Process control in microwave heating of concrete - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 192

The accuracy of the FBG measurements can also be verified using the
surface temperatures recorded by the infrared thermal camera. The tem-
peratures at the same locations monitored by the FBG sensors were com-
pared with those measured using the infrared camera and are shown in
Figures 5.12 and 5.13. A comparison of the maximum temperatures mea-
sured by the FBG sensors and the temperatures at the same locations moni-
tored using the infrared thermal camera again confirm that the accuracy of
the bare FBG sensors is satisfactory for use in processes associated with the
microwave heating of concrete.
5.4 SUMMARY
Temperature is one of the most important process-monitoring and feedback
control measures for microwave-assisted concrete processing. However,
accurate measurement of temperatures in the presence of a strong elec-
tromagnetic field is difficult. This is mainly because most conventional
metal-based temperature sensors are either dysfunctional or lack preci-
sion when used in an electromagnetic field. In this chapter, the suitability
of some of the more common temperature sensors for use in temperature
monitoring and feedback control in microwave-assisted processing of con-
crete was discussed. It was noted that conventional thermocouples are not
suitable as sensors in microwave-processing applications as the readings
of the thermocouples in the presence of an electromagnetic field are not
accurate and mostly overestimate the actual temperature of the concrete.
On the contrary, we observed that the temperature of concrete in a micro-
wave field can be accurately monitored using bare FBG sensors. Therefore,
bare FBG sensors may be used as reliable process feedback control tools
in microwave-processing applications. However, it should be noted that
commercially packaged, off-the-shelf FBG sensors currently used for strain
and temperature measurements of concrete in normal field environments
cannot be used in the presence of strong microwave fields. This is mainly
because of the self-heating of the outer polymer sheath present in such FBG
sensors. Also, there is uncontrolled electromagnetic interference caused by
the metallic connectors present in such optical fibres.
REFERENCES
1. Leung, C.K.Y. and Pheeraphan, T., Very high early strength of microwave
cured concrete . Cement and Concrete Research , 1995, 25 (1):136-146.
2. Leung, C.K.Y. and Pheeraphan, T., Determination of optimal process for micro-
wave curing of concrete . Cement and Concrete Research , 1997, 27 (3):463-472.