Sensitivity analysis of simplified diffusion-based corrosion initiation model of concrete structures exposed to chlorides

This paper presents the results of a sensitivity analysis of the diffusion-based corrosion initiation model for reinforced concrete structures built in chloride-laden environments. Analytical differentiation techniques are used to determine the sensitivity of the time to corrosion initiation to the four governing parameters of the model, which include chloride diffusivity in concrete, chloride threshold level of steel reinforcement, concrete cover depth, and surface chloride concentration. For conventional carbon steel, the time to corrosion initiation is found to be most sensitive to concrete cover depth, followed by chloride diffusion coefficient, with normalized sensitivity coefficients of about 2 and − 1. For corrosion resistant steels, the time to corrosion initiation is most sensitive to the surface chloride concentration and chloride threshold level followed by the concrete cover depth and chloride diffusion coefficient. The results of this sensitivity analysis are discussed in detail, including the variations in predicted time to corrosion initiation induced by variations of the four model parameters and their implications for the design and maintenance of concrete structures built in corrosive environments.     

Corrosion damage diagnosis of a reinforced concrete beam after 40 years natural exposure in marine environment

A detailed investigation of the chloride induced corrosion damage was performed on a 40 years old reinforced concrete beam exposed in marine environment. Visual observations, electrochemical measurements, carbonation depth, total chloride content were carried out. Half-cell potential measurements were used to locate corrosion areas. It appeared that the interpretation based on gradient of the potential was in good concordance with real state of damage. Complementary destructive methods are applied to observe the real corrosion state of steel rebars and characterize the corrosion products and the steel/concrete interface (optical and electronical microscopy tools (XRD, SEM, EDS and μ-Raman).All these data indicate that on the beam, one may distinguish two types of areas: “high-corrosion zones” and “low-corrosion zones.” Given the fact that the “high corrosion zones” were found to be close to corrosion induced cracks and that they have a different morphology, this contribution concludes that the position of these areas did not shift in time.     

Corrosion performance of conventional (ASTM A615) and low-alloy (ASTM A706) reinforcing bars embedded in concrete and exposed to chloride environments

Conventional reinforcing steel is used in the majority of reinforced concrete structures. In general, steel reinforcement meeting ASTM A615 specifications has been the predominant reinforcement used for these structures. Low-alloy reinforcing steel (ASTM A706) was developed and is being marketed to improve ductility and weldability deficiencies associated with the ASTM A615 reinforcement. Several State Highway Agencies have adopted the use of these low-alloy reinforcing steels. Limited research has been performed on the corrosion characteristics of the steel reinforcement meeting ASTM A706 specifications. This paper presents results from a laboratory study on the critical chloride threshold, macrocell corrosion rates, and mass loss testing for ASTM A706 and ASTM A615 reinforcing steels embedded in concrete and exposed to chloride solution. Results from this study indicate that ASTM A706 reinforcing steel exhibits lower critical chloride threshold levels and higher corrosion rates than ASTM A615 reinforcing steel when embedded in cementitious materials.     

Chloride-induced corrosion of reinforced concrete bridge decks

A closed-form solution is developed to predict the corrosion initiation time of reinforced concrete bridge decks using measured time varying surface chloride accumulations. The data base for the surface chlorides are core measurements at a shallow depth below the surface of 15 bridge decks in the snow belt region. The data base was collected during the bridges' biennial inspections over a period of 15 years. Regression analysis is used to represent the surface chlorides by an exponential variation with time. The time predicted to initiate corrosion is computed for different values of the effective diffusion coefficient and the concrete cover thickness. The results are compared to the constant surface accumulation model commonly used in the literature. As expected, the corrosion initiation based on constant chloride accumulation at the surface is faster (in some case by up to 100%) than the initiation time calculated from actual chloride concentration data. Such results are useful for the realistic estimation of the service lives of bridge decks and for scheduling bridge deck maintenance and rehabilitation programs.     

Threshold chloride concentration for stainless steels activation in concrete pore solutions

The threshold chloride concentration for stainless steels activation in concrete environments depends not only on chemical composition of the steel and on pH level of concrete pore solution but is also considerably affected by the superficial state of the steel. The presence of scales leads to significant decrease of corrosion resistance. This study evaluates the corrosion resistance of various stainless steels exposed to pore solutions of fresh concrete by means of electrochemical methods. All tested materials with bare surface, except for martensitic chromium steel FeCr12, proved to be completely resistant to fresh concrete pore solution containing chlorides. If the surface is scaled, the identically high resistance cannot be expected even for the most alloyed stainless steels.     

Characteristic service life for concrete exposed to marine environments

A statistical treatment has been applied to a deterministic service life model of concrete structures in marine environments. The chloride ingress model based on Fick's second law of diffusion was assumed. The quality of concrete was quantified in terms of three factors, namely, an apparent diffusion coefficient at 1 year (D_a), surface chloride concentration (C_s) and a critical chloride level (C_cr). The standard deviation of service life can be estimated from standard deviations of the four factors, namely, C_s, D_a, C_cr and cover depth. The effect of the severity of environment on service life was also demonstrated. With data from the literature and an approximation of the inverse error function, sensitivity analyses were carried out. Service life was found to be more sensitive to cover depth than the diffusion coefficient, and more sensitive to surface chloride concentration than the critical chloride level. Characteristic service life of a range of normal Portland cement (NPC) concrete grades was evaluated as a function of 28-day strength and cover depths for a nominated confidence level. Such characteristic service life can be readily used and appreciated by design engineers.     

Laboratory investigation of reinforcement corrosion initiation and chloride threshold content for self-compacting concrete

Time-to-corrosion (T_i) of reinforcement in concrete and chloride threshold content (C_th) are important service life determinants for reinforced concrete structures in chloride-laden environments. In this study, the two determinants of a series of self-compacting concretes (SCC) and regular concretes were experimentally investigated. A new sampling approach for C_th determination (milling powder from corrosion active site at the rebar/concrete interface) was adopted to accurately express chloride content resulting in corrosion occurrence. It is found that the T_i and C_th follow the 3-parameter Weibull distribution. The results indicate that the corrosion initiation of rebar in concrete slabs depends upon both cement alkalinity and superplasticizer. Rebar, embedded in high alkalinity cement SCC, exhibits better corrosion resistance as indicated by the longer T_i, higher C_th and larger Weibull modulus, m. A larger Weibull modulus indicates that anti-corrosion performance of rebar in slabs is more stable and less scattered. The effects of specific superplasticizer on rebar corrosion resistance are discussed from the viewpoint of air void amount and size distribution at the rebar/concrete interface.     

Corrosion inhibitors for chlorides induced corrosion in reinforced concrete structures

Reinforcements corrosion is the most important cause of premature failure on reinforced concrete structures. Phenomena promoting corrosion are the ingress of chlorides and the reaction of atmospheric CO₂ with cement paste. Aim of this paper is the investigation on the effectiveness of three organic commercial inhibitors in preventing carbon steel chlorides induced corrosion in concrete, since there is not yet a clear knowledge on the real effectiveness of these products. Inhibitors were added to the concrete mixture in dosage suggested by the manufacturers. Chlorides were added in the concrete mixture or penetrated from outside by “ponding” cycles with a 3.5% sodium chloride solution. The effectiveness of the inhibitors has been evaluated by long-term rebar corrosion monitoring in reinforced concrete and by rebar visual inspection after three years tests. Also solution tests were performed in order to verify the effectiveness of inhibition. Results give information about corrosion prevention ability of analysed commercial inhibitors.     

Critical chloride content in reinforced concrete — A review

Chloride induced corrosion as the major cause for degradation of reinforced concrete has been the subject of great research efforts over the last fifty years. The present literature review summarises the state of the art by presenting the concept of the critical chloride content, discussing influencing factors, and assessing available measurement techniques. A large number of published chloride threshold values together with the respective experimental details are collected. While today's experience is mostly based on Portland cement, more modern studies with non-traditional binders often reported contradictory results. The present literature evaluation highlights the strong need for a practice-related test method, and, in this regard, focuses especially on experimental procedures by discussing advantages and drawbacks of methods and setups. It clearly emerges that many of the setups used to determine critical chloride contents are not suited to give realistic results.     

Effect of calcium nitrite-based corrosion inhibitor in preventing corrosion of embedded steel in concrete

Due to the corrosion inhibition effect and compatibility with concrete properties, calcium nitrite solution has been widely used, in North America and Asia,. However, investigation has often been restricted to tests using a macrocell corrosion monitoring or measuring corrosion rate in an aqueous condition. This study concerns the assessment of the inhibition effect of calcium nitrite-based corrosion inhibitor using a polarisation method and its influence on the chloride transport, compressive strength and setting time of concrete.The calcium nitrite-based corrosion inhibitor significantly reduced the corrosion rate of steel in chloride contaminated mortar and raised the chloride threshold level, ranging from 0.22% to 1.95% by weight of cement, while nitrite-free specimen produced the threshold level ranging from 0.18% to 0.33%. It was observed that concrete specimens containing the corrosion inhibitor produced the higher total charge passed in a rapid test for chloride ion permeability. An increase in the dosage of corrosion inhibitor resulted in a decrease in the concrete setting time. In addition, the compressive strength at early ages was increased by corrosion inhibitor, but in a long term (900 days), decreased to the level for 28 days.     

Effect of corrosion on bond in reinforced concrete under cyclic loading

Cyclic loading can result in severe deterioration in the bond between reinforcing steel bar and the surrounding concrete, especially when the reinforcement is corroded. In this study, tests were carried out for bond stress-slip response of corroded reinforcement with concrete under cyclic loading. Parameters investigated include: corrosion level, confinement, bar type, and loading history. The results revealed that bond behaviour was significantly reduced under cyclic loading. Degradation in bond was significantly less for deformed bars than for smooth bars at the initial loading cycle, but the difference was diminished with loading. The bond reduction was more substantial for unconfined steel bars than for confined bars. The relatively high level of corrosion caused degradation primarily in the initial five cycles, the effect of corrosion being decreased with loading. It was also demonstrated that the cyclic bond stress-slip curves depended on loading history.     

Analyzing crack width to predict corrosion in reinforced concrete

Our aim in this paper is to introduce a set of relationships linking the distribution of reinforcement corrosion and the width of cover crack that results from such corrosion. This work is based on experimental results obtained on the longitudinal reinforcements of two beams naturally corroded over periods of 14 and 17 years. We first compared these experimental results with existing models linking crack width and attack penetration. Noting that such models only partially predict actual experimental data, we put forward a new model using the parameter of reinforcement cross-section loss.     

A migrating corrosion inhibitor evaluated in concrete containing various contents of admixed chlorides

The performance of a surface-applied migrating corrosion inhibitor (MCI) based on an alkylaminoalcohol was evaluated on concrete specimens containing reinforcing steel bar (rebar) segments. Two water/cement ratios (w/c's), various chloride contents and two exposure conditions were investigated. The inhibiting efficiency was followed over a period of 1000 days by means of parameters such as corrosion potential, corrosion current and electrical resistance. Results show that when concrete is exposed to the marine environment, the inhibitor is able to reduce the corrosion rate (CR) only when the initial chloride content is below 0.16 wt.% relative to cement content. Efficiency increases as the w/c increases. There is no beneficial effect when the initial chloride content is greater than 0.43%. When concrete is immersed in a saline solution, no beneficial effect associated to the use of the inhibitor could be appreciated, regardless of w/c or initial chloride content in concrete.     

An electrochemical and analytical approach to the inhibition mechanism of an amino-alcohol-based corrosion inhibitor for reinforced concrete

Laboratory investigations were performed in order to assess the effectiveness and the inhibition mechanism of an amino alcohol-based inhibitor currently used as admixture to prevent corrosion of steel in concrete. The investigation was performed in the presence of chloride ions, using solutions simulating the concrete interstitial solution. Electrochemical measurements allowed to conclude that, an inhibitor film is formed on the surface hindering the anodic activity. Furthermore, the analytical investigation through the use of X-ray photoelectron spectroscopy (XPS) shows that the inhibitor film is able to complex with the chloride ion.     

Probabilistic approach for durability design of reinforced concrete in marine environment

A probabilistic approach to durability is proposed for the design of reinforced cover of a concrete immersed in sea water. It uses a non-linear chloride diffusion model for a saturated medium, which is first exposed with its physical parameters. Parameter variability is estimated and used through the probabilistic approach to assess the probability of reaching a critical chloride concentration near the reinforcement for a given service life. Based on the concrete formulation and the cement chemical composition, the model parameters are evaluated with their associated random distribution. The corrosion risk is then estimated through the Hasofer-Lind reliability index. Currently, the methodology is available for Ordinary Portland Cement (OPC) and for saturated media. A practical application is proposed to underline the applicability of the methodology. The decrease in the reliability index with time is computed for a cover depth coming from actual design rules. The probabilistic sensitivity factors are also assessed and show the importance of some model parameters called “durability indicators”.     

Development of a galvanic sensor system for detecting the corrosion damage of the steel embedded in concrete structures: Part 1. Laboratory tests to correlate galvanic current with actual damage

The correlation between sensor output and corrosion rate of reinforcing steel was evaluated by laboratory electrochemical tests in saturated Ca(OH)₂ with 3.5 wt.% NaCl. In this paper, two types of electrochemical probes were developed: galvanic cells containing of steel/copper and steel/stainless steel couples. The corrosion behavior in saturated Ca(OH)₂ solution with and without 3.5 wt.% NaCl addition for the different electrodes was investigated by potentiodynamic test. Weight loss measurement and galvanic corrosion test were conducted to obtain the corrosion rate of reinforcing steel and the charge of sensor in saturated Ca(OH)₂ solution with 3.5 wt.% NaCl addition, respectively.

The results of the potentiodynamic test indicated the possibility of detecting an ingress point of chlorides by measuring the galvanic current. In galvanic corrosion tests, the galvanic current of steel/copper couple was higher than that of steel/stainless steel couple, i.e., the steel/copper sensor is more suitable for high resistance environment. The steel/stainless sensor showed a better linear correlation than the steel/copper sensor. Through the relationship between the sensor system output and the weight loss (mg/cm²) of steel, real corrosion damage of the steel embedded in concrete can be detected.     

Corrosion measurements of steel reinforcement in concrete exposed to a tropical marine atmosphere

As part of a broad research program, reinforced concrete cylinders of different qualities were exposed to the marine atmosphere of North Peninsula of Yucatán. The cylinders were electrochemically monitored over a period of 56 months. The time for the onset of active corrosion was shorter for rebars in concretes with a high water-to-cement (w/c) ratio compared to that for rebars in low w/c ratio concrete. Results also indicate, as expected, that for equal periods of exposure, nominal corrosion current density (i_corr) values were generally higher for rebar in concrete with higher w/c ratio than those for rebar in low w/c ratio concrete. Analysis of the observed impedance spectra in terms of a modified Randles circuit (in which the ideal capacitor is replaced by a constant phase element (CPE)) appeared to be a reasonable approximation.     

Modeling of chloride transport coupled with enhanced moisture conductivity in concrete exposed to marine environment

In this research, the chloride penetration in concrete is modeled under marine environment loadings. Moisture migration model is enhanced to simulate the sorption flux under submerged wetting after long exposure to drying. The non-ideal viscosity of flow in the porous media is modified according to the nature of micro-pore structure by modeling the sensitivity of different porous networks (dense and coarse) to react towards the external environment. The strong sorption flux generated as a result of wetting and drying cyclic exposure is modeled by applying hydraulic pressure at the exposed surface. Chloride profiles are then simulated by coupling the enhanced moisture conductivity model with chloride transport model.