Everybody knows what “Curing”
is. What exactly happens to concrete when it is not “cured” property?
Sketch below gives results of
insufficient curing
From the diagram it is seen
that generally when the curing period under water reduces and period in air
increase, the strength gain reduces. In particular,
- With first 14 days in water & remaining period in air the strength is 92 percent.
- With only first 7 days in water and remaining period in air, the strength reduces to 85 percent.
- With first 7 days in air and remaining period in water, the strength is even lower at 75 percent.
- With first 27 days in air and one day in water the strength is still lower at 59 percent.
Similarly, other combinations
can give varying strength levels. The above values are general only and based
on laboratory tests and can become even lesser in server weather conditions.
A concrete slab exposed to sun,
on normal day (350C) in dry weather (RH 50% approx.) with wind
blowing slowly, can loose about ONE litre of water per every square metre
surface area in only one hour. This is to be noted in appropriate steps taken
in such drastic cases.
The term ‘Curing’ means to
create conditions to promote the hydration of cement in the concrete, after
placement in forms. There are several methods of doing this and the commonest
of course is to pond the surface of concrete with water or cover with wet
hessian. Curing compounds can be advantageous in areas of water scarcify.
Like all chemical reactions,
temperature has an accelerating effect on hydration reaction. However, in
concrete technology when time strength relationships are referred to, it
assumes moist curing conditions and normal temperatures.
Accelerated curing is resorted
to, especially pre-cast products; this involves keeping the products in a
curing chamber where steam is introduced so that the temperature is gradually
increased to 750 C to 800 C and maintained for about 8
hours. Increase in temperature or subjecting to steam starts after a delay
period 4 hours or so and the rate of temperature rise is not more than 100C
per hour.
This steam curing regime can
result in attaining a concrete strength level of approximately seven days
normally cured strength. On the other hand, at low temperatures, say just above
50C, the concrete might gain strength only after several days.
Hence, special precautions are called for high strength and high performance
concrete which use supplementary cementitious materials, silica fume admixture
and are made with low water-cement ratio. Curing is critically important and
needs to be very clearly spell out and performed to achieve the desired
results.
The twin objective of curing
are to prevent the loss of moisture and to control the temperature of concrete
for a period sufficient to achieve the desired strength. Curing can be achieved
by spraying water or ponding or immersion; moisture-retaining materials offer
some cooling to the concrete through evaporation, which is beneficial to the
concrete especially in hot weather. It is possible to adopt moisture loss
prevention methods of curing through the means of curing compounds or
impervious sheets like polythene.
