Keywords: pulse-echo, non-metallic, ultrasound, non-destructive, emitting, receiver.

This paper shows some experimental results concerning the ultrasound wave propagation in asphalt. In the field of non-destructive evaluation of construction materials for roads the velocity is usually measured and utilized to predict or correlate the strength of material. The pulse-echo method is a suitable technique for flaw detection in the structural material, based on stress wave propagation. A transient stress pulse is introduced into the sample by a pulse generator with an ultrasound transducer placed on the element surface. Using this test method we can determine the ultrasonic pulse velocity, propagated into specimen. It conforms to the testing standard BS1881: Part 203; ASTMC597 [1,2]. The pulse-echo method is presented for flaw detection in non-metallic materials based on stress wave propagation [3,4]. Using this method the travel path of the wave into the material can be determined, as well as the material thickness. Also presented are the calculus relationships to determine the elasticity coefficients, and propagation velocities for applications in plate-like structures.

The equipment has a pulse generator, an emitting transducer and a sensitive receiver, as a piezoceramic transducer. The piezoceramic emitting transducer is excited by electrical pulses of high voltage amplitude. The emitting transducer (ET) converts the electrical signal in mechanical vibrations, which propagate into the specimen. The output signal of the receiver transducer (RT), namely the piezoelectric sensor, is displayed on the memory digital oscilloscope. The results of the signal analysis in time and frequency can be used to determine the attenuation coefficient. An important application is the asphalt thickness determination. The active element of the emitting transducer and receiver transducer is a piezoceramic disc. Typically the piezoceramic disc is used in acoustic transducer applications [5], yielding an omni-directional pattern. The vibration movement of the piezoelectric element is described by the differential equation of the radial vibration disc [6,7]. Therefore, a model of piezoceramic disc is analysed and used to predict the resonance frequencies of the piezoelectric transducer, and its performance. Considering the resonance frequencies as solutions of the transcendental equation we could determine these specific frequencies, and other characteristics. As the application of the pulse-echo method is a set-up using an ultrasonic generator, equipped with an ultrasonic emitting transducer of 50 kHz and a sensitive receiving transducer. As a result, this pulse-echo method can be applied to non-metallic materials with rugged and non-homogeneous structures, such as asphalt plates.

1

ASTM C 215-97e1, Standard Test Method for Fundamental Transverse, Longitudinal and Torsional Frequencies of Concrete Specimens, 04.02.

2

ASTM C 597, Standard Test Method for Pulse Velocity through Concrete, 04.02.

3

N.J. Carino, M. Sansalone, "Impact-Echo: A New Method for Inspecting Construction Materials", Proceedings, Conference on NDT&E for Manufacturing and Construction, Aug. 1988, Urbana, IL., H.L.M. dos Reis, Ed., Hemisphere Publishing Corp., 209-223, 1990.

4

M. Sansalone, N.J. Carino, "Stress Wave Propagation Methods", Handbook on Nondestructive Testing of Concrete, V.M. Malhotra and N.J. Carino, eds., CRC Press, Inc., 275-304, 1991.

5

B.B. Auld, Acoustic Fields and Waves in Solids, Wiley Interscience, New York, 2, 63-104, 1973.

6

A.H. Meitzeler, H.M. OBryan Jr., H.F. Tiersten, IEEE Trans. On Sonics and Ultrasonics, SU-20, 233, 1973.

7

G. Amza, D. Barb, F. Constantinescu, Ultra-acoustical Systems. Calculus, design, applications in technique, Ed. Th., Bucharest, 1988.

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