The early-age short-term creep of hardening cement paste: AC impedance modeling

Impedance spectra were monitored at early ages on hydrating Portland cement pastes subjected to a sustained load. The pastes were prepared with two different water-cement ratios (0.35 and 0.50). The experiments were conducted in a controlled chamber maintained at (96±2)% relative humidity. The three ages at loading investigated were 18, 24 and 30 hrs. Real-time changes in paste microstructure due to sustained load were followed through the coupling of an AC impedance frequency analyzer with a miniature loading system. Cement paste specimens were in the form of T-shaped columns with a minimum thickness value (for the web and flanges) less than 1.25 mm. The impedance analysis included an assessment of the relevance of the high frequency arc depression angle to an understanding of the creep and shrinkage behavior of cement paste. Electrical models were developed in order to predict the creep coefficient of normal (w/c = 0.50) and high strength (w/c = 0.35) cement pastes from early age data.     

A coupled AC impedance—creep and shrinkage investigation of hardened cement paste

The effect of three pre-drying treatments (prior to resaturation with synthetic pore fluid) on time dependent deformation was investigated. The pre-drying treaments comprised methanol and isopropanol exchange (followed by vacuum heating at 37°C) and vacuum drying at 37°C alone. Real-time changes in microstructure due to sustained load were followed through the coupling of an A.C. impedance frequency analyzer with a miniature loading system. Cement paste specimens were in the form of T-shaped columns with minimum thickness value around 1 mm. The impedance analysis included an assessment of the relevance of the high frequency arc depression angle to creep and shrinkage behavior of cement paste.

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Résumé L’effet de trois traitements de pré-séchage (suivi d’une resaturation avec une solution interstitielle synthétique) sur les déformations différées a été investigué. Les traitements de pré-séchage consistent en un séchage sous vide à 37°C d’éprouvettes avant et après immersion dans un solvant (méthanol ou isopropanol). Les modifications causées par la charge appliquée sur la microstructure étaient suivies en temps réel par le biais du couplage d’un analyseur de fréquences d’impédance avec un système de chargement miniature. Les éprouvettes de pate de ciment étaient de petites colonnes en forme de T ayant une épaisseur minimale d’environ 1 mm. L’analyse des spectres d’impédance inclue une évaluation de la pertinence de l’angle de dépression sur le fluage et le retrait de la pate de ciment.

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Editorial Note Laval University (Canada) is a RILEM Titular Member. Prof. J. Marchand was awarded the 2000 Robert L’Hermite Medal. He is Editor in Chief forConcrete Science and Engineering and Associate Editor forMaterials and Structures. Jacques Marchand participates in RILEM TC 186-ISA ‘Internal Sulfate attack’.     

Electrical properties of cement paste obtained from impedance spectroscopy

The impedance spectra of hydrating Portland cement paste showed a small part of a large depressed arc and a single depressed arc at low- and high-frequency regions, respectively. A new equivalent circuit was proposed for hydrating cement paste, which consists of a serial combination of a resistor and a constant phase element (CPE) with a resistor in parallel for the bulk effects, and a parallel combination of a resistor and a CPE for the electrode effects. The microstructural evolution in hydrating cement paste was associated with the changes in the resistances, capacitance and the critical relaxation time.     

Interpretation of impedance spectroscopy of cement paste via computer modelling

Computer simulation of impedance spectroscopy (IS) of hydrating cement paste, using a three-dimensional, four-phase model, is described. Two puzzling features of experimental IS results, the possible offset resistance in the Nyquist plot and the sharp decrease in normalized conductivity within the first 50 h of reaction, have been studied using the computer simulation model. Insight is provided into these features using the ability of the model to compare quantitatively microstructure and properties. It is concluded that the offset resistance is an experimental artefact, and does not directly relate to microstructure. The drop in conductivity during the first 50 h is shown to be a consequence of a gradual shift from parallel-dominated to series-dominated behaviour of the electrical conductivity, as microstructural modifications take place during hydration, causing the capillary pore structure to become more tortuous. This tortuousity can also explain the high-frequency impedance behaviour in terms of a two-arc response.Part II of this has been previously published in volume 29 issue 19, pages 4984–4992 of this journal.     

Interpretation of the impedance spectroscopy of cement paste via computer modelling

The d.c. conductivity, , and low-frequency relative dielectric constant, k, of Portland cement paste were monitored, using impedance spectroscopy, during cooling from room temperature down to -50 °C. Dramatic decreases in the values of and k, as great as two orders of magnitude, occurred at the initial freezing point of the aqueous phase in the macropores and larger capillary pores. This result provides strong experimental support for the dielectric amplification mechanism, proposed in Part II of this series, to explain the high measured low-frequency relative dielectric constant of hydrating Portland cement paste. Only gradual changes in the electrical properties were observed below this sudden drop, as the temperature continued to decrease. The values of and k of frozen cement paste, at a constant temperature of -40 °C, were dominated by properties of calcium-silicate-hydrate (C-S-H) and so increased with the degree of hydration of the paste, indicating a C-S-H gel percolation threshold at a volume fraction of approximately 15%–20%, in good agreement with previous predictions. Good agreement was found between experimental results and digital-image-based model computations of at -40 °C. Freeze-thaw cycling caused a drop in the dielectric constant of paste in the unfrozen state, indicating that measurements of k could be useful for monitoring microstructural changes during freeze-thaw cycling and other processes that gradually damage parts of the cement paste microstructure.     

Interpretation of the impedance spectroscopy of cement paste via computer modelling

Dielectric properties of cement pastes are measured using impedance spectroscopy, and the effective dielectric constants of the low frequency bulk arcs are reported. The unusually high values thereby obtained, and their dependence on reaction time and water:cement ratio, are explained by the presence of microstructural features that serve to amplify the dielectric constants of the individual material phases. The dielectric properties of three-dimensional cement paste models and of simple two-dimensional models of the hypothesized microstructural features are analysed. The model results provide insight into the proposed dielectric amplification mechanism in real cement paste.     

Evaluating ion diffusivity of cracked cement paste using electrical impedance spectroscopy

Cracking can significantly accelerate mass transport in concrete and as such, impact its durability. This paper is aimed at quantifying the effect of saturated cracks on ion diffusion. Electrical conductivity, measured by electrical impedance spectroscopy (EIS), was used to characterize the diffusion coefficient of fiber-reinforced cement paste disks that contained one or two through-thickness cracks. Crack widths in the range 20–100 μm were generated by controlled indirect tension test. Crack profiles were digitized and quantified by image analysis to determine crack volume fraction and average crack width. Crack connectivity (e.g., inverse of tortuosity) was calculated from the conductivity results measured by EIS. The results suggest that the diffusion coefficient of cracked samples is strongly and linearly related to the crack volume fraction; but is not directly dependent on crack width. Crack tortuosity does reduce the ion diffusion through cracks, but its impact is not very significant. Overall, the most important parameter governing ion diffusion in saturated cracked concrete is the volume fraction of cracks. Theoretical justifications of these observations are also provided.     

Coupled thermomechanical analysis of transformation-induced plasticity in multiphase steels

The thermomechanical response of low-alloyed multiphase steels assisted by transformation-induced plasticity (TRIP steels) is analyzed taking into account the coupling between the thermal and mechanical fields. The thermomechanical coupling is particularly relevant since in TRIP steels the phase transformation that occurs during mechanical loading is accompanied by the release of a considerable amount of energy (latent heat) that, in turn, affects the mechanical response of the material. The internal generation of heat associated with the martensitic phase transformation and the plastic deformation are modeled explicitly in the balance of energy. The momentum and energy equations are solved simultaneously by using a fully-implicit numerical scheme. The simulations are conducted using a micromechanical formulation for single crystals of austenite and ferrite. The characteristics of the model are illustrated by means of simulations for a single crystal of austenite and an aggregate of austenitic and ferritic grains. For a single crystal of austenite, it is found that the increase in local temperature due to transformation actually hinders further transformation and, instead, promotes plastic deformation. However, for an aggregate of austenitic and ferritic grains in a multiphase steel, the increase in temperature due to transformation is limited since the heat generated in the austenite is conducted to the ferritic matrix, effectively lowering the temperature in the austenitic phase.     

Dielectric spectra of fresh cement paste below freezing point using an insulated electrode

The spectra of complex dielectric constant were measured on a fresh cement paste with a water/cement ratio of 0.4 sandwiched between insulated electrodes in the frequency range 10 kHz–1 MHz and temperature range between 0 °C and — 30 °C. The bulk dielectric constant, 30–20, and conductivity, 6.14×10^–5–0.65×10^–5, in the temperature range –10 to –28 °C were much lower than those at room temperature, owing to the great decrease of ionic mobility caused by freezing the cement paste. The activation energy of 0.31 eV for the ionic conduction in fresh cement paste was obtained from an Arrhenius plot of conductivity at subzero temperature.     

Heat deformations of cement paste phases and the microstructure of cement paste

The paper presents the results of an experimental investigation of the influence of thermal deformations of phases present in hardened cement paste on its microstructure. For the observation of the thermal deformation course in the range of temperature20–800°C, a method of complex thermal analysis (DTA, DTD, TD) was employed. The microstructure of heated cement pastes was observed by both optic and electron microscopes. From an analysis of the results of the investigation it has been found that chemical processes stimulated by temperature in particular phases of cement paste have a significant influence on the thermal deformation course. It is shown that micracracks caused by different thermal deformations, appear first in the aregas of Ca (OH)₂ concentration (ca.300°C), and next (ca.400°C) as well as in the areas of occurrence of unhydrated large clinker grains.

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Résumé On rend compte d'une étude expérimentale sur les causes de la destruction de la microstructure de la pate de ciment portland exposée à la chaleur dans une gamme de températures allant de20 à800°C. On a prêté une attention particulière aux déformations thermiques de chacune des phases reconnues de la pate de ciment durci. On a utilisé une méthode d'analyse thermique complexe (DTA, TD, DTD) pour l'étude de l'évolution de déformations thermiques. On a observé les microstructures de la pate de ciment chauffée au microscope optique et électronique. On a étudié les phases suivantes de la pate de ciment: clinkers (C₃S, β-C₂S, C₃A, C₄AF), minéraux hydratés de ciment (hydrates C₃S, β-C₂S, C₃A, C₄AF), et produits d'hydratation [Ca(OH)₂, ettringite]. Les résultats de l'étude ont permis à l'auteur de déterminer l'allure de la déformation thermique des phases que présente la pate de ciment durcie sous températures croissantes. On a constaté que les processus chimiques que l'élévation de température induit dans chacune des phases de la pate de ciment exercent une influence notable sur l'évolution des déformations thermiques. Les microfissures causées par différentes déformations thermiques des phases apparaissent dans les zones de concentralisation de Ca(OH)₂ à environ300 et400°C aussi bien que dans les zones où se rencontrent des grains de clinker non hydraté.

     

Frost action in cement paste

An earlier theory of frost action is further developed and its implications considered, particularly with regard to testing. Factors determining durability of paste, dimensions of sample, degree of saturation and rate of cooling are discussed. It is shown that cumulative residual expansion is a good indicator for testing frost resistance: samples can be evaluated after 10 to 15 freeze-thaw cycles. The results are discussed in relation to existing tests.     

Water in hardened cement paste

Summary Some properties of adsorbed water in hardened cement paste and in the three completely hydrated main constituents of portland cement have been examined by means of differential thermoanalysis, thermo-gravimetry and nuclear magnetic resonance. When a dried specimen is re-wetted, the adsorbed water first contributes to the build-up of a mono-layer. Only in the higher humidity range is the sorbed water partly bound as inter-layer hydrate water and as hydrate water. According to the first results of the nuclear magnetic resonance measurements the water adsorbed in the monolayer behaves like a two-dimensiional Van-der-Waals-gas.

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Résumé Quelques propriétés de l'eau adsorbée dans la pate de ciment durcie et dans les trois constituants principaux complètement hydratés du ciment Portland ont été étudiées avec l'analyse thermique différentielle, la thermo-gravimétrie et la résonance magnétique nucléaire. Lorsqu'une éprouvette séchée est réhumidifiée, l'eau adsorbée contribe d'abord à la formation d'une mono-couche. L'eau adsorbée, en tant qu'eau d'hydratation, n'est partiellement liée dans la structure du sel que pour les humidités les plus élevées. Selon les tout premiers résultats fournis par les mesures de la résonance magnétique nucléaire, l'eau adsorbée dans la mono-couche se comporte comme un gaz bi-dimensionnel de Van-der-Waals.

     

Coupled elastoplasticity and viscoplasticity under thermomechanical loading

This paper is a contribution to material behaviour modelling. Total strain is decomposed into elastoplastic and viscoplastic strains. Both parts are analysed separately and put together by the principle of superposition. The spring‐slider model controlled by either stress or strain enables elastoplasticity modelling under constant or variable temperature with the Prandtl operators. Viscoplasticity is taken into account, if temperature exceeds creep temperature, by adding a nonlinear damper to existing spring‐slider models, otherwise just elastoplasticity is considered. The material parameters result from isothermal strain‐controlled low cycle fatigue (LCF) tests. Hysteresis loops are assumed to be stabilized. The high speed of computation that is characteristic of Masing and memory rules is retained. Solving of differential equations is not required. The model developed so far is uniaxial, but a multiaxial extension is possible.     

氧化石墨烯对水泥净浆流动度及水泥石结构和性能的影响

采用Hummers法及超声破碎分散法制备了氧化石墨烯(GO)纳米相分散液,研究了GO对水泥净浆流动度和水泥石微观结构的影响,用FT-IR、AFM、XRD及SEM对GO及水泥石结构进行了表征。结果表明GO的掺入降低了净浆流动性,每增加0.01%的GO需要增加0.02%的聚羧酸系减水剂(PCs)以保持水泥净浆流动度在3 h内在200 mm以上,GO的掺入能够使水泥石的微观结构发生明显的变化,当GO/PCs掺量为0.01%/0.24%~0.03%/0.28%时,水化龄期7 d的水泥石出现了大量分散均匀的花状微晶体,当GO/PCs掺量为0.05%/0.32%~0.07%/0.36%时,同龄期水泥石中出现大量的片状晶体,在水化龄期延长到28 d时有转化为密实结构的趋势,结果说明GO具有调控水泥水化产物形貌的能力及增强增韧的作用,此研究结果对于提高水泥基材料的力学性能具有重要意义。     

Hygral stress in hardened cement paste

A specially constructed stress cell, permitting variation of degree of restraint, was used to measure the hygral stress produced in thin-walled hardened cement paste cylinders due to water absorption. The effects of porosity, relaxation and relative humidity on the hygral stress were investigated using a Portland and a Portland composite cement. It was found that capillary suction transports water to the gel pores causing an initial rapid stress development. This is followed by a gradual increase over ca. 3 days governed by the redistribution kinetics of water molecules in the cement gel. The hygral stress developed is proportional to the volume fraction of cement gel. The cement gel itself produces a uniaxial stress of ca. 8 MPa for degrees of restraint above ca. 80%. About 70% of the stress is caused by changes in surface energy at the gel particle/pore water interface; the remainder is due to the disjoining pressure. A change in surface energy of 0.17 J/m² was estimated based on measurements of specific surface and porosity. The development of hygral stress is also controlled by stress relaxation. This appears to be enhanced by the effect of the disjoining pressure which weakens bonds between gel particles to create a more mobile structure under stress.     

Porosity and liquid absorption of cement paste

The moisture behaviour of building materials exposed to the natural climate is largely dependent on their water absorption. In contrast to most building stones, cementitious materials like concrete do not exhibit a water absorption that is proportional to the square root of time. There must be a slowing-down effect which is related to water because the absorption of organic liquids, such as hexane, is quite normal. Measurements of the porosity of hardened cement paste determined by helium pycnometry and water saturation show that water molecules can enter spaces in the microstructure which are not accessible to the smaller helium atoms. Considering the results of dilatation tests both before and after water and hexane saturation, it seems possible that a contraction of capillary pores due to moisture-related swelling of the cement gel leads to the non-linear water absorption over the square root of time

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Résumé Le comportement hydrique des matériaux de construction exposés au climat naturel dépend largement de leur absorption d'eau. Contrairement à la pierre de construction, les matériaux à base de ciment comme le béton ne démontrent pas une absorption d'eau proportionnelle à la racine carrée du temps. Il doit y avoir un effet ralentisseur qui se rapporte à l'eau, car l'absorption des liquides organiques tel que l'hexane est complètement normale. Des mesures de la porosité de pate de ciment durcie déterminées par pycnomètrie d'hélium et par saturation en eau montrent que les molécules d'eau peuvent entrer dans des espaces de la microstructure qui ne sont pas accessibles aux atomes d'hélium, plus petits. Si l'on prend également en considération les résultats des essais de dilatation, avant et après saturation en eau et en hexane, il apparait possible qu'une contraction des pores capillaires due au gonflement du gel de ciment, et qui se rapporterait à l'eau, soit responsable de la non linéarité de l'absorption de l'eau sur la racine carrée du temps.

     

Lateral creep of maturing cement paste

This investigation was conducted to study systematically the lateral creep of maturing cement paste under constant stress-strength ratio. The age at loading ranged from 0.75 to 28.75 days. It has been found from these tests that lateral creep is much more for maturing cement paste than for mature cement paste. When hydration is stopped by cooling to ?11°C, the creep values are the same. The creep Poisson's ratio values tend to be higher for earlier age of loading.     

Electrical conductivity of drying cement paste

Previous research has shown that electrical measurements can be used to monitor moisture movement inside concrete. The interpretation of these measurements is frequently based on empirical relationships between moisture changes and electrical properties of concrete. As such, these empirical relationships can limit the application of the electrical measurements to a specific material or exposure history. To facilitate the development of a general method that is applicable to a concrete member in service, this paper characterizes the electrical conduction in cement paste subjected to drying (desorption) and moisture absorption. The paper quantifies how the electrical conductivity is dependent on the volume and connectivity of the moisture inside the pores and the conductivity of pore solution. This paper also presents a procedure to quantify the contribution of the surface (solid-pore) conduction on the overall conductivity of the cement paste. The results of this investigation contribute to the development of an embedded relative humidity sensor that can be used to monitor changes in the internal humidity of concrete during its service life.