**2. Impact-Echo Method**

The Impact-Echo (IE) method is a nondestructive technique invented at the turn of '70 s and '80 s for the testing of solid concrete and multilayer [6–9] elements. This method is based on application of impact of a steel ball at the surface of the structure to evoke an elastic stress wave, which later reflects and is recorded with a piezoelectric transducer. The

**Citation:** Sawicki, B.; Piotrowski, T.; Garbacz, A. Development of Impact-Echo Multitransducer Device for Automated Concrete Homogeneity Assessment. *Materials* **2021**, *14*, 2144. https://doi.org/ 10.3390/ma14092144

Academic Editor: Mercedes Sánchez Moreno

Received: 23 March 2021 Accepted: 20 April 2021 Published: 23 April 2021

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received signal in time domain is changed to frequency domain by Fast Fourier Transform (FFT) and then it is analyzed and interpreted to obtain the thickness of the element and, possibly, the depth of reinforcement and flaws (Figure 1).

**Figure 1.** Scheme of impact-echo method with an example of waveform time-domain spectrum and corresponding frequency spectrum when defect in concrete is observed.

A hammer or, most commonly, a steel ball is used as an impactor. The diameter of the ball governs the contact time [7,10], and thus, the frequency content of produced wave. The expected resonance frequency of the tested element should be included in the produced frequency range [11]. Usually, steel balls of diameters 4 mm to 30 mm are used, which allows for the testing of the concrete element of thickness up to1m[12]. Another wave source, like an air gun [13], can be used as well.

An elastic stress wave produced with an impactor contains three components: (I) Rayleigh (R) surface wave, (II) Shear (S) wave and (III) Compression (P) wave [9,11]. The P wave travels the medium and reflects from lower and upper faces of the element, as well as from any faults or borders of underlying layers. This phenomenon is caused by a difference in the mechanical impendences of materials in the element, and especially between air and concrete [7,12]. The periodicity of echoes produces resonance frequencies.

These periodic reflections are recorded with a piezoelectric transducer. A device with a large bandwidth between 2 kHz and 50 kHz is used. The abovementioned frequencies correspond to average thickness of concrete element between 5 cm and 1 m [12]. Another technique, e.g., laser interferometry [14] or microphone [15,16], can be used in the aircoupled approach.

For sake of interpretation, the registered signals are usually presented in frequency domain using fast Fourier transform [17]. The use of frequency domain allows for observation of multiple reflections of wave between upper and lower face of the element, making the measurement insensitive to the distance between impactor and receiver [9]. The wavelet analysis can be used in the analysis as well [18].

The thickness of the element can be computed knowing both: (I) the resonance frequency of P wave, and thus the travelling time, and (II) the velocity of P wave. The velocity of the P wave can be either measured [12] or obtained empirically [6]. A similar procedure is adopted for any layers' interfaces or faults present in the tested element, visible as peaks in the frequency spectrum (Figure 1).

IE is very similar to the ultrasonic method, but two significant differences are present: (I) ultrasonic waves are linear, which enables measurement only in line of excitation, while stress-waves in the IE propagate spherically; and (II) the frequency of waves in the IE method is lower, and therefore, heterogeneity of concrete has neglectable influence on the results. Additionally, the spherical wave propagation allows for separation of the excitor and receiver, making IE more flexible and capable of testing bigger areas and volumes, thus making it faster in use.

Beside measurement of the thickness of element, the Impact-Echo method has been used to detect: flaws [8], cracks [19], voids and debondings [7], honeycombing [20], delaminations and quality of interfaces [21,22] or voids in grouted tendon ducts [23].

The numerical finite element methods are important to ensure the IE development [8]. They have been used extensively to simulate different types of flaws and understand the results obtained with IE testing. From point of view of this paper, the sensitivity of IE to size of faults [23] and fusion of results from multiple sensors [24] are especially important.

Several trials with automated NDT using IE [25,26] and its fusion with other techniques [27] has been undertaken before. The commercial devices were also developed, e.g., Olson Engineering INC-Bridge Deck Scanner (BDS) [28] or BAM NDT Stepper [29].

### **3. Preparation and Validation of Finite Element Model**

To obtain reliable results from the numerical model, verification and validation is needed [30]. The parametrical studies by variating modulus of elasticity, Poisson ratio and density of concrete to change wave velocity, as well as finite element size, were conducted. The results were verified against analytical solution as described below and good agreement was found. For sake of brevity, detailed results are not presented here, and are available in [31].

### *3.1. Experiment for Validation*

To validate the numerical model, an experiment on solid concrete slab was used. The slab of dimensions 50 cm × 50 cm × 7 cm was casted using C20/25 class concrete according to Eurocode and supported on 5 cm by 5 cm washers in four corners. After curing, 30 IE tests using commercial DOCter® Mark IV commercial device [32] were executed. A 2 mm ball was used to evoke the wave in a distance of 5 cm from the receiver. All the measurements show the thickness of 70 mm, so no internal faults were present. The signal was probed approximately every 2 × <sup>10</sup>−<sup>3</sup> ms, and the total length of the obtained signal was 1.9 ms per measurement. All the results were recorded and saved for further comparison with the numerical model.
