Ultrasonic crosshole testing
Ultrasonic crosshole testing requires ultrasonic transducers, a depth
encoding device and a field computer with proper software.
Ultrasonic systems measure the time taken by ultrasonic waves to pass
between parallel tubes cast into the concrete element being tested. The
test relies on the fact that waves propagate at different velocities in
media with different values of the elastic modulus. In concrete, the
velocity of sound is constant, being of the order of 4,000 m/sec. Since
the velocity of propagation is much lower in concrete containing soil
inclusions, gravel, bentonite or honeycombing phenomena, the presence of
such irregularities becomes immediately obvious. Ultrasonic systems also
measure the energy of the signal that pass between the transducers which
provide additional information for the analysis.
The test is carried out as follows: The tubes are filled with water. The
probes (emitter and receiver) and meter wheel are connected to the data
acquisition logger. The probes are then inserted into the tubes. The
ultrasonic emitter is lowered to the bottom of one of the tubes, and the
ultrasonic receiver, to the bottom of the other, both to the same depth.
The test begins when the two probes of the pair are raised to the top of
the tubes (pile). As the probes rise, ultrasonic waves pass through the
concrete to test the integrity of the pile.
Each signal is modulated to a series of black and white lines. Any
defect shows up clearly on the screen and later in the printed log.
An important advantage of the "waterfall" log compared to the first wave
arrival graph is that it does not hide defects. The software that
support first wave arrival graphs uses filters that often smooth the
graph and hide irregularities. In addition the software enable the user
to change the time of the first wave arrival for any given depth and by
that the analysis of the graphs may be unreliable.
For more info about pile testing see : testing_piles.pdf
