Civil Engineering Reference
In-Depth Information
60
50
40
B-Proceq
Portland Cement
Early strength
Blast furnace
Average Curve
30
20
10
20
25
35
40
45
50
Rebound value R
1.7 Radar systems
There are three fundamentally different approaches to using radar to
investigate concrete structures.
a
Frequency modulation: in which the frequency of the transmitted radar
signal is continuously swept between pre-defined limits. The return
signal is mixed with the currently transmitted signal to give a difference
frequency, depending upon the time delay and hence depth of the
reflective interface. This system has seen limited use to date on relatively
thin walls.
b
Synthetic pulse radar: in which the frequency of the transmitted radar
signal is varied over a series of discontinuous steps. The amplitude and
phase of the return signal is analysed and a 'time domain synthetic pulse'
is produced. This approach has been used to some extent in the field and
also in laboratory transmission line studies to determine the electrical
properties of concrete at different radar frequencies.
3
Impulse radar: in which a series of discrete sinusoidal pulses within
a specified broad frequency band are transmitted into the concrete,
typically with a repetition rate of 50 kHz. The transmitted signal is
often found to comprise three peaks, with a well-defined nominal centre
frequency.
