Improved impact-echo approach for non-destructive testing and evaluation (original) (raw)
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This study examines rationale of correction factor β in the formula of thickness resonant frequency, fundamental to the impact-echo (IE) approach in non-destructive testing and evaluation for integrity appraisal and damage diagnosis of infrastructure systems. It shows the role of the factor in the resonant frequency which is typically obtained with average characteristic from traditional fast Fourier transform or FFT data analysis of IE recordings. A time-frequency data analysis termed Hilbert-Huang Transform or HHT is then introduced to overcome the shortage of FFT analysis in identifying the resonant frequency from IE recordings. With the FFT and HHT analyses of five data sets of sample IE recordings from sound and damaged concrete structures and comparison with referenced ones, this study reveals that the proposed IE approach with HHT data analysis not only eliminates the use of correction factor in the formula, it also improves greatly the accuracy in the IE approach.
European Journal of Environmental and Civil Engineering, 2011
The study presented here aims at evaluating the bulk elastic Young modulus of six different concrete mixes as a function of the water content and degradations due to carbonation or chloride ingress. The frequency analysis of ultrasonic waves in concrete after the impact of a steel ball (impact echo method) is commonly used to measure the thickness of large slabs, to detect voids in concrete structural elements considered as infinite. In the research project, this method was employed on reduced sized slabs (0.5x0.25x0.12 m 3 ). As a consequence, it was necessary to identify the frequencies corresponding to resonance modes or pseudo-stationary modes. This modal analysis was validated by several simplified models (for thin or semi-thick slabs or beam) and used to calculate the dynamic Young modulus E dyn and the Poisson ratio. This last parameter is varying from 0.17 to 0.24, classical values for concrete. The dispersion of the Poisson ratio is too important and the values can not be compared to destructive nor non destructive test results. Otherwise once inverted for all concrete mixes, the E dyn -modulus is compared to static Young modulus E stat measured by destructive testing.
Crack Depth Determination using Advanced Impact-Echo Techniques
Concrete cracking resulting in stiffness loss and also corrosion of the steel reinforcement is an important aspect in reinforced concrete structures. Normally the concrete cover will protect the steel from corrosion. However, if cracks occur and become large the corrosion protection is not given anymore. The corrosion rate strongly depends on the crack width and the crack depth so there is the need to measure and to characterize its dimensions. Only if the crack dimensions exceed a certain level, a repair is necessary. To increase the lifetime of the structure and to optimize the repair this deterioration level has to be known. One way to do this is with the Impact-Echo method. The impact-echo technique is a useful tool for the detection of different faults in concrete structures. Unfortunately, the existing instruments and analysis tools designed for measurements were lacking of several features in the past detaining the extensive use. For many applications as for inspections of large structures, it is essential that the equipment for Impact-Echo measurements along profiles uses a scanning technique and a fast and reliable data acquisition. A new concept for Impact-Echo testing systems is presented in this paper. Therefore, a new device was developed, which is small and easy to handle, robust and unproblematic regarding transportation. The system utilizes advanced impact generation for fast scanning techniques and reproducible impacts. The data acquisition, filtering and visualization of data are optimized for the inspection of large structures obtaining data at many measurement points. A new option is the automatic estimation of crack parameters like crack depth. The crack depth determination is a combination of the well known time-of-flight method with new analysis methods that were made in the time domain. Therefore the acoustic wave and the energy content will be further analysed. In this paper fundamental principles of the different measurement techniques as well as details of the hardware and software functionality of the measurement system are described.
Mechanical Systems and Signal Processing, 2018
code-compliant structure is furthermore expected to withstand, with minor or light damage, moderate ground motions that could occur several times during its life time. Minor or light damage in RC structures means concrete cracking; severe damage is accompanied by plastic deformations in the steel reinforcing bars. In any case, seismic codes assume that moderate or severe ground motions will inevitably produce some damage in conventional RC frame structures. To enhance the overall energy dissipation capacity of the RC frame and to prevent structure collapse, current codes also prescribe that damage (i.e. concrete cracking and plastic strain in reinforcing steel) must be concentrated at the ends of beams and the bases of columns of the first story. The damage of beams and columns is predominantly concentrated around the so-called plastic hinges. Plastic hinges are the zones of the RC members where plastic deformations localize, usually close to the regions of maximum bending moment. The formation and development of a plastic hinge involves concrete cracking and plastic deformations of concrete and steel reinforcement. Micro-cracking in concrete initiates in the highly stressed regions when the tensile stress exceeds the tensile strength of the material, and it results in a stress transfer from concrete to steel and the activation of the later. As the external load increases, cracks propagate and grow forming macro-cracks, involving complex mechanisms that include internal friction between the surface of the steel and the surrounding concrete [3] and the yielding of the former. The emerging elastic waves and the corresponding AE waveforms provide then information about these processes. The above mentioned behavior of code-compliant RC frame structures was recently assessed experimentally through shaking table tests by some of the authors [4,5]. Determining quantitatively, through nondestructive techniques, the level of damage in a structure after a moderate or severe earthquake is of foremost importance when assessing whether the activity of a building must be interrupted, and to determine the level of seismic retrofitting required. The importance of condition monitoring of concrete structures has been widely stated. One of the most relevant technologies used for real time diagnosis of structures, in nondestructive conditions, is the acoustic emission method (AE) [6-8]. AE technology is similar to seismology except AE is in the scale of engineered structures and in a different frequency range. Seismic phenomena occur on low frequencies (0-40 Hz) while AE operates on high frequencies (commonly from 20 kHz to 1 MHz). This governs the applicability of sensors involved. AE is constituted by elastic waves generated by the sudden internal stress-strain field redistribution in materials or structures when external load is applied. This occurs in crack initiation and growth, crack opening and closure, deformation, dislocation movement, void formation, interfacial failure, corrosion, fibre-matrix debonding in composites, etc. These waves propagate through the material and eventually reach the surface, producing small temporary surface displacements. The elastic waves are of low amplitude and of high frequency, normally ultrasonic. Sources of AE are commonly related with damage. Thus, its detection and analysis can be used to evaluate the behavior of the material under load conditions and so, to predict its failure. AE signals are bursts with a high frequency range, non-stationary and with very low amplitude. Traditional AE methods performed a parameter-based analysis (amplitude, duration, rise-time, RMS-root mean squared). Contrarily, actual quantitative AE methods deal with the waveform and the wave propagation inside the material and/or the signal processing of the recorded data by means of advanced techniques such as the Wavelet Transform (WT) which is used in the present paper [9-11]. The success of the application of the method of AE lies basically in making a correct implementation of two important aspects during data analysis (AE signals): (1) correctly extract the data providing useful information, i.e. separate the genuine AE produced by concrete cracking (primary sources) and that produced by other secondary sources; (2) define and verify reliable criteria and indexes (damage indexes) to make a correct evaluation of the accumulated damage in the structure. Many AE signal features and criteria have been proposed to address both challenges. Signal amplitude, duration, risetime, number of counts, number of AE signals, and others are the classical AE features extensively studied and applied for these purposes. Other combinations, like angle ratio (RA) and the average frequency (AF) have been widely used for classification of cracking modes in concrete, reporting a successful separation between tensile and shear cracks [12-16]. Several evaluation criteria have been based on the Kaiser effect, i.e. the absence of detectable AE until previously applied load stress levels are exceeded. The lack of this physical phenomenon has been considered as a damage indicator [8,17]. Other evaluation criteria are based on the use of the relaxation ratio analysis of the AE signals, the calm and load ratios and the severity and historic indices [18-21]. One of the most used features is the signal AE peak amplitude, typically quantified in AE decibels. The traditional b-value, calculated from amplitude distribution AE data, has been proved as a good indicator of the level of damage of RC structures. This is computed on the basis of the power-law relation between the number of events surpassing a given amplitude, and the amplitude of these events. This relationship is commonly known as Gutenberg-Richter [22]. It has been successfully applied for assessing the damage of reinforced concrete beams subjected to cyclic loading [23,24], health monitoring of retrofitted RC structures [25] and to evaluate cracks in concrete and cement mortar [26]; these studies suggest a limit b-value that determines the transition from micro-crack growth to macro-crack formation in concrete. Shiotani and collaborators incorporated statistical values of amplitude distribution analysis and defined the so-called improved b-value (ib-value) [19,27-31]. Most of the papers have applied this index to specimens subjected to static and quasi-static (cyclic) tests. However, few works have been reported about its application to dynamic earthquake-type loading [31]. Nevertheless, although peak amplitude is closely related to the magnitude of fracture, not many studies have investigated the implementation of AE energy to develop damage qualification procedures. It is reasonable to suggest that energy content in the AE signal is correlated with the plastic strain energy of the associated deformation [32]. Energy physically expresses severity (intensity) of an AE event, so it is expected that large crack growth will emit AE signals with high energy and microcracks growth will emit AE signals with low energy. It should be also noticed that the type of crack (tensile cracks, shear
Application of Impact-Echo techniques for crack detection and crack parameter estimation in concrete
Existing instruments designed for measurements involving the Impact-Echo (IE) technique are usually of limited use in field tests when investigating large concrete structures like buildings or tunnels. In 2001 demand for such instruments increased significantly particularly in Germany, as German authorities made quality control tests of state road tunnel constructions mandatory. For this task, it is essential that the equipment used for Impact-Echo measurements along profiles uses the scanning technique and is easy to work. A new concept for impact-echo testing systems is presented. The new device is small and easy to handle, robust and unproblematic regarding transportation. The system utilizes advanced impact generation, measurement, data acquisition, filtering, and visualization techniques. Regarding the software state-of-the-art requirements were implemented for both, scientific applications and field-tests. A new option is the flaw detection and estimation of crack parameters like crack width and depth out of the impact-echo signals. Some first results of in-situ measurements are shown.
2009
Impact echo method is an effective way to assess flaw in concrete. The commercially available impact echo analysis is interpreted manually, thus, to evaluate large area by the method is neither practical nor cost effective. In order to enhance the method in application to plate structure such as bridge decks, where duration of investigation plays substantial influence on traffic activity, data analysis and interpretation should not only be rapid and objective but also provide automatic mapping of deck quality at the end of investigation. This paper represents the development of algorithm for automatic impact echo data analysis in frequency domain and verification on the validity of the proposed method. The algorithm is developed to compensate the in field variation of thickness frequency (f T ) of the investigated no defect portion of deck where a range of frequency at which f T would migrate is allowed. As a result, a software program can be developed based on this algorithm to eliminate the presence of skilled attendance during field investigation of slabs.
Tutorial. Signal processing aspects of structural impact testing
1992
! analysis analysis frequency (Hz) ! sample digital sample ra te (Hz) !l.f frequency re solution (Hz) G�F (w, �) auto spectrum of transient force G '1'x (w, T) auto spectrum of vibration re sponse signal G�x (w, T) cross spectrum of force and vibration response signal G� (ro, T) auto spectrum of general transient signal H(ro) theoretical frequency response function
Lamb Wave Basis for Impact-Echo Method Analysis
Journal of Engineering Mechanics-asce, 2005
The impact-echo method has been developed over the past 20 years and is now widely used in the nondestructive evaluation of concrete. However, some practical issues remain unresolved, such as the physical basis for the empirical correction factor ͑͒ used to obtain thickness mode frequency. A new approach based on guided wave theory is proposed in this paper: that the impact-echo resonance in plates corresponds to the zero-group-velocity frequency of the S 1 Lamb mode. A numerical model is developed, verified by experiment, and then shown to adequately simulate the dynamic response of a concrete plate. Using this model the thickness resonance mode is identified and found to accurately match that particular Lamb mode in terms of shape and frequency. New values for  based on the Lamb mode model are computed and dependence on material Poisson's ratio is demonstrated.
Evaluation of the concrete characteristics by measurements of sonic wave velocity
The seismic vulnerability of the existing RC buildings and their retrofit are two important topics in modern seismic engineering (both search and professional practice). The related problems with assessment of material and structural characteristics have been extensively studied in last years and then incorporated in seismic codes. In particular, in order to define new effective methodologies and techniques many studies have been performed. Moreover, economic problems regarding these techniques must to be considered. Commonly, in situ tests on RC buildings are subdivided in Non Destructive Test (NDT) and Destructive Tests (DT). The advantage of NDTs tests is that no damage is caused to the structural elements and moreover provide a large amount of data with relatively low cost. In this paper we propose a fast NDT methodology based on the measure of the sonic wave velocity within the RC structural elements. This technique has been developed by means of a wide experimental campaign (carried out in Laboratory of University of Basilicata) on RC elements extract by existing RC buildings. An impulse is generated by a hammer on the surface of the considered elements. Then the average velocity of the sonic waves was estimated. Finally, the results were compared with those obtained by the classical ultrasonic method. From the analysis of the preliminary applications, the proposed technique seems to be able to describe the concrete characteristics with few economic costs due to unexisting damage on non structural components.
Use of sound for the interpretation of impact-echo signals
This paper describes the development and evaluation of techniques for using sound to aid in the interpretation of signals obtained from the nondestructive testing of concrete using the impact-echo method. The impact-echo method and the significance of using sound for the field engineer are introduced. The auditory representation scheme developed and the software used are described. Psychological experiments that evaluate the effectiveness of the representation scheme are discussed. Results indicate the success of using sound to enhance signal interpretation in real-time and also suggest ways of using sound to train field engineers in the proper use of the impact-echo method.