Accelerating strength gain

Accelerating strength gain

Accelerating strength gain aims to create a favorable temperature for hydration to occur.

Steam curing
This method is useful in cold weather or when early strength gain is important. There are two methods of curing:

live (or low pressure) steam at atmospheric pressure, and
high-pressure steam curing in autoclaves.

Steam at atmospheric pressure:
This is used for enclosed cast-in-place structures and large precast units. In the precast concrete industry, steam curing allows increased production by a more rapid turnover of molds and formwork.

Maximum temperatures range from 104°to 212°F (40° to 100°C), with the optimum being in the 150°to 175°F (65°to 80°C) range. The higher the initial temperature of the curing environment, the lower the ultimate strength. This means that the temperature used will be a compromise between the rate of strength gain and the ultimate strength.

During curing, the concrete is taken through a curing cycle consisting of the following stages:

an initial delay prior to steaming
a period for increasing the temperature
a period for holding the maximum temperature constant
a period for cooling

With careful temperature selection, the strength of concrete after 3 days can exceed the 28-day strength of normally cured concrete. Besides the early strength gain, steam-cured concrete has reduced drying shrinkage and creep as compared to normally cured concrete.

High pressure steam:
This method is used for small manufactured units. The curing is done in autoclaves at temperatures in the 325°to 375°F range and pressures of 80 to 170 psi. This environment changes the chemistry of the hydration producing a concrete that has the following modified properties:

a 24 hour strength equal to 28-day strength under ambient curing,
less creep and shrinkage,
better sulfate resistance,
no efflorescence,
lower moisture content after curing.

Insulating blankets
When temperatures fall below freezing, layers of dry porous material such as straw may be used to insulate the concrete from the cold. Formwork can also be economically insulated with commercial blanket. Framed enclosures of canvas, polyethylene or other material can also be used and heated by space heaters or steam.

In these cases care needs to be taken to ensure that temperatures do not rise excessively.

Electrical, oil and infrared curing
These methods are used primarily in the precast concrete industry. Electrical methods include a variety of techniques such as using the concrete itself as a conductor, using the steel as the conductors, and heated steel forms. The last method is the most common at present.

Hot oil may also be circulated through steel forms to heat the concrete. Infra-red rays may also be used to heat concrete under a covering or steel forms.