Impact Echo (IE) test is used to determine pavement thickness as well as to evaluate pavement debonding and integrity.
Basic Concept: Impact Echo on pavements is done with small metal spheres or electronic solenoid sources. The impact, however it is made, sends acoustic energy into the pavement which resonates at a frequency whose wavelength is the thickness of the pavement.
Data Acquisition: Methods based on the original work by Sansalone use metal spheres of varying sizes to get a source impact of the proper strength and frequency, requiring a certain amount of experimentation in making the measurements. Systems that use electric solenoid sources rely signal processing techniques to determine the resonant frequency from a single source hit. Care must be taken when making these measurements that the true depth is being measured and not the depth to an inhomogeneity in the pavement.
Several companies manufacture instruments for IE; two sets of such equipment are illustrated below in figure 75.
Data Processing: Using frequency analysis techniques amplitudes of the frequencies in the data are computed. The velocity of the pavement must be measured (or assumed).
Data Interpretation: The frequency spectrum of the receiver is used to determine the depth of reflectors according to:
D = Vp / (2 x fr)
where D is the reflector depth, fr is the large dominant frequency peak identified in the response, and Vp is the compressional wave velocity. If the velocity of the concrete is known or can be measured, then the depth of a reflector can be calculated from the reflection echo peak frequency. The wave speed Vp can be measured by observing the travel time of a compressional wave between two transducers held a fixed distance apart on the concrete surface or by performing a calibration test on a slab of known thickness and observing the dominant frequency.
The highest amplitude frequency peak is the main indicator of a reflector depth (thickness echo). The presence of additional echo peaks can also be significant, indicating the presence of possible defects or other interfaces in the concrete.
Advantages: This technique does not require coring.
Limitations: Impact Echo does not give accuracy to the level of 0.64 or 0.32 cm as is sometimes sought for certain engineering applications.
Figure 75. Example Impact Echo Systems: (a) Impact Echo Instruments, LLC, and (b) Germann Instruments A/S.