Interfacial Transition Zone Volume Fraction Research Articles

ITZ volume fraction and thermal conductivity of concrete: A unified random packing model for gravels and crushed rocks

This work proposes a mesostructure-guided computational framework to evaluate the effective thermal conductivity (ETC) of concrete affected by aggregate and interfacial transition zone (ITZ) configurations at a mesoscopic scale. An unified aggregate shape descriptor for gravels and crushed rocks is characterized by superellipsoid covering from convexity to non-convexity, as well as random static and dynamic packings of superellipsoidal aggregates (SAs) are carried out to generate tri-phase concrete mesostructures consisting of homogeneous mortar, densely packed aggregates and their surrounding ITZs. A closed-form expression for the ITZ volume fraction is theoretically developed in the present concrete mesostructure, which is also validated using the one-point probability function simulation. Moreover, an optimized Browanian motion numerical strategy is developed to estimate the ETC of tri-phase concrete. This strategy is confirmed to be an accurate numerical technique by comparing against the Mori-Tanaka model. The present framework reveals the quantitative responses of various configurations on the ETC, which manifest rigorous “constituent-mesostructure-ETC” relations. The results are available for tailoring components to optimize ITZ structures for the improvement of thermal efficiency and durability of buildings.

Numerical Investigation of Sulfate Diffusion Characteristics in Recycled Aggregate Concrete Based on Mesoscale Multiphase Analysis

Sulfate-induced deterioration can reduce the service life of recycled aggregate concrete (RAC). To demonstrate the transport behavior of sulfate ions in RAC, a two-dimensional, six-phase numerical model of RAC is proposed in this study. Sulfate ion diffusion coefficients were defined for different phases [attached mortar, new mortar, interfacial transition zone (ITZ) between attached mortar and original aggregate, ITZ between the new mortar and original aggregate, and ITZ between the new mortar and attached mortar] of RAC. The effects of ITZ width and porosity on ion transport behavior in RAC were analyzed. The results show that the numerical model agrees with the experimental data. Numerical results indicate that the six-phase model of RAC provides more-accurate simulation results because tortuosity is reflected in the model. The width and porosity of ITZs both affect the diffusion of sulfate ions in concrete. However, due to the low volume fraction of ITZ in concrete, just improving ITZ performance has no significant effect on ion attack resistance of concrete. The ITZ is a crucial zone where damage may originate and develop.

Proposing a three-phase model for predicting the mechanical properties of mortar and concrete

The interfacial transition zone (ITZ) around aggregates is known as the weakest zone in mortar and concrete, which highly influences the mechanical properties. In this study, the influence of ITZ on mechanical properties such as Young’s modulus and Poisson’s ratio of cementitious materials is evaluated using a multi-scale model (developed in five hierarchical levels: nano-scale cement hydrates∼ scale of concrete). In the proposed model, the microstructure of mortar/ concrete is considered as a three-phase material: fine/ coarse aggregates, ITZ of aggregates and bulk paste/ mortar. The primary input is the microstructure of cement paste and ITZ, which are predicted as the function of curing time using coupled cement hydration-thermodynamic model. The ITZ volume fraction is analytically computed based on aggregate particle size distribution. Multi-level homogenization methods (based on three-phase sphere model for two-phase composite material) are finally implemented to predict the effective properties of mortar and concrete. Here, the equivalent matrix consisted of fine/ coarse aggregate, bulk paste/ mortar and ITZ are obtained with the first and second levels homogenization procedures for mortar/ concrete. To validate the predictability of the models for mortar and concrete, predicted values are compared with independent experimental data sets. The results show a reasonable agreement with experimental results of Poisson’s ratio and Youngs modulus (in the error range of 5 GPa). The influences of ITZ on properties of mortar and concrete are also discussed based on the outcomes.

Modeling and Software Development of the Interfacial Transition Zone of Ellipsoidal Aggregate in Cement-Based Composites

The interfacial transition zone (ITZ) between the aggregates and the bulk paste is the weakest zone of ordinary concrete, which largely determines its mechanical and transporting properties. However, a complete understanding and a quantitative modeling of ITZ are still lacking. Consequently, an integrated modeling and experimental study were conducted. First, the theoretical calculation model of the ITZ volume fraction about the rotary ellipsoidal aggregate particles was established based on the nearest surface function formula. Its calculation programs were written based on Visual Basic 6.0 language and achieved visualization and functionalization. Then, the influencing factors of ITZ volume fraction of the ellipsoidal aggregate particles and the overlapping degree between the ITZ were systematically analyzed. Finally, the calculation models of ITZ volume fraction on actual ellipsoidal aggregate were given, based on cobblestones or pebbles particles with naturally ellipsoidal shape. The results indicate that the calculation model proposed is highly reliable.

A study on ITZ percolation threshold in mortar with ellipsoidal aggregate particles

The percolation of interfacial transition zone (ITZ) in cementitious materials is of great importance to the transport properties and durability issues. This paper presents numerical simulation research on the ITZ percolation threshold of mortar specimens at meso-scale. To simulate the meso-scale model of mortar as realistically as possible, the aggregates are simplified as ellipsoids with arbitrary orientations. Major and minor aspect ratios are defined to represent the global shape characteristics of aggregates. Some algorithms such as the burning algorithm, Dijkstra\'s algorithm and Connected-Component Labeling (CCL) algorithm are adopted for identification of connected ITZ clusters and percolation detection. The effects of gradation and aspect ratios of aggregates on ITZ percolation threshold are quantitatively studied. The results show that (1) the ITZ percolation threshold is mainly affected by the specific surface area (SSA) of aggregates and shows a global decreasing tendency with an increasing SSA; (2) elongated ellipsoidal particles can effectively bridge isolated ITZ clusters and thus lower the ITZ percolation threshold; (3) as ITZ volume fraction increases, the bridging effect of elongated particles will be less significant, and has only a minor effect on ITZ percolation threshold; (4) it is the ITZ connectivity that is essentially responsible for ITZ percolation threshold, while other factors such as SSA and ITZ volume fraction are only the superficial reasons.

Quantification of the influences of aggregate shape and sampling method on the overestimation of ITZ thickness in cementitious materials

The microstructure of the interfacial transition zone (ITZ) surrounding the aggregate in a cementitious composite is quite different from that of the bulk matrix, because of its distinct physical nature including relatively high porosity and low rigidity. The thickness and volume fraction of the ITZ play a major role in determining the transport and mechanical behavior of cementitious composites. However, the ITZ thickness may be overestimated when undertaking sectional plane analysis of these composites. Analysis of Platonic particles has previously shown that the sphericity of the particle is an important parameter in determining the overestimation of the ITZ thickness, but this raises the question of whether sphericity is sufficient to uniquely characterize the influence of aggregate shape. This paper investigates the influence of particle shape on overestimation of ITZ thickness for aggregate shapes which have the same sphericity values as Platonic particles; specifically, spheroids of differing geometries. A normal line sampling algorithm, which is designed to replicate the practical experimental process used in ITZ determination, is employed to obtain the apparent ITZ thickness. The influences of particle shape, sampling method and particle size distribution are investigated in terms of the overestimation of the ITZ volume fraction, and the effective diffusivity within three-phase composites, using the differential effective medium approximation.

Aggregate shape effect on the overestimation of interface thickness for spheroidal particles

Interfacial transition zone (ITZ) between matrix and aggregate often plays a major role in transport and mechanical behaviors of cementitious composites as its physical nature of relatively high porosity and low rigidity. However, the ITZ thickness may be overestimated via sectional plane analysis technologies, this is attributed to a random sampling plane rarely passing through the normal to the surface of aggregates. So, it is necessary to quantify the overestimation degree of the ITZ thickness. In this study, a methodology to construct the ITZ layer around ellipsoidal aggregate particles is proposed. Then, a systematic line sampling algorithm is employed to obtain the statistical mean value of the ratio of the apparent to the actual ITZ thickness. The influence of aggregate shape and particle size distribution is discussed on the overestimations of the thickness and volume fraction of the ITZ, and the effective diffusivity of three-phase composites using the differential effective medium approximation. Results show that the aggregate shape has a significant impact on the apparent ITZ thickness, which mainly reflects in the sphericity of the aggregate shape. And the ranking of the overestimation of ITZ thickness, ITZ volume fraction and diffusivity for two types of spheroidal aggregate which have the same sphericity with the specified Platonic particle follow: Platonic particle > Oblate ellipsoid > Prolate ellipsoid. Sphericity is not a unique parameter characterizing the influence of particle shape on the overestimation degree of ITZ thickness. For quantification, other shape parameter needs to be developed in the future. We hope this article will be helpful to guide the practice of evaluating the contribution of aggregate particle shape to the ITZ microstructure and the influence of the ITZ on macroscopic performance of composite materials.

Multi-scale modelling for diffusivity based on practical estimation of interfacial properties in cementitious materials

The interfacial transition zone (ITZ) between matrix and aggregate is a weak region because of its physical nature of relatively high porosity and low rigidity. ITZ microstructural configurations such as its thickness and volume fraction play an important role in transport and mechanical behaviours of cementitious composites. However, such ITZ characteristics are usually overestimated since random sampling planes from conventional sectional plane analysis technologies rarely pass through the normal to the surface of anisotropic aggregates. In this study, we first propose a normal line sampling (NLS) algorithm to evaluate the overestimation degrees of these microstructures for complex geometrical shaped aggregates like Platonic particles. The NLS algorithm applies the sampling lines along the normal of aggregate boundary at the given section plane that is significantly different from the previous methods in the literature. Then, the ITZ volume fraction can be determined by the ITZ thickness according to a generalized formula. We also extend these practical ITZ characteristics to estimate the effective diffusivity of cementitious composites by the classical effective medium approximation. The derived results show that the ratio of the apparent ITZ thickness to the actual ITZ thickness is dependent on the sphericity which is a shape descriptor of particles. Moreover, the overestimation degree of ITZ volume fraction and diffusivity of three-phase composites are governed by aggregate volume fraction, particle size distribution and particle shape. Finally, the results reflect that the aggregate shape has a significant effect on the apparent ITZ thickness, and the sampling process of ITZ thickness by using NLS algorithm is similar to the sampling techniques in experiment.

Aggregate shape effect on the overestimation of ITZ thickness: Quantitative analysis of Platonic particles

Quantitative image analysis approach was often employed to obtain the apparent interfacial transition zone (ITZ) thickness around aggregate particles. However, the ITZ thickness may be overestimated via sectional plane analysis method, and exaggerate the ITZ's contribution to the macro-properties of cementitious composites. Until present, little work was done to derive the quantitative relationship between apparent and actual ITZ thickness, especially on non-spherical particle. This paper has proposed a methodology to construct the ITZ layer around Platonic particles and has employed a systematic line sampling algorithm to estimate the thickness of ITZ, called apparent ITZ thickness. The influence of particle shape unified by sphericity on the estimation of ITZ thickness unified by degree of overestimation is discussed where the degree of overestimation represents the ratio of apparent ITZ thickness to actual ITZ thickness. The particle shape effect is discussed for a single particle, for multi-dispersed spherical systems and for Platonic particle systems, respectively. Finally, the unexpected contribution of ITZ thickness overestimation on ITZ volume fraction and diffusivity of concrete are analysed.

Effective medium method for predicting the chloride diffusivity in concrete with ITZ percolation effect

In predicting the chloride diffusivity in concrete as a three-phase material, composed of aggregates, interfacial transition zone (ITZ), and bulk cement paste, not only the volume fractions, physical properties, and interactions of the three phase constituents but also the ITZ percolation effect should be taken into account. This paper attempts to develop an effective medium method to achieve this. Based on the stereological analysis of the aggregate size distribution and the statistical geometry of composites, the ITZ volume fraction is formulated analytically for the general aggregate gradation. By taking the ITZ-coated aggregate as an “equivalent aggregate”, the three-phase concrete is reduced to a two-phase composite material. An analytical solution is then derived for the chloride diffusivity in concrete applying the general effective medium theory. The primary advantage of the derived analytical solution is that the ITZ percolation effect and the Hashin–Shtrikman bounds are taken into account. Finally, the validity of the derived analytical solution is verified with two sets of experimental data and the effects of the ITZ thickness of and the chloride diffusivity in ITZ on the chloride diffusivity in concrete are evaluated through sensitivity analysis. The paper concludes that the presented analytical solution can predict the chloride diffusivity in concrete with an average relative error of 6%.

Effective Medium Approach for Evaluating the Oxygen Diffusivity of Concrete

In view of the importance of the diffusion of oxygen molecules in concrete to the durability assessment and design of reinforced concrete structures located in a marine or deicing salt environment, it is essential to determine its oxygen diffusivity. This paper presents an effective medium approach for evaluating the oxygen diffusivity of concrete. Since the aggregate, bulk cement paste, and interfacial transition zone (ITZ) have different morphological characteristics and contributions to the diffusion of oxygen molecules in concrete, concrete is modeled as an isotropic three-phase composite material. Based on the stereological analysis of aggregate size distributions and the statistical geometry of composites, the ITZ volume fraction is formulated analytically. By introducing a hypothetical homogeneous medium of nonzero oxygen diffusivity and applying the general effective medium approach, an analytical solution is derived for the oxygen diffusivity of concrete. The primary advantage of the approach is that the ITZ percolation effect is taken into account and the oxygen diffusivity bounds are satisfied. Finally, some numerical comparisons are made to show the good consistency between the proposed approach and experimental results obtained from the research literature.

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.

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.

ITZ volume fraction in concrete with spheroidal aggregate particles and application: Part II. Prediction of the chloride diffusivity of concrete

Owing to its importance in the durability assessment of reinforced concrete structures located in a marine or de-icing salt environment, it is essential to determine the chloride diffusivity of concrete. The intention of the present paper is to present an analytical solution for the chloride diffusivity of concrete with aggregate shape effect. A three-phase composite spheroid model for the concrete matrix is proposed to represent the heterogeneous nature of concrete and to take into account the aggregate shape effect. By reducing the three-phase composite spheroid to two two-phase composite spheroids, an analytical solution for the chloride diffusivity of concrete is derived. After the validity of the derived analytical solution is verified with two sets of experimental results, the effects of main factors that affect the chloride diffusivity of concrete are evaluated in a quantitative manner. It is found that the chloride diffusivity of concrete increases with the increase of the thickness and chloride diffusivity of the interfacial transition zone (ITZ), but decreases with the increase of the aspect ratio of spheroidal particles and the maximum aggregate diameter. It is also found that the aggregate gradation has a significant influence on the chloride diffusivity of concrete. The paper concludes that the analytical solution derived here can predict the chloride diffusivity of concrete with reasonable accuracy.