Shrinkage of concrete - Plastic, Drying and Thermal shrinkage

 Shrinkage of concrete means volumetric change in concrete which affects the long term strength and durability of concrete. It is an inherent property of concrete. It appears in form of cracks.

  Shrinkage of concrete is classified in three category -

1. Plastic shrinkage
2. Drying shrinkage
3. Thermal shrinkage

Plastic shrinkage - This type of shrinkage occurs when concrete is in plastic stage or we can say it occurs just after the concrete is placed.

Plastic shrinkage takes place due to

• loss of water by evaporation or absorption by aggregate.
• poorly made concrete with high water - cement ratio.
• unintended vibration or yielding of formwork.
• extreme bleeding at the same time rapid drying.

Plastic shrinkage crack can be reduced by

• preventing the rapid loss of water on surface. This is done by covering the surface with polythene sheet after placing the concrete immediately.
• by working at night.
• by applying mono-molecular coating by fog-spray.

Drying shrinkage - After placing the concrete, it undergoes through drying conditions. And in this state, the shrinkage occurs is called drying shrinkage. It is an everlasting phenomena.

• Due to loss of free water contained in gel pores, a change in volume happens that results in drying shrinkage.
• It depends on constituents of concrete, size of member and environmental conditions.
• Concrete made with smaller size aggregate shrinks more than the concrete which made with larger size of aggregate.
• The magnitude of drying shrinkage is also a function of fineness of gel. Finer the gel, more the shrinkage.
• For design purpose, total shrinkage is taken as 0.0003.

Thermal shrinkage - Due to decrease in temperature, the concrete is subjected to volumetric change which is called thermal shrinkage. It mostly takes place in the summer season. In day time, roof slab or road pavement expandS due to high temperature and shrinks in the night with decrease in temperature. This process happens on daily basis which results in thermal shrinkage.

Factors affecting shrinkage cracks -

1. Intrinsic factors - water/cement ratio, degree of hydration, age of concrete paste, moisture content, admixtures and aggregate proportions and content.
2. Extrinsic factors - level of applied stresses, duration of load, age of loading, relative humidity and temperature.

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