ABSTRACT
Non-destructive techniques based on sonic wave propagation are often used in structural diagnosis. They are applied directly in the field for investigations of a wide range of structures and infrastructures, and in laboratory for the characterisation of materials. Non-destructive sonic methods (SMs) are based on measurements of the velocity v of acoustic waves propagating through the material. They are preferentially carried out applying the Direct Transmission Technique (DTT), in which the wave is transmitted by a transducer (Emitter) through the test object and received by a second transducer (Receiver) on the opposite side. This allows to measure the time t that the wave needs to travel through the object’s thickness, from the emitter to the receiver, along a path of length l; the average velocity of the wave is then obtained from the ratio l/t. The wave velocity is directly related to structure’s elastic parameters, thus its analysis provides information crucial for inspections of structures’ stability and lastingness. The DTT is very effective, since the broad direction of wave propagation is perpendicular to the source surface and the signal travels through the entire thickness of the item. Standards concerning the determination of waves velocity in structures-e.g. European EN 12504-4-suggest, therefore, the application of this kind of signals transmission. Nevertheless, there are many kind of structures, e.g. slabs, retaining walls, piers, in which the DTT cannot be performed, because only one side of the item is accessible. In these cases the Indirect Transmission Technique (ITT), in which both the emitter and the receiver transducers are placed on the same side of the investigated object, might be used. ITT is less effective than the DTT because the amplitude of the received signal is lower, and the pulse propagates in a concrete layer just
beneath the surface. These remarks have since now not allowed the ITT systematic development, and the scientific literature concerning ITT use is still quite poor. Despite that, ITT skills of ease to be performed, high potential to evaluate the quality and the characteristics of concrete covering on site, immediacy and low cost, claim to thorough examine its suitability in concrete diagnosis on site, and then to develop studies concerning the standardisation of its application.
