Analysis of coherent surface wave dispersion and damping for non destructive testing of concrete (original) (raw)
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Maintenance and rehabilitation of concrete structures affected by alkali-aggregate reaction (AAR) require conducting detailed assessment of the concrete conditions, mainly close to the surface where the damage is more severe. This paper presents in situ investigations by surface wave testing of near-surface AAR damage in two hydraulic structures. The survey was carried out using a non-intrusive multi-sensor method that involves frequency–wavenumber analysis of surface waves. The method allows solving Rayleigh surface wave propagation modes required for the determination of the shear wave velocity in terms of depth. The variation of Young’s modulus with concrete depth can be estimated from the obtained shear wave velocity profile. Two different cases of surface wave propagation, typical of concrete structures, are discussed in this paper. The tests were conducted from the concrete surface only and the subsurface quality was mapped up to a depth of 1.50 m. The applications show that the proposed surface wave method is a potential non-destructive evaluation method that can be used to detect and locate near surface damage in concrete structures. La maintenance et la réhabilitation des structures en béton atteintes de Réaction Alcalis-Granulats (RAG) requièrent une étude détaillée de la qualité du béton, principalement près de la surface où l’endommagement est le plus sévère. Cet article présente les résultats d’investigation par ondes de surface de l’endommagement attribuable à la RAG dans deux structures hydrauliques. Les mesures ont été effectuées en utilisant une méthode non intrusive à plusieurs capteurs, qui intègre l’analyse fréquence-nombre d’onde des ondes de surface. Cette méthode permet de résoudre les modes de propagation des ondes Rayleigh de surface, nécessaires pour la détermination de la vitesse des ondes de cisaillement en fonction de la profondeur. La variation du module d’élasticité avec la profondeur peut être estimée à partir du profil de vitesse des ondes de cisaillement obtenu. Deux cas différents de propagation des ondes de surface, typiques pour les structures en béton, sont discutés dans cet article. Les tests ont été effectués à partir de la surface du béton seulement, et la qualité du béton a été cartographiée jusqu’à une profondeur égale à 1.5 m. Les présentes applications montrent une méthode non destructive potentielle qui peut être utilisée pour détecter et localiser l’endommagement près de la surface du béton.
Stress Wave Scattering: Friend or Enemy of Non Destructive Testing of Concrete?
Journal of Solid Mechanics and Materials Engineering, 2008
Cementitious materials are by definition inhomogeneous containing cement paste, sand, aggregates as well as air voids. Wave propagation in such a material is characterized by scattering phenomena. Damage in the form of micro or macro cracks certainly enhances scattering influence. Its most obvious manifestation is the velocity variation with frequency and excessive attenuation. The influence becomes stronger with increased mis-match of elastic properties of constituent materials and higher crack content. Therefore, in many cases of large concrete structures, field application of stress waves is hindered since attenuation makes the acquisition of reliable signals troublesome. However, measured wave parameters, combined with investigation with scattering theory can reveal much about the internal condition and supply information that cannot be obtained in any other way. The size and properties of the scatterers leave their signature on the dispersion and attenuation curves making thus the characterization more accurate in case of damage assessment, repair evaluation as well as composition inspection. In this paper, three indicative cases of scattering influence are presented. Namely, the interaction of actual distributed damage, as well as the repair material injected in an old concrete structure with the wave parameters. Other cases are the influence of light plastic inclusions in hardened mortar and the influence of sand and water content in the examination of fresh concrete. In all the above cases, scattering seems to complicate the propagation behavior but also offers the way for a more accurate characterization of the quality of the material.
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The French national research project SENSO aims at providing a methodology combining several non destructive testing methods to evaluate indicators required for assessing the durability of concrete cover. A large set of 0.5m x 0.25m x 0.12m slabs has been built for various water/cement ratios, aggregate sizes and type. Those slabs have been subsequently studied under controlled water saturation level,
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Results from an experimental study concerning wave propagation in cementitious materials are presented in this paper. Narrow band pulses at several frequencies were introduced into specimens of cement paste, mortar and concrete allowing direct measurement of longitudinal wave velocities and amplitude for each frequency. It is shown that aggregate content play an important role in wave propagation increasing considerably the wave velocity, while the aggregate size seems to control the attenuation observed. Slight velocity variations observed with frequency are discussed in relation to the degree of inhomogeneity of the materials.
Effect of Aggregate Size on Attenuation of Rayleigh Surface Waves in Cement-Based Materials
Journal of Engineering Mechanics, 2000
This research uses laser ultrasonic techniques to study the effect of aggregate size on the attenuation of Rayleigh surface waves in cement-based materials. The random, multiphase, and heterogeneous nature of cement-based materials causes a high degree of material attenuation in the ultrasonic waves that propagate in these materials. Physically, these attenuation losses are due to a combination of absorption and the scattering losses due to material heterogeneity. Laser ultrasonics is an ideal methodology to measure attenuation in these materials because of its high fidelity, large frequency bandwidth, and absolute, noncontact nature. To investigate the effect of aggregate size on attenuation, this research uses a dual-probe, heterodyne interferometer to experimentally measure attenuation losses (as a function of frequency) in five different material systems (each with a different microstructure). These experimental results show that absorption, not scattering from the aggregate, is the dominant attenuation mechanism present in cement-based materials. As a result, aggregate size does not dominate attenuation.
ndt.net
We are concerned by the design of a non destructive ultrasonic method quantifying porosity of cover concrete. Modification of porosity is a major cause of reinforcing bar corrosion that induces bar swelling and macro-cracks which may cause the ruin of the structure. It is then necessary to characterize the porosity in the first cm above the steel bars. For surface measurements the most energetic mode is the Rayleigh wave whose investigation depth is around a half wavelength. A large spectrum, with frequency between 50 kHz and 600 kHz, is used in order to obtain a porosity depth profile of the concrete cover. We consider the concrete as a two phases media composed of aggregates with various radii embedded in a homogeneous mortar. The porosity of mortar is modeled through wave damping.For strongly heterogeneous media, the wave field can be analyzed as the superposition of a coherent part and an incoherent part. Here we focused on the coherent field by using dynamical homogenization theories. The model is coupled to dispersion relation of Rayleigh waves in order to depict the behavior of coherent surface waves. This model allows us to evaluate the sensitivity of surface waves to a variations of mortar properties such as an increase of its porosity.
Influence of water gradient on surface wave measurements in concrete
2015
The knowledge of water gradient in cover concrete is of primary importance for the estimation of concrete durability. In this paper the influence of water gradient on surface wave phase velocity dispersion curves is studied on concrete slabs during water absorption and drying. The comparison of surface wave phase velocity dispersion curves measured at various times corresponding to different water gradients shows that surface waves are sensitive to water ingress. Furthermore, taking information from embedded sensors and gammadensimetry, and using master curves established for an homogeneous water content, it is possible to predict the surface wave phase velocities variations. This suggest that, eventhough the variations of surface wave phase velocity are only a few %, the comparison to a reference state should make it possible to follow variation of water gradient.
Spectral Analysis of surface waves for evaluating actual concrete structures at early ages
The Group of Mechanics of Solids and Structures (TEP167) of the Department of Structure from the University of Granada, in charge of Professor Rafael Gallego Sevilla, develops lines of work of non-destructive methods in predictive structural monitoring based on the analysis of vibrations, inverse problems for identification of damage in structures based on static-dynamic response, and ongoing development of a line of research of theoretical and experimental structural analysis based on wave propagation by vibration impact for the determination of the elastic properties and mechanical strength of real structures of concrete at early age curing. In collaboration with Fernando M. Soto, PhD student, researcher applies the technology of the uniaxial piezoelectric accelerometers charge, Pulse Lan-XI analyzer and impact hammers of Brüel & Kjaer in the study of the art multi-channel SASW "Spectral Analysis of Surface Waves" (ACI228.2R-98) for the development of a numerical model of the behavior of the concrete during the curing process a t early ages (9h-60days), used to determine the dynamic elastic properties of the material (Young´s modulus, Poisson´s ratio, Shear modulus and compressive strength) from the experimental dispersion curve in order to reduce curing times in real concrete structures.