Approximate migration coefficient of percolated interfacial transition zone by using the accelerated chloride migration test

In this study, the electrochemical technique is applied to accelerate chloride ion migration in cement-based material to estimate its migration coefficient. In order to investigate the chloride migration coefficient of percolated interfacial transition zone (ITZ) on the chloride migration coefficient of specimen, specimens with cylindrical aggregates of the same height as the specimen were cast and tested. In this study, the volume fraction of aggregate is constant and the varied lateral surface area of the aggregate cylinder was obtained by using different diameters and number of aggregate. The chloride migration coefficient of cement-based material was determined experimentally as a function of the lateral surface area of aggregate. A model obtained for the migration coefficient of cement-based material and the regression analysis are used to determine the approximate chloride migration coefficient of the percolated ITZ. Based on the experimental and regression analytical results, the approximate percolated ITZ migration coefficient is 40.6, 35.5, and 37.8 times of the altered migration coefficient of matrix mortar for the water/cement (w/c) ratio of 0.35, 0.45, and 0.55, respectively.     

Influence of the migration of chloride ions on the electrochemical impedance spectroscopy of mortar paste

During the last decade, electrochemical impedance spectroscopy (EIS) started to be effectively exploited to investigate the electrical properties of hydrated cement pastes.This paper is devoted to the study of the high- and medium-frequency regions of the impedance spectrum and focused on determining the relationship between chloride migration and impedance spectroscopy in accelerated diffusion tests carried out on samples of mortar.After introducing an experimental protocol based on a four-electrode arrangement, we present the results of a parametric study which is related to the thickness and water-cement ratio (W/C) of the samples.Firstly, all our measurements show that the presence of chloride ions modifies the impedance response of mortar and reveals a small loop as of modification of the composition of the upstream solution. This loop is probably due to the development of interfacial phenomena between material and the solution.Secondly, as the migration of chloride process progresses, an increase followed by a decrease of the bulk electrical resistance is observed, whereas the second loop, due to the presence of chlorides, remains constant.An equivalent electrical circuit is then proposed to fit the different experimental data.     

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.     

Magnetic resonance imaging of H, Na, and Cl penetration in Portland cement mortar

The first magnetic resonance imaging profiles of chloride content in low- and high-permeability Portland cement mortar have been obtained using a novel material science imaging technique. The penetration of water as well as chloride and sodium ions, into mortar specimens was monitored for a period of 72 h. Marked differences in penetration depth were observed in low- and high-permeability mortar. These preliminary experiments show significant differences between the extent and nature in the penetration of chlorides compared to water and sodium.     

Improvement of the chloride ingress resistance of OPC mortars by using spent cracking catalyst

The influence of the incorporation of spent cracking catalyst (FC3R) on the chloride ingress resistance has been evaluated. Thermogravimetric analyses have shown that the pozzolanic reaction of FC3R yields higher contents of hydrated calcium aluminates and silicoaluminates, so chloride binding capacity of mortars was highly improved. Mercury intrusion porosimetry analyses demonstrated that FC3R produces a significant reduction of capillary pore volume. As a result non-steady-state and steady-state chloride diffusion coefficients were reduced, enhancing the chloride ingress resistance of mortars incorporating FC3R. Additionally, the corrosion behaviour of steel embedded in Portland cement mortars partially substituted by spent cracking catalyst (FC3R) under chloride attack has been studied. Results showed that the incorporation of FC3R decreased the corrosion rates of steels and increased chloride thresholds for corrosion. For this reason, FC3R is an interesting pozzolanic material that can be used in reinforced concrete for civil engineering applications exposed to the action of chlorides.     

Numerical simulation of hydration-driven moisture transport in bulk and interface paste in hardening concrete

In real concrete two types of cement paste can be distinguished, i.e., bulk paste and interface paste. Initially the paste in the interface zone will generally contain more water than the bulk paste and will therefore hydrate differently. Differences in relative humidity and associated differences in pore water pressure will result as well. If the interface paste and the bulk paste could hydrate individually, a situation will result where a relatively porous water-rich interfacial zone coexists with a relatively dry bulk paste. However, due to gradients in porosity, permeability, relative humidity and pore water pressure, a flow of moisture will start from the water-rich interfacial zone to the bulk paste. It will be shown how the moisture transport can be simulated numerically and how this transport phenomenon influences the overall rate of hydration of cement in concrete. Numerical results are compared with experimental data presented in literature. The relevance of modelling of this kind of transport phenomena is briefly dealt with.     

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.     

Aggregate influence on chloride ion diffusion into concrete

An attempt is made to predict the probable effect of the aggregate on chloride ion diffusion into saturated concrete. It is shown that if the chloride ion diffusion coefficient of an aggregate ranges from 0.2 to 10 times that of the cement paste matrix, then this could result in variations in the concrete chloride ion diffusion coefficient of up to 10:1. Such a variation is equivalent to a change in free water-cement ratio from 0.77 to 0.45.     

Determination of chloride diffusion coefficient of concrete using open-circuit potential measurements

The rate of chloride ion ingress into concrete is of great importance for the performance of reinforced concrete structures exposed to chloride-contaminated environments. The service life of reinforced concrete structures subjected to such exposure conditions is closely related to the rate of chloride ion diffusion through the concrete. This paper presents the determination of the apparent chloride diffusion coefficient of concrete using open-circuit potential measurements. The chloride diffusion coefficients obtained are in the range of 6.4×10⁻⁸ to 12.4×10⁻⁸ cm²/s for a simulated seawater tidal condition, which is quite consistent with those reported in the literature. This indicates that open-circuit potential measurements can be considered as an approximate but simple method of assessing the diffusivity of chloride through concrete. Limited with the testing conditions and the characteristics of concrete used, results indicated that the time necessary for corrosion initiation of concrete with a cover depth of 7 cm ranges from 3 to 6 years for the seawater exposure, whereas it is only 1.5 years for a 3% sodium chloride exposure.     

Assessing the influence of ITZ on the steady-state chloride diffusivity of concrete using a numerical model

In this study, the influence of the aggregate-cement paste interfacial transition zone (ITZ) on the steady-state chloride diffusivity of mortars and concretes was examined using a semi-empirical, three-phase composite sphere model. Mortars and concretes were modelled as three-phase composites consisting of the aggregate, bulk cement paste and an inhomogeneous ITZ. The latter was divided into a series of homogenous concentric shell elements of equal thickness. The initial porosity and cement gradients at the ITZ were first estimated from the overall water/cement ratio (w₀/c). The evolution of the porosity, solid hydration products and remnants of unreacted cement were then calculated from the hydration degree and local water/cement ratio (w/c) using Powers' empirical model. Based on the Laplacian equation, an element transfer matrix was derived analytically to predict the steady-state chloride diffusivity. The model was calibrated using the available experimental data and then applied to perform a sensitivity analysis to evaluate the effects of aggregate content, water/cement ratio, curing period, ITZ width, maximum aggregate size and aggregate gradation on diffusivity. Some of these variables are impractical to quantify by laboratory experimentation. Implications of the findings with regard to the role of ITZ on mass transport properties are discussed.     

Estimation of service life of coated brickwork mortar joint

This work reports an investigation into the permeation of chloride in brick masonry mortar joints in three different media (water from rain, runoff, and sea). The coated and uncoated (control) masonry prisms were subjected to 3% chloride solutions for periods of 1-5 weeks each, and the chloride concentration at depth of 20 mm was determined (ranging from 0.012 to 0.327 for immersion days of 7 to 35, respectively, for coated and uncoated prisms of 0.03 to 0.887, respectively). The results show that the service life of the coated brick mortar joint has been increased by 36 years for the sea water sample.     

Influence of aggregate size and volume fraction on shrinkage induced micro-cracking of concrete and mortar

In this paper, the influence of aggregate size and volume fraction on shrinkage induced micro-cracking and permeability of concrete and mortar was investigated. Nonlinear finite element analyses of model concrete and mortar specimens with regular and random aggregate arrangements were performed. The aggregate diameter was varied between 2 and 16 mm. Furthermore, a range of volume fractions between 0.1 and 0.5 was studied. The nonlinear analyses were based on a 2D lattice approach in which aggregates were simplified as monosized cylindrical inclusions. The analysis results were interpreted by means of crack length, crack width and change of permeability. The results show that increasing aggregate diameter (at equal volume fraction) and decreasing volume fraction (at equal aggregate diameter) increase crack width and consequently greatly increases permeability.     

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.     

On the mathematics of time-dependent apparent chloride diffusion coefficient in concrete

This paper provides an improved mathematical analysis of chloride penetration into concrete employing a time-dependent diffusion coefficient for the solution of Fick's second law of diffusion. In the paper the possible errors caused by the application of oversimplified mathematical expressions used in some models for the evaluation of service life of reinforced concrete structures are discussed. The results from this mathematical analysis demonstrate that some models based on the oversimplified error function complement (ERFC) solutions may easily overestimate the service life by orders of magnitude, especially when the age factor is high. Some chloride profiles after up to 10 years' field exposure were used to compare the oversimplified with the improved models. The results show that both the oversimplified and the improved models fairly well predict the 10 years' chloride ingress in Portland cement concrete, but the oversimplified ERFC model significantly underestimates the chloride ingress in concrete with fly ash.     

Experimental study of gas and liquid permeability of a mortar

Eight samples cored from the same mortar were used to investigate their respective gas, ethanol and water permeability. Two gas and liquid permeability cells, using special devices for measuring the injected flow under steady conditions, were designed and presented in this paper. The obtained results showed that water permeability was systematically lower (in an order of magnitude from 1 to 2) than gas permeability whereas ethanol permeability was intermediate between these two values. Nevertheless, ethanol and gas permeabilities were found of the same order and, when gas permeability is corrected from the Klinkenberg (or slippage) effect, the results given by these two fluids are virtually identical and can be considered to be the intrinsic permeability value. Thus, the differences observed between water and gas permeability values have to be explained by other phenomena such as rehydration, dissolution and migration of fine elements or water adsorption in the thinnest pores.     

Influence of lightweight aggregate on the microstructure and durability of mortar

The results of an investigation on the effect of dry and prewetted lightweight aggregates on the microstructure and durability of mortar are presented in this paper. The results are compared with those obtained for normal aggregate mortar. There appears to be only a small difference in the microstructure of the interfacial transition zone (ITZ) between dry and prewetted lightweight aggregate mortars. The porous ITZ surrounding lightweight aggregate appears to extend for about 10 and 15 μm from the aggregate surface for dry and prewetted lightweight aggregates, respectively. The ITZ for dry and prewetted lightweight aggregates seems to be surrounded by dense paste that extends from 10 to about 50 μm from the aggregate surface. This dense paste has lower porosity than that observed in the bulk paste located 50 μm and farther from aggregate surface. The normal aggregate mortar prepared with the same water/cement ratio appears to have porous ITZ that extends beyond 35 μm from the aggregate surface. The dry and prewetted lightweight aggregate mortars seem to have a lower sorptivity and electrical conductivity than does the normal aggregate mortar. Lightweight aggregate mortars also appear to have excellent resistance to sulfate attack as compared with normal aggregate mortar.     

Application of √t-type, logarithmic and half-time methods in desorptive measurements of diffusivity in narrow humidity ranges

This paper concerns the use of non-stationary desorptive measurement techniques for defining the mass diffusivity of cement based materials. Three different procedures are presented: √t-type calculation; logarithmic; and half-time procedures. Cement mortars of different water to cement ratios (w / c), equal to 0.50, 0.65 and 0.80 were selected as the model environment for testing the usability of the above-mentioned desorptive techniques. The study was carried out at the temperature (T) of 20 °C within narrow relative humidity (φ) ranges: from φ₁ = 30% to φ₂ = 12%, and 50% → 30%, 75% → 50%, 85% → 75%, 97% → 85%. The results obtained are used to evaluate the conformity of these methods. The conformity is analyzed with regard to each mortar in all the above humidity ranges Δφ. The values of diffusivity D_m, defined by means of the √t-type calculation and the logarithmic procedure, demonstrated rather high conformity, all relative differences between D_m(√t) and D_m(ln) did not exceeded 20%. However, the half-time procedure can be applied for rough estimation of the diffusivity only. That is because deviations between D_m(t_1 / 2) and the values found by means of the two other methods were too large.     

Linear and non-linear analysis of desorption processes in cement mortar

Based on the gathered experimental data concerning adsorption/desorption processes in cement mortar, it has been stated that the rate of these processes changes in time even if they proceed in stable conditions. In this paper an attempt is made to describe such processes by applying linear and non-linear diffusion theories for comparison. The main aim of these studies is to determine the diffusion coefficient by correlating the theoretically determined desorption isotherms with the experimental ones. The validation of the diffusion coefficient was accomplished through comparison of the theoretical desorption curves with the experimental data for narrow and broad ranges of the air humidity changes. The final conclusion is that the moisture transfer in hygroscopic porous materials for broad ranges of the air humidity changes should be modeled by the non-linear diffusion theory, in which the diffusion coefficient is a function of moisture content. The new material in this paper concerns very long time measurements in desiccators, and evaluation of the diffusion coefficient by an advanced optimization algorithm.     

Effects of metakaolin, water/binder ratio and interfacial transition zones on the microhardness of cement mortars

Variations in the microhardness of the hydrated cement matrix component of model mortars have been investigated as functions of the distance from the aggregate surfaces for specimens in which the binder was Portland cement or a blend of Portland cement and metakaolin.Microhardness measurements were made using a Knoop indenter at distances of up to 120 μm from the aggregate. The microhardnesses of the paste-aggregate interfacial transition zones (ITZs) were found to be between 14% and 22% lower than those of the corresponding bulk cement pastes at the lower water/binder ratios investigated, i.e. 0.4 and 0.5 for samples prepared with Portland cement and 0.4 for samples prepared with a binder comprising Portland cement and metakaolin.Metakaolin increased the mean microhardness of specimens prepared at the higher water/binder ratios of 0.5 and 0.6 by 13% and 54%, respectively.