Shrinkage of concrete, types, factors affecting it

Shrinkage of concrete refers to the reduction in the volume of freshly placed concrete as it dries and hardens over time. This shrinkage is caused by the loss of moisture from the concrete, which leads to a decrease in the volume of the solid material.

Types of shrinkage of concrete

4 types of shrinkage occur in concrete as follows

1. Plastic shrinkage 
2. Drying shrinkage
3. Autogenous shrinkage
4. Carbonation shrinkage

Plastic shrinkage

This occurs when the concrete is still in a plastic state and is caused by the evaporation of water from the surface of the concrete. As the surface dries, it shrinks and can cause cracking.

Drying shrinkage

This occurs as the concrete continues to dry and harden over time. The moisture inside the concrete escapes through the capillaries and pores, causing the concrete to shrink. 

This can lead to cracking & other forms of damage.

Autogenous shrinkage

It refers to the self-induced shrinkage of concrete due to the chemical reaction that occurs during the hydration process. As water is consumed in the reaction, the concrete can shrink without the loss of moisture. 

This can lead to cracking and other forms of damage if the concrete is not properly reinforced or restrained.

Carbonation shrinkage

It occurs as a result of the reaction between carbon dioxide in the air and the hydrated cement paste in the concrete. This reaction causes the paste to lose some of its calcium hydroxide, which can lead to shrinkage and cracking. 

This type of shrinkage is generally more pronounced in concrete that is exposed to the atmosphere, such as in bridges, tunnels, and other outdoor structures.

Factors affecting shrinkage of concrete

• Mix Design: The proportion of cement, water, aggregates, and other additives in the concrete mix can affect the shrinkage of concrete. A higher proportion of cement and water can result in higher shrinkage.

• Water-Cement Ratio: The water-cement ratio is one of the most critical factors affecting shrinkage. A higher water-cement ratio leads to higher shrinkage.

• Cement Type: The type of cement used can also affect the shrinkage of concrete. Different types of cement have different chemical compositions that can influence shrinkage.

• Aggregate Type: The type of aggregate used can affect the shrinkage of concrete. Coarser aggregates tend to lead to higher shrinkage.

• Temperature and Humidity: The temperature and humidity during the curing process can affect the shrinkage of concrete. Higher temperatures and lower humidity levels can result in higher shrinkage.

• Concrete Age: The age of the concrete can affect its shrinkage. The longer the concrete has been curing, the lower the shrinkage.

• Reinforcement: The presence of reinforcement in the concrete can affect shrinkage. Reinforcement can reduce the overall shrinkage of the concrete.

• Admixtures: The use of admixtures in the concrete mix can affect the shrinkage. Some admixtures can increase or decrease the shrinkage of concrete.

• Finishing: The finishing of the concrete surface can affect shrinkage. A smoother surface can result in higher shrinkage.

• Formwork: The type and condition of the formwork used can affect the shrinkage of concrete. Properly maintained formwork can help reduce the overall shrinkage of concrete.

• Curing: The curing process can have a significant impact on the shrinkage of concrete. Proper curing techniques can help reduce the overall shrinkage of concrete.

Effects of shrinkage of concrete in strictures

1.  Cracking

Shrinkage can cause cracking in concrete structures, which can compromise their integrity and lead to water infiltration.

2. Reduced Durability

Cracking due to shrinkage can lead to a reduction in the durability of the structure, as water and other harmful substances can penetrate the cracks and cause further damage.

3. Loss of Bond

Shrinkage can cause a loss of bond between the concrete and any reinforcing steel or other elements within the structure.

4. Increased Deflection

Shrinkage-induced cracking can lead to increased deflection, which can lead to further cracking and potential failure of the structure.

5. Structural Instability 

In extreme cases, shrinkage can cause structural instability, leading to the partial or total collapse of the structure.

6. Curling

Shrinkage can cause curling of the slab edges, which can lead to tripping hazards and unevenness in flooring systems.

7. Differential Settlement

Differential shrinkage can cause uneven settling of the structure, which can lead to cracking and structural instability.

Prevent shrinkage of concrete

1. Use low shrinkage concrete mix design, with a lower water-cement ratio and a suitable amount of cementitious materials like pozzolans and slag.
2. Properly curing the concrete for an extended period of time using methods like water curing, curing compounds or membranes, and steam curing can help to reduce shrinkage.
3. Adding shrinkage-reducing admixtures to the concrete mix can significantly reduce the amount of shrinkage.
4. Avoiding the use of very dry or wet mixes can help reduce shrinkage.
5. Proper placement and consolidation of the concrete can help reduce the formation of voids, which can lead to increased shrinkage.
6. Limiting the size of the concrete pour can help reduce shrinkage, as larger pours are more prone to cracking.
7. Using reinforcement to limit the size of the cracks that may occur due to shrinkages, such as fiber reinforcement, welded wire reinforcement, or rebar.
8. Designing joints properly, with sufficient width and depth, and placing them in areas where shrinkage is likely to occur, can help limit cracking due to shrinkage.
9. Using formwork that is properly braced and secured to prevent movement during the concrete pour.
10. Designing with appropriate thickness and cross-sectional area of members can help reduce the impact of shrinkage on structural performance.

Also read: What is the difference between creep and shrinkage?