Luping Tang

Concentration dependence of diffusion and migration of chloride ions: Part 2. Experimental evaluations

Abstract This paper series includes two parts: (1) theoretical considerations and (2) experimental evaluations. In Part 1, the chloride ion diffusion and migration processes in a pure solution system as well as in a solution-concrete system have been theoretically analyzed. The relationships between different diffusion coefficients from the conventional diffusion or migration tests have been established. In this, the second part, the parameters influencing the “effective” diffusion coefficient, such as activity coefficient, counter-electrical field, and the friction coefficient due to the ionic interaction, are quantitatively evaluated with available experimental data. The results show that the concentration dependence of chloride diffusion and migration can be quantitatively described by two decisive parameters: Kτ and f; the former describes the effect of counter-electrical field and the latter describes the friction effect. The theories fit the experimental data fairly well. Some phenomena of ion diffusion in concrete can be explained by these theories, such as the differences in diffusion coefficient between diffusion and migration tests and between cations and anions.

Concentration dependence of diffusion and migration of chloride ions: Part 1. Theoretical considerations

Abstract This paper series includes two parts: (1) theoretical considerations and (2) experimental evaluations. In Part 1, the chloride ion diffusion and migration processes in concentrated solutions are theoretically analyzed. The quantitative relationship between diffusion coefficient and migration coefficient is established. Two important parameters that are inherent in the cement based materials (i.e., ion-selective semipermeability and chloride binding) are discussed. The theoretical analysis shows that the “effective” chloride diffusion coefficient determined from the conventional diffusion or migration tests is not a constant, but rather is a complicated function of concentration. This function contains four main factors: (1) the parameter K τ , describing the effect of counter-electrical potential; (2) the ratio of cation velocity to anion velocity β v ; (3) the friction coefficient f , which reflects the ionic interaction; and (4) the activity coefficient γ, which is also a function of concentration.

Resistance of concrete to chloride ingress: Testing and modelling

Chloride ingress in reinforced concrete induces corrosion and consequent spilling and structural weakness, and it occurs world-wide and imposes an enormous cost. Yet it can be resisted by using test methods and relevant models for service life prediction. Resistance of Concrete to Chloride Ingress sets out current understanding of chloride transport mechanisms, test methods and prediction models. It describes basic mechanisms and theories, and classifies the commonly used parameters and their units which expressing chloride and its transport properties in concrete. Laboratory test methods and in-field applicable test methods, including precision results from inter-laboratory comparison tests, are then outlined. Some of the fundamentals of models are explained, and the different types of models are then analyzed theoretically and critically. Analytical and probabilistic approaches are used to analyze the sensitivity of various models and the results from a benchmarking evaluation of different models are presented and discussed. Guidelines for the practical use of test methods and models are given, including tests for in-situ applications, and test methods validated by the precision results are detailed. The book draws to a large extent on the Chlortest project, which involved seventeen partners from ten European countries, and serves as an authoritative guide.

Recurrent studies of chloride ingress in uncracked marine concrete at various exposure times and elevations

Uncracked reinforced concrete slabs were field exposed mounted on a floating pontoon and partly submerged for 5 years at the Swedish west coast. The total chloride ingress was analysed at various exposure times at 3 elevations representing a submerged, a splash, and an atmospheric exposure zone. The concrete mixtures varied in w/c ratio, type of cement, and amount and type of pozzolan used in the binder. The data is unique as it represents recurrently measured total chloride penetration profiles at various exposure ages, providing a foundation for the prediction of chloride ingress in concrete in a given environment. The results after 5 years of exposure confirmed the expected inverse relationship between water-to-binder ratio and chloride ingress. The use of 5–10% silica fume in the binder had a very positive effect on reducing the chloride ingress, but little or no benefit at all was found for concrete with fly ash in the binder as compared to the use of 5% silica fume. The chloride penetration rate as expressed by a calculated effective chloride diffusivity has a tendency to decrease over time. High- performance concrete with w/c ≤ 0.4 and a minimum of 5% silica fume added as a well dispersed slurry exhibited an effective chloride diffusivity in the range of 1 × 10−13 to 5 × 10−13 m2/s after 5 years exposure in the splash zone.

A new approach to the determination of pore distribution by penetrating chlorides into concrete

In this paper, a new concept of pore distribution is discussed and consequently a new approach to the determination of pore distribution by penetrating chlorides into concrete is proposed. This approach involves 1) accelerating chloride penetration into concrete specimens by applying an electrical field, 2) determining the profile of total chloride content by using the dry-grinding technique and chemical analysis, and 3) calculating the pore distribution of concrete from the profile of total chloride content. A number of specimens have been tested. Preliminary results show that the pore distributions determined by using the new approach are qualitatively comparable with those determined by using conventional mercury intrusion technique.

Field study of the penetration of chlorides and other ions into a high quality concrete marine bridge column

The transport of ions related to the penetration of chlorides into concrete has been studied in the field by drilling 100-mm concrete cores from a marine bridge column. A 4-year-old concrete column in Sweden was selected. The concrete was of high quality (i.e., frost- and sulfate-resistant, with a low-heat, low-alkali portland cement with a maximum water-cement ratio of 0.40) according to new Swedish recommendations. Concrete cores were drilled from the submerged, splash, and atmospheric zones. Selective grinding from the concrete surface(profile grinding) revealed concentration profiles of acid-soluble chlorides, carbonates, sulfates, and water-soluble alkalies. Selected parts of the concrete surface were examined by SEM and thin-section microscopy for microstructural studies. Laboratory estimates of chloride diffusivities were carried out on 6-month-old laboratory concrete of similar mix proportions, and also on unexposed parts of drilled concrete cores. Chloride diffusivities obtained from laboratory exposure were then compared with the values obtained from the field concentration profiles, from both the bridge column and a field station, using Ficks second law of diffusion. Maximum chloride diffusivities calculated from the field profiles after 4 years of exposure were more than ten times lower than those obtained from the same concrete in the laboratory. Clearly, there are important mechanistic problems associated with laboratory procedures, resulting in serious misjudgments, if such laboratory tests are used for linear extrapolation of the service life for marine concretes.* (Less)

A Numerical Method for Prediction of Chloride Penetration into Concrete Structures

In this study, the mathematical modelling of chloride penetration into concrete structures is suggested to consider two main phenomena: 1) chloride penetration through the pore solution and 2) the distribution of the total chloride content in concrete. In this model, the chloride binding capacity was taken into account and the diffusivity was considered as a function of depth, age, and temperature. Some parameters, such as changes in surface concentration and temperature, degree of hydration, pore content, and binder content in concrete were also taken into consideration. The intrinsic diffusion coefficient, which can be determined by using a rapid method developed at the Chalmers University of Technology, was introduced into the model.

Slab test-freeze/Thaw resistance of concrete—Internal deterioration

This method is used for determining the resistance of concrete to internal deterioration when subjected to repeated freezing-and-thawing. 3.0% NaCl solution is used as a freezing medium for concrete in an environment with de-icing salt, and demineralised water is used as a freezing medium for concrete in an environment without de-icing salt. RILEM TC 176-IDC: ‘Internal damage of concrete due to frost action’ Final Recommendation

Transport Mechanisms in Porous Materials

The paper gives an introduction to the basic laws, mainly on a macroscopic scale, for the transport processes which occur in a pore and void system during the service life or the testing of a porous building material. Basic equations for pure potential mass flow, convective flow, and some combined flow mechanisms are presented and discussed. Some versions of the mass balance equation are shown and the binding properties required for solving the mass balance equation are discussed. Apparent transport properties frequently applied when using Fick’s 2nd law are derived. Basic laws describing the distribution profiles during steady state and non-steady state flow processes are summarized. Finally, theoretical and empirical relationships between some transport properties are dealt with.

Validation of models for prediction of chloride ingress in concrete exposed in de-icing salt road environment

This paper presents the results from validation of models for prediction of chloride ingress in concrete exposed in de-icing salt road environment. Three models including the simple error-function complement (ERFC) model, the DuraCrete model and the ClinConc model, were evaluated using the measurement data collected from both the field exposure site after over ten years exposure and the real road bridges after 25–30 years in service. The sensitivity of input parameters in each model is analysed. The results show that, among different input parameters, the age factor is the most sensitive one. The simple ERFC model significantly overestimates chloride ingress. The DuraCrete model, if the input parameters are properly selected, may give a reasonably good prediction, otherwise often underestimates chloride ingress. The ClinConc model in general gives fairly good predictions for chloride ingress in de-icing salt road environment with heavy traffic at high speed.