Concrete curing

If concrete curing is done correctly, the final product will be durable and reliable; otherwise, concrete will be weak. After pouring concrete, the appropriate temperature and moisture should be maintained for curing. Although curing is necessary for all kinds of concrete, the process requires engineering knowledge about and using building codes. Anyways, in this research, we discuss this process in detail.

Concrete curing definition

When it comes to curing, people simply think about maintaining moisture on the surface of the concrete. The main objective is to achieve the desired compressive strength rather than merely retaining moisture on the concrete surface. Concrete strength depends on the growth of crystals inside the concrete matrix. Crystals grow through the reaction between Portland cement and water, called hydration. If the water amount is not sufficient, crystals cannot grow, and concrete does not gain enough strength.

Another critical aspect of curing is temperature; concrete should not be too cold or too warm. By cooling fresh concrete, the hydration reaction becomes slow. The concrete temperature is critical here, not the air temperature. Hydration would be very slow under 50 F, but high temperatures increase the setting rapidity. Hydration causes a temperature difference in concrete due to its exothermic nature, which results in cracking. Cement with a quick reaction does not allow crystals enough time to form, so they lack strength.

Concrete curing objective

The reaction between cement and water is called hydration. It is an exothermic reaction (accompanied by the release of heat). After adding water to the concrete mixture, hydration begins, which causes concrete to dry faster due to an exothermic reaction. Concrete retains moisture to complete the hydration process and reach its maximum strength faster.

The main curing objectives are as follows:

• It maintains concrete moisture to keep its strength.
• It delays concrete shrinkage until the concrete is strong enough to resist cracking.
• Curing improves durability and strength against erosion.
• It is an essential process to enhance the compressive strength and stiffness of concrete.
• Concrete retains its functionality and durability.
• The microstructure of concrete improves by curing.
• It increases concrete strength against cracks, plant growth, chemical attacks, etc.
• It is essential for improving concrete’s desired properties, such as impenetrability, strength, durability, etc.

• It prevents concrete from drying prematurely due to wind, sun, etc.

Concrete curing time

The total time for the curing process is around one month. Each project has a different curing time due to variations in weather, concrete type, and concrete pouring methods. However, the following items should be taken into account:

• 24 to 48 hours: Molds can be removed after the primary setting.
• 7 days: The movement of people and devices is permitted after a partial recovery.
• 28 days: In this step, concrete reaches its ultimate strength.

Concrete pouring and curing are accompanied by bleeding water. The water will then reach the surface of the concrete due to aggregate sedimentation. Concrete bleeding depends on different factors, such as water-to-cement ratio, compaction method, thickness, etc.

After full evaporation, concrete drying begins. It is the appropriate time for the primary curing of concrete. Thus, primary curing minimizes the loss of surface water and prevents early drying. It prevents cracking and concrete downfall before and during the process.

Concrete curing time can be determined according to the moisture evaporation value. A longer curing time results in higher concrete strength. The main factors that affect curing time include:

• Concrete structure dimensions
• Concrete grade
• Concrete mix design
• Concrete properties
• Evaporation rate (wind intensity, sunlight, and air humidity)

The minimum curing time is according to the table below:

Minimum curing time to achieve half the strength of the concrete.

Curing is never stopped, and its strength grows over time. After 2 or 3 days of concrete pouring, walking is safe over it. After concrete pouring, concrete compressive strength is increased rapidly for 3 to 7 days and gradually for 3 future weeks.

Curing time is between 24 to 48 hours, which is a safe time for ordinary traffic. After one week, the concrete is ready to control the construction resumption, including traffic of heavy vehicles. Notably, concrete gains its full strength 28 days after pouring.

Concrete curing effective factors

• water-to-cement ratio
• Concrete additives
• Durability
• Size and form of the structural member (ceiling, column, or beam)
• Weather
• Wind speed
• Surface evaporation value
• Concrete mix temperature during the poring
• Environment temperature and moisture

The impact of incorrect curing

• Reduction of compressive and flexural strength of concrete
• Formation of the crack in the concrete surface
• Increasing the carbonation speed
• Reduction of durability due to increasing permeability
• Reduction of concrete strength properties against the air, chemical substances, plant growth, etc

How is the curing done?

Curing methods are as follows:

1- Shading

Using this method, water evaporation from the concrete surface, so to speak, is locked. It also protects the concrete surface against heat, wind, and other environmental conditions. In addition, it prevents concrete freezing by retaining the cement hydration heat in cold regions.

2- Covering the surface

This method avoids water waste and damage to concrete by covering the surface in wet nylons and sacks. It is the most popular and inexpensive method in concrete curing.

3- Sprinkling Water

In this method, water is sprinkled on concrete at proper distances using nozzles. This method has a short-term effect due to the difficulty of maintaining moisture in the concrete surface all the time and is not appropriate in terms of water usage.

4- Membrane Curing

In this method, the concrete surface is covered using anti-water membranes or sealing combinations, such as bitumen emulsion, wax, rubber latex emulsion, hydrophobic materials, plastic films, etc.

Conclusion

Curing is the process of protecting the concrete surface moisture and increasing cement hydration. The physical properties of concrete depend on the cement setting. If curing is not done properly, the concrete cannot gain its full strength. Incorrect curing can also lead to cracking and insufficient strength in concrete.