Interfacial Transition Zone Thickness Research Articles

Strengthening mechanism of fracture properties by nano materials for cementitious materials subject to early-age frost attack

With the increasing interest and utilization of nano materials to cementitious materials, the mechanical properties of the nano materials-modified cementitious materials under the severe environmental conditions is critical for their successful application. This study investigates the fracture toughness of nano materials-strengthened cement paste by nano silica particles and carbon nanotubes at the micro and macro scales by using nano-scratch technique and the conventional three-point bending test, respectively. The aim is to better understand the strengthening mechanism of these two nano materials on the fracture properties of cementitious materials subject to early-age frost attack. The paste sample is designed subject to freeze-thaw cycles at the age of 1, 3, and 7 days, and then sealed cured after the 7 days’ freeze-thaw period until the age of 28 days. The micro fracture toughness of the unreacted clinker, the hydrates, and the interfacial transition zone (ITZ) between the unreacted clinker and the surrounding hydrates is measured as well as the thickness of ITZ. The effect of nano silica and carbon nanotubes on the fracture properties are discussed and compared, and the different strengthening mechanism of these two nano materials are recognized. The results of this study are beneficial for designing mixtures to achieve better early age frost resistance that satisfies the engineering applications.

Mesoscale damage modelling of concrete by using image-based scaled boundary finite element method

As a thin layer surrounding aggregates, the interfacial transition zone (ITZ) has a significant influence on the mechanical performance of concrete, a large number of small elements is required to discretize the ITZ and its vicinity owing to its small thickness. In this contribution, the mesostructure of concrete is explicitly built by a novel method named improved random walking algorithm (IRWA), with which the grading, content, shape of aggregates and the thickness of ITZs, can be conveniently controlled by users, and high volume fraction of aggregate and remarkable efficiency can be achieved. In the framework of the scaled boundary finite element method (SBFEM), the image-based quadtree decomposition is employed to automatically generate the multi-level mesh for the concrete composites, with no trouble in dealing with hanging nodes or the dramatically changing element size in the vicinity of the material interface. The nonlocal damage model in integral format is extended to simulate progressive damage of concrete at mesoscale with good mesh independence. Four benchmarks are modelled to demonstrate the performance of the proposed approach, and the effects of model parameters on the structural responses are also discussed.

Statistical analysis of the critical percolation of ITZ around polygonal aggregates in three-phase concrete materials

Concrete is generally regarded as a kind of three-phase granular material composed of aggregates, interfacial transition zone (ITZ) and bulk cement paste, among which ITZ is an inescapable transport path of aggressive agents due to its high porosity. As the weakest part in concrete, the existence of ITZ usually triggers an unfavorable change of the holistic properties of concrete structures. In particular, the percolation behavior of ITZ is likely to be an important factor that accelerates the penetration of aggressive agents. Previous studies about the percolation of ITZ around aggregates are mainly carried out based on the models of circles, spheres and ellipsoids, which are very different from the real features of aggregates. In this work, the regular polygons which are more similar to the geometrical shapes of fine and coarse aggregates are applied to represent the aggregate particles. The ITZ around these aggregates is simplified to the soft interphase layer with identical thickness and the models of concrete structures can be modeled by the random sequential addtion (RSA) algorithm. By combining with the continuum percolation models of hard core/soft shell, the percolation properties of ITZ around different 2D multi-sized aggregates are studied in detail and the effect of both aggregates and ITZ on the percolation thresholds ϕagg,c of ITZ is quantified. Based on the obtained values of ϕagg,c here, an approximately numerical formula of ϕagg,c is proposed and its reliability is further verified by comparing with the results in the literature. The comparison confirms that the proposed model can be utilized to predict the critical thresholds of ITZ around various polygonal particles with the broad particle size range and ITZ thickness. The research findings may provide a helpful guidance for the design and optimization of concrete materials.

Experimental and Theoretical Prediction Model Research on Concrete Elastic Modulus Influenced by Aggregate Gradation and Porosity

In this research, we developed a four-phase model, which takes the aggregate gradation and porosity into account in the prediction of the elastic modulus of concrete, based on the micromechanical theories. The model has been verified with experimental results. First, using the Mori Tanaka and the differential self-consistent (DSC) methods, the pores in both the mortar and interfacial transition zone (ITZ) were homogenized. Then, the continuously graded aggregates were divided into finite aggregate size intervals. Further, using the generalized self-consistent model and multiphase composite model derived from the Mori Tanaka method, an aggregate gradation model for the prediction of the elastic modulus of concrete was developed. By simulating the pores in concrete with expanded polystyrene sphere (EPS) grains, the effect of overall porosity on the elastic modulus of concrete was investigated. The research results show that aggregate gradation and porosity have remarkable influence on the elastic modulus of concrete, and the proposed model is effective to estimate the elastic modulus of concrete, the deviation between the predicted elastic modulus and experimental elastic modulus is less than 8%. The elastic modulus decreases with increasing ITZ porosity. However, for ITZ porosity exceeding 40%, the decrease in the elastic modulus is large with increasing ITZ porosity. For a fixed overall porosity, the ITZ porosity owned more influences than the mortar porosity on the elastic modulus of concrete. Enhancing the ITZ elastic modulus and decreasing the ITZ thickness are efficient in increasing the elastic modulus of concrete.

Nanoindentation-based micromechanical characterisation of ultra-high-performance concrete exposed to freezing–thawing

The durability of ultra-high-performance concrete (UHPC) exposed to freezing–thawing (F–T) is closely associated with its micromechanical properties. In this study, the nanoindentation technique is employed to characterise the micromechanical properties of UHPC at different F–T cycles. It is observed that the indentation moduli of low-density calcium silicate hydrate, high-density calcium silicate hydrate, and calcium hydroxide in the studied UHPC decrease moderately with the decreasing ranges of 11.58, 14.56 and 3.3%, respectively, after undergoing 1500 F–T cycles. More micropores gradually form, converge and connect, and the volume fraction of micropores increases up to about 120%. The interfacial transition zone (ITZ) between steel fibre and cement paste is more prone to act as the weakest zone than that between fine sand and cement paste. The thickness of ITZ enlarges from 22 μm of the unconditioned stage to 60 μm at 1500 F–T cycles. Based on the micromechanical properties obtained from the nanoindentation test, the modulus of elasticity values of UHPC under different F–T cycles are predicted using the multiscale homogenisation model. It is proved that the combined micromechanical characterisation and multiscale homogenisation technique is capable of predicting the macromechanical properties of UHPC and their changes under F–T action.

Influence of the ITZ Thickness on the Damage Performance of Recycled Concrete

The interfacial transition zone (ITZ) is a weak region of recycled concrete. To further study the influence of the ITZ on the damage performance of recycled concrete, a formula for calculating the ITZ thickness is derived, and the ITZ thickness is calculated for various recycled concrete mixtures with a recycled aggregate content of 30%. The concrete model test method and finite element analysis software were used to numerically simulate common concrete and recycled concrete with ITZ thicknesses of 0.05 mm, 1 mm, 2 mm, 3 mm, and 4 mm. The results show that the thickness of the ITZ has an obvious influence on the cracking strength of recycled concrete and that the cracking strength of recycled concrete decreases with increasing ITZ thickness. The cracks in recycled concrete initiate from the ITZ, and the damage degree increases with increasing ITZ thickness. Compared with that of common concrete, the cracking strength of recycled concrete with an ITZ thickness of no more than 2 mm (i.e., the replacement rate of the recycled aggregates is no more than 30%) is not significantly reduced.

A Three-Dimensional Random Walk Algorithm for Estimating the Chloride Diffusivity of Concrete.

The chloride diffusivity of concrete is an important parameter for assessing the long-term durability of coastal concrete structures. The purpose of this paper is to present a three-dimensional random walk algorithm (RWA) for estimating the chloride diffusivity of concrete. By analyzing the size distribution of aggregates, the equivalent interfacial transition zone (ITZ) thickness is derived in an analytical manner. Each aggregate is combined with the surrounding ITZ to construct an equivalent aggregate model (EAM) and the chloride diffusivity is formulated. It is found that the equivalent ITZ thickness decreases with the increase of practical ITZ thickness and aggregate volume fraction. The aggregate gradation influences the equivalent ITZ thickness to a certain extent. The relative chloride diffusivity of the equivalent aggregate is almost directly and inversely proportional to the equivalent ITZ thickness and the aggregate radius, respectively. The numerical results show that, when the EAM is adopted, the computational time is greatly reduced. With the EAM, concrete can be modeled as a two-phase material and the chloride diffusivity is estimated by applying the RWA. It is shown that, with the increase of mean square displacement and number of Brownian particles, the average chloride diffusivity of concrete approaches a stable value. Finally, through comparison with experimental data, the validation of the RWA is preliminarily verified.

Multifunctional application of nanoscratch technique to characterize cementitious materials

This study aims to further explore the multifunctional applicability of nanoscratch technique to cementitious materials for comprehensive characterization. Based on the investigation of deformation, applied force, and fracture toughness of hardened cement paste during scratching, the methods of phase identification, fracture toughness of individual phase, and the interfacial transition zone (ITZ) thickness quantification were proposed and discussed. After redefining the vertical loading rate of scratching, the slope of the square scratching depth vs. scratching length is recognized as a reliable parameter to distinguish the individual phases for the first time. A fracture toughness-based method is proposed to quantify the thickness of ITZ between clinker and hydrates. Meanwhile, the applicability of phase identification by the scratch method is discussed for different transverse scratching speeds and the vertical loading rates. The findings will benefit the investigation and mechanism understanding of cementitious materials at the microscale by utilizing nanoscratch technique.

Nano- and micro-scale characterisation of interfacial transition zone (ITZ) of high volume slag and slag-fly ash blended concretes containing nano SiO2 and nano CaCO3

There is a strong demand of higher volume replacement of ordinary Portland cement (OPC) in concrete and cementitious composites by supplementary cementitious materials such as blast furnace slag (BFS) and fly ash (FA) to reduce their carbon footprint. This paper presents the effect of addition of nano calcium carbonate (NC) and nano silica (NS) on improving the interfacial transition zone (ITZ) of high volume slag (HVS) and high volume slag-fly ash (HVS-FA) blended concretes. Nanoindentation, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) techniques were used to characterise the nano- and microstructure of the ITZ area between aggregates and matrix. Results show that the addition of NC and NS improved the modulus and hardness of hydration products in the ITZ area of matrix of HVS and HVS-FA concretes. The thickness of ITZ is also reduced in those concretes due to addition of NS and NC. SEM and EDS analysis also confirm the above observation where dense microstructure with less voids and micro cracks and higher peaks of Ca, Si and Al were observed. The observed improvement of 28 days compressive strengths of the HVS and HVS-FA concretes due to addition of NC and NS also correlated well with the improvement of microstructures of ITZ area.

Multi-scale peridynamic formulations for chloride diffusion in concrete

A multi-scale peridynamic (PD) model of concrete with three material phases is developed to study the influences of the properties of the interfacial transition zone (ITZ) on chloride diffusion. In the PD model, the quadtree algorithm is employed to generate the multi-scale grid and two correction schemes based on the background grid are developed to calibrate the volume and length of PD bonds. A comparison with the uniform grid PD model and the finite element model is carried out to validate the proposed multi-scale PD model. The results of the multi-scale PD model are consistent with that of the two models. In addition, the effects of ITZ properties, including ITZ diffusivity and ITZ thickness, on chloride diffusion are investigated. These results demonstrate that the penetration depth and the chloride concentration increase with the increasing ITZ diffusivity and ITZ thickness, and the increase rate of chloride concentration decreases with the exposed time.

Areal analysis induced bias on interface thickness around ovoidal particles

Two-dimensional image analysis is usually used to obtain the thickness of the interfacial transition zone (ITZ) around aggregates. However, if the two-dimensional section plane crosses through the ITZ region but not through the corresponding aggregate, this region (called invalid region) will be excluded from the analysis. This exclusion will induce bias in estimating the thickness t of the ITZ. This paper proposes a methodology to quantify the bias on the ITZ thickness induced by areal analysis. The approach includes three parts: (1) Uniform ITZ layers around ovoidal aggregates are generated based on the Minkowski sum; (2) A systematical cap sampling method (SCSM) is proposed to estimate the volume of the invalid region VITZiv; (3) By combining with VITZiv, the underestimation degree of the ITZ thickness for dilute mono- and multi- packing systems are calculated. The results reveal that the underestimation degree increases with the increasing sphericity of aggregates.

In-situ measurement of nominal compressive elastic modulus of interfacial transition zone in concrete by SEM-DIC coupled method

Due to the small dimensions of interfacial transition zone (ITZ) in concrete, the measurement of ITZ is subjected to difficulties of deformation measuring at macro-scale and loading at micro-scale. This work presents a coupled strategy between the scanning electron microscope (SEM) technology and the digital image correlation (DIC) method to experimentally study the mechanical properties and thickness of ITZ. Such the SEM-DIC coupled method not only requires a relatively uncomplicated preparation of samples, but also overcomes the difficulty of measuring the nominal compressive elastic modulus (ENC) of ITZ. A cuboid sample composed of aggregates and mortar is designed to make it convenient to conduct in-situ compressive tests by the SEM-DIC coupled method. The ENC of ITZ is obtained by analyzing the stress measured by an in-situ loading device and the strain acquired by DIC method. Results reveal that the ENC of ITZ can be well measured by the proposed method and the values are obviously less than that of aggregates and mortar. Also, the distribution of ENC is described with a quadratic polynomial function. With this coupled method, the effects of aggregate type and water-cement ratio (w/c) on the mechanical properties and thickness of ITZ are further explored. It is indicated that ITZ areas with limestone aggregates and high water-cement ratio deform with more tendency than those with granite aggregates and low water-cement ratio.

The bias of the interface thickness and diffusivity of concrete comprising Platonic aggregates induced by areal analysis

The micro-structure of three-dimensional (3D) interfacial transition zone (ITZ) in concrete is usually investigated by one-dimensional (1D) and two-dimensional (2D) analyses. However, the information obtained in 1D and 2D cannot be directly used to characterize the actual 3D microstructure. If the two-dimensional section plane crosses through the ITZ region but not through the corresponding aggregate, this invalid region will be excluded from the analysis. Due to this exclusion, the ITZ thickness is underestimated and ultimately lead to a biased estimation of the contribution of the ITZ to the overall performance of the concrete. In this work, the underestimation of the ITZ thickness of concrete comprising mono- and multisized Platonic aggregates are investigated. By combining the underestimated ITZ thickness with a differential effective medium approximation, the bias on diffusivity of concrete induced by the areal analysis is evaluated. The results reveal that the bias increases with the increasing sphericity of aggregates.

Numerical modelling of interfacial transition zone influence on elastic modulus of concrete

Concrete is a highly complex and heterogeneous three-phase composite composed of aggregates, matrix and the interfacial transition zone (ITZ) between them. As the weakest structure of concrete, the ITZ is often ignored due to difficulty in its modelling using the finite-element method. In this paper, concrete models with high-volume fractions of aggregates are established, in which the ITZ structure is considered. The numerical results for compressive elastic modulus are in good agreement with those obtained from experiments. The influence of the volume fraction, the size of aggregate and the thickness and compressive elastic modulus of ITZ on the compressive properties of concrete have been investigated. The numerical analyses show that the increase of the aggregate volume fraction, ITZ compressive elastic modulus, or the decrease of the ITZ thickness can all lead to the increase of the concrete compressive elastic modulus, while the aggregate size only has a mild effect on it.

Influences of multiple factors on the chloride diffusivity of the interfacial transition zone in concrete composites

The corrosion of reinforcement caused by chloride attack can degrade the durability and strength of reinforced concrete (RC) structures and shorten the structural service life. The interfacial transition zone (ITZ) in concrete composites is generally considered a weak phase of chloride ingress due to its higher porosity compared to mortar and coarse aggregate. In this paper, the influences of multiple factors, including the water-cement ratio, ITZ thickness, and coarse aggregate properties, on the ITZ chloride diffusivity in concrete composites are elaborated. The results indicate that the normalized chloride diffusivity of the ITZ is affected by the factors of ITZ thickness, coarse aggregate volume fraction and coarse aggregate grading but is independent of the water-cement ratio. In particular, during the investigations of this paper, an experiential model is built to assess the normalized chloride diffusivity of the ITZ in concrete composites considering multiple factors of the ITZ thickness, coarse aggregate volume fraction and maximum coarse aggregate size. The precision accuracy for this developed experiential model is validated and examined based on mesoscopic numerical simulation using finite element analysis to determine the diffusion concentrations of chloride ions in concrete composites.

Interfacial characteristics of nano-engineered concrete composites

This study investigates the interfacial characteristics between aggregates and cement paste matrix in nanofillers modified concrete. A three-point bend test on the specimens composed of two pieces of aggregates bonded with a thin layer of cement pastes with/without nanofillers was carried out to characterize the interfacial bond strength of the composites. The scanning electron microscope observations and energy dispersive x-ray spectrometry analysis were also performed to characterize the interfacial microstructures and compositions of the composites. The experimental results indicated that the nanocomposites have higher interfacial bond strength and narrower interfacial transition zone thickness as well as more optimized intrinsic compositions and microstructures than that of composites without nanofillers. Specifically, the interfacial bond strength of nanocomposites can reach 7.67 MPa, which is 3.03 MPa/65.3% higher than that of composites without nanofillers. The interfacial transition zone thickness of nanocomposites ranges from 9 μm to 12 μm, while that of composites without nanofillers is about 18 μm. The ratio of CaO to SiO2 in the interface of composites without nanofillers is 0.69, and that of nanocomposites increases to 0.75–1.12. Meanwhile, the nanofiller content in nanocomposite interface is 1.65–1.98 times more than that in the bulk matrix. The interfacial microstructures of nanocomposites are more compact and the content and crystal size of calcium hydroxide were significantly reduced compared with that of composites without nanofillers.

Validation of Micromechanical Model for Prediction of ITZ Thickness of High-Strength Concrete Containing Secondary Cementitious Materials

The mechanical properties of a cementitious composite are strongly affected by interfacial transition zone (ITZ) between the matrix and the aggregates, mainly by its strength and thickness. A micromechanical model based on Mori-Tanaka scheme coupled with an estimation of deviatoric stress in ITZ was developed for evaluation of the effect of selected secondary cementitious materials (SCMs – silica fume, fly ash and metakaolin) on the properties of ITZ in high-strength concrete (HSC). The model was validated by means of comparison of predicted ITZ thickness with direct ITZ thickness measurements performed by a combination of scanning electron microscopy and grid nanoindentation. Very good agreement between the theoretical and experimental results was reached, therefore the developed micromechanical model can be used for further research and optimization of HSC containing SCMs. Silica fume was determined to be the most efficient supplementary cementitious material from the point of view of ITZ thickness reduction.

Modeling interfacial transition zone of RAC based on a degenerate element of BFEM

To study the properties of the interfacial transition zone (ITZ) of recycled aggregate concrete (RAC), a RAC model was presented that can mesh directly inside the aggregate. The characteristics of the ITZ can be accurately identified and simulated based on this model. Additionally, as for comparative specimens, several projection mesh models of RAC were also set up to prove the accuracy and efficiency of this model. Further, a two-dimensional (2D) element was introduced to simulate the ITZ element, which is degenerated from a three-dimensional (3D) element on the base force element method (BFEM). Hereafter, these RAC models were subjected to a displacement loading to simulate the uniaxial compression and tension tests based on the BFEM. The effect of ITZ thickness on the compressive strength, tensile strength, peak/ultimate strain, and crack development was studied. Besides, the influence of the relative properties between ITZs and the corresponding mortar was also investigated. The obtained results demonstrate that the present model is an efficient way to analyze the influence law of ITZ. Besides, the multiple influencing factors can be fixed via this model; therefore, their effects can be studied by varying one single factor at a time. The degenerate element is a new approach to investigate the element with a large aspect ratio with sufficient accuracy and efficiency.

Effect of fine aggregate size on the overlapping of interfacial transition zone (ITZ) in mortars

The interfacial transition zone (ITZ), existing between the cement matrix and aggregates, is commonly viewed as a weak link for its highly porous microstructure. Enormous research has been conducted experimentally, analytically, and numerically on ITZ from the aspects of its forming mechanisms and influencing factors. In this paper, the effect of fine aggregate size on the formation of ITZ was investigated as well as the overlapping of ITZ between adjacent aggregate particles. Four mortar mixes were prepared with varying fine aggregate sizes. Via quantitative analysis on the backscattered electron image (BSE), the porosity and thickness of the ITZ was experimentally determined. Nearest surface function was further introduced to calculate the volume fraction of the ITZ based on the obtained thickness. Also, the inflection point for overlapping was roughly estimated from the differential volume fraction distribution curve of ITZ. The spacing factor between adjacent aggregate particles was defined and numerically calculated, which was further compared with value of ITZ thickness and inflection point for overlapping to understand the role of aggregate size on the overlapping phenomenon of ITZ.

Influence of Graphene Oxide on Interfacial Transition Zone of Mortar

In this paper, the influence of graphene oxide (GO) on the microstructure of interfacial transition zone (ITZ) in cement mortar was investigated through image analysis (IA) of backscattered electron (BSE) micrographs. The results showed that the incorporation of GO significantly reduced the thickness of ITZ. The porosity in ITZ and bulk paste decreased due to the introduction of GO; meanwhile, the compressive strength of the mortar samples was improved. The addition of GO also narrowed the gap between the porosity of ITZ and bulk paste, and therefore, the entire microstructure of mortar became more homogenous. Based on the above results, the model to predict the compressive strength of mortar was modified for better precision. The improved prediction model indicated that the difference between the compressive strength of ITZ and bulk paste was reduced upon the refinement of ITZ by GO.