Why Keep Concrete Wet When Curing?

This article will answer the question “Why keep concrete wet when curing?” and will cover topics such as the definition of curing and the chemical reaction that happens during its process. The factors that affect the curing process as well as the methods and techniques to optimize curing results will also be tackled in this blog post.

Why keep concrete wet when curing?

It is important to keep concrete mixture wet during curing to avoid excessive water loss that could compromise the strength of the structure in the long run. When the surface of a concrete slab dries out during the process, it suffers from plastic shrinkage which causes excessive tensile stress to the matrix. If the stress surpasses the tensile strength of the concrete structure, the integrity of the structure is compromised from the formation of concrete cracks.

What is Curing?

Curing is defined as the process where concrete develops and attains structural strength. The process is normally mistaken for a simple drying phenomenon, despite the stark differences. Drying is the physical separation of appreciable amounts from a material due to mass and heat transfer while curing is a chemical reaction between concrete components to produce crystalline compounds that are strong and durable to support the structure’s strength. During curing, the drying process may occur especially on the surface of a fresh concrete that is widely exposed to the atmosphere. However, it is incorrect to assume that the hardening and the gaining of strength of the structure is attributed from the loss of water alone.

Curing as a Chemical Reaction

As mentioned, curing is a chemical reaction between water and cementitious materials in the concrete mixture. The reaction is technically referred to as a hydration reaction wherein water molecules are consumed in the process to produce crystalline strength-giving compounds.

Cement is not a pure substance but a mixture of several solid components that may react with water. Among its components, the most active silicate compounds in cement mixture are tricalcium silicate and dicalcium silicate. The tricalcium silicate assumes the first reaction and provides the initial strength of the structure approximately during the first week of the reaction. The reaction of tricalcium silicate with water results in the sequential formation of calcium hydroxide followed by calcium silicate hydrate which are both critical in triggering the formation of other crystals. At the latter part of the process where the condition is much favorable, the reaction of dicalcium silicate with water then proceeds.

Factors that Affect Concrete Curing

Generally, the environmental conditions that dictate the evaporative capacity of a concrete surface greatly affect the curing process. Water evaporates due to heat and mass transfer which are dependent on temperature, humidity, and air velocity.

Temperature

The temperature of the surrounding medium affects the amount of water that could escape the concrete surface through heat transfer. High temperature difference between the concrete surface and air implies the transfer of thermal energy from air to water. As water accepts heat, the activity of its molecules also increases, creating motions that trigger its escape and transformation from the liquid phase to gaseous phase. More and more molecules undergo phase change from the added heat, resulting in more water loss. Because water is required in curing, it is important to control this factor in order to avoid this moisture loss.

Humidity

Humidity is defined as the amount of moisture present in air. Highly humid environments such as those of tropical countries feel hot because the sweat could not naturally escape the body. This is exactly what happens on the surface of a concrete slab. When the humidity of the surrounding medium is high, it implies that it has no capacity to draw water molecules from the concrete surface. On the other hand, when the humidity is low, it requires more moisture in order to attain equilibrium. This can be achieved by transfer of moisture from the concrete surface to the surrounding medium.

Air Velocity

Air velocity could affect the transfer of moisture from the concrete to the surrounding air by the concept of mass transfer. As mentioned previously, the difference in the moisture content between air and the concrete surface allows the transfer of water in order to achieve equilibrium. Having high velocity air promotes this transfer by clearing the newly transferred moisture from the surface away from its destination, leaving the surrounding air still in need of more moisture. The cycle continues until the water loss in the concrete mixture affects the curing process.              

Curing Techniques

By establishing the need for the right amount of water during concrete curing, the techniques to be discussed next focus on either maintaining the moisture content within the matrix or reducing the water loss through evaporation. 

Spraying

Spraying is often associated with terms like fogging or misting which are all techniques that both replenish the lost moisture from evaporation and decrease the evaporation rate of the mixture. When the temperature of the surrounding environment is above the freezing point while the humidity is low, the application of tiny droplets of water by spraying or misting could increase the air humidity. As discussed previously, when the humidity of the surrounding air humidity is high, the condition is no longer favorable for the transfer of moisture from the concrete surface. This implies the reduction in water loss as well as the gradual addition of water from deliberate spraying.

Wet Cloth Coverings

Wet coverings are normally installed on the surface of a concrete structure that has already gained sufficient hardness in order to prevent significant surface damages. The materials used in this technique vary from cotton, rugs, burlap, and any other non-abrasive material that has enough absorptive capacity for water. The use of damp coverings in concrete slabs prevents the evaporation of the internal concrete water, as the surface exposed in the surrounding medium is the one evaporated first.

Membrane-forming Compounds

Membrane-forming compounds are usually composed of resins or wax that either undergo emulsification with water or dissolution with a solvent. The emulsified or dissolved compound is then applied onto the surface of a concrete structure. Its exposure with the surrounding air allows the evaporation of the solvent or water, leaving a layer of protective membrane atop the surface. This membrane prevents the moisture of the concrete from escaping the matrix, thereby maintaining the water content of the mixture necessary for the curing process.

Conclusion

This blog post answered the question “Why keep concrete wet when curing?” It was clearly explained that it is important for a curing concrete to be wet during the process in order to replenish the lost water from evaporation and chemical reaction. Dry concrete surface is not desirable as it induces high tensile stress within the matrix which could cause shrinkage cracks in the long run.

It was discussed in the article that curing is a chemical reaction between the components of the concrete mixture. The reaction is called a hydration reaction which takes up water to form sturdy crystalline compounds that supports concrete’s strength and stability. As mentioned in the blog post, the factors that affect the curing process are normally environmental conditions in the location such as air temperature, air velocity, and temperature.

Finally, the techniques to optimize the curing process were taught in the latter section of the article. It was mentioned that spraying, covering with wet cloth, and the application of membrane-forming compounds could maintain moisture in the matrix during curing.

For any questions and suggestions about this article, please feel free to submit your thoughts in the comment section below.

Frequently Asked Questions (FAQs): Why Keep Concrete Wet When Curing?

How long should fresh concrete be kept wet?

Normally, seven days is enough time to regularly replenish the vaporized water from a curing concrete. Although, it is important to understand that the duration varies depending on environmental conditions such as humidity and air temperature.

Why is my concrete always wet?

There are several reasons why your concrete is “always wet”, but the most logical reason for that is that the concrete mixture has an abnormally high amount of water content. Because of the density difference between concrete components, water naturally floats onto the surface and gradually evaporates into the atmosphere. Now, if the surface remains wet without any water replenishment, the water cement ratio is critically high which could compromise the integrity of the structure in the long run.

Does concrete ever stop curing?

In theory, concrete curing should not stop. It normally attains a logarithmic development for the entire curing process, wherein the initial days show remarkable gaining of strength which then tapers down to a slower rate as time goes by.

Do you have to wet concrete while curing?

Yes, it is important that a concrete slab attains a certain moisture content during curing. Curing involves a chemical reaction known as hydration which consumes the water content of a concrete mixture. The evaporation of water from the surface of the structure also contributes to the total water loss of the concrete mixture. Loss of water without proper replenishment normally results in reduced structural strength and being vulnerable to concrete cracks.

Does spraying water on concrete help on curing?

Yes, the gradual and regulated addition of water onto a curing concrete structure could help the process attain its optimized results. Water is essential during curing as it is consumed to form minerals that contribute to the overall strength of the structure.

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