Steam curing of concrete is the method of curing the concrete elements in water vapor. This curing is performed either under atmospheric pressure or high-pressure steam. The steam curing method is preferred where there is a need for accelerated strength gain and the concrete requires additional heat to perform hydration. This is especially performed in regions where the temperature is low.
Some of the important features of steam curing, methods along with its pros and cons are explained briefly in this article.
Need for Steam Curing
The main reason to opt steam curing is to obtain high early strength. High early strength is very much desirable for the manufacture of precast and prestressed concrete units. These units demand forms and stress beds which is very costly.
Fast removal of concrete forms and moving the finished units to the storage yards is only possible by early strength gain of concrete units. The proper use of steam curing meets this purpose as it accelerates the early gain of strength.
Uncontrolled use of steam curing can result in excessive volumetric changes that can affect the strength gain in the early stages.
Methods of Steam Curing
Steam Curing can be performed by two methods:
- Steam at Atmospheric Pressure
- High-pressure steam curing in autoclaves
1. Steam Curing of Concrete at Atmospheric Pressure
The steam curing process is performed within a temperature range of 70 to 90F per hour which is merely dependent on the size and the shape of the concrete element. The method is employed for the manufacture of large precast units and for enclosed cast-in-place structures.
Phases of Steam Curing of Concrete
The four phases employed during the steam curing of concrete are:
1. Initial Delay before Starting the Steaming Process: In this phase, the concrete is held under a temperature maintained for 180 minutes.
2. An Increasing Temperature Process: In this phase, the curing temperature is increased up to 60 degree Celsius with an approximate time period of 120 minutes.
3. Maintaining a Maximum Temperature: In this phase, the concrete is held at a maximum curing temperature not more than 70 degrees Celsius and it is maintained for 120 minutes.
4. Decreasing the Temperature: In this phase, the concrete elements are allowed to cool for a time period of 120 minutes.
Steam Curing Procedure
The steam curing of concrete is performed in an enclosed chamber in order to avoid any sort of heat loss and moisture. The enclosed space is normally made of tarpaulins.
1. The properly hardened concrete is applied to the steam enclosure after a delay period of 3 hours. It is not exposed directly to the steam just after the hardening.
2. Once placed in the steam enclosure, it is kept for 3 to 5 hours. Normally, this time range would make concrete gain its maximum early strength.
Control of Steam Curing
The start to the end of the steam curing process must be followed with utmost control. In order to gain maximum control over the whole curing process, necessary limits have to be provided to the following variables:
- The time duration of steam curing
- The steam curing temperature
- The time between the concrete mixing and the start of steam curing. This is called delay time
- The maximum rate of temperature increase
- The maximum rate of temperature decrease
Also Read: Curing of Cement Concrete-Time and Duration
The concrete cured by this method gains:
- Less drying shrinkage value
- Lesser Creep
- Strength at 3 days after curing exceeds the 28th-day strength
2. Steam Curing of Concrete by High Pressure
The method is used for small manufactured units. This curing process is performed with autoclaves at a temperature range of 3250F to 3750F and a pressure range of 80 to 170psi.
The above-mentioned temperature and pressure-controlled environment change the chemistry of the hydration process of the concrete. The concrete produced by this method ought to have:
- No efflorescence
- Low moisture content after the curing process
- Better sulfate resistance
- Less creep and shrinkage
- The strength gained at 24 hours is equal to the 28th-day strength under ambient curing conditions.
Advantages of Steam Curing
The main advantages of steam curing are:
- Best solution for curing concrete during cold weather conditions that facilitates rapid hardening of concrete.
- High early age strength can be achieved.
- Construction Speed is increased.
- Steam curing is fast compared to other curing methods.
Disadvantages of Steam Curing
The main disadvantages of steam curing are:
- The method does not work efficiently for large surface.
- The process must be carried out only by skilled labors.
- The initial cost of steam curing process is very high.