Internal Defects Of Concrete Research Articles

Defect detection using spatial spectral entropy for noncontact acoustic inspection method

Inspection of concrete structures is important for maintaining social infrastructure such as tunnels and viaducts. Therefore, noncontact acoustic inspection method using acoustic irradiation induced vibration and a scanning laser Doppler vibrometer (SLDV) has been devised. Since this method uses flexural resonance, it can be said to be an alternative method for a hammering test. Using our method, internal defects of concrete can be detected up to a depth of about 10 cm from the surface of concrete, even at a distance of about 30 m. However, in a closed space such as an underground cavity, the signal-to-noise (S/N) ratio decreases due to reverberation from the surroundings, making it difficult to detect defects compared to open spaces such as viaducts. This problem can be solved by using spatial spectral entropy (SSE), which extends the concept of spectral entropy to the two-dimensional space of the measurement plane. In other words, by SSE analysis, the resonance frequency of both the defect and the SLDV can be detected and separated because of the difference in the spatial distribution characteristics of the vibration velocity spectrum for each the frequency. Therefore, it is possible to detect a defective part even in a place where the S/N ratio is not sufficient.

A Review of Research on Detection Methods for Internal Defects in Concrete

This article introduces the commonly used detection methods for internal defects in concrete; A review was conducted on the research progress of impact echo method and infrared thermal imaging method. The principles of impact echo method and infrared thermal imaging method were studied, and the problems existing in existing methods were pointed out. The results indicate that the detection methods for internal defects in concrete have made certain progress in both theoretical research and engineering applications. However, further improvement and refinement are still needed. The most important step in the improvement plan is to determine the damage situation of reinforced concrete components. From multiple perspectives, non-destructive testing is undoubtedly the main development object for detecting concrete damage in the future.

Detection of Internal Defects in Concrete and Evaluation of a Healthy Part of Concrete by Noncontact Acoustic Inspection Using Normalized Spectral Entropy and Normalized SSE.

Non-destructive testing, with non-contact from a remote location, to detect and visualize internal defects in composite materials such as a concrete is desired. Therefore, a noncontact acoustic inspection method has been studied. In this method, the measurement surface is forced to vibrate by powerful aerial sound waves from a remote sound source, and the vibration state is measured by a laser Doppler vibrometer. The distribution of acoustic feature quantities (spectral entropy and vibrational energy ratio) is analyzed to statistically identify and evaluate healthy parts of concrete. If healthy parts in the measuring plane can be identified, the other part is considered to be internal defects or an abnormal measurement point. As a result, internal defects are detected. Spectral entropy (SE) was used to distinguish between defective parts and healthy parts. Furthermore, in order to distinguish between the resonance of a laser head and the resonance of the defective part of the concrete, spatial spectral entropy (SSE) was also used. SSE is an extension of the concept of SE to a two-dimensional measuring space. That is, based on the concept of SE, SSE is calculated, at each frequency, for spatial distribution of vibration velocity spectrum in the measuring plane. However, these two entropy values were used in unnormalized expressions. Therefore, although relative evaluation within the same measurement surface was possible, there was the issue that changes in the entropy value could not be evaluated in a unified manner in measurements under different conditions and environments. Therefore, this study verified whether it is possible to perform a unified evaluation for different defective parts of concrete specimen by using normalized SE and normalized SSE. From the experimental results using cavity defects and peeling defects, the detection and visualization of internal defects in concrete can be effectively carried out by the following two analysis methods. The first is using both the normalized SE and the evaluation of a healthy part of concrete. The second is the normalized SSE analysis that detects resonance frequency band of internal defects.

Elastic Wave Velocity and Attenuation Tomography Using Randomly-Induced Excitations for Damage Detection of RC Slab

As it is imperative for the damage evaluation of infrastructures to establish an efficient non-destructive testing (NDT), an acoustic emission (AE) tomography technique has been developed. Authors has been studying tomography techniques based on elastic-wave and acoustic emission (AE) to visualize the internal defects in concrete. In the result of AE tomography, it can reasonably be assumed that lower elastic-wave velocity corresponds to heavier deterioration. Thus, the AE tomography estimates wave velocity distribution, which is supposed to be decreased as the damage progresses, inside the reinforced concrete. The AE tomography combines an iterative AE source location algorithm with travel-time tomography to produce a 3D visualization of the elastic wave velocity. However, since the computation for the elastic wave ray-trace algorithm considering all potential detours of elastic waves takes up much time, and in the case that only a few AE signals are detected, AE tomography technique does not always work efficiently. In this paper, AE signals induced by random hammering and rain droplets, which provide elastic waves with a variety of frequency, on surface of RC deck are utilized as elastic waves’ excitations, and wave velocity and attenuation tomography assuming linear ray paths are performed in conjunction with AE source locations. Accordingly, random hammering lead to a hundred of AE events after in-situ measurement for a few minutes and rain-induced elastic waves also does to thousands of those as well for an hour. Consequently, the 3D tomography results show accurate and time-saving analysis, compared to the above-mentioned conventional AE tomography technique, for quantifying the damage of RC slab.

Evolution of Fatigue Damage in Wheel-Loading Tests Evaluated by 3D Elastic-Wave Tomography

Currently, for proper maintenance of infrastructures, preventive and proactive measures for prognosis of infrastructures are preferable in comparison with reactive/corrective maintenances of structures that are highly deteriorated. This is so because vast sums are generally necessary for recovering the performance of the highly damaged structures. Therefore, prognostic maintenance must be conducted to establish economic and efficient management systems for the existing concrete infrastructures to complete their designed service life and to even extend them. Severe deterioration of aging infrastructures is currently a critical issue. In particular, the damage and deterioration of concrete slabs in bridges and highways are regarded as a critical issue worldwide. These components are often so fatigue-damaged under conditions of heavy traffic that repair and retrofit work definitely require regulating the traffic, thereby severely disrupting their function for the users. Consequently, preventive and proactive maintenances of concrete slabs that are in service are being urgently demanded for establishing the prognosis for civil engineering. To decide the maintenance systems based on the prognosis of concrete slabs, evolution of the fatigue damage and internal defects should be evaluated properly, if possible, visually. In this respect, Acoustic Emission (AE) tomography and elastic-wave tomography is under investigation and development as innovative nondestructive testing (NDT) methods. By determining the three-dimensional velocity distribution inside a slab via the above methods, the damaged or deteriorated areas are identified. Until now, regulated on-site visual inspections are only performed for the slab components of in-service infrastructures. However, the recent methods can predict the internal defects before the deteriorations physically emerge on the surface. Therefore, inspection methods to identify internal defects in concrete are to be readily implemented prior to the repair works. In the present work, a comparative study is performed during the internal progress of the fatigue damage induced by wheel-loading to identify the damaged area quantitatively via elastic-wave tomography, followed by a comparison with resultant surface crack conditions. The results show a good agreement between the predicted low-velocity zones and the damaged areas estimated by crack distributions, displacements, and strains. In particular, at locations where cracks are intensely observed, the velocities decrease below 3400 m/s. Furthermore, the areas with velocities below 2700 m/s are also observed in the slab corresponding to the attainment of the fatigue limit.

Detecting the Defects in Concrete Components with Impact-Echo Method

With the large-scale application of the prestressed concrete structure, the quality of the concrete component defects and pipeline grouting has increasingly become the focus of attention. The impact-echo scanner uses the nature of wave, which pass though different media at different velocities, to distinguish internal defects of concrete, pipe filling density and so on. In this paper, using the impact-echo method to detect the concrete block with prefabricated defects of shape, location, and size explores the effect of defect properties, parameter settings and detection environment to impact-echo preliminarily and also explores the relationship of pipeline filling status and impact-echo image. Based on this study, the article raised the problem met during this non-destructive testing methods applied to engineering, and accumulated a certain amount of available engineering data. The experiment results show that using the impact-echo method to identify the defects of concrete components and to test the quality of pipeline grouting is a more convenient and effective non-destructive testing method. Especially, with the radar method in the pipeline grouting quality inspection which complement each other to make up for the shortcomings the lightning wave in case of the metal medium total reflection phenomenon, cannot detect metal pipe grouting plumpness.