Process control in microwave heating of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 184

e Metallic Splicer
Bulb Creation next to
the Metallic Splicer
Figure 5.9 Combustion of the plastic coating next to the metallic splicer after micro-
wave heating.
when selecting FBG sensors for temperature measurements in the pres-
ence of a microwave field. The coated optical fibres usually used for strain
and temperature measurements in civil engineering applications are gener-
ally not suitable for temperature monitoring in a microwave field. This is
because microwaves may heat the coating to different extents compared to
the concrete specimen, depending on the difference between the dielectric
properties of the coating material and concrete.
In summary, to select an appropriate FBG type for use in microwave-
assisted concrete processing applications, the following two important
points should be taken into account:
1. The FBG sensor should not contain any metallic components,
whether in the form of fibre splicers, within the cover, or as part of
the sensor's structure.
2. The covering on the fibre should be derived from microwave-
transparent materials. Bare fibres are suitable but should be handled
with care because of their fragile nature.
After selection of the most suitable FBG sensors and prior to installation
of sensors on the concrete component, FBG sensors should be calibrated.
The calibration process allows for the determination of the thermal-optic
coefficient K T of the sensor. The calibration may be performed by placing
FBGs together with two or more thermocouples in a conventional oven.
The change in the wavelength of the fibres and the temperatures moni-
tored by the thermocouples should then be recorded using an FBG inter-
rogator system and a data logger, respectively, as the oven's temperature is
increased from room temperature, say 20°C, to the estimated maximum
temperature measurable with the FBG. The use of multiple thermocouples