Enhancement of barrier properties of cement mortar with graphene nanoplatelet

The transport properties of cement mortar with graphene nanoplatelet (GNP) are investigated experimentally in this study. GNP, a low cost carbon-based nano-sheet, was added to mortar at contents of 0, 2.5, 5.0 and 7.5%, by weight of cement. The water penetration depth, chloride diffusion coefficient and chloride migration were determined for cement mortar with GNP and compared with plain cement mortar specimens. Test results showed that the addition of 2.5% GNP can cause significant decrease of 64%, 70% and 31% for water penetration depth, chloride diffusion coefficient and chloride migration coefficients respectively. The reduced water and ions ingress can be partially attributed to a reduction in the critical pore diameter of about 30%. This refinement of the microstructure by the GNP is validated by the mercury intrusion porosimetry (MIP) results. The impermeable GNP also contributes to the reduced permeability due to the increased tortuosity against water and aggressive ions ingress.     

On the relationship between pore structure and chloride diffusivity from accelerated chloride migration test in cement-based materials

In this study the electrochemical technique is applied to accelerate chloride ion migration in concrete to determine the chloride ions in anode cell, and the pore characteristic of the same mortars in concrete was obtained from the mercury intrusion porosimetry (MIP). The plain ordinary Portland cement and the constant aggregate with eight w/c ratios ranging from 0.3 to 0.65 were used. A good linear relationship between the steady-state migration coefficient and non-steady-state migration coefficient based on the same experimental setup and specimens was obtained. Both of the steady-state migration coefficient and non-steady-state migration coefficient were linearly related to the capillary pore volume and the critical pore diameter.     

Influence of traversing crack on chloride diffusion into concrete

This study examined the effects of traversing cracks of concrete on chloride diffusion. Three different concretes were tested: one ordinary concrete (OC) and two high performance concretes with two different mix designs (HPC and HPCSF, with silica fume) to show the influence of the water/cement ratio and silica fume addition. Cracks with average widths ranging from 30 to 250 μm, were induced using a splitting tensile test. Chloride diffusion coefficients of concrete were evaluated using a steady-state migration test. The results showed that the diffusion coefficient of uncracked HPCSF was less than HPC and OC, but the cracking changed the material behavior in terms of chloride diffusion. The diffusion coefficient increased with the increasing crack width, and this trend was present for all three concretes. The diffusion coefficient through the crack D_cr was not dependent of material parameters and becomes constant when the crack width is higher than  80 μm, where the value obtained was the diffusion coefficient in free solution.     

Fixed bed modelling for acid dye adsorption onto activated carbon

The adsorption of three separate acid dyes onto activated carbon has been studied using fixed bed adsorption. A film‐pore diffusion model was developed and applied to the experimental breakthrough curve data. A sensitivity analysis showed that pore diffusion was the dominant mass transport mechanism. Pore diffusion coefficients were determined by an optimization routine with a minimization of the sum of errors squared. The external mass transfer coefficients were sensitive to the external fluid dynamic parameters, such as liquid flow rate and mean particle size. These fluid dynamic parameters did not affect the effective diffusion coefficient. The effective diffusion coefficient was not affected by changes in the fluid dynamic parameters but did change with differing initial dye concentrations. This may be due to a contribution from surface diffusion to the effective diffusion coefficient. Copyright © 2003 Society of Chemical Industry     

Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions

Electrochemical chloride extraction (ECE) is used for the rehabilitation of chloride-contaminated concrete. Anions such as chloride and hydroxide are pushed away from the cathode (reinforcement), and cations such as sodium, potassium and calcium are attracted to the cathode. During ECE an increase of the concrete resistance can be observed. The results of a petrography study on ECE-treated concrete are presented in this paper. It also investigates the influence of pore size on ion migration using a concrete substitute model with known pore size. Findings showed that the pore size and pore size distribution of concrete are altered due to ECE. It is therefore suggested that concrete acts as active migration medium in the migration process by releasing ions into the pore solution. Moreover, small pores hinder the migration of ions, which may partially be responsible for changes in concrete resistance.     

Determination of the activation energies of diffusion of organic molecules in poly(ethylene terephthalate)

Poly(ethylene terephthalate) (PET) is a highly inert packaging material that exhibits low interaction with foodstuff and consequently a limited diffusion of migrants. Migration modeling can therefore be used as an alternative to experimental migration tests in order to confirm compliance of PET packaging materials with food laws. The most important factor for predicting migration using mathematical models is the diffusion coefficient of the migrant in PET. However, current models that predict this parameter are typically based on worst‐case scenarios and thereby significantly over‐estimate the degree of migration. The key parameter for developing more realistic migration models is the activation energy of diffusion of potential migrants in PET, but experimental data on this are scarcely available in the scientific literature. The aim of the present study was therefore to develop a fast and precise method for determining diffusion coefficients and activation energies of diffusion of organic compounds in PET. Activation energies of diffusion for 13 organic compounds in PET were determined via their diffusion coefficient temperature dependencies. The molecular weight and activation energy of diffusion for the compounds investigated in this study were correlated, offering a basis for a new approach in predicting diffusion coefficients for use in migration modeling. The proposed method is a suitable tool to establish the datasets needed to refine the current migration model. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

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.     

Evaluation of chloride penetration in high performance concrete using neural network algorithm and micro pore structure

Chloride attack is one of the major causes of deterioration of reinforced concrete structures. In order to evaluate the chloride behavior in concrete, a reasonable prediction for the diffusion coefficient of chloride ion, which governs mechanism of chloride diffusion inside concrete, is basically required. However, it is difficult to obtain chloride diffusion coefficients from experiments due to time and cost limitations.In this study, a numerical technique for chloride diffusion in high performance concrete (HPC) using a neural network algorithm is proposed. In order to collect comparative data on diffusion coefficients in concrete with various mineral admixtures such as ground granulated blast-furnace slag (GGBFS), fly ash (FA), and silica fume (SF), a series of electrically driven chloride penetration tests was performed. Seven material components in various mix designs and duration time are selected as neurons in a back-propagation algorithm, and associated learning of the neural network is carried out. An evaluation technique for chloride behavior in HPC using the obtained diffusion coefficients from the neural network algorithm is developed based on, so-called, Multi-Component Hydration Heat Model (MCHHM) and Micro Pore Structure Formation Model (MPSFM). The applicability of the developed technique is verified by comparing the analytical simulation results and the experimental results obtained in this study. Furthermore, this proposed technique using the neural network algorithm and micro modeling is applied to available experimental data for verification of its applicability.     

Extension of the pore diffusion approach for modelling binary adsorption of lead and arsenic ions in a fixed‐bed column packed with atlantic cod fish scales

The removal of lead and arsenic ions using Atlantic Cod (Gadus morhua) fish scale involves ion exchange, chemisorption, and precipitation of its constituents and is competitive with respect to adsorption onto the negatively charged heterogeneous substrates. The nonlinear sorption equilibrium concept is preceded by pore diffusion of the solute and is utilised for numerical modelling of this binary adsorption. Numerical simulation data demonstrate reasonable agreements with experimental results of dynamic column tests. Sensitivity analyses confirm that parameters such as porosity, adsorption coefficient, mobility of ions, and number of sorption sites contribute significantly to breakthrough interval of contaminant in dynamic columns. At lower pH values (7–8.04), the adsorption coefficients of cations such as lead are significantly lower than at pH value of 11.0.     

The use of bulk diffusion tests to establish time-dependent concrete chloride diffusion coefficients

The proper determination of chloride diffusion values, including how they change with time, is important for service life modelling. Currently, there are two major approaches for using chloride diffusion coefficients to predict the service life of structures. The average diffusion coefficient or the instantaneous diffusion coefficient can be used. Using instantaneous diffusion coefficients is a more flexible technique, but requires a more advanced evaluation of bulk diffusion test results to establish material parameters. This paper describes an analytical procedure for determining the instantaneous chloride diffusion value as a function of time for a concrete using data from bulk diffusion tests. The importance of interpreting the data correctly is illustrated with simulated bulk diffusion test data, generated using a finite-difference model for diffusion. In addition, the application of this procedure for evaluating diffusion values is illustrated with experimental data.     

Improvement in concrete resistance against water and chloride ingress by adding graphene nanoplatelet

Graphene nanoplatelet (GNP) is a cheap impermeable carbon-based nanoplatelet with large surface-to-volume ratio and has been exploited in polymer materials to improve their transport resistance. Experimental investigation on the transport properties under chloride and water exposure was carried out on concrete containing up to 2.5% of GNP at 0.5% increment. The pore structure was inferred using mercury intrusion porosimetry and significant reduction in pore sizes was measured. Concrete with 1.5% of GNP showed the greatest reduction in transport; water penetration depth, chloride diffusion, and migration coefficients were reduced by 80%, 80%, and 37%, respectively. The barrier effects of GNP were characterized and it was found that more than 50% of the improvement in transport resistance can be attributed to tortuosity while the rest to pore refinement. However, further improvement did not take place at GNP content higher than 1.5% due to limitation in dispersing the nanoplatelet clusters.     

The effect of undrained heating on a fluid-saturated hardened cement paste

The effect of undrained heating on volume change and induced pore pressure increase is an important point to properly understand the behaviour and evaluate the integrity of an oil well cement sheath submitted to rapid temperature changes. This thermal pressurization of the pore fluid is due to the discrepancy between the thermal expansion coefficients of the pore fluid and of the solid matrix. The equations governing the undrained thermo-hydro-mechanical response of a porous material are presented and the effect of undrained heating is studied experimentally for a saturated hardened cement paste. The measured value of the thermal pressurization coefficient is equal to 0.6 MPa/°C. The drained and undrained thermal expansion coefficients of the hardened cement paste are also measured in the heating tests. The anomalous thermal behaviour of cement paste pore fluid is back analysed from the results of the undrained heating test.     

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:...     

Time dependent diffusion in concrete—three laboratory studies

The rate of chloride diffusion into concrete decreases with time due to issues such as continued hydration and chloride binding. Data are presented from three projects where the specimens were continuously exposed to chloride solutions for periods ranging from 28 days to 3 years. Chloride profiles obtained from the specimens at various time intervals were used to calculate diffusion values using Crank's solution to Fick's second law. The time-dependant reduction coefficient, m, was determined using three methods, yielding different values. Since the reduction of calculated diffusion coefficients with time has great impact on service life predictions, and values are sensitive to the method used, it is imperative to also know how the diffusion and time-dependant reduction coefficients were obtained.     

混凝土孔结构及其分形维数与氯离子扩散性能的关系

氯离子扩散系数是研究海洋环境下混凝土结构耐久性的重要参数之一。通过开展不同水胶比混凝土的压汞试验和盐雾扩散试验,研究了混凝土内部孔隙率、孔径分布及临界孔径对氯离子扩散系数的影响规律。结合Menger海绵体模型,建立孔体积分形维数与氯离子扩散系数的关系。结果表明:孔隙率和临界孔径与无量纲化氯离子扩散系数的相关性很高,可作为反映混凝土氯离子扩散性能的重要参数;通过数学分析计算得到的孔表面分形维数分布在2.56~3.86之间,孔体积分形维数分布在2.85~2.98之间;基于压汞法和分形理论计算得到的孔体积分形维数可以作为评价氯离子扩散系数的指标,在孔径小于10 nm、10～100 nm、100～1 000 nm以及大于1 000 nm四类区间,氯离子扩散系数随孔体积分形维数的增加而下降。     

Electrically induced chloride ion transport in alkali activated slag concretes and the influence of microstructure

Chloride ion transport in alkali silicate powder and liquid activated slag concretes, and the influence of the material microstructure are discussed. Increasing the Na₂O-to-slag ratio (n) results in a reduction in the rapid chloride permeability (RCP) and non-steady state migration (NSSM) coefficients (D_nssm) of solid sodium silicate activated slag concretes. Increasing the SiO₂-to-Na₂O ratio (M_s) of the activator beneficially influences the transport parameters of liquid sodium silicate activated concretes. The chloride transport parameters are related to the reaction product microstructure and composition evaluated using mercury intrusion porosimetry (MIP) and Fourier Transform Infrared (FTIR) spectroscopy respectively. A reduction in the critical pore sizes reduces the transport coefficients to a larger extent than porosity reduction, which is responsible for the better performance of solid sodium silicate activated concretes. The total chloride concentration determined from the charge passed and the ionic transference numbers is used to predict an apparent non-steady state transport coefficient.     

Phenomenological mass-balance-based model of migration tests in stationary conditions: Application to non-steady-state tests

In order to understand the phenomena that take place during a migration test, and to obtain a complete picture of the system through the pursuit of the transference numbers, a phenomenological mass-balance-based model of the evolution of all ionic species, has been developed. The model has been built on a series of steady-state migration tests and has been experimentally validated. Afterwards, it has been applied to migration tests in non-stationary conditions, and the resulting deductions have also been checked with experimental results. This has allowed the development of a new simplified way to determine the non-steady-state diffusion coefficients, D_ns, just from the registration of the current intensity circulating and the analysis of the surface concentration, C_s, after the test. In addition, the possibility of the tabulation of the different C_s for different external chloride concentrations and the application of different “binding factors” for each kind of binder is posed, which will allow the calculation of the D_ns from a simple migration test of the AASHTO type (ASTM C1202-91).     

Study of chloride binding and diffusion in GGBS concrete

Ordinary Portland cement (OPC) and OPC/ground granulated blastfurnace slag (GGBS) 70%, with or without 5% sulfates, were widely investigated in respect to their pore structures, chloride diffusion coefficients, internal and external chloride-binding capabilities by expression method and leaching method and the microstructure analysis on Friedel's salt such as differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It can be concluded that GGBS can improve the pore structure of OPC and decrease the chloride diffusion coefficient greatly, and that sulfates do not do good for the pore structure and chloride diffusion for GGBS. GGBS increases the chloride-binding capability greatly without reference to the internal or external chloride and sulfates decrease the chloride-binding capability of GGBS greatly. It can be also found that the maximum intensity peak corresponding to Friedel's salt appears at about 8.0 Å, its endothermic effect appears at about 360 °C, its morphology is hexagonal slice whose size is about 2-3 μm; that the output of Friedel's salt formed by GGBS is much more than OPC; and that sulfates influence the output of Friedel's salt greatly. The corresponding mechanism was also analyzed.     

Experimental study of dispersion in a consolidated sandstone

Dispersion experiments were conducted on a Berea sandstone core having a permeability of 0.28 (μm)², using chloride ion as the water soluble tracer to provide quantitative information useful to the prediction of the motion of reservoir fluids in enhanced oil recovery. Both single phase (water only) and two phase (water plus oil) experiments were completed under identical operating conditions in order to observe flow differences. The theory of Ebach and White (1958) was used to determine the dispersion coefficients. It has been found that over a wide range of fluid velocities, low concentrations of oil have no effect on the dispersion of chloride ions in the water phase but high concentrations of oil (S₀ ? 0.55) resulted in a dispersion coefficient 2.2 times higher than for chloride ions in water alone. This work indicates that the presence of an immiscible phase has a pronounced affect on the dispersion coefficient in porous media and must be accounted for in quantitative modeling of reservoir flows.     

Effect of crack width on chloride diffusion coefficients of concrete by steady-state migration tests

The purpose of the present study is to explore the diffusion characteristics of cracked concrete according to the width of cracks. Major test variables include crack width, concrete strength, fly ash addition, and maximum aggregate size. The diffusion characteristics have been measured by steady-state migration test. The present study indicates that the diffusion coefficients do not increase with increasing crack widths up to the so-called “threshold crack width.” The threshold crack width for diffusion is found to be around 55–80 μm. Above this threshold value, the diffusion coefficients start to increase with crack width. A composite model with the introduction of “crack geometry factor” was derived to identify the diffusion coefficient in cracked concrete. It was shown that the crack geometry factor ranges from 0.067 to 0.206. Finally, the effects of concrete strength, fly ash addition and maximum aggregate size on diffusion coefficients are also discussed.