The use of ultrasonic (or microseismic) method represents, together with the sclerometric one, the most popular test to assess concrete mechanical resistance. The waves, having a higher frequency than 20.000 Hz, radiated by an emitting probe, are picked-up by an accelometer placed on the opposite part of the face (direct measuring) or placed on the same side (indirect measuring). Both the devices are linked to an amplifier and analogic-digital converter to display and record the results on a computer. The elastic waves propagate through a solid material favouring the directions having a higher density and deviate in correspondence of vacuum or discontinuity. Thus, the tests with ultrasounds, carried out on masonry structures, being heterogeneous and anisotropic elements, allow to identify cavities, cracks or portions of material having different characteristics, intercepted along the wave transmission path.
The investigation is carried out according to UNI EN 12504-4:2005 regulation and can be performed according to three procedures:
- by direct transmission of the impulse – the transducers are placed in correspondence of two opposite levels of the element to be investigated.
- by semidirect transmission of the impulse – the transducers are placed in correspondence of two adjacent, perpendicular levels of the element to be investigated.
- by indirect or superficial transmission of the impulse – the transducers are placed on the same level of the element to be investigated.
The ultrasonic test has various advantages among which being non invasive, the rapidity of the execution and the repeatability in more parts of the structure.
- Measuring the elastic wave propagation speed in the element.
- Morphologic, qualitative assessment of the section identifying the discontinuities (in case for example of masonry having more faces) and any likely anomaly and imperfection such as inclusions or cavities.
- Obtaining information about the concrete resistance, about the static and dynamic elastic modulus, about the material homogeneity.
- Assessment of the deteriorating state and the presence of microcracks.
- Assessment of the efficacy of a consolidating intervention such as an injection.