Surface Chloride Concentration Research Articles

Analytical solution and experimental validation for dual time-dependent chloride diffusion in concrete

An analytical solution for dual time-dependent chloride diffusion within concrete was developed by taking into account the time-dependent behaviour of both surface chloride concentration and chloride diffusion coefficient. Firstly, the time-varying chloride diffusion coefficient was transformed into a constant one by introducing the equivalent diffusion time. Then an analytical solution for dual time-dependent chloride diffusion within concrete was presented based on the Duhamel’s theorem. Finally, the accuracy and applicability of the proposed analytical solution were validated by comparing with field data, existing approximate analytical solutions and numerical results from finite element analysis. Analysis results show that the influence of time-dependent behaviour of chloride diffusion coefficient on the chloride concentration distribution within concrete is much larger than that of surface chloride concentration. Furthermore, the proposed solution is applicable for various kinds of (such as square-root, exponential, logarithmic and power-law, etc.) time-varying models of surface chloride concentration with high accuracy.

Probabilistic analysis of chloride penetration in reinforced concrete subjected to pre-exposure static and fatigue loading and wetting-drying cycles

Abstract Reinforced concrete (RC) structures are subjected to environmental actions and loading that could significantly affect their serviceability and safety. Among these actions these paper focuses on the interaction between concrete cracking produced by static and fatigue loading and chloride ingress. Given the complexity of the problem and its related uncertainties, we carried out an experimental study that provided information for the probabilistic characterization of chloride ingress model parameters by accounting for static and fatigue loading effects. In the experimental part of the study RC specimens (beams) were subjected to pre-exposure loading and wetting-drying chloride exposure. At the end of the simulated chloride environmental attack, ϕ 100 × 120-mm cylinders were drilled out from the bottom side of the central zone of the beams and used to measure chloride profiles within the concrete cover. The probabilistic analysis of test results showed that static and fatigue loading increases both the mean and standard deviation of chloride content for concrete depths larger than 15 mm. In order to evaluate the effect of loading on modeling chloride ingress processes, experimental data was used to fit the surface chloride concentration and the chloride diffusion coefficient for an analytical chloride ingress model as random variables. The identified random variables are then used to determine the distribution of the time to corrosion initiation for the present experimental configuration and to evaluate the long-term probability of corrosion initiation for other environmental exposures and cover configurations. The overall results confirm that considering pre-exposure loading conditions has an important effect in lifetime assessment. This effect depends on the aggressiveness of the surrounding environment.

A time-variant model of surface chloride build-up for improved service life predictions

Current service life models for predicting the time to chloride-induced corrosion initiation of steel reinforcement in concrete structures are based on a hypothesized surface chloride concentration, Cs, as a boundary condition. This is either assumed to be constant or varies with time, generally disregarding other factors that influence Cs. For example, Fickian models use a constant Cs and existing time-variant models assume Cs is only a function of time. In this paper, an improved time-variant Cs model is hypothesized using general physical concepts and is then validated by an empirical study. The proposed model, as opposed to the existing time-variant models, not only accounts for the variability of Cs with exposure time but also incorporates the effects of time to exposure and the effects of the concentration of chlorides in the exposure environment. The model assumes that Cs is sigmoidal in shape with an asymptote that is a function of the concentration of chlorides in the environment. The input variables for the proposed model were selected based on best subset sampling analysis on the results of the experimental work to determine the influence of water-to-cement ratio, time to exposure, the concentration of chlorides in the exposure environment, and exposure time on Cs. The accuracy of the proposed model is assessed versus existing time-variant Cs models and the results indicate that the proposed model better predicts the Cs in concrete exposed to chlorides. Moreover, results of service life predictions using the proposed model show that the proposed model yields more accurate results, better protecting owners from financial and engineering risks.

Mechanical behavior and chloride penetration of high strength concrete under freeze-thaw attack

This paper proposes an experimental study to investigate the freeze-thaw (FT) effects on mechanical behavior and chloride diffusion in high strength concrete. Sets of high strength concrete cubes are cyclically frozen in range from +18°C to −20°C. The mechanical property and the chloride penetration tests are performed on the specimens after 0, 5, 10…95, and 100 FT cycles. FT effects on surface scaling damage, compressive strength, elastic modulus, stress-strain relationship and chloride resistance of high strength concrete are clarified. Also, a simple model is discussed and verified to predict the chloride concentration of high strength concrete attacked by FT cycles. Results show that the behavior of high strength concrete depends on the number of FT cycles. Visible damages appear in concrete after about 40 FT cycles. Before 25–60 FT cycles, no significant difference exists in FT-caused weight loss between normal strength concrete and high strength concrete. Beyond this, the FT-caused weight loss in normal strength concrete becomes faster than that in high strength concrete. FT damage decreases the compressive strength, the elastic modulus and the chloride resistance of concrete, but increases its ductility. In the present test, the compressive strength and elastic modulus decrease about 32% and 24% after 100 FT cycles, respectively. The surface chloride concentration and chloride diffusion coefficient increase to about twice of the control values.

Investigation of computational model for surface chloride concentration of concrete in marine atmosphere zone

Current computational models for the surface chloride concentration of concrete exposed to marine atmosphere zone cannot comprehensively take into account the influence of marine environmental conditions and material parameters of concrete. To overcome this limitation, the transportation process of chloride ions from the marine atmosphere into bulk concrete were explored. Then an improved computational model for the surface chloride concentration of concrete exposed to the marine atmosphere zone was developed on the basis of a large amount of field data. Both environmental conditions and material parameters were considered in the proposed model, including the distance from the sea, wind speed, water-to-binder ratio, binder type and exposure time. The accuracy and applicability of the proposed model were validated by comparing with current computational models and field investigation data. Results show that the proposed model is consistent with the process of transportation and accumulation of chloride ions from sea air into bulk concrete, which overcomes the aforementioned limitation in current models.

Probabilistic time-dependent sensitivity analysis of HPC bridge deck exposed to chlorides

A robust finite element based reinforced concrete bridge deck corrosion initiation model is applied for time-dependent probabilistic sensitivity analysis. The model is focused on uncertainties in the governing parameters that include variation of high performance concrete (HPC) diffusion coefficients, concrete cover depth, surface chloride concentration, holidays in reinforcements, coatings and critical chloride threshold level in several steel reinforcements. The corrosion initiation risk is expressed in the form of probability over intended life span of the bridge deck. Conducted study shows the time-dependent sensitivity analysis to evaluate the significance of governing parameters on chloride ingress rate, various steel reinforcement protection and the corrosion initiation likelihood. Results from this probabilistic analysis provide better insight into the effect of input parameters variation on the estimate of the corrosion initiation risk for the design of concrete structures in harsh chloride environments.

Impacts of Global Warming and Sea Level Rise on Service Life of Chloride-Exposed Concrete Structures

Global warming will increase the rate of chloride ingress and the rate of steel corrosion of concrete structures. Furthermore, in coastal (atmospheric marine) zones, sea level rise will reduce the distance of concrete structures from the coast and increase the surface chloride content. This study proposes a probabilistic model for analyzing the effects of global warming and sea level rise on the service life of coastal concrete structures. First, in the corrosion initiation stage, an improved chloride diffusion model is proposed to determine chloride concentration. The Monte Carlo method is employed to calculate the service life in the corrosion initiation stage; Second, in the corrosion propagation stage, a numerical model is proposed to calculate the rate of corrosion, probability of corrosion cracking, and service life. Third, overall service life is determined as the sum of service life in the corrosion initiation and corrosion propagation stages. After considering the impacts of global warming and sea level rise, the analysis results show that for concrete structures having a service life of 50 years, the service life decreases by about 5%.

Acelerated Test Methods: Determining the Diffusion Coefficient of Concretes in the Saturated Condition

The chloride diffusion coefficient indicates the capacity of a particular type of concrete to resist chloride penetration and is therefore used to predict the service life of a particular reinforced concrete structure exposed to environments containing this type of aggressive agent. Its experimental determination is time-dependent and time-consuming. For that reason, our study analyzes the characteristic behavior of the diffusion coefficient (D) of concretes in the saturated condition by testing higher NaCl concentrations and lower contamination ages than those used in standardized tests, in addition, the objective is to analyze the behavior of surface chloride concentration (Cs) over time. Therefore, it was concluded that for concrete dosed with pozzolanic cement, the Cs value varied with increasing tendency at higher ages. In addition, the D value obtained by the proposed method presented values ​​close to those obtained by standardized tests at contamination ages of 21 and 35 days.

Chloride transport in concrete under sustained flexural loading

Chloride transport in structural concrete has been found to be both stress- and time-dependent. Several models have been developed to take into account the time dependency of the chloride diffusivity of concrete. However, less is understood about the stress dependency and the interaction between the two dependencies. Experiments were conducted to investigate chloride transport in concrete subjected to different tensile stress levels and different periods of wet–dry cycling exposure (56, 112, 168 and 224 d). Two key parameters were investigated – surface chloride content (Cs) and the apparent chloride diffusion coefficient (Da). It was found that regardless of the tensile stress level, the values of Da and Cs respectively decreased and increased with time due to the continuous hydration of cement and the periodic surface wetting with chloride solution. However, both the decreasing and increasing rates were found to be dependent on the tensile stress levels. Both Da and Cs increased with the tensile stress level due to the opening of pores and the development of microcracks in concrete. Practical models were developed to reflect the above time and stress dependency of Da and Cs, as well as their interactions. The models were validated through comparisons of the experimental and predicted chloride profiles of all the tested specimens.

REDUCING ROAD SALT APPLICATION BY CONSIDERING WINTER MAINTENANCE NEEDS IN PARKING LOT DESIGN

INTRODUCTION Winter snow and ice can have a significant impact on our mobility, whether on foot or by car. Alongside plowing, arguably the greatest tool in combating snow and ice is salt. The most commonly used salt for winter maintenance is Sodium Chloride (NaCl), the same salt used in food and water softeners, is applied to roads, sidewalks, and parking lots as it is an effective deicer when temperatures are between 0°C and −12°C. Studies have shown that deicing with salt reduces accidents by 88% and injuries by 85% (Salt Institute 2017). The effectiveness of road salt, as well as its relative affordability, means that as much as four million tonnes may be applied annually in Canada for deicing (Environment Canada 2012). However, while salt is relatively inexpensive to purchase, there are a number of external costs that are becoming increasingly apparent. These include corrosion of vehicles and infrastructure like concrete, bridges, and water mains; damage and staining to the interior and exterior of bu...

Depassivation time estimation in reinforced concrete structures exposed to chloride ingress: A probabilistic approach

Estimation of depassivation time is a key issue in corrosion prevention. A method to get a probabilistic model from a deterministic one is presented and applied to three simple models: square root of time (SRT), error function (EF) and constant flux (CF) models. Probability distributions of the involved random variables are needed as input parameters. Experimental data have been obtained from a concrete structure exposed to the atmospheric marine environment. These data are analysed to obtain the probability distributions of chloride transport parameters: penetration velocity k (SRT model), diffusion coefficient D (EF and CF models), surface chloride concentration CS (EF model), and chloride ingress flux J (CF model). These distributions are used to calculate the depassivation time probability distributions according to the three models and the orientation of the samples respect to the sea. This allows to estimate depassivation time for a given depassivation probability.

Estimation of Corrosion-Free Life for Concrete Containing Ground Granulated Blast-Furnace Slag under a Chloride-Bearing Environment

The rate of chloride transport by diffusion in concrete containing ground granulated blast-furnace slag (GGBS) was mathematically estimated to predict the corrosion-free service life of concrete structures exposed to seawater environment. As a factor to corrosiveness of steel embedment, replacement ratio of GGBS was selected, accounting for 25 and 50% to total binder. As a result, it was found that an increase in the GGBS content resulted in an increase in the chloride binding capacity, which would give rise to a lower chloride diffusion rate, thereby reducing the risk of chloride-induced corrosion. When it comes to the sensitivity of parameters to service life, the effective diffusivity showed a marginal influence on serviceability, irrespective of GGBS contents while surface chloride content and critical threshold concentration revealed more crucial factors to long term chloride diffusion. As the GGBS replacement increased, the variation in service life has become less influential with changing parameters. Substantially, GGBS concrete at high replacement ratio enhanced the service life due to a combination of dense pore structure and enhanced chloride binding capacity.

Comparison of chloride penetration into surface-treated concrete in artificial and natural environments

This study investigated the penetration of chloride into surface-treated high-performance concrete and normal concrete in natural and accelerated environments. Both high-performance concrete and normal concrete were applied in a real port. Concrete specimens that were cast together with the concrete port were transported to the laboratory and subjected to wetting and drying cycles with NaCl solution. The chloride contents of the specimens in the laboratory and the in situ components were tested. The chloride diffusion coefficients and surface chloride contents were calculated based on Fick’s second law. The results show that high-performance concrete and surface treatment clearly slow the chloride penetration into the concrete both in the laboratory and in situ. The chloride contents on the surface and in the concrete in the components of the concrete port are higher during the summer than during the winter. The chloride penetration performance in the concrete of real structures cannot be inferred from its performance in specimens under artificial environments in the laboratory.

Service life variation in RC columns exposed to chloride attack: a time-dependent deterministic and probabilistic examination

AbstractService life prediction for reinforced concrete (RC) is usually performed through deterministic or probabilistic methods in regard to anticipating the phenomenon of chloride attack. The field investigation results from long-term exposure can provide reasonable data for evaluating service life, but corrosion can progress differently due to local conditions, even with the same mix proportions and exposure conditions. This paper uses two methods (deterministic and probabilistic) to explore the variation of the service life of RC columns which have been exposed to chloride attack for 3.5–4.5 years. Fieldwork was carried out on four RC columns exposed to tidal and atmospheric zones, and various chloride profiles from 25 cores were obtained. The chloride diffusion coefficients and surface chloride contents were analysed in relation to the heights at which the cores were taken and the corresponding exposure conditions. Increasing surface chloride content and decreasing diffusion coefficient with time are...

Evaluation of Additional Protection Methods to Control Reinforcement Corrosion

The prevention of reinforcement corrosion is primarily achieved by using high quality concrete, adequate concrete cover and suitable casting and curing. Additional prevention methods are adopted when severe environmental conditions occur on structures requiring very long service life. While existing standards, EN 206 or Eurocode 2, adopt prescriptive approach, performance based methods can improve the evaluation of the service life and enable the comparison of available design options. The reliability of the prediction is very important and even widely used models (e.g. fib Model Code) still have open questions. In this paper different additional protection are evaluated: corrosion inhibitors, stainless steel rebars, concrete coatings. A simplified approach is used, considering that the serviceability limit state, i.e. initiation of corrosion in chloride containing environments, can be modeled through the analytical solution of the 2nd Fick’s law of diffusion. The main factors influencing corrosion initiation (critical chlorides concentration, chlorides diffusion coefficient, surface chloride concentration, concrete cover) are represented by distribution of values and the probability distribution of service life is evaluated by Monte Carlo simulation.

Development of a New Concrete Marine Exposure Site on the Arabian Gulf-East Coast of Saudi Arabia

A concrete durability site in the Eastern Province of Saudi Arabia that exposes Saudi concrete to harsh marine conditions on the Arabian Gulf and salty air conditions was constructed. The site contains 32 concrete blocks formed from eight different mixtures made with Cement Type I, Cement Type V, Cement Type I with 6% silica Fumes, 25% fly Ash, 70% Ground-granulated blast-furnace slag (GGBFS), Migrate Corrosion Inhibitors (MCI), Calcium Nitrite based inhibitors (CNI) and Caltite. Each mixture formed 4 blocks; 2 were reinforced with black steel and two were plain concrete. The concrete blocks were 1200 mm high, 460 mm wide and 230 mm thick placed vertically. The concrete blocks were placed at three zones to achieve different exposure conditions which are atmospheric, splash or spray zone and tidal zone. Samples will be taken periodically to measure the rate of chloride ingress in each concrete mixture under the different exposure conditions. Embedded steel reinforcement in specimens in atmospheric zone have not recorded corrosion activities at six months of exposure. The steel bars in three zones will be monitored for corrosion activity through linear polarization test. The measured chloride profiles in the exposure site will be compared to the measured concrete transport properties from the companion laboratory specimens. Standard compressive test and bulk diffusion test were conducted for 150x300 mm and 75x150 standard cylinders, respectively. All of the cylinders’ strength exceeded the designed compressive strength of 28 MPa. Whereas for 35 days standard bulk diffusion test, mix contained 100% cement type I had the least chloride concentration, and mix 5 that had 70% GGBFS and 30% cement type I had the highest surface chloride concentration. Overall chloride profiles for all the mixes were within ranges reported in literature. Further tests of chloride binding capacity and bulk diffusion for different intervals will be undertaken.

Durability of Concrete under Combined Exposure Conditions of Chlorides and Sulfates

Concrete structures are generally subjected to serious deterioration under harsh environment, even though the concrete materials exhibit inherently higher durability than any other construction materials. The service life of concrete structures is directly affected by the durability performance under various conditions. In this regard, many studies have been conducted on the deterioration of concrete structures under various environmental conditions. However, previous studies were confined mostly to the deterioration of concrete structures under single deteriorating factor such as chloride ingress only or sulfate attack only, although real environment is actually a combination of such factors. The purpose of the present study is, therefore, to explore the effects of combined deterioration due to chlorides and sulfates in concrete structures. To this end, comprehensive experimental program has been set up to observe the chloride penetration behavior under combined deterioration conditions of chlorides and sulfates. The test results indicate that the chloride penetration is more pronounced for the case of combined attacks of chloride and sulfates than the case of single chloride attack. The surface chloride content is found to increase with time and this phenomenon is also more pronounced under the combined exposure of chlorides and sulfates. The present study may allow more realistic assessment of durability for such concrete structures which are subjected to combined attacks of chlorides and sulfates.

Long-term field study of chloride ingress in concretes containing pozzolans exposed to severe marine tidal zone

Chloride ingress in concrete is the most deteriorating factor for reinforced concrete structures exposed to marine environment. This phenomenon depassivates reinforcing steel in concrete and causes steel corrosion. To improve the durability and serviceability of concrete structures in harsh environments with high relative humidity, temperature and concentration of chloride ions such as in the Persian Gulf, the addition of silica fume and other pozzolans have been recommended by several investigations. In this paper, the chloride ion diffusivity and mechanical performance of concretes containing silica fume, metakaolin and natural zeolite, and those of concretes with different water-cement ratios (w/c) under in-situ tidal exposure condition were investigated. To achieve this objective, concrete specimens with w/c of 0.35, 0.40, 0.45 and 0.50 were fabricated. In addition, to examine the performance of pozzolans, other specimens with constant water-binder ratio (w/b) of 0.40 containing 5% silica fume, 5% metakaolin and 10% natural zeolite were prepared. All of the specimens were subjected to laboratory and tidal exposure in the field conditions. The results indicate that 10% natural zeolite, 5% metakaolin and 5% silica fume have similar effects on concrete strength and durability in harsh marine environments. In addition, it is concluded that at long time field exposure, chloride diffusion coefficient and surface chloride content of different mixtures reach to relatively similar amounts.

Predictive convection zone depth of chloride in concrete under chloride environment

Based on the assumption that chloride concentration and water influential depth change linearly in convective zone, the chloride convection zone depth is used in Fick’s diffusion law to investigate the chloride diffusion characteristic of concrete at tidal zones. In this study, an artificial simulation environment and field tests were carried out to validate and calibrate the rationality of the chloride convection zone depth model. The distribution model of surface chloride concentration in concrete was determined by exploring the correlations among chloride concentration, elevation, time and linear distance from seaside. The results illustrated that the depth of chloride convection zone on the concrete surface varied and manifested in terms of a function of water influential depth. The relation between the surface chloride concentration and the linear distance from the seaside followed an S-curve function equation. The similarities and differences regarding the characteristic of chloride diffusion in concrete between experimental and in-situ test were discussed for surface chloride concentration, chloride convection zone depth and chloride diffusion coefficient.

Chloride ingress into marine exposed concrete: A comparison of empirical- and physically- based models

In establishing the reliability of performance-related design methods for concrete – which are relevant for resistance against chloride-induced corrosion - long-term experience of local materials and practices and detailed knowledge of the ambient and local micro-climate are critical. Furthermore, in the development of analytical models for performance-based design, calibration against test data representative of actual conditions in practice is required. To this end, the current study presents results from full-scale, concrete pier-stems under long-term exposure to a marine environment with work focussing on XS2 (below mid-tide level) in which the concrete is regarded as fully saturated and XS3 (tidal, splash and spray) in which the concrete is in an unsaturated condition. These exposures represent zones where concrete structures are most susceptible to ionic ingress and deterioration. Chloride profiles and chloride transport behaviour are studied using both an empirical model (erfc function) and a physical model (ClinConc). The time dependency of surface chloride concentration (Cs) and apparent diffusivity (Da) were established for the empirical model whereas, in the ClinConc model (originally based on saturated concrete), two new environmental factors were introduced for the XS3 environmental exposure zone. Although the XS3 is considered as one environmental exposure zone according to BS EN 206-1:2013, the work has highlighted that even within this zone, significant changes in chloride ingress are evident. This study aims to update the parameters of both models for predicting the long term transport behaviour of concrete subjected to environmental exposure classes XS2 and XS3.