Curing is a critical process that ensures the concrete’s durability, strength, and longevity. It helps to accelerate the hydration of cement by controlling moisture and temperature. There are different concrete curing methods, and steam curing is one of them. Steam curing is an excellent way to push the concrete to reach its maximum potential faster. Primarily, curing aims to prevent moisture loss from concrete and keep it saturated with water to aid the hydration process. Steam curing at atmospheric pressure makes curing the concrete with water vapor.

What is Steam Curing?

Steam curing is a process of concrete curing that uses water vapor at atmospheric or higher pressure. The steam chamber temperature reaches between 40 – 70 degrees Celsius at atmospheric pressure. The temperature varies due to the size and shape of concrete precast elements. This method provides excellent benefits in manufacturing large precast units and enclosed cast-in-place structures. Steam curing can accelerate the strength gain process of concrete by increasing the temperature and providing moisture simultaneously. Steam curing is a standard process used in producing precast concrete products and elements for high productivity. The leading reason steam curing is used is its ability to provide high early strength in concrete. It is used where high early strength is necessary, and heat is essential for hydration, like in colder climates.

In prestressing and precast plants, steam curing methods can provide early strength without demerits. It enables the usage of rapid demolding and reuse of forms. There are two types of steam curing:

1. Steam curing at atmospheric pressure
2. Steam curing at high pressure

Difference Between Steam Curing at High Pressure and Atmospheric Pressure

Steam curing at atmospheric pressure uses steam that is below 100 degrees Celsius. This method can also be labeled as hot water curing. Furthermore, this method is performed under an open atmosphere. On the other hand, steam curing at high pressure is quite different. High-pressure steam curing is performed in closed chambers. Superheated steam is used at pressures reaching 80 – 170 psi and between 160 – 190 degrees Celsius. This method of concrete curing is commonly known as “Autoclaving.”

Steam Curing Phases of Concrete at Atmospheric Pressure

Steam curing at atmospheric pressure requires the concrete to be prepared in four phases. These phases allow the process to be carried out in a standard and effective method.

1. Delay before Starting the Steam Curing Process: Concrete is placed under high temperature for 180 minutes in this phase. It creates a good environment in the concrete for hydration to occur.
2. Increasing Temperature Phase: The steam curing begins at this phase. Curing temperature is increased to 60 degrees Celsius by maintaining 120 minutes.
3. Maximum Temperature Phase: In this phase, the steam curing process reaches its maximum temperature, typically more than 70 degrees Celsius, and is maintained for around 120 minutes.
4. Declining Temperature Phase: In the last phase, concrete products are allowed to cool down for a time of 120 minutes.

Source: The Constructor

How the Concrete is Cured by Steam at Atmospheric Pressure?

The steam curing method requires an enclosed space to avoid moisture and heat loss. This makes the whole process more efficient. Generally, water repellant membranes like tarpaulins form the enclosed space.

Before applying steam, the concrete must be hardened enough to sustain the vapor. It should be delayed for at least three hours or as long as the concrete is required to harden after its final placement. After placing the concrete elements in the steam enclosure, these should remain there for three to five hours. This time range makes sure the concrete reaches its desired high early strength.

The steam curing process of concrete is sophisticated, where all variables play a crucial part. Ignoring one factor might bring negative results for the longevity and durability of the concrete. Control plays a big part in steam curing concrete. The following variables should be carefully examined to avoid any shortcomings.

• The period of steam curing
• Steam curing temperature
• Delay period of the time between concrete mixing and the starting of the curing process
• The rate at the temperature increases in the enclosure
• The maximum rate of temperature decrease

Usability of Steam Curing

Precast concrete elements are highly benefited by steam curing at atmospheric pressure. Modern structures are primarily dependent on concrete elements. This method is very useful in developing countries where large infrastructural projects are ongoing. Bridges require precast prestressed girders. These girders are cured at steam for faster development of strength and durability. Furthermore, large construction sites require a lot of precast blocks that can achieve strength faster than traditional curing methods that require 28 days. So, steam curing is highly useful in these scenarios.

However, steam curing on the construction site is difficult to execute. But at some construction sites, it can be executed by forming a steam jacket by trapping the vapor with tarpaulin or thick polythene sheets. But these methods are sometimes costly and do not show effective results as steam can escape making the concrete drier than expected.

The steam curing method is beneficial in precast elements, which can be stored in enclosures, ensuring steam cannot escape. At atmospheric pressures, raising the temperature by more than 70 degrees is not recommended as it does not help with the strength development of concrete. Raising temperature more than the limit may not be economically viable, and also it can induce damage in the concrete if not appropriately supervised. If the internal temperature of concrete exceeds 70 degrees, it can introduce delayed expansion and reduce its ultimate strength gain. The type of cement used to manufacture concrete also plays a vital role in the ultimate strength gain process of concrete. So, choosing the right cement aids in the steam curing of concrete.

One of the most important things to avoid during steam curing is excessive heating and cooling rates. This can introduce a weakened bond on the concrete. The temperature in the enclosure should always stay in the recommended range; otherwise, the concrete can get damaged. The range might fluctuate due to the shape and size of the concrete element.

Advantages of Steam Curing Process

The key advantages of using the steam curing process are:

• Steam curing provides the best solution in colder climates as it can facilitate the rapid hardening of concrete without sacrificing any strength.
• High-early strength can be achieved.
• Construction pace can be increased.
• Steam curing provides one of the fastest strength developments in concrete.

So, time-sensitive construction projects are highly benefited from using the steam curing method.

Disadvantages of Steam Curing Process

A few disadvantages of using the process are:

• This method is not suitable for on-site curing methods.
• Not effective on large surfaces.
• The process demands skilled workers.
• The initial cost of steam curing is higher compared to other methods.

 Summary

Steam curing is a highly effective method but costly. It is beneficial when dealing with concrete precast elements. Adequate curing after placing concrete is a vital requirement, as it can bring high strength, durability, volume stability, water-seal, and abrasion resistance. Adequately cured concrete does not crack under extreme weather conditions. Steam curing at atmospheric pressure is a suitable curing method in which high early strength is required and colder climates.

Frequently Asked Questions (FAQs)

What is the minimum curing period?

Ans: To get maximum strength, 28 days of curing is recommended.

When is steam curing preferred over other methods?

Ans: Steam curing is preferred in colder climates and requires high early strength.

Why is an enclosure required for atmospheric pressure steam curing?

Ans: An enclosure ensures minimum moisture loss, and the whole process becomes much more efficient.

What is the fastest way to cure concrete?

Ans: Steam curing at high pressures can yield the best results in gaining high early strength.

Can I use steam curing for building construction?

Ans: It is not an efficient process for on-site curing. Concrete precast elements are usually steam cured.