Interfacial Transition Zone In Concrete Research Articles

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.

Effect of Mud Powder on the Performance of Bridge Deck Leveling Concrete in a Seasonally Frozen Region.

Mud powders in aggregates are often found to cause deterioration of concrete properties. Based on a study of the mechanical properties of bridge deck leveling concrete (BDLC) containing different mud powders at various ages, the effects of mud powders on concrete durability were evaluated through rapid chloride permeability testing, freeze-thaw testing, and the coupling of salt solution and a freeze-thaw test. The properties of the interfacial transition zone (ITZ) were also investigated via microhardness testing. The test results showed that mud powder reduced the compressive strength, static compressive elastic modulus, and bond strength at early stages of curing. Moreover, mud powder was found to reduce the tensile properties and durability of concrete, with clay powder causing a greater reduction than mud powder in river sands and coarse aggregate. In addition, the width of the ITZ of concrete containing mud powder was found to increase by 23.1-48.3%. A significant correlation between the ITZ and the tensile properties, as well as the durability of concrete, was also observed. Therefore, in order to improve the tensile properties and durability of BDLC in seasonally frozen regions, the content of mud powder in the aggregates should be minimized according to the different compositions of mud powders. The coupling effect of salt solution and a freeze-thaw cycle should also be taken into consideration.

Impact of Natural Pozzolan and the Degree of Circularity of Aggregates on the Thickness, Strength and Durability of ITZ in Concrete (Modeling and Experimentation)

The interfacial transition zone (ITZ) has a significant negative impact on the structural performance of concrete. This negative impact can be ameliorated by introducing mineral additions into the concrete. In this study, different forms (degree of circularity) of coarse aggregates and three contents (10 %, 15 % and 20 %) of mineral addition (natural pozzolan) were used to prepare ordinary concrete. This article aims to determine the optimal and effective mixture (the best form of aggregate and addition). This article aims to determine the optimal and efficient mixture (the best form of aggregate and addition) on the basis of mechanical (compressive strength), and chemical (immersion in 3.5 % H2SO4 solution) tests. Furthermore, the effect of the interaction between the aggregate and the modified cementitious matrix was analyzed and discussed using the scanning electron microscope (SEM). The results show that the effect of the addition of 20 % pozzolana improves the aggregate/matrix transition zone, which made it possible to develop a maximum compressive strength at 60 days (51 MPa). In addition, the effect of mineral additions varies depending on the shape of the coarse aggregate. Thus, an effective numerical modeling of the compressive strength is based on the approach of the full factorial design taking into account the degree of circularity of the coarse aggregates and the content of the pozzolan.

Experimental study of the relationship between bond strength of aggregates interface and microhardness of ITZ in concrete

• A research on the acquisition of macro-mechanical data of the ITZ in the meso-mechanical calculation of concrete. • The mechanical properties of the ITZ with different mix ratios were measured, the relationship between the bond strength and the microhardness of the ITZ in concrete is established. • A two-parameter linear relationship between the splitting tensile strength of the interface bond and the age and the W/B ratio was established. • The priority order of the calculated value of the interface strength in the numerical analysis of concrete meso-mechanics is proposed. The problem of taking macro-mechanical data of the interface between aggregates and slurry in the calculation of meso-mechanics of concrete was studied in this paper. Through a systematic standard interface test study where the interfacial bond splitting tensile strength method and the interfacial transition zone (ITZ) microhardness method were used, mechanical properties were investigated at macro-scale (bond strength), and followed by ITZ microhardness at meso-scale, moreover the relationships between macro- and meso-mechanical properties were established. The results show that interfacial bond splitting strength of aggregates-slurry gradually increases with increasing hydration age and decreases with increasing water-binder ratio; the ITZ thickness decreases and the ITZ microhardness valley value increases with the extension of curing age. There is a certain correlation between macro-scale mechanical strength of interface and meso-scale mechanics of ITZ. There is a significant linear relationship between the splitting tensile strength of the interface bond and the splitting tensile strength of the cement slurry, so the interface strength can be calculated according to the splitting tensile strength of the slurry substrate. The interfacial bond splitting tensile strength and ITZ microhardness valley have different degrees of correlation. The interfacial bond splitting tensile strength has a particularly significant linear relationship with the microhardness value of the cement paste substrate. Therefore, the calculation value of the interface strength in the numerical analysis of concrete micromechanics should preferably adopt the linear relationship formula with the microhardness of the cement paste substrate.

Influence of thermally activated coal gangue powder on the structure of the interfacial transition zone in concrete

Coal gangue is a kind of solid waste produced in the coal mining and preparation. Abundant SiO 2 and Al 2 O 3 are contained in coal gangue, which has potential cementitious activity. Therefore, the preparation of cementitious materials from coal gangue powder has broad application prospects. The effect and mechanism of the cementitious activity of thermally activated coal gangue powder on the pore structure, microscopic morphology , and mineral composition of the interfacial transition zone in concrete were studied. The results showed that the flexural and compressive strengths of concrete prepared with cement replaced by 15 wt% thermally activated coal gangue powder at 800 °C were 0.39% and 5.94% lower than those of pure cement concrete and 7.47% and 20.17% higher than those of raw coal gangue powder concrete. Mainly because abundant active Al 2 O 3 and SiO 2 released by thermally activated coal gangue powder at 800 °C reacted with the cement hydration product calcium hydroxide, calcium silicate hydrate gel was produced, and then the pores in the interfacial transition zone were filled and transformed into smaller pores, resulting in the pore structure and microscopic morphology of the interface transition zone being optimized, and the bond between the matrix and aggregate becoming closer.

Microstructural characteristics and nano-modification of interfacial transition zone in concrete: A review

Abstract The interfacial transition zone (ITZ) has long been considered as a zone of weakness in concrete. Many scholars have carried out relatively scattered tentative explorations to improve the performance of ITZ. The application of nanomaterials to enhance ITZ in concrete is a promising research. In order to further study the basic principles and practical applications of this field, it is urgent to systematically sort out the existing results. In this study, the nano-modification of ITZ in concrete is systematically reviewed and summarized. First, this study reviews the microscopic characterization of ITZ, including hydration products, porosity, and microhardness. Then, the influence of nanomaterials on ITZ is reviewed from the view of the above three aspects. Finally, the method and mechanism of the ITZ modified by nanomaterials were preliminarily clarified, which provided theoretical and empirical data support for the team’s next experimental work. A large number of research results show that nanomaterials improve the mechanical properties and microstructure of the ITZ, thus enhancing the mechanical properties and durability of concrete. The results of this article will provide source materials for the development of this field, a broader research basis for peer research, and a basis for further development of green engineering materials.

Mechanical properties measurement and micro-damage characterization of ITZ in concrete by SEM-DIC method

Due to the limitation of current experimental measurements, there is an almost insurmountable challenge to determine the mechanical tensile properties of interfacial transition zone (ITZ) in concrete. The present work aims to experimentally characterize micro-mechanical properties of ITZ under tensile load with the combined strategy of scanning electron microscope (SEM), digital image correlation (DIC), nanoindentation and wavelet packet technology. A bi-material sample composed of aggregate and mortar in the same volume and shape is designed for convenience to conduct tensile experiments using the proposed SEM-DIC method. By analyzing the stress obtained from the loading device and full-field strain calculated through DIC technique, the mechanical properties including tensile strength, nominal fracture energy (GNF) and nominal tensile elastic modulus (ENT) of ITZ are evaluated. Nanoindentation tests are also conducted, and their results are briefly compared with that of SEM-DIC method. The micro-damage distribution in ITZ area is further evaluated by wavelet packet analysis. Results reveals that a region with apparently lower ENT values can be characterized as the ITZ area, in which the average ENT value is about 55% of that of the mortar. In addition, growing path of microcracks can be estimated by the direction of maximum principal strain, and the degree of micro-damage could be well characterized by energy change rate (ECR) through wavelet packet analysis.

Particle size distribution of aggregate effects on the reinforcing roles of carbon nanotubes in enhancing concrete ITZ

By virtue of their superior mechanical properties and huge surface area, carbon nanotubes (CNTs) have become a potential representative in the field of the nanoengineered concrete, which provides an excellent opportunity to reinforce concrete interfacial transition zone (ITZ) characteristics. Adjusting the most reasonable aggregate gradation of concrete to maximize the enhancement effects of CNTs to ITZ is the key to popularizing the application of nano reinforcement in practical engineering. Hence, in this work, we investigated the reinforcing roles and mechanisms of CNTs on the concrete ITZ, and the corresponding influence on the mechanical performance and impermeability properties under different aggregate particle size distributions. The results reveal that CNTs can promote nucleation effects to generate hydration products growth and act as the refined templates to produce finer microstructure in concrete ITZ. Compared with plain concrete, the reinforcing ratios of CNTs on the ITZ characteristics, mechanical performance and impermeability present a quadratic function correlation with the Talbot index. The optimal effects of CNTs on the reinforcement of concrete ITZ characteristics appear at 0.8 Talbot index, reducing about 30% ITZ width and 22.8% abrasion crack width in ITZ. The corresponding mechanical properties and impermeability improve 20.7–35.6% and 38.1–43.6%, respectively. Low Talbot gradation index would result in the uneven dispersion of CNTs among the capillary pores. In contrast, high Talbot gradation index may enlarge the ITZ range, increase the amount and width of abrasion cracks in ITZ, and further weaken the reinforcing effects of CNTs.

Interfacial Transition Zone (ITZ) Study of Concrete with Polyethylene Terephthalate (PET) Plastic

Concrete is unquestionably the most desirable construction material, but, like any coin, italso has a negative side. In one way or another, the raw materials used in the production of concretehave a negative impact on the environment. Cement production releases carbon dioxide into theatmosphere, while aggregate production releases dust. The geology of the region where coarseaggregates were mined is also influenced by their extraction. Natural fine aggregates were replacedwith polyethylene terephthalate (PET) bottles in this analysis. PET Plastic aggregates wereproduced by little cutting of waste plastic bottle. Plastic is the most serious environmental problem,and it is having a rapid effect. Shredded waste plastic was used in concrete as a partial substitute fortraditional fine aggregate at 0%, 5%, 10%, 15%, and 20% by weight. The aim of this research wasto determine the compressive strength, microstructure of concrete, and the interfacial transition zoneof concrete (ITZ). The concrete with 5% fine aggregate replacement showed a possible outcome of21 MPa compressive strength, the control mix had the lowest percentage of 1.3 percent, and theconcrete with 20% PET plastic had the highest percentage of 9.8 percent. It can be concluded thatrecycled plastic aggregates can be used to replace fine aggregate in concrete up to 5% of thereplacement and can be used in non-load bearing structures where lightweight materials arepreferred

Influence of different calcined clays on the water transport performance of concretes

This study compares the pozzolanic activity of kaolinitic and illite clays after calcination as they are the most abundant types of clays worldwide. The impact of calcined clays on microstructure development was tested by determining the porosity of cement paste as well as by phase and optical analysis of the cement matrix and the interfacial transition zone of concrete. As water transportation with damaging carbonate or chloride ions is the main reason for reduced durability of reinforced concrete, the results were complemented with water absorption tests on blended concretes. At 28 days, the system with calcined kaolinitic clay revealed a higher densification of its microstructure with a lower water absorption rate compared with plain concrete and concrete with calcined illite clay, which is related to its higher pozzolanic reactivity. Nonetheless, calcined illite clays can be considered as a low-cost clinker replacement as a significant pozzolanic contribution is detectable and after 90 days, the water absorption behaviour is comparable to those of the other systems investigated.

Evolution of ITZ and its effect on the carbonation depth of concrete under supercritical CO2 condition

In this paper, supercritical carbonation tests of concrete specimens with different water-to-cement ratios are carried out. In the test, the thickness of interfacial transition zone (ITZ) of the concrete is determined by the distribution of Ca/Si ratio across the interface between the coarse aggregate and cement paste. The microhardness distribution, microstructure and porosity of the ITZ before and after supercritical carbonation are analyzed. A geometrical and physical model considering the distribution of porosity, coarse aggregates, ITZ, and the supercritical carbonation of concrete is proposed, by which cracks, pores, calcium carbonates, and C-S-H gel at the interface of coarse aggregates and cement paste can be studied. The overall microstructures are relatively compacted after supercritical carbonation. The thickness of ITZ of concrete is reduced from 47-79 μm to 35–51 μm after supercritical carbonation. The average value and variance of carbonation depth of concrete increase with the increase of the thickness and porosity of ITZ. Comparing the carbonation results of concrete with different thicknesses and porosity of ITZ, it appears that porosity of ITZ has greater impact on the carbonation depth of concrete.

A cohesive contact algorithm to evaluate the mechanical behaviour of concrete ITZ at different roughness conditions

The capability of modelling damage evolution in concrete materials represents a topic of relevance for improving the design of concrete structures and for the recovery of existing components. The present work aims to investigate the collapsible behaviour of the Interfacial Transition Zone (ITZ) around coarse inclusions, by using a new cohesive contact law able to take into account the coupling between normal/peeling and shear stresses, specifically calibrated to take into account the roughness of various aggregate types. A series of numerical analyses have been carried out considering different material inclusions and distributions within concrete samples, so proving the capabilities of the proposed formulation.

Experimental study of the chemo-mechanical properties of the interfacial transition zone of concrete

This article presents an experimental study of the chemical and mechanical properties of the cement paste / aggregate interphase. Interfacial Transition Zone (ITZ) represent the contact zone of the cement paste with the aggregate with a thickness of about 20 - 50 μm. This zone has different mechanical and chemical properties compared to the bulk paste. Cement paste / limestone aggregate composite samples were performed to allow the study of the chemical and mechanical properties of ITZ. The cement paste is made with Portland cement with a water/cement mass ratio of 0.4. Calcium concentration profiles in the ITZ show a high concentration of calcium due to the presence of portlandite and ettringite. The nano-indentation tests indicate weakness of ITZ compared to bulk cement paste.

Experimental Study on Interface Strength Between Hardened Cement Paste-Aggregate

The interfacial transition zone (ITZ) is a weak part of concrete, which is the main reason that the mechanical properties of concrete are far lower than those of hardened cement paste and aggregate itself. In order to improve the mechanical properties of concrete ITZ, the effects of cement varieties, water-binder ratio, mineral admixtures and aggregate types on the strength of hardened cement paste-aggregate ITZ were studied by orthogonal test. A linear regression model between interfacial bonding splitting tensile strength and splitting tensile strength of hardened cement paste was proposed. The results show that the water-binder ratio has the greatest impact on the mechanical properties of ITZ, followed by aggregate. The interfacial bonding splitting tensile strength of five kinds of aggregate and cement paste is in the order of marble > coral > granite > basalt > quartzite. The optimal interface test combination is as follows: water-binder ratio 0.25, aggregate is marble, P•O52.5 cement, slag content 15%, fly ash content 20% and silica fume content 10%. The optimal cement slurry combination is: water-cement ratio 0.25, basic magnesium sulfate cement, slag content 15%, fly ash content 20% and silica fume content 10%. The linear regression model between interfacial bond splitting tensile strength and hardened cement paste splitting tensile strength is established by dividing aggregate, which is of great significance to study the law of interfacial strength.

Using artificial neural network to predict the fracture properties of the interfacial transition zone of concrete at the meso-scale

Concrete is a multi-phase heterogeneous material in which the interfacial transition zone (ITZ) between aggregates and mortar significantly affects the cracking behaviour of concrete, especially under tensile load. In this paper, the artificial neural network (ANN) method is applied in predicting the fracture properties of ITZ in concrete. To form the data pool for the training of the ANN, a large number of two-dimensional (2D) meso-scale fracture simulation of concrete under direct tensile load is conducted. Cohesive crack elements are used in simulating arbitrary cracking in the ITZ and mortar. After the verification of the trained ANN model, the tensile strength and fracture energy of the concrete ITZ are predicted by using the RILEM direct tensile test results, i.e., stress–displacement curve, as the input for the ANN. It has been found that, the trained ANN performs well in predicting the ITZ properties and the computed stress–displacement curve together with the optimized ITZ fracture parameters has a good agreement with that from the RILEM test results. The randomness of aggregates has little effect on the predicted ITZ tensile strength while it becomes slightly bigger on the predicted ITZ fracture energy. The tensile strength ratio of ITZ to mortar are calculated 0.38–0.47 and the fracture energy ratio of ITZ to mortar are 0.18–0.58. These inversely predicted fracture properties of concrete ITZ can be useful complementation to the existing dataset and 2D fracture simulation of concrete structures.

Roles of carbon nanotubes in reinforcing the interfacial transition zone and impermeability of concrete under different water-to-cement ratios

Concrete plays a central role in fundamental construction and engineering. However, the interfacial transition zone (ITZ), as the area between the aggregates and cementitious pastes, is the weakest component in concrete and always limits the properties and application of the concrete. Here, we reveal the potential of carbon nanotubes (CNTs) as superior additives for reinforcing the ITZ in concrete and the corresponding impermeability enhancement against various of water-to-cement (W/C) ratios. The results suggest that CNTs not only promote higher hydration reaction and generate more hydrated products to dense ITZ area, but also form net-like distributions to inhibit flaws propagation at the microscale. Compared with the plain concrete specimens, the addition CNTs micrified the ITZ width by 14.3–30% and reduced the abrasion crack width by 19.5–35%. The reinforcing effects of CNTs on ITZ characteristics are consistent with Gauss fitting and achieve the highest enhancement at 0.4 W/C ratio. The corresponding impermeability properties of concrete present a linear correlation with both ITZ width and abrasion crack width, which can be strengthened by approximately 10.9%-35.8% assisted by CNTs. Furthermore, selecting the appropriate W/C ratios is the key to make full use of CNTs roles in strengthening the ITZ of concrete. High W/C ratios may lead to the re-agglomeration of CNTs and low W/C ratios would promote CNTs to act as pulled-out roles and weaken their ability to inhibit flaws propagation.

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.

Numerical Simulation on Size Effect of Fracture Toughness of Concrete Based on Mesomechanics.

The fracture performance of concrete is size-dependent within a certain size range. A four-phase composite material numerical model of mesofracture considering a mortar matrix, coarse aggregates, an interfacial transition zone (ITZ) at the meso level and the initial defects of concrete was established. The initial defects were assumed to be distributed randomly in the ITZ of concrete. The numerical model of concrete mesofracture was established to simulate the fracture process of wedge splitting (WS) concrete specimens with widths of 200–2000 mm and three-point bending (3-p-b) concrete specimens with heights of 200–800 mm. The fracture process of concrete was simulated, and the peak load (Pmax) of concrete was predicted using the numerical model. Based on the simulating results, the influence of specimen size of WS and 3-p-b tests on the fracture parameters was analyzed. It was demonstrated that when the specimen size was large enough, the fracture toughness (KIC) value obtained by the linear elastic fracture mechanics formula was independent of the specimen size. Meanwhile, the improved boundary effect model (BEM) was employed to study the tensile strength (ft) and fracture toughness of concrete using the mesofracture numerical model. A discrete value of β = 1.0–1.4 was a sufficient approximation to determine the ft and KIC values of concrete.

Acidification at rebar-concrete interface induced by accelerated corrosion test in aggressive chloride environment

The rebar-concrete interface plays a critical role in initiating corrosion of steel reinforcing bars embedded in concrete. The intimate physical contact between a steel rebar and the Ca(OH)2-rich interfacial transition zone of concrete provides physical protection and buffering action against corrosion. However, the mechanism of the buffering action is not well understood and a systematic study has not yet been conducted. This paper investigates acidification at the rebar-concrete interface, buffering capabilities of concrete made with 100% Portland cement and with 50% fly ash or ground-granulated slag as cement replacement, and corrosion behavior of the embedded rebar. It was found that a simple technique with filter paper wrapped around the steel rebar under a high impressed anodic current and a severe chloride environment allowed for interface solution to be accumulated, squeezed out, and collected for analysis. The solutions had pH values less than 2, thus proving acidification at the interface during the accelerated corrosion test. With a high content of Ca(OH)2 in the matrix, the concrete with 100% cement as binder was shown to have a higher buffering capacity as compared to the matrix with 50% fly ash or slag as cement replacement. This acidification phenomenon promoted steel dissolution and subsequent corrosion activity, although the presence of cracks in the concrete cover could also play a dominant role in controlling the subsequent corrosion rates. Acidification was also shown to occur under natural corrosion of lower corrosion activity.

Research on Improving Concrete Durability by Biomineralization Technology

The interfacial transition zone (ITZ) around aggregates in concrete is a weak area with higher porosity than the matrix; it breaks easily under stress and is not conducive to the durability of concrete. However, the ITZ in concrete is full of calcium hydroxide crystals, which can provide the calcium source required for biomineralization. In view of this, this study aims to use the biological activity (i.e., biomineralization technology) existing in nature to enhance the ITZ in concrete and repair concrete cracks to improve the strength and durability of concrete. In this study, the bacterial strain Sporosarcina pasteurii, which is environmentally friendly, was selected. In addition, lightweight aggregate was used as a bacterial carrier. The bacteria were first sporulated. To protect the strains, the biological species were fixed in porous lightweight aggregates. These lightweight aggregates were then used as concrete aggregates. The planned tests included concrete engineering properties (i.e., compressive strength, chloride ion penetration, and water permeability tests) and residual strength after crack repair. The test results show that the use of lightweight aggregate as a carrier and the implantation of Sporosarcina pasteurii can induce biomineralization, strengthen the ITZ, and repair small internal cracks in concrete, thereby improving the strength and durability of the concrete.