Concrete shrinkage is inevitable under normal circumstances unless we use a special type of construction cement. Non-shrinkage concrete made with cement and which does not lose volume after pouring may be the only solution for this.
However, non-shrink concrete cannot be used in general construction because it is produced in small quantities and is expensive to use.
Therefore, we need to deal with the problem of concrete shrinkage during construction and take necessary measures to reduce the impact on the durability of concrete .
What is concrete shrinkage?
Shrinkage is the variation in volume over time, which leads to a reduction in the dimensions of the concrete.
The volume of concrete changes during the curing process due to the hydration of the cement and the concrete drying process, causing the concrete paste to lose water.
Volume reduction in a three-sided concrete structure occurs in the vertical direction by reducing the height of the concrete.
Types of shrinkage in concrete
There are essentially six types of concrete shrinkage. We list them.
- Plastic shrinkage
- Chemical shrinkage
- Autogenous shrinkage
- Dry shrinkage
- Thermal shrinkage
- Shrinkage due to carbonation
Plastic shrinkage
Plastic shrinkage occurs at the surface of fresh concrete due to evaporation from the surface or absorption from the concrete or bed.
It can also be observed whether the evaporation rate is greater than the bleeding rate in the concrete.
This very fast process takes place in the first few hours of Concreto .
During this process, the fresh concrete moves towards each other. Shrinkage in the surface layer impedes the movement of internal parts and leads to the development of tensile stresses in the fresh concrete.
If these tensile stresses are greater than the tensile strength of the fresh concrete, plastic cracks may occur.
These cracks would be about 1-2 mm wider and 300-500 mm long. The depth of the crack can be around 20-50 mm.
The main cause of plastic shrinkage cracks is the evaporation of water from the concrete surface. If this can be avoided, concrete shrinkage can be minimized.
The most important thing we can do is cure the concrete properly and in a timely manner. The article Concrete Hardening Methods For more information on curing, see.
In addition, it should be noted that Curing time to avoid plastic shrinkage of concrete.
Chemical shrinkage
A reduction in volume occurs in concrete when water chemically binds during the cement hydration process.
Furthermore, several chemical transformations occur in the cement paste during the hydration process. These processes reduce the volume of concrete and are called chemical shrinkage.
Autogenous shrinkage
Autogenous shrinkage is also called hydration shrinkage because it is directly related to the use of concrete irrigation water for the hydration reaction.
Furthermore, there is no clear distinction between autogenous shrinkage and chemical shrinkage.
If there is not enough moisture for the hydration reaction, the concrete will swell.
In this situation, water is removed from the capillary openings through the hydration reaction, resulting in concrete shrinkage.
The process of removing irrigation water from capillaries for hydration is called self-drying.
Dry shrinkage
Shrinkage of hardened concrete due to loss of capillary water in the concrete is called drying shrinkage of concrete.
Drying shrinkage occurs 3-4 days or even several months after concreting.
Thin components with a large surface area are susceptible to dry shrinkage cracks in concrete. Other reasons for shrinkage will be explained later in this article.
Drying shrinkage causes cracks in concrete when the movement of concrete is restricted by elements such as subgrade, foundation, parts of the structure, etc.
Let's discuss the factor that affects shrinkage during drying.
Factors Affecting Shrinkage During Drying
- Concrete mix design – excessive cement content, higher water content, etc. affect drying shrinkage.
- Add excess water to construction sites.
- Insufficient armor to absorb internal stresses due to internal limitations
- Insufficient concrete hardening.
- Lack of attention to controlling water evaporation from concrete surfaces
- No expansion/retraction joints. Gaskets must be provided in accordance with regulations. Joint spacing must comply with section requirements and be based on the desired reinforcement area.
- The geometry of concrete.
- Use of chemical and mineral additives. Chemical additives in small doses have no influence on the drying shrinkage of concrete. However, using a high dosage will have a greater impact on drying shrinkage.
- Insufficient concrete compaction can lead to concrete shrinkage.
Concrete shrinkage can be minimized if we consider the above factors when appropriate and necessary.
Given that cracking of this type has a direct impact on the Durability of concrete, it must receive maximum attention.
The following figure shows the variation in drying and autogenous shrinkage.
Thermal shrinkage
Thermal shrinkage is also known as thermal shrinkage and is a type of concrete shrinkage due to the thermal movement of the concrete.
The temperature of concrete fluctuates. When it goes up, it expands and when it goes down, it shrinks.
If this continues in the concrete and internal stresses arise in the concrete, the concrete will crack due to the stresses created within it.
Seasonal temperature fluctuations can cause concrete to crack due to thermal contraction of the concrete.
Additionally, it may be due to the increase in temperature during the hydration process.
Shrinkage due to carbonation
Due to the carbonation of concrete, carbonation shrinkage occurs.
Carbonation is a concrete decomposition process that leads to reinforcement corrosion. This happens because carbon dioxide is formed in the atmosphere, which damages hardened concrete.
During this process, the weight of the concrete increases while its volume decreases. This causes the concrete to shrink.
Let's discuss some of the important factors that affect all types of shrinkage.
Factors Affecting Concrete Shrinkage
Let's discuss the main factors that need to be controlled to avoid/minimize shrinkage.
- Lateral limits : Stress in concrete is due to foundations, parts of the structure, etc. This can be controlled by regular shrinkage joints in the concrete. Appropriate expansion joints also minimize concrete cracking due to reduced lateral loads.
- Relative humidity : It is a well-known fact regarding the relationship between relative humidity and evaporation.
- Elastic modulus of aggregates : The higher the aggregate modulus, the less possibility of cracking or shrinkage.
- Water-cement ratio : Cement content and water content directly affect shrinkage. Higher cement content results in greater shrinkage.
- The geometry of the concrete element
