The ecometric test (else sonic impedance on pole) requires the employment of an apparatus designed to detect a pole’s behaviour following an impulsive stress provoked by a mechanic blow of a hammer. It is an investigation with low deformation level based on the analysis of an elastic wave’s propagation with the aim to establish the presence of anomalous reflections due to geometry variations, earthy inclusions or parts of poor-quality concrete. Those variations of physical, mechanical and geometrical features entail a variation of the mechanic impedance and the resulting partial reflection of the elastic waves.
This test is performed by applying an accelerometer on the element’s top and stressing the pole’s head through a hammer, in order to generate a compression wave, whose speed – usually included between 3500 m/s and 4000 m/s (for poured in place poles) – is related to the concrete’s quality. The wave that reached the pole’s basis is partially reflected to the top (basis reflection), recorded by the accelerometer, amplified and then transmitted to the control unit where it is processed in form of signal in the time domain (reflexogram).
In addition to the analysis of reflexograms representing the amplitude ratio variation of the pole head, some softwares allow to analyse the obtained signal in accordance also with the response frequency of the stressed structure, by applying the Fast Fourier Transform (FFT).
If a pole is mechanically stressed, it will oscillate with frequencies having constant interval; such interval is related to the pole’s length and to the mechanic waves transmission speed, according to the following math relation:
- L is the pole’s length
- V is the transmission speed of the waves within the pole’s constituent material;
- ΔF is the frequency’s constant interval among the various modalities of pole’s vibrating
- In addition, the first vibrating modality (fundamental frequency) is related to the level of the pole’s ends constraint (pole fitted at the tip or suspended pole).
- Establishing the foundation poles’ length
- Individuation of discontinuity, section reduction, concrete expansions, crawl space, inclusions extraneous to the jet.
- Non-invasive technique
- Establishing immediately the test values
- Performing a large number of tests in a short time.