Strength Of Interfacial Transition Zone 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.

Graphical analysis-based calculation method of concrete strength

This study established a three-dimensional random spherical aggregate model of concrete. Models with various volume contents of coarse aggregate were sliced, and slice areas were statistically analyzed. To study the effects of the coarse aggregate volume content and the interfacial transition zone (ITZ) strength on concrete’s compressive strength, a concrete strength calculation method based on graphical analysis was proposed. The ratio of crown area B to sectional area A of the spherical aggregate model was found to be close to 1.5. At the ITZ strength below 70% of the mortar strength, the increased volume content of coarse aggregate reduced the compressive strength of concrete, with the opposite trend observed at the ITZ strength exceeding 70% of the mortar strength.

Interfacial mechanical bond characterization between cement pastes and porous aggregates through a coupled XCT and DVC technique

Interfacial transition zone (ITZ) between cement matrix and porous aggregates plays an important role in affecting overall behavior of lightweight aggregate concrete (LWAC) due to mechanical interlocking effect. However, existing characterization techniques could not assess interfacial bond performance of LWAC in a three-dimensional and non-destructive way. The present study proposed a novel technique involving both in-situ X-ray computed tomography (XCT) and digital volume correlation (DVC) to quantitatively assess interfacial bond behavior between cement pastes and porous aggregates. In the test program, samples with various interfacial conditions were loaded to generate necessary displacement differences among different constituents. Volumetric deformations traced by XCT were transformed into volumetric strain components through DVC computation. Furthermore, cement-aggregate interactions were evaluated by analysis of XCT results. Eventually, strain profiles were used to characterize mechanical bond properties of the interfaces. Through the investigations, it was found that not all the penetrated cement paste could contribute to interfacial bond and that the proposed method could evaluate the effective area providing mechanical interlocking. The test results also showed that overall concrete strength could not represent the strength of ITZ.

Uniaxial Compression Failure and Size Effect of Recycled Aggregate Concrete Based on Meso-Simulation Analysis.

Recycled aggregate concrete (RAC) is a kind of five-phase composite material at the meso-level. It has a more complex interfacial transition zone (ITZ) than ordinary aggregate concrete (NAC), which is an important factor affecting the meso-failure of RAC. In addition, the maximum aggregate size plays an important role in the nonlinear mechanical behavior of concrete, which is closely related to the size effect. In this paper, a 2D random aggregate model of RAC is established based on meso-mechanics. The mechanical properties and failure modes of RAC under uniaxial compression are simulated using a plastic damage constitutive model. Through variable parameter analysis, the effects of the properties and thickness of ITZ on the elastic modulus and peak stress of RAC are studied, and the effect of the maximum aggregate size on the size effect of the compressive strength of RAC is discussed. The results show that the ITZ strength has a positive linear correlation with the peak stress and elastic modulus of RAC, while the ITZ thickness has a negative linear correlation with the peak stress and elastic modulus of RAC. Under the same specimen size (D = 100 mm, 150 mm, 200 mm, 300 mm), with an increase in the maximum aggregate size (dmax =20 mm, 25 mm, 30 mm, 35 mm), the nominal compressive strength of RAC increases by 6–10%, and the size effect is gradually weakened. When the maximum aggregate size reaches 30 mm, a decrease in the size effect tends to slow down compared with the maximum aggregate size of 20 mm. The classical Bažant size effect law is applicable to describe the compressive properties of RAC under different maximum aggregate sizes, and has a certain guiding significance for the prediction of the size effect of RAC in practical engineering.

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.

Mesoscale investigation on concrete creep behaviors based on discrete element method

Studying creep behaviors of concrete materials at a mesoscale level is crucial to understanding their mechanism. Discrete element method (DEM) could provide an effective numerical tool to explain the mesoscale mechanism of concrete materials. In this study, the creep characteristics of concrete are analyzed by combining DEM and Burgers model. Firstly, a multi-phase concrete numerical model consisting of matrix, aggregate, and interfacial transition zone (ITZ) was developed. Then, the sensitivity of the Burgers model parameters was conducted. Finally, to investigate concrete creep behaviors at a mesoscale level, a series of numerical simulations were carried out. Simulation results show that Burgers model can reflect the creep behavior of concrete well. The porosity gradually decreased as the loading duration increase, and deceleration became stable at the later stage. The creep contribution of ITZ decreases with the increase of the ratio of ITZ strength to matrix strength. Force-chain strength in ITZ was greater than that in matrix, and viscoelastic sliding or squeezing occurs at Burger contact. Meanwhile, the direction distribution of force-chain strength in the matrix is regular and the main direction of the contact distribution is about 90° or 270°. In addition, the probability function of contact force is exponential, and the strength probability distribution of matrix and ITZ contact reached the peak at 0.7 f/f¯ and then decreased, and the proportion of strong contact and weak contact in matrix and ITZ was about 70% and 30%, respectively. The current study can bridge the gap between simulation and experimental research.

Numerical insights into the effect of ITZ and aggregate strength on concrete properties

Mechanical properties of concrete are highly dependent on its components. Consequently, a comprehensive understanding of the effects of aggregate and interfacial transition zone on concrete properties is important to applications. In this paper, a mesoscale investigation on the effects of coarse aggregate and interfacial transition zone strength on mechanical and fracture behaviours of concrete is investigated using discrete element modelling. The results show that the elastic modulus of concrete is insensitive to the strength of ITZ and aggregate, whereas the compressive strength and strain-to-damage highly depend on these two factors. An increase in ITZ/aggregate strength leads to a decrease of cracks in ITZ/aggregates and little change of cracks in mortar. Aggregate size, under certain conditions, can alter the crack growth process and result in distinct crack patterns.

A study on micro-morphology and impacts of curing temperature on bond strength of interfacial transition zone through scanning electron microscope

The influence of curing temperature on the bond strength of ITZ was studied in this paper. It is found that, Increasing curing temperature can improve the bond strength of ITZ in the early age, but it will significantly decreases the growth trend of the bond strength. The micro-morphology characteristics were examined by the method of scanning electron microscopy, It was found that, the higher the curing temperature is, the more uneven the hydration products distribution of ITZ is, and also the looser the ITZ structure is.

A 3D FEM Mesoscale Numerical Analysis of Concrete Tensile Strength Behaviour

A three-dimensional (3D) finite element method (FEM) based on an inserted cohesive element numerical analysis procedure was developed for concrete mesoscale systems on the ABAQUS platform with python scripts. Aggregates were generated based on dividing the existing geometrical element algorithms to randomize arbitrary spheres. Simultaneously, randomizations of the maximum aggregate size and uniform distributions of aggregate particles were also considered. An FEM for the mortar phase in concrete mesoscale systems was generated along with the interfacial transition zone (ITZ) by inserting a cohesive element. Numerical parameter analyses were performed for nine different concrete systems by varying the coarse aggregate volume fraction (α) and the ITZ tension strength (ITZ-S). The mechanical performance of concrete systems with the coupling effects of α and ITZ-S was evaluated for simulated tensile loading. The results of the numerical simulations for mechanical properties, such as the simulated tensile strengths and tension damage behaviour of concrete systems, were verified with experimental results. The proposed aggregate and ITZ generation approach and numerical simulation procedure can be used by researchers to better understand how aggregate volume fraction and ITZ strength affect the tensile behaviour of concrete mesoscale systems.

Beneficial effect of nanomaterials on the interfacial transition zone (ITZ) of non-dispersible underwater concrete

Non-dispersible underwater concrete (NUC) is a kind of widely used construction engineering material. It has added flocculant so that concrete can have the characteristics of inhibiting dispersion of cement and aggregate in an underwater environment. However, soluble salts (such as sodium chloride and calcium sulphoaluminate) in seawater can infiltrate the interfacial transition zone (ITZ) of concrete, and then the ionic binding ability of the crystal surface will be reduced, which will cause mass loss of bonding materials in an ITZ and lead to the reduction of mechanical properties of concrete. In order to solve the problem of weak mechanical properties of ITZ, in this study, the influences of three types nanomaterials (nano-SiO2, nano-metakaolin and nano-Al2O3) were studied. These materials were added into concrete to improve the bond and hardness strength of ITZ, which could be characterized by a push-out test, nano indentation test, X-ray diffraction (XRD) and scanning electron microscope (SEM). The results indicated that the nano-SiO2 and nano-metakaolin could significantly increase the bond strength of ITZ at 3 d by 134.12% and 87.70%, respectively, and by 108.54% and 43.41%, respectively, at 28 d). When the addition of nano-SiO2 and nano-metakaolin was 3%, the modulus of ITZ was improved by 54.39% and 71.03%, respectively, compared with plain NUC. The XRD data displayed the fact that nanomaterials could promote the formation of calcium hydroxide (CH) at early stage of hydration, while they had a pozzolanic effect on the reduction of content of CH at later stage of hydration. The nano-SiO2 and nano-metakaolin could obviously improve the microscopic morphology. This study revealed that nano-SiO2 and nano-metakaolin could effectively enhance mechanical properties of NUC, thus enhancing anti-erosion and anti-scouring ability of NUC. It is believed that these findings could fulfill a better understanding of the role of nanomaterials in NUC.

Impact failure mechanism and mechanical characteristics of steel fiber reinforced self-compacting cementitious composites containing silica fume

An experimental investigation was performed to study the benefits of partially replacing ordinary Portland cement with pozzolanic material on the fresh and hardened characteristics of high-strength self-compacting (HSSC) cementitious composite reinforced by randomly distributed wavy steel fiber. The four wavy fiber volume fractions (0.25%, 0.50%, 0.75%, and 1%) and silica fume contents of 5 and 15 wt% (wt%) of cement were investigated. A total of fifteen mixtures, including different combinations of the fiber and silica fume, were designed and assessed in terms of the passing ability, water absorption, tensile, compressive, flexural, and impact tests. The energy-dispersive X-ray microanalysis (EDX) was applied to specify the cementitious matrix's chemical compositions quantitatively. Moreover, the wavy fiber’s surface topography in the nano-metric scale was specified using an atomic force microscope. The results reveal that the patterns of cracks, impact energy at first crack and failure mechanism, increment in the number of blows after first-cracks, flexural strength, and splitting tensile behavior of mixtures were greatly influenced by adding wavy steel fibers. It was also found that the improvement in toughness and post-peak behavior of HSSC cementitious composites was appreciably governed by the bridging effect of fibers and an increase in bonding strength of interfacial transition zone between fine aggregate and cement paste due to an increase in the content of silica fume. The benefit of the present investigation is to better understand the failure performance of fibrous self-compacting composites.

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.

Bond-slip characterization of the interfacial transition zone of cement concrete and granite rock using push-out test

Push out tests of the rock (in the forms of rods) embedded in the cement mortar have been conducted to measure the bond strength of the interfacial transition zone (ITZ) in Mode-II. Locally available granite rock which contains the medium size of Quartz, Feldspar grains have been collected from the near vicinity of Mandi (HP, India) town. Cylindrical specimens of size 18 mm in diameter have been obtained using drill machine. A modified push out test setup developed for the experiments. It observed in the trial tests that due to the roughness of the rock specimens, the contribution of the frictional stresses is very high at the ITZ. This frictional contribution has made the crack propagation difficult in Mode-II. Hence the failure is extended through the mortar phase. Different combinations have been tried and finally, an optimized push out test setup geometry has been developed. Next, the effect of the water-cement (w/c) ratios (i.e.0.25, 0.30 and 0.35) on the Mode-II strength of ITZ for the age of 7 days has been studied. It is observed that the Mode-II strength of ITZ is sensitive to the water-cement ratio. The strength has increased by 37% when the w/c ratio is reduced from 0.35 to 0.25.

Experimental study on the compressive strength, damping and interfacial transition zone properties of modified recycled aggregate concrete.

At present, many modification methods have been proposed to improve the performance of recycled aggregate concrete (RAC). In this study, tests on the compressive strength and damping properties of modified RAC with the addition of different proportions of recycled coarse aggregate (RCA) (0, 50, 100%), rubber powder (10, 15, 20%), steel fibre (5, 7.5, 10%) and fly ash (15, 20, 5%) are carried out. To elucidate the effect of the modification method on the interfacial transition zone (ITZ) performance of RAC, model ITZ specimens are used for push-out tests. The results show that when the replacement rate of RCA reaches 100%, the loss factor of the RAC is 6.0% higher than that of natural aggregate concrete; however, the compressive strength of the RAC decreases by 22.6%. With the addition of 20% rubber powder, the damping capacity of the modified RAC increases by 213.7%, while the compressive strength of the modified RAC decreases by 47.5%. However, with the addition of steel fibre and fly ash, both the compressive strength and loss factor of the RAC specimens increase. With a steel fibre content of 10 wt%, the compressive strength and loss factor of the RAC increase by 21.9% and 15.2%, respectively. With a fly ash content of 25 wt%, the compressive strength and loss factor of the RAC increase by 8.6% and 6.9%, respectively. This demonstrates that steel fibre and fly ash are effective in improving both the damping properties and compressive strength of RAC, and steel fibre is more effective than fly ash. Two methods were used for modification of the RAC: reinforcing the RCA through impregnation with a 0.5% polyvinyl alcohol (PVA) emulsion and nano-SiO2 solution, and strengthening the RAC integrally through the addition of fly ash as an admixture. Both of these techniques can improve the ITZ bond strength between the RAC and new mortar. Replacing 10% of the cement with fly ash in the new mortar is shown to be the best method to improve the ITZ strength.

The Effect of Homogenization Procedure on Mechanical Properties of High-Performance Concrete with Partial Replacement of Cement by Supplementary Cementitious Materials

High-performance concrete is a very specific type of concrete. Its production is sensitive to both the quality of compounds used and the order of addition of particular compounds during the homogenization process. The mechanical properties were observed for four dosing procedures of each of the three tested concrete mixtures. The four dosing procedures were identical for the three mixes. The three mixes varied only in the type of supplementary cementitious material used and in water content. The water content difference was caused by variable k-value of particular additives. The water-to-binder ratio was kept constant for all the concretes. The additives used were metakaolin, fly ash and microsilica. The comparison of particular dosing procedures was carried out on the values of basic mechanical properties of concrete. The paper compares compressive strength and depth of penetration of water under pressure. Besides the comparsion of macro-mechanical properties, the effect of microsilica and fly ash additives on micro-mechanical properties was observed with the use of scanning electron microscopy (SEM) and nanoindentation data analysis. Nanoindentation was used to determine the thickness and strength of interfacial transition zone (ITZ) for different sequence of addition of cement, additive and aggregate. The thickness obtained by nanoindentation was further investigated by SEM EDS line scanning.

Impact of silica fume, fly ash, and metakaolin on the thickness and strength of the ITZ in concrete

The interfacial transition zone (ITZ) has a major detrimental impact on the structural performance of concrete. This negative impact can be modulated by introducing mineral admixtures to a concrete mix, which fill the excessive voids within ITZ and react with portlandite to form more compact products. The approach described here, consisting of characterization of phases and micromechanical modeling, enabled assessment of the effect of silica fume, fly ash, and metakaolin on ITZ thickness and strength. The proposed model was based on the Mori-Tanaka scheme coupled with an estimation of deviatoric stress within ITZ. This study suggests that silica fume is efficient in reducing ITZ thickness, while the addition of fly ash more significantly contributes to ITZ strength. Moderate replacements of Portland cement for silica fume or fly ash, up to 20%, can positively influence concrete performance; in case of metakaolin, replacement up to 10% is recommended.

Microstructural damage evolution and its effect on fracture behavior of concrete subjected to freeze-thaw cycles

Concrete structures in cold regions are exposed to cyclic freezing and thawing environment, leading to degraded mechanical and fracture properties of concrete due to microstructural damage. While the X-ray micro-/nano-computed tomography technology has been implemented to directly observe concrete microstructure and characterize local damage in recent years, the freeze-thawed damage evolution processes and its effect on overall mechanical performance are not well understood. In this paper, the X-ray nano-computed tomography technology and micro-scale cohesive zone model are combined to quantitatively investigate microstructural damage evolution and its effect on fracture behavior of freeze-thawed concrete samples in three-point bending tests. A two-level micro-to-macro scale finite element model is developed based on computed tomography microstructural images with microcracks due to freeze-thaw cycles. The macroscopic load–deflection curves and fracture energies are simulated and compared favorably with experimental results. Simulation results demonstrate that microcracks caused by freeze-thaw actions are the primary reason for degradation of concrete mechanical properties. Fracture behaviors of frost-damaged concrete with different mortar and interfacial transition zone strength and fracture constants are also simulated and discussed. The combined X-ray nano-computed tomography technology and cohesive zone model proposed is effective in characterizing fracture behavior of concrete and capturing freeze-thaw cycle-induced microstructural damage evolution and its effect on fracture process of concrete.

A theoretical method to determine the tortuous crack length and the mechanical parameters of concrete in direct tension—A particle size effect analysis

Cementitious materials, mainly in the form of concrete, belong to a class of macroscopically heterogeneous materials. The building technology of mass concrete structures enables large-sized coarse aggregate to be used. The maximum coarse aggregate size, in conjunction with the mechanical properties of the interfacial transition zone (ITZ), have great influences on the mesoscopic crack path and the macroscopic mechanical behavior of concrete. Herein this study, based on some pioneering work (Stroeven, 2000; Rosselló and Elices, 2004, Rosselló et al., 2006; Elices and Rocco, 2008), the effects of varying the maximum coarse aggregate size and the ITZ strength on the global mechanical properties of concrete in direct tension are theoretically modelled from a mesoscopic point of view. The particle size effect behavior of concrete is analyzed based on the proposed theoretical method, and some general conclusions can be drawn as: (1) The degree of tortuosity of the mesoscopic crack path in direct tension increases with increasing maximum coarse aggregate size and with decreasing ITZ strength; (2) The fracture energy and the tensile strength of concrete increase dramatically with increasing ITZ strength; (3) For normal strength concrete, the fracture energy and the tensile strength decrease with increasing maximum coarse aggregate size, while for relatively high strength concrete, the trends are the opposite; (4) The properties of ITZ could significantly affect the homogeneity of concrete. High quality ITZ could make the mechanical properties of concrete more homogeneous and get a relatively high strength concrete, consequently a short characteristic length and a high brittleness number are resulted; (5) For normal strength concrete, the characteristic length increases with increasing maximum coarse aggregate size, while for relatively high strength concrete, it is fairly insensitive to the maximum coarse aggregate size. It is hoped that this theoretical method may be helpful in the study of size effect and in the design of concrete and future cementitious materials.

Study on microstructure and bond strength of interfacial transition zone between cement paste and high-performance lightweight aggregates prepared from ferrochromium slag

In this study, high-performance lightweight aggregates (HPLWA) are successfully fabricated by using ferrochromium slag, fly ash and bauxite as raw materials. Besides, the high-belite sulphoaluminate clinker is prepared and coated on the surface of HPLWA by one-step sintering technique to obtain an outstanding surface reactivity of aggregates. The results show that the prepared HPLWA are made up of two parts, the inner porous ceramics which consist mainly of cordierite and spinel and the outer modified layers composed of belite and sulphoaluminate. The inner porous ceramics afford the HPLWA high compressive strength more than 6MPa and low water absorption less than 2wt%. Furthermore, the outer modified layers would weaken the compressive strength and increase the water absorption of HPLWA, but they endow a dense microstructure and high bond strength of interfacial transition zone (ITZ) between HPLWA and cement paste.

Numerical modeling tensile failure behavior of concrete at mesoscale using extended finite element method

Uniaxial tensile behavior of concrete plays an important role in the behavior of concrete specimens as well as structural elements. A 2D mesoscale analysis is performed to investigate the fracture process of concrete subjected to uniaxial tension by using the extended finite element method. The concrete considered includes the hydrated cement paste, aggregate particles, and the interfacial transition zones. In the cracked domain, to represent the discontinuities of a displacement field, the Heaviside jump function is added to the common finite element approximation for the local enrichment based on the framework of partition of unity. A user-defined subroutine for the extended finite element method is implemented with FORTRAN codes and embedded into the commercial software ABAQUS. The failure process of the L-specimen is studied, and good agreement is obtained between the experimental observation and the present simulation results. Based on the extended finite element method, the fracture process of the concrete under uniaxial tension is investigated subsequently. The influences of aggregate distribution, aggregate size, and aggregate shape as well as the strength of interfacial transition zone on the failure process and the global deformation process of concrete are analyzed. The crack initiation, propagation, and failure mode of concrete specimens are also discussed in detail. The simulation results indicate that the macroscopic mechanical properties of concrete are merely dependent on the aggregate distribution, but dependent on aggregate shape, size, and strength of the interfacial transition zone.