Non-steady State Migration Test Research Articles

Effects of load-induced micro-cracks on chloride penetration resistance in different types of concrete

Chloride penetration resistance of concrete is one of the key parameters for the durability design of reinforced concrete structures located in chloride-bearing environments. In all the current available durability models, service life is evaluated considering concrete in uncracked conditions, which is rarely found in practice. This work investigates chloride penetration resistance of concrete in uncracked and micro-cracked configurations, evaluated in terms of chloride migration coefficient through non-steady state migration test (Rapid Chloride Migration test). Prismatic specimens were manufactured considering six different concrete types and two different times of curing. In micro-cracked configuration, cracks were obtained with a specifically developed loading procedure. Micro-cracks were characterized at the end of the exposure test, in terms of crack width at the exposed surface and crack depth. Results showed that cracks were 5–70 μm wide and up to 40 mm deep, always causing an increase in chloride penetration, that should be evaluated considering both crack width and crack depth, with respect to sound conditions. The effects on the chloride penetration seemed to be more pronounced on the more impervious concretes.

Statistical modelling of the resistance to chloride penetration in concrete with recycled aggregates

Steel corrosion is one of the biggest threats to the service life of reinforced concrete structures and chloride penetration favours the corrosion process. Due to sustainability and environmental issues, the use of recycled aggregate in concrete production has been growing. The aim of this study is to develop analytical models that allow predicting the performance of concrete incorporating recycled aggregates regarding chloride penetration. Multiple linear regression analysis was applied to a dataset collated from the literature. The dataset comprised 942 case studies from 33 publications. The analyses allowed identifying the explanatory variables and developing two mathematical models. One of these models is intended to predict the chloride diffusion coefficient from accelerated non-steady state migration tests, while the other predicts the charge transfer in rapid chloride permeability tests. The obtained correlation coefficients and analysis of variance (ANOVA) test results evidence the adequacy of both models. Thus, it is possible to estimate the resistance to chloride penetration in concrete incorporating recycled aggregates through the proposed models.

A refined, self-consistent Poisson-Nernst-Planck (PNP) model for electrically induced transport of multiple ionic species through concrete

A fundamental re-look of the use of Poisson-Nernst-Planck (PNP) model for predicting electrically accelerated ionic transport through concrete is carried out. The PNP model is augmented with concentration-based diffusion coefficients and explicit considerations of transport path in the formulations since the electric field is distributed only along the connected pore path. The geometric tortuosity obtained from electrical property measurements is modified with a multiplicative correction factor that is a function of the ratio of dominant pore sizes in the specimen. Idealized models of the pore structure are used to arrive at this correction factor, which ranges between 1.5-to-3.0 for typically used concretes. Time-dependent changes in boundary conditions induced by the electrode reactions are also accounted for. The effect of chloride binding as a time-dependent process during the test, is also ascertained. The model is verified on a series of concretes of different water-to-cement ratios and containing different cement replacement materials (fillers and/or reactive materials) subjected to the non-steady state migration test.

Experimental and Numerical Investigations of the Influence of Real Cracks on Chloride Ingress in Concrete

Abstract Experimental and numerical results of a chloride ingress study conducted on samples drilled from different locations of a reinforced concrete slab, previously loaded until failure, are presented. The experimental part was carried on following the NT Build 492 standard for the non-steady state migration test, then a 3D model was developed using the Abaqus/Standard software based on the FEM in order to simulate chloride ingress in both uncracked and cracked concrete.

Damage of Reinforced Concrete Structures due to Steel Corrosion

Reinforced concrete (RC) became one of the most widely used modern building materials. In the last decades a great interest has been shown in studying reinforcement corrosion as it became one of the main factors of degradation and loss of structural integrity of RC structures. The degradation process is accelerated in the case of RC structures situated in aggressive environments like marine environments or subjected to de-icing salts. In this paper it is shown how steel corrosion of the embedded rebars occurs and how this affects the service life of reinforced concrete structures. Also, an experimental study regarding the combined effect of carbonation and chloride ingress was realized. Samples with and without rebars were drilled from a RC slab which was stored in the laboratory for two years. Non-steady state migration tests were realized in order to determine the chloride profile, while the carbonation depth was measured using the colorimetric method based on phenolphthalein spraying. It was concluded that carbonation has a significant effect on chloride ingress, increasing it.

The influence of alkali content on the electrical resistivity and transport properties of cementitious materials

This paper examines the role of alkali content on the transport properties of cementitious materials. While cement with high alkali content may be a concern due to the potential for alkali silica reaction (ASR), the alkali content also influences the electrical properties of pore solution and the bulk cementitious system. Higher concentrations of alkalis decrease the resistivity of the pore solution. However, the alkalis may also impact the microstructure formed during hydration. This paper examines the role of alkali content on both the pore solution and on the microstructure that forms. Water vapor desorption measurements indicate that systems with a higher alkali content have a lower overall porosity. This dense microstructure is not caused simply by a higher degree of hydration (DOH) (i.e., an increase in reacted products), as suggested by loss on ignition (LOI), but rather possibly from the improved morphology and distribution of calcium hydroxide, as observed with scanning electron microscope (SEM).These improvements in the microstructure alter the electrical resistivity and the results of steady and non-steady state migration tests. This indicates that with higher alkali content, cement may have a beneficial influence on reducing chloride ion transport. Furthermore, alkali leaching during saturated curing is observed, quantified, and discussed as it relates to the interpretation of electrical measurements and microstructure formed.

Non-steady state chloride migration and binding in cracked self-compacting concrete

We adopted a notch method to study the influence of crack width (macro level) on chloride transport and binding of cracked concrete under a non-steady state migration test. The results show that migration coefficient of cracked concrete increases with increasing crack width up to a critical value (0.43 mm), for the whole concrete or the area close to crack; the increase of migration coefficient could be independent from crack parameter when a critical crack width is reached. For chloride binding, Langmuir isotherms of cracked concrete samples exhibit the similar decreasing trend as crack width increases from 0.27 to 1.96 mm. The increased current value could be responsible for the trend based on the hypothesis of electric force.

An electrical impedance investigation into the chloride ion transport resistance of alkali silicate powder activated slag concretes

The chloride transport resistance of alkali silicate powder activated slag concretes is evaluated. Two different Na2O-to-source material ratios (n) and two SiO2-to-Na2O ratios of the activator (Ms) are used on concretes proportioned using two slag contents (300kg/m3 and 400kg/m3). Rapid chloride permeability (RCP) and non-steady state migration (NSSM) tests are used to evaluate the chloride transport behavior. Alkali silicate powder activated concretes demonstrate comparable or better chloride transport resistance than OPC concretes when evaluated using RCP and NSSM tests. The relationships between the activator parameter, n·Ms, and the critical pore sizes (dc) or the RCP and NSSM values demonstrate similar trends showing the influence of dc in determining the ionic transport response. Electrical impedance spectroscopy (EIS) is used to relate the material response before and after the transport tests, and equivalent circuit models are used to extract the parameters that relate to the pore structure. EIS coupled with circuit modeling clearly brings out the differences between the RCP and NSSM test on their influences in the microstructure of alkali activated slag concretes.

Influence of Crack Depth on Chloride Transport of Cracked Self-Compacting Concrete under Non-Steady State Migration Test

From a physical and mechanical point of view, concrete cracking is hard to avoid. So, it is meaningful to study the influence of crack depths on chloride transport in cracked self-compacting concrete. Following the NT BUILD 492 chloride migration test method, water soluble chloride conetnt, acid soluble chloride content and penetration depth were determined on cracked concrete samples which were prepared by artificial crack method. Afterwards, the migration coefficients were obtained by curve fitting on water soluble chloride contents and penetration depths seperately. The results show that the influence of crack depth on chloride concentration of concrete is significant for the concrete zone deeper than 20 mm from the surface. Secondly, the influential zone caused by the crack is limited to a distance of 10 mm at both sides from the crack. Thirdly, the existing equation of chloride transport is not applicable to concrete with different crack depths.

Electrical impedance analysis based quantification of microstructural changes in concretes due to non-steady state chloride migration

Non-steady state migration (NSSM) test is commonly employed to evaluate chloride ion ingress into concretes. This paper quantifies the microstructural changes in concretes subjected to NSSM tests, induced by the chemical constitution of the cement and replacement materials. Electrical impedance spectroscopy (EIS) and equivalent electrical circuit modeling are used to obtain characteristic features of the concrete pore structure that aid in this quantification. A methodology is provided in this paper to convert the bulk resistances of the specimens under the action of any applied voltages to the ones corresponding to a 30 V applied potential. The relationships between: (i) bulk resistance of the specimens before the NSSM test and the NSSM coefficient D nssm, (ii) resistance of the connected pores and the microstructural parameter ϕβ (product of porosity and pore connectivity), and (iii) bulk resistance after the NSSM test corresponding to a 30 V potential and the chloride penetration depths, demonstrate the effect of chloride binding. The capacitance related to the pore–solid interface obtained from the equivalent circuit is used to indicate that the chloride binding products are formed at the interfaces and influence the tortuosity of the pore system. The experiments and the model suggest an average reduction of about 10% in ϕβ values after the NSSM test.

Chloride transport in fly ash and glass powder modified concretes – Influence of test methods on microstructure

This paper discusses the influence of rapid chloride transport test methods on the microstructure and the measured chloride penetration resistance of concretes containing fly ash or fine glass powder as partial cement replacement materials. Rapid chloride permeability (RCP), non-steady-state migration (NSSM), and steady state conduction (SSC) tests are performed on plain and modified concretes. The glass powder modified concretes demonstrate similar or lower RCP values as compared to the fly ash modified concretes of the same cement replacement level whereas the steady state conductivities are lower for the fly ash modified mixtures. The NSSM coefficients are lower for the fly ash modified concretes even when the initial conductivities are similar to those of plain or glass powder modified concretes. Chloride binding under the conditions of NSSM test, that influence the microstructure and the transport parameter, is quantified using thermal analysis and XRD patterns as well as an electrical circuit model for the impedance response. The resistance of connected pores (Rc) extracted from the model adequately captures the changes in microstructure with time and with chloride transport. The changes in Rc between the start and finish of the NSSM test also indicate microstructural alteration in fly ash modified concretes.

On the time dependency of the chloride migration coefficient in concrete

From studies of the chloride penetration behaviour in laboratory tests and real structures, a clear time dependency of the chloride diffusion coefficient is observed. In order to get insight in this complicated problem, laboratory tests were performed with the non-steady state migration test, described in NT BUILD 492, up to 5 years concrete age. The influence of the type and amount of cement and filler and the amount of water on the migration coefficient and the so-called age factor of 16 self-compacting concrete mixes (SCC) and four traditional vibrated concrete mixes (TC) were investigated. The capillary porosity was introduced as parameter to describe the influence of the concrete composition. An increasing capillary porosity leads to an increased non-steady state migration coefficient and a decreasing age factor.

Influence of cracks and crack width on penetration depth of chlorides in concrete

ABSTRACT Chloride induced reinforcement corrosion is the main durability problem for concrete structures in a marine environment. If the chlorides reach the reinforcement steel, the latter will depassivate and start to corrode in presence of air and water. Since the corrosion products have a larger volume than the initial products, concrete stresses are induced, leading to spalling and degradation of the concrete structures. If cracks are present in concrete, the penetration of chlorides is much faster than in uncracked concrete. In this way, the corrosion process is initiated earlier and the service life is decreasing drastically. In order to investigate the effect of cracks on the chloride penetration, a testing program was carried out. Firstly, a method was developed to create cracks in concrete. Afterwards, chloride penetration tests with the non-steady state migration test described in NT BUILD 492 were carried out. From the penetration profiles, the influence of the crack width on the maximum penetration depth and the extent of the crack influencing zone were investigated. This leads to the conclusion that for increasing crack width, the maximum penetration depth is increasing and that the extent of the crack influencing zone is depending on the crack width.

Influence of cracks and crack width on penetration depth of chlorides in concrete

Chloride induced reinforcement corrosion is the main durability problem for concrete structures in a marine environment. If the chlorides reach the reinforcement steel, the latter will depassivate and start to corrode in presence of air and water. Since the corrosion products have a larger volume than the initial products, concrete stresses are induced, leading to spalling and degradation of the concrete structures. If cracks are present in concrete, the penetration of chlorides is much faster than in uncracked concrete. In this way, the corrosion process is initiated earlier and the service life is decreasing drastically. In order to investigate the effect of cracks on the chloride penetration, a testing program was carried out. Firstly, a method was developed to create cracks in concrete. Afterwards, chloride penetration tests with the non-steady state migration test described in NT BUILD 492 were carried out. From the penetration profiles, the influence of the crack width on the maximum penetration depth and the extent of the crack influencing zone were investigated. This leads to the conclusion that for increasing crack width, the maximum penetration depth is increasing and that the extent of the crack influencing zone is depending on the crack width.

Influence of Cracks on the Service Life of Concrete Structures in a Marine Environment

Chloride initiated reinforcement corrosion is the main durability problem for concrete structures in a marine environment. If the chlorides reach the reinforcement steel, it will depassivate and start to corrode in presence of air and water. Since the corrosion products have a larger volume than the initial products, concrete stresses are induced, leading to spalling and degradation of the concrete structures. If cracks, caused by early drying, thermal effects, shrinkage movements or overstress, are present in the concrete, the penetration of chlorides is much faster compared to uncracked concrete. In this way, the corrosion process is initiated earlier and the service life is decreasing drastically. In order to study the influence of existing cracks in concrete structures on the penetration of chlorides a test program was set up at the Magnel Laboratory for Concrete Research of Ghent University, Belgium in cooperation with the “Politehnica” University of Timisoara, Romania. The first part of the test program consists of concrete specimens with artificial cracks. The chloride penetration into the concrete was realised with a non-steady state migration test and modelled with the finite element method COSMOS/FFE Thermal software. Based on the experimental and numerical results, a crack influencing factor was determined. With this factor, the resulting service life of the cracked concrete construction is determined and compared with the original service life.

Experimental and numerical determination of the chloride penetration in cracked concrete

One of the major causes of deterioration of reinforced concrete structures is chloride-induced corrosion of the reinforcing steel. This phenomenon is significantly influenced by the presence of cracks. This paper presents the experimental and numerical results on the influence of cracks on chloride penetration in concrete structures. The experimental results were obtained with the non-steady state migration test, described in NT BUILD 492, using an electrical field and artificial cracks. The numerical simulation results were obtained using a transient finite element analysis. A variable diffusion coefficient with total chloride concentration was considered in the simulation. A good agreement between experimental and the numerical chloride penetration results was obtained.

Effect of the application of electrical fields on the interactions between the chloride ions and the cementitious matrix

The application of electrical fields to determine the chloride diffusion coefficients through concrete has become a need in order to obtain the required results in reasonable periods of time. However, this acceleration could modify the complex interactions between the chloride ions and the cementitious matrix, being the coefficients obtained by migration tests not representative of those of natural conditions. This subject is undertaken in this paper by performing steady and non-steady state migration tests for chlorides and natural diffusion for oxygen and chloride ions. As a result it has been stated that when applying an electrical field (until 10.5 V/cm), the transport of chloride ions take place through the same kind of pores, but the interaction to combine chlorides is smaller than when driven by natural diffusion. This is a point to be taken into account when designing the experimental conditions for migration tests to determine the non steady state diffusion coefficient of chloride ions.