2.2. Types of Shrinkage

• Plastic Shrinkage
  • Occurs within few hours after placing concrete.
  • Caused by rapid evaporation of surface water.
  • Results in cracking due to differential shrinkage.
  • Preventable by curing, windbreaks, fogging.

Plastic shrinkage is a type of shrinkage that occurs shortly after concrete is poured and before it has fully set. This phenomenon happens when water from the surface of the wet concrete evaporates quickly, particularly in hot and windy conditions. When this happens, the surface of the concrete begins to shrink while the inner parts are still wet and not contracting, leading to cracks. To prevent plastic shrinkage, methods such as curing the concrete, using windbreaks, or applying a fog spray can be employed to retain moisture.

Imagine trying to apply a thin layer of icing on a cake that is starting to dry out. If you don’t work quickly, the icing will start to crack as you apply it. Similarly, if concrete isn't protected from rapid evaporation, it can suffer from plastic shrinkage cracks, just as the icing suffers while attempting to set on a drying cake.

• Drying Shrinkage
  • Occurs when hardened concrete loses water from capillary pores to the environment.
  • Most significant type.
  • Can continue for months to years.
  • Heavily influenced by humidity, aggregate type, and curing.

Drying shrinkage refers to the gradual loss of moisture from hardened concrete, which leads to volume reduction. This type of shrinkage can continue for extended periods (months or even years) after the concrete has cured. The loss of water occurs mainly due to evaporation into the surrounding environment, and factors like ambient humidity, the types of aggregate used, and the effectiveness of curing all play critical roles in determining the extent of drying shrinkage. Lower humidity increases drying rates, resulting in more significant shrinkage.

Think of a wet sponge. As it sits out in the open air, it gradually dries out and shrinks as it loses moisture. In concrete, similar behavior is observed after it hardens. The more it 'breathes' moisture into the air, the more it shrinks, just like that sponge does as it dries.

• Autogenous Shrinkage
  • Volume reduction due to chemical reactions (hydration) in low water-cement ratio concretes.
  • Prominent in high-strength concrete (w/c < 0.4).
  • Can cause early-age cracking.

Autogenous shrinkage happens in situations where concrete has a low water-cement ratio. This type of shrinkage occurs due to the hydration reactions between cement particles and water altering the microstructure of the concrete. With less water available, the chemical reactions draw water from the paste and cause the paste to contract without any external drying influence. This is often a concern in high-strength concrete mixes, which may have a water-cement ratio below 0.4, leading to risks of early-age cracking.

Consider a cake that is made with very little liquid compared to the flour. As it bakes, the minimal water content evaporates or combines with the flour, causing the cake to shrink or sink in the center. In a similar manner, when concrete with a minimal water-cement ratio undergoes hydration, it can shrink internally, leading to cracks.

• Carbonation Shrinkage
  • Caused by reaction of CO₂ with calcium hydroxide in concrete to form calcium carbonate.
  • Leads to minor long-term shrinkage.
  • Prominent in exposed concrete surfaces.

Carbonation shrinkage occurs when carbon dioxide (CO₂) from the atmosphere reacts with calcium hydroxide present in hydrated cement paste, creating calcium carbonate. This reaction can lead to minor volume reduction in concrete over time. It's particularly significant in areas where concrete surfaces are continuously exposed to the environment, leading to potential long-term shrinkage concerns as the process progresses.

Think about how a piece of chalk dissolves slightly over time when left in an open area where it's exposed to carbon dioxide in the air. Just like chalk subtly changes because of environmental interactions, concrete can also change its structure over time due to the carbonation process, leading to shrinkage.

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