4.2. Types of Shrinkage

• Plastic Shrinkage:
  • Occurs within few hours of placing.
  • Caused by rapid evaporation of surface water.
  • Results in surface cracking.

Plastic shrinkage is a phenomenon that happens shortly after concrete is poured. In this stage, the surface of the concrete can lose moisture very quickly, especially in hot or windy conditions. When this moisture evaporates, it causes the concrete to shrink before it has fully set. As a result, cracks can form on the surface of the concrete. These cracks are often small and can appear randomly.

Imagine a wet sponge left out in the sun. Just as the water evaporates quickly from the sponge’s surface, causing it to shrink and possibly crack, freshly poured concrete can 'shrink' and crack when it loses moisture to the air too fast.

• Drying Shrinkage:
  • Most common type.
  • Due to moisture loss from hardened concrete.
  • Significant in the first few months after curing.

Drying shrinkage is the most frequent type of shrinkage in concrete. It occurs after the concrete has cured for a while, when moisture continues to escape from the hardened concrete. This moisture loss leads to a gradual reduction in volume, which can cause developing cracks as the concrete shrinks. This type of shrinkage predominantly affects concrete in the initial months after it has set.

Think of a freshly baked loaf of bread. As it cools, it shrinks slightly, pulling away from the edges of the pan. Similarly, after curing, concrete continues to lose moisture, which leads to gradual shrinking.

• Autogenous Shrinkage:
  • Results from internal chemical reactions.
  • Prominent in low water-cement ratio mixes.

Autogenous shrinkage is the shrinkage that occurs due to chemical processes within the concrete itself, even in the absence of moisture loss. This type of shrinkage is especially prominent in concrete mixes that contain a low water-to-cement ratio. Here, the hydration process consumes water, leading to a volume change as the internal structure of the concrete develops.

Consider a sponge that has a lot of dry material inside. As it absorbs water and expands, if it later releases that water, it will shrink internally. Autogenous shrinkage acts similarly, where the concrete's own hydration reactions cause it to decrease in volume.

• Carbonation Shrinkage:
  • Due to reaction between CO₂ and calcium hydroxide.
  • Occurs at the surface over long periods.

Carbonation shrinkage is related to a chemical reaction that occurs over time when carbon dioxide from the air reacts with calcium hydroxide present in concrete. This reaction diminishes the amount of calcium hydroxide and can lead to volume changes at the concrete's surface as it becomes denser. While carbonation is a slow process, its effects can accumulate over long periods.

Think of how apple slices turn brown when exposed to air. This browning represents a reaction with oxygen. Similarly, carbonation is a long-term reaction between concrete and carbon dioxide, leading to changes in the concrete's structure, particularly on the surface.

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