Dimitrios G. Aggelis

Acoustic Emission and Ultrasound for Damage Characterization of Concrete Elements

The acoustic emission (AE) technique is widely used for real time damage detection in concrete. It uses stress waves emerging from nucleation and propagation of cracks recorded on the surface of the material by suitable sensors. In the present study, AE is used to monitor the deterioration progress of reinforced concrete beams subjected to four-point bending. The specimens consist of two layers of plain and fiber concrete. At different loading steps, ultrasonic pulse velocity measurements were also conducted to obtain the transient three-dimensional tomographic reconstruction of the internal structure. The AE source location is in good agreement with the velocity structure visualization and the results are confirmed by visual observation of the actual cracks developed. The results show that the AE technique and velocity tomography are useful tools to study the failure progress of concrete.

Experimental study of surface wave propagation in strongly heterogeneous media

In the present paper, the propagation of Rayleigh waves in a strongly heterogeneous medium is discussed. Scattering of stress waves is a difficult scientific problem. Specifically, the interaction of surface waves with distributed inhomogeneity seems highly complicated due to the existence of two displacement components. Rayleigh waves undergo significant attenuation and velocity change depending on the frequency and the inhomogeneity content. The aim of this study is to highlight the dispersive behavior of concrete, especially when damaged, and increase the experimental data in an area where the work is limited.

An acousto-ultrasonic approach for the determination of water-to-cement ratio in concrete

A novel nondestructive procedure for the evaluation of water-to-cement (w/c) ratio in concrete is presented. The experimental setup is based on the method of acousto-ultrasonics; data analysis, however, and recognition of concrete composition from the waveform transmitted through specimen thickness, are achieved by simple time and frequency domain schemes used in this work. Experiments were performed in a number of concrete specimens made at various w/c ratios and at a number of ages starting from 2 up to 90 days. Recognition results were satisfactory and the algorithm introduced was successful in identifying the correct w/c ratio in more than 90% of the test cases. The use of existing spectral analysis techniques such as the coherence function has also proved to be more efficient and fits the purpose. The possibility of water content determination in fresh paste is also discussed along with preliminary evidence from initial tests.

Detecting the Activation of a Self-Healing Mechanism in Concrete by Acoustic Emission and Digital Image Correlation

Autonomous crack healing in concrete is obtained when encapsulated healing agent is embedded into the material. Cracking damage in concrete elements ruptures the capsules and activates the healing process by healing agent release. Previously, the strength and stiffness recovery as well as the sealing efficiency after autonomous crack repair was well established. However, the mechanisms that trigger capsule breakage remain unknown. In parallel, the conditions under which the crack interacts with embedded capsules stay black-box. In this research, an experimental approach implementing an advanced optical and acoustic method sets up scopes to monitor and justify the crack formation and capsule breakage of concrete samples tested under three-point bending. Digital Image Correlation was used to visualize the crack opening. The optical information was the basis for an extensive and analytical study of the damage by Acoustic Emission analysis. The influence of embedding capsules on the concrete fracture process, the location of capsule damage, and the differentiation between emissions due to capsule rupture and crack formation are presented in this research. A profound observation of the capsules performance provides a clear view of the healing activation process.

Mechanical property distribution of CFRP filament wound composites

Abstract The extending use of composite materials and the simultaneous need for reliability-based design of structural components has introduced the stochastic modelling of material properties, as implied by several design codes and standards. Therefore, for using such materials in advanced applications, the stochastic character of their properties needs to be taken into account. In this work the statistical aspects of strength and elastic properties of carbon/epoxy (C/Ep) filament wound composites are determined through a series of static tensile and compressive tests. Six candidate statistical distributions were used in order to model the strength and elastic properties of the composite, but none of them was proved to be a better fit to the experimental results, as revealed by the Kolmogorov–Smirnov test. Therefore, anyone of them can be used equally well to model mechanical property distributions. Although not expected, the experimental investigation revealed considerable scattering on the elasticities, besides strength, indicating that the variation of the elastic properties should be implemented in an adequate reliability analysis.

The influence of propagation path on elastic waves as measured by acoustic emission parameters

Apart from the quantitative parameters of acoustic emission testing, such as the total activity or the location of the sources, much more information can be exploited by qualitative characteristics of the signals. The shape of the waveform strongly depends on the source, supplying information on the type of cracks. Shear cracks which normally follow tensile during fracture, emit signals with longer rise time as well as lower average frequency. However, due to the inherent inhomogeneity of the media, which is enhanced by the nucleation of cracks, each pulse suffers strong dispersion which results in serious alteration of the waveform shape. Therefore, classification of cracks based on acoustic emission parameters would be probably misleading in case the separation distance of the sensors is long or the material contains many cracks. In the present study, numerical simulations were conducted in order to examine the influence of distance on the shape distortion of an excited wave inside concrete. Results are compared with actual experiments on steel fiber reinforced concrete, showing that the distance between the source crack and the acquisition point should not exceed a threshold value in order to lead to reliable crack classification.

Assessment of Construction Joint Effect in Full-Scale Concrete Beams by Acoustic Emission Activity

In the present paper the mechanical and acoustic emission (AE) behaviors of full-scale reinforced concrete beams are evaluated. One of the beams was constructed in two parts, which were assembled later in order to evaluate the effect of the joints in the structural behavior. The load was applied by means of a four-point-bending configuration. It is revealed that at initial stages of loading, the conventional measurements of strain and deflection, as well as pulse velocity, do not show any discrepancy, although the structural performance of the two beams is eventually proven to be quite different. On the contrary, AE parameters, even from early load steps, indicate that the damage accumulation is much faster in the assembled beam. This is confirmed by the calculated sources of AE events which are close to the construction joints. The results show that the AE technique is suitable to monitor the deterioration process of full-scale structures and yields valuable information that cannot be obtained at the early stages of damage by any other way.

Effect of Inhomogeneity Parameters on Wave Propagation in Cementitious Material

In this study, the relationship between inhomogeneity in cementitious material and stress wave parameters is investigated by measuring parameters of through transmission measurements of ultrasonic waves. Except from the inherent inhomogeneity of this type of material, the presence of damage in the form of cracks can lead to even more highlighted velocity dispersion and attenuation phenomena for specific bands of frequencies. Therefore, different contents of crack-like, film-shaped particles are included during casting of concrete to evaluate the contribution of distributed damage in the observed wave parameters. Experiments are carried out using low- and high-frequency sensors with the range of frequencies also covering those used for in-place application.

Crack sealing and damage recovery monitoring of a concrete healing system using embedded piezoelectric transducers

The autonomous healing performance of concrete is experimentally verified by applying a technique based on the ultrasonic pulse velocity method using embedded piezoelectric transducers. Crack opening which deteriorates the mechanical capacity of concrete infrastructure is traditionally studied by different monitoring techniques that adequately provide a direct estimation of damage. Conversely in this research, an ultrasonic pulse velocity method is applied in order to monitor the crack closure and sealing of small-scale concrete beam elements. Short glass capsules filled with healing adhesive break due to crack formation and release those healing additives which fill the crack void and reset the element continuity. The damage index based on the early part of the wave arrival observes any emitted signal shape differentiation indicating the crack formation and development under two-cycle three-point bending loading tests (in the first cycle, the crack forms and healing release takes place, and consequently, after few hours of curing and crack reset, the beam is reloaded leading to crack reopening).

Rupture of anterior cruciate ligament monitored by acoustic emission

The scope of this study is to relate the acoustic emission (AE) during rupture of human soft tissue (anterior cruciate ligament, ACL) to the mechanisms leading to its failure. The cumulative AE activity highlights the onset of serious damage, while other parameters, show repeatable tendencies, being well correlated with the tissues mechanical behavior. The frequency content of AE signals increases throughout the experiment, while other indices characterize between different modes of failure. Results of this preliminary study show that AE can shed light into the failure process of this tissue, and provide useful data on the ACL reconstruction.