How to Cure Concrete by Steam Curing at Atmospheric Pressure?
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Curing is the procedure used for accelerating the hydration of cement and consists of control of temperature and of the moisture movement from and into the concrete. In order to develop good concrete properties, the placing of an appropriate mix must be followed by curing in a suitable environment during the early stages of hardening. Suitable properties of concrete are achieved by changing the proportion of concrete ingredients, as well as by using the various curing methods, etc.
More specifically the function of curing is to prevent the loss of moisture from the concrete and to keep concrete saturated, or as nearly saturated as possible until the originally water-filled space in the fresh cement paste has been filled with the desired products of hydration of cement.
What is Steam Curing?
Steam curing is basically, curing in water vapour at atmospheric or higher pressures. It is used where early strength gain is required and where heat is needed for hydration, such as in cold weather. The primary reason for using steam curing for concrete is to produce a high early strength. Steam curing can be done at high pressure or at atmospheric pressure. We have already discussed the concrete steam curing at high pressure. Here, in this article we are going to focus on concrete steam curing at atmospheric pressure.
Difference Between Concrete Steam Curing at Ordinary Pressure VS High Pressure
In the steam curing of concrete at atmospheric pressure, the temperature of the steam is below 100°C. The steam will get converted into the water; hence it can also be called in a way as hot water curing. This is done in an open atmosphere. The high-pressure concrete steam curing is different from ordinary steam curing, here the curing is carried out in a closed chamber. The super-heated steam at high pressure about 80 to 170 psi (5.6 to 12 kg/cm2) and high temperature approximately 325°F to 375°F (160°C to 190°C) is applied on the concrete. This process is called “Autoclaving”.
In 41st Annual Meeting of the Highway Research Board, Washington, Richard R. Merritt and James W. Johnson (1962) stated that in their project HR-40 of the Iowa Highway Research Board the steam curing accelerated the gain in strength at early ages, but the uncontrolled use of steam may seriously affect the growth in strength at later age.
Now, let’s first understand the cycle of steam curing that is carried out at ordinary pressure.
Typical Cycle of Steam Curing
A typical cycle of steam curing consists of the following phases, and each phase has different timings that you can set.
01. An initial delay prior to steaming.
During this phase, hold the concrete at ambient temperature for approximately 180 minutes.
02. A phase for increasing the temperature.
In this phase, the curing temperature is increased up to 60°C (140°F) within approximately 120 minutes.
03. A phase for holding the maximum temperature constant.
During this phase, hold the concrete in maximum temperature (not more than 70°C (160°F)) for approximately 120 minutes..
04. A phase for decreasing temperature.
In this phase, cool the concrete for approximately 120 minutes.
These are the phases that are carried out step by step during the steam curing of concrete.
Let’s understand the process in more details.
Steam Curing of Concrete at Ordinary/Atmospheric Pressure
Steam curing at ordinary pressure is generally done in an enclosure to minimize heat losses and moisture. Tarpaulins are frequently used to form the enclosed space.
Application of steam to the enclosure should be delayed for at least 3 hours or delayed until it has sufficiently hardened after final placement of concrete. 3 to 5 hours prior to steaming will achieve maximum early strength.
This method of curing is often carried out for precast concrete elements, specially prestressed concrete sleepers which are being introduced in the entire Indian Railways.
Large numbers of bridges are being built for infrastructural development. There are requirements for the casting of precast prestressed girders. These girders are steam cured for faster development of strength which has many other associated advantages.
Steam curing on the construction site is a little difficult task. However, at some places, it has been tried for on-site construction by forming a steam jacket with the help of tarpaulin or thick polyethylene sheets. However, this method of application of steam for in situ work is found to be wasteful, and the intended rate of development of strength and subsequent benefits are not really achieved.
The steam temperature in the enclosure should be kept at about 60°C (140°F) until the desired concrete strength has developed. Strength will not increase significantly if the maximum steam temperature is raised from 60°C to 70°C (140°F to 160°F). Steam-curing temperatures above 70°C (160°F) should be avoided. They are uneconomical and may damage. It is recommended that the internal temperature of concrete should not exceed 70°C to avoid heat-induced delayed expansion and undue reduction in ultimate strength.
We have also try to explain test to check compressive strength of concrete.
As per Türkel, Selcuk & Alabas, Volkan, (Published in: The effect of excessive steam curing on Portland composite cement concrete, 2005), The type of Cement, as well as curing period and temperature, are essential parameters in the steam-curing process.
Hence, excessive rates of heating and cooling should be avoided to prevent damaging volume changes. Temperatures in the enclosure surrounding the concrete should not be increased or decreased more than 22°C to 33°C (70°F to 90°F) per hour depending upon the size and shape of the concrete element.
The curing temperature in the enclosure should be maintained until the concrete has reached the desired strength. The time required will depend on the concrete and steam temperature in the enclosure.
To sum up, adequate curing of the concrete during its initial days after placing is essential in order to achieve its strength including other qualities such as durability, volume stability, water-tightness and abrasion resistance. Proper curing also prevents the concrete surface from cracking, caused by insufficient binding due to the non-availability of water. Steam curing at atmospheric pressure is carried out in cold weather and when early high strength is required.
We have also written articles on various methods of curing, have a look,