Impact Echo
Impact-echo
is a method for nondestructive evaluation of concrete and masonry
structures, based on the use of impact-generated stress (sound)
waves that propagate through a structure and are reflected by
internal flaws and external surfaces.
-
It provides independent, nondestructive
measurements of the thickness of concrete slabs with an accuracy of 3%
or better.
-
It can determine the location and extent
of flaws such as cracks, delaminations, voids, honeycombing, and
debonding in plain, reinforced and post-tensioned concrete structures.
-
It can locate voids in the subgrade
beneath slabs and pavements.
-
For masonry it can determine thickness and
locate cracks, voids and other defects where the brick or block units
are bonded together with mortar.
-
It is not adversely affected by the
presence of steel reinforcing bars.
How
It Works
A
short duration impact, produced by tapping a small steel sphere
against a concrete or masonry surface, is used to generate low-frequency
stress waves (1 to 60 kHz) that propagate into the structure
and are reflected by flaws and external surfaces. The use of
long-wavelength, low-frequency stress waves distinguishes impact-echo
from traditional ultrasonic methods.
These stress waves propagate through concrete as though it were
a homogeneous elastic medium. Surface displacements caused by the
arrival of reflected waves at the impact surface are recorded by a
transducer, located adjacent to the impact site, producing an analog
voltage signal proportional to displacement. The resulting voltage-time
signal, called a waveform, is digitized and transferred to the computer
memory, where it is transformed mathematically into a spectrum of
amplitude vs frequency.
Both
the waveform and spectrum are plotted on the computer screen.
The dominant frequencies, which appear as peaks in the spectrum,
are associated with multiple reflections of stress waves within
the structure, and they provide information about the thickness
of the structure, its integrity, and the location of flaws.
The
Impact-Echo Test System
The principal
components of a test system are a cylindrical hand-held transducer
unit, a set of spherical impactors, a notebook computer, a high-speed
analog/digital data acquisition system, and a software system
that controls and monitors the tests and displays the results
in numerical and graphical form. Power is provided by internal
batteries in the notebook computer and data acquisition system.
Instrument
Configurations Used in Testing :
