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strength development of various mixes with different compositions and w/c
ratios prior to microwave curing of actual precast concrete components.
2.5.2.4 Effect of microwave-curing duration
Identifying the optimal duration of microwave curing to achieve the desired
strength development rate is crucial to prevent overheating. Excessive micro-
wave curing can negatively affect the long-term properties of the concrete
and result in unnecessarily high energy consumption and associated emis-
sions. Figure 2.34 compares the compressive strength development curve
of the concrete specimens microwave-cured for 15 minutes and 30 minutes
at similar constant microwave powers. As shown, not only does excessive
microwave curing not result in any improvement in early concrete strength
development rate, but it may also negatively affect the long-term strength of
the microwave-cured samples. This is probably because an excessively long
microwave-curing duration leads to a lack of free water available for hydra-
tion and an increase in capillary voids. The reduction in the w/c ratio of
mortar specimens subjected to microwave heating as reported by Wu et al
.
(1987) is shown in Table 2.4.
Based on the results summarised in this table and Figures 2.21 and 2.22,
one may conclude that the compressive strength of mortar (with an initial
w/c ratio of 0.5) decreases when its final w/c ratio decreases to less than 0.40.
450
425
400
375
350
325
300
275
Microwave curing for 15 mins
Microwave curing for 30 mins
250
225
200
1
10
Age (days)
100
Figure 2.34
Effect of extended microwave curing on the compressive strength of cement
paste. (From Rattanadecho, P., Suwannapum, N., et al., Development of
compressive strength of cement paste under accelerated curing by using a
continuous microwave thermal processor.
Materials Science and Engineering
,
2008,
427
:299-307. With permission.)
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