Process control in microwave heating of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 171

Process control in microwave
heating of concrete
5.1 INTRODUCTION
A number of potential applications of microwave heating in the production,
demolition, and recycling of concrete were discussed in the previous chap-
ters. In Chapter 2, we observed that uniform microwave heating at a lower
industrial, scientific, and medical (ISM) microwave frequency range may be
used to accelerate the curing of concrete. However, we also observed that
the uniformity of microwave heating is a key factor affecting curing qual-
ity. Differential heating of concrete components as a result of the selection
of an inappropriate microwave frequency or inappropriate design of the
microwave applicator may result in the development of undesirable thermal
stresses within the cured concrete components. Such differential stresses
can negatively affect the mechanical properties and durability of the con-
crete components being cured by contributing to the growth of microcracks
present in concrete.
To achieve a uniform microwave heating pattern, besides choosing an
appropriate microwave frequency, the microwave-curing chamber should
also be properly designed. However, there are as yet no guidelines in the
selection of reliable design techniques to achieve uniformity of microwave
power applied across a typical microwave-curing chamber, and the few
designs that have been adopted are usually based on the engineer's or
practitioner's experience. Hence, a process control technique is required
to monitor and ensure uniformity of microwave heating throughout the
curing process.
In addition to the uniformity of heating requirement, an important
requirement to ensure optimal microwave curing of concrete components
is to maintain a level of heating that is optimal throughout the microwave-
curing process. It has been reported that overheating during curing may
negatively affect the long-term properties of the microwave-cured concrete
[1]. However, maintaining the concrete temperature at the desired range
during the microwave-curing process is difficult and requires continuous
monitoring and feedback control. This is because the temperature of the
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