Géraldine Villain

Analysis of coherent surface wave dispersion and attenuation for non-destructive testing of concrete

Rayleigh waves measurements are used to characterise cover concrete and mortar in the frequency range 60-180 kHz. At these frequencies, the wavelength is comparable to the size of the aggregates, and waves propagate in a multiple scattering regime. Acquired signals are then difficult to interpret due to an important incoherent part. The method proposed here is the study of the coherent waves, obtained by averaging signals over several configurations of disorder. Coherent waves give information on an equivalent homogeneous medium. To acquire a large amount of measurements with accuracy, an optimised piezoelectric source is used with a laser interferometer for reception. Adapted signal processing technique are presented to evaluate the coherent phase and group velocities and also the coherent attenuation parameter. The sensitivity of these three parameters with the properties of concrete is discussed, as well as the necessity to use coherent waves to obtain accurate results.

Assessment of transport properties of cementitious materials : A major challenge as regards durability ?

ABSTRACT In this paper, the liquid water permeability has been investigated by various methods in laboratory conditions. A comparison has been performed between the “intrinsic” permeabilities thus obtained. The values have also been compared to the results obtained from gas permeability measurement. The results point out that the concept of intrinsic permeability, independent of the permeating fluid, is not relevant for cementitious materials, when liquid water is regarded, except maybe for highly permeable ones. The various physicochemical processes likely to occur during the tests can explain the significant differences observed. The apparent chloride diffusion coefficient has been assessed by means of diffusion and electrical migration tests. The differences recorded have been discussed. They can be explained in particular by the various microstructural changes that occur during these two types of tests, as illustrated by MIP and 29Si NMR results.

Modélisation du séchage des bétons : Analyse des différents modes de transfert hydrique

ABSTRACT A modelling of drying of concrete is presented. « Intrinsic » liquid water (Kl) and gas (Kg) permeabilities are distinguished, since the concept of intrinsic permeability, which is independent of the fluid nature, is not relevant for a cementitious material. New laws for gas transfers are introduced. Thus, a law gives the effective diffusion coefficient of water vapour as a function of porosity and degree of liquid water saturation. In the same way, a new function, expressing the relative permeability to gas according to this degree of saturation, is calibrated thanks to experimental results. In order to describe the global movement of gas, viscous and slip flows are taken into account according to the Klinkenbergs concept. A numerical study shows, on the one hand, that a gas depression (below the atmospheric gas pressure) can be observed and, on the other hand, that transfers of water in the gas phase can significantly contribute to the drying of cementitious materials in addition to liquid water transport by capillarity. Simplified approaches are presented. Finally, a calibration of Kl is carried out thanks to drying experiments.

Comparaison et validité des méthodes de mesure de la carbonatation

ABSTRACT The modeling of concrete carbonation needs physical parameters well determined, especially those of the real carbonation front and the quantity of calcium carbonate formed after carbonation. This paper proposes a comparison between different carbonation measurement methods. Obtained results are slightly different from used method. Furthermore, the interest and the limit of each technique are showed.

Assessment of Concrete by a Combination of Non-Destructive Techniques

Abstract: The evaluation of the properties of concrete is an important objective for managers and engineers of the diagnosis of structures. The contribution of non-destructive evaluation methods (NDE) is essential because it allows us to obtain information on the quality of the material or its durability without damaging the material examined. This evaluation includes two objectives: (a) an evaluation of mechanical indicators (modulus, mechanical strength, stress), and (b) an evaluation of durability indicators (water content, chloride content, porosity, carbonate depth, etc.).

Construction of Conversion Models of Observables into Indicators

Abstract: Non-destructive techniques can be used in concrete structures for detection, localization (1D, 2D or 3D) or quantification. These three sets of objectives correspond to objectives of increasing ambition: – in the first case, it is about detecting the presence of a defect, a singularity or a local variation of property: presence of a rebar, thickness change, compactness defect, etc. It is based on the sensitivity of the non-destructive measurement to the desired change; – in the second case, we aim to map an observable (the result of non-destructive measurement), for example the resistivity which varies according to the water content of the cover concrete, or the velocity of the radar wave. As in the first case, it exploits the sensitivity of the measurement to the expected contrast and can also take advantage of the velocity of acquisition of non-destructive measurements to rapidly map large areas; – the third objective is more ambitious and more difficult to satisfy: it is a matter of estimating a quantity, called an Indicator (I), from the non-destructive measurement of an Observable (O). The sensitivity of the value of I to any change of O is of course necessary, but it is also necessary to have a model M which will make it possible to deduce the value of I from the measurement of O in the form I = M(O).

Applications In Situ

Abstract: For several years research efforts have been under way to: – develop non-destructive evaluation methods (NDE), their quality, their exploitation and the analysis of the observables obtained for the ultrasonic (US), electromagnetic (EM) and electrical methods; – improve empirical and physical conversion models (also called calibration models), linking observables of NDE and durability indicators through destructive (DE) and non-destructive (NDE) laboratory evaluations; – combine on-site observables and conversion models to eliminate biases and assess indicators more accurately as well as monitor parameters for degradation of the study; – structure the transfer methodology of all these tools developed in the laboratory to the auscultation of reinforced concrete structures, as proposed during the ACDC project financed by the French Civil and Urban Network and the French Ministry of Ecology and Sustainable Development, as explained of this book.