Study of the influence of the interfacial transition zone on the elastic modulus of concrete using a triphasic model

Concrete can be considered as a three-phase composite material composed of cement matrix, aggregate particles, and interface transition zone (ITZ). Generally, the ITZ presents particular characteristics that can reduce the properties of concrete and therefore limits its performance. Thus, with such complex structures, this zone is the weakest zone of the composite. The evaluation of the effective behavior of composite using predictive models requires a consideration of this zone. In this context, an approach based on the model of double inclusion and on the Mori–Tanaka theory to predict the elastic modulus of concrete has been used. This approach will be compared with some analytical biphasic model such as Reuss model, Voigt model, the Voigt and Reus combined models, and Hashin and Shtrikman models. Many experimental results are considered in the confrontation. So the developed model predicts very satisfactorily the elastic modulus of the concrete unlike other models in which a discrepancy in the results is demonstrated in the majority of cases.     

A study on characteristics of interfacial transition zone in concrete

The aim of this study was to observe the behavior of the interfacial transition zone (ITZ) of high-performance concrete that was under curing in saturated lime water. From the Scanning Electron Microscope (SEM), it was found that the pores and hydration products at the ITZ, within 100 μm between the paste and aggregate, permuted each other during the early hydration stage, and then appeared as a large lump or strip. They gradually became irregular and small lumps for the further curing age. At the curing age of 56 days, the pores almost concentrated within an area of 0-15 μm from the aggregate edge. The hydration products were much denser with the increase in its distance from the aggregate edge.     

Influence of compaction on the interfacial transition zone and the permeability of concrete

The interfacial transition zone (ITZ) is regarded as a key feature for the transport properties and the durability of concrete. In this study one self-compacting concrete (SCC) mixture and two conventionally vibrated concrete (CVC) mixtures are studied in order to determine the influence of compaction on the porosity of the ITZ. Additionally oxygen permeability and water conductivity were measured in vertical and horizontal direction. The quantitative analysis of images made with an optical microscope and an environmental scanning electron microscope shows a significantly increased porosity and width of the ITZ in CVC compared to SCC. At the same time oxygen permeability and water conductivity of CVC are increased in comparison to SCC. Moreover, considerable differences in the porosity of the lower, lateral and upper ITZ are observed in both types of concrete. The anisotropic distribution of pores in the ITZ does not necessarily cause anisotropy in oxygen permeability and water conductivity though.     

A multiscale model for effective moduli of concrete incorporating ITZ water-cement ratio gradients, aggregate size distributions, and entrapped voids

This paper develops a model for the effective elastic properties of concrete, which is a function of the volume fractions, size distributions, and elastic properties of fine aggregate (FA) and coarse aggregate (FA) and entrapped voids. Furthermore, the model is a function of the overall water-cement ratio and specific gravity of cement. Explicitly modeled are the water-cement ratio gradients through the interfacial transition zone (ITZ), which, in turn, affect the variation of the cement paste elastic properties through the ITZ, while maintaining the total fractions of cement and water consistent with the overall water-cement ratio. The ITZ volume is also conserved.     

Application of nanoindentation testing to study of the interfacial transition zone in steel fiber reinforced mortar

The characteristics of the profiles of elastic modulus and hardness of the steel fiber-matrix and fiber-matrix-aggregate interfacial zones in steel fiber reinforced mortars have been investigated by using nanoindentation and Scanning Electron Microscopy (SEM), where two sets of parameters, i.e. water/binder ratio and content of silica fume were considered. Different interfacial bond conditions in the interfacial transition zones (ITZ) are discussed. For sample without silica fume, efficient interfacial bonds across the steel fiber-matrix and fiber-matrix-aggregate interfaces are shown in low water/binder ratio mortar; while in high water/binder ratio mortar, due to the discontinuous bleeding voids underneath the fiber, the fiber-matrix bond is not very good. On the other hand, for sample with silica fume, the addition of 10% silica fume leads to no distinct presence of weak ITZ in the steel fiber-matrix interface; but the effect of the silica fume on the steel fiber-matrix-aggregate interfacial zone is not obvious due to voids in the vicinity of steel fiber.     

Aggregate-cement paste transition zone properties affecting the salt-frost damage of high-performance concretes

The influence of the cement paste-aggregate interfacial transition zone (ITZ) on the frost durability of high-performance silica fume concrete (HPSFC) has been studied. Investigation was carried out on eight non-air-entrained concretes having water-to-binder (W/B) ratios of 0.3, 0.35 and 0.42 and different additions of condensed silica fume. Studies on the microstructure and composition of the cement paste have been made by means of environmental scanning electron microscope (ESEM)-BSE, ESEM-EDX and mercury intrusion porosimetry (MIP) analysis. The results showed that the transition zone initiates and accelerates damaging mechanisms by enhancing movement of the pore solution within the concrete during freezing and thawing cycles. Cracks filled with ettringite were primarily formed in the ITZ. The test concretes having good frost-deicing salt durability featured a narrow transition zone and a decreased Ca/Si atomic ratio in the transition zone compared to the bulk cement paste. Moderate additions of silica fume seemed to densify the microstructure of the ITZ.     

Approximate migration coefficient of percolated interfacial transition zone by using the accelerated chloride migration test

In this study, the electrochemical technique is applied to accelerate chloride ion migration in cement-based material to estimate its migration coefficient. In order to investigate the chloride migration coefficient of percolated interfacial transition zone (ITZ) on the chloride migration coefficient of specimen, specimens with cylindrical aggregates of the same height as the specimen were cast and tested. In this study, the volume fraction of aggregate is constant and the varied lateral surface area of the aggregate cylinder was obtained by using different diameters and number of aggregate. The chloride migration coefficient of cement-based material was determined experimentally as a function of the lateral surface area of aggregate. A model obtained for the migration coefficient of cement-based material and the regression analysis are used to determine the approximate chloride migration coefficient of the percolated ITZ. Based on the experimental and regression analytical results, the approximate percolated ITZ migration coefficient is 40.6, 35.5, and 37.8 times of the altered migration coefficient of matrix mortar for the water/cement (w/c) ratio of 0.35, 0.45, and 0.55, respectively.     

Influence of aggregate size, water cement ratio and age on the microstructure of the interfacial transition zone

This paper presents the results of an investigation on the effect of water-cement ratio (w/c), aggregate size, and age on the microstructure of the interfacial transition zone (ITZ) between normal weight aggregate and the bulk cement paste. Backscattered electron images (BSE) obtained by scanning electron microscope were used to characterize the ITZ microstructure. The results suggest that the w/c plays an important role in controlling the microstructure of the ITZ and its thickness. Reducing w/c from 0.55 to 0.40 resulted in an ITZ with characteristics that are not distinguishable from those of the bulk paste as demonstrated by BSE images. Aggregate size appears to have an important influence on the ITZ characteristics. Reducing the aggregate size tends to reduce the ITZ porosity. The evolution of the ITZ microstructure relative to that of the bulk paste appears to depend on the initial content of the unhydrated cement grains (UH). The results suggest that the presence of a relatively low amount of UH in the ITZ at early age may cause the porosity of the ITZ, relative to that of the bulk paste, to increase with time. The presence of relatively large amount of UH in the ITZ at early ages may cause its porosity, relative to that of the bulk paste, to decrease with time.     

Variation of shear modulus through the interfacial bond zone of an adhesive

The variations in shear modulus occurring through the interfacial bond zone (IBZ) of AF 453 adhesive bonded to aluminium have been determined. Acoustic surface (Rayleigh) waves at ultrasonic frequency were propagated along surfaces of the adhesive sequentially exposed from the bulk through the IBZ towards the bondline. The propagation speed of these sound waves over the adhesive surfaces was then used to obtain the shear modulus of the material at each layer. With AF 453 adhering to an aluminium substrate prepared according to an industrial standard surface treatment procedure, a linear variation was found for the shear modulos near the adhesive/ adherend interface, independent of cure time. (Cure temperature was that specified by the manufacturer). The range of the IBZ was larger for a longer than normal period of cure.     

Justification of Fédération International de Béton, fib, 2000 model for elastic modulus of normal and high-performance concrete, HPC

This article outlines an experimental and theoretical comparison between the fib 2000 model for elastic modulus of normal concrete, NC, and HPC and 144 laboratory tests. Two dimensions of specimen were studied (diameters 56 and 100 mm) and one climate (20 °C, sealed or with ambient relative humidity, RH, 60%). The age dependence did not seem to be well correlated between the derived and the measured elastic modulus. The fib 2000 model overestimated the elastic modulus of mature concrete and in contrast slightly underestimated the elastic modulus of young NC. As a whole, the derived modulus when using the fib model was about 110% of the elastic modulus measured at loading. A relationship to strength was found for this diversity with an increasing overestimation of the E-modulus at lower strength.     

A re-examination of the elastic modulus dependence on crystallinity in semi-crystalline polymers

The elastic modulus versus crystallinity linear relationship in Polyethylene (PE) is re-examined via meticulous measurements over a wide set of PE. First, large discrepancies to linearity are observed; moreover, Raman spectroscopy revealed that the content of the so-called interphase exhibits considerable variations over the set of PE. Therefore a novel strategy based on DMA is developed for a better identification of the modulus of each phase along the temperature.On the one hand, below the α and β relaxations, the young modulus proved to be linearly dependant on the sum of the crystal and interphase content. On the other hand, between the α and β relaxations, the interphase appears surprisingly as stiff as the crystal. In addition, the quenched samples exhibit a particular behavior. A simple model has lead to the conclusion that their mechanical coupling and/or amorphous modulus are significantly different as compared with all others materials.     

Modelling of elastic modulus of a biphasic ceramic microstructure using 3D representative volume elements

In this work a methodology to reconstruct three-dimensional microstructures, representative of real biphasic ceramics using Neper free software is proposed. Finite element analysis in Ansys was implemented in order to calculate the effective elastic modulus of the simulated microstructures.Fine grained and dense zirconia toughened alumina (ZTA) materials with 5 and 40 vol.% of Yttria Stabilized Zirconia (YTZP) have been chosen to validate the proposed methodology. First, the effects of the size of the representative volume elements (RVEs) and the characteristics of the grain shapes are analysed. Second, the compliance with the isotropic condition is also verified.Agreement between the numerical and experimental values of the elastic modulus of the considered ZTA materials has been found. For these materials, zirconia fractions higher than 10 vol.% lead to bi-continuous microstructures which make the elastic properties deviate from the Voigt limit due to the increased number of contacts between zirconia grains.     

Water-cement ratio gradients in mortars and corresponding effective elastic properties

Water-cement ratio gradients are modeled through the interfacial transition zone (ITZ) of a mortar with spherical inclusions. The model is a function of the over-all water-cement ratio, volume fraction and radius of sand, specific gravity of cement and thickness of ITZ. Based on experimental data from the literature, the dependence of saturated, homogeneous cement paste is modeled as a function of water-cement ratio. Subsequently, the effective bulk and shear moduli for mortars are determined using a generalized self-consistent method. Finally, application of the model to data in the literature pertaining to elastic wave speeds in saturated mortars composed of 20-30 screened sand with an overall water-cement ratio of 0.3 yielded a mean ITZ thickness of 48.3 μm.     

Prediction equation of drying shrinkage of concrete based on composite model

In this study, the prediction equation of drying shrinkage of concrete is obtained with two-phase composite model as aggregate and matrix. In order to obtain the input values for this prediction equation easily, the experimental formula of drying shrinkage and Young's modulus of cement paste are obtained, and the estimation method of Young's modulus of aggregate are proposed with easy test using cement paste, mortar and concrete. According to the experimental results, this equation can predict the drying shrinkage at any age in error by less than about 100 μm.     

Assessing the influence of ITZ on the steady-state chloride diffusivity of concrete using a numerical model

In this study, the influence of the aggregate-cement paste interfacial transition zone (ITZ) on the steady-state chloride diffusivity of mortars and concretes was examined using a semi-empirical, three-phase composite sphere model. Mortars and concretes were modelled as three-phase composites consisting of the aggregate, bulk cement paste and an inhomogeneous ITZ. The latter was divided into a series of homogenous concentric shell elements of equal thickness. The initial porosity and cement gradients at the ITZ were first estimated from the overall water/cement ratio (w₀/c). The evolution of the porosity, solid hydration products and remnants of unreacted cement were then calculated from the hydration degree and local water/cement ratio (w/c) using Powers' empirical model. Based on the Laplacian equation, an element transfer matrix was derived analytically to predict the steady-state chloride diffusivity. The model was calibrated using the available experimental data and then applied to perform a sensitivity analysis to evaluate the effects of aggregate content, water/cement ratio, curing period, ITZ width, maximum aggregate size and aggregate gradation on diffusivity. Some of these variables are impractical to quantify by laboratory experimentation. Implications of the findings with regard to the role of ITZ on mass transport properties are discussed.     

Prediction of diffusivity of concrete based on simple analytic equations

Proposed is a simple analytic model that can predict realistically the diffusivities of concrete and mortar. The basic concept of the model comes from the relation between the diffusivities and the microstructure of concrete. The microstructure that affects the diffusivity includes the interfacial transition zone (ITZ) between aggregates and cement pastes as well as the microstructure of cement paste itself. The general effective media (GEM) equation was introduced to derive the diffusivity of cement paste. The effective diffusivity of concrete is derived on the basis of the composite sphere assemblage model, which considers the diffusivities of both ITZ and cement paste. The main parameters in the proposed model are the microstructural properties of cement paste such as capillary porosity and pore structure parameter, solid phase diffusivity, aggregate volume fraction, and interfacial zone properties. To validate the proposed model, many series of concrete and mortar specimens have been tested to measure the diffusivities. The major test variables include the water-to-binder ratios, the types and amount of mineral admixtures on the diffusivities. The effects of compressive strength, water-to-binder ratio, and mineral admixtures have been investigated comprehensively. The comparison of the proposed theory with the test data exhibits reasonably good correlation. The proposed model allows more accurate prediction of diffusion process and, thus, more realistic durability design of concrete structures.     

An inverse method to determine the elastic properties of the interphase between the aggregate and the cement paste

Concrete is a three-phase material consisting of cement paste matrix, discrete inclusions of rock (aggregate), and an interfacial transition zone (ITZ) between the matrix and the inclusions. We model the material as a composite formed by a matrix with embedded spherical particles; each surrounded by a concentric spherical shell. Effective elastic moduli of this composite are evaluated on the basis of the generalized self-consistent scheme (GSCS). This formulation is used to solve the inverse problem of determining the elastic moduli of the ITZ from experimentally known elastic properties of the composite.     

Effects of end conditions on compressive strength and static elastic modulus of very high strength concrete

The use of bonded and unbonded caps in testing very high strength concrete cylinders has been investigated experimentally. A hundred and ninety-two concrete cylinder specimens of 150-mm diameter and 300-mm height were cast and tested using packing with softboard, neat cement paste, neoprene pad and sulfur mortar. The design strength level of 75-100 MPa was achieved using water-cementitious material ratios of 0.22, 0.26 and 0.31. The results of the study were compared considering compressive strength and static elastic moduli values. A two-way analysis of variance was performed at a .01 level of significance in order to compare the effect of end conditions. It was found that the overall mean compressive strengths of specimens capped with neat cement paste, neoprene pad sulfur mortar were not significantly different. The packed specimens exhibited a significant difference from the others. On the other hand, there was no statistical difference in the static elastic moduli values when different capping types were used. Several modulus prediction equations were also examined. Experimental values were consistently higher than the predicted values.     

Effects of metakaolin, water/binder ratio and interfacial transition zones on the microhardness of cement mortars

Variations in the microhardness of the hydrated cement matrix component of model mortars have been investigated as functions of the distance from the aggregate surfaces for specimens in which the binder was Portland cement or a blend of Portland cement and metakaolin.Microhardness measurements were made using a Knoop indenter at distances of up to 120 μm from the aggregate. The microhardnesses of the paste-aggregate interfacial transition zones (ITZs) were found to be between 14% and 22% lower than those of the corresponding bulk cement pastes at the lower water/binder ratios investigated, i.e. 0.4 and 0.5 for samples prepared with Portland cement and 0.4 for samples prepared with a binder comprising Portland cement and metakaolin.Metakaolin increased the mean microhardness of specimens prepared at the higher water/binder ratios of 0.5 and 0.6 by 13% and 54%, respectively.