Properties Of Interfacial Transition Zone Research Articles

Two scale models for fracture behaviours of cementitious materials subjected to static and cyclic loadings

This study employs a lattice fracture model to simulate static and fatigue fracture behaviour of Interfacial Transition Zone (ITZ) at microscale and mortar at mesoscale. The heterogeneous microstructure of ITZ and mesostructure of mortar are explicitly considered in the models. The initial step involves calibrating and validating the microscopic model of the ITZ through micro-cantilever bending tests. Subsequently, this validated ITZ model serves as a constitutive law to simulate the fracture behavior of mortar at the mesoscale using an uncoupled upscaling method. The influence of microstructural features, such as w/c ratio and microscopic roughness, on the fracture behaviour of ITZ is investigated. Moreover, the effect of ITZ properties and stress level on the fracture performance and fatigue damage evolution of mortar is also studied. The simulation results for both the ITZ and mortar demonstrate good agreement with experimental results. The proposed two models provide insights into the fracture mechanisms and fatigue damage evolution in cementitious materials subjected to static and cyclic loadings.

Improved concrete ITZ and performance via pre-soaking of inert recycled red brick aggregates in carbonation solution

Compared to recycled concrete aggregate (RCA), the high-water absorption of recycled red brick aggregate (RBA) has limited its utilization in concrete production. However, this study takes the advantage of its high-water absorption capacity as a carrier for carbonation solutions, aiming to compensate the strength loss by enhancing the interfacial transition zone (ITZ) properties of incorporated concrete. The effects of carbonation solutions (e.g., CO2 and NaHCO3) by varying the CO2 injection times (10 s, 60 s and 600 s) and NaHCO3 dosages (1%, 3% and 5%) on the physical (i.e. density and water absorption) and mechanical (i.e. compressive and tensile bond strengths) properties of concrete, in particular focus on the ITZ changes, were systematically investigated. In addition, the microstructure changes of concrete ITZ were studied using thermogravimetric analysis (TGA), scanning electron microscope-backscattered electron (SEM-BSE) image analysis and mercury intrusion porosimetry (MIP). The results indicated that it was feasible to enhance the ITZ properties by using CO2 solution as the pre-soaking agent of RBA. The adoption of CO2 solution could significantly improve the bond strength of ITZ by up to 22.5% and resulted 8.6% improvement in 28-day compressive strength. The CO2 solution contributed to the formation of CaCO3 can accelerate the cement hydration of surrounding matrix and hence densified the microstructure of ITZ. For the case of pre-soaking with NaHCO3, although the release of solution from RBA could improve the early strength of concrete, the later strength significantly reduced due to the effect of sodium ions in NaHCO3 on the growth of outer C–S–H gel.

Experimental study of mechanism properties of interfacial transition zones in steel fiber reinforced concrete

To investigate the effects of steel fibers on the macroscopic mechanical properties, fracture properties and microstructure of the interfacial transition zones (ITZs) in concrete, composite spilt tensile and three-point bending specimens with three different volume fractions of steel fibers (0.5%, 1.0%, and 1.5%) and two types of rock aggregates (Limestone, Granite) were tested, and the fiber- cement paste and reinforced cement paste-aggregate interfacial transition zone microstructure by scanning electron microscopy (SEM). The results show that SF affects the mechanical properties of ITZ on different rock surfaces by different mechanisms, and the high volume content of steel fibers (≥1.5%) significantly reduces the tensile strength, fracture energy and fracture toughness in the transition zone.

Determination of the Thickness and Fracture Toughness of Shale Interfacial Transition Zone Near Various Micromineral Aggregates Using Nano-Scratch Tests

Summary Heterogeneously distributed micromineral aggregates (MMAs) are common in shale reservoirs. The interfaces between these MMAs and the surrounding minerals significantly affect fracture propagation behaviors during shale oil exploitation. In this paper, the concept of interfacial transition zone (ITZ) between the MMAs (e.g., striped barite, pyrite, calcite, apatite blocks, and bedding plane) and the surrounding mineral in shale is introduced. Due to the small thickness of the ITZ, its thickness and mechanical properties are very difficult to determine by traditional standard methods. To address this issue, this paper proposes a method combining nano-scratch tests, scanning electron microscopy (SEM), and quantitative evaluation of minerals by SEM (QEMSCAN) techniques to investigate the thickness and fracture toughness of the ITZ near typical MMAs. The results show that the thickness of the ITZ determined by the transverse force FT and scratch depth (-d2) varies from 3.2 μm to 17.3 μm. In addition, the fracture toughness of both MMAs and ITZs is characterized by high heterogeneity ranging from 0.1 MPa·m0.5 to 2.1 MPa·m0.5. Moreover, a formula evaluating the fracture toughness of the ITZ is proposed taking into account the type and content of minerals in the ITZ. A strong linear relationship is observed between the thickness of the ITZ and the average fracture toughness ratio. In addition, a relationship is established between the fracture toughness of the ITZ and MMAs, and an evaluation equation is derived. This study is helpful for understanding the characteristics (i.e., mainly thickness and fracture toughness) of the ITZ within shales.

Effect of pore water pressure on mechanical performance of recycled aggregate concrete under triaxial compression

The pore water pressure in concrete can significantly increase due to volume compression. Recycled aggregate concrete (RAC) possesses a more complex microstructure compared to natural aggregate concrete (NAC). Understanding the porosity and micromechanical properties of RAC is crucial for analysing its failure mechanism under the influence of coupled confining pressure and pore water pressure. This study compares the constituent proportions and micromechanical properties of interfacial transition zones (ITZs) and the adjacent paste matrix in NAC and RAC. Compressive stress-strain curves were obtained for concrete under coupled confining pressure and pore water pressure. The results indicate that the newly formed ITZ, which bonds to old mortar, outperformed the one bonded to natural aggregate when considering the same water-cement ratio. Compressive strength, ductility, and maximum volumetric strain gradually increased with increasing confining pressure. However, when pore water pressure was removed, compressive strength decreased while elastic modulus improved. Due to the inferior microstructures of RAC compared to NAC, the supportive effect of pore water becomes more pronounced. This is evident in the gradual increase in peak strain with increased pore water pressure for the stress-strain curves of RAC (100 % replacement ratio). Finally, a failure criterion and stress-strain theoretical model considering pore water pressure are proposed, and satisfactory fitting results are obtained.

Multiscale study of enhancing the fracture properties of interfacial transition zone: Insights from molecular dynamics and finite element simulations

Cementitious composites are widely used in architecture and construction field, but usually show quasi-brittle failure at macroscale under tensile or bending loads. In contrast, the calcium silicate hydrates (C-S-H) exhibits noticeable ductile response at nanoscale. In this article, we focus on the interfacial transition zone between cement paste and aggregate, which consists of the C-S-H/quartz and the calcium hydroxide (CH)/quartz interface structures. To comprehend how a nanoscale design might improve concrete’s macro mechanical qualities, molecular dynamics and finite element simulations are performed at the nanoscale and macroscale, respectively. The elastic properties, modulus, fracture toughness and critical energy release rate of C-S-H/CH/graphene oxide (GO)/quartz composites are calculated using the molecular dynamics method at nanoscale. These parameters then are used in macro-simulation and calculating mechanical properties of macro concrete structures. The incorporation of GO sheets into the interface results in a 38.40% to 58.06% increase in the fracture toughness of interfacial transition zone (ITZ) systems and an 80% and 9.81% improvement in the tensile strength and tensile modulus of the concrete model, respectively. This fundamental insight into nanoscale interfaces notably allows for more accurate predictions of macroscopic mechanical behavior, paving the way for innovative and enhanced cement-based materials in the future.

Study on the influence of the properties of interfacial transition zones on the performance of recycled aggregate concrete

Applying recycled aggregate (RA) aligns with the development concept of green and environmental protection. Nevertheless, introducing RA has resulted in a complex interface structure, which will deteriorate the performance of recycled aggregate concrete(RAC). To investigate the influence of the properties of the interfacial transition zones(ITZs) on the performance of RAC, and the digital image correlation(DIC) technology was used to track the strain variation law of each interface during the failure process. The porosity and mechanical properties of the ITZs were quantitatively characterized by backscattered electron(BSE) imaging and microhardness testing. By controlling a single variable, the different ITZs of RAC were improved by targeted strengthening, including mortar and aggregate strengthening.The influence of the ITZs properties on the macroscopic properties and mesoscopic failure behavior of RAC was analyzed. The experimental results show that the mechanical properties of the ITZs were enhanced and the porosity was reduced. Strengthening treatment improved the strength of target interface, causing a change in the relative strength of the internal interface of concrete, and reducing the probability of failure of the strengthened interface. This is consistent with the results observed in SEM images, the more compact and stable the structure at the interface, the lower the probability of failure under load. A comprehensive evaluation of mortar strengthening and aggregate strengthening was carried out with the help of the linear weighted sum method, and the results showed that the mortar strengthening was more effective.

A comprehensive investigation on sulphate resistance of geopolymer recycled concrete: Macro and micro properties

As a potential alternative to ordinary Portland cement concrete (OPC), geopolymer recycled concrete (GRC) is provided with great research value as it not only significantly reduces carbon dioxide (CO2) emissions but also promotes the resourceful use of construction and demolition waste (CDW). However, concrete is frequently subjected to extreme circumstances such as sulphate attack during service. To comprehensively explore the structural and performance damage mechanisms of GRC caused by sulphate assault, this study is conducted to erode GRC specimens with recycled aggregate (RA) replacement rate of 0%, 30%, 50%, 70% and 100% by using magnesium sulphate solution with 10% mass fraction. The macroscopic properties of GRC subjected to sulphate attack are examined, including mass loss rate, relative modulus of elasticity and compressive strength. The microstructure and phase evolution of GRC subjected to sulphate attack is analyzed by SEM, EDS and XRD. The micro-mechanical properties of interfacial transition zones (ITZs) in GRC are measured by nanoindentation. The results show that there is no loss of mass in all GRC specimens throughout the erosion age, and the relative dynamic elastic modulus of GRC first increases and then decreases. GRC with 30% RA substitution rate has the best resistance to sulphate attack. The degradation of GRC by sulphate attack is closely related to its internal structural changes. The deterioration of the micromechanical properties of three ITZs by sulphate erosion is not significant, which indicates the RA has favorable performances with geopolymer binder contributing to excellent resistance of GRC to sulfate attack. In addition, a prediction model for the evolution of GRC resistance to sulfate erosion with erosion age for different RA substitution rates is developed, which provides a theoretical guidance for the application of GRC.

Effect of internal curing and shrinkage-mitigating materials on microstructural characteristics of fiber-reinforced mortar

Internal curing (IC) and other shrinkage mitigating materials are employed to reduce shrinkage and risk of cracking. This study investigates the efficiency of individual versus combined use of IC and shrinkage mitigating materials on key characteristics of fiber-reinforced mortar (FRM). The investigated mixtures include a 25% pre-saturated lightweight sand (LWS) that is added individually and combined with 10% CaO-based expansive agent (EA) and 2% shrinkage reducing admixture (SRA). This elucidates the synergistic effect of high content IC and EA/SRA on macro- and micro-mechanical characteristics of FRM, especially at the interface of the matrix with fibers. Mechanical properties, microstructural characteristics, and fiber–matrix bonding of FRM made with 0.5% steel fibers are investigated. The results show that the use of IC with EA and SRA completely compensates for shrinkage at 56 days. The highest compressive and fiber pull out strengths are observed for FRM with IC and without EA/SRA due to the densification of the interfacial transition zone (ITZ) confirmed by microstructural analysis. Such improvement is associated with the lower porosity of the cement paste and longer silicate chain—higher Si/Ca—obtained by FRM made with LWS. Although the combination of both EA and SRA with IC leads to 180 μstrain expansion after 56 days, the corresponding mixture presents the weakest ITZ and inferior mechanical properties.

Influence of the interfacial transition zone between a steel inclusion and cement-based composite on the fracture response of a bent specimen

The influence of the interfacial transition zone (ITZ) between a steel inclusion and fine-grained cement-based composite on fracture behaviour of the specimen with the inclusion is investigated. Three-point bending fracture tests were conducted and the mechanical fracture properties of the composite were evaluated based on the results of the tests. The numerical analysis of the fracture test was performed and related to the experimental data obtained during crack propagation, and the microstructure of fracture surfaces. The crack propagation conditions were assessed based on the criterion of the average value of tangential stress calculated over certain distance in dependence on the polar angle (GMTS criterion). We concluded that the fracture toughness of the interface KIc, ITZ is equal to 33 % of the fracture toughness of matrix KIc, MTX. The poor fracture properties of ITZ are responsible for the reduction of load-bearing capacity of whole specimen with inclusion in comparison to the sample without the inclusion.

The interfacial transition zone microstructure of ground ferronickel slag incorporated self-compacting concrete investigated by nanoindentation

The effect of using ground ferronickel slag (GFNS) as a supplementary binder on the interfacial transition zone (ITZ) and strength properties of self-compacting concrete (SCC) were investigated. Cement was replaced by 20–50% GFNS, and mechanical properties of the SCC were assessed. The ITZ was characterized by EDS line scanning. The elastic modulus and hardness properties of ITZ were determined by grid nanoindentation tests. Microstructural changes were followed up by XRD, SEM and EDS analysis, while the macro-level characterization was carried out by compressive and splitting tensile strengths. The strengths showed a decreasing trend with increasing GFNS content. The 28-day compressive strength of the mix with 35% GFNS was 66 MPa as compared to 79 MPa for the control mix. Moreover, up to 35% replacement level, this study finds a compatible ITZ, with a thickness of around 55 μm. The elastic modulus and the hardness values from the nanoindentation test suggest a decrease of the calcium silicate hydrate with the increase of GFNS that decreased strength. XRD data confirmed this finding. The line scanning data from EDS reveals that the presence of Al, Fe and S in ITZ was not significant. Hence, the Ca/Si ratio plays a pivotal role in determining the ITZ characteristic, which increases with the increase of GFNS content in the binder. Strength diminishes gradually with increasing Ca/Si ratio, possibly due to the damaged silicate skeleton and the hydrolytic weakening effect. Therefore, microstructural observations and nanoindentation results well elucidate the strength results, and these experimental outcomes will provide a strong base for utilizing GFNS in SCC.

An improved understanding of the influence of w/c ratio and interfacial transition zone on fracture mechanisms in concrete

The interfacial transition zone (ITZ) is significantly influenced by the water/cementitious materials (w/cm) ratio and governs the overall strength and fracture properties of concrete. Even though an ample number of studies on this topic are available in the literature, there is a lack of unified conclusions pertaining to the influence of w/cm ratios on various fracture characteristics and the underlying mechanisms. In this study, an attempt was made to investigate fracture characteristics (such as the size of the fracture process zone (FPZ), fracture energy, traction-free cracks (TFCs), toughening mechanisms) for varying ITZ properties and water/cement (w/c) ratios. Plain concrete beams of size 700 × 150 × 80 mm (l × d × b) with varying w/c ratios were tested under centre-point loading by controlling the crack mouth opening displacement. The acoustic emission (AE) technique was used to assess the internal damage parameters. A novel method was developed for identifying the TFC tip and the FPZ size using AE event location data at different stages of loading. The role of the ITZ on the fracture mechanisms of different types of concrete was also assessed. Evaluation of the fracture energy revealed that its relation with the w/c ratio is affected by the type of concrete.

Micromechanical properties of interfacial transition zone between carbon fibers and UHPC matrix based on nano-scratching tests

Ultra high performance concrete (UHPC) can be prepared by using carbon fiber (CF), instead of steel fiber, as reinforcement and indeed avoid the corrosion of steel fiber. The properties of CF/matrix interface play a key role on the composite performances. However, the properties of interface between CFs with different surface treatments and UHPC matrix are not yet clear. The denser matrix of UHPC makes the CF/matrix interface obviously different from that of ordinary concrete and more difficult to be characterized quantitatively. In this study, micromechanical properties of CF/matrix interface in UHPC were quantified using nano-scratch technique. Fracture toughness of the interface between UHPC matrix and CFs modified with electrochemical oxidation technique was higher than that of unmodified CF by about 51%. Moreover, the thickness of the interface transition zone decreased by more than 50%. Compared with the UHPC matrix without adding silica fume, 15% (by weight of cement) silica fume added to the UHPC matrix make the fracture toughness of CF/matrix interface increased by 17% and the interfacial thickness decreased by 37%.

Roles of carbonated recycled fines and aggregates in hydration, microstructure and mechanical properties of concrete: A critical review

CO2 mineralisation by recycled concrete is a novel approach of carbon capture and utilisation to improve the sustainability of concrete products, which has attracted increasing attention in recent years. This paper presents a comprehensive review on carbonation mechanisms related to CO2 mineralisation and hydration, microstructure and multiscale mechanical properties of concrete containing carbonated recycled concrete fines and aggregates, with special focus on the chemistry-microstructure-property relationships. The carbonation kinetics of recycled fines in aqueous environment exhibit a typical particle size effect, while the high pozzolanic reactivity of carbonated recycled fines can facilitate the nucleation and stabilization of hydration gels and contribute to the rapid hydration of cement paste. Carbonated recycled aggregates are favourable in enhancing the micro-mechanical properties of interfacial transition zone in concrete through the synergic effects of physical interlocking and chemical bonding. Compared to concrete with plain recycled aggregates, up to 33.0%, 12.1% and 28.7% rises in compressive, splitting and flexural strengths of concrete can be achieved under 100% replacement of carbonated recycled aggregates. Due to the Stefan effect, concrete with carbonated recycled aggregates has higher dynamic peak stress but lower strain-rate sensitivity of elastic modulus than concrete with plain recycled aggregates. The global CO2 mitigation capacity of recycled concrete varies from region to region and thus reasonable transportation network and high-efficient carbonation process are essential to ensure a balance between the environmental and economic benefits. This review summarises the recent advances in the field and discusses some future research opportunities to develop low-carbon concrete with recycled concrete as a carbon sink.

Microstructure and micromechanical properties of interfacial transition zone in green recycled aggregate concrete

Green recycled aggregate concrete (GRAC) is considered one of the sustainable building materials, which is made up of recycled aggregates (RAs) and green binding materials (alkali-activated materials, i.e., AAMs). However, the GRAC has not been used in building engineering due to its uncertain reaction mechanisms, especially the formation of interfacial transition zone (ITZ) between RAs and AAMs. Thus, the objective of this paper is to systematically investigate the microstructure and micromechanical properties of ITZ in GRAC. Backscattered electron microscopy (BSE) and energy dispersive spectroscopy (EDS) were used to investigate the microstructure and ITZ, whereas nanoindentation was used to study the micromechanical properties. The results indicate that the elastic modulus of ITZ in GRAC incorporated with crushed concrete (34 GPa) is higher than that with unbound stone (22 GPa) and crushed brick (24 GPa). This is due to the presence of C-A-S-H gels with superior micromechanical properties and compacted microstructure. Additionally, the elastic modulus and hardness of ITZ were found to be higher than the paste matrix in GRAC regardless of RAs due to the desired bonding between paste and RAs. It signifies the great potential of utilizing industrial by-products as well as construction and demolition waste to produce low-carbon and sustainable GRAC.

Tensile Fracture Property of Concrete Affected by Interfacial Transition Zone

As a weak link between aggregate and mortar in concrete, interfacial transition zone (ITZ) usually plays a key role in concrete fracture. To investigate the tensile fracture property of concrete affected by the mechanical properties of ITZ numerically, the geometrical models of heterogeneous concrete were established with the parameterization modeling. They include three phases, namely, mortar, ITZ, and randomly distributed aggregates with distinct sizes and orientations. The cracking behaviors of mortar and ITZ were characterized by the bilinear cohesive zone constitutive model. Based on the experiments, the mechanical properties of ITZ were mediated by changing the water–cement ratio of mortar, the aggregate surface roughness and the content of silica fume in interfacial agent. A series of numerical simulations were conducted on the concrete models in tension after the numerical modeling method was validated. The macroscopic tensile fracture properties of concrete were quantitatively connected with some microscopic variables, including the water–cement ratio of mortar, the aggregate surface roughness and the silica fume content in interfacial agent. It was found that the tensile fracture properties of concrete have negative linear correlations with the water–cement ratio of mortar, while the effects of the aggregate surface roughness and the silica fume content in interfacial agent are very complex. The tensile fracture mechanical properties of concrete have a bilinear relationship with the aggregate surface roughness and an approximate quadratic parabola relationship with the content of silica fume in the interfacial agent. This study is beneficial to improve the fracture resistance of concrete by some interface handling measures.

Flexural strength and fatigue properties of interfacial transition zone at the microscale

In this study, the flexural strength and fatigue properties of interfacial transition zone (ITZ) were experimentally investigated at the micrometre length scale. The hardened cement paste cantilevers (150 × 150 × 750 μm3) attached to a quartzite aggregate surface were prepared and tested under the monotonic and cyclic load using a nanoindenter. The measured flexural strength of the ITZ (10.49–14.15 MPa) is found to be one order of magnitude higher than the macroscopic strength of ITZ reported in literature. On the other hand, the fatigue strength of the ITZ is lower than that of bulk cement paste at same length scale, measured previously by the authors. The microscopic mechanical interlocking and the electrostatic interaction between aggregate surface and hydration products are thought to contribute to the bond strength of ITZ. This study provides an experimental basis for the development of multiscale analysis of concrete subjected to both static and fatigue loading.

Comparison of lightweight and normal-weight aggregate concrete

In this experimental study, lightweight aggregate concrete (LWAC) and normal-weight aggregate concrete (NWAC) were compared within the contexts of the size of calcium hydroxide (CH) crystals in interfacial transition zone (ITZ), compressive strength, and oven-dry density. Six LWAC and NWAC mixtures were prepared for this study. Thirty-six images obtained from Scanning Electron Microscope (SEM) were used to determine the size of CH crystals in ITZ of LWAC and NWAC. Eighteen test specimens (three for each of the six LWAC and NWAC mixtures) were prepared in 150x300 mm sizes and in the form of cylinders for the compressive strength tests and also eighteen test specimens in 100x100x100 mm sizes and in the form of cubes for the oven-dry density tests. It was determined that the size of CH crystals in ITZ of LWAC is 8.43% less than (on average), compressive strength of LWAC is 39.09% more than (on average), and oven-dry density of LWAC is 10.97% less than (on average) the NWAC’s that has the same volumetric proportions of ingredients. The findings of this study show that lightweight aggregate that has high particle density, angular shape, rough surface texture, and a structure that enables chemical reaction with CH crystals will be beneficial for the ITZ microstructure and properties of concrete. It is considered that these properties should be taken into consideration in the selection of lightweight aggregate for structural concrete production.

Microstructural Characterization of ITZ in Copper Slag Concrete Composite

The use of copper slag as an alternative to natural fine aggregates in concrete construction has two advantages: effective management of hazardous waste and assistance in conserving natural resources. In order to build confidence among stakeholders and to improve the utilization of copper slag in concrete construction to the required extent, this article evaluates the interfacial transition zone (ITZ) of hardened copper slag concrete composites through various experimental studies. This includes tests on compressive and split-tensile strengths, sorptivity, microhardness, and microcrack properties of ITZ. This paper also examines the surface morphology of ITZ along with its elementary compositions using field emission scanning electron microscopy (FESEM) imaging and energy dispersion spectroscopy (EDS) analysis. The results show that copper slag improves the ITZ characteristics of hardened concrete due to its higher level of pozzolanic reactivity compared to natural sand.

Multi-scale creep analysis of river sand and manufactured sand concrete considering the influence of ITZ

This paper investigated the multi-scale prediction of long-term creep in river sand (RS) concrete and manufactured sand (MS) concrete and analyzed the effect of the interfacial transition zone (ITZ) around the aggregate on the creep properties of concrete. Uniaxial compressive creep tests were conducted on the concrete for a duration of 360 d. The creep kinetic properties of the paste matrix and ITZ were obtained by microindentation test. The properties of ITZ were determined by the backscattered electron (BSE) image threshold segmentation technique and the Lu & Torquato model. Finally, the predicted concrete creep modulus was obtained through the calculation of creep homogenization, and the accuracy of multi-scale creep prediction when considering different ITZ cases was discussed. The results show that under the same water-binder ratio, the thickness of ITZ around MS was larger, the creep modulus was smaller and the volume fraction was larger compared with that of ITZ around RS. In the multi-scale homogenization calculation of the concrete long-term creep, the prediction accuracy of the computational model increased with the raised of the considered ITZ types. The prediction accuracy obtained when all ITZs were considered was improved by more than 46.3% compared to that when no ITZs were considered.