Proceq Pundit Lab - Page 18

© 2013 by Proceq SA
17
5.9. Minimum Dimensions
Minimum dimensions are recommended for accurate test results.
Minimum path length
100 mm for concrete with maximum aggregate size of 20 mm or less.
150 mm for concrete with maximum aggregate size between 20 mm and 40 mm.
Minimum lateral dimensions of the sample
This is dependent on the transmission wavelength and the pulse velocity. The minimum path
length should be equal to or greater than the transmissio
n wavelength, or a severe reduction in
the pulse velocity may be detected. The following table taken from BS 1881: Part 203 shows
the minimum recommended lateral dimensions for the test object.
T
x F
r
e
q
u
e
n
c
y
k
H
z
P
u
l
se
v
e
l
o
c
i
ty
3500 m/s
Pulse velocity
4000 m/s
Pulse velocity
4500 m/s
Minimum Lateral Dimensions
of T
est Object
2
4
1
4
6
m
m
1
6
7
m
m
1
8
8
m
m
5
4
6
5
m
m
7
4
m
m
8
3
m
m
1
5
0
2
3
m
m
2
7
m
m
3
0
m
m
5.10. Transducer Selection Guide
Generally, lower frequencies allow more depth of penetration. Higher frequencies allow better
resolution in the measurements.
The inhomogeneities in concrete influence the propagation of an ultrasound pulse. This influ-
ence can be significantly
reduced by choosing the frequency, f, such that the wave length,
λ
, is
at least twice as large as the aggregate size.
λ
is given by:
λ
= c/f,
where c is the pulse velocity (speed of sound) in concrete. The table below shows typical ag-
gregate sizes and the
corresponding maximum recommended f
requency
, respectively:
c
(
m
/
s
) 3
5
0
0
4
0
0
0
4
5
0
0
aggregate
size (mm)
8
1
6
3
2
8
1
6
3
2
8
1
6
3
2
f
m
a
x
(
k
H
z
)
2
1
9
1
0
9
5
5
2
5
0
1
2
5
6
3
2
8
1
1
4
1
7
0
5.11. 250 kHz Shear Wave Transducers
When measurements with the 250 kHz shear wave transducers are performed, it is crucial to
use the special shear wave coupling paste, otherwise shear waves cannot be properly trans-
mitted into the object under test. The shear wave coupling paste is a non-toxic, water soluble
organic substance of very
high viscosity.
Furthermore, we highly recommend using Pundit Link’s waveform display in order to manually
locate the onset of the shear wave echo. Since the latter is always preceded by a relatively
weak longitudinal echo (see Figure below), the transit time determined by Pundit Lab, would
correspond to the longitudinal instead of the shear wave.