Keywords: concrete, longitudinal stress wave, nondestructive testing, pulse velocity, ultrasonic testing,, ICS Number Code 91.100.30 (Concrete and concrete products)
Doplňujúce informácie
Significance and Use
5.1 The pulse velocity, V, of longitudinal 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 pulse velocity, and this factor must be
considered when evaluating test results (Note 1). In addition, the pulse velocity
in saturated concrete is less sensitive to changes in its relative
quality.
Note 1:The pulse velocity in saturated concrete may be up to
5 % higher than in dry concrete.3
5.4 The 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 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 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’ pulse velocity is
calculated based on the distance between the transducers. Not all
forms of deterioration or damage actually change the pulse velocity
of the material, but they affect the actual path for the pulse to
travel from transmitter to receiver. For example, load-induced
cracking will increase the true path length of the pulse and thus
increase the measured 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 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 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 pulse velocity in steel
is up to double that in concrete, the 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 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.
1.3This 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 and health practices and determine the
applicability of regulatory limitations prior to use.
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