What causes concrete shrinkage

Concrete shrinkage

·         The volumetric change of concrete structures due to the loss of capillary water by evaporation is known as concrete shrinkage or shrinkage of concrete.

·         It is a time dependent phenomenon which reduces the volumes of concrete without the impact of external forces.

Types of shrinkage

1.       Plastic shrinkage: plastic shrinkage occurs very soon after curing the concrete in the forms. The hydration of cement results in reduction to the volumes 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 known 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 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 in increased strength and permeability.

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

Factors Affecting shrinkage

i)                    Water-cement Ratio: It increase with the increase in water cement ratio.

ii)                  Relative Humidity: It is one of the major factor affecting shrinkage. It increases with decrease in the relative humidity. There will be no shrinkage if relative humidity is 100%.

iii)                 Time: The rate of shrinkage rapidly decreases with time but shrinkage strain increases 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.

iv)                 Type of aggregates: Aggregates with moisture movement and low elastic modulus cause large shrinkage. The rate of shrinkage generally decrease with the increase of the size of aggregates.

v)                  Admixtures: The shrinkage increases with the addition of accelerating admixtures due to the presence of calcium chloride (CaCl2) in it.

vi)                 Composition and fineness of cement: It have variable effect on the shrinkage of cement paste.

vii)               Strength of concrete: High strength of concrete have less shrinkage as compared to low strength concrete.

viii)             Method of curing: Shrinkage does depends upon the method of curing adopted different methods of curing have different rate of shrinkage.

ix)                 Dimension of the element casted


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