbonated or acidic in nature. This process is called as carbonation.
This creates shrinkage that is observed on the surface. It is may be observed during the service period of the concrete structure. The carbonation shrinkage is observed in areas that have intermediate humidity conditions.
The carbonation process will result in the decomposition of some of the cement compounds. The carbonates that are formed by the carbonation will result in filling up of the pores and hence decrease the permeability.
As the permeability is decreased, the strength will increase. But when the shrinkage is restrained either partially or fully due to the internal or the external restraints, cracking is developed. These cracks are due to the tensile stresses produced due to the restraints.
Suitable joints can be provided in the structure during its casting for contraction and expansion movements. This kind of shrinkage will help to group the steel tightly which in turn helps to increase the bond.