Why is Concrete Cured? (5 reasons)

This blog will answer the question “why concrete is cured”, and cover the topics on the chemical process involved in concrete curing, factors involved in the curing process, different methods for curing concrete and appropriate implementations of each, difference between curing and drying and results of properly cured cement. 

Why is concrete cured?

Concrete is cured because it needs to achieve high-compressive strength. This strength is primarily the reason why concrete is the basic foundation of construction. Mixing water, cement, and aggregates alone simply would not suffice the needed  strength to withstand extreme forces. Proper setting conditions or curing must also be achieved after concrete pouring in order for the material to sustain its robust strength. 

Hydration process in high-quality concrete

As mentioned in the previous paragraph, concrete is made by mixing water, cement, and aggregates together. Diving deeper in a molecular point of view, when water and cement interact, it triggers a reaction initiating the growth of crystals within the matrix binding the elements together, better known as the hydration process. It is the extent to which these crystals grow that defines the strength of concrete. Thus, it is necessary to maintain the optimum conditions during this setting time to produce high-quality products in a process called concrete curing.

Optimizing the foundation of a foundation

Since concrete is the foundation of construction, and hydration is the foundation for high-grade concrete. Simply define the curing process as optimizing the foundation of a foundation. High-compressive strength of concrete is mainly brought about by its curing process. It is the process of maintaining adequate conditions to ensure the right environment is achieved while the cement develops its essential properties.

Curing and Drying: What’s the difference?

Many often interchange the terms concrete curing and concrete drying . Concrete curing is linked to the hardening of the concrete while concrete drying refers to the removal of moisture content present in the concrete. Concrete curing comes first before concrete drying. The concrete must harden first before unwanted or excess moisture may be removed.

Curing of concrete usually takes a month for the hydration process to be at its maximum if proper conditions are met, however, drying of concrete may last several months depending on the environmental factors such as relative humidity and temperature. It is best to know the difference between the two so confusion is avoided. It is best to note that this blog mainly focuses on curing concrete.

Factors involved in the curing process

It was noted that the cement curing process must achieve certain conditions over a period of time. These conditions are satisfied by monitoring and controlling three main factors—moisture, temperature, and time. Observation and control of these necessary elements is done continuously until hydration is complete. To better understand, below are the importance of each during the curing process:

Moisture

Water added initially in the mixture is used up partially in the hydration reaction, and some are lost during natural evaporation as, normally,  concrete is exposed in the atmosphere. To compensate for the water loss, the concrete must be sustained with water until the swelling of crystals comes to its optimum growth creating a compact matrix, and, thus, increased concrete strength and durability.

Temperature

Temperature for cement curing must not be too hot nor too cold. As the newly-poured concrete starts to cool down, the rate of hydration eventually decreases and unfortunately comes to a stop. At temperatures below 50F, expect that the hydration process is very slow, and may come to a stop at 40F. Thus, too cold environment is not favourable for the setting time of concrete. On the other hand, if the concrete is too hot, then the rate of evaporation of water increases taking away the needed moisture for  the completion of the  process. With this, the ideal working range for cement curing is between 50 to 85F.

Time

Given that proper hydration is sustained and temperature is at operational working condition, the time at  which these sufficient conditions must be retained should also be considered. It is best to know that the curing process starts initially after the concrete is poured. After 24-48 hours, forms can be removed, the concrete has set to its shape. After 7 days, partial curing is only achieved, and the entire process lasts up to 28 days. The entire curing process may take a month in order for the compressive strength of concrete to be at its maximum. Figure below shows the compressive strength versus days cured of a moist concrete sample

Cement curing methods

In order to maintain the appropriate moisture and temperature requirements for the curing of cement. Here are some of the examples done over a period of time:

Spraying

From the word itself, mist is supplied over the concrete continuously to increase the humidity of the surface. This technique is suited for temperatures which are above freezing in a relatively low humid environment. Nozzles or pipes with holes are used to supply the water over the concrete.

Ponding

This method is suited for concrete floors or flat surfaces to supply water and maintain ambient temperatures. This is simply done by immersing the concrete unit in water. Just like spraying, this method ensures that there is adequate moisture during the completion of the curing process.

Wet covering

Spraying ang ponding requires time and attention. If these extra efforts cannot be met, another alternative is to cover the concrete with a material that will lock-in moisture and slow down the water’s evaporation rate. Rocks or slabs are placed on top of the covering, ensuring that the covering may not peel off or be accidentally removed throughout the course of the day.

Steam curing

The methods mentioned above are distinctly for environments above freezing. Hence, for cold environments, it is necessary that water molecules do not freeze-up so as to sustain the curing process. So in this method, live steam is introduced into the concrete unit to attain desirable working temperature and moisture contents. It must be noted that as mentioned in the previous paragraph, at temperatures below 40F, the curing process may stop.

Right time to cure concrete

Wondering when is the right time to start curing concrete? The answer is immediately. A new born individual is guided and taken care of as soon as it comes out of the mother’s womb until it is capable and independent. Similarly, concretes, after being poured, must be given adequate conditions instantly for the hydration process to foster until the concrete becomes durable and robust.

Results of properly cured cement

To those who have skipped the arduous curing process of cement, they may not able to reap from the benefits of it, and may be left with a material with unacceptable strength and poor cosmetic quality. But for those who continued with the process, below are the benefits of a properly-cured cement:

• Compressive strength is at maximum. As seen in the figure above, compressive strength is only at 40% for uncured concrete.
• Concrete durability is increased because porosity is reduced due to proper moisture supply. With this, thermal plastic shrinkage is avoided making the concrete impermeable to liquid chemical attacks.
• Concrete microstructure intertwined effectively. Since the hydration process progressed, crystal growth is at optimum reaching sufficient size for durability and high-strength.

Conclusion

This blog answered the question “Why is concrete cured?”, and discussed that concrete is cured to optimize properties such as concrete durability, compressive strength, and appearance. This blog also elaborated on the main process involved in curing, the difference between concrete curing and drying, the factors involved in curing—moisture, temperature, and time, the different curing methods, and the result of a properly cured cement. 

Frequently Asked Questions (FAQs):Why is concrete cured?

What is concrete curing?

Concrete curing is the process of maintaining adequate conditions such as moisture and temperature over a period of time. These factors affect the interaction of water and cement, known as the hydration process, in order to maximize the properties of concrete in terms of durability, strength, and cosmetic properties.

What happens if concrete is not cured? 

If concrete is not cured. The finished material will be powdery in texture with lots of cracks in terms of visual aspect, but most importantly the concrete will be weak and its durability will be decreased by multiple factors.

How long until concrete is fully cured?

After pouring, the curing process starts and may take up to 28 days for the curing process to complete. Completion of concrete may be tested by means of different methods.

How curing of concrete is done?

Curing of concrete is done by providing the sufficient conditions for the hydration process to occur. By providing the right moisture supply and temperature over a period of time, the properties of concrete may be maximized. Below are some of the widely-used curing methods:

• Spraying
• Ponding 
• Wet covering
• Steam curing

Bibliography

Balina, C. (N.D.)  Curing Concrete- Normal, Hot, and Cold Water. Retrieved from https://www.engr.psu.edu/ce/courses/ce584/concrete/library/construction/curing/curing.html on December 27, 2020

Basham, K., (2015) Curing Part I—Why Cure?, Concrete Contractor Magazine. Retrieved from  http://www.forconstructionpros.com/article/12151464/curing-part-i-why-cure.com on December 27, 2020

Hearns, A. (2019). Curing Techniques for Improving the Compressive Strength of Concrete. Retrieved from https://www.giatecscientific.com/education/curing-techniques-for-measuring-the-compressive-strength-of-concrete/ on December 27, 2020

Kosmatka, S. et al (2002). Design and Control of Concrete Mixtures. Steve Kosmatka et al,  Design and Control of Concrete Mixtures, 15th Edition, EB001, PCA Engineering Bulletin EB 001, Portland Cement Association , Skokie, IL 2002

Kulkarni, S.B. & Pereira, C. (2011). Significance of curing of concrete for durability of structures. Retrieved from https://www.nbmcw.com/tech-articles/concrete/25057-significance-of-curing-of-concrete-for-durability-of-structures.html on December 27, 2020

Mindess, S. & Young, J.F (1981). Concrete. Prentice-Hall, Inc., Englewood Cliffs, NJ, pp. 671.

Rothstein, D. (2017). The How’s and Why’s of Concrete Curing. Retrieved from https://www.forconstructionpros.com/concrete/equipment-products/technology-services/article/12169059/the-hows-and-whys-of-concrete-curing on December 27, 2020

Zemajtis, J. (N.D. ) Role of Concrete Curing. Retrieved from https://www.cement.org/learn/concrete-technology/concrete-construction/curing-in-construction on December 27, 2020