Microwave-assisted accelerated curing of precast concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 48

100

90

80
70

60

50

40
30

20

10
0

0 3 7 14 28

Duration of Moisture Curing (days)

Continuous

Figure 2.5 Effect of duration of water curing on the long-term strength of concrete.

moist cured. The provision of additional water during the curing process is
especially important in concretes with low w/c ratios as the hydration may
cease when the relative humidity falls below 80%. Provision of sufficient
water for thorough moist curing is sometimes difficult in concretes with
w/c ratios lower than 0.3 because of the low permeability of such concretes.
The strength development rate of concrete at ambient conditions var-
ies with the moist-curing scheme applied. The strength development rate
slows considerably when the moist-curing period ends. Moreover, continu-
ous moist curing considerably increases the 28-day strength as well as the
long-term strength development of the concrete. The ultimate compressive
strength of concrete also varies considerably with the duration of moist cur-
ing. Figure 2.5 shows that the long-term compressive strength of a concrete
that has not been moist cured can be as low as 40% of that of a continu-
ously moist-cured concrete. As shown, this can increase to about 85% and
95% by increasing the duration of moist curing from 3 days to 14 days
and 28 days, respectively.
The curing process also affects the durability of concrete considerably.
Two of the most important parameters influencing the durability of con-
crete are permeability and absorptivity, which are related to the porosity
of concrete and interconnectivity of capillary pores. Adequate water curing
to ensure continuation of hydration can result in partial or complete fill-
ing of some of the capillary pores with time, thereby reducing the perme-
ability of the concrete. A decrease in permeability can result in an increase
in the durability of concrete by preventing the ingress and transport of
harmful substances into the concrete. Figure 2.6 shows that the coefficient