Sonic testing
(Low-strain pile testing)
A plane pressure wave is
induced along the entire length of the concrete element to be tested by
lightly striking the element head with a hammer. The wave induced at the
element head travels rapidly through the concrete, reaches the bottom of
the element, is reflected by it and returns, fully or in part, to the
element head. Discontinuities in the properties of the medium and
discontinuities in the element's cross-section affect the reflections of
the wave resulting from its passage through different layers of the
material. The returning waves are sensed by an accelerometer applied to
the element head. The signal is transmitted from the accelerometer to an
electronic unit, which stores the signal. The signal that is presented
as a reflectogram shows the first signal of the hammer blow, the wave
returning from the bottom of the element, and other reflections
resulting from anomalies in the element's geometry, inclusions of
unwanted material or discontinuities in the material.
By recording all the
waves returning to the accelerometer and displaying them on the
reflectogram, it is possible to analyze the integrity of the concrete
and detect most of the geometric changes in the element. The reflected
waves are modified by discontinuities in the physical properties
(density, elastic modulus, wave propagation velocity) of the concrete
and of the surrounding materials, as well as by geometric differences in
the element's cross-section.
In sonic testing of
concrete, the results will be influenced by a number of factors:
-
Physical properties
of the concrete
-
Geometry of the
element
-
Nature of the ground
layers underlying the element and surrounding it along its entire
length
-
Length to diameter
ratio of the element
Both ultrasonic and gamma
ray testing involve placing probe tubes along the entire length of the
element under inspection. While this entails a relatively large outlay
as compared with sonic testing, the results are more precise, more
complete and more reliable. Moreover, both ultrasonic and gamma ray
testing can pinpoint the precise location of any fault discovered in the
element.
If the soil engineer
wishes to make sure that all the elements on the site are sound, we
recommend selecting a test method based on inspection tubes.
For more information about pile testing
see: testing_piles.pdf
