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2w
shorter time; (b) due to shorter period of curing, production is increased and cost reduced as
also (c) storage space in the factory. The temperature can be raised by placing the concrete in
steam, hot water or by passing an electric current through the concrete. In the hydration
process of cement at higher temperatures, the released calcium hydroxide reacts with finely
divided silica, present in the coarse and fine aggregates and forms a strong and fairly insoluble
compound which results in higher strengths. Since free calcium hydroxide content is reduced,
the leaching and efflorescence are minimised. The hydrating dicalcium silicates and tricalcium
alluminates react together at high temperatures to form sulphate resisting compounds.
Consequently autoclaved products show higher resistance to sulphate attack. The initial drying
shrinkage and moisture movements are also considerably reduced. However, high-pressure
steam-curing reduces the bond strength by about 50 per cent.
The concrete members are heated by steam at 93 °C either at low pressure or high pressure.
In low pressure steam curing about 70 per cent of the 28 day compressive strength of concrete
can be obtained in about 16-24 hours. High pressure steam curing is usually applied to precast
concrete members and gives 28 day compressive strength at 24 hours. The effect of curing
temperature on compressive strength is shown in Fig. 10.12. It also results in increased resistance
to sulphate action and to freezing and thawing. The mixes with low water-cement ratio respond
more favourably to steam curing than those with higher water/cement ratio.
An early rise in temperature at the time of setting of concrete may be detrimental because the
green concrete may be too weak to resist the air pressure set up in the pores by the increased
temperature. The rate of increase or
decrease of temperature should not
exceed 10 to 20° C per hour to avoid
thermal shocks. The higher the wa-
ter/cement ratio of concrete, the
more adverse is the effect of an early
rise in temperature. Therefore, to
meet the requirement of compres-
sive strength of concrete, the tem-
perature and/or time required for
curing can be reduced by having a
lower water/cement ratio. While in
identical time cycle, the higher the
maximum temperature, greater is
the compressive strength. The ad-
vantages of curing above 70°C are
negated by dilational tendencies due
to the expansion of concrete.
Steam curing should be followed
by water curing for a period of at
least 7 days. This supplementary wet
curing is found to increase the later-
age strength of steam-cured concrete by 20 to 35 per cent. In most cases, steam curing is
employed only for achieving 50 to 70 per cent of specified strength in a short period instead of
full treatment for 2 to 3 days required to obtain specified strength.
p 2IH IP s"22g" 2"
2'2 '22g