Concrete is one of the most widely used materials in construction because of its strength, durability, and versatility. However, like all materials, concrete has its own limitations. One common issue with concrete is shrinkage. If not properly accounted for, shrinkage can lead to cracks, reducing the structure’s strength and lifespan. This blog explains what shrinkage of concrete is, the types of concrete shrinkage, and some practical tips to minimise its effects.

What Is Shrinkage of Concrete?

Shrinkage of concrete refers to the reduction in volume of a concrete element as it hardens and dries. This volume change happens due to loss of moisture from the concrete or changes in its internal structure. Since concrete is weak in tension, shrinkage often causes cracks if the material is restrained or unable to move freely.

Shrinkage is an important factor to consider in concrete design because cracks not only look bad but can also allow water and chemicals to penetrate, leading to corrosion of reinforcement and damage to the structure over time.

Why Does Concrete Shrink?

The main reasons behind concrete shrinkage are:

• Loss of water due to evaporation (drying shrinkage)
• Chemical changes during cement hydration
• Temperature changes causing contraction
• Settlement of solids in fresh concrete

Understanding these mechanisms helps engineers and contractors take preventive measures.

Types of Concrete Shrinkage

There are several types of concrete shrinkage, each with different causes and characteristics. Let’s look at them in detail:

1. Plastic Shrinkage

This occurs when concrete is still fresh (plastic state). It happens when water on the surface of freshly placed concrete evaporates faster than it can rise from below. As a result, small cracks appear on the surface, especially in hot, windy, or dry weather.

Key points:

• Happens within a few hours of placing concrete.
• Appears as shallow cracks on the surface.
• Can be minimised by proper curing and preventing rapid evaporation.

2. Drying Shrinkage

Drying shrinkage happens as hardened concrete loses moisture over time. Since the cement paste contracts as it dries, it pulls against aggregates, and if restrained, cracks develop. This is the most common type of concrete shrinkage.

Key points:

• Long-term shrinkage.
• Can continue for months or even years.
• Depends on water-cement ratio, aggregate type, and curing.

3. Autogenous Shrinkage

Autogenous shrinkage occurs in high-strength concrete with low water-cement ratios. In such mixes, most of the water is used up in hydration reactions, leaving less free water. This leads to self-drying and internal shrinkage.

Key points:

• Significant in high-performance concretes.
• Does not depend on external drying.
• Can cause internal micro-cracking.

4. Carbonation Shrinkage

This type occurs when hardened concrete reacts with carbon dioxide (CO₂) from the air. CO₂ combines with calcium hydroxide in the concrete to form calcium carbonate, which slightly reduces volume. While generally small, carbonation shrinkage can add to other shrinkage effects.

Key points:

• Slow process, happening over years.
• More in thin sections and exposed surfaces.
• Can reduce alkalinity and promote steel corrosion.

5. Thermal Shrinkage

Although not always classified as shrinkage, temperature changes can cause concrete to contract (shrink) when it cools. This is particularly important in mass concrete pours or in regions with large temperature fluctuations.

Key points:

• Caused by cooling of hot concrete.
• Can lead to thermal cracking if restrained.

How to Reduce Concrete Shrinkage

Here are some simple ways to minimise shrinkage and cracking:

• Use proper curing techniques to maintain moisture.
• Avoid excessive water in the mix (low water-cement ratio).
• Use shrinkage-reducing admixtures.
• Provide movement joints to allow expansion and contraction.
• Select proper aggregates with low shrinkage properties.

Careful mix design and site practices go a long way in controlling shrinkage-related problems.

Importance of Understanding Concrete Shrinkage

Knowing the types of concrete shrinkage helps engineers and builders plan better and ensure durable structures. For example, shrinkage is especially critical in large slabs, pavements, and walls where restraints are high. Designing with shrinkage in mind can save costly repairs in the future.

Conclusion

Concrete shrinkage is a natural phenomenon, but if left unchecked, it can lead to cracks and compromise structural durability. Understanding the types of shrinkage—plastic, drying, autogenous, carbonation, and thermal—helps in choosing the right materials and techniques to control it. Always aim for a proper mix, good curing, and thoughtful detailing to ensure your concrete performs well for years to come.

FAQs on Concrete Shrinkage

Q1. Why does concrete crack due to shrinkage?
Because concrete is strong in compression but weak in tension. When it shrinks but is restrained (by reinforcement or supports), tensile stresses build up, leading to cracks.

Q2. Which type of shrinkage is most common?
Drying shrinkage is the most common type as it occurs in almost all concrete over time.

Q3. Can shrinkage be eliminated completely?
No, it cannot be completely eliminated, but it can be significantly reduced through good mix design, curing, and construction practices.

Q4. Does higher water-cement ratio increase shrinkage?
Yes, mixes with high water content tend to shrink more because they lose more water during drying.

Q5. How does curing help control shrinkage?
Curing keeps the concrete moist, reducing water loss and allowing proper hydration, which reduces drying shrinkage and surface cracking.