5.1?The
ultrasonic pulse velocity, V, of longitudinal ultrasonic stress
waves in a concrete mass is related to its elastic properties and
density according to the following relationship:
5.2?This
test method is applicable to assess the uniformity and relative
quality of concrete, to indicate the presence of voids and cracks,
and to evaluate the effectiveness of crack repairs. It is also
applicable to indicate changes in the properties of concrete, and
in the survey of structures, to estimate the severity of
deterioration or cracking. If used to monitor changes in condition
over time, test locations are to be marked on the structure to
ensure that tests are repeated at the same positions.
5.3?The
degree of saturation of the concrete affects the ultrasonic pulse
velocity, and this factor must be considered when evaluating test
results (Note 1). In
addition, the ultrasonic pulse velocity in saturated concrete is
less sensitive to changes in its relative quality.
Note 1:?The ultrasonic pulse velocity in saturated concrete
may be up to 5 % higher than in dry concrete.3
5.4?The
ultrasonic pulse velocity is independent of the dimensions of the
test object provided reflected waves from boundaries do not
complicate the determination of the arrival time of the directly
transmitted pulse. The least dimension of the test object must
exceed the wavelength of the ultrasonic vibrations (Note 2).
Note 2:?The wavelength of the vibrations equals the ultrasonic
pulse velocity divided by the frequency of vibrations. For example,
for a frequency of 54 kHz and a pulse velocity of 3500 m/s, the
wavelength is 3500/54000 = 0.065 m.
5.5?The
accuracy of the measurement depends upon the ability of the
operator to determine precisely the distance between the
transducers and of the equipment to measure precisely the
ultrasonic pulse transit time. The received signal strength and
measured transit time are affected by the coupling of the
transducers to the concrete surfaces. Sufficient coupling agent and
pressure must be applied to the transducers to ensure stable
transit times. The strength of the received signal is also affected
by the travel path length and by the presence and degree of
cracking or deterioration in the concrete tested.
Note 3:?Proper coupling can be verified by viewing the shape
and magnitude of the received waveform. The waveform should have a
decaying sinusoidal shape. The shape can be viewed by means of
outputs to an oscilloscope or digitized display inherent in the
device.
5.6?The
measured quantity in this test method is transit time, from which
an apparent ultrasonic pulse velocity is calculated based on the
distance between the transducers. Not all forms of deterioration or
damage actually change the ultrasonic pulse velocity of the
material, but they affect the actual path for the ultrasonic pulse
to travel from transmitter to receiver. For example, load-induced
cracking will increase the true path length of the ultrasonic pulse
and thus increase the measured ultrasonic pulse transit time. The
true path length cannot be measured. Because the distance from
transmitting to receiving transducer is used in the calculation,
the presence of the cracking results in a decrease in the apparent
pulse velocity even though the actual ultrasonic pulse velocity of
the material has not changed. Many forms of cracking and
deterioration are directional in nature. Their influence on transit
time measurements will be affected by their orientation relative to
the pulse travel path.
5.7?The
results obtained by the use of this test method are not to be
considered as a means of measuring strength nor as an adequate test
for establishing compliance of the modulus of elasticity of field
concrete with that assumed in the design. The longitudinal
resonance method in Test Method C215 is recommended for determining the
dynamic modulus of elasticity of test specimens obtained from field
concrete because Poisson's ratio does not have to be known.
Note 4:?If circumstances warrant, a velocity-strength (or
velocity-modulus) relationship may be established by the
determination of ultrasonic pulse velocity and compressive strength
(or modulus of elasticity) on a number of specimens of a concrete.
This relationship may serve as a basis for the estimation of
strength (or modulus of elasticity) by further pulse-velocity tests
on that concrete. Refer to ACI 228.1R4
for guidance on the procedures for developing and using such a
relationship.
5.8?The
procedure is applicable in both field and laboratory testing
regardless of size or shape of the specimen within the limitations
of available pulse-generating sources.
Note 5:?Presently available test equipment limits path lengths
to approximately 50-mm minimum and 15-m maximum, depending, in
part, upon the frequency and intensity of the generated signal. The
upper limit of the path length depends partly on surface conditions
and partly on the characteristics of the interior concrete under
investigation. A preamplifier at the receiving transducer may be
used to increase the maximum path length that can be tested. The
maximum path length is obtained by using transducers of relatively
low resonant frequencies (20 to 30 kHz) to minimize the attenuation
of the signal in the concrete. (The resonant frequency of the
transducer assembly determines the frequency of vibration in the
concrete.) For the shorter path lengths where loss of signal is not
the governing factor, it is preferable to use resonant frequencies
of 50 kHz or higher to achieve more accurate transit-time
measurements and hence greater sensitivity.
5.9?Because the ultrasonic pulse velocity in
steel is up to double that in concrete, the ultrasonic pulse
velocity measured in the vicinity of the reinforcing steel will be
higher than in plain concrete of the same composition. If possible,
avoid measurements close to steel parallel to the direction of
pulse propagation.
1. Scope
1.1?This
test method covers the determination of the propagation velocity of
longitudinal ultrasonic stress wave pulses through concrete. This
test method does not apply to the propagation of other types of
stress waves through concrete.
1.2?The
values stated in SI units are to be regarded as standard. No other
units of measurement are included in this standard.
1.3?This standard does not purport to
address all of the safety concerns, if any, associated with its
use. It is the responsibility of the user of this standard to
establish appropriate safety, health, and environmental practices
and determine the applicability of regulatory limitations prior to
use.
1.4?This international standard was
developed in accordance with internationally recognized principles
on standardization established in the Decision on Principles for
the Development of International Standards, Guides and
Recommendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
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