Microwave-assisted accelerated curing of precast concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 84

Table 2.4 w/c Ratio (after microwave curing) versus the duration
of microwave curing

Curing duration

Microwave power
(watts) 0 min 15 min 30 min 60 min 90 min 120 min

150 0.44 0.416 0.342 0.290 0.222

300 0.44 0.404 0.351 0.215 0.110

This is consistent with the theory of hydration, which indicates that a w/c
ratio equal to or greater than 0.42 is required to ensure the availability of
sufficient free water to achieve complete hydration of the cement. Powers
et al . (1958) showed that, at w/c ratios of about 0.38, all the water will be
in the form of combined water and gel water, and no capillary water will
be available if the cement is fully hydrated [23,24]. This means that at
w/c ratios below 0.38, achieving full hydration is difficult. Therefore, fine-
tuning the microwave-curing process to prevent excessive heating is critical
to maintain the final w/c ratio of the mix at levels sufficient to have free
water available for hydration.

2.6 SUMMARY

Accelerated curing at elevated temperatures is an effective method to
increase the early strength development rate of concrete, which is highly
appealing to the precast concrete industry. However, conventional elevated-
temperature accelerated curing methods, including low-pressure steam cur-
ing, high-pressure steam curing, and conduction/convection heating-based
curing methods, have a number of major drawbacks that limit their wide-
spread application in the precast concrete industry. Conventional elevated-
temperature curing methods usually result in some nonuniformity in the
degree of hydration of cement throughout the concrete element because of
the inherent thermal insulating properties of concrete and the difficulty of
conventional heating sources in achieving uniform heating. Nonuniform
heating of concrete may result in the development of differential thermal
stresses that negatively affect the long-term properties of the concrete by
introducing new nano- and microcracks and accelerating the growth rate of
the existing cracks present in the concrete. In addition, another important
disadvantage of conventional accelerated curing methods is their relatively
long processing durations. Even the shortest conventional accelerated cur-
ing processes, such as steam curing, require about 10 hours to complete
before demoulding is possible.
Microwave curing is believed to have great potential to revolutionise the
curing of precast concrete. Microwave heating at lower ISM frequencies