Numerische Simulation von Temperaturgradienten und thermisch induzierten Eigenspannungen in Natursteinplatten infolge von Sonneneinstrahlung

The interaction between micro-organisms and natural stone surfaces has been studied in great detail in the past. The destructive mechanisms of micro-organisms can be roughly subdivided into (a) chemical attack and (b) physical attack. Microorganisms may physically destroy the structure of stones by creating an inter-granular swelling pressure. Recently, it has been shown experimentally that black spots formed by yeast-like fungi lead to a local temperature increase by selective absorption of solar radiation. The maximum temperature observed for clean marble surfaces remained below the maximum temperature observed on inoculated surfaces. As a consequence, thermal dilatation of inoculated marble was shown to be more important. Destruction of the heated stone occurs predominantly if there exists a thermal gradient. In this paper, temperature distribution in clean and stained marble plates have been simulated numerically. Thermal eigenstresses have been determined. It is shown that tensile stresses of up to 5 N/mm2 can be expected. This may cause damage in weak zones of the surface. A sudden driving rain leads to much higher stresses. Cyclic thermal loading may eventually destroy the surface by fatigu

Rechnerische Bestimmung der lonenbeweglichkeit in Ziegelstein unter Zuhilfenahme von Ergebnissen aus nichtstationären Diffusionsversuchen

Ion concentration profiles in sandwich-brick specimens have been determined experimentally at different degrees of water saturation. Based on the experimental data, ion diffusion coefficients were determined by inverse analysis. The diffusion equation served as a basis for an optimization technique. It has been shown that the ion diffusivity in bricks decreases markedly as the moisture content decreases. The interface between two adjacent bricks slows down the ion migration at low water content in particular. It is planed to investigate the interface between mortar and bricks in further experimental and numerical studie

Untersuchung von Feuchtetransport mittels numerischer Modellierung und Neutronentransmissionsanalyse

Moisture transport in porous building materials is described by a transport model that expresses the action of the several transport mechanisms of liquid water and water vapor in the porous system by a single material specific transfer coefficient. This parameter has to be determined by experimental measurement of moisture distributions inside samples exposed to known initial and boundary conditions. Neutron radiography is employed as experimental method because of its high sensitivity and good spatial resolution. The moisture distribution is determined from the raw data obtained in the measurement by a calculation procedure based on a signal transfer model of the neutron transmission. This approach is illustrated by two examples: the drying of a plain brick sample and the drying of a composite sample consisting of two brick elements separated by a layer of morta

Ultrasonic propagation of reflected waves in cancellous bone: Application of Biot theory.

International audienceAn ultrasonic propagation in cancellous bone is considered using the Biot theory modified by the Johnson et al. Numerical simulations of reflected waves in the time domain are worked out by varying the modified Biot parameters. The sensitivity of different mechanical parameters : Young modulus and the Poisson ratio of the skeletal frame and physical parameters , porosity , tortuosity and viscous characteristic length are studied showing their effect on the reflected ultrasonic waves of the bone sample. The sensitivity of the modified Biot parameters with respect to the reflected wave depends strongly on the coupling between the solid and fluid phases of the cancellous bone. We show from these simulations that some parameters such as porosity and tortuosity play an important role on reflected wave ; the remaining parameters have low sensitivity compared with the porosity and tortuosity. Experimental results for reflected waves by human cancellous bone samples are given and compared with theoretical predictions

Acoustic characterization of porous materials with a rigid structure in the low frequency regime

International audienceAn acoustic method is proposed for measuring the low frequencies parameters of porous materials having a rigid frame. The Biot equivalent fluid model is considered, in which the viscous losses are described by the the static viscous permeability and an additional viscous parameter. The inertial interactions between fluid and structure are taken into account by the inertial factor. The thermal couplings are modeled using the thermal permeability and the thermal tortuosity. The proposed method is based on a solution of the direct and inverse problems for the propagation of transient audible frequency waves in a homogeneous isotropic slab of porous material having a rigid frame. The physical parameters are determined fromthe solution of the inverse problem using experimental reflected and transmitted waves by porous samples, in a wave guide (pipe). The minimization of the objective function, defined as the difference between experiment and theory, is done in the time domain. Tests are performed using industrial plastic foams. Experimental and theoretical data are in good agreement. Furthermore, the prospects are discussed

Determination of the flow resistivity and thickness of porous materials with rigid frames via transmitted waves at Darcy's regime

International audienceAn acoustic method is proposed for measuring the flow resistivity and the thickness of air-saturated porous materials. The conventional methods [14, 16, 17] for the measurement of the flow resistivity (or the viscous permeability) require the prior knowledge of the porosity. The method presented in this work is based on a temporal model of the direct problem in which a simplified expression (independent of frequency and porosity) of the transmission coefficient at the Darcy’s regime (low frequency range) is established, this expression depends only on the viscous permeability (or the flow resistivity) and the thickness of a porous sample. The inverse problem is solved based on the leastsquare numerical method using experimental transmitted wave in time domain. Tests are performed using two samples of different thicknesses to same industrial plastic foam, thereby enabling the determination the thickness and flow resistivity of foam plastic. This method has the advantage of being simple, fast and efficient

Evaluation of UHPFRC activation energy using empirical models

The influence of thermal curing on the evolution of the material properties and the UHPFRC behaviour was investigated. Tests results showed a beneficial effect of a high temperature curing on the early age material properties due to the thermo-activation effect on the hydration process. However, an inverse effect was observed at long-term. In our study, activation energy of UHPFRC was evaluated from experimental data by means of empirical models. The traditional maturity-function based on Arrhenius law, generally used to describe thermally activated physical or chemical processes, was used to predict the evolution of the UHPFRC autogenous shrinkage and to validate the applicability of this concept for such cement-based materials. Results showed that the concept based on Arrhenius law could describe correctly temperature effects on UHPFRC for temperature lower than 30°

Détermination de l'épaisseur et la résistivité au passage de l'air d'un matériau poreux à structure rigide en utilisant les ondes transmises.

National audienceUne méthode acoustique est proposée pour mesurer la résistivité au passage de l’air et l’épaisseur d’un échantillon poreux à structure rigide. Les méthodes classiques 3,4 permettant la mesure de la résistivité (où la perméabilité visqueuse) nécessitent la connaissance préalable de la porosité. La méthode présentée dans ce travail est basée sur un modèle temporel du problème direct dans lequel une expression simplifiée (indépendante de la fréquence et de la porosité) du coefficient de transmission dans le régime de Darcy (très basses fréquences) est établie. Cette expression ne dépend que de la perméabilité visqueuse (où la résistivité au passage de l’air) et de l’épaisseur d’un échantillon poreux. Le problème inverse est résolu en minimisant, l’écart entre le signal transmis théorique et expérimentale, permettant ainsi la détermination de l’épaisseur et de la perméabilité visqueuse (où la résistivité) d’une mousse en plastique. Cette méthode présente l’avantage d’être simple, rapide et efficace

Ultrasonic waves Reflected at Oblique Incidence by Porous Rigid Medium.

International audienceAn ultrasonic reflected wave at oblique incidence by porous medium with rigid frames is considered using equivalent fluid model. The viscous and thermal losses of the medium are described by two susceptibility kernels which depend on the viscous and thermal characteristic lengths. Analytical derivation of reflection coefficient is given in frequency domain. The simulated reflected wave is obtained at time domain by convolution between the reflected operator and the incident field. Experimental results for plastic foam samples of air-saturated porous media are given and compared with theoretical prediction