低应力水平下混凝土中氯离子扩散行为多尺度分析方法

混凝土的扩散渗透性能与其微观结构（包括细观尺度上粗骨料颗粒与砂浆之间的界面过渡区及砂浆本身的微观结构等）密切相关。在微观尺度上,砂浆和界面过渡区均可视为由无孔砂浆基质和孔隙水夹杂相组成的两相复合材料,二者的主要区别表现为孔隙率不同。在外荷载作用下,砂浆和界面过渡区的毛细孔隙率及孔隙连通性会发生改变,从而改变混凝土的扩散渗透性能。基于此,该文建立了低应力水平下混凝土中氯离子扩散行为多尺度理论分析方法,获得了混凝土表观扩散系数与外荷载（以体应变表征）及砂浆和界面过渡区当前孔隙率的定量关系。分析所采用的主要参数为砂浆和界面过渡区的毛细孔隙率、无孔砂浆基质和骨料相的力学参数、骨料相和界面过渡区的体积分数、外荷载等。与已有文献数据对比知,该文分析结果与之吻合良好,表明了理论分析方法的合理性与准确性。此外,基于该方法,探讨分析了混凝土微/细观结构对其宏观扩散性能的影响。     

Influence of internal curing using lightweight aggregates on interfacial transition zone percolation and chloride ingress in mortars

The microstructure of the interfacial transition zone (ITZ) between cement paste and aggregate depends strongly on the nature of the aggregate, specifically its porosity and water absorption. Lightweight aggregates (LWA) with a porous surface layer have been noted to produce a dense ITZ microstructure that is equivalent to that of the bulk cement paste, as opposed to the more porous ITZ regions that typically surround normal weight aggregates. This ITZ microstructure can have a large influence on diffusive transport into a concrete, especially if the individual ITZ regions are percolated (connected) across the three-dimensional microstructure. In this paper, the substitution of LWA sand for a portion of the normal weight sand to provide internal curing (IC) for a mortar is examined with respect to its influence on ITZ percolation and chloride ingress. Experimental measurements of chloride ion penetration depths are combined with computer modeling of the ITZ percolation and random walk diffusion simulations to determine the magnitude of the reduced diffusivity provided in a mortar with IC vs. one with only normal weight sand. In this study, for a mixture of sands that is 31% LWA and 69% normal weight sand by volume, the chloride ion diffusivity is estimated to be reduced by 25% or more, based on the measured penetration depths.     

Penetration of cement pastes into sand packings during 3D printing: analytical and experimental study

3D printing processes of concrete and cement based materials could bring architectural and structural innovation in construction industry. Additive manufacturing and digital fabrication methods in civil engineering have recently been developed at laboratory scale. Among the 3D printing processes that could bring new perspectives in innovative and designed architectural elements, one of the most interesting is called the selective paste intrusion method. The component is built layer by layer by selectively applying cement paste on an aggregate packing using a 3D printer nozzle and a subsequent penetration of the paste into the aggregate layer. The implementability of the selective paste intrusion method requires the prediction of the flow of a yield stress fluid through a porous media. The rheological behaviour of the cement paste must be adapted for its penetration through the porous network of the aggregate particle packing. An adequate penetration depth of the cement paste produces homogeneous materials that are capable of sustaining a high mechanical stress. We show in this paper that the compressive strength of component made by such a technique is directly linked to the penetration depth of the cement paste into the aggregate layer; consequently, this paper aims at predicting the penetration depth of cement pastes into sand layers. A theoretical framework has been developed to propose an evaluation of penetration depth as a function of the average sand grain diameter and the yield stress of the cement paste, which is experimentally validated with specific penetration measurements. Finally, we stress that the prediction of penetration with an analytical model is an effective technique to ensure building homogeneous cement based materials with the 3D printing selective binding method.     

浓度对超声检测颗粒两相流的影响

讨论了颗粒两相流检测中常用的超声单散射模型和复散射模型 ,研究了浓度对超声检测颗粒两相流的影响。通过实验测得了超声在体积浓度为１ %～１０ %的ＴｉＯ２-水悬浊液中传播时的衰减系数。结果表明 ,悬浊液中的超声衰减系数随着浓度和频率的升高而增大。实验的结果和使用的超声散射模型的理论值在低浓度时吻合较好 ,而在高浓度时有所偏离。文中对这种偏差的原因进行了讨论。     

Diffusivity evolution under decalcification: influence of aggregate natures and cement type

Since the decalcification of cement paste has been largely reviewed, we focus our studies on the influence of aggregate nature on this phenomenon in relation to the type of cement used, Ordinary Portland Cement or blended cement with fly ash and slag. Some characteristics of similar mortar mixtures where only aggregate nature differs (lime and siliceous sand) are therefore compared for the two types of cement before and after chemical decalcification induced by ammonium nitrate attack: mechanical strength, microstructure (porosity observed by mercury intrusion and profiles of oxide content trough degraded and sound zones determined by electronic microprobe analysis), transport properties (chloride ions diffusivity, gas and water permeabilities). The characterization of sound mortars underlines that siliceous aggregates promote less porous cementitious matrix. The duplication of ammonium nitrate attacks on same material allows testing the experimental parameters governing the degradation. The flows of calcium leached, the microstructure and the evolution of transport properties with decalcification suggest that limestone aggregates are not inert material. Consequently, for the mortars incorporating siliceous sand, the cementitious matrix is more decalcified and this leads to an amplification of ionic transports, especially through blended cement paste.     

The influence of mixing on the microstructure of the cement paste/aggregate interfacial zone and on the strength of mortar

The influence of mixing on the microstructure of the cement paste/aggregate bond has been investigated. Back-scattered electron microscopy was used in conjunction with quantitative image analysis to examine the microstructure of the interface between limestone aggregate and the cement matrix in a series of mortars. The distribution of porosity and anhydrous material along the paste/aggregate interface was shown to be dependent upon the relative abundance of water at the aggregate surface during mixing. Improvements in the interfacial microstructure were shown to correlate with improvements in strength and fracture properties. The interfacial zones seen in the limestone mortars were compared with a model interfacial system. A new classification system for two types of interfacial regions in mortar is proposed.     

宽频超声衰减法测量河流泥沙粒径分布

为了研究宽频超声波通过河流泥沙悬浊液的声衰减谱中包含泥沙的颗粒粒度信息,采用超声耦合相模型和散射模型的叠加描述河流泥沙悬浊液中超声衰减行为,宽频超声换能器测量得到2～7.5MHz间泥沙悬浊液声衰减谱,结合独立模式的最优正则化反演算法,获得了相同体积浓度、不同粒径分布下和相同粒径分布、不同体积浓度下泥沙的颗粒粒径分布。结果表明,宽频超声衰减法能较好地区分3种不同粒径分布的泥沙,并且在体积浓为3.8%～8.5%范围间,测量得到的平均粒径受浓度变化影响较小。将超声法测量结果与显微镜图像法测量结果相比较,两种方法得到的粒径分布趋势较为一致,平均粒径较为吻合。     

Thermal properties of cement-based materials: Multiscale estimations at early-age

The prediction of the thermomechanical response of cement-based materials depends on the adequate determination of the thermal properties. At early-age, these properties depend on the evolution of the microstructure due to hydration processes and the multiscale nature of CBMs. In this paper, the heat capacity, the thermal conductivity and the coefficient of thermal expansion of cement-based materials are estimated by means of analytical homogenization. Estimates at the scales of cement paste, mortar and concrete are provided. At cement paste scale, the evolution of the volume fractions of clinker minerals and hydrates are taken into account. Interface Transition Zone (ITZ) is considered at mortar scale. A discussion about the effective properties of the “pore phase” and C-S-H as well as their influence on the effective behaviour is provided. A formulation accounting for the mechanical coupling in the determination of the heat capacity is used. Finally, the model estimations prove to be consistent with experimental data from the literature.     

Determination of saturated surface-dry condition of clay–sand mixed soils for soil–cement concrete construction

Saturated surface-dry condition of soil needs to be known when soil–cement concrete mix is rationally designed. Based on the effective water concept, determination of the saturated surface-dry moisture content (hereafter referred to as w _ssd) of clay–sand mixed soil, necessary for determining unit water of soil–cement concrete construction, is dealt with in this study. The saturated surface-dry condition of soil, where soil water exhibits the same chemical potential as that of cement paste, can be determined both with drying rate method and pF method. Mixed soils were prepared in combination with six cohesive soils, standard sand and a recycled fine aggregate. Drying rate method and pF method were proven to be effective in determining w _ssd of any combinations of the soils. Relationship between volume fractions of cohesive soil and sand and w _ssd was found to be a linear equation.     

Monitoring Early Age Properties of Cementitious Material Using Ultrasonic Guided Waves in Embedded Rebar

The evaluation of early age properties of concrete is critical for ensuring construction quality. This paper presents a sensing method to use ultrasonic guided waves in a rebar for monitoring the early age properties of cementitious material. An EMAT sensor was used to excite the longitudinal mode L(0,1) wave in a rebar embedded in cement/mortar, and an ultrasonic transducer was used for receiving the echo signals. Guided wave dispersion curves were developed to select appropriate frequency range. The leakage attenuations of the L(0,1) mode wave from the rebar to the surrounding cement materials were continuously monitored for the first 10 h. The evolution of the shear wave velocity was also monitored simultaneously. The leakage attenuation from experimental measurements was compared with the theory-predicted attenuation in both time and frequency domains, and showed good agreement. Experiments were performed on three cement paste samples and three mortar samples. The results indicated that attenuation is nearly linearly related to the shear wave velocity, and shear wave velocity is linearly related to the penetration resistance (ASTM C403) in logarithmic scale. These results suggest that mechanical properties and hardening process of cement materials can be monitored by using the ultrasonic guided waves in a rebar.     

Ultrasonic determination of the particle concentration in model suspensions and ice slurry

The development of ice slurry for refrigeration systems and the enhancement of its efficiency depend on an accurate control of the ice concentration. We present here an ultrasonic method capable to measure precisely the particle concentration in ice slurry. To calibrate the ultrasonic measurement, we first determine the sound velocity and attenuation in two model suspensions (glass beads/polyethylene glycol and polyethylene beads/vaseline oil) for different particle volume fractions. The experimental results show a good agreement with the predictions of the two-component models in the long-wavelength limit. Additionally, the sound attenuation reveals a clear signature of the aggregate formation in the nearly iso-dense suspension. We next conduct the measurement of the sound velocity in the polypropylene glycol ice slurry where the ice concentration changes with temperature. The ice concentrations extracted from our sound velocity measurements are well consistent with the values determined from the binary phase diagram.     

Microstructural monitoring by laser-ultrasonic attenuation and forward scattering

Laser generation and reception techniques have been used for non-contact measurement of frequency-dependent ultrasonic attenuation in steels with different microstructures. In addition, the forward-scattered ultrasonic energy from microstructural discontinuities has been detected and characterized with reference to its amplitude and frequency content. It was found that fine microstructure (small grain size) causes less high frequency attenuation than a coarse microstructure, in agreement with the results of previous studies using contact piezoelectric transducers. Furthermore, the coarser microstructure produced higher amplitude forward scattering in the frequency range 4–25 MHz. These results could form the basis of a non-destructive method for monitoring materials quality during manufacture.     

Micromechanical finite element framework for predicting viscoelastic properties of asphalt mixtures

A micromechanical finite element (FE) framework was developed to predict the viscoelastic properties (complex modulus and creep stiffness) of the asphalt mixtures. The two-dimensional (2D) microstructure of an asphalt mixture was obtained from the scanned image. In the mixture microstructure, irregular aggregates and sand mastic were divided into different subdomains. The FE mesh was generated within each aggregate and mastic subdomain. The aggregate and mastic elements share nodes on the aggregate boundaries for deformation connectivity. Then the viscoelastic mastic with specified properties was incorporated with elastic aggregates to predict the viscoelastic properties of asphalt mixtures. The viscoelastic sand mastic and elastic aggregate properties were inputted into micromechanical FE models. The FE simulation was conducted on a computational sample to predict complex (dynamic) modulus and creep stiffness. The complex modulus predictions have good correlations with laboratory uniaxial compression test under a range of loading frequencies. The creep stiffness prediction over a period of reduced time yields favorable comparison with specimen test data. These comparison results indicate that this micromechanical model is capable of predicting the viscoelastic mixture behavior based on ingredient properties.     

煤粉气-固两相流的超声衰减特性

以颗粒相在整个检测空间内均匀分布为前提,采用McClements理论和Bouguer-Lambert-Beer定律共同描述煤粉气-固两相流中超声衰减特性,建立超声衰减系数与气-固两相流相关参数的理论关系,通过数值模拟,分析超声衰减随着颗粒相体积分数、超声频率、颗粒粒径变化的规律。结果表明,超声频率越高衰减系数越大;声衰减系数随着颗粒相体积分数的增大线性递增;选用某固定频率检测,测得2个体积分数下的声衰减系数即可确定衰减-体积分数曲线斜率,从而实现任意衰减系数对应的颗粒相体积分数测量;在相同的体积分数下,煤粉颗粒粒径为10～200μm,声衰减系数随着颗粒粒径的增大单调递减,当煤粉颗粒粒径大于200μm时,声衰减系数对煤粉颗粒粒径不再敏感。     

Effect of aggregates on the diffusion properties and microstructure of cement with slurried silica fume based materials

Diffusivity of cement-based materials is an important factor regarding durability and the service life prediction of concrete structures. The present research focuses on investigating the influence of aggregates on tritiated water diffusivity of cement-based materials containing slurried silica fume. Effective diffusion coefficients of mortars with several sand volume fractions varying from 0 to 65% were determined by through-out diffusion tests. Microstructure was examined by scanning electron microscopy associated to energy dispersive spectrometry analysis, thermogravimetric analysis, water and mercury porosimetry, and BET adsorption analysis. It was found that large agglomerated particles of silica fume observed in cement paste and mortar with a low sand content (here 10%), reduce pozzolanic reactivity and thus affect the effectiveness of silica fume on the materials sustainability parameters. The clusters present in these formulations are mainly due to the interaction of silica fume with calcium hydroxide of the mixing solution and not to the initial state of the slurry, which was well stirred and whose particles size were checked before use. However, the presence of high content of aggregates (more than 30% of sand volume fraction) during mortar's mixing improves the dispersion of slurried silica fume particles and helps to ‘shear’ and break up agglomerates of silica fume providing a better homogenization of the material and improving the microstructural and diffusivity parameters. The addition of superplastizer in mortars with more than 50% sand content may also participate in dispersing silica fume.     

Pore structure of hardened cement paste and mortar

The paper describes an experimental investigation into the pore structure of hardened cement paste and mortar. The pore structure was studied using mercury porosimeter. Ordinary portland cement and natural river sand were used. Pore structure determination was carried out for both the cement paste and mortar mixes over four hydration periods and five water-cement ratios. The threshold radius which was found to be prominent in the hardened cement paste, flattens out as the fine aggregate content increased.     

A model of phase stability,microstructure and properties during leaching of portland cement binders

A new model of 3D cement paste microstructure development is described and used to simulate the influence of leaching on hydrated cement pastes. In contrast to recent leaching models that have used empirical rules for phase dissolution, this model uses continual thermodynamic speciation and phase stability calculations to guide the microstructural changes that happen throughout hydration and subsequent exposure to low-pH solutions. This novel aspect of the model enables it to predict not only the well-known phase instability of calcium hydroxide at the onset of leaching, but also the detailed compositional and volumetric changes of C–S–H gel and other calcium, aluminate, and sulfate phases. Besides tracking the compositional and microstructural changes, we use the evolving microstructure as input to calculate changes in the relative diffusivity and effective Young’s modulus of the binder using established finite difference and finite element models. The results are broadly consistent with previous experimental and modeling investigations of leaching. In particular, the leaching process can be roughly divided into initial, intermediate, and final stages, each of which has distinct degradation characteristics and consequences for mechanical and transport properties. The thermodynamic basis of the model makes it readily extensible to simulate a wide range of cementitious materials and degradation phenomena, so we discuss its potential as a virtual microprobe for use with continuum-scale service life models of concrete elements.     

A stochastic second‐order and two‐scale thermo‐mechanical model for strength prediction of concrete materials

A stochastic thermo‐mechanical model for strength prediction of concrete is developed, based on the two‐scale asymptotic expressions, which involves both the macroscale and the mesoscale of concrete materials. The mesoscale of concrete is characterized by a periodic layout of unit cells of matrix‐aggregate composite materials, consisting of randomly distributed aggregate grains and cement matrix. The stochastic second‐order and two‐scale computational formulae are proposed in detail, and the maximum normal stress is assumed as the strength criterion for the aggregates, and the cement paste, in view of their brittle characteristics. Numerical results for the strength of concrete obtained from the proposed model are compared with those from known experiments. The comparison shows that the proposed method is validated for strength prediction of concrete. The proposed thermo‐mechanical model is also employed to investigate the influence of different volume fraction of the aggregates on the strength of concrete. Copyright © 2016 John Wiley & Sons, Ltd.     

Performance simulation and quantitative analysis of cement-based materials subjected to leaching

A 3D numerical modelling platform (MuMoCC) developed in a previous work by the authors is applied in this paper to investigate the effect of leaching of some solid phases of cement paste (portlandite and hydrated aluminates or sulfoaluminate phases) on the mechanical and diffusivity performances of cement paste and mortar. The platform is based on a multi-scale approach and uses two numerical tools. First, NIST’s CEMHYD3D code is used to simulate 3D Representative Volume Elements of cement paste and mortar. Then mechanical and diffusivity behaviour of the numerical materials are simulated using ABAQUS software. The proposed three-dimensional heterogonous model presents at least two advantages. Firstly, it is able to capture the complexity of the random microstructure of cement-based materials. Secondly, only a few parameters have to be fitted compared to the other existing models, which indicates the relevance of the model. The numerical simulations of leached cement paste and mortar performance highlight and quantify the significant effect of portlandite and hydrated aluminate and sulfoaluminate phases’ dissolution on the decrease of elastic modulus and compressive strength and on the increase of ductility and diffusivity. The numerical results show that the leaching of portlandite decreases the compressive strength of a w/c = 0.4 cement paste by a factor of 1.33. The dissolution of portlandite and hydrated aluminates or sulfoaluminate phases involves a decrease by a more important factor (1.86).The leaching of portlandite phase involves an important increase, by a factor of 31, of the effective diffusion coefficient.Using the developed multi-scale modelling and knowing the leaching kinetics values, the mechanical and diffusion performances of cement-based materials can be estimated correctly according to leaching duration.     

Moisture flow by micro structural contraction

Material properties of building and construction elements are requested to last for an increasingly longer service life. For cementitious materials in particular, the “birth” of these properties emerge at the early stage of the hardening process. It is the development of the microstructure that determines the load bearing capacity as well as the quality in the material in the future. The quality of the microstructure realized at early ages with this affect the performance of a structures current and futures performance. As research areas focus more and more on micro-structural densification and durability related issues, deformational changes become increasingly decisive for a high quality guarantee.When considering the different research levels of concrete, the multi-scale modelling approach can be considered. When considering the paste captured in between two aggregate particles, the ribbon-model can be applied to simulate the hardening behaviour of this piece of paste.The article shows numerical simulations about the effect of capillary water expelled from the microstructure due to syneresis processes on the one hand and contraction of the microstructure due to hardening shrinkage on the other. Simulations of cement pastes, composed of different water/cement ratios, clearly demonstrate a movement of capillary water throughout the microstructures and its influencing on shrinkage. The simulation results show a slight influence of syneresis on water expulsion and on the deformation of the ribbon-paste.