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abstract

For reliable modelling of microwave heating of concrete its complex permittivity has to be known precisely within the full range of working temperatures. Dielectric characterization of dry concrete cured with different water-to-cement (w/c) ratios and concrete samples from nuclear power plant constructions was performed during heating and cooling cycles from room temperature to 700
C and back. On average, higher permittivity values are found for concretes cured with smaller w/c ratio (more dens and less porous) as compared to concretes cured with higher w/c ratio (lighter and more porous). Samples from nuclear power plant reveals a permittivity close to the concrete prepared with lowest w/c ratio. Permittivity change along increasing temperature correlates with moisture loss and thermal decomposition reactions. These reactions are irreversible that lead to a permittivity divergence in heating and cooling scenarios. The variations of concrete permittivity because of w/c ratio, water transport and decomposition reactions are discussed.

6. Conclusions

Dielectric characterization of dry concrete cured with different w/c ratios as well as of concrete cut from a nuclear power plant construction was performed at temperature cycle 25
C / 700
C / 25
C. The measured dielectric constant and loss factor vary as 3.5< εr 0 <5.5 and 0.05< εr 00 <0.30 depending on the temperature and heating history. Since the amount of residual moisture in dry concrete is higher when the w/c ratio is lower, denser concretes M1 (w/c ¼ 0.40) and BK28 (from reactor) being exposed to elevated temperature from 25 to 400
C show two to three times higher dielectric loss factor as compared with lighter concretes prepared with w/c ¼ 0.55 (M2) and 0.70 (M3). Accordingly, the microwave penetration depth for BK28 and M1 grades is about two times shorter than for M2 and M3 grades. It means that the concretes cured with lower w/c ratio (including the concrete from nuclear power plant) may be easier ablated/demolished/broken with microwaves than the concretes cured with the higher w/c ratio. All concretes under investigation demonstrate almost similar dynamics of permittivity along increasing temperature. Their permittivities first decrease because of water evaporation, then get saturated at 300 < T < 400
C, and thereafter increase due to the increase of permittivity of the solid constituents. Moreover, at the initial phase of heating ðT(100
CÞ the following peculiarity is found: the denser concretes, BK28 and M1, unlikely to lighter concretes M2 and M3, demonstrate the increase of permittivity with temperature. This effect d favorable for more effective and more localized microwave heating d was qualitatively explained in the framework of Debye model. In the cooling scenario the permittivity of all concretes at T < 350
C is found much lower as compared with the permittivity measured in the heating scenario at the same temperatures. The reason for that is the irreversible release of water and carbon dioxide from concretes at heating scenario. Dielectric properties of the concrete taken from real nuclear power plant constructions (BK28) are mostly close to the concrete of grade M1 fabricated with w/c ¼ 0.4. Note, the higher percentage of calcareous aggregates in BK28 as compared with lab-made M1M3 grades.