Keywords: Compressive Strength, Concrete, Standards, Ultrasound
This paper was presented at the International Symposium Non-DestructiveTesting in Civil Engineering (NDT-CE) 26.-28.09.1995 in Berlin. NDT-CE, Full Program or the Ultrasound Part
Most nations have standardized procedures for the performance of this test ( 1). Out of these standards, the following eight are analyzed in this paper:
The comparison and analysis of the test methods are followed by a critical evaluation. This is necessarily subjective; nevertheless, it is hoped that it will help in the improved use of the ultrasonic pulse velocity method and contribute to the improvement of future specifications.
Several standards use the term "measurement" (Messung) or equivalent, of pulse velocity. This is not exactly correct because only the distance between the two transducers and the transit time are measured directly. (The transit time is the time taken for the onset of the pulse to pass through the concrete.) The pulse velocity is calculated from these two. Nevertheless this misnomer does not cause much confusion.
For the sake of clarity, the text dealing with specifications in the DIN/ISO stanard are written in italics, everything else is written as plain text.
Goal and Use
The scope of DIN/ISO is restricted to the determination of the velocity of longitudinal ultrasonic waves in concrete. This so-called "pulse velocity" may be used for the assessment of the uniformity of concrete in a structure, measurement of layer thickness of a concrete of inferior quality, monitoring changes in oncrete with time, and detection of defects and anisotropy. It is also permissible to use it for the assessment of the strength of concrete if reliable calibration curves are available. It is pointed out, however, that this ultrasonic test is not an acceptable substitute for the standard, destructive strength determination. Determination of the elastic constants is not mentioned in the DIN.
Similar restrictions and uses are given in the other standards, especially in the ASTM and RILEM standards. Most of them, however, permit the assessment of the elastic constants from pulse velocity measurements. The BS also offers explanations for the various uses of pulse velocity.
Basic Principles of the Test
The method specified in all standards is based on the same principle. Pulses of longitudinal ultrasonic waves are generated by an electro-acoustical transducer that is held in contact with the surface of the concrete under test. After traversing through the concrete, the pulses are received and converted into electrical energy by a second transducer. The velocity v is calculated from the distance 1 between the two transducers, and the electronically measured transit time t of the pulse as v = l/t.
Generally, the apparatus consists of a pulse generator, a pair of transducers, an amplifier, and an electronic timing device for measuring the transit time. According to the DIN, the generator should have: a precision of +/-1% in time measurements, short rise time, a capacity for a low frequency generation, and field worthiness. For short path lengths the use of transducer of high frequency (60 to 200kHz) is recommended; for long path lengths, low frequency (10 to 40 kHz) is recommended Transducers with in the frequency range of 40 to 60 kHz are acceptable in most cases. The timing device should be sensitive enough to be triggered by low amplitude pulses.
The ASTM also specifies that
The British standard offers a mettod for checking the the precision of the transit it
measurement. The Hungarian specification requires a 0.1µs precision of the time measurement.
According to the GOST, the limits If the permissible absolute error of the measurements of transit time of the standard specimens should not be more than delta = +/-(0.01t + 0.1), where t is the transit time in µs. Also, the deviation If the individual measurements of the transit tinge of a specimen from the average value of the measurements of the same specimen should not be more than 2%.
According to the STN, the precision of the test apparatus on the reference bars should be +0.01 As if the surroundingtemperatureranges from -10 to +45°C, and the humiditvis not more than 80%. RILEM provides details about transducer characteristics.
The DIN describes three possible arrangementsfor the transducers for velocity determination. These are:
Whenever possible, the direct transmission arrangement should be used and on the surfaces that were in contact with the mold.
It is essential that there be adequate acoustical coupling between the concrete and the face of each transducer. For most concrete, the surfaces are usually smooth enough to secure a good transfer of ultrasound if a thin layer of an appropriate coupling agent is applied. The accuracy of the transit time measurements should be checked against a calibration device before very series of measurements. The distance between the two transducers should be measured vith the precision of +/-1%, and the transit time should be recorded to three significant digits.
The most detailed description of the measurements with any of the three transducer arrangements is presented in the BS. The details deal with calibration, accessories, such as cathode ray oscilloscope, digital instruments, etc. According to ASTM, repeat measurements should be made at the same location to minimize erroneous readings due to poor contact. It is suggested in the RILEM as well as in the Hungarian specification that before testing, the concrete surface be smoothed when it is rough. RILEM also provides details about the transit time measurements with an oscilloscope both by the maximum amplitude technique and by the fixed amplitude technique. Both the BS and the STN warn that the indirect transmission produces lower pulse velocities than the direct transmission method. The GOST specifies that the maximum depth and diameter of voids on the contact area must not exceed 3 mm and 6 mm respectively, and the maximum height of any proturbance should not be more than 0.5 mm.
All standards specify that the pulse velocity v should be calculated as
The values of coefficients k2 and k3 are dependent on the value of dynamic Poisson's ratio pcu, and can be obtained, as follows:
The GOST permits the use of the transit time t, instead of the velocity when the value of 1 is kept constant.
The DIN/ISO provides detailed instructionsfor the preparation of the test report. This includes: a description of the tested structure or specimen; specificationsfor the concrete; concrete composition; curing condition; and age; the testing apparatus and procedure; arrangement of the transducers; location of the reinforcement; properties of the concrete surface; estimated moisture content; path length; pulse velocity in various directions; and other meaningful information.
The requirements of other standards are shorter but cover essentially the same items for the report. The ASTM requires the measured transit time and also the adjusted transit time. The BS specifies the recording of the date, time and place of the investigation. The Hungarian specification requires the name of the client, the purpose of the testing, the names of the performers of the measurements, the apparatus used, visual observations, and details of the sampling.
The DIN/ISO states in the Appendix that the precision of the transit time should be checked. If this checking takes place with a calibration bar, the transit time should be known with the precision of +/-0.2s. Measured values should not differ more than +/-0.5% from the known value of the calibration bar.
According to the ASTM precision statement, tests involving three test instruments and five operators have indicated that, for path lengths from 0.3 m to 6 m through sound concrete, different operators using the same instrument, or one operator using different instruments, will achieve repeatability of time test results within 2%. In the case of deteriorated concrete, the variation of results are substantially increased. In such cases, however, calculated velocities will be sufficiently low as to indicate clearly the presence of distress in the concrete tested.
The effects of two additional factors are discussed below.
Size and Shape of the Specimen
The dimension of the concrete specimen in the direction of pulse propagation should be at least 80 mm when tested with ultrasound of 40- to 60-kHz frequency. Smaller specimens should be used with caution.
The STN regulates the ultrasonic wavelength according to the shape and dimensions of the tested elements. The shapes are defined, as follows:
Effect of Steel Reinforcement on Pulse Velocity
Steel reinforcement increases the measured pulse velocity, when it is in close proximity to the pulse path. This influence is especially strong when the reinforcement is parallel to the direction of pulse propagation. The increase, however, is negligible if the distance between the steel surJace and the path is more than a sixth of the measured length. The influence of the steel reinforcement perpendicular to the direction of the measurement is very small, with the exception of heavy reinforcement.
If it is not possible to avoid wave propagation paths which are parallel to the reinforcing bars, and the path is in the vicinity (a/I < 0.25) of a steel bar, the British Standard recommends the following correction of the measured pulse velocity:
Eq. 6 may be modified to give the following:
The effects of reinforcing bars which have axes perpendicular to the direction of wave propagation and with diameter less than 20 mm, may be ignored. The GOST also specifies that the measurements of the transit time should be made in the direction perpendicular to the direction of steel reinforcement. The concentration of the reinforcement along the path of the wave propagation should be less than 5%. Measurements along the path parallel to the direction of steel reinforcement are permissible if the distance between the path and the steel surface is more than a sixth of the measured length.
The STN also states that measurements perpendicular to the direction of the reinforcement are preferred. In this case, the effect of the steel bars is negligible, unless the steel concentration S is
The RILEM specification presents somewhat different formulas for the effects of parallel and perpendicular reinforcement.
The Slovak standard severely restricts the velocity measurement if the path would be parallel to the direction of the reinforcement. In such cases, the location of transducers should be outside of the area influenced by the reinforcement. The assumed influenced area is a cylindrical surface with an approximate diameter of l/6.
Much more important from engineering point of view is the writers' main objection against the analyzed standards. This is that the standards do not warn the user about the pitfalls of the assessment of concrete properties from longitudinal pulse velocity. Most standards list about half a dozen possible applications of this ultrasonic test, such as assessment of strength, elastic constants, defect detection, etc., frequently supplemented with formulas However, none of the standards rate these applications according to their reliability. This is unfortunate because it gives the impression that the pulse velocity test is equally suitable for all these applications, which is not the case (4,5). In fact, the best, and perhaps the only reliable, applications of the longitudinal pulse velocity are for (a) checking the uniformity of concrete, and (b) monitoring the changes in a concrete over time. Strength estimation is possible only within a 20% accuracy, and even this can be achieved only under strict laboratory conditions with an established calibration curve. This low accuracy is not improved by supplementing the pulse velocity measurement with other tests, such as the rebound hammer test (6) The other suggested applications of pulse velocity (defect detection, crack depth measurement, etc.) are even less reliable.
The present state of ultrasonic concrete testing clearly needs improvement. The first step toward this may be a warning in the standards about the uncertainties of the use of the standardized longitudinal pulse velocity method. Further improvement should come from a better understanding of the theory of ultrasonic pulse propagation in concrete. This may lead to the use of surface and other guided waves, and advanced signal processing techniques (7,8). Unfortunately, these writers do not know of any standards dealing with such tests.
It also follows that the present state of ultrasonic concrete testing needs improvement. Since further improvement may come from the use of surface and other guided waves, advanced signal processing techniques, etc., development of standards for these is timely.
This paper was partially sponsored by the U.S. - Slovak Science and Technology Program.
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