CONCRETE SHRINKAGE AND ITS TYPES

Concrete Shrinkage

The volumetric changes of concrete structures due to the loss of moisture by evaporation is known as concrete shrinkage.Concrete shrinkage is the change in length per unit length and is, therefore  a dimensional number expressed as percent.It is a time dependent deformation which reduces the volume of concrete without the impact of external forces and is usually quantified in term of micro strain which is equal to 1x10^-6 in/in or 1x10^-6 m/m.

Types of Concrete Shrinkage

1. Plastic Shrinkage
2. Drying Shrinkage
3. Carbonation Shrinkage
4. Autogenous Shrinkage

1. Plastic Shrinkage

Plastic shrinkage occur very soon after pouring the concrete in the forms.The hydration of cement result in a reduction in the volume of concrete due to evaporation from the surface of concrete, which leads to cracking.

2. Drying Shrinkage

The shrinkage that appears after the setting and hardening of the concrete mixture due to loss of capillary water is know as drying shrinkage.Drying shrinkage generally occurs in the first few months and decreases with time.

3. Carbonation Shrinkage

Carbonation shrinkage occurs due to the reaction of carbon dioxide (CO2) with the hydrated cement minerals, carbonating Ca(oh)2 to CaCo3.The carbonation slowly penetrates the outer surface of the concrete.This type of shrinkage mainly occurs at medium humidity and results increased strength and reduced permeability.

4. Autogenous Shrinkage

Autogenous shrinkage occurs due to no moisture movement from concrete paste under constant temperature.It is a minor problem of concrete and can be ignored.

Factors Affecting Shrinkage

The shrinkage of concrete depends on several factors which are

1. Water-Cement Ratio

Shrinkage is mostly influenced by the water-cement ratio of concrete.It increases with the increases in the water-cement ratio.

2. Environmental Condition

It is one of the major factor that affect the total volume of shrinkage.Shrinkage is mostly occurred due to the drying condition of the atmosphere.It increases with the decrease in the humidity.

3. Time 

The rate of shrinkage rapidly decreases with time.It is found that 14-34% of the 20 years shrinkage occurs in two weeks, 40-80% shrinkage occurs in three months and the rest 66-85% shrinkage occurs in one year.

4. Types of Aggregate

Aggregate with moisture movement and low elastic modulus cause large shrinkage.The rate of shrinkage generally decreases with the increase of the size of aggregates.It is found that concrete made from sandstone shrinks twice than the concrete of limestone.

5. Admixtures

The shrinkage increases with the addition of accelerating admixtures due to the presence of calcium chloride (CaCl2) in it and it can be reduced by lime replacement.  

Other Factors

• The method of curing.
• The strength of concrete.
• The dimension of elements
• The type and quantity of cement.
• Granular and microbiological composition of aggregate.