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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