Effect Of Interfacial Transition Zone Research Articles

A multiscale approach to estimate the interfacial transition zone effects on the temperature rise of heterogeneous cement based materials at early age

This paper presents a novel numerical model for solving the thermochemical problem of hydration of cement paste and its combination with mortar aggregates, with the aim of determining the temperature rise in massive structures considering adiabatic and semi-adiabatic problems considering the Interfacial Transition Zone (ITZ) effects. The model presented here takes advantage of recent advances in the computer-aided homogenization technique known as FE2 to fully model the massive structure based on the description of the microstructure. Each integration point of the massive structure is associated with an RVE whose matrix consists of cement paste and the fillers (inhomogeneities) of sand particles, which also takes into account the Interfacial Transition Zone (ITZ) effects, given by a thermal resistance. The presented strategy allows the prediction of the macroscale thermal behavior based only on the microscale thermal properties and heat generation. Despite the large variability/uncertainty in cement and mortar (chemical–physical) mixes, thermal properties, and ITZ effects, a validation study is also performed to show the adequacy of the proposed approach. Macroscale and microscale predictions are presented for mortar recipes and for different boundary conditions unveiling the combined effect of convection, volume fractions of sand in mortars, and ITZ on the predicted temperature and hydration evolution, showing that an imperfect interface between the matrix and the fillers causes a faster hydration reaction in the cement paste.

Quantifying the evolution of mechanical behaviours of concrete materials resting on realistic microstructural characteristics: A micro-to-macro validated combination analysis method

Understanding the physical and chemical essence of concrete material corrosion and the evolution process of mechanical properties is crucial for the study of durability. However, there is currently no suitable universal method to obtain the results of mechanical property evolution through physical and chemical essence. The purpose of this paper is to bridge durability parameters and mechanical behaviors through the micro-to-macro-scale, resting only on realistic microstructural characteristics. This model took the essential parameters of durability degradation such as phase change, porosity, and corrosion degree after corrosion as input parameters. A structure with a corroded layer wrapping the uncorroded inner core was proposed. This structure is closer to the actual corroded structure to a greater extent. This method quantifies and unravels the deterioration mechanisms of mechanical behaviour in concrete caused by external sulfuric acid and sulphate attack. The remarkable features are that: (1) corrosion products form only in outer C-S-H layers through “ink-bottle” theory; (2) considers interfacial transition zone effects and (3) combines mean-field homogenization method with the finite element analysis. Additionally, the mechanical properties obtained from the multi-scale model have been experimentally verified, demonstrating the robustness and reliability of the model.

A numerical framework for the ITZ percolation, effective fraction and diffusivity of concrete systems considering the nonuniform ITZ

The critical percolation and effective fraction of interfacial transition zone (ITZ) around the aggregates are generally considered as two important structural factors in the prediction of macroscopically transport properties in concrete, and their solutions are very susceptible to the geometric features of both aggregate particles and ITZs. Understanding the influence of aggregate features and ITZ thickness on the variation of these ITZ configurations and their effects on the macroscopic performance of materials would have great significance for the optimization and adjustment of concrete materials. However, in most existing numerical studies, both the coarse and fine aggregates are generally simplified as the hard particles of the same morphology and the ITZ thickness around them is also set to be uniform, which don’t reflect the reality perfectly and may lead to the great deviation. To clarify the variation of ITZ features and ionic diffusivity with the ITZ effect in a more realistic way, the polydispersity of aggregate shapes is taken into consideration by assuming coarse and fine aggregates as polygons and ovals, respectively. Moreover, a differential ITZ thickness is also assigned for the aggregates according to their sizes and W/C ratio. Based on the above premises, a numerical framework for the prediction of the effect of individual phases on the ITZ percolation, effective fraction and diffusivity of concrete is developed sequentially in this paper. The whole framework can be divided roughly into three parts: (1) the numerical simulation and quantification of the critical percolation of nonuniform ITZs; (2) the statistical evaluation and quantitative analysis of the effective ITZ fraction by statistical geometry; (3) the continuum percolation-based effective medium prediction for the relative diffusivity of polyphase concrete systems. According to the systematically numerical investigation, it can be found that the oversimplification of both ITZ thickness and aggregate shape may result in the serious deviation for the evaluation of ITZ percolation, effective fraction and macroscopic properties, especially the ITZ thickness. The numerical results show that when the ITZ thickness is assumed to be uniform for all the aggregates, the ITZ percolation threshold ϕagg,c, its effective fraction ϕITZ and ionic diffusivity Dcon of concrete system would generally be overestimated, underestimated and underestimated compared with the ones with the nonuniform ITZs, respectively. The proposed models here may give more realistic results than the previous prediction models in 2D case.

Compressive strength prediction of crumb rubber mortar based on mesoscale model

A mesoscale model was developed to study the compressive performance of crumb rubber mortar (CRM). The internal structure of CRM was considered as a three-phase composite consisting of rubber, mortar, and interfacial transition zone (ITZ). The effects of ITZ, rubber aggregate distribution, mortar matrix strength, rubber aggregate shape, and treating rubber as pores on the compressive strength of CRM were studied. The influence of the presence of ITZ on the compressive strength of CRM was lower than 2 % and was negligible.The location of rubber particles has an effect on the strength of CRM. The use of small-sized rubber particles resulted in a more uniform internal structure of the CRM samples and less variation in strength among different samples. Adding rubber reduces the compressive strength of mortar, and the strength reduction rate is mainly affected by the rubber content. Rubber particles can be equivalent to pores to analyze their influence on the compressive strength and failure mode of CRM. The stress concentration that occurs around the position of the rubber particles is a major factor that causes the strength of CRM to decrease. A prediction formula was presented to calculate the compressive strength of CRM, which agreed well with the experimental results.

Quantitative study on the influence of fly ash content, saturation degree and interface transition zone on the permeability of cement-based materials

The interfacial transition zone (ITZ) and fly ash content affect the transmission performance of concrete by altering its pore structure, while varying water content also has distinct effects on transport characteristics. Therefore, this paper investigates the effects of ITZ, fly ash content, and saturation degree on the gas transport performance of concrete materials through gas penetration tests, mercury injection tests, scanning electron microscopy (SEM), and backscattered electron (BSE) imaging. The influences of these factors on the intrinsic permeability of concrete materials are quantitatively analyzed through methods such as parallel test comparative analysis, regression analysis, function fitting, and fixed parameter analysis. The results indicate that the incorporation of fly ash can refine the pore structure of concrete, but it also leads to an increase in intrinsic permeability. The relationship between saturation degree of concrete and intrinsic permeability conforms to an exponential function. Under the same volume fraction, the intrinsic permeability of ITZ is hundreds of thousands of times higher than that of mortar matrix.

Mechanical tensile behaviour of cement paste/aggregate bond exposed to leaching

An experimental program is oriented to the mechanical behaviour of the Interfacial Transition Zone (ITZ) exposed to leaching, for a single mix. Leaching kinetics of the degraded depth, measured through grey level on cross sections, reveals a maximum gap of 10 % between ITZ and bulk cement paste. ITZ, with a main thickness of 15 µm suffers a significant decalcification, which is reflected on the mechanical properties of the bond. Grey level, Calcium/Silicon (Ca/Si) molar ratio and Young’s modulus in the degraded zone present similar evolution. Effect of ITZ causing cracking and then debonding is noticed, through chemical analysis, image observation and mechanical tests.

Mesoscale Analysis of the Effect of Interfacial Transition Zone on the Compressive Damage of Concrete Based on Discrete Element Method.

The coarse aggregate-mortar interface transition zone (ITZ) has a great influence on the mechanical properties of concrete, which cannot be easily studied using laboratory tests in the mesoscale. In this paper, a series of axial compression tests were conducted using the discrete element method (DEM) on concrete specimens for four phases: coarse aggregates, mortars, aggregate-mortar interface transition zones, and voids. The effects of ITZ strength on macroscopic stress and microscopic cracks under different strength reduction factors were investigated through axial compression testing. With the increase in interface transition strength, the compressive strength of the concrete becomes stronger; moreover, the number of cracks decreases, and the anisotropy of contact orientation becomes weaker. Meanwhile, the direction of crack development and the damage mode of compressed concrete specimens were also dependent on the coarse aggregate-mortar interface strength coefficient.

Effect of interfacial transition zone on creep behavior of steel fiber-reinforced concrete

• A SFRC creep prediction model considering ITZ was established. • The role of ITZ in SFRC was revealed. • The effects of relevant factors on SFRC creep were analyzed. • The key factors affecting SFRC creep were determined. To explore the interface transition zone (ITZ) effect on the creep behavior of steel fiber-reinforced concrete (SFRC), this study proposed a three-phase creep prediction model based on the mesomechanical shear lag theory. The model verification was performed using available creep data, and three influencing factors were analyzed. The proposed creep prediction model well described the creep behavior of SFRC, revealing the ITZ's role in it. The specific creep values of SFRC at the loading age of 365d with 1, 2, and 3% steel fiber contents exceeded those assessed without the ITZ effect account by 9.8, 6.3, and 54.0%, respectively. The effects of the ITZ elastic modulus and thickness, as well as the steel fiber length-to-diameter ratio on the SFRC creep, were analyzed via the proposed model. The grey relational analysis theory can characterize the correlation between related factors and various variations, so as to determine the key factors. Through this theory, it was revealed that the ITZ elastic modulus variation had the most significant effect on the SFRC creep.

The role of aggregate and cement paste in the deterioration of the transitional interface zone of pervious concrete during freeze-thaw cycles

Improving the performance of the cement matrix is an effective method to enhance the freeze-thaw (F-T) damage resistance of pervious concrete. However, the interfacial transition zone (ITZ) is the weakest component. The purpose of this paper is to evaluate the effect of ITZ on the deterioration of pervious concrete under F-T circumstances. The evolution of the compressive strength of samples under water and salt F-T conditions was obtained by uniaxial compression tests. Additionally, the development of the ultrasonic velocities and the relative dynamic modulus of elasticity were examined. Changes in the microstructure and the pore structure were also continuously monitored. The results demonstrate that as the number of F-T cycles rises, compressive strength and dynamic elastic modulus diminish, and the deterioration of samples in 3% NaCl solution is more severe than that in water. Within 100 F-T cycles, cement paste samples with the identical w/c as pervious concrete showed no discernible strength deterioration, and detrimental porosity did not alter considerably. Scanning electron microscopy (SEM) results showed that the performance difference between aggregate and ITZ and the expansion stress lead to debonding at the interface of aggregate and cement, which is the primary culprit for the worsening of pervious concrete under F-T conditions. This work serves as a reference for the deterioration mechanism and durability enhancement of pervious concrete under F-T circumstances.

Effect of Interfacial Transition Zone on diffuse ultrasound in thermally damaged concrete

The correlation between Interfacial Transition Zone (ITZ) and the diffusion of ultrasound for the Non Destructive Evaluation of thermally damaged concrete is investigated in this study. As most damages first emerge from the ITZ, it is necessary to understand the ultrasonic wave/ITZ interaction. This paper focuses on the use of the diffusivity parameter to characterize thermal damage within the microstructure. A numerical model accounting for real ITZ characteristics and validated through a previous study is used to assess the effects of ITZ on diffusivity values. Diffusivity measurements are performed on real concrete specimens with different thermal damage states. Comparison of experimental and numerical data exhibits a strong correlation between the ultrasonic diffusivity, the thermal damage level and the ITZ contact level. A direct correlation between diffusivity and ITZ characteristics is suggested. This result is a major step toward the development of quantitative ultrasonic methods for the NDE of concrete.

Phase field modeling scheme with mesostructure for crack propagation in concrete composite

Tracking the crack propagation in concrete at the mesoscopic level is of great importance for revealing the crack pattern and failure mechanism, due to the huge threat of cracking to structure safety. This paper develops a phase field modeling scheme with mesostructure of concrete composite to investigate its crack propagation behavior. With the classical phase field model and the staggered algorithm implementation by three-dimensional (3D) finite element method (FEM), crack propagation within concrete’s mesostructure composed of coarse aggregate, mortar matrix, interfacial transition zone (ITZ) and initial defects including micropores and microcracks is modeled. The proposed modeling scheme is tested on single-aggregate samples and complex mesostructured concrete samples subjected to uniaxial loading, showing reasonable accordance with the experimental observation and common failure modes of concrete material. The results indicate that mesostructural configurations are important factors affecting the crack pattern of concrete. The presence of coarse aggregates is prone to initiate and induce interfacial cracking in the ITZ effect, but also exerts the blocking effect on the crack growth. And the initial defects (micropores and microcracks) are also the important influential factor on crack initiation, propagation and the formation of the major crack. It is found that the higher the volume fractions of aggregates and initial defects, the cracking of concrete initiates earlier, propagates faster and penetrates more easily when subjected to the same loading condition.

A numerical model for permeability of cement mortar considering the interfacial transition zone

A numerical model for evaluating the permeability of cement mortar is proposed. The numerical model simulates the permeation process of cement mortar under water pressure considering the cement paste connectivity and the connectivity of interfacial transition zone (ITZ). To illustrate the connectivity of the ITZ, a critical volume fraction of fine aggregates is proposed to indicate the threshold at which percolation of the ITZ occurs in the mortar penetration process. The numerical model was validated using empirical experimental results of a series of water penetration pressure tests. The phenomena in which the permeability ratio increases with the volume fraction of aggregate and a dramatic increase appears at a volume content of 35–45% are explained through the interconnection and percolation of ITZs. Finally, using the proposed numerical model, the influence of the ITZ on vapour water permeability is evaluated, which shows that the ITZ effect has very little influence on vapour moisture transport.

Effect of interfacial transition zone on the transport of sulfate ions in concrete

Long-term external sulfate attack can result in significant concrete degradation. The purpose of this study is to develop an experimentally validated reactive transport model to investigate the transport behaviour of sulfate ions in concrete containing interfacial transition zone (ITZ). The model takes into account the spatially dependent pore size in ITZ and the change of diffusion coefficient of sulfate ions due to the formation of ettringite and gypsum in concrete pores resulting from the chemical reaction between sulfate ions and concrete components. The developed model was firstly validated by performing a series of experiments involving concrete specimens and sodium sulfate solution. Then, a series of parametric studies were carried out to identify the critical parameters that govern the transport of sulfate ions in concrete. The backscattered electron microscopy (BSE) results show that the porosity of concrete within ITZ is relatively higher than that in bulk paste with a six-fold higher porosity in inner ITZ (5 µm from aggregate surface) than that in outer ITZ (50 µm from aggregate surface). The model predictions suggested that the concentration of sulfate ions initially increases rapidly in concrete during the first 20 days and then the rate of increase gradually decreases due to the reduction in the diffusion coefficient of ions as a result of the ettringite and gypsum formation in concrete pores. In addition, it demonstrates that self-induced electric field has little influence in the transport of sulfate ions.

Effect of the Interfacial Transition Zone on Basic Mechanical Properties of a Solid Composite Propellant

The evaluation of basic mechanical properties are shown to be of importance for the solid composite propellant development and application. The numerical analysis approach termed the three-phase Voronoi cell finite element method, was proposed to evaluate of the interfacial transition zone effect. Numerical results showed that the performance of the effective modulus could be essentially enhanced by increasing the zone thickness. The module of the composite with the inhomogeneous zone was higher than that of the composite with the homogeneous one. The effect of different zones on volume fractions and matrix moduli was also calculated. The Voronoi method can also be helpful for analyzing the characteristic properties of other three-phase composites.

Effect of interfacial transition zone on the Young's modulus of carbon nanofiber reinforced cement concrete

Identifying the properties of the region where the cement paste meets the aggregate surface (interfacial transition zone, ITZ) is critical to understanding the strength and fracture behavior of carbon nanofiber (CNF) reinforced cement concrete. In this study, the finite element method is employed to investigate the effect of the ITZ on the Young's modulus of cement concrete made with CNF. The numerical models for cement concrete with and without CNF are constructed based on the digital image processing technique. To consider the interface effect, the concept of “effective aggregate” is put forward, namely, the Young's modulus and Poisson's ratio for each aggregate particle are replaced by an effective Young's modulus and effective Poisson's ratio, in which the effect of thickness and the Young's modulus of ITZ is taken into account in an averaged manner. Then, the quantitative nanomechanical mapping (based on atomic force microscopy) technique is adopted to measure the Young's modulus and thickness of ITZ with and without CNFs, which are further used as the input parameters in the numerical model. The numerical simulation results are verified by experimental testing, which indicates that the ITZ effect should be considered when implementing the numerical simulation. In addition, this analysis shows that compared with the plain cement concrete, CNFs can greatly enhance the mechanical properties of ITZ, which will in turn improve the Young's modulus of cement concrete significantly. Finally, the effect of thickness and Young's modulus of ITZ on the Young's modulus of CNF reinforced cement concrete is also discussed.

Stochastic micromechanical predictions for the effective properties of concrete considering the interfacial transition zone effects

A stochastic micromechanical framework for predicting the concrete probabilistic behavior is proposed considering the interfacial transition zone effects in this paper. The volume fraction of the interfacial transition zone is analytically calculated based on the aggregate grading. Multilevel homogenization schemes based on the direct interaction micromechanical solutions are presented to predict the concrete effective properties considering the aggregate and interfacial transition zone effects. By modeling the volume fractions and properties of the constituents as stochastic, we extend the deterministic framework to stochastic to incorporate the inherent randomness of effective properties among different concrete specimens. With the moments of the effective properties, the probability density function is approximated using the exponential polynomial for concrete material. Numerical examples including limited experimental validations, comparisons with existing micromechanical models, commonly used probability density functions, and the direct Monte Carlo simulations indicate that the proposed models provide an accurate and computationally efficient framework in characterizing the material’s effective properties. Finally, the effects of the randomness of interfacial transition zone and aggregate on the materials’ macroscopic probabilistic behaviors are investigated based on our proposed stochastic micromechanical framework.

Modelling of interfacial transition zone effect on resistance to crack propagation in fine-grained cement-based composites

In this paper, the attention is paid to investigation of the importance of the interfacial transition zone (ITZ) in selected fine-grained cement-based composites for the global fracture behaviour. This is a region of cement paste around the aggregate particles which specific features could have significant impact on the final behaviour of cement composites with a crack tip nearby this interface under applied tension. The aim of this work is to show the basic interface microstructure by scanning electron microscopy (SEM) done by MIRA3 TESCAN and to analyse the behaviour of such composite by numerical modelling. Numerical studies assume two different ITZ thicknesses taken from SEM analysis. A simplified cracked geometry (consisting of three phases – matrix, ITZ, and aggregate) is modelled by means of the finite element method with a crack terminating at the matrix–ITZ interface. ITZ’s modulus of elasticity is taken from generalized self-consistent scheme. A few conclusions are discussed based on comparison of the average values of the opening stress ahead of the crack tip with their critical values. The analyses dealing with the effect of ITZ’s properties on the stress distribution should contribute to better description of toughening mechanisms in silicate-based composites.

Effect of Interfacial Transition Zone on the Carbonation of Cement-Based Materials

AbstractThis study dealt with an analysis of the effects of the interfacial transition zone (ITZ) on the carbonation of cement-based material. To this end, a series of experiments dealing with past...

Micromechanical framework for saturated concrete repaired by the electrochemical deposition method with interfacial transition zone effects

An improved micromechanical framework with interfacial transition zone (ITZ) effects is proposed for saturated concrete repaired by electrochemical deposition method (EDM) based on our recent studies. A multiphase micromechanical model with ITZs is proposed based on the material microstructure and a new multilevel homogenization scheme with inter-particle interactions is employed to predict the effective properties of repaired concrete considering the ITZ effects. The equivalent particle, composed by the water, deposition product and ITZ, is obtained by modifying the generalized self-consistent model. The mechanical properties of the healed concrete are calculated by micromechanical homogenization considering the inter-particle interactions. Moreover, modification procedures considering the ITZ effects are presented to attain the properties of repaired concrete in the dry state. To demonstrate the feasibility of the proposed micromechanical model, predictions in this study are compared with those of the existing models and the experimental data. Finally, the influences of ITZ on the equivalent particle and repaired concrete are discussed based on the proposed micromechanical framework.

Influence of leaching on the local mechanical properties of an aggregate-cement paste composite

This paper presents an experimental study to characterize the mechanical behaviour, at the local scale, of sound and degraded concrete after leaching. An experimental protocol was developed to study the effects of the calcium leaching mechanism at the interfacial transition zone (ITZ) on the mechanical properties of the cement-aggregate interface and bulk paste of model material. The process of lixiviation with deionised water occurs very slowly. The experimental study in the laboratory was accelerated by replacing the water with an ammonium nitrate solution. To quantify the development and kinetics of degradation at the cemented bond, the concrete leaching fronts were characterized at different levels of degradation using phenolphthalein and local mechanical tests (compression and tensile) and performed on samples consisting of two aggregates bound by cement paste. Tests were performed on sound samples at different hydration times and on chemically degraded samples. The results show the effect of leaching on the mechanical properties of the samples and making it possible to correlate the progress of leaching to the evolution of these locally considered properties. The experimental results show that there is an ITZ effect on the alteration of the mechanical properties due to leaching.