Porosity Of Interfacial Transition Zone Research Articles

Evolution of LWAC–OC interfacial properties during sulfate dry–wet cycles: Bonding strength and micro–properties

The interface properties between lightweight aggregate concrete (LWAC) and ordinary concrete (OC) have drawn significant research interest. However, its evolution during sulfate dry–wet cycles has yet to be elucidated. Therefore, the LWAC–OC composite specimens were subjected to 15 cycles of dry and wet cycles test, using 5 wt% sodium sulfate solution. The dry–wet cycling method consisted of complete immersion for 7 days, followed by natural drying for 8 days. Moreover, tests for splitting tensile, direct shear, scanning electron microscopy and microhardness were conducted on the LWAC–OC interfaces at varying numbers of dry and wet cycles. The results showed that the failure mode of specimens changed from mixed cohesive failure to interfacial bonding failure during sulfate dry–wet cycles. Direct shear and splitting tensile strength slightly increased and then markedly decreased as the number of dry–wet cycles increased. The ratio of direct shear strength to splitting tensile strength showed a distinct negative linear relationship with the number of sulfate dry–wet cycles. Moreover, when the number of dry–wet cycles increased, the porosity of interfacial transition zone first decreased and then increased. The mean value of the interfacial micro–hardness first increased and then decreased. Interfacial micro–properties correlated well with bonding strength throughout the process.

Effect of nanosilica and fiber on mechanical properties and microstructure of recycled coarse aggregates road concrete

Recycled coarse aggregate has its own characteristics of large porosity, high water absorption and high crushing index, which adversely affects the mechanical strength and durability of recycled aggregate concrete. In this study, the mechanical properties, durability and microstructure of recycled aggregate concrete (RAC) modified with nano-SiO2 (NS) and fibres were investigated to prepare road RAC, using NS polypropylene fibres (PPF) and basalt fibres (BF) as test variables. Compared with RAC only incorporating NS, NS and fibre synergism can significantly enhance the mechanical properties of recycled concrete, compressive strength increased by 27–51 %, flexural strength increased by 12 %, and abrasion resistance increased by 25–72 %. The synergistic effect of NS and PPF significantly improves the frost resistance of recycled concrete. NS has a volcanic ash effect which can react with calcium hydroxide to produce a large number of C-S-H in the secondary hydration reaction, making the internal structure of the cement matrix denser. Interfacial transition zone microstructure is more compact, thus interfacial transition zone porosity is effectively reduced. The hydration products are attached to the fibre surface, which enhances the bonding ability between the fibers and matrix. Therefore, the fibres effectively inhibit the generation and development of internal micro-cracks. NS optimises the internal pore structure of recycled aggregate concrete and enhances the interfacial transition zone, and NS synergistically with PPF can improve the strength and durability of various mechanical properties of recycled concrete. The research results provide the scientific basis for the application of road RAC.

Properties and Microstructure of an Interfacial Transition Zone Enhanced by Silica Fume in Concrete Prepared with Coal Gangue as an Aggregate.

The wide application of concrete prepared with coal gangue (CG) as an aggregate (CPCGA) is limited because of the low mechanical strength and strong water absorption capacity of CG. This paper used silica fume (SF) to improve the performance of the interfacial transition zone (ITZ) in CPCGA and revealed the enhancement mechanism. The results showed that the compressive strength of CPCGA prepared with cement replaced by a suitable amount of SF at the age of 28 days increased by more than 30%, and the flexural strength increased by over 20%. The SF could effectively reduce the porosity and micropore size in the ITZ of CPCGA, and the porosity of the ITZ in CPCGA added with 7.50% SF decreased by 44.22, 46.16, and 24.46% at distances from the aggregate surface of 10, 50, and 100 μm, respectively, compared with CPCGA without SF. Further research showed that Ca(OH)2 (CH) generated in the cement hydration reaction reacted with a large amount of active SiO2 in SF to restrain the formation of coarse CH in the ITZ of CPCGA, and the calcium silicate hydrate (C-S-H) gel generated filled the ITZ micropores to reduce the ITZ porosity further. Moreover, the reaction of SF and CH helped to promote the hydration reaction of cement to proceed thoroughly in CPCGA, thus improving CPCGA performance.

Numerical Investigation of Sulfate Diffusion Characteristics in Recycled Aggregate Concrete Based on Mesoscale Multiphase Analysis

Sulfate-induced deterioration can reduce the service life of recycled aggregate concrete (RAC). To demonstrate the transport behavior of sulfate ions in RAC, a two-dimensional, six-phase numerical model of RAC is proposed in this study. Sulfate ion diffusion coefficients were defined for different phases [attached mortar, new mortar, interfacial transition zone (ITZ) between attached mortar and original aggregate, ITZ between the new mortar and original aggregate, and ITZ between the new mortar and attached mortar] of RAC. The effects of ITZ width and porosity on ion transport behavior in RAC were analyzed. The results show that the numerical model agrees with the experimental data. Numerical results indicate that the six-phase model of RAC provides more-accurate simulation results because tortuosity is reflected in the model. The width and porosity of ITZs both affect the diffusion of sulfate ions in concrete. However, due to the low volume fraction of ITZ in concrete, just improving ITZ performance has no significant effect on ion attack resistance of concrete. The ITZ is a crucial zone where damage may originate and develop.

Factors affecting uniaxial compressive strength and ultrasonic pulse velocity of meso-scale heterogeneous concrete

Ultrasonic pulse velocity test (UPV) and uniaxial compressive strength (UCS) is widely used for building conversion model in concrete non-destructive test. However, the stability and accuracy of test results is always a questionable issue because concrete is heterogeneous on the meso-scale. In this study, a meso-scale simulation methodology aimed at specifying the influencing factors of UCS and UPV is proposed. Firstly, a meso-scale finite element model of concrete consisting of mortar, interfacial transition zone (ITZ) and randomly distributed aggregates is developed. Furtherly, by using L18 orthogonal array, the effects of significant influencing factors were analyzed: aggregate content, aggregate type, maximum aggregate size, thickness and porosity of ITZ, and water-cement ratio. The analysis of variance (ANOVA) results show water-cement ratio is the only significant influence of UCS. But significant factors for UPV including water-cement ratio, aggregate type and ITZ porosity. In addition, the rationality of the simulation methodology is validated with low relative errors (4.72% for UCS and 3.21% for UPV) is achieved on the experimental data and theoretical formulations.

Effects of belite incorporation on mechanical and microstructural properties of OPC mortars cured under carbonation condition

This study investigates the effects of various dosages of belite replacing ordinary Portland cement (OPC) on properties of OPC binders using carbonation curing. Mechanical strength was examined, pore-related test and microstructural/mineralogical analyses including water absorption, scanning electron microscopy (SEM), thermogravimetric analysis and 29Si nuclear magnetic resonance (29Si-NMR) were carried out. Extra attention was paid on the changes in the thickness and porosity of interfacial transition zone (ITZ) of mortars. Compared to normal curing, carbonation curing increased the compressive strength of all mortar mixes, especially given a longer period of curing and/or with belite addition. The greatest increase (73.18%) in the compressive strength was seen in the specimen with 35% of belite after 28 days of carbonation curing compared its peer under normal curing condition. Besides, specimens with 5% and 15% belite obtained higher flexural strength than that of the control after 7 and 28 days of carbonation curing. Carbonation curing led to about 92.75% increment in the flexural strength of the specimen with 15% belite. Regardless of curing conditions, in most cases, belite modified mortars had higher fractural toughness, probably due to the formation of more C–S–H gels instead of Portlandite. Compressive and flexural strength are more sensitive towards the thickness of ITZ than its porosity and carbonation curing strengthened this sensitivity. Carbonation curing densified the bulk OPC binders, especially after a long period of curing incorporated with belite. Microstructural analyses indicate that although in general belite hinders the hydration process of OPC, a suitable amount (5 wt%) improved the hydration degree of OPC when cured in carbonation environment. The polymerization degree of C–S–H gels could be enhanced when belite is added irrespective of curing conditions.

Modeling and experimental study on chloride migration of interfacial transition zone based on its pore structure

In this work, chloride migration coefficient of the interfacial transition zone (ITZ) between mortar and coarse aggregate was investigated. A three-phases-in-parallel model was employed to determine the chloride migration coefficient of ITZ (DITZ) from the migration coefficients of bulk mortar (Dmor) and concrete (Dcon). The ITZ porosity distribution and microscopic pore structure were measured by backscattered electron image (BSE) and mercury intrusion porosimetry (MIP), respectively. A porosity-dependent model was proposed to predict the DITZ. The results indicate that the ITZ thickness ranges from 20 to 40 µm and DITZ is almost 25 to 55 times larger than Dmor. Addition of fly ash remarkably decreases the ITZ thickness and DITZ, and also slightly decreases the ratio of DITZ to Dmor。The ratio of DITZ to Dmor predicted by the model ranges from 25 to 60, in good agreement with the experimental results. MIP validates that addition of fly ash decreases the total porosity primarily by lowering the pore volume of pores >10 nm. Sensitivity analyses indicate that the Dcon is more sensitive to the variation of pores between 10 and 100 nm compared with other pores.

Influence of fiber orientation on the microstructures of interfacial transition zones and pull-out behavior of steel fiber in cementitious composites

The reinforcement of steel fibers on cementitious composites is mainly influenced by the bonding between the fiber and matrix. The steel fibers and matrix bonding depend on the interfacial transition zone (ITZ). A major cause of ITZ formation is microbleeding, i.e., the flow of free water around steel fibers at the early stage of bleeding may cause the redistribution of water and cement particles around the fibers, leading to the formation of ITZs. The fiber orientation in the matrix affects the flow of water and further influences the microstructure of the ITZ around the steel fiber and the bonding properties. This study proposes a theoretical model based on fluid mechanics to investigate the influence of fiber orientation on cement-water suspension migration around steel fibers. The microstructure of the ITZ around the steel fiber is then estimated based on the Powers model according to the water-cement ratio distribution. Finally, the interrelationships among the fiber orientation, ITZ porosity and bond strength are established by curve fitting. The results indicate that since free water with small particles flows past the steel fiber by flowing past circular (or elliptical) cylinders, a difference in the ITZ microstructures at the upper and lower surfaces of the steel fiber arises. As the inclined angle of the fiber increases, the upper surface porosity and the average porosity of the ITZ around the steel fiber decrease, leading to an increase in the fiber-matrix bond strength.

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.

Prediction on Permeability of Engineered Cementitious Composites

Permeability of concrete is regarded as a basic indicator of its durability. This paper proposed a simple model to predict the permeability of engineered cementitious composites (ECC), which are fiber reinforced cementitious composites with extremely high ductility and toughness. The permeability of cement paste in ECC was firstly determined based on the general effective media theory. The needed microstructure information of cement paste was obtained from a simulated microstructure. Porosity of the interfacial transition zone (ITZ) was obtained with an ITZ porosity model, and then used to calculate the permeability of ITZ. The permeability of the matrix was determined according to the general self-consistent scheme, and the influence of fiber was simplified with its volume fraction. The calculated permeability of ECC was verified with results from water permeability tests and the accuracy of the model was acceptable for cement-based 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.

Damage mechanism and interfacial transition zone characteristics of concrete under sulfate erosion and Dry-Wet cycles

The durability of concrete structures under sulfate attack is of paramount importance to the structural safety and serviceability. Previous relevant researches have considered concrete as a single phase and homogeneous material in experiments without analyzing its multi-phase and heterogeneous properties. In addition, the damage mechanism of concrete under sulfate attack at the microscopic level, i.e., interfacial transition zone (ITZ), was not fully explored in literature. Therefore, this paper provided a comprehensive investigation on the damage mechanism and ITZ characteristics of concrete under sulfate attack and dry-wet cycles. Two typical sulfate attack conditions were examined in an experimental study, which includes 10% Na2SO4 and 10% MgSO4 solutions (by mass), respectively. The deterioration process of concrete was investigated by inspecting the visual change, corrosion resistance coefficient, and ions transportation. Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Atomic Force Microscopy (AFM) were also utilized to analyze the micro-topography change and corrosion products of concrete. The results indicate that the porosity of ITZ increases with the dry-wet cycles in tap water, and the concrete microstructure can be compensated by sulfate solutions, in which cases the compensation of C30 concrete is more remarkable than that of C60. 10% Na2SO4 solution is found to exhibit a stronger compensation effect than 10% MgSO4 solution. However, such compensation effect cannot offset the sulfate erosion damage on the microstructure of concrete. As a result, the porosity and initial cracks of ITZ continue to expand, and finally accelerate the cracking process and deterioration of concrete. It is also found that that under the combined effect of sulfate corrosion and dry-wet cycles, the roughness of ITZ increases with exposure time together with the largest increase in tap water. This study provides new insights and deeper understanding into the deterioration process and ITZ-link characteristics of concrete under sulfate erosion, the results of which serve as a decision basis for more durable concrete structure design.

Influence of aggregate surface treatment on corrosion resistance of cement composite under chloride attack

The high porosity of interfacial transition zone (ITZ) could significantly influence the durability of offshore concrete structures. This study aims to investigate the effects of aggregate surface by coating slag and silica fume on the ITZ microstructure, ultimately the corrosion resistance of the cement composite materials exposed to chloride attack. First, a series of experimental studies on chloride transport were carried out. Then, an experimentally validated model was developed to further understand the fundamental reactive-transport behaviour of chlorides in concrete in relation to different aggregate surface coating methods. The experimental results show that aggregate surface coating using silica fume can significantly reduce the porosity of interfacial transition zone (e.g. more than 40% reduction of porosity 5 μm from aggregate surface). Most importantly, the numerical predictions indicate that the slag and silica fume coating could potentially prolong the initiation time of the steel bar corrosion in offshore structures by more than 60% and 170%, respectively.

An innovation study on chloride and oxygen diffusions in simulated interfacial transition zone of cementitious material

This study simulated interfacial transition zone (ITZ) by controlling aggregate in type, size, shape, amount and placement to experimentally investigate diffusion coefficients in ITZ in comparison of mortar using an accelerated chloride diffusion test and a newly developed oxygen diffusion test. The water-to- cement ratios (w/c) of 0.55, 0.50, 0.45, 0.40 and 0.35 were used to evaluate w/c effect on diffusion. The results indicated that the simulated ITZ was successfully used to investigate the effect of w/c on chloride and oxygen diffusion properties in ITZ. The diffusion coefficient in ITZ was much larger than that in mortar. An inflexion of diffusion coefficient in ITZ was found with w/c reducing. A critical index M (ratio of DITZ and Dmor) was used to determine whether the porosity or the ratio of ITZ porosity and mortar porosity dominated diffusion coefficient in ITZ. The relationship between effective diffusion coefficient (overall diffusion coefficient) and M was proposed. Additionally, oxygen diffusion coefficient was much higher than chloride diffusion coefficient.

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.

Meso-scale modelling of stress effect on chloride diffusion in concrete using three-phase composite sphere model

At mesoscopic scale of modelling, concrete can be conceived as a three-phase composite material which is composed of coarse aggregate, mortar matrix and interfacial transition zone (ITZ) between the coarse aggregate and the mortar. This paper proposes a numerical model of stress effect on chloride diffusion in concrete from the meso-scale perspective. The concrete is idealised by using the three-phase composite sphere model. Based on the theory of elasticity, the constitutive relationships between the porosity of mortar, porosity of ITZ and the stress of concrete are established. Subsequently, the relationship among apparent chloride diffusion coefficient of concrete, chloride diffusion coefficients of mortar and ITZ, and volume fractions of three phases is established, and an apparent chloride diffusion coefficient model with respect to the elastic deformation stage of concrete is obtained. Finally, to extend the applicability of the model to a wide range of concrete stress levels under tension and compression, the model is modified to cater for the inelastic deformation stage of concrete. The modified model is applied to evaluate the apparent chloride diffusion coefficient of experimental specimens in the literature, and good agreement between experimental data and computed results is achieved. For concrete under tensile stress, the maximum prediction error of the modified model in the elastic stage and inelastic stage of concrete is 8.1% and − 11.6%, respectively. For concrete under compressive stress, the maximum prediction error of the modified model in the elastic stage and inelastic stage of concrete is − 8.2% and − 9.3%, respectively. Hence, the modified model is of high accuracy and can serve as a useful tool for durability design and service life prediction of concrete structures, especially those susceptible to chloride attack.

The effect of rough vs. smooth aggregate surfaces on the characteristics of the interfacial transition zone

Due to its higher porosity, the interfacial transition zone (ITZ) between cement paste and aggregates in cementitious composites is often viewed as a weak link in the microstructure. Sectional plane analysis based on back-scattered electron scanning electron microscope (SEM-BSE) images is used to quantify the ITZ porosity gradient, yet not much research has focused on the effect of aggregate surface morphology on the ITZ composition. In this paper, a model concrete specimen with a single spherical ceramic particle acting as an aggregate is used to study the properties of the ITZ and its uneven distribution around the aggregate based on quantitative analysis of SEM-BSE images. A careful treatment of the statistics of the ITZ was employed. The average porosity of the ITZ at a smooth part of the aggregate surface was smaller than that found at a rough part of the aggregate surface. A method is proposed to describe the local surface roughness (SR) at the pixel level and relate this quantity to other ITZ properties. The relationship between the surface roughness and the ITZ porosity gradient within 30 μm of the aggregate surface was analyzed with the “K-means” clustering method. The role of gravity was seen, since more porous ITZ regions tended to form underneath the aggregate than above the aggregate, with intermediate results at the aggregate sides.

The impact of carbonation on bulk and ITZ porosity in microconcrete materials with fly ash replacement

Scanning electron microscopy was used to evaluate the porosity patterns found in microconcrete materials to better understand the gaseous diffusion pathways and reaction of CO2 within the bulk cement paste and interfacial transition zone (ITZ) of microconcrete materials containing different fly ash replacement types. Image segmentation was applied to evaluate the porosity as a function of distance from an exposed surface in the bulk cement paste and from aggregate boundaries in the ITZ in both carbonated and non‑carbonated microconcrete materials. The ITZ region remained more porous compared to the bulk cement paste for all microconcrete types after carbonation despite a change in the porosity profile across the ITZ region. Results indicate that both the carbonation reaction capacity and porosity before carbonation impact the decrease in porosity observed as a result of carbonation in the ITZ and bulk cement paste regions.

Bonding in cementitious materials with asphalt-coated particles: Part II – Cement-asphalt chemical interactions

Reclaimed asphalt pavement (RAP), when used as an aggregate in concrete, will reduce bulk concrete strength and modulus. While Part I of this study investigated the properties of the interfacial transition zone (ITZ), Part II focuses on the nature of the cement-asphalt bond. Several chemical oxidative treatments of the asphalt were found to improve the interfacial cement-asphalt bond energy without affecting the ITZ porosity and size. Based on surface free energy measurements, the failure mode was estimated to occur preferentially as asphalt cohesion rather than cement-asphalt adhesion or ITZ cohesion. Based on the findings from Parts I and II, RAP aggregates reduce concrete strength and modulus because of: (1) the higher porosity in the ITZ, which produces a lower bulk modulus and allows for easier crack initiation, and (2) the preferential asphalt cohesion failure, which occurs rather than adhesive failure of the cement-asphalt interface or cohesive failure of the ITZ.

Experimental Study of Diffusivity of the Interfacial Transition Zone between Cement Paste and Aggregate

AbstractThe purpose of this paper is to investigate the diffusivity of the interfacial transition zone (ITZ) experimentally using cementitious composite materials made of rod-shaped aggregates and cement mortar. Two types of tests were conducted: the porosity distribution of the ITZ at different distances from aggregate surface was measured by backscattered electron images, and the water vapor diffusivity of the composites was measured by the penetration rate of water vapor through thin slides of specimens. The surface roughness of rod-shaped aggregates was considered as a testing parameter. The diffusivity of ITZ was calculated based on the measured diffusivities of the composite and it was also modeled according to the measured porosity distribution within the ITZ. The results showed that the porosity of ITZ decreases with increasing distance from the aggregate surface, a rougher aggregate surface decreases the porosity of ITZ in the first 10 μm from the aggregate surface, and the ratio of averaged wate...