Nicos Martys

Capillary transport in mortars and concrete

Abstract Results are presented of a study concerning capillary transport of water in concretes and mortars as a function of water/cement ratio, sand size distribution, and curing. Our studies indicate that the capillary sorption (1) of water in concrete exhibits a complex time dependence. At early times, of the order of one hour, the total water uptake increased with the t 1 2 behavior of typical capillary sorption theories. At longer times, of the order of tens of days, a slow crossover regime is seen as the rate of capillary suction decreases. At very long times, of order hundreds of days, a slower uptake which may be driven by the capillary forces in the smaller gel pores is found. Again the t 1 2 behavior is recovered but with a much smaller sorptivity coefficient. We describe an empirical fitting form for data which describes capillary suction at both short and long times. Aspects of the experimental design including sample drying and exposure to air are discussed. The utility of service life predictions from such measurements is discussed.

Computer simulation study of the effective viscosity in Brinkman’s equation

Brinkman’s equation is often used to match solutions of Stokes’ equation to solutions of Darcy’s law at free‐fluid:porous medium interfaces by the introduction of an effective viscosity parameter, μe. Theoretical predictions of the dependence of μe on the porosity of the porous medium have given conflicting results. A finite difference solution of Stokes’ equation in three dimensions was used to study fluid flow near the interface between a free fluid and a porous medium. It was found that in order to match solutions of Brinkman’s equation to the numerical solutions, the value of μe had to be greater than the free‐fluid viscosity. Within numerical precision, the effective viscosity μe was monotonically increasing with decreasing porosity. Good fits to the numerical fluid velocity profiles were obtained for porosities ranging from 50% to 80%.

Pulsed gradient NMR measurements and numerical simulation of flow velocity distribution in sphere packings

The displacement of water molecules associated with the flow of water inside a nonconsolidated packing of 800 μm OD glass spheres has been measured by a pulsed gradient NMR technique. Using a stimulated spin‐echo sequence, mean displacements of up to 300 μm corresponding to measurement times of up to 200 ms can be analyzed. The measurement can be quantitatively calibrated using the pure molecular self‐diffusion of water at zero flow conditions. For molecular displacements much smaller than the pore size, the distribution of the flow velocity component along the mean flow direction is determined at Reynolds numbers high enough so that longitudinal molecular diffusion is negligible. An exponential decay of the probability distribution of the displacements is observed at large distances. The results are very similar to those obtained by numerical solution of the Stokes equation in random sphere packings. At longer displacement distances, a secondary peak of the displacement distribution is observed: It is in...

Microstructure and transport properties of porous building materials. II: Three-dimensional X-ray tomographic studies

Three-dimensional X-ray microtomography is used to obtain three-dimensional images of the microstructure of two types of brick. The images are processed to remove the noise (random and circular pattern) and then thresholded to match the porosity determined experimentally. The 3-D binary images are then analyzed to estimate their vapor diffusivity and air permeability to compare to experimental data published in part one of this report. Care must be taken in obtaining the tomographic images at a resolution that both enables isolation and quantification of the pores of interest and provides a representative elementary volume for the transport property calculations. In general, the agreement between computed and measured properties is reasonable, suggesting that X-ray microtomography can provide valuable information on the characteristics and properties of the pore networks developed in these porous building materials. A preliminary evaluation indicates that the Katz-Thompson relationship between permeability, diffusivity, and pore size is valid for these materials.RésuméLa microtomographie à rayons X synchrotron, est utilisée pour obtenir des images tridimensionnelles de la microstructure de deux types de briques. Les images sont tout d’abord traitées pour éliminer le bruit (anneaux aléatoires_ et ensuite seuillées par ajustement avec la porosité déterminée expérimentalement. À partir des images binaires 3D, on estime numériquement la diffusivité à la vapeur et la perméabilité à l’air, les valeurs obtenues sont ensuite comparées avec les données expérimentales publiées dans la partie I de cette communication. Dans le cadre d’une telle procédure, la résolution des images doit à la fois rendre possible la discrimination et la quantification de tous les pores importants vis-à-vis du phénomène étudié et fournir un volume élémentaire représentatif pour le calcul des propriétés de transport. L’accord satisfaisant obtenu entre les valeurs calculées et mesurées montre que la microtomographie X peut fournir des informations pertinentes sur les caractéristiques et les propriétés du réseau poreux des matériaux de construction. Une évaluation préliminaire indique que la relation de Katz-Thompson entre la perméabilité, la diffusivité et la taille des pores est applicable pour ces matériaux.

Study of a dissipative particle dynamics based approach for modeling suspensions

Synopsis In this paper, a dissipative particle dynamics sDPDd based approach for modeling suspensions is examined. A series of tests is applied comparing simulation results to well established theoretical predictions. The model recovers the dilute limit intrinsic viscosity prediction of Einstein and provides reasonable estimates of the Huggins coefficient for semidilute suspensions. At higher volume fractions, it was necessary to explicitly include lubrication forces into the algorithm as the usual DPD interactions are too weak to prevent overlaps of the rigid bodies and account for other related effects due to lubrication forces. Results were then compared with previous studies of dense hard sphere suspensions using the Stokesian dynamics method and experimental data. Comparison of relative viscosity values determined from strain controlled shearing versus stress controlled shearing simulations are also given. The flow of spheroidal objects is studied. The rotation of a single spheroid under shear is consistent with the predictions of Jeffery. Simulations of sheared spheroids at higher volume fractions produce an apparent nematic phase. An example is given of the application of DPD to model flow in another geometry, gravitational driven flow between parallel cylinders, which is of practical interest.

Using Impedance Spectroscopy to Assess the Viability of the Rapid Chloride Test for Determining Concrete Conductivity.

The suitability of using the initial current from the rapid chloride test (ASTM C 1202) to determine specimen conductivity is tested using impedance spectroscopy with a frequency spectrum of 10 Hz to 1 MHz. The specimen conductivity has an analytical relationship to specimen diffusivity and so is a useful quantity in service life prediction. Measurements made on specimens of different lengths indicate that the total charge passed during the six hour conduction test carried out according to ASTM C 1202 is not a direct measure of specimen conductivity. Further, ohmic heating during the 6 hour test makes it nearly impossible to directly measure any specimen transport property from the results. The total charge passed during the 6 hour conduction test is, therefore, not a reliable quantity for service life prediction. Results indicate that the direct current (dc) measurement of resistance using a voltage of 60 V is sufficient to overwhelm polarization effects, thereby yielding an accurate estimate of the true specimen conductivity. Impedance spectroscopy measurements also indicate that corrosion may form on the brass electrodes, adding bias to a conductivity estimate based upon a dc measurement.

Microstructure and transport properties of porous building materials

To successfully predict the performance of building materials exposed to a degradative environment, transport properties must be either measured or estimated. The development of relationships between microstructure and transport properties for these materials should allow accurate prediction of the latter and an increased understanding of how microstructure influences transport. Here, two microstructural characterization techniques, mercury intrusion porosimetry and scanning electron microscopy, are combined with computer modelling techniques to compute the vapor diffusivity and air permeability of three building materials commonly exposed in building facades, two types of brick and a natural sandstone. In general, the computed values compare favorably to those measured experimentally, thus demonstrating the capability of employing microstructural characterization to predict transport properties.RésuméPour obtenir une prédiction fiable des performances des matériaux de construction exposés à un environnement agressif, leurs propriétés de transport doivent être soit mesurées soit estimées. Le développement de relations entre la microstructure et les propriétés de transport devrait permettre une prédiction exacte de ces dernières et une meilleure compréhension de l’influence de la microstructure sur le transfert. Dans cet article, deux techniques de caractérisation de la microstructure (porosimétrie au mercure et microscopie électronique à balayage) sont combinées avec des techniques de simulation par ordinateur pour calculer la diffusivité à la vapeur d’eau et la perméabilité à l’air de trois matériaux de construction traditionnellement utilisés en facades: deux types de brique et un grès naturel. En général, les valeurs calculées sont proches de celles mesurées expérimentalement. Ces résultats démontrent les possibilités des outils de caractérisation de la structure pour prédire les propriétés de transport.

Improved approximation of the Brinkman equation using a lattice Boltzmann method

In this Brief Communication, a new approach to generalize the lattice Boltzmann method to produce fluid flow consistent with the Brinkman equation is presented. The method described in this communication both eliminates second order errors in velocity and improves stability over that of a previously proposed lattice Boltzmann-based model.

Transport and diffusion in three-dimensional composite media

We will review recent progress in our understanding of classical transport in porous media. Theoretical concepts will be illustrated with two distinct kinds of calculations. The first involve the grain consolidation model and are based on a particular multisize packing of spherical grains. The computational methods developed here are sufficiently accurate that we propose to combine them with direct measurements of the pore and grain geometry based on X-ray microtomography. Our preliminary results indicate that this approach may well play an important part in future studies of transport in porous media.

Hydraulic radius and transport in reconstructed model three-dimensional porous media

Methods for reconstructing three-dimensional porous media from two-dimensional cross sections are evaluated in terms of the transport properties of the reconstructed systems. Two-dimensional slices are selected at random from model three-dimensional microstructures, based on penetrable spheres, and processed to create a reconstructed representation of the original system. Permeability, conductivity, and a critial pore diameter are computed for the original and reconstructed microstructures to assess the validity of the reconstruction technique. A surface curvature algorithm is utilized to further modify the reconstructed systems by matching the hydraulic radius of the reconstructed three-dimensional system to that of the two-dimensional slice. While having only minor effects on conductivity, this modification significantly improves the agreement between permeabilities and critical diameters of the original and reconstructed systems for porosities in the range of 25–40%. For lower porosities, critical pore diameter is unaffected by the curvature modification so that little improvement between original and reconstructed permeabilities is obtained by matching hydraulic radii.