Shrinkage In Concrete
- Concrete undergoes volume changes as it changes phase from plastic to solid and this process is called Shrinkage.
- Shrinkage is usually expressed as a linear strain (mm/mm).
- Shrinkage and temperature effects are similar.
- Shrinkage and creep are not independent phenomena. But for convenience, it is normal practice to treat their effects as separate, independent, and additive.
- Unlike creep, shrinkage strains are independent of the stress condition of the concrete.
- Shrinkage is reversible to a great extent.
- The total shrinkage of concrete depends upon the constituents of concrete, size of the member, and environmental conditions etc.
- For a given humidity and temperature, the total shrinkage of concrete is most influenced by the total amount of water present in the concrete at the time of mixing and to a lesser extent by the cement content.
- Shrinkage has detrimental effects on the serviceability and durability of concrete. Shrinkage has been divided into five types as per different mechanisms.
Types of Shrinkage
- Chemical Shrinkage
- Autogenous Shrinkage
- Plastic Shrinkage
- Drying Shrinkage
- Carbonation Shrinkage
This is due to chemical reactions in concrete. As the absolute volume of hydration products is less than the total volume of unhydrated volume of cement and water before hydration, in the plastic phase the chemical shrinkage results in overall reduction of specimen volume and at a later stage it creates pores within the mix structure.
It is a volume reduction of the concrete with no moisture transfer with the outer environment. It does not include the volume change due to loss or ingress of substances, temperature variation, application of external force, and restraint. Autogenous shrinkage is mainly due to self-desiccation of cement resulting in rise in capillary suction pressure.
Sometimes chemical shrinkage and autogenous shrinkage are treated as same.
Chemical Shrinkage induces interval voids and autogenous shrinkage results in element shortening.
In this type of shrinkage, the cracks develop primarily in the top surface of the freshly laid (Plastic) concrete due to the volumetric contraction of the cement paste which is accelerated by loss of surface bleed water via evaporation or by absorption by aggregate or subgrade. During mixing, heat of hydration is generated and the exposed surface of concrete attain the ambient temperature faster than the inside region.
This include cracking on the top surface due to the shortening of the concrete as sufficient tensile strength has not been developed in the concrete. It is a short-term process.
It is the contraction of a hardened concrete due to loss of water from the concrete pores. It is similar to autogenous shrinkage as both occurs due to loss of water. However, in drying shrinkage the water is transferred to the outside and in autogenous shrinkage the water is transferred within the pore structure. Drying shrinkage is a long-term process.
It is the result of reaction between calcium hydroxide Ca(OH)2 present in the concrete and carbon dioxide in the atmosphere with the existence of moisture. Carbon dioxide (CO2) and moisture from carbonic acid (H2CO3),
H2CO3 + Ca(OH)2 = CaCO3 + 2H2O
The above reaction lowers the pH in concrete and results in corrosion of reinforcement.
The rate of carbonation shrinkage is low.
Carbonation reaction cause the rearrangement of the concrete microstructure, decrease the porosity, and create a differential volume change between the top surface and bulk concrete.
Carbonation of concrete also results in increased strength of concrete possibly because water released by carbonation promotes the process of hydration.
Methods to Reduce Shrinkage
- Moisten subgrade and forms to prevent absorption.
- Dampen dry aggregate.
- Use evaporation retarder.
- Use of spray to increase humidity.
- Prolonged curing.
- Providing reinforcement, fibres etc.
- Using shrinkage compensation concrete, also known as K-concrete.