Civil Engineering Reference
In-Depth Information
properties of concrete, including durability properties, is also essential to
ensure the long-term durability of the concrete structure. As discussed in
the previous section, the strength and durability of concrete are affected
considerably by the degree and rate of hydration. A sufficient level of
hydration is required to reduce the porosity of concrete so that the desired
strength and durability of concrete are attainable. It is well known that
achieving a sufficient level of hydration requires the availability of an ade-
quate supply of water. The surface of concrete is particularly susceptible to
incomplete hydration because it dries faster than the interior. With this in
mind, curing processes were initially developed to ensure the availability of
sufficient water for the complete hydration of cement. The results of numer-
ous studies have shown that curing is a crucial stage in the concrete life
cycle to ensure that the optimal properties of concrete are developed. The
strength, durability, abrasion resistance, water tightness, volume stability,
and freeze-thaw resistance of concrete all increase with adequate curing.
Because of such significant effects, the curing of concrete is sometimes con-
sidered as a separately paid professional service in especial circumstances.
It is theoretically impossible to achieve fully complete hydration of all
cement present in the concrete. This is mainly because C-S-H produced dur-
ing hydration tends to cover some of the larger cement grains, preventing
further hydration reaction. With this in mind, the objective of conventional
standard curing is to achieve as much hydration as possible at a reasonable
cost. In theory, at w/c ratios equal to or greater than 0.42, there should be
sufficient water for achieving complete hydration of all the cement present
in the concrete. However, in practice, a considerable amount of the water
present in the concrete is evaporated or absorbed by aggregates or forms.
Hydration of cement does not require a fully saturated condition and may
occur at a relative humidity that is below 100%. This is because cement
can use the water held through surface tension in the larger capillary pores.
However, it is well known that the rate of hydration tends to be slower at
lower relative humidity and ceases when the relative humidity falls below
80%. The water present in concrete is first used for the localised hydra-
tion of the adjacent cement particles; thus, the areas with a faster hydration
rate tend to experience lower relative humidity with time. If the concrete
is maintained at a saturated condition by providing additional water, the
water could flow toward the areas with higher consumption rates. However,
when the concrete is not maintained at a saturated condition, the flowability
of water in the capillary pore system decreases with the gradual decrease in
the water content of capillary pores, leading to a considerable decrease in the
flow rate of water toward the areas in need of more water. Therefore, even
if a concrete is sealed against moisture loss, because of the reduction in the
relative humidity with hydration and the subsequent reduction in the ability
of the water present in the capillary pores to flow, the rate of hydration and
strength development will be slower than for a concrete that is continuously
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