Study of wave attenuation in concrete
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M. Ben Souda and J. L. Robert Laboratoire Central des Ponts et Chaussees, Centre de Nantes, Section Essais Non Destrucifs, BP 19, 44340 Bouguenais, France (Received 25 September 1992; accepted 18 April 1993)
The experimental study of wave attenuation in concrete has been achieved in the case of the propagation of plane waves in concrete rods. Different mortars and concretes have been investigated. A transmitter transducer coupled to one of the ends of the concrete rod generates the propagation of a plane wave in the rod. The receiver transducer, similar to the previous one, is coupled to the other end of the rod. The experimental results lead to an analytical expression for wave attenuation as function of the concrete composition, the propagation distance, and the wave frequency.
I. INTRODUCTION Work in the field of acoustic emission developed at the Laboratoire Central des Ponts et Chaussees (LCPC) concerns the monitoring of civil engineering constructions and the analysis of damage in concrete. Initially, investigation of damage in concrete has been considered in relation with fracture mechanics.1'2 So as to define more clearly dimensions of the damage area and to specify the processes of damage initiation and propagation, the acoustic emission technique was improved 38 in the case of mechanical tests on concrete CT (compact tension) specimens. The results obtained show that a better understanding of the wave attenuation in concrete is required. In this paper, the importance of wave attenuation is emphasized in the context of the acoustic emission process. Then an experimental study of wave attenuation is performed in the case of the propagation of plane waves. II. ACOUSTIC EMISSION PROCESS Acoustic emission is the general name given to the processes of emission and propagation of strain waves, resulting from localized modifications of materials. In the case of concrete test specimens or structures, submitted to mechanical loading, acoustic emission results from fracture mechanisms. Fracture processes (called acoustic emission events) create localized discontinuities which generate waves which propagate through materials (Fig. 1). These waves can be detected on the surface of specimens or structures by piezoelectric transducers which convert the mechanical vibrations to electric signals (the acoustic emission signals). These signals can be recorded and analyzed to obtain information about the fracture mechanisms. 2344
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J. Mater. Res., Vol. 8, No. 9, Sep 1993
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So the acoustic emission process can be regarded as a system whose input e(t) is the event and whose output s(t) is the acoustic emission signal. This system can be divided into three transfer functions associated with the physical processes induced: (1) a material function hm{t)\ (2) a geometrical function hg{t); (3) a transducer function hc(t). In the case of linear behavior, the relation between signal and event can be expressed in the form of convolution as:
s(t) = hm{t) * hJt) * hc(t) * e(t)
(1
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