The apparent and effective chloride migration coefficients obtained in migration tests

The apparent (D_app) and effective (D_eff) migration coefficients obtained in chloride migration tests are investigated in this study. The presented D_app profiles in concrete show that the apparent migration coefficient is strongly concentration-dependent. As demonstrated, the binding of chlorides during the migration tests is very low at low free-chloride concentrations and therefore the chloride penetration front progresses throughout the concrete only slightly retarded by the binding. The diffusion flux during migration tests is shown to be insignificant compared to the migration flux. The D_RCM obtained from the Rapid Chloride Migration (RCM) tests are found to be equal to the computed D_app at the locations of the chloride penetration fronts, which gives an indication that the D_RCM represents only the migration coefficient at the front. A linear correlation is found between the D_RCM obtained from the traditional RCM model and the D_eff obtained from the chloride transport model which includes non-linear chloride binding and concentrations in non-equilibrium.     

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.     

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.     

Diffusion of chloride ions from seawater into concrete

The rates of chloride diffusion from seawater into concrete made with different w/c ratios and different types of cements were studied. The effect of cathodic protection on the chloride penetration was also investigated. The effect of w/c ratio on the chloride content was limited mainly to a surface layer of the concrete and to short durations of exposure. For longer durations of exposure and for greater depths of penetration, cement type had a greater influence on the depth of chloride penetration than the w/c ratio. Portland cements gave as much as two to five times higher chloride penetration than blended cements. For the Portland cements as much as 8.6% C₃A was not adequate for reducing the chloride penetration compared with zero-content of C₃A. It appears that the diffusion of chloride ions into concrete is dependent not only on the permeability and the capacity of chloride binding but also on the ion exchange capacity of the system.     

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.     

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.     

氯离子扩散系数快速测试方法综述

氯离子扩散系数是评价混凝土结构抗氯离子渗透性能和使用寿命的重要指标,选择一种快速、准确的测试方法至关重要.本文以非稳态电迁移实验(Rapid Chloride Migration,RCM)为基准,与国内外常用的几种氯离子扩散系数快速测试法比较,总结每种方法的特点及适用范围,建立RCM法与其他测试方法的相关性.结果表明:...     

Field indicator of chloride penetration depth

Chloride-induced corrosion of steel reinforcement is causing serious damage to many concrete structures. A number of methods to evaluate the chloride penetration into concrete have been developed. The most common practice in measuring the chloride profile is very time consuming. A simple colourimetric method of measuring the depth of chloride penetration into concrete by spraying a 0.1-N AgNO₃ solution is very attractive. But some questions have been raised about its sensitivity. In this article, the results from colourimetric tests and the corresponding quantity of chloride detected at the colour-change boundary, determined for more than 70 concrete samples, are given. The magnitude and variation of chloride concentrations are compared with those reported in the literature and evaluated in relation to typical chloride threshold values. In addition, a relationship between the colourimetric penetration depth and the charge passed during testing to ASTM C1202 is shown.     

固体废弃物作集料对混凝土氯离子传输性质的影响

采用再生集料、钢渣作集料,研究了集料种类及体积掺量对不同胶凝材料组成配制混凝土氯离子传输行为的影响。研究发现,随集料体积掺量的增加,不同胶凝材料组成配制普通集料混凝土和钢渣集料混凝土的氯离子迁移系数先减小后增大,在集料体积掺量为45%时达到最小值;再生集料混凝土的氯离子迁移系数逐渐增大。但钢渣集料替代普通集料并未对混凝土抗氯离子渗透性能产生明显影响。在不同胶凝体系中,掺合料的掺入显著降低了混凝土的氯离子迁移系数,其中矿粉的效果优于粉煤灰。     

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.     

Laboratory investigation of reinforcement corrosion initiation and chloride threshold content for self-compacting concrete

Time-to-corrosion (T_i) of reinforcement in concrete and chloride threshold content (C_th) are important service life determinants for reinforced concrete structures in chloride-laden environments. In this study, the two determinants of a series of self-compacting concretes (SCC) and regular concretes were experimentally investigated. A new sampling approach for C_th determination (milling powder from corrosion active site at the rebar/concrete interface) was adopted to accurately express chloride content resulting in corrosion occurrence. It is found that the T_i and C_th follow the 3-parameter Weibull distribution. The results indicate that the corrosion initiation of rebar in concrete slabs depends upon both cement alkalinity and superplasticizer. Rebar, embedded in high alkalinity cement SCC, exhibits better corrosion resistance as indicated by the longer T_i, higher C_th and larger Weibull modulus, m. A larger Weibull modulus indicates that anti-corrosion performance of rebar in slabs is more stable and less scattered. The effects of specific superplasticizer on rebar corrosion resistance are discussed from the viewpoint of air void amount and size distribution at the rebar/concrete interface.     

Effect of hydroxyl ions on chloride penetration depth measurement using the colorimetric method

In this paper, the effect of hydroxyl ions on chloride penetration depth measurement using the colorimetric method was studied. Equivalent silver nitrate solution (i.e. Ag⁺ = Cl⁻) was added to the NaCl + NaOH solution with different concentrations, then the amount of precipitated silver chloride and silver oxide were determined by chemical methods, and the color of the precipitated products was examined. Results show that the amount of silver chloride formed decreases linearly as OH⁻ to Cl⁻ ratio (r) increases. Thus, the chloride concentration at color change boundary changes with the pH value of the concrete. AgCl has a white color, while Ag₂O has a dark brown color. When the value of r exceeds 4, the color of the mixture looks brown, and color change boundary cannot be easily distinguished.     

The experimental investigation of concrete carbonation depth

Phenolphthalein indicator has traditionally been used to determine the depth of carbonation in concrete. This investigation uses the thermalgravimetric analysis (TGA) method, which tests the concentration distribution of Ca(OH)₂ and CaCO₃, while the X-ray diffraction analysis (XRDA) tests the intensity distribution of Ca(OH)₂ and CaCO₃. The Fourier transformation infrared spectroscopy (FTIR) test method detects the presence of C-O in concrete samples as a basis for determining the presence of CaCO₃. Concrete specimens were prepared and subjected to accelerated carbonation under conditions of 23 °C temperature, 70% RH and 20% concentration of CO₂. The test results of TGA and XRDA indicate that there exist a sharp carbonation front. Three zones of carbonation were identified according to the degree of carbonation and pH in the pore solutions. The TGA, XRDA and FTIR results showed the depth of carbonation front is twice of that determined from phenolphthalein indicator.     

Radial gas mixing in a fluidized bed using response surface methodology

Radial gas mixing in a fluidized bed was studied using response surface methodology (RSM), which enables effect examinations of parameters with a moderate number of experiments. All experiments were conducted in a 0.29-m ID fluidized-bed cold model. The gas dispersion process within the bed is described using the dispersed plug flow model. Pure carbon dioxide was used as the tracer gas, continuously injected into the center of the bed by a point source. The downstream radial tracer concentration profile was measured using a gas chromatograph.The radial gas dispersion coefficient, D_r, was well correlated with operating parameters and the particle and gas properties: (U−U_mf)/U_mf, H_s/d_b, φ_d, and Ar, with a determination coefficient R² of 0.966. Effect test indicates that the dimensionless characteristic velocity, (U−U_mf)/U_mf, has the most significant influence on D_r, while the static bed height to bed diameter ratio, H_s/d_b, is less remarkable. The interactions of (U−U_mf)/U_mf with the distributor open-area ratio, φ_d, and with the Archimedes number, Ar, both play important roles. An evolutive response surface model was proposed to describe the radial gas mixing in the bubbling/slugging fluidization regimes.     

Cure depth for photopolymerization of ceramic suspensions

The cure depth of a series of photopolymerizable SiO₂ and Al₂O₃ ceramic suspensions was measured as a function of energy dose to determine the sensitivity parameter D_p and its dependence on ceramic volume fraction, type and concentration of photoinitiator and inert dye. As predicted by an Absorption Model for D_p, 1/D_p is a linear function of photoinitiator concentration and dye concentration. The molar extinction coefficients derived from the cure depth measurements using an Absorption Model were compared with coefficients determined via spectrophotometry.     

Dynamic light scattering measurements of polystyrene in semidilute theta solutions

Measurements of the co-operative diffusion coefficient, D_c, and a centre of mass translational diffusion coefficient, D_s, have been made by dynamic light scattering for the polystyrene-cyclohexane theta system as a function of molecular weight and concentration. For semidilute solutions it is established that D_s∝N^?2c^?3 which is in agreement with the predictions from scaling arguments for the self-diffusion coefficient. However, if the co-operative mode is interpreted by D_c∝N^xc^y, the results of 0 < x < 0.7 and 0 < y < 0.5 are in disagreement with scaling predictions of D_c∝N⁰c¹. A discussion of the assumptions and potential shortcomings of the blob model which is used in the derivation of the power law predictions and the dynamic scattering equations is included. In addition, monomeric friction coefficients have been obtained from the D_s results within the framework of Doi-Edwards model. A comparison is made of the concentration dependence of the monomeric friction coefficient from the present data to that from similar experiments on a good solvent (tetrahydrofuran) system and from shear relaxation modulus measurements on the polystyrene in Aroclor 1248.     

Expression of the transportation of corrosive media into epoxy coating using nano-indentation

Nano-hardness (H_nano) and nano-elastic modulus (E_nano) of corroded coating surface layer were investigated using nano-indentation after epoxy coated specimens were immersed in 5% NaCl solution. The penetration depths (d) of corrosive media into epoxy coating were calculated fitting the curves of (H/E)_nano vs. indentation depth (h). EIS spectra of epoxy coated specimens immersed in 5% NaCl solution for different time were measured. Finally the relationship of capacitance (C_f,T) of corroded epoxy coating with the penetration depth of corrosive media was studied. The experimental results showed that the transportation process of corrosive media into epoxy coating was studied through the profiles of (H/E)_nano with indentation depth. The penetration depth of corrosive media into the epoxy coating increases rapidly with immersed time firstly, and then increases slowly. The capacitance of corroded epoxy coating has a linear function relationship with the penetration depth of corrosive media into epoxy coating. Nano-indentation can be used to study the transportation process of corrosive media into organic coating.     

Knudsen diffusion of various gases through a barium sulphate packed bed

The diffusion coefficients, D_k, of helium, krypton, carbon dioxide and air through a packed bed of barium sulphate calculated from the Knudsen diffusion flow equation were found to be a cumulative parameter obtained from the contribution of the diffusion coefficient through the voids of the packed bed, D_v, and the diffusion coefficient through the pores, D_p, within barium sulphate.The surface area of compacted and uncompacted material obtained from low temperature gas adsorption and the Brunauer, Emmett and Teller equation (BET), together with the void sizes and pore sizes in compacted material obtained by Barrett, Joyner and Halenda (B.J.H.) isotherm analysis, when compared with the surface area values obtained by Knudsen diffusion and permeametry, indicated that the coefficients D_v and D_p evaluate the void and pore areas in compacted barium sulphate, respectively.     

Photopolymerization of acrylate suspensions with visible dyes

Polymerization behavior of silica loaded diacrylate suspensions in the presence of color changing dyes was studied via cure depth measurements and photorheology. The dyes have their maximum absorption peak in the visible range; however, spectrophotometry revealed significant absorption also in the UV range where the photoinitiators are active. Thus, the dyes behave as other inert dyes and they affect the curing behavior. The effect of the dye concentration on the sensitivity D_p, critical energy dose E_c and time parameters determined via photorheology was studied and the data was fit to theoretical models. The measured values of 1/D_p, E_c and the photorheology time parameters linearly increase with increasing dye concentration c_D as predicted by the models.