Experiments and micromechanical modeling of electrical conductivity of carbon nanotube/cement composites with moisture

Carbon nanotube (CNT)/cement composites have been proposed as a multifunctional material for self-sensing and traffic monitoring due to their unique electric conductivity which changes with the application of mechanical load. However, material constituent and environmental factors may significantly affect the potential application of these materials. Therefore, it is necessary to understand an influence of material constituent such as porosity and dispersion of CNT and environmental factor such as moisture on the electrical conductivity of CNT/cement composite. This paper investigates the effect of moisture on the effective electrical conductivity of CNT/cement composites. To prepare the specimens, multi-walled carbon nanotubes (MWCNTs) are well dispersed in cement paste, which is then molded and cured into cubic test specimens. By drying the specimens from the fully saturated state to the fully dry state, the effective electrical conductivity is measured at different moisture contents. As the water in the specimen is replaced by air voids, the electrical conductivity significantly decreases. Different ratios of MWCNTs to cement have been used in this study. Micromechanical models have been used to predict the effective electrical conductivities. A comprehensive model is proposed to take into account the effects of individual material phases on the effective electrical conductivity of CNT/cement composites with moisture effect.     

Thermal dilation and internal relative humidity of hardened cement paste

The coefficient of thermal dilation (CTD) of hardened cement paste and concrete is a function of the state of internal moisture in the pore system. It has been theorized that changes in the pore fluid pressure induced by temperature change causes additional dilation when the material is partially saturated. Drying shrinkage stresses in early-age concrete also evolve from changes in the pore fluid pressure. The Kelvin-Laplace equation relates changes in the pore fluid pressure to the measured internal relative humidity (RH). This research investigated the role of pore pressure changes on the CTD through internal RH measurements. A maximum change in humidity (ΔRH) due to temperature change (ΔT) was measured when the initial humidity was at an intermediate value. Likewise, the maximum CTD was also measured at an intermediate initial RH. Based on these findings, the additional thermal strain caused by changes in pore fluid pressure was modelled using internal RH measurements as a primary parameter.     

Modeling the influence of the interfacial zone on the DC electrical conductivity of mortar

The interfacial zone separating cement paste and aggregate in mortar and concrete is believed to influence many of the properties of these composites. The available experimental evidence, obtained on artificial geometries, indicates that the DC electrical conductivity of the interfacial zone, because of its higher porosity, may be considerably larger than that of the bulk cement paste matrix. This paper presents the theoretical framework for quantitatively understanding the influence of the interfacial zone on the overall electrical conductivity of mortar, based on realistic random aggregate geometries. This understanding is also used, via an electrical analogy with Darcy's law, to make predictions about the effect of the interfacial zone on fluid permeability. The results obtained for mortar should also pertain to concrete.     

Electrical conductivity based characterization of plain and coarse glass powder modified cement pastes

This paper describes the use of electrical conductivity to characterize plain and coarse glass powder modified cement pastes. It is observed that the glass powder addition facilitates improved hydration of the cement grains. For the proportions investigated in this study, and the particle size of glass powder, this advantage is negated by the reduced amount of hydration products, i.e., the dilution effect. The variation of electrical conductivity and its derivative with time can be related to the various phases in the microstructural development of the paste. It is observed from the time derivative of conductivity plots that the addition of glass powder results only in minor changes in the setting time of the pastes. Higher the glass powder content, higher the normalized conductivity (ratio of conductivity at a certain glass powder content to that of plain paste) at very early times, and then it falls to a value closer to or less than 1.0 at later times. A parallel model is used to represent effective conductivity as a function of the pore solution conductivity, porosity, and pore connectivity factor. The pore solution conductivity increases with increase in glass powder content. The porosity of the pastes reduces with increase in glass powder content at early ages and increases at later ages. A reduced pore connectivity factor is observed for pastes with higher glass powder content at later times. However, this does not imply increased volume of hydration products as is commonly interpreted for normal pastes, but the electrical conduction pathways are made more tortuous by the relatively large volume of un-reacted filler material in the pore structure.     

Volume change and cracking in internally cured mixtures made with saturated lightweight aggregate under sealed and unsealed conditions

Low water to cement ratio (w/c) concrete exhibits increased occurrences of early-age shrinkage cracking. These occurrences have spawned innovative developments in concrete mixture design that reduce the risk of cracking. One such development is internal curing, which utilizes the inclusion of saturated lightweight aggregate (SLWA). The SLWA supplies additional moisture to cement paste as it hydrates thereby counteracting the effects of self-desiccation. This paper presents results from a study that examined mortar systems with different volumes of SLWA under sealed and unsealed conditions. Specifically, the results indicate the influence of SLWA volume and type on the internal humidity, autogenous shrinkage, and restrained shrinkage cracking behavior. The performance of each system is shown to be related to the cavitation of vapor-filled space in the paste, the SLWA spacing, and the pore structure of the paste in relation to the LWA.     

Characterisation of pore structure development of alkali-activated slag cement during early hydration using electrical responses

This paper describes the results of a study investigating early age changes in pore structure of alkali-activated slag cement (AASC)-based paste. Capillary porosity, pore solution electrical conductivity and electrical resistivity of hardened paste samples were examined and the tortuosity determined using Archie's law. X-ray computed micro-tomography (X-ray μCT) and Scanning electron microscope (SEM) analysis were also carried out to explain conclusions based on electrical resistivity measurements. AASC pastes with 0.35 and 0.50 water-binder ratios (w/b) were tested at 3, 7, 14 and 28 days and benchmarked against Portland cement (PC) controls. Results indicated that for a given w/b, the electrical resistivity and capillary porosity of the AASC paste were lower than that of the PC control, whilst an opposite trend was observed for the pore solution conductivity, which is due to AASC paste's significantly higher ionic concentration.Further, capillary pores in AASC paste were found to be less tortuous than that in the PC control according to estimations using Archie's law and from the results of X-ray μCT and SEM analysis. In order to achieve comparable levels of tortuosity, therefore, AASC-based materials are likely to require longer periods of curing. The work confirms that the electrical resistivity measurement offers an effective way to investigate pore structure changes in AASC-based materials, despite threshold values differing significantly from PC controls due to intrinsic differences in pore solution composition and microstructure.     

Computational results of a model for chloride ingress in concrete including convection, drying-wetting cycles and carbonation

Over the past decades a considerable research effort has been attributed to the modelling of chloride ingress into reinforced concrete in order to predict its durability. Traditional models are based on the error-function. In the present paper computational results of an advanced model are presented, which takes environmental temperature and humidity fluctuations, chloride binding, diffusion and convection, as well as carbonation effects into account. These qualifications make the model particularly suitable for simulating drying-wetting cycles, an example of which is included. The good performance of the moisture sub-model is demonstrated in an example concerning drying cement paste. Another example deals with a simple submersion case. The examples are based on laboratory and real-life experiments with documented concrete or paste compositions and environmental conditions, as well as the eventual measured chloride profiles. In general, the computational results are in good agreement with the measurements. The paper concludes with a comparison between simulation results of the present model and the error-function model for the case of cyclic drying-wetting exposure. It appeared that the implicit negligence of the moisture fluctuations by the error-function model, forced that model to deploy strongly deviating material characteristics to reach agreement with the measured chloride profile.     

Dielectric and electrical properties of ordinary Portland cement and slag cement in the early hydration period

The dielectric constant and electrical conductivity of ordinary Portland cement (OPC) with water-cement ratios (w/c) of 0.30, 0.35 and 0.40 were measured for the first 30 h hydration, using a microwave technique in the frequency range 8.2–12.4 GHz. It was found that both the dielectric constant and electrical conductivity of the cement paste are sensitive to the water-cement ratio, the higher the w/c value, the greater the dielectric constant and electrical conductivity, and the longer the hydration time. We also found that the higher the frequency the greater the electrical conductivity but the smaller the dielectric constant. The dielectric constant and electrical conductivity of high- and low-slag cement with water-solid ratio (w/s) of 0.40 were measured in the first 30 h after mixing. The changes in dielectric constant and electrical conductivity of low-slag cement with time are similar to that of OPC, but the high-slag cement shows very different dielectric and electrical properties compared with OPC and low-slag cement. The relationship between the dielectric and electrical properties of cement paste and cement hydration was also discussed.     

导电混凝土的导电性能及影响因素研究进展

导电混凝土是具有导电、电热、电磁屏蔽等诸多特性或功能的复合材料,在道路融雪化冰、电气设备接地、结构健康监测以及电磁屏蔽等领域具有广阔的应用前景。导电混凝土应具有适宜的导电性能和电阻率稳定性,但是导电材料类型、形态和掺量以及导电混凝土含水率和环境温湿度等诸多因素都可能导致导电性能和电阻率稳定性降低,从而制约导电混凝土的工程应用。分类对比了常见导电材料的性能差异以及用不同导电材料制备的导电混凝土的导电性能差异,在此基础上,探讨了导电混凝土的导电性能和制备方法的研究现状,较为系统地分析了导电材料类型和掺量等因素对导电性能的影响,并提出了改善导电性能和电阻率稳定性的建议。     

Coefficient of thermal expansion of cement paste and concrete: Mechanisms of moisture interaction

Crack sensitivity calculations for young concrete are strongly influenced by the coefficient of thermal expansion (CTE) values for the concrete. This paper demonstrates the strong effect of moisture content on CTE, and discusses the mechanism(s) based on experimental results on mature cement paste. The temperature variation of the relative humidity (RH) exerted by the pore water is quantified and used to explain the high CTE of partly dried specimens. The relevance for early age crack sensitivity is that the concrete should be kept as wet as possible. This minimizes CTE and will largely eliminate autogenous shrinkage. However, trustworthy procedures to separate autogenous- and thermal deformations require a better fundamental understanding of moisture effects and the nature of delayed deformations.     

混凝土硬化过程中的应力-应变发展和基于内部湿度的开裂风险预估

建立了一种对水泥混凝土开裂风险进行评估的新方法。模型基于硬化过程中混凝土的应变-约束应力发展, 研究了开裂时间与收缩变形率和混凝土内部湿度降低率的关系。研究结果表明:混凝土开裂风险与收缩变形速率有关, 开裂时间与开裂时的临界收缩率存在唯一对应关系。该研究根据混凝土湿度与变形的关系, 提出了一个基于混凝土内部湿度降低速率的混凝土早期开裂预估方法。该方法避免了工程实际中为确定混凝土开裂风险而需要进行的变形测量及复杂的需要考虑徐变、应力松弛、强度和模量的发展等因素但又未必准确的应力计算过程, 简化了混凝土硬化过程中的开裂风险预测。     

An analytical solution for hydraulic conductivity of concrete considering properties of the Interfacial Transition Zone (ITZ)

As a composite material, hydraulic conductivity of concrete depends on conductivity of its components that are the mortar, aggregates and the Interfacial Transition Zone (ITZ). Since hydraulic conduction is analogous to heat and electrical conduction, analytical models from these analogous areas relating effective conductivity of composite to conductivity of its components can be used to find the effective hydraulic conductivity of concrete as a function of properties of its components, i.e., aggregate, mortar and the ITZ. However, effect of the conduction in the ITZ has not been considered in these models. This paper presents an analytical solution for the hydraulic conductivity of concrete as a three-phase composite material. The solution is an extension to the model originally proposed for conduction of composite media with randomly suspended spheres. Results of the proposed model compare well against the experimental results and those obtained from rigorous numerical analysis using the Finite Element (FE) method. The principal significance of this study lies in the development of a versatile analytical model that can be employed as a quick tool for assessment of hydraulic conductivity of concrete without the need for sophisticated FE models at the meso-scale level. It offers more insight into effect of different components of concrete on its overall conductivity.     

Effect of composition,environmental factors and cement-lime mortar coating on concrete carbonation

The paper describes the physicochemical processes of concrete carbonation and presents a simple mathematical model for the evolution of carbonation in time, applicable under constant relative humidity higher than 50%. The model is based on fundamental principles of chemical reaction engineering, and uses as parameters the ambient concentration of CO₂, the molar concentratrations of the carbonatable constituents, Ca(OH)₂ and CSH, in the concrete volume, and the effective diffusivity of CO₂ in carbonated concrete. The latter is given by an empirical function of the porosity of hardened cement paste and of relative humidity, derived from laboratory diffusion tests. The validity of the model for OPC or pozzolanic cement concretes and mortars is demonstrated by comparison of its predictions with accelerated carbonation test results obtained in an environment of controlled CO₂ concentration, humidity and temperature. The mathematical model is extended to cover the case of carbonation of the coating-concrete system, for concrete coated with a cement-lime mortar finish, applied either almost immediately after the end of concrete curing or with a delay of a certain time. Parametric studies are performed to show how the evolution of carbonation depth with time is affected by cement and concrete composition (water/cement or aggregate/cement ratio, percentage OPC or aggregate replacement by a pozzolan), environmental factors (relative humidity, ambient concentration of CO₂), the presence and the time of application of a lime-cement mortar coating and its composition (water/cement, aggregate/cement and lime/cement ratios of the mortar, percentage OPC or aggregate replacement by a pozzolan).     

炭纤维水泥石压敏传感器的温湿度补偿方法

介绍了适合混凝土结构局部压应力、 压应变状态监测的嵌入式炭纤维水泥石压敏传感器的概念及其传感原理, 并研究了环境温度和湿度两个因素对炭纤维水泥石压敏传感器零点输出的影响; 同时从传感器电路设计角度探讨了温度、 湿度的补偿方法及其在结构中的应用方式。研究表明: 炭纤维水泥石压敏传感器的零点输出对环境温度、 湿度的变化比较敏感, 温度、 湿度的最大影响分别约为6%和200%; 对炭纤维水泥石压敏传感器采用"测试-补偿"对耦设置构成的补偿电路可以消除因环境温度、 湿度以及极化作用产生的噪声信号对传感器零点输出的影响。      

Influence of aggregate fraction in the mix on the reliability of the rapid chloride permeability test

The present paper elucidates the influence of aggregate content of the mix on the reliability of rapid chloride permeability test (RCPT) results. For this purpose, test specimens prepared with mixes varying in total aggregate content were subjected to soaking test, RCPT and electrical resistivity measurements, and the results from these tests were compared and conclusions drawn. The RCPT results indicated the plain cement concrete to be relatively more resistant against chloride penetration than the plain cement mortar, whilst the opposite was true according to the 90-day soaking test results. The above trend did not change despite the addition of silica fume (SF) to the concrete and mortar mixes. The lower aggregate content or higher paste content of plain cement mortar and the mortar with SF is shown to mislead the RCPT results. The higher paste content in the above mix promotes the conduction of higher charge as a result of lower electrical resistivity. Thus the results derived from the present investigation emphasize the need to consider the volume fraction of aggregate in the mix with and without SF while interpreting the RCPT results. Furthermore, regardless of the total aggregate and SF content in the mix, the total charge passed (from the RCPT) through the mix decreased exponentially with increasing electrical resistivity. On the other hand, for those mixes containing either SF or a high volume fraction of aggregates the linear correlation between the total charge passed and chloride penetration coefficient (K) was poor. However, for the mix with relatively lower aggregate content and with no SF the charge passed was well correlated linearly with K.     

Nature and behaviour of cement bonded particleboard: structure, physical property and movement

The paper records work carried out to analyse the structure and determine the behaviour of cement bonded particleboard (CBPB). The structure was quantified with respect to the structural parameters (distribution, size, shape and occupied area) of and interaction between individual components (pore, wood particle and cement paste), and the movement was analysed with respect to the structure of CBPB and the nature of the cement paste and wood particles. The results showed that: (1) The volume of CBPB occupied by the detectable pores is less than 1%, the areas occupied by wood particles are about 42% and 38% in vertical and horizontal surfaces respectively, and those by cement paste about 58% and 62%. (2) Orientation, size and shape of wood particles are very different between across the thickness and along the transverse directions of CBPB. (3) Equilibrium moisture content (EMC) and density have been complicated by the penetration of cement paste into wood particles and interfacial region between wood and cement paste. (4) CBPB was unstable under both constant and changing environmental conditions: exposure to a constant environmental condition resulted in an increase in mass and decrease in dimensions. Under a single change in relative humidity, the changes both in mass and dimensions on both adsorption and desorption consisted of two distinct stages: a significant change in the early stage of exposure and a gradual change in the later stage. Cycling under changing environmental regimes resulted in corresponding changes in mass and dimensions, and both reversible and irreversible behaviour occurred, giving rise to a series of displaced hysteresis loops which are very dissimilar to those for other materials. (5) The movement of CBPB has been attributed to the combined effects of moisture reaction, carbonation and degradation of CBPB, and all of these parameters gave rise to the development of incompatible stresses, which aggravated the above effects. Both mass and dimensional changes were essentially Fickian and non-Fickian. Models have been developed and able to predict both mass and dimensional changes effectively and efficiently. (6) The change of CBPB also reflected the change of wood particles and cement paste, and the strain and sorption of the CBPB have been successfully quantified in terms of moduli and volume/mass concentration of the wood particles and cement paste (the rule of mixtures).     

Influence of cement content and environmental humidity on asphalt emulsion and cement composites performance

Asphalt and cement concrete are the most popular materials used in the construction of roads, highways, bridge deck surface layers and pavements in airports and other areas with heavy wheel roads. Whereas asphalt possesses, compared to concrete, the advantages of a short curing period, high skid resistance and easy maintenance, it also shows lower fatigue durability, ravelling and rutting due to repeated concentrated loads and susceptibility to temperature changes and moisture. On the other hand, concrete pavements are initially more expensive, have lower driving comfort and are susceptible to cracking due to volume changes and to salt damage. A material with low-environmental impact and with advantages of both asphalt and concrete may be obtained by combining bitumen emulsions and a cementitious material. In this paper, cold asphalt mixtures with different amounts of cement were tested with Marshall stability tests. Selected mixtures were also cured at different environmental relative humidity (35, 70 and 90 % RH). By monitoring the mass of the specimens and estimating the water bound by the cement, the total water remaining in the mixtures was calculated. Details of the microstructure in the mixtures were examined with X-ray microtomography. According to the results of the present study, cement contributes to the hardening of cold asphalt mixtures both by creating cement paste bridges between the aggregates and by removing water from the mixtures through cement hydration. Asphalt and cement composites appear to be promising materials for implementation in real pavements, although their rate of hardening needs to be improved further.     

Experimental validation of a framework for hygro-mechanical simulation of self-induced stresses in concrete

An accurate simulation of the internal stresses that develop in concrete structures in service demands the explicit consideration of the internal heat and moisture movements that induce strains, which in turn have the potential to cause relevant internal stresses. Following a previous exploratory work from the authors, in which a thermo-hygro-mechanical simulation framework had been proposed, the present paper focuses on the validation of the proposed methodology through an experimental program targeted to assist prediction of the evolution of bulk drying shrinkage (and the corresponding internal stresses) in three specimens of different sizes. This paper presents the entire experimental program, which involved extensive concrete characterization using three distinct sizes of specimens, namely: moisture diffusion with internal humidity sensors, compressive strength, E-modulus, creep and shrinkage on three distinct specimen sizes. The program also included characterization of cement paste shrinkage, which was accomplished employing a newly proposed methodology based on the use of very thin specimens monitored with a handheld microscope. Finally, the obtained experimental data was used for partial validation of predictive capacities of the above mentioned simulation framework, mostly in regard to the prediction of bulk shrinkage. It is noted that the validation does not focus on thermal modelling, since this is a matter that has been addressed by the authors in previous works.     

Determination of moisture content of concrete by microwave-resonance method

The moisture content of concrete is one of its important properties. A new method of measuring the moisture content of concrete and other building materials is described. The method is based on the measurement of the dielectric constant at microwave frequencies. Studies on concrete have shown that a linear relation exists between the evaporable water content of concrete and the instrument reading. The main advantages of this method are i) it is nondestructive, ii) rapid response of the instrument, iii) humidity gradients in the specimen can be studied iv) measurements are independent of the dielectric losses and the conductivity of the material, v) sharp resonance peak of the microwaves allows very accurate measurements.     

Influence of curing at different relative humidities upon compound reactions and porosity in Portland cement paste

The reduction in relative humidity within drying concrete depends mainly on the depth from the exposed surface, exposure duration, temperature and environmental humidity; this limits hydration and coarsens pore structure, thus impairing durability. OPC paste, of 0.59 water/cement ratio, was cured for 2 days and then exposed to controlled relative humidity environments. After 14 and 90 days exposure, hydration of the individual compounds was measured using quantitative X-ray diffraction and thermogravimetry. Methanol adsorption was used to monitor porosity and gel formation. Even a small drop in relative humidity below 95% r.h. significantly limited cement hydration. Gel porosity increased with amount of hydration and, for a given level of hydration, decreased with drying. Curing below 80% r.h. produced a coarsened pore structure with a large-diameter porosity three times greater than that obtained with saturated curing.