New method to measure the rapid chloride migration coefficient of chloride-contaminated concrete

The apparent chloride diffusion coefficient, D_app, which is obtained by fitting chloride profiles as the result of time-consuming immersion tests can be substituted in a model on chloride ingress by the rapid chloride migration (RCM) coefficient of concrete, D_RCM, which is determined under electrically accelerated conditions. Until now, it was not possible to measure D_RCM of chloride-contaminated concrete, as already inherent chlorides interfere with the common colorimetric indicator used for penetration depth measurements. Furthermore, carbonation may also interfere with the penetration depth reading. To overcome these problems, the regular test has been modified by using iodide as penetrating ion and iodate-starch acetic acid as indicator. A strong linear relationship between the regular RCM test and the newly developed Rapid Iodide Migration (RIM) Test was found. Carbonated specimens can be tested using the RIM test without additional interference. Thus, the new method enables the quantification of the actual concrete quality sampled from existing structures during a condition assessment.     

Nitrite diffusivity in calcium nitrite-admixed hardened concrete

The apparent diffusivities (D_app) of nitrite in concrete were estimated by monitoring time-dependent concentrations of nitrite leached into water from calcium nitrite-admixed hardened concrete specimens. Experiments were conducted with five different concrete mixes and with deionized water (DI), limewater, or synthetic seawater as the leaching agents. The D_app in Type II Portland cement concrete for long curing times and a w/c ratio of 0.40 was ∼1.7×10⁻⁸ cm²/s when leached at 22 °C with limewater. The D_app was relatively insensitive to nitrite dosage and to DI or limewater as the leaching agent, but an increase in the w/c ratio to 0.49, or an increase in temperature by ∼14 °C, increased D_app by ∼50%. A 20% Type F fly ash cement replacement reduced the apparent diffusivity by ∼60%. The D_app decreased with concrete curing time. The magnitude of the D_app and its dependence on concrete and exposure parameters were comparable to those observed in the transport of chloride ions in concrete.     

Solid-liquid contacting effectiveness in trickle bed reactors

Internal and external effective wetting of a porous catalyst in a trickle bed reactor was considered. Experimental tests were carried out based on the analysis of the response curves of the reactor to a step decrease of the tracer inlet concentration.The pore filling of the catalyst pellets was practically total even at very low liquid flow rates, probably due to capillarity.The external effective wetting was interpreted in terms of an apparent internal diffusivity (D_i)_app, determined on the basis of a model which assumes a total external wetting of the catalyst. The values of (D_i)_app increased with the liquid flow rate, tending to the actual internal diffusivity D_i determined in a full reactor. The found values of (D_i)_app/D_i were used to interpret the ratio of the apparent kinetic rate constant determined in a trickle reactor k_app and that for a totally wetted catalyst k_v.The calculated values agree substantially with those proposed by Satterfield[3] from kinetic tests.     

Characteristic service life for concrete exposed to marine environments

A statistical treatment has been applied to a deterministic service life model of concrete structures in marine environments. The chloride ingress model based on Fick's second law of diffusion was assumed. The quality of concrete was quantified in terms of three factors, namely, an apparent diffusion coefficient at 1 year (D_a), surface chloride concentration (C_s) and a critical chloride level (C_cr). The standard deviation of service life can be estimated from standard deviations of the four factors, namely, C_s, D_a, C_cr and cover depth. The effect of the severity of environment on service life was also demonstrated. With data from the literature and an approximation of the inverse error function, sensitivity analyses were carried out. Service life was found to be more sensitive to cover depth than the diffusion coefficient, and more sensitive to surface chloride concentration than the critical chloride level. Characteristic service life of a range of normal Portland cement (NPC) concrete grades was evaluated as a function of 28-day strength and cover depths for a nominated confidence level. Such characteristic service life can be readily used and appreciated by design engineers.     

Spinodal decomposition as a probe to measure the effects on molecular motion in poly(styrene-co-acrylonitrile) and poly(methyl methacrylate) blends after mixing with a low molar mass liquid crystal or commercial lubricant

The effects on molecular motion observed through early stage phase separation via spinodal decomposition, in melt mixed poly(styrene-co-acrylonitrile) (SAN) containing 25% by weight of acrylonitrile (AN) and poly(methyl methacrylate) (PMMA) (20/80 wt%) blends after adding two low molar mass liquid crystals (CBC33 and CBC53) and two lubricants (GMS and zinc stearate) were investigated using light scattering techniques. The samples were assessed in terms of the apparent diffusion coefficient (D_app) obtained from observation of phase separation in the blends. The early stages of phase separation as observed by light scattering were dominated by diffusion processes and approximately conformed to the Cahn-Hilliard linearised theory. The major effect of liquid crystal (LC) was to increase the molecular mobility of the blends. The LC generally increased the Cahn-Hilliard apparent diffusion coefficient, D_app, of the blend when added with concentrations as low as 0.2 wt%. GMS and zinc stearate can also improve the mobility of the blend but to a lesser extent and the effect does not increase at higher concentration. On the other hand, the more LC added, the higher the mobility. In all systems the second derivative of the Gibbs free energy becomes zero at the same temperature. The improved mobilities therefore seem to arise from changes in dynamics rather than thermodynamic effects.     

Experimental study of swelling and shrinking kinetics of spherical poly(N,N-diethylacrylamide) gel with continuous phase transition

The kinetics of swelling and shrinking of spherical particles of poly(N,N-diethylacrylamide) (DEAA) gel, prepared with static mixing technology, was investigated experimentally. The shrinking process was separated into late shrinking and initial shrinking by the skin layer effect. When the final temperature of temperature swing experiments was above the lower critical solution temperature (LCST) of polyDEAA, the apparent increase in the response rate constant (k_v) and the polymer diffusion constant (D) were due to reduction of the friction coefficient between polymer chains and water, arising from decrease in the water viscosity. When the final temperature was below the LCST, the main factor determining k_v and D was the change in the network structure of DEAA particles caused by the final temperature. The initial temperature affected the values of k_v and apparent diffusion constant D_app only in initial shrinking, in which the decrease in D with increase in the initial temperature came mainly from the spatial limitation.     

Effect of decreasing film thickness on the electrochemical responses of cations adsorbed in clay-modified electrodes

Clay-modified electrodes ranging in thickness from 3.4 μm to 8 nm, as estimated from the clay loadings, were prepared using three different smectites by spin-coating, solvent evaporation or electrophoretic deposition. For all three clays, the voltammetic waves obtained for [Ru(bpy)₃]²⁺ or [Os(bpy)₃]²⁺ adsorbed in these CMEs were independent of the film thickness for all films thicker than 100 nm. Only in very thin films, ≤40 nm were significant decreases in the peak currents observed. However, when the contributions to the peak currents from the electroactive concentrations, C* and effective diffusion coefficients, D_eff were separated, the values of C* were found to increase with decreasing film thickness, while D_eff decreased by several orders of magnitude. This was attributed to increase contributions to the electrochemical responses from less mobile electrostatically bound cations in the thinner films. Similar variations in C* and D_eff were obtained in films prepared by solvent evaporation. However, C* obtained in 20 nm thick electrodeposited films were significantly lower than in 40 nm spin-coated films. For [Ru(NH₃)₆]³⁺, the peak currents increased rapidly with the film thickness. However, no significant changes in the values of C* and D_eff with film thickness were found for this ion. This is consistent with the greater mobility of [Ru(NH₃)₆]³⁺ in clays films that allows a larger fraction of the adsorbed ions to remain electroactive even in thicker films. Results obtained for [Fe(bpy)₃]²⁺ were intermediate. While, the peak currents were independent of film thickness, the values of C* or D_eff obtained for this ion were also independent of the clay loadings.     

Comparative study of methods used to estimate ionic diffusion coefficients using migration tests

Ionic diffusion coefficients are estimated rapidly using electromigration tests. In this paper, electromigration tests are accurately simulated by numerically solving the Nernst-Planck (NP) equation (coupled with the electroneutrality condition (EN)) using the finite element method. Numerical simulations are validated against experimental data obtained elsewhere [E. Samson, J. Marchand, K.A. Snyder, Calculation of ionic diffusion coefficients on the basis of migration test results, Materials and Structures/Matériaux et Constructions 36 (257) (2003) 156-165., H. Friedmann, O. Amiri, A. Aït-Mokhtar, A direct method for determining chloride diffusion coefficient by using migration test, Cement and Concrete Research 34 (11) (2004) 1967-1973.]. It is shown that migration due to the non-linear electric potential completely overwhelms diffusion due to concentration gradients. The effects of different applied voltage differences and chloride source concentrations on estimations of chloride diffusion coefficients are explored. We show that the pore fluid within concrete and mortar specimens generally differs from the curing solution, lowering the apparent diffusion coefficient, primarily due to interactions of chloride ions with other ions in the pore fluid. We show that the variation of source chloride concentration strongly affects the estimation of diffusion coefficients in non-steady-state tests; however this effect vanishes under steady-state conditions. Most importantly, a comparison of diffusion coefficients obtained from sophisticated analyses (i.e., NP-EN) and a variety of commonly used simplifying methods to estimate chloride diffusion coefficients allows us to identify those methods and experimental conditions where both approaches deliver good estimates for chloride diffusion coefficients. Finally, we demonstrate why simultaneous use and monitoring of current density and fluxes are recommended for both the non-steady and steady-state migration tests.     

Influence of morphology on the transport properties of polystyrene/polybutadiene blends: 2. Modelling results

Unsteady-state and steady-state models are used to predict the effective diffusion coefficients, D_eff, of small molecules in randomly interspersed polystyrene-polybutadiene blends and comparisons are made with experimental values obtained by sorption measurements. It is found that the polybutadiene and polystyrene phases are not topologically equivalent. This fact is responsible for the discrepancies between the experimental values of D_eff and the predictions from standard steady-state models.Simulations of sorption experiments based on an unsteady-state model have shown that values of D_eff obtained by different methods from the same sorption experiment differ consistently from each other. This supports the hypothesis that the experimentally observed differences between the various values of D_eff, for a given blend-permeant pair, can be justified solely on the basis of morphological arguments.     

Experimental investigation and numerical modeling of chloride penetration and calcium dissolution in saturated concrete

Chloride penetration and calcium dissolution have been investigated for a saturated concrete after exposure to a 0.5 mol/L NaCl solution for a period of up to 3150 days. Simultaneous ion transport model (SiTraM) that allows the transport of chloride and calcium ions to be simultaneously simulated in a hydrated cement system has been used to verify the experimental results.Self-compacting concrete (SCC) with a water to cement ratio of 0.3 resulted in a limited chloride penetration depth while the calcium dissolution was also reduced within the near surface zone. Increased unit water content for normal concrete resulted in higher chloride penetration depth and larger dissolution front of Ca(OH)₂ regardless of having the same water to cement ratio.It was revealed that the SiTraM can predict the profiles of chloride and calcium for self-compacting concrete. It was also found that the primary factor to control chloride penetration front and the dissolution front of Ca(OH)₂ was the pore structure characteristic of concrete.     

Evaluation of the bond properties between concrete and reinforcement as a function of the degree of reinforcement corrosion

In order to investigate the effect of reinforcement corrosion on the bond properties between concrete and reinforcement, the pullout tests were conducted using reinforcements embedded in concrete specimens, which were corroded by an accelerated electric corrosion method. A finite element method (FEM) analysis was also carried out on the basis of the results of the pullout tests. The maximum bond strength (τ_max) and the bond rigidity (D_s) of specimens decreased in proportion to the increase of corrosion percentage (Δ_w), respectively. Also, the curves of bond stress-free end slip could be analyzed by the FEM, if the τ_max and D_s were determined as a function of corrosion percentage. The equations for calculating the maximum bond strength and the bond rigidity necessary for an FEM analysis of RC members with corroded reinforcements were obtained by the experiments and the FEM analysis of the pullout tests.     

Gas-liquid interface temperature rise in the case of temperature-dependent physical,transport and reaction properties

This paper anlyzes the problem of gas-liquid interface temperature rise for a first order gas-liquid reaction when the diffusion coefficient, solubility and reaction rate coefficient are dependent upon temperature. The analysis indicates that as long as the quantity ?_eff, which is numerically equal to ?_S − (? + ?_R/2 (wherein ?_S, ?_D and ?_R are the dimensionless temperature-solubility, temperature-diffusion coefficient and temperature-reaction rate constant coefficients) is zero, Danckwerts' analysis [1–3] is useful for the calculations of large rise in gas-liquid interface temperature. For ?_eff ≠ 0, the paper presents approximate empirical correlations for calculating gas-liquid interface temperature rise with the knowledge of Danckwerts' analysis and the magnitude of ?_eff.     

Sorption of dissolved organics from aqueous solution by polystyrene resins—II. External and internal mass transfer

External and internal mass transfer were characterized for the sorption of 2,4-dichlorophenol and p-nitrophenol on polystyrene resins. An evaluation method is presented which estimated the external mass-transfer coefficient, κ_L, from the concentration uptake in an integral fixed-bed reactor. The experimental κ_L values are compared with values predicted using four different empirical correlations. The effect of axial dispersion on the values of κ_L was shown to be small under the given conditions.To quantify the intraparticle mass transfer, an effective diffusion coefficient, D_eff, was estimated using the pore diffusion model interpretation of data from a differential column reactor within a recirculated batch system. The D_eff values depend on both resin structure and solute properties. Values of D_eff between 0.3 × 10⁻⁹ and 1.8 × 10⁻⁹ m²/s were observed for macroporous resins, which exceed values that are characteristic of pore diffusion, and a value of 0.6 × 10⁻¹¹ m²/s was estimated for a microporous resin.     

Drying kinetics and microstructure evolution of nano-zirconia under microwave pretreatment

The effects of microwave power and sample quality on microwave drying kinetics and characteristics of zirconia were studied. It is found that by increasing the microwave power and decreasing the sample mass, the surface diffusion coefficient (D_eff) appears to an upward tendency. The corresponding value D_eff at a sample mass of 10, 20, 30, and 40g are 1.849E-14, 2.443E-14, 3.210E-14, and 3.278E-14 m²/s, respectively. The corresponding value D_eff at a microwave power of 300, 400, 500, 600, and 700W are 1.270E-14, 1.784E-14, 2.619E-14, 3.392E-14, and 4.497E-14 m²/s, respectively. Besides, the materials were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FT-IR) to evaluate the changes of materials before and after drying. The results show that microwave accelerates the drying of zirconia and increases its dispersibility. The heat conduction direction of microwave drying is the same as that of moisture diffusion, which avoids being affected by heat inertia and heat transfer loss. The drying process is fast and efficient, and the microwave directly penetrates the product, avoiding the disadvantage of slow evaporation caused by the temperature gradient.     

Effect of citric acid and blanching pre-treatments on drying and rehydration of Amasya red apples

Drying behaviour of red apples was experimentally examined in a laboratory dryer at drying temperatures of 55, 65 and 75 °C and a constant air velocity of 2.0 m/s. Two pre-treatments (blanching and 0.5% citric acid) were applied to prior to drying process. It was observed that both the drying temperature and pre-treatment affected the drying time. The shortest drying times were obtained from pre-treated samples with citric acid solution. Blanched samples have higher rehydration ratios than other samples. Five mathematical models namely, Newton, Henderson and Pabis, Page, Logarithmic and Parabolic models were evaluated in the kinetics research. The fit quality of the proposed models was evaluated by using the determination of coefficient (R²), reduced chi-square (χ²), root means square error (RMSE) and mean relative percent error (P). The Parabolic model provided the best representation of data. Effective moisture diffusivity (D_eff) computed on the basis of Fick's second law. D_eff value of pre-treated samples with citric acid solution was higher than the other samples.     

Influence of the applied voltage on the Rapid Chloride Migration (RCM) test

This study addresses the influence of the applied voltage (electrical field) on the value of the chloride migration coefficient, as determined with the Rapid Chloride Migration (RCM) test, and on other properties of cement based mortars. It is shown that in the investigated ranges of applied voltages, the chloride migration coefficients, computed from two different chloride transport models, are relatively constant. However, other properties of mortars are changing due to the application of the electrical field. It is shown that the resistance of the test samples increases during the migration test (therefore the DC current during the test decreases). Moreover, the mass of the samples increases and this increase is found to be proportional to the chloride penetration depth. The pH of the catholyte solution (10% NaCl water solution) increases significantly during the migration test, thus the test conditions change as the OH^? to Cl^? proportion changes. Furthermore, the measured values of the polarization of the electrodes confirm the value of 2 V, assumed in the guidelines for the RCM test. Also, a dark coloration is observed on samples split after the test, prior to spraying with the colorimetric chlorides indicator. This coloration is attributed to a liquid-saturation of the samples only in the colored region and not in the entire volume of the sample, as is assumed after performing the vacuum-saturation prior to the migration tests.     

Corrosion resistance and chloride diffusivity of volcanic ash blended cement mortar

This article reports the results of an investigation on the chloride diffusivity and corrosion resistance of volcanic ash (VA) blended cement mortars with varying curing times of up to 1 year. The mortars had 20% and 40% VA as cement replacement and water/binder ratio of 0.55. The accelerated chloride ion diffusion (ACID) test was used to calculate the chloride ion (Cl⁻) diffusion coefficient (D_i) of the mortars using the Nernst-Plank equation for steady state condition. In addition, electrical resistivity, mercury intrusion porosimetry and differential scanning calorimetry (DSC) tests were conducted. Electrochemical measurement such as linear polarization resistance was used to monitor the corrosive behavior of the embedded steel bars. The chloride ingress into the mortars was also studied. Good correlations were found among D_i, total pore volume (TPV) and electrical resistivity of the mortars. It was also found that blending cement with VA significantly reduced the long-term D_i and hence increased the long-term corrosion resistance of mortars. This fact was also supported by the presence of lower quantity of Ca(OH)₂ and higher quantity of Friedel's salt in the VA blended mortars as observed from the DSC tests. Mortars with 40% VA showed better performance in terms of Cl⁻ diffusivity, chloride ingress and passivation period of embedded steel compared with the control mortar with 0% VA.     

Apparent diffusion coefficient of intercalated species measured with PITT: A simple formulation

Apparent values (D_ap) of chemical diffusion coefficients (D) of intercalated species are measured with the potentiostatic intermittent titration technique (PITT) due to the usual approximations of infinitely fast charge-transfer kinetics at the electroactive interface and negligible Ohmic drop effects. A simple expression is derived in this work that enables one to predict the parametric dependence of the diffusion coefficient ratio (D_ap/D) on the ‘key-factor’ (Λ) for insertion/extraction processes. The parameter Λ can be either experimentally determined from electrochemical impedance spectra, or predicted from theoretical expressions of insertion/extraction reaction rate. The measurement error of diffusion coefficient is predicted theoretically due to slow charge-transfer kinetics at the interface, Ohmic drop effects and bulk interactions in the host material. The influence of electrode or particle geometry is also analyzed. In most cases, but not in all cases, a minimum value of apparent diffusion coefficient is observed near the standard potential of insertion material. The measurement error of D becomes very important in the presence of attractive interactions in host materials, together with slow charge-transfer kinetics at the electroactive interface. Finally, the superposition between spurious behavior of the chemical diffusion coefficient and its dependence on the guest ions interactions is investigated within the framework of Frumkin insertion isotherm.     

Nonlinear phase‐separation behavior of poly(methyl methacrylate)/poly(styrene‐co‐maleic anhydride) blends

The nonlinear phase‐separation behavior of poly(methyl methacrylate)/poly(styrene‐co‐maleic anhydride) (PMMA/SMA) blends over wide appropriate temperature and heating rate ranges was studied using time‐resolved small‐angle laser light scattering. During the non‐isothermal process, a quantitative logarithm function was established to describe the relationship between cloud point (T_c) and heating rate (k) as given by T_c = Alnk + T₀, in which the parameter A, reflecting the heating rate dependence, is much different for different compositions due to phase‐separation rate and activation energy difference. For the isothermal phase‐separation process, an Arrhenius‐like equation was successfully applied to describe the temperature dependence of the apparent diffusion coefficient (D_app) and the relaxation time (τ) of the early stage as well as the late stage of spinodal decomposition (SD) of PMMA/SMA blends. Based on the successful application of the Arrhenius‐like equation, the related activation energies could be obtained from D_app and τ of the early and late stages of SD, respectively. In addition, these results indicate that it is possible to predict the temperature dependence of the phase‐separation behavior of binary polymer mixtures during isothermal annealing over a range of 100 °C above the glass transition temperature using the Arrhenius‐like equation. © 2012 Society of Chemical Industry