Process control in microwave heating of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 172

concrete being microwave heated at a particular microwave frequency and
power is controlled by the size of the concrete component and the dielec-
tric properties of the concrete present. The size and dielectric properties of
concrete components fabricated in a concrete prefabrication plant may vary
considerably depending on the type and number of precast concrete compo-
nents produced at any particular time. In addition, the dielectric properties
of the concrete may vary considerably during the microwave-curing process
with the changes in the concrete's water content. Unless the components are
sealed completely, the water content of the concrete being microwave cured
is likely to reduce with time as the available free water present in the con-
crete evaporates as a result of the microwave heating.
With this in mind, continuous ine-tuning and feedback adjustments
of microwave power may be required to avoid localised overheating and
underheating throughout the concrete mass or volume during the micro-
wave-curing process. Therefore, a monitoring and feedback control mecha-
nism to ensure the essential optimal level and pattern of heating throughout
the curing process is necessary to guarantee the quality of the microwave-
cured precast concrete products. Undoubtedly, the simplest and most logi-
cal way to control a heating process is via temperature monitoring. Leung
and Pheeraphan showed that an optimal microwave-curing regime that
provides high strength at both the early and later stages can be achieved
with the help of a temperature control system [2]. Figure 5.1 illustrates how
temperature monitoring can be used as a feedback control mechanism for
the accelerated microwave curing of concrete.
In addition, in Chapter 3, we observed that in microwave-assisted selec-
tive demolition of concrete, the thermal stresses and pore pressure devel-
oped in the concrete component may vary as a function of temperature and
its gradient. This suggests that monitoring the temperature of concrete may
serve as a simple means to monitor the thermal stresses and pore pressure
developed within the concrete and that temperature feedback control may
be used effectively in microwave-assisted selective demolition of concrete.
The surface temperature of the concrete component being heated using a
microwave-assisted selective demolition tool may be used to predict the
heating duration needed to achieve the desired removal depth. Accordingly,
based on real-time feedback, the microwave power can be adjusted contin-
uously to minimise the differences between the actual and planned removal
depths and rates. Figure 5.2 illustrates how temperature monitoring can
be used as a feedback control mechanism for microwave-assisted selective
concrete demolition systems.
In Chapter 4, a microwave separation method for reducing the mortar
content of recycled concrete aggregates (RCAs) through partial or complete
removal of the mortar adhering to the RCA particles was introduced. We
observed that the separation of mortar is achieved through development of
relatively high differential thermal stresses in the adhering mortar and its