Interfacial Transition Zone Thickness Research Articles

IMPROVED RANDOM AGGREGATE MODEL FOR NUMERICAL SIMULATIONS OF CONCRETE ENGINEERING SIMULATIONS OF CONCRETE ENGINEERING

In numerical simulations, concrete is usually considered as a three-phase material consisting of an aggregate, a cement matrix, and an interfacial transition zone (ITZ). Three-dimensional modeling of concrete usually requires extremely large computational requirements. In this study, an improved random aggregate model for numerical simulations of concrete is developed, which can minimize the number of elements, optimize the ITZ thickness, and create internal cracks and holes. Numerical investigations on the cracks form as well as deflection and tensile strength are also conducted based on three-point bending tests. The simulation results agree well with the experimental results.

A three-phase model for predicting the effective chloride migration coefficient of ITZ in cement-based materials

Specimens (water/cement ratio of 0·35) with various volume fractions (0, 0·1, 0·2, 0·3, 0·4 and 0·5) of fine aggregates were cast and tested to investigate the behaviour of the interfacial transition zone in mortar and the effect of the interfacial transition zone on the chloride migration coefficient. The double-inclusion method and Mori–Tanaka theory were used to predict the migration coefficient of three-phase composite materials, which are cement paste, interfacial transition zone and fine aggregate. There are two stages to calculate the overall migration coefficient of mortar. The first stage, the equivalent migration coefficient of fine aggregate and the interfacial transition zone, was calculated by Hori and Nemat-Nasser's double-inclusion model. The second stage, the equivalent migration coefficient, was used to calculate the overall migration coefficient of the mortar composite by Mori–Tanaka theory. A comparison was conducted between the theoretical and experimental data in this study. The results indicated that the migration coefficient of the interfacial transition zone is 1·5 to 10 times the migration coefficient of cement paste when an interfacial transition zone thickness of 20 to 30 μm is assumed.

Numerical calculation on the porosity distribution and diffusion coefficient of interfacial transition zone in cement-based composite materials

Based on the cement particles distribution characteristic of interfacial transition zone (ITZ) in cement-based composite materials and modified powers model, a numerical calculation model is proposed to predict the porosity distribution, average porosity and effective diffusion coefficient of chloride ions in ITZ. The model takes into account the influence of water-to-cement ratio, the thickness of ITZ and the degree of hydration on the porosity distribution of ITZ. The results show that the ratio of the capillary porosity of ITZ to that of bulk cement paste (ϕITZ/ϕB) and the ratio of the diffusion coefficients of chloride ions in ITZ to that in bulk cement paste (DITZ/DB) are almost consistent with those in published papers. In addition, the results of numerical calculation further confirm the diffusion coefficient of ITZ is 10 times as much as that of bulk cement paste.

Numerical calculation and influencing factors of the volume fraction of interfacial transition zone in concrete

The determination of volume fraction of interfacial transition zone (ITZ) is very important for investigating the quantitative relationship between the microstructure and macroscopical property of concrete. In this paper, based on Lu and Torquato’s most nearest surface distribution function, a calculating process of volume fraction of ITZ is given in detail according to the actual sieve curve in concrete. Then, quantitative formulas are put forward to measure the influencing factors on the ITZ volume fraction. In order to validate the given model, the volume fractions of ITZ obtained by numerical calculation are compared with those by computer simulation. The results show that the two are in good agreement. The order of the factors influencing the ITZ volume fraction is the ITZ thickness, the volume fraction of aggregate and the maximum aggregate diameter for Fuller gradation in turn. The ITZ volume fraction obtained from the equal volume fraction (EVF) gradation is always larger than that from the Fuller gradation for a given volume fraction of aggregate.

Prediction of the Effective Diffusion Coefficient of Chloride Ions in Cement-Based Composite Materials

AbstractA multiscale model is established for effective diffusion coefficient of chloride ions in cement-based composite materials. The model takes into account the relationship between the diffusivity and microstructure of cement-based materials, where the microstructure includes the interfacial transition zone (ITZ) between the aggregate particles and the bulk cement pastes, and the microstructure of the bulk cement paste itself. In addition, the model also includes the parameters such as water-to-cement ratio, hydration degree of cement, the thickness of ITZ, the volume fraction, and gradation of aggregate. A model for predicting the porosity distribution in ITZ is proposed based on the cement-particle distribution and modified Powers model, and then effective chloride diffusivities of ITZ can be estimated. Thus, the effective diffusion coefficient is predicted by n-layered inclusion theory in which mortar and concrete is considered as four-phase materials consisting of matrix phase, aggregate, ITZ, an...

ITZ volume fraction in concrete with spheroidal aggregate particles and application: Part I. Numerical algorithm

When the interfacial transition zone (ITZ) is treated as a separate phase, the ITZ volume fraction plays an essential role in determining the physico-mechanical properties of concrete. The intention of the present paper is to develop a numerical algorithm for the ITZ volume fraction in concrete with spheroidal aggregate particles. By applying a contact function for two spheroidal aggregate particles and introducing periodic boundary conditions, the distribution of spheroidal aggregate particles with various sizes within a cubic element is implemented. The Monte Carlo method is then adopted to evaluate the ITZ volume fraction. After the validity of the developed numerical algorithm is verified with the analytical solution for the ITZ volume fraction in concrete with spherical aggregate particles, the effects of various factors that affect the ITZ volume fraction are evaluated through sensitivity analysis. It is found that the ITZ volume fraction increases with the increase of the ITZ thickness, but decreases with the increase of the maximum aggregate diameter and the aspect ratio of spheroidal aggregate particles. It is also found that the aggregate gradation has a significant influence on the ITZ volume fraction. The paper concludes that the numerical algorithm developed in the paper can predict the ITZ volume fraction with reasonable accuracy.

Numerical Method for Predicting Young’s Modulus of Concrete with Aggregate Shape Effect

Owing to its importance to the structural analysis and design of cement-based materials, it is essential to determine Young’s modulus of concrete. This paper presents a numerical method for predicting Young’s modulus of concrete with aggregate shape effect. In the numerical method, aggregate particles are assumed to be elliptical and distributed within a rectangular concrete element with periodic boundary conditions by introducing an overlap criterion for two elliptical aggregate particles. The method modifies the lattice model to take account of the mechanical properties of each phase constituent of concrete in the analysis of stress. After the validity of the developed numerical method is verified with three independent sets of experimental results, the effects of the maximum aggregate diameter, aggregate gradation, interfacial transition zone (ITZ) thickness, and aggregate shape on Young’s modulus of concrete are evaluated in a quantitative manner. It is found that Young’s modulus of concrete increases with the increase of the maximum aggregate diameter and aggregate aspect ratio but decreases by increasing the ITZ thickness. It is also found that the aggregate gradation has a significant influence on Young’s modulus of concrete. The paper concludes that the numerical method developed in the paper can predict Young’s modulus of concrete with an average relative error smaller than 7%.

Effect of the Interfacial Transition Zone on the Migration Behavior of Cations under the Accelerated Lithium Migration Technique

This paper researches the effect of the interfacial transition zone (ITZ) on the migration behavior of cations under the Accelerated Lithium Migration Technique (ALMT). Cylindrical mortars with different aggregate volume fractions (Vf) and w/c=0.5 were used. The result shows that when Vf≤ 20 %, the Li+steady state migration coefficient (Ds) is influenced by the dilution and tortuosity effects of the aggregate. The combined effect of ITZ and percolation occurs at Vf= 30 %, and the effects increase with the increase in Vf. Based on the experimental and regression analysis results, when the assumed thickness of ITZ = 20 μm, the approximate Dsof ITZ and percolated ITZ are 46.4 and 52.6 times the matrix migration coefficient.

Quantitative analysis and affecting factors of the overlapping degree of interfacial transition zone between neighboring aggregates in concrete

Based on the nearest surface function formula, a quantitative formula to measure the overlapping degree between the interfacial transition zone (ITZ) of neighboring aggregate particles was put forward. The formula was further deduced to quantitatively analyze the influence of the volume fraction of aggregate, ITZ thickness and the maximum aggregate diameter on the overlapping degree between neighboring ITZ. The volume of ITZ was quantitatively calculated in actual concrete by comparing the nearest surface function formula with an approximate method, that is the surface area of the aggregates multiplied by the uniform thickness of the ITZ layers. The results showed that the influencing order of these three factors on the overlapping degree between neighboring ITZ in turn was the interface thickness, aggregate volume fraction and the maximum aggregate diameter; As long as the interface thickness <50 μm and the aggregate volume fraction <50%, the calculated error between two methods mentioned above is about 10 %.

Evaluation of sub-microstructure in concrete with low water-binder ratio by SEM-BSE image analysis

The coarse pore system, interfacial transition zone (ITZ) between aggregate and paste matrix and volume fraction of unhydrated cement in concrete (w/c=0.3) containing mineral admixtures were quantitatively characterized by the scanning electron microscope-backscattered electron (SEM-BSE) image analysis technique. The experimental results show that compound addition of slag and fly ash decreases the coarse porosity from 10.17% to 3.74% and the threshold diameter of coarse pore size from 345 μm to 105 μm compared with concrete (w/c=0.30) without mineral admixtures; Moreover with compound addition of fly ash and slag, the volume proportion of unhydrated cement in paste matrix is reduced by 30%, the maximum amount of coarse pores in the ITZ between aggregate and paste decreases from 13.11% to 5.57% and the thickness of ITZ is reduced by 37%, compared with concrete without mineral admixtures.

Influence of the Interfacial Transition Zone Properties on Chloride Corrosion in Reinforced Concrete-Characterization of ITZ

Influence of the Interfacial Transition Zone Properties on Chloride Corrosion in Reinforced Concrete-Characterization of ITZ

A numerical algorithm for the ITZ area fraction in concrete with elliptical aggregate particles

In view of the importance of the interfacial transition zone (ITZ) to the macro-properties of concrete, it is essential to determine the ITZ area fraction in concrete. Once the ITZ area fraction is known, the physico-mechanical properties of concrete can be predicted based on the three-phase composite model. The intention of the present paper is to develop a numerical algorithm for the ITZ area fraction in concrete with elliptical aggregate particles. By introducing a contact function for two ellipses, the elliptical aggregate particles with various sizes are distributed within a square element with periodic boundary conditions. The Monte Carlo method is then adopted to evaluate the ITZ area fraction. After verifying the developed numerical algorithm with the analytical solution for the ITZ area fraction in concrete with circular aggregate particles, the effects of key factors that affect the ITZ area fraction are examined in a quantitative manner. It is found in the paper that the ITZ area fraction increases with the increase of the ITZ thickness, but decreases with the increase of the maximum aggregate diameter and the ratio between the major and minor axes of elliptical aggregate particles. It is also found that the aggregate gradation has a significant influence on the ITZ area fraction. The paper concludes that the numerical algorithm presented in the paper can predict the ITZ area fraction with reasonable accuracy.

Interfacial interactions in concretes with silica fume and SBR latex

This paper deals with the effect of silica fume and styrene-butadiene latex (SBR) on the microstructure of the interfacial transition zone (ITZ) between Portland cement paste and aggregates (basalt). Scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis system (EDX) was used to determine the ITZ thickness. In the plain concrete a marked ITZ around the aggregate particles (55 μm) was observed, while in concretes with silica fume or latex SBR the ITZ was less pronounced (35–40 μm). However, better results were observed in concretes with silica fume and latex SBR (20–25 μm).

Three-Phase Composite Sphere Model for the Prediction of Chloride Diffusivity of Concrete

In predicting the chloride diffusivity of concrete as a three-phase material, the morphological characteristics of the three phases, the physical properties of each phase constituent material, and the interactions of the three phases in the concrete matrix should all be taken into account. The present paper attempts to develop an analytical method to achieve this. A three-phase composite sphere model for the concrete matrix is proposed to represent the heterogeneous nature of concrete and a closed form solution for the chloride diffusivity of concrete is derived. After verifying the derived closed form solution with experimental results, the effects of key factors that affect the chloride diffusivity of concrete, namely the chloride diffusivity and thickness of the interfacial transition zone (ITZ), the maximum aggregate diameter, and the aggregate gradation are examined in a quantitative manner. It is found in the paper that the most important factor influencing the chloride diffusivity of concrete is the chloride diffusivity of ITZ. When the aggregate volume fraction is equal to 0.8, the relative chloride diffusivity of concrete to cement paste for a given relative chloride diffusivity of ITZ to cement paste at 10 is almost four times that for a given relative chloride diffusivity of ITZ to cement paste at 2. The second most important factor is the thickness of ITZ. When the aggregate volume fraction is equal to 0.8, the relative chloride diffusivity of concrete to cement paste for a given ITZ thickness at 0.05 mm increases by 76% as compared to that for a given ITZ thickness at 0.02 mm. The least important factors are the maximum aggregate diameter and aggregate gradation. The extent to which they influence the chloride diffusivity of concrete is in the range of 10 — 55%. The paper concludes that the closed form solution derived here can predict the chloride diffusivity of concrete with reasonable accuracy.

Overestimation of the interface thickness around convex-shaped grain by sectional analysis

Sectional analysis is commonly used to characterize qualitatively and quantitatively the interfacial transition zone (ITZ) of concrete, especially its thickness. However, the normal of the sectioning plane is in general not perpendicular to the normal of the aggregate surface due to the irregularity of the shape of the aggregates. Therefore, the sectional approach overestimates ITZ thickness. Calculating the degree of overestimation is key to obtain a precise measure of ITZ thickness and therefore the influence of the ITZ on the macro-properties of composites. However, the irregularity of the aggregates makes this a difficult task. In this contribution, some related theorems and results from geometrical probability were used to study the relationship between the statistical value of the apparent ITZ thickness and actual ITZ thickness. General solutions are derived for an arbitrary convex-shaped aggregate grain for both two- and three-dimensional cases.

Effect of silica fume and SBR latex on the pasteaggregate interfacial transition zone

This paper deals with the effect of silica fume and styrene-butadiene latex (SBR) on the microstructure of the interfacial transition zone (ITZ) between Portland cement paste and aggregates (basalt). Scanning Electron Microscope (SEM) equipped with energy dispersive x ray analysis system (EDX) was used to determine the ITZ thickness. In the plain concrete a marked ITZ around the aggregate particles (55 µm) was observed, while in concretes with silica fume or latex SBR the ITZ was less pronounced (35-40 µm). However, better results were observed in concretes with silica fume and latex SBR (20-25 µm).

Effects of fiber curvature on the microstructure of the interfacial transition zone in fresh concrete

The study on the interfacial transition zone (ITZ) of concrete has received lots of attention in the last decade. However, no information is available on the influence of the curvature of a rigid surface on the microstructure of ITZ. This paper employed computer simulation technology to study the influence of fiber curvature on the initial microstructure of ITZ in concrete. For the sake of simplification, the investigation was first focused on the mono-size spherical particle packing system around a cylindrical fiber with different diameters. An algorithm of serial cylindrical sectioning was developed. The curve of the solid volume fraction versus the distance to the surface of the fiber was used as a parameter to characterize the microstructure of the ITZ. Then, the influence of the ratio of fiber diameter to particle diameter on the initial ITZ’s microstructure was studied. These curves were compared with the ones from flat aggregate surface on which mono-size spherical particles were packed. Furthermore, the multi-size spherical particles system was further investigated. The simulation results demonstrate that no matter whether the spherical particle system is mono-size or multi-size, the fresh ITZ’s microstructure is irrelevant to the curvature of the fiber. The shape of the curve of solid volume fraction versus the distance from the surface of the fiber is similar to that around a flat aggregate surface. In all cases, the horizontal coordinates of the first peak of the curves are located at around half the mean weight diameter of the particles. The thickness of ITZ reduces slightly with the decrease in water/cement ratio. Therefore, one may use the ITZ’s microstructure around a flat aggregate surface to represent the ITZ’s microstructure around a cylindrical fiber in the fresh state, and vice versa.

Avaliação da zona de transição interfacial pasta-agregado leve em concretos com sílica ativa e látex SBR

Este trabalho aborda o efeito da sílica ativa e do látex de estireno-butadieno (SBR) na zona de transição entre a argila expandida (agregado leve) e a matriz de cimento Portland. Para a avaliação da zona de transição utilizou-se um microscópio eletrônico de varredura (MEV) associado a um sistema de análise quantitativa (espectrografia de Raios-X por dispersão de energia - EDS). No concreto de referência a zona de transição apresentou espessura de 30mim. Com a inclusão de sílica ativa ou de látex SBR nos concretos observou-se a redução da espessura da zona de transição para valores entre 20 e 25mim. Entretanto os menores valores da espessura da zona de transição, entre 8 e 15mim, foram obtidos para os concretos com sílica ativa e látex SBR.

On the relative use of micro-mechanical lattice analysis of 3-phase particle composites

In this paper results obtained from analyses made using a beam lattice model are presented and discussed. The material system studied is a 3-phase particle composite where stiff and strong aggregate particles are embedded in a weaker and softer cement matrix. A third phase exists between these two phases, namely a thin zone enveloping the aggregate particles. This thin zone of extremely weak material is the interfacial transition zone (ITZ). In the composite either the ITZ phase or the matrix phase may percolate. It is shown that the macroscopic behavior of the composite differs significantly when the internal material geometry varies, either by changing the particle density or by varying the ITZ thickness. The results from these analyses could, after an extensive verification/falsification experimental phase, help to construct realistic macroscopic fracture models, [Van Mier JGM. Reality behind fictitious cracks? In: Li VC, Leung CKY, Willam KJ, Billington SL, editors. Proceedings of the 5th international conference on ‘Fracture of concrete and concrete structures’ (FraMCoS-V). Evanston, (IL): IA-FraMCoS; 2004. p.11–30], and might point towards new ways to improve the material.

Prediction of the chloride diffusion coefficient of concrete

It has been experimentally verified that the structure of the interfacial transition zone (ITZ) in concrete differs from that of bulk cement paste. As such, concrete should be modeled as a three-phase material at a mesoscopic level. This paper presents a three-phase composite model for predicting the chloride diffusion coefficient of concrete. Taking the inclusion as aggregate and the matrix as cement paste, the composite circle model is established by adding an ITZ layer in between the inclusion and the matrix. Solving the asymmetrical problem analytically, a closed-form solution for the chloride diffusion coefficient of concrete is derived. After verifying this model with experimental results, the effects of the aggregate area fraction, the chloride diffusion coefficient of ITZ, the ITZ thickness, the maximum aggregate diameter and the aggregate gradation on the chloride diffusion coefficient of concrete are evaluated in a quantitative manner. It is found that the chloride diffusion coefficient of concrete decreases with the increase of the aggregate area fraction and the maximum aggregate diameter, but increases with the increase of the chloride diffusion coefficient and thickness of ITZ. It is also found that the aggregate gradation has a significant influence on the chloride diffusion coefficient of concrete.