g PTQLow pressure steam curing at atmospheric pressure can be continuous or intermittent.
The maximum curing temperature is limited to 85 to 90°C. In the normal steam curing procedure,
it is advisable to start the steam a few hours after casting. A delay of two to six hours, called the
presteam or presetting period, depending upon the temperature of curing, is usual. The
presetting period helps to achieve a 15 to 30 per cent higher 24 hour strength than that obtained
when steam curing is resorted to immediately. The rate of initial temperature rise after presetting
period is of the order of 10 to 20°C per hour.
In the case of normal steam-curing at atmospheric pressure, the ultimate strength of concrete
may be adversely affected if the temperature is raised rapidly. This difficulty can be overcome
by employing the steam at a pressure of 8 atmospheres. The process is termed high-pressure
steam-curing. The increase in temperature allowed is up to 50°C in the first hour, up to 100°C
in second hour and up to 185°C in the third hour. The period of treatment under full pressure
depends upon the strength requirements. This period of treatment is 7 to 10 hours for hollow
block products and 8 to 10 hours for slab or beam elements the period may be increased with
the thickness of concrete.
Curing by Infra Red Radiation: A much more rapid gain of strength can be obtained with the
help of infra red radiation than even with steam curing. The rapid initial rise of temperature
does not affect the ultimate strength. It is particularly suitable for the manufacture of hollow
concrete products in which case the heaters are placed in the hollow spaces of the product. The
normal operative temperature is 90°C.
Electrical Curing: Concrete products can be cured by passing alternating current of low voltage
and high amperage through electrodes in the form of plates covering the entire area of two
opposite faces of concrete. Potential difference between 30 and 60 V is generally adopted.
Evaporation is prevented by using an impermeable rubber membrane on the top surface of the
concrete. By electrical curing, concrete can attain the normal 28-day strength in a period of 3
days. The technique is expensive.
Chemical Curing: Chemical membranes can be sprayed on to cure concrete. Liquid membrane-
forming curing compounds such as sodium silicate (water glass) solution retard or prevent
evaporation of moisture from concrete. They form a film, fill the pores, seal the surface voids
and prevent evaporation. The application should be made immediately after the concreting
has been finished. If there is any delay, the concrete should be kept moist until the membrane
is applied. Membrane curing compound should not be applied when there is free water on the
surface, because this water will be absorbed by the concrete and the membranes broken. Nor
should the compound be applied after the concrete has dried out since it will be absorbed into
the surface of the concrete and a continuous membrane will not be formed. The correct time to
apply the membrane is when the water sheet disappears from the surface of the finished
concrete. Adequate and uniform coverage of curing compounds is essential. In most cases two
applications are required. Chemical membranes are suitable not only for curing fresh concrete
but also for further curing of concrete after removal of forms or after initial moist curing.
p
Concrete is basically used because of its high compressive strength. However, the finish of the
ultimate product is not that pleasant. In past couple of decades efforts have been made to
develop surface finishes to give a better appearance to concrete surfaces and are as follows.