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› Mae 9
› SONIC TEST ON MASONRY
The principle of the sonic tests on masonry
is the same as principle of ultrasound
tests on concrete. The only difference
is the method to produce the
longitudinal elastic wave used for the
measurement: a hammer is used instead
of an ultra-sonic pulse. The frequencies
thus produced (a few hundreds
of Hz), lower than those of the
ultra-sounds (a few tens of KHz) are
able to also cross scarcely compact
masonry walls, such as brick-faced
or brick walls, thus allowing to evaluate
the conservation state, through
the speed and/or mitigation values of
the wave produced. This type of test
can also be applied on concrete structures,
which sizes do not allow to use
ultra-sounds, because these can no
longer be measured at a few metres
from the source. The sonic tests give
less accurate values compared to the
tests with ultra-sounds, but they can
be performed also on scarcely compact
materials and/or at a distance of
a few metres.
› PILE INTEGRITY TEST
The sonic echo test is a survey method
consisting in the measurement
of the reflection speed of compression
waves, to assess the integrity of
a pole. The method was conceived in
Holland during the 70s, as instrument
to check the quality of pre-fabricated
foundation poles in concrete, extensively
used in that country. Due to the
regularity of the surfaces of the prefabricated
poles, the sonic echo test
could be used with a high level of reliability.
For this type of test, a compression
wave is propagated until the base
of the pole and is reflected towards its
head. The compression wave is generated
with an impact on the pole head
and the signal recorded by a geophone
is displayed in a graph in real time. If
discontinuities are present in the pole
such as variations of the section or
cracks, these cause reflections. If the
discontinuities are fairly significant,
such as for example a complete fracture
of the concrete, these cause an
almost complete fracture, thus preventing
to detect the base of the pole.
In order to execute the test properly,
the pole head must be accessed. The
geophone is placed on the concrete
surfaced, properly prepared, through
a coupling material and by placing
on top a load of a few kg. When the
head of the pole is hit in a selected
point, the impact enables the acquisition
of the signal produced by the geophone,
which is shown immediately on
the screen. In order to eliminate the
background noise caused from the
construction site activities, the acquisition
can be repeated a few times to
calculate the averages.