Steam curing is beneficial where early strength gain in concrete is important or where additional heat is required to accomplish hydration, as in cold weather.
A Typical Cycle Consists of Following Phases:
- An initial delay prior to steaming
- A phase for increasing the temperature
- A phase for holding the maximum temperature constant
- A phase for decreasing temperature
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 at least 3 hours or delayed until it has get enough hardened after final placement of concrete to allow for some hardening of the concrete. However, 3 to 5 hour delay period prior to steaming will achieve maximum early strength.
This method of curing is often carried out for prefabricated concrete elements, specially prestressed concrete sleepers. Concrete sleepers are being introduced on the entire Indian Railway.
Large numbers of bridges are being built for infrastructural development in India. There are requirements for casting of innumerable precast prestressed girders. These girders are steam cured for faster development of strength which has many other associated advantages.
Application of steam curing on site construction will be 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 benefit is not really achieved.
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 result damage. It is recommended that the internal temperature of concrete not exceed 70°C to avoid heat induced delayed expansion and undue reduction in ultimate strength.
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 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 mixture and steam temperature in the enclosure.