Chloride Profiles Research Articles

Combining experimental and modelling approaches to monitor the transport of an artificial tracer through the hyporheic zone

AbstractIn order to advance methodologies used in the investigation of Hyporheic Zone (HZ) mixing processes, this article combines experimental and modelling tools to follow a tracer injected into the river and infiltrating into the HZ. A highly concentrated sodium chloride solution was injected into the river; (i) the river conductivity, (ii) the riverbed resistivity by Electrical Resistivity Tomography (ERT) and (iii) vertically distributed chloride concentrations within the HZ were monitored. Both ERT and concentration measurements showed an infiltration depth of the tracer of 35 cm, and a partial recovery after injection, which was faster within the superficial layer that was found to be more resistive according to the ERT initial image. The modelling approach used the HydroGeoSphere code to model the coupling between river surface flows and HZ groundwater flows and transport processes. The model set‐up involved a 50 cm high existing riverbed step, a vertical contrast in HZ saturated hydraulic conductivity and the aquifer discharge flux. Fitting the vertical chloride profile, the adjusted values were 5 × 10−2 m s−1 for the saturated hydraulic conductivity of the first highly permeable layer below the riverbed, and 4 × 10−6 m s−1 for the aquifer discharge flux. The bottom layer saturated hydraulic conductivity was found to be at least 10 times lower than the value within the first layer. Numerical simulations showed that the two main parameters controlling the mixing within the HZ were the groundwater discharge and the saturated hydraulic conductivity first sediment layer of the riverbed. The riverbed step was found to be less significant here compared to these two parameters. The combination of experimental and modelling tools allowed us to quantify the aquifer discharge flux, which is complicated to investigate in the field without any model. Results of this study showed that combining modelling with ERT and vertically distributed chloride sampling allows the quantification of the main factors controlling the hyporheic exchange.

Effects of coupled heat and moisture and load damage on chloride transport in concrete

This paper presents thoroughgoing research on chloride transport in damaged concrete. The effects of temperature and temperature gradient on chloride transport were investigated, along with the effects of relative humidity, humidity gradient, concrete damage and exposure time. The higher the temperature and the greater the humidity gradient were, the quicker the chloride transport was. Moisture transport increased as concrete damage increased, while chloride transport decreased incrementally. Considering the effect of coupled heat and moisture on chloride transport in concrete, a chloride transport model was established and verified by experiments. Chloride profiles in damaged concrete were related to temperature, temperature gradient, relative humidity and humidity gradient. The chloride attack rate decreased with increasing concrete damage and exposure time. Hence, coupled heat and moisture as well as concrete damage had significant effects on chloride transport in damaged concrete, and the effects of concrete damage on chloride transport should be considered when determining chloride profiles in damaged concrete.

Effect of leaching on the composition of hydration phases during chloride exposure of mortar

Mortar specimens were exposed to either a 3% NaCl solution or a 3% NaCl+KOH solution for up to 180 days. Exposure to the NaCl solution provoked much more leaching than the NaCl+KOH exposure. Leaching strongly impacted the chloride ingress profiles. The extended leaching led to a maximum total chloride content almost three times higher and a deeper chloride penetration than exposure with limited leaching after 180 days. The higher maximum chloride content seems to be linked to the enhanced binding capacity of the C-S-H and AFm phases upon moderate leaching as determined by SEM-EDS. The total chloride profile appears to be governed by multi-ion transport and the interaction of chloride with the hydration phases. Service life prediction and performance testing both rely on total chloride profiles and therefore ought to take these interactions into account.

Chloride Profiles in Carbonated Concrete

Chloride Profiles in Carbonated Concrete

Chloride profiles with a peak – why and what are the consequences for predictions?

Chloride ingress profiles do almost always have a peak at some depth but most prediction models are missing this peak. Some prediction models, such as the fib model, simply “cut off” a slice of the concrete up to the peak in further predictions. Other prediction models use data only from the profiles beyond the peak but include the concrete up to the peak as if it has the same properties as the rest of the concrete. A physical model has been developed to quantify the local changes because of leaching and the consequences of these changes with time. The model uses Fick’s 1st law for chloride diffusion and linear chloride binding. The depth of leaching with time is modelled with a simple square-root equation. The consequences of leaching are assumed to be linear from the surface into the maximum depth of leaching. The consequences of leaching are modelled as depth-dependent changes of porosity, chloride binding and the diffusion coefficient in Fick’s first law.

A methodology for comparing the chloride penetration severity across different tidal zones

Corrosion of steel in reinforced concrete structures exposed to the marine tidal environment severely affects the durability of structures. Comparing the severity of corrosion vulnerability across tidal zones requires a systematic procedure encompassing various environmental factors affecting tidal zones. A mathematical framework for comparing the severity of chloride penetration in concrete structures exposed to tidal zones at different coastal stations is proposed in this study. Validated moisture and chloride transport models are used to simulate the long-term moisture and chloride profiles in concrete exposed to tidal wetting and drying. The boundary conditions are formulated using real-time tide heights and several meteorological parameters: temperatures, wind velocities, and relative humidity values. Various penetration indices are used to characterize the severity of chloride and moisture penetration in concrete from the viewpoint of corrosion initiation. The developed framework has been implemented for the case of two Indian seaports located at Visakhapatnam and Mumbai. The proposed methodology may be extended to conduct a comparative corrosion vulnerability assessment of various coastal stations and classify them into durability zones.

Procedure for determining the remaining time to initiation of chloride induced reinforcement corrosion for existing concrete structures

For efficient asset management of reinforced concrete infrastructure, owners want to know when to expect corrosion in structures. At present, no widely accepted procedures for testing of existing structures for the remaining time to corrosion initiation are available. This paper describes such a procedure, based on the authors’ long-time experience. From about 20 years age, the concrete contains its mature response to actual environmental loads, e.g. chloride ingress. By measuring the mean and standard deviation of the actual cover depth, taking chloride profiles, assuming a few parameters and a simple, pragmatic model, the expected time to corrosion initiation for a particular test area can be predicted. Required numbers of cores and samples per core are given. Uncertainties are taken into account by subtracting a safety margin from the measured concrete cover depth. Results of six cores per test area are classified and interpretations are given. Because of large variability, the results are classified in three ranges of time to corrosion initiation: five years or less, five to fifteen years, or more than 15 years. The procedure has been approved by the relevant national Standards committees and was issued as a CUR Recommendation in 2018. It is illustrated based on a field case and the obtained results are discussed.

Ert Results Analysed with Two Inversion Methods to Determine Chloride Profiles on BFS Concrete Having Very High Electrical Resistivity

Ert Results Analysed with Two Inversion Methods to Determine Chloride Profiles on BFS Concrete Having Very High Electrical Resistivity

Artificial Weathering Resistance Test Methods for Building Performance Assessment of Profiles Made of Natural Fibre-Reinforced Polymer Composites.

A growing popularity of profiles made of natural fibre-reinforced polymer composites in civil engineering encourages determining test methods relevant for building performance assessment. Weathering resistance is among the key aspects that condition the durability of building structures. The paper includes a comparative analysis of two artificial weathering resistance test methods. Polyvinyl chloride and wood flour composite profiles were tested. They were subjected to UV and spraying (X-exposure) and UV, spraying and wetting by condensation (F-exposure), both at different exposure times. The influence of the applied weathering procedures on the composite’s microstructure and its mechanical characteristics were analysed. No changes in the microstructure of brittle fractures were observed. However, surface morphology changes were revealed, noticeably greater following X-exposure than F-exposure. F-exposure exerted significant influence on the mechanical properties of brushed profile, including, but not limited to, flexural modulus. Whereas X-exposure exerted more influence on the mechanical properties of non-brushed profile.

Evaluating the Chloride Absorption of Unsaturated Cover-Concrete Using Electrical Measurements

To avoid chloride induced corrosion of steel in reinforced concrete structures, it is firstly important that the chloride amount in cover concrete is quantified. This study investigates the influence of chlorides on the electrical conductivity of Portland cement concretes through a chloride absorption test conducted over a wide frequency range (10 H z–1 MHz). To this end, the concretes were exposed to 10% NaCl solution for up to 48 h. In addition to calculating the sorptivity of concrete, its electrical conductivity was de-embedded from the impedance measurements, fitted to the Jonscher equation to determine the bulk conductivity, and presented in the frequency domain to investigate the main conduction processes with chloride penetration. Further, chloride profiling was performed to quantify the chloride concentration and identify the transport behaviour within concrete. Ionic conduction was shown to be the main conduction process in concrete containing absorbed chlorides, and the amount of absorbed chlorides was shown to have a linear relationship with the conductivity. In addition, the acceptable frequency range for representing the bulk conductivity of Portland cement concretes was confirmed to be 450 Hz–1 kHz. Moreover, the moisture content was shown to be an important factor in the impedance measurement.

Impact of leaching on chloride ingress profiles in concrete

To investigate the effect of leaching on chloride ingress profiles in concrete and mortar, we exposed concrete and mortar specimens for 90 and 180 days to two different exposure solutions: 3% NaCl, and 3% NaCl with KOH added to limit leaching. The solutions were replaced weekly. After exposure, we determined total chloride profiles to investigate the chloride ingress, and portlandite profiles to assess the extent of leaching. The results showed that leaching during exposure greatly affects the chloride ingress profiles in mortar and concrete. We found that leaching leads to considerably higher maximum total chloride content and deeper chloride penetration into the concrete than in the specimens where leaching was limited. We recommend therefore that leaching should be taken into account in standard laboratory testing and that more mechanistic service life models should be used to take into account the impact of leaching.

Durability assessment of a tunnel structure with two‐sided chloride ingress—A case study located in a tropical environment

AbstractReinforced concrete structures are the backbone of the transportation infrastructure. Their structural safety and functional performance are critical to sustaining a globalized economy. Corrosion of steel reinforcement is typically triggered by chloride contamination and carbonation, becoming a primary factor in reinforced concrete deterioration. This paper presents a durability assessment of a tunnel structure located in a tropical marine environment. The condition assessment included visual examination, crack survey, cover depth measurements, half‐cell potential mapping, and concrete coring. The chloride profiles showed that the tunnel walls are subjected to chloride attack from both surfaces, an unusual finding in the literature. A stochastic approach is implemented to estimate the probability of corrosion initiation for chlorides and carbonation, including an innovative model for two‐sided chloride diffusion. The results indicate that two‐sided chloride exposure significantly reduces the corrosion initiation time.

The role of chloride binding mechanism in the interpretation of chloride profiles in concrete containing limestone powder

The effect of limestone powder addition on the chloride binding capacity of concrete was evaluated. The results indicate that the chloride binding capacity becomes weaker as the limestone content increases due to decreased physical adsorption and chemical binding. XRD patterns indicate that adding limestone restrains the formation of monosulfate (AFm), thereby undermining the chemical binding. Thermogravimetric analyses (TGA) validate that despite limited participation of calcite in the hydration reaction, it is significant enough to change the stoichiometry of the cementitious systems and restrain the formation of Friedel's salt. The ratio between the apparent and effective chloride diffusion coefficients increases with decreasing chloride binding. A modified diffusion model was proposed based on Fick’s second law coupled with Freundlich adsorption isotherm on water-accessible pore surfaces to predict the chloride profile. The model exhibits a consistent trend that an enhanced chloride binding, higher tortuosity and lower porosity contribute to a lower chloride content.

Influence of non-linear chloride binding on the determination of apparent chloride diffusion coefficient for cement paste with mineral additives

The determination of apparent chloride diffusion coefficients (Da) in standard codes employs the linear chloride binding assumption. However, this assumption is too simplified as chloride binding is actually non-linear. This paper intends to investigate the influence of non-linear chloride binding on the determination of Da values in the bulk diffusion test. To achieve this aim, a semi-analytical method for the semi-infinite diffusion problem was firstly established. This method can be used in data fitting to obtain the Da values considering non-linear chloride binding. Then, bulk diffusion test and mercury intrusion porosimetry were conducted on four different cement pastes with fly ash and blast furnace slag. Based on chloride profiles, the chloride binding isotherms were obtained through data fitting. Meanwhile, the Da values considering the linear and non-linear chloride binding assumption were calculated respectively and compared. The results show that the chloride binding isotherms in the bulk diffusion test fit well with the Freundlich isotherm but are not so non-linear as expected. The proposed method can calculate the effective diffusion coefficient which cannot be obtained in standard codes. Da values will vary along the chloride ion diffusion direction rather than be a constant once the non-linear chloride binding is considered. The comparison between different cement paste mixtures indicates that both the apparent diffusion coefficient calculated from free chloride profiles or total chloride profiles can only be regarded as a qualitative description of resistance to chloride diffusion for a certain mixture. The comparison between different mixtures based on these values is not reliable.

General model for service life analysis of reinforced concrete structures subject to chloride penetration

The conventional procedure for chloride profiles modeling is based only on the diffusion zone considering the solution of Fick's second law of diffusion, however, the extrapolation of this model to the convection zone is not capable to represent the chloride concentrations. Therefore, the present work analyzes the applicability of the Modified Holliday Equation in profiles obtained from field structures built in different parts of the world (composed of different materials and submitted to specific local environmental characteristics). The main goal of this work is to validate the applicability of the Modified Holliday Equation for service life analysis of field reinforced concrete structures subject to chloride penetration. A mathematical proposal for the determination of the peak’s chloride concentration position, delimiting the convection and diffusion zones, is also part of the scope of this work. The results showed that the empirical model based on the Modified Holliday Equation is capable of representing the chlorides concentration considering both, the data from the convection zone and the diffusion zone. Furthermore, in many analyzed cases, a better statistical fit between the proposed model and the chloride profile concentrations was obtained, when compared to the modeling through the solution of Fick's second law. The mathematical proposal presented for the peak determination has shown to be simple and efficient in cases where there are, at least, three neighboring points, forming a sharp peak in the profile. Finally, the results attested to the possibility of applying the Modified Holliday Equation to represent chloride concentration of field structures in different marine aggressive zones, in order to predict the service life of these structures in a more accurate way.

Analytical solutions of moisture‐chloride ion coupled transport in unsaturated concrete

AbstractStudies of chloride‐induced corrosion in reinforced concrete structures are performed based on the reliable predictions of chloride and moisture distributions in concrete. This paper adopted the Laplace Transformation and the inverse Laplace Transformation to obtain the analytical solutions of the moisture‐chloride two‐way coupled transport processes. Additionally, the model has involved with realistic material parameters to predict the internal distributions of relative humidity and free chloride ions in concrete samples. To illustrate the accuracy of the present model, the calculations were compared with the available experimental results, and good agreements were obtained. It indicates that the present analytical model can successfully characterize the two‐way coupling effects between chloride transport and moisture diffusion in concrete; and the model can provide the distribution profiles of both chloride and relative humidity in concrete, which are useful for predicting the service life of reinforced concrete structures.

Deep North Atlantic Last Glacial Maximum Salinity Reconstruction

AbstractWe reconstruct deep water‐mass salinities and spatial distributions in the western North Atlantic during the Last Glacial Maximum (LGM, 19–26 ka), a period when atmospheric CO2 was significantly lower than it is today. A reversal in the LGM Atlantic meridional bottom water salinity gradient has been hypothesized for several LGM water‐mass reconstructions. Such a reversal has the potential to influence climate, ocean circulation, and atmospheric CO2 by increasing the thermal energy and carbon storage capacity of the deep ocean. To test this hypothesis, we reconstructed LGM bottom water salinity based on sedimentary porewater chloride profiles in a north‐south transect of piston cores collected from the deep western North Atlantic. LGM bottom water salinity in the deep western North Atlantic determined by the density‐based method is 3.41–3.99 ± 0.15% higher than modern values at these sites. This increase is consistent with: (a) the 3.6% global average salinity change expected from eustatic sea level rise, (b) a northward expansion of southern sourced deep water, (c) shoaling of northern sourced deep water, and (d) a reversal of the Atlantic's north‐south deep water salinity gradient during the LGM.

Permeation Characteristics and Surface Accumulation of Chloride in Different Zones of Concrete along Altitude in Marine Environments

The accumulation characteristics of surface chloride in concrete in different zones are different in the marine environment. A series of laboratory experiments were conducted to investigate the surface chloride and permeation characteristics of concrete in a simulated marine environment. The experimental results indicated that the surface chloride and chloride profiles of concrete in different zones of marine environment decreased in the following order: tidal zone > splash zone > submerged zone > atmospheric zone. The width of the ascent zone of Cl− concentration at tidal and splash zones was far less than that of the influential depth of moisture transport (IDMT), and the range of convection zone was dependent on the IDMT. Cl− at splash and tidal zones penetrated into concrete as a bulk liquid by non-saturated permeation driven by a humidity gradient. The change of chloride profiles in concrete along the altitudinal gradient was consistent with that of the cyclic water absorption amount (CWAA). The transport rate of chloride was the highest at the highest point of the tide.

On the Tortuosity-Connectivity of Cement-Based Porous Materials

In this work, the transport equations of ionic species in concrete are studied. First, the equations at the porescale are considered, which are then averaged over a representative elementary volume. The so obtained transport equations at the macroscopic scale are thoroughly examined and each term is interpreted. Furthermore, it is shown that the tortuosity-connectivity does not slow the average speed of the ionic species down. The transport equations in the representative elementary volume are then compared with the equations obtained in an equivalent pore. Lastly, comparing Darcy’s law and the Hagen–Poiseuille equation in a cylindrical equivalent pore, the tortuosity-connectivity parameter is obtained for four different concretes. The proposed model provides very good results when compared with the experimentally obtained chloride profiles for two additional concretes.

Alkaline hydrogels as ion‐conducting coupling material for electrochemical chloride extraction

AbstractOne method to rehabilitate chloride‐contaminated reinforced concrete structures is electrochemical chloride extraction, by which chloride ions are extracted from the pore system of the concrete using an externally applied voltage. Although many factors influence the efficacy of electrochemical chloride extraction, a number of these, such as the concrete cover and the chloride profile, are predefined by the structure. One factor with major influence that can be improved is the coupling material. In this contribution, we present a suitable coupling material in the form of a highly alkaline hydrogel, with which the applied voltage can be reduced from 40–50 to 1 V under laboratory conditions. The reduction is due to the high conductivity of the gel and its excellent electrolytic contact with concrete. In a field test on chloride‐infested car‐park columns, more than 95% of the chloride could be extracted with 2–3 cycles, each comprising of 21 days of extraction and 7 days of rest. During the entire treatement, the gel did not require any attending to, such as replacing evaporated moisture.