Shrinkage in Concrete: Types, Causes, Effects & Design Considerations Explained

• It results from the volume change of concrete
• Like creep, shrinkage introduces time-dependent strain in concrete, unlike creep, as shrinkage strains are independent of applied stress.

Generally, there are five types of shrinkage

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

1) Chemical shrinkage:

Absolute volume of unhydrated materials > Absolute volume of hydrated materials

Due to chemical reaction i.e. hydration of cement
In the early stage → volume reduction
In the later stage → voids creation.

2) Plastic shrinkage:

Due to loss of moisture from the top surface, a short-term process

Cracks generally develop on the top surface

Shrinkage occurred before the concrete hardened.

3) Autogenous shrinkage:

• Volume reduction if no moisture transfer outside
• Mainly due to self self-desiccation of cement (extreme state of dryness)
• Resulting in a rise in capillary pressure
• Increases as the grade of concrete increases
• Generally occurs in the early days
• short-term process

4) Drying shrinkage:

• Contraction of the hardened concrete due to loss of water from the pores
• The most important type of shrinkage
• Decreases as the grade of concrete ↑
• Long-term process

5) Carbonation shrinkage:

As the Ca(OH)₂ from concrete ↓, its pH ↓ and chance of corrosion ↑

Major factors that affect carbonation shrinkage are permeability of concrete, moisture content, RH and amount of CO₂ in the air.

The rate of carbonation shrinkage is very slow.

Reaction:

$$\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3$$CO2​+H2​O→H2​CO3​

(atm. concrete)$$\text{H}_2\text{CO}_3 + \text{Ca(OH)}_2 \rightarrow \text{CaCO}_3 + 2\text{H}_2\text{O}$$H2​CO3​+Ca(OH)2​→CaCO3​+2H2​O

Effect of shrinkage:

(Diagram showing original and after-shrinkage member with tensile R.F. at bottom & compressive R.F. at top)

In an unrestrained RCC member, the shrinkage of concrete results in shortening of the member. However, the embedded R/F resist the shortening, and as a result, the compressive stress is developed in both compressive R/F and tensile R/F.

For symmetrically placed R/F (that is, equal compressive and tensile R/F), shrinkage does not result in any curvature of the member fora determinant structure.

However, in an indeterminant structure, shrinkage results in an overall change of geometry.

• In restrained RCC member Shrinkage develop Tensile Stress in concrete and compressive stress in both tensile and Compressive R/F.
• For unsymmetrically placed R/F (line in the flexural member) curvature results due to differential shrinkage.
• Both flexural curvature and shrinkage curvature are additive; hence the long-term deflection increases due to shrinkage.

(Diagram showing shrinkage curvature)

Factors affecting shrinkage:

• All factors related to material property, composition, mix, curing and environmental conditions.
• Member size and age that affect creep also affect shrinkage.
• Also shrinkage is reversible to great extent i.e. alternative volume (in dry–wet condition) will cause alternating volume change in concrete.

Shrinkage for design:

• For normal concrete, generally drying shrinkage dominates.
• Generally, half of the total shrinkage occurs in first month and 3/4 in first 6 months.
• In the absence of test data the absolute value of shrinkage strain for design shall be taken as 0.0003 (3 × 10⁻⁴).

Shrinkage in Concrete: Types, Causes, Effects & Design Considerations Explained

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