Impact-echo Testing Research Articles

Effect of Hammer Type on Generated Mechanical Signals in Impact-Echo Testing.

The impact-echo diagnostic method is a well-known nondestructive pulse compression test method, which can be relatively easily used for the testing of concrete and reinforced concrete elements. The evaluation of the measurement with this method is based on the analysis of the signal itself in the time and frequency domains. This allows acquisition of information on the velocity of the mechanical wave, the resonant frequency of the specimen or on the presence of internal defects. The ability to interpret these measurements depends on the experience of the diagnostic technician. The advent of classification algorithms in the field of machine learning has brought an increasing number of applications where the entire interpretation phase can be considerably simplified with the help of classification models. However, this automated evaluation procedure must be provided with the information of whether the signal acquired by the test equipment has actually been measured under optimally set conditions. This paper proposes a procedure for the mutual comparison of different measuring setups with a variable tip type, hammer handle and impact force. These three variables were used for a series of measurements which were subsequently compared with each other using multi-criteria evaluation. This offers a tool for the evaluation of measured data and their filtering. As an output of the designed method, each measurement is marked by a score value, which represents how well the acquired signal fit the weight demands for each observed feature of the signal. The method allows the adjustment of selected demands for a specific application by means of set thresholds. This approach enables the understanding of characteristics of the signal in the automated pre-processing of measured data, where computing power is limited. Thus, this solution is potentially suitable for remote long-term observations with sensor arrays or for acoustic emission signals pre-processing.

An effective crack-identification approach for impact echo signals using MWT spectrograms and scaled FFT spectra

When performing an impact-echo (IE) test to detect internal cracks in concrete structures, the fast Fourier transform (FFT) of the measured surface responses is used to extract frequency peaks of major periodic events and obtain crack depth information. However, reflections from reinforcing bars located at approximately one-half the crack depth may result in comparable frequencies. It is then difficult to determine whether a certain peak frequency is associated with a crack or a bar based solely on the FFT frequency. Our research group suggested using the Morlet wavelet transform (MWT) to facilitate distinguishing between crack responses and bar responses. It was found that the proposed idea is feasible for identifying relatively large cracks. This paper extends the applicability of the previous study by incorporating two analytical models into the methodology: an attenuation model for describing the decrement trend of the amplitude histograms extracted from the MWT spectrograms; and a method for normalizing and scaling the frequency amplitudes. Thus, an enhanced scheme for identifying cracks and bars has been developed, in which the evaluation procedures consist of a quick diagnosis stage for large cracks and a confirmation stage for bars. Through numerical studies and experimental measurements, this study demonstrates the logic in how to validate the two-stage procedures with appropriate benchmark values. Hence, this paper provides the analysts with an effective scheme to distinguish between bar and crack echoes in IE signals.

Interpretation of Impact-Echo Testing Data from a Fire-Damaged Reinforced Concrete Slab Using a Discrete Layered Concrete Damage Model.

The main objectives of this study are to investigate the spectral responses of a fire-damaged concrete slab using Impact-echo (IE) testing, and to develop a simplified model for interpreting the frequency shift due to heat-induced concrete damage after the fire. For these purposes, a reinforced concrete slab specimen (1000 mm (width) by 5000 mm (length) by 210 mm (thickness)) was fabricated in the laboratory. Heat damage in the concrete slab specimen was induced by exposing the bottom of the specimen to the temperatures corresponding to the standard fire curve described in the ASTM E 119 for 3 h. Impact-echo testing was performed on the bottom surface of the concrete slab specimen before and after inducing the fire damage. It was observed that the spectral responses of the fire-damaged concrete were dominated by several non-propagating waves, which resulted in main peak frequencies around 4500 Hz and 5100 Hz. A discrete layered concrete damage model developed in this study was used to reconstruct the variation of the P-wave velocity with the depth of the fire-damaged concrete. It was demonstrated that the predicted P-wave velocity profile using the simplified model showed a good agreement with the measured values from the five core samples, which measured 100 mm (diameter) by 200 mm (height) cylinders, using ultrasonic pulse velocity (UPV) measurements at eight different depths. In addition, the peak frequencies predicted by the simplified model were consistent with the measured peak frequencies. The experimental results in this study demonstrated that IE testing is effective for evaluating the post-fire damage of reinforced concrete slabs. Particularly, the simplified model in this study can be effective for better interpreting the spectral responses of fire-damaged concrete slabs by IE testing.

Effect of impact source on detection quality in impact echo testing of sleeve grouting

Abstract The quality of grouting is important for the mechanical performance of prefabricated buildings. The impact echo method is used to study the internal condition of grouting sleeves. Mode analysis of the sleeves is carried out by finite element methods. Impact loads with durations of 12 μs, 20 μs and 40 μs were applied to the model for impact echo studies, and the echo signals were compared by FFT transformation and wavelet transform. In impact echo experiments, steel balls with different diameters were used to impact the grouting sleeve. It was found that the characteristic frequency polarization of the echo signal with a diameter of 5 mm was the most pronounced. The research shows that for smallsized inspected objects such as grouting sleeves, the object’s own vibration frequency has a major influence on the detection result.

Effect of impact source on detection quality in impact echo testing of sleeve grouting

Abstract The quality of grouting is important for the mechanical performance of prefabricated buildings. The impact echo method is used to study the internal condition of grouting sleeves. Mode analysis of the sleeves is carried out by finite element methods. Impact loads with durations of 12 μs, 20 μs and 40 μs were applied to the model for impact echo studies, and the echo signals were compared by FFT transformation and wavelet transform. In impact echo experiments, steel balls with different diameters were used to impact the grouting sleeve. It was found that the characteristic frequency polarization of the echo signal with a diameter of 5 mm was the most pronounced. The research shows that for smallsized inspected objects such as grouting sleeves, the object’s own vibration frequency has a major influence on the detection result.

Concrete density estimation of linac bunker walls using impact-echo testing.

To estimate the concrete density of a newly constructed bunker using impact-echo testing prior to the installation of the linear accelerator. A newly constructed bunker showed visible honeycombing after the removal of the construction formwork. Impact-echo testing, which is based on the propagation and reflection of elastic waves in solids, was applied to confirm the bunker shielding integrity. A mechanical impact on the bunker wall generates a stress pulse, which propagates through the wall and is reflected or refracted by voids or changes in material characteristics such as density. Surface displacements caused by the reflected waves are recorded by a transducer, located near the impact point. The resulting displacement-time curves are analysed in the frequency domain for anomalies. The dominant frequencies are related to the depths from which stress waves are reflected within the structure. If the dynamic elastic modulus and Poisson ratio of the concrete are known, then the measured velocity of the so-called P-wave can be related to the concrete density. Validation measurements on a wall with known concrete density gave an estimate within 3% of the true density. Measured velocities on the honeycombed wall ranged from 3750m/s to 4300m/s, corresponding to densities of 2894kg/m3 and 2201kg/m3 respectively, with the majority of estimated densities ranging from 2307kg/m3 to 2544kg/m3. A radiation survey after the installation of the linear accelerator confirmed adequate shielding. Impact-echo testing presents a viable solution to confirm bunker shielding integrity before the installation of a linac.

Application of Tooth Gear Impact-Echo System for Repeated and Rapid Data Acquisition

Developments in air-coupled testing hardware in impact-echo (IE) tests have enabled new levels of scanning tests for concrete bridge decks. A tooth gear IE system has been developed using tooth gears as impactors and microelectromechanical systems (MEMS). Since the tooth gear moves and generates impacts itself, this system collects a large amount of test data across the field continuously. The contact duration of two different tooth gears is evaluated and the contact mechanism is compared to a conventional steel ball impactor by a high-speed camera. The data measurements were carried out on concrete slabs, where artificial delaminations were embedded at different depths. Based on our IE experiments, reducing the pitch or increasing the number of teeth was required to decrease the contact duration and generate the thickness mode frequency from deep delaminations. Rapidly obtained time domain data were transferred to the frequency-time domain using spectrograms to identify the dominant frequency band of the signal set. The results show that the developed system enabled us to acquire high-quality data during air-coupled IE tests and spectrogram analysis provided meaningful frequency information and verified its repeatability.

Preliminary Study on Assessing Delaminated Cracks in Cement Asphalt Mortar Layer of High-Speed Rail Track Using Traditional and Normalized Impact-Echo Methods.

The severe deterioration of a cement asphalt (CA) mortar layer may lead to the movement of the upper concrete slab and impair the safety of the speedy train. In this study, a test specimen simulating the structure of high-speed rail track slabs was embedded with delaminated cracks in various lateral sizes inside the CA mortar layer. Impact–echo tests (IE) were performed above the flawed and flawless locations. In present study, the IE method is chosen to assess defects in the CA mortar layer. Both traditional IE and normalized IE are used for data interpolation. The normalized IE are the simulated transfer function of the original IE response. The peak amplitudes in the normalized amplitude spectrum and the peak frequency in the traditional amplitude spectrum for the top concrete overlay were used to develop simple indicators for identifying the integrity of the CA mortar layer. The index was based on the difference of the experimental peak amplitude and frequency of the ones calculated from previously developed formulas for plates without substrates. As a result, the technique does not require an experimental baseline for the crack assessment. A field test and analysis procedure for evaluating high-speed rail slab systems are proposed.

Guidelines for Impact Echo Test Signal Interpretation Based on Wavelet Packet Transform for the Detection of Pile Defects

Nonlinear amplification is typically done on velocity signals from low-strain pile integrity tests to enhance weak echoes and superimpose any peak reflections. This conventional method may sometimes fail to untangle the hidden information within the signal that is obscured by the presence of noise. In this study, a pile defect identification system based on the conventional nonlinear amplification method and the wavelet packet transform (WPT) was proposed to easily detect the presence of any geometric or material defects by identifying feature parameters. Diagnostic rules, which have been lacking in the literature, were presented to serve as a guide in interpreting decomposed signals and in analyzing various characteristics of peak waveforms that are associated with certain types of defects. In this study, the finite element method was used to simulate the impact echo test of nine cases of defective piles. To verify the proposed scheme, six data sets of the nine cases of defective piles were made, in which a total of 54 piles were analyzed. The results of the study showed that the identification method based on WPT could detect defects 87.04% of the time compared to the conventional method, which only detected defects 64.81% of the time.

Evaluation of Steel Corrosion in Concrete Structures Using Impact-Echo Method

The impact-echo method is commonly used for detection of flaws in concrete elements based on the shift in the thickness frequency of a plate-like member. However, there is a need to develop this efficient technique for other applications. This paper investigates the feasibility of using the impact echo-method for identifying corrosion of steel reinforcement in concrete structures. For this purpose, 180 reinforced concrete cubes were cast and tested. The main parameters studied were the amount of recycled aggregate (i.e. 0%, 25%, 50% and 100%), nanosilica (1.5% and 3%) and the steel bar diameter (12 and 20mm). Different levels of corrosion were electrochemically induced by applying impressed voltage technique for 2, 5, 10 and 15 days. The impact-echo results were correlated against the actual corrosion levels obtained by the mass loss method. The experimental results showed that the response of impact echo in terms of frequency peaks is found to be sensitive to the high and moderate levels of corrosion. However, no clear trend was observed at the initial stage of corrosion. It is demonstrated that the impact-echo testing can be effectively used to qualitatively detect the damage caused by corrosion phenomenon in reinforced concrete structures.

Evaluation of Delamination in Concrete by IE Testing Using Multi-Channel Elastic Wave Data.

The main objectives of this study are to develop a non-destructive test method for evaluating delamination defects in concrete by the Impact-echo test using multi-channel elastic wave data and to verify the validity of the proposed method by experimental studies in the laboratory. First, prototype equipment using an eight-channel linear sensor array was developed to perform elastic wave measurements on the surface of the concrete. In this study, three concrete slab specimens (1500 mm (width) by 1500 mm (length) by 300 mm (thickness)), with simulated delamination defects of various lateral dimensions and depth, were designed and constructed in the laboratory. Multi-channel elastic wave signals measured on the three concrete specimens were converted to the frequency-phase velocity image by using the phase-shift method. A data processing method was proposed to extract the dominant propagating waves and non-propagating waves from the dispersion images. The dominant wave modes were used to evaluate delamination defects in concrete. It was demonstrated that the surface wave velocity values were useful for characterizing the shallow delamination defects in concrete. In addition, the peak frequency of non-propagating wave modes extracted from the dispersion images gives information on the lateral dimensions and depths of the delamination defects. This study also discussed the feasibility of combined use of the results from propagating and non-propagating wave modes to better understand the information on delamination defects in concrete. As will be discussed, the multi-channel elastic wave measurements enable more accurate, consistent, and rapid measurements and data processing for evaluation of delamination defects in concrete than the single-channel sensing method.

Distinction between crack echoes and rebar echoes based on Morlet Wavelet Transform of impact echo signals

In an Impact-Echo (IE) test, the Fast Fourier transform (FFT) of the impacted surface responses reveals frequency peaks of major periodical events and can be used to determine depth information, such as the thickness of a target specimen and the depth of a delaminated crack. Since rebar interfaces also result in comparable spectral frequencies but with different depth characteristics, IE testers might have difficulty to determine whether a certain peak frequency is associated with a crack or a rebar. This paper aims to illustrate the use of the time-frequency analysis with Morlet Wavelet transform (MWT) to discriminate the crack response from the rebar response. In additional to the traditional FFT spectrum, the time-frequency technique provides more informative data to assist the analysis task.The proposed method for recognizing crack and rebar can be carried out first by analyzing the peak frequencies in the FFT spectrum. Next, the amplitude histogram at the selected frequency is extracted from the time-frequency spectrogram. Through evaluating the decay pattern of the amplitude curve, an index representing the “duration of amplitude-decay” is defined. Based on the comparison between results of the two studied interfaces, namely concrete-void and concrete-steel, it is preliminarily summarized that the reflected signals are generally less significant at the concrete-steel interface. A rebar leads to weaker reflective energy and thus shortens duration of amplitude-decay. Therefore, it is concluded that the proposed methodology can be applicable to distinguish the difference between the crack echoes and the rebar reflections based on the spectrogram obtained with the time-frequency analysis. Accordingly, this innovated technique provides analysts with an alternative scheme in the recognition/verification of cracks and rebars via the IE tests.

HHT-Based CEEMD to Improve an Impact Echo Test

Abstract An impact echo (IE) test is a common, low-strain integrity test used to evaluate the thickness and inner condition of concrete plates. The vibration signal propagating along the thickness of a plate is of concern. However, the measured signal includes other waves, such as surface waves, reflective waves, noises from the environment, and so on. These unwanted signals affect the detection of plate integrity. The Hilbert–Huang transform (HHT), which is based on complementary ensemble empirical mode decomposition (CEEMD), has been found to distinguish and interpret IE signals effectively. This method breaks signals down into several meaningful ingredients called intrinsic mode functions (IMFs). CEEMD can also substantially reduce mode mixing and the final residue. The critical parameters of CEEMD are studied by coherence in the frequency domain, and their performance is demonstrated in this study. A Hilbert spectrum analysis is also proposed to distinguish various types of IE signals and enhanced to explain its properties. After obtaining the IMF of the echo wave, the corresponding frequency of the peak magnitude could be used to calculate the interface position. In addition, an approach is also proposed to calculate the depth interface using the IMF phase information, and the result shows that it can be evaluated successfully.

A multidisciplinary approach to calibrating advanced numerical simulations of masonry arch bridges

Abstract This paper proposes a robust multidisciplinary method that combines geomatic procedures (terrestrial laser scanning and reverse engineering), geophysical methods (ground-penetrating radar and multichannel analysis of surface waves), sonic and impact echo tests, and ambient vibration approaches to generate accurate numerical simulations of masonry arch bridges. These methods are complemented by a robust finite element model updating method based on metamodeling global sensitivity analysis and a robust calibration strategy. The results obtained corroborate the feasibility of the proposed methodology with an average relative error in frequencies of 1.21% and an average modal assurance criterion of 0.93.

2D finite element simulation of air-coupled impact echo testing on concrete slab

Impact-echo is an effective method in identifying and detecting delamination in concrete structures. Recently, the contact sensor is replaced with air-coupled sensor due to the interest in rapid testing method. This paper presents a study of the optimum test configuration of air-coupled impact echo and conventional concrete-coupled impact echo tests by means of numerical simulation. Two-dimensional finite element analyses were performed on concrete slab to investigate the effect of impact-to-sensor distance and sensor height from concrete surface. Two concrete slabs, one without delamination and one with delamination, were modelled. For intact concrete, both sensors were effective in detecting concrete thickness at any point within the specified location range. However, for air-coupled IE performed on concrete with 40 mm delamination depth, optimum sensor location is 60 to 100 mm away from impact point at height of 10 mm.

Automated Air-Coupled Impact-Echo Testing of a Concrete Bridge Deck from a Continuously Moving Platform

The objective of this work was to develop an automated air-coupled impact-echo testing device for mapping the occurrence of delamination in a concrete bridge deck from a continuously moving platform with a fast, repeatable excitation mechanism and algorithms for collecting and analyzing the acoustic data. The apparatus developed in this research included an impactor unit, a moving platform, a microphone for air-coupled sensing, a distance measurement instrument, and signal processing modules. Given the absence of an existing universal threshold for differentiating among intact and delaminated areas using the new device, a delamination detection threshold value was determined in a first field demonstration also involving chain dragging and coring of a concrete bridge deck. While the maps of the air-coupled impact-echo data and chain-dragging data exhibit a high degree of similarity and are highly consistent with the results of coring, a quantitative method of comparison also demonstrates the utility of the new device across a broad range of delamination percentages. Specifically, the results indicate that the percentage of the deck area determined to be delaminated using impact-echo testing was within 3 percentage points of that determined to be delaminated using chain dragging for 10 of the 13 deck test sections, which generally exhibited delamination percentages ranging from 7 to 21%. Finally, testing with the new device was more than seven times faster than chain dragging.

Evaluation of the Impact Echo Method for Concrete Bridge Decks with Asphalt Overlays

Asphalt overlays have been commonly used to extend the service life of deteriorated concrete bridge decks. Nonlinear, viscoelastic asphalt has properties that differ considerably from the properties of the underlying portland cement concrete (PCC) deck. The impact echo (IE) method has been extensively used to nondestructively evaluate PCC structures. The method, however, performs differently on asphalt overlays, especially when a membrane is placed between the overlay and the deck. The general recommendation has been to perform IE tests on asphalt overlays in cold weather. The temperature range under which IE can be used, however, is still uncertain. This study was conducted to systematically examine IE performance on asphalt overlays. Three PCC specimens with three types of asphalt overlays―asphalt with a liquid membrane, asphalt with a sheet membrane, and asphalt without a membrane―were tested in a temperature chamber. The three PCC specimens had identical designs, materials, and artificial defects. The nondestructive evaluation (NDE) results from the tests indicate that: (1) 32°F and below is the appropriate temperature range for IE to detect defects in the underlying deck; and (2) membranes can significantly reduce the propagation of waves into the underlying deck.

Application of Impact Echo Test Method to Detection of Separation Void between Layers of Concrete Slab Track Foundation

Application of Impact Echo Test Method to Detection of Separation Void between Layers of Concrete Slab Track Foundation

Effects of elastic supports and flexural cracking on low and high order modal properties of a reinforced concrete girder

A number of nondestructive evaluation (NDE) methods are available to detect cracks and damage in reinforced concrete structures. Methods such as ultrasonic, impact echo, or X-ray testing have high resolution but are time consuming to perform and therefore only used locally. Since the early 1970s, researchers have also studied the modal properties of structures for damage detection and identification. In this paper, we discuss the effects of elastic supports and flexural cracking on the dynamic response of a large-scale laboratory reinforced concrete girder. For this purpose, an instrumented hammer used for impulse response testing was employed to initiate vibrations and to subsequently estimate the natural frequencies and modes of a large-scale laboratory concrete girder. Natural frequencies up to the 13th mode were successfully extracted from the frequency response function (FRF). In addition, the mode shapes were extracted up to the 6th mode due to the limitation of the linearity of the FRF response. These modal properties were determined from the measured accelerations of two points on the girder using a roving hammer. The test procedure was performed on the uncracked beam and repeated after the beam had been cracked. The experimental measurements were verified with a finite element (FE) model consisting of one-dimensional beam elements that consider both flexural and shear deformations. Support flexibility was modeled as linear-elastic springs, and was found critical in order to explain experimentally-obtained measurements. The effects of cracking were found to be most obvious in the higher modes. Additionally, utilizing the modal flexibility method on the first mode (natural vibration frequency and mode shape) was an accurate predictor of flexural crack locations.

Frequency Wave Number–Domain Analysis of Air-Coupled Impact-Echo Tests in Concrete Slab

AbstractAn air-coupled (also known as noncontact or contactless) sensing technique in the impact-echo method has been developed and its practical application to the field has been significantly imp...