Monitoring of drying of cement screed with the help of ultra-wideband microwaves and air-coupled antennas

The drying behaviour of freshly prepared anhydrite screed and cement screed samples was investigated non-destructively and in contact-free way using ultra-wideband microwave signals in the frequency range between about 1 GHz and 10 GHz. The measurements have been performed using a vector network analyser and air-coupled wideband horn antennas in reflection mode. Both entry echo and back wall echo at the interfaces of the samples were analysed. The amplitudes of the entry signals decreased within few weeks as the real part of the relative permittivity decreased with drying. Finally a plateau level was reached. At the beginning of the tests the back wall signal of the specimen was not recognizable since the attenuation of the microwaves resp. the imaginary part of the permittivity in the moist sample was too high. After a few weeks of drying, a back wall signal was detected. It increased with continued desiccation and thus lowered attenuation, until here again a constant level was reached. At the same time, the temporal position of the back wall signal moved to smaller times since the propagation velocity of the microwaves increased until a constant value was reached as well.