Kenneth A. Snyder

Protected paste volume in concrete: Extension to internal curing using saturated lightweight fine aggregate

Abstract One difficulty in the field use of high-performance concrete is the extensive self-desiccation and autogenous shrinkage that may occur due to its low water/cement ratio and the addition of silica fume to the mixture proportions. Several researchers have proposed the use of saturated lightweight aggregates to provide “internal” curing for the concrete. In this communication, simple equations are developed to estimate the replacement level needed to ensure adequate water for complete curing of the concrete. Additionally, a three-dimensional concrete microstructural model is applied to determine the fraction of the cement paste within a given distance from the lightweight aggregate surfaces. The simulation results are compared with analytical approximations developed previously. This new concept for curing is similar to the protected paste volume concept conventionally applied to characterizing air void systems in air-entrained concrete.

Percolation and pore structure in mortars and concrete

Abstract The cement paste in concrete and mortar has been shown to have a pore size distribution different than that of plain paste hydrated without aggregate. For mortar and concrete, additional porosity occurs in pore sizes larger than the plain pastes threshold diameter as measured by mercury intrusion. Based on the assumption that these larger pores are essentially present only in the interfacial zones surrounding each aggregate, an experimental program was designed in which the volume fraction of sand in a mortar was varied in a systematic fashion and the resultant pore system probed using mercury intrusion porosimetry. The intrusion characteristics were observed to change drastically at a critical sand content. Similar results are observed for a series of mortar specimens in which the cement paste contains 10% silica fume. To better interpret the experimental results, a hard core/soft shell computer model has been developed to examine the percolation characteristics of these interfacial zone pores. Using the model, interfacial zone percolation in concretes is also examined. Finally, the implications of interfacial zone percolation for transport properties and durability of mortar and concrete are discussed.

Calculation of ionic diffusion coefficients on the basis of migration test results

Migration tests are now commonly used to estimate the diffusion coefficients of cement-based materials. Over the past decade, various approaches have been proposed to analyze migration test results. In many cases, the interpretation of test data is based on a series of simplifying assumptions. However, a thorough analysis of the various transport mechanisms that take place during a migration experiment suggests that some of them are probably not valid. Consequently, a more rigorous approach to analyze migration test results is presented. The test procedure is relatively simple and consists in measuring the evolution of the electrical current passing through the sample. Experimental results are then analyzed using the extended Nernst-Planck-Poisson set of equations. A simple algorithm is used to determine for each experiment the tortuosity factor that allows to best reproduce the current curve measured experimentally. The main advantage of this approach resides in the fact that the diffusion coefficients of all ionic species present in the system can be calculated using a single series of data. Typical examples of the application of this method are given. Results indicate that the diffusion coefficients calculated using this approach are independent of the applied voltage and depends only slightly on the concentration level and the chemical make-up of the upstream cell solution.RésuméLes essais de migration sont maintenant couramment utilisés pour estimer les coefficients de diffusion des matériaux cimentaires. Récemment, différentes approches ont été proposées pour analyser les résultats de l’essai de migration. Dans la plupart des cas, l’analyse des mesures est basée sur une série d’hypothèses simplificatrices. Cependant, une étude détaillée des mécanismes de transport des ions présents durant l’essai de migration révèle que certaines de ces hypothèses sont probablement incorrectes. Une approche plus rigoureuse de l’analyse des résultats de l’essai de migration est donc présentée. La méthode consiste à mesurer les courants électriques traversant l’échantillon durant l’essai. Ces résultats sont ensuite analysés à l’aide du système d’équations Nernst-Planck—Poisson. Un algorithme numérique permet de trouver pour chaque essai le facteur de tortuosité permettant de reproduire au mieux la courbe de courant mesurée expérimentalement. L’avantage principal de cette méthode est qu’elle permet de calculer le coefficient de diffusion de chacune des espèces ioniques présente dans le matériau sur la base de cette seule mesure de courant. Des exemples d’utilisation de la méthode sont décrits. Les résultats montrent que les coefficients de diffusion évalués selon cette approche sont indépendants du voltage appliqué au cour de l’essai et qu’ils ne dépendent que très légèrement du niveau de concentration et du type de solution utilisé dans le bac amont du montage.

Estimating the electrical conductivity of cement paste pore solutions from OH-, K+ and Na+ concentrations

A proposed method for estimating the electrical conductivity of cement paste pore solution at 25 °C is based on the concentrations of OH−, K+ and Na+. The approach uses an equation that is a function of the solution ionic strength, and requires a single coefficient for each ionic species. To test the method, the conductivity of solutions containing mixtures of potassium hydroxide and sodium hydroxide with molar ratios of 4:1, 2:1 and 1:1, and having ionic strengths varying from 0.15 to 2.00 mol/l were measured in the laboratory and compared to predicted values. The proposed equation predicts the conductivity of the solutions to within 8% over the concentration range investigated. By comparison, the dilute electrolyte assumption that conductivity is linearly proportional to concentration is in error by 36% at 1 mol/l and in error by 55% at 2 mol/l. The significance and utility of the proposed equation is discussed in the context of predicting ionic transport in cement-based systems.

Using Impedance Spectroscopy to Assess the Viability of the Rapid Chloride Test for Determining Concrete Conductivity.

The suitability of using the initial current from the rapid chloride test (ASTM C 1202) to determine specimen conductivity is tested using impedance spectroscopy with a frequency spectrum of 10 Hz to 1 MHz. The specimen conductivity has an analytical relationship to specimen diffusivity and so is a useful quantity in service life prediction. Measurements made on specimens of different lengths indicate that the total charge passed during the six hour conduction test carried out according to ASTM C 1202 is not a direct measure of specimen conductivity. Further, ohmic heating during the 6 hour test makes it nearly impossible to directly measure any specimen transport property from the results. The total charge passed during the 6 hour conduction test is, therefore, not a reliable quantity for service life prediction. Results indicate that the direct current (dc) measurement of resistance using a voltage of 60 V is sufficient to overwhelm polarization effects, thereby yielding an accurate estimate of the true specimen conductivity. Impedance spectroscopy measurements also indicate that corrosion may form on the brass electrodes, adding bias to a conductivity estimate based upon a dc measurement.

Dimensional analysis of ionic transport problems in hydrated cement systems: Part 1. Theoretical considerations

The validity of the local equilibrium assumption in hydrated cement systems that a particular chemical reaction is instantaneous with respect to transport is examined using a dimensional analysis of electrochemical transport in cementitious materials. The transport equation parameters are scaled, resulting in a dimensionless equation. The dimensionless coefficient for each reaction/transport term determines its relative contribution to the overall process. The diffusion of ions in a reactive porous medium can be fully described on the basis of six independent dimensionless numbers. The analysis demonstrates that the kinetics of the reaction determine the appropriate time constant for the analysis. The formalism is applied to the dissolution of calcium hydroxide under an electrochemical potential gradient. The results are in agreement with previous observations and demonstrate quantitatively the local equilibrium hypothesis is valid in most practical cases where ions are transported by diffusion through a saturated material.

Effect of speciation on the apparent diffusion coefficient in nonreactive porous systems

A combined theoretical and experimental study of the effect that concentration and ionic speciation have on the apparent diffusion coefficient is performed using a nonreactive porous material in a divided cell diffusion apparatus. Varying the ionic species concentration over two orders of magnitude changes the apparent diffusion coefficient by no more than 20% for the systems studied. By contrast, at fixed ionic concentration, varying the ionic species changes the initial apparent diffusion coefficient by a factor of two. Over longer periods of time, the apparent diffusion coefficient varies in time, increasing by a factor of ten or more. For one system, the macroscopic diffusion potential across the specimen induces a transient negative apparent diffusion coefficient; iodide ions are transported from regions of low iodide concentration to regions of high iodide concentration. The theoretical analysis shows that, in nonreactive porous systems, the behavior of all the concentrations and species studied can be completely characterized by an electro-diffusion system of equations that contain two time-independent constants: the porosity and the formation factor. The relationship between these results and the prediction of concrete performance in the field is discussed.

Interfacial zone percolation in concrete: effects of interfacial zone thickness and aggregate shape

Previously, a hard core/soft shell computer model was developed to simulate the overlap and percolation of the interfacial transition zones surrounding each aggregate in a mortar or concrete. The aggregate particles were modelled as spheres with a size distribution representative of a real mortar or concrete specimen. Here, the model has been extended to investigate the effects of aggregate shape on interfacial transition zone percolation, by modelling the aggregates as hard ellipsoids, which gives a dynamic range of shapes from plates to spheres, to fibers. For high performance concretes, the interfacial transition zone thickness will generally be reduced, which will also affect their percolation properties. This paper presents results from a study of the effects of interfacial transition zone thickness and aggregate shape on these percolation characteristics.

Anticipating the Setting Time of High-Volume Fly Ash Concretes Using Electrical Measurements: Feasibility Studies Using Pastes

AbstractOne common concern limiting the proliferation of high-volume fly ash (HVFA) concrete mixtures is the significant delay in setting that is sometimes encountered in field concrete mixtures. While several methods to mitigate the delayed setting times of HVFA mixtures have been demonstrated, a related issue is the prediction of setting times in field mixtures, so that construction operations including finishing and curing can be anticipated and properly scheduled. This paper presents a feasibility study evaluating the employment of simple electrical measurements to predict the setting time of paste mixtures on which concurrent Vicat needle penetration testing was performed. Electrical, setting, and accompanying calorimetry tests are conducted at three different temperatures, each under quasi-isothermal conditions to minimize the confounding influence of temperature variation on the obtained results. Electrical resistance (or heat flow) measurements can be used to adequately predict a mixture’s initial...

Viscosity Modifiers to Enhance Concrete Performance

The hazard rate function for concrete structures is often portrayed as a “bathtub”-shaped curve, with a finite ever-decreasing probability of early-age failures being followed by a life with a relatively low constant probability of failure that ultimately increases dramatically as the end of service is reached. Ideally, new concrete technologies should reduce the failures occurring at both ends of this service-life spectrum. VERDiCT (viscosity enhancers reducing diffusion in concrete technology) is one such strategy based on increasing the pore solution viscosity. This approach has the potential to reduce the propensity for early-age cracking while also reducing long-term transport coefficients of deleterious ions such as chlorides. In this paper, the performance of a typical VERDiCT admixture—a viscosity modifier/shrinkage-reducing admixture— is investigated in mortar and concrete, both as an addition to the mixing water and as a concentrated solution used to pre-wet fine lightweight aggregates. A reduction in early-age cracking is achieved by eliminating autogenous shrinkage stresses that typically develop in lower water-cementitious material ratio (w/cm) concrete. By substantially increasing the viscosity of the pore solution in the concrete, the resistance to ionic diffusion is proportionally increased relative to a control concrete without the VERDiCT admixture. Herein, chloride ion diffusion coefficients are evaluated for two types of concrete containing typical substitution levels of supplementary cementitious material —namely, either 25% fly ash or 40% slag by mass. For the eight concrete mixtures investigated, the effective diffusion coefficient was reduced by approximately 33% by adding the VERDiCT admixture which, in practice, may imply a 50% increase in their service life, while the autogenous shrinkage was virtually eliminated. However, these benefits in early-age cracking resistance and long-term durability are tempered by up to a 20% reduction in compressive strength that may need to be accounted for at the design stage.