Approximate migration coefficient of interfacial transition zone and the effect of aggregate content on the migration coefficient of mortar

In this study, the electrochemical technique is applied to accelerate chloride ion migration in mortar to estimate its transport properties. In order to investigate the effect of aggregate content on the chloride migration coefficient of mortar, specimens with different fine aggregate volume fractions were cast and tested. The chloride migration coefficient of mortar was determined experimentally as a function of the volume fraction of aggregate. The chloride migration coefficient of mortar is used to assess the dilution, tortuosity and interfacial transition zone (ITZ) effects of aggregate in the cement-based composites. A model modified from the Bruggeman theory for the migration coefficient of mortar is used, and the regression analysis is used to determine the approximate chloride migration coefficient of ITZ. Based on the experimental and regression analytical results, the approximate ITZ migration coefficient is 2.83, 1.76 and 1.55 times of the matrix migration coefficient for the ITZ with the thickness of 20, 40 and 50 μm, respectively.     

骨料对混凝土干缩性能的影响

采用收缩试验针对骨料对混凝土干缩的影响进行研究。主要分析不同骨料体积含量下骨料的约束作用、过渡区湿扩散作用对干缩影响的权重,以及骨料最大粒径和砂率对于缩影响。结果表明：骨料体积含量小于66％时,约束起主导作用;骨料体积含量介于66％-68％之间时,约束与过渡区湿扩散共同起主导作用;骨料体积含量大于68％时,约束再次起主导作用。骨料最大粒径和砂率的变化对干缩几乎没有影响,只有在单位用水量变化时对于缩产生影响。     

基于随机细观骨料的钢筋表面氯离子浓度概率分布研究

氯盐侵蚀诱发钢筋锈蚀是导致钢筋混凝土结构耐久性失效的主要因素之一。研究钢筋表面氯离子浓度概率分布对于预测海洋工程的服役性能具有十分重要的意义。通过引入随机厚度的界面过渡区,建立了包含骨料、界面过渡区以及砂浆的三相混凝土细观模型。在此基础上,结合氯离子扩散理论,建立了氯离子扩散细观数值模型。研究结果表明,骨料对氯离子的扩散具有阻碍作用,增加了氯离子的扩散路径。通过对6 000个钢筋表面氯离子浓度的模拟结果进行统计分析,发现:当骨料率为40%(体积分数)时,钢筋表面氯离子浓度呈双峰分布;当骨料率小于40%时,钢筋表面氯离子浓度近似呈正态分布;随着骨料率的增加,钢筋表面氯离子浓度不断减小,变异系数不断增大。     

Approximate migration coefficient of percolated interfacial transition zone by using the accelerated chloride migration test

In this study, the electrochemical technique is applied to accelerate chloride ion migration in cement-based material to estimate its migration coefficient. In order to investigate the chloride migration coefficient of percolated interfacial transition zone (ITZ) on the chloride migration coefficient of specimen, specimens with cylindrical aggregates of the same height as the specimen were cast and tested. In this study, the volume fraction of aggregate is constant and the varied lateral surface area of the aggregate cylinder was obtained by using different diameters and number of aggregate. The chloride migration coefficient of cement-based material was determined experimentally as a function of the lateral surface area of aggregate. A model obtained for the migration coefficient of cement-based material and the regression analysis are used to determine the approximate chloride migration coefficient of the percolated ITZ. Based on the experimental and regression analytical results, the approximate percolated ITZ migration coefficient is 40.6, 35.5, and 37.8 times of the altered migration coefficient of matrix mortar for the water/cement (w/c) ratio of 0.35, 0.45, and 0.55, respectively.     

Meso-scale computational modeling of the plastic-damage response of cementitious composites

Concrete is considered as a 3-phase composite material; mortar matrix, aggregates, and interfacial transmission zone (ITZ). In order to investigate the contribution of each phase to the strength and damage response of concrete, 2-D and 3-D meso-scale simulations based on a coupled plasticity-damage model are carried out. The aggregates are modeled as a linear-elastic material, whereas the mortar matrix and ITZ are modeled using a coupled plasticity-damage model with different tensile and compressive mechanical behavior. Aggregate shape, distribution, and volume fraction are considered as simulated variables. The effect of the ITZ thickness and the strength of the ITZ and mortar matrix are also evaluated. It is shown that the behavior of concrete is merely dependent on the aggregate distribution and the strength of the mortar matrix, but dependent on aggregate shape, size, and volume fraction, and the thickness and strength of the ITZ.     

混凝土界面渗流的集料体积率阈值算法及其影响因素评价

提出了混凝土界面渗流的集料体积率阈值算法并定量评价了影响该阈值的因素.通过引入周期性边界条件,在立方体单元内模拟混凝土中集料和界面的分布.基于模拟的混凝土细观结构,给出了集料体积率阈值算法并讨论了模拟单元边长对界面渗流概率的影响,在该算法的有效性获得实验验证后,定量评价了界面厚度、最大集料直径和集料级配对集料体积率阈值的影响.结果表明:集料体积率阈值随着界面厚度的增大而减小,但随着最大集料直径的增大而增大;集料级配对集料体积率阈值有很大的影响,其影响程度在20%左右,最大集料直径对集料体积率阈值的影响仅为7%.     

Enhancement of barrier properties of cement mortar with graphene nanoplatelet

The transport properties of cement mortar with graphene nanoplatelet (GNP) are investigated experimentally in this study. GNP, a low cost carbon-based nano-sheet, was added to mortar at contents of 0, 2.5, 5.0 and 7.5%, by weight of cement. The water penetration depth, chloride diffusion coefficient and chloride migration were determined for cement mortar with GNP and compared with plain cement mortar specimens. Test results showed that the addition of 2.5% GNP can cause significant decrease of 64%, 70% and 31% for water penetration depth, chloride diffusion coefficient and chloride migration coefficients respectively. The reduced water and ions ingress can be partially attributed to a reduction in the critical pore diameter of about 30%. This refinement of the microstructure by the GNP is validated by the mercury intrusion porosimetry (MIP) results. The impermeable GNP also contributes to the reduced permeability due to the increased tortuosity against water and aggressive ions ingress.     

Influence of lightweight aggregate on the microstructure and durability of mortar

The results of an investigation on the effect of dry and prewetted lightweight aggregates on the microstructure and durability of mortar are presented in this paper. The results are compared with those obtained for normal aggregate mortar. There appears to be only a small difference in the microstructure of the interfacial transition zone (ITZ) between dry and prewetted lightweight aggregate mortars. The porous ITZ surrounding lightweight aggregate appears to extend for about 10 and 15 μm from the aggregate surface for dry and prewetted lightweight aggregates, respectively. The ITZ for dry and prewetted lightweight aggregates seems to be surrounded by dense paste that extends from 10 to about 50 μm from the aggregate surface. This dense paste has lower porosity than that observed in the bulk paste located 50 μm and farther from aggregate surface. The normal aggregate mortar prepared with the same water/cement ratio appears to have porous ITZ that extends beyond 35 μm from the aggregate surface. The dry and prewetted lightweight aggregate mortars seem to have a lower sorptivity and electrical conductivity than does the normal aggregate mortar. Lightweight aggregate mortars also appear to have excellent resistance to sulfate attack as compared with normal aggregate mortar.     

混凝土中邻近集料表面最近间距分布的计算机模拟

混凝土中邻近集料问界面过渡区的相互影响程度、混凝土中原生裂纹的尺度范围以及中心质假说中集料效应圈的范围等都涉及到混凝土中最邻近集料表面间距分布的问题。由于常规的实验方法无法给出混凝土中集料空间分布的信息,同时以往的计算机模拟方法由于采用随机分布方式分布粒子,导致无法得到较高集料体积分数的模型混凝土结构,而采用具有粒子动态混合密实功能的SPACE系统,模拟了高集料体积分数混凝土的结构。在假定模型混凝土中集料的最小粒径为1mm的前提下,以符合Fuller分布为例,研究了模型混凝土中集料粒径分布和集料体积分数对邻近集料表面最近间距分布的影响。     

Assessing the ITZ microcracking via scanning electron microscope and its effect on the failure behavior of concrete

The influence of aggregate size and water-to-cement (w/c) ratio of the matrix on the structure of interfacial transition zone (ITZ) and the interaction between the ITZ and the matrix on the failure process of concrete under uniaxial compression were studied. The ITZ microcracking and the failure process of concrete were investigated experimentally by means of compressive and indirect tensile testing, stress-volumetric strain measurements and microscopic analyses on the model concrete containing single spherical steel aggregate with three different w/c ratios. At low w/c ratios, the rigid and smooth surface texture aggregates made by the ITZ have a significant structural difference compared to the mortar. This was more pronounced for larger aggregates. Higher structural differences between the mortar matrix and ITZ in low w/c ratio composites resulted in accelerated ITZ microcracking at high stress level. The effect of condensed microcracking in a narrower ITZ was reflected in the lower critical stress levels for the low w/c ratio composites with larger aggregates.     

Diffusion in liquid-filled pores of activated carbon. I. Pore volume diffusion

The adsorption isotherms on activated carbon were measured for the following six systems: benzaldehyde, phenol, and potassium chloride in water; benzene, isopropylbenzene, and phenol in cyclohexane. The systems isopropylbenzene and benzene each in cyclohexane and potassium chloride in water proved to be slightly adsorbing systems, whereas the benzaldehyde and phenol each in water were highly adsorbing systems. The rate of diffusion of the slightly adsorbing solutes was interpreted by assuming that the intraparticle diffusion was due to pore volume diffusion. The results indicated that the tortuosity factor for activated carbon is 3.5. The effective pore volume diffusivity and the tortuosity factor were not affected by the concentration of the solute, solute molecular size and the particle diameter. The external mass transfer resistance was negligible when pore volume diffusion was the controlling intraparticle diffusion mechanism.     

Link between microstructure and tritiated water diffusivity in mortars: Impact of aggregates

The present study aims to investigate the influence of microstructure, particularly the effect of aggregates content and interfacial transition zone (ITZ), on tritiated water (HTO) diffusivity in mortars.To this end, three different series of mortars were prepared and HTO diffusion tests were conducted. Variables are water-to-cement ratio, sand volume fractions, and particle size distribution. In parallel, the microstructure of these materials was characterized by water porosimetry, mercury porosimetry, and by backscattered electron microscopy associated to images analysis.It was observed that at low sand content (0%–50%), diffusion properties of mortars are dominated by aggregates dilution effect. Beyond 50% of standard sand, other effects related to the large number of sand grains appear, such as air voids and porous areas mainly due to the difficulty of obtaining well-compacted materials.     

Prediction of diffusivity of concrete based on simple analytic equations

Proposed is a simple analytic model that can predict realistically the diffusivities of concrete and mortar. The basic concept of the model comes from the relation between the diffusivities and the microstructure of concrete. The microstructure that affects the diffusivity includes the interfacial transition zone (ITZ) between aggregates and cement pastes as well as the microstructure of cement paste itself. The general effective media (GEM) equation was introduced to derive the diffusivity of cement paste. The effective diffusivity of concrete is derived on the basis of the composite sphere assemblage model, which considers the diffusivities of both ITZ and cement paste. The main parameters in the proposed model are the microstructural properties of cement paste such as capillary porosity and pore structure parameter, solid phase diffusivity, aggregate volume fraction, and interfacial zone properties. To validate the proposed model, many series of concrete and mortar specimens have been tested to measure the diffusivities. The major test variables include the water-to-binder ratios, the types and amount of mineral admixtures on the diffusivities. The effects of compressive strength, water-to-binder ratio, and mineral admixtures have been investigated comprehensively. The comparison of the proposed theory with the test data exhibits reasonably good correlation. The proposed model allows more accurate prediction of diffusion process and, thus, more realistic durability design of concrete structures.     

The interface between natural siliceous aggregates and geopolymers

The reaction products as well as the formation mechanisms of alkali-activated binders, or geopolymers, have been studied intensively. However, the interface between mineral aggregates, such as sand and/or natural rocks, and geopolymers has not been studied. This paper reports the microstructure and the bonding strength (Mode I bending) of the interface between natural siliceous aggregates and fly ash-based geopolymers. It was found that when the activating solution that contained no or little soluble silicates, the compressive strengths of the geopolymeric binders, mortars and concretes were significantly weaker than those activated with high dosages of soluble silicates. The presence of soluble silicates in the initial activating solution was also effective in improving the interfacial bonding strengths between rock aggregates and geopolymeric mortars. No apparent interfacial transition zone (ITZ) could be identified near the aggregates if the systems were free from chloride contamination. Chloride (KCl) was found to decrease the interfacial bonding strength between the aggregates and the binders probably by causing gel crystallisation near the aggregate surfaces, which resulted in debonding.     

Investigations about the effect of aggregates on strength and microstructure of geopolymeric mine waste mud binders

Geopolymeric binders appear to be an alternative to traditional Portland cement, due to high mechanical performances and environmental advantages. Some aspects related to the effect of aggregates in the microstructure and mechanical behaviour of geopolymeric mine waste mud (GMWM) binders are reported in the present study. Compressive and tensile strength of mine waste mud binders were analyzed. The factors investigated were the aggregate/binder ratio, the aggregate dimension and aggregate type, schist, granite and limestone.Test results showed that GMWM binders have a very high strength at early ages and also possess a very high tensile strength. It's suggested that behaviour may be due to the dissolution of quartz and alumina in the presence of alkalis enhancing bonding between paste and aggregates.The aggregate dimension showed only significant effect on tensile strength. Limestone aggregates showed a chemical bond to the alkali-activated paste but presented higher shrinkage. It was also found that no traditional porous ITZ was detected in GMWM binders.     

Micromechanics of ITZ‐Aggregate Interaction in Concrete Part II: Strength Upscaling

Cracking in concrete typically starts in the immediate vicinity around the aggregates, i.e., in the region of the so‐called interfacial transition zones (ITZs), but the process is still not fully understood. Notably, crushing of concrete in compression results in fragments with interesting aggregate surface textures. Part of the aggregate surfaces is cleanly separated from the ITZ, while another part of the aggregate surfaces remains covered with a thin layer of cement paste. This suggests two different types of failure: ITZ‐aggregate separation and ITZ failure; which we here study based on the continuum micromechanics approach of the companion paper (part I). It provides access to both traction vectors acting on aggregate surfaces and three‐dimensional stress states within representative ITZ volumes for loading states below the elastic limit of concrete. When inserting these microtractions and microstresses into Rankine‐type strength criteria for the aggregate‐ITZ interface and for the ITZ, respectively, the micromechanics model allows for upscaling this microscopic failure behavior to concrete‐level criteria for crack onset. Comparing the latter to corresponding experimental results, reveals that under tension‐dominated loading both ITZ failure and ITZ‐aggregate separation appear to be realistic, while under compression‐dominated loading ITZ failure appears as the more likely mechanism. Also, comparing model and experiments shows that the ITZ‐aggregate separation strength amounts to at least half of the internal ITZ cohesion strength, but may be much larger than the latter.     

Utilisation of steel furnace slag coarse aggregate in a low calcium fly ash geopolymer concrete

This paper evaluates the performance of steel furnace slag (SFS) coarse aggregate in blended slag and low calcium fly ash geopolymer concrete (GPC). The geopolymer binder is composed of 90% of low calcium fly ash and 10% of ground granulated blast furnace slag (GGBFS). Mechanical and physical properties, shrinkage, and detailed microstructure analysis were carried out. The results showed that geopolymer concrete with SFS aggregate offered higher compressive strength, surface resistivity and pulse velocity than that of GPC with traditional aggregate. The shrinkage results showed no expansion or swelling due to delayed calcium oxide (CaO) hydration after 320 days. No traditional porous interfacial transition zone (ITZ) was detected using scanning electron microscopy, indicating a better bond between SFS aggregate and geopolymer matrix. Energy dispersive spectroscopy results further revealed calcium (Ca) diffusion at the vicinity of ITZ. Raman spectroscopy results showed no new crystalline phase formed due to Ca diffusion. X-ray fluorescence result showed Mg diffusion from SFS aggregate towards geopolymer matrix. The incorporation of Ca and Mg into the geopolymer structure and better bond between SFS aggregate and geopolymer matrix are the most likely reasons for the higher compressive strength observed in GPC with SFS aggregate.     

Assessing the influence of ITZ on the steady-state chloride diffusivity of concrete using a numerical model

In this study, the influence of the aggregate-cement paste interfacial transition zone (ITZ) on the steady-state chloride diffusivity of mortars and concretes was examined using a semi-empirical, three-phase composite sphere model. Mortars and concretes were modelled as three-phase composites consisting of the aggregate, bulk cement paste and an inhomogeneous ITZ. The latter was divided into a series of homogenous concentric shell elements of equal thickness. The initial porosity and cement gradients at the ITZ were first estimated from the overall water/cement ratio (w₀/c). The evolution of the porosity, solid hydration products and remnants of unreacted cement were then calculated from the hydration degree and local water/cement ratio (w/c) using Powers' empirical model. Based on the Laplacian equation, an element transfer matrix was derived analytically to predict the steady-state chloride diffusivity. The model was calibrated using the available experimental data and then applied to perform a sensitivity analysis to evaluate the effects of aggregate content, water/cement ratio, curing period, ITZ width, maximum aggregate size and aggregate gradation on diffusivity. Some of these variables are impractical to quantify by laboratory experimentation. Implications of the findings with regard to the role of ITZ on mass transport properties are discussed.     

Improvement of the chloride ingress resistance of OPC mortars by using spent cracking catalyst

The influence of the incorporation of spent cracking catalyst (FC3R) on the chloride ingress resistance has been evaluated. Thermogravimetric analyses have shown that the pozzolanic reaction of FC3R yields higher contents of hydrated calcium aluminates and silicoaluminates, so chloride binding capacity of mortars was highly improved. Mercury intrusion porosimetry analyses demonstrated that FC3R produces a significant reduction of capillary pore volume. As a result non-steady-state and steady-state chloride diffusion coefficients were reduced, enhancing the chloride ingress resistance of mortars incorporating FC3R. Additionally, the corrosion behaviour of steel embedded in Portland cement mortars partially substituted by spent cracking catalyst (FC3R) under chloride attack has been studied. Results showed that the incorporation of FC3R decreased the corrosion rates of steels and increased chloride thresholds for corrosion. For this reason, FC3R is an interesting pozzolanic material that can be used in reinforced concrete for civil engineering applications exposed to the action of chlorides.