Cover cracking of reinforced concrete due to rebar corrosion induced by chloride penetration

Cracking of concrete cover due to corrosion induced expansion of steel rebar is one of the major causes of the deterioration of reinforced concrete (RC) structures exposed to marine environments and de-icing salts.This paper presents two models that deal with the chloride-induced corrosion and subsequent cracking of concrete cover in RC structures. The former analyses the chloride diffusion within partially saturated concrete. A comprehensive model is developed through the governing equations of moisture, heat and chloride-ion flow. Nonlinearity of diffusion coefficients, chloride binding isotherms and convection phenomena are also highlighted. The latter describes the internal cracking around the bar due to expansive pressures as corrosion of the reinforcing bar progresses. Once a certain chloride concentration threshold is reached in the area surrounding the bar, oxidation of steel begins and oxide products are generated, which occupy much greater volume than the original steel consumed by corrosion. An embedded cohesive crack model is applied for cracking simulation.Both models are incorporated in the same finite element program. The models are chained, though not explicitly coupled, at first instance. Comparisons with experimental results are carried out, with reasonably good agreements being obtained. The work is a step forward for the integration of the two traditional phases (initiation and propagation) widely used in the literature and usually analysed separately. The estimation of the service life of the structure needs to evaluate the associated time for each one.     

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 process and structural performance of a 17 year old reinforced concrete beam stored in chloride environment

The long-term corrosion process of reinforced concrete beams is studied in this paper. The reinforced concrete elements were stored in a chloride environment for 17years under service loading in order to be representative of real structural conditions. At different stages, cracking maps were drawn, total chloride contents were measured and mechanical tests were performed. Results show that the bending cracks and their width do not influence significantly the service life of the structure. The chloride threshold at the reinforcement depth, used by standards as a single parameter to predict the end of the initiation period, is a necessary but not a sufficient parameter to define service life. The steel-concrete interface condition is also a determinant parameter. The bleeding of concrete is an important cause of interface de-bonding which leads to an early corrosion propagation of the reinforcements. The structural performance under service load (i.e.: stiffness in flexure) is mostly affected by the corrosion of the tension reinforcement (steel cross-section and the steel-concrete bond reduction). Limit-state service life design based on structural performance reduction in terms of serviceability shows that the propagation period of the corrosion process is an important part of the reinforced concrete service life.     

The effect of silane-based hydrophobic admixture on corrosion of reinforcing steel in concrete

The influence of a hydrophobic admixture based on silane on the corrosion resistance of steel reinforcement in concrete was studied. Sound or deliberately pre-cracked concrete specimens were manufactured with w/c of 0.45 and 0.80, both in the presence and in the absence of silane. The specimens were fully immersed in a 3.5% NaCl aqueous solution.The results, in terms of electrochemical measurements, visual observations, and weight loss measurements of steel reinforcement, show that silane blocked corrosion process in uncracked concrete specimens. On the other hand, in cracked concrete specimens, corrosion of steel reinforcements was unexpectedly more severe in hydrophobic specimens rather than in the corresponding not hydrophobic ones. It is believed that oxygen, which is needed to feed the corrosion process, diffuses faster in a gaseous phase through the open concrete porosity in the hydrophobic concrete, whereas in concrete without silane, oxygen diffuses much more slowly through the water filling the pores of the saturated concrete.     

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.     

Corrosion behaviour of new stainless steels reinforcing bars embedded in concrete

The activities concerned with the evaluation, repair and restoration of structures are estimated to amount to 35% of the total volume of the work in the building sector and this continues to increase. The corrosion of rebars in the reinforced concrete structures (RCS) is the main reason for their degradation, so the use of reinforcing stainless steels seems to be one of the possible solutions with most probabilities of solving this problem. In this work, in order to demonstrate the advantages of using reinforcing stainless steels, the corrosion behaviour of AISI 304 and 316 reinforcements embedded in concrete slabs (C35/45 and C60/70 concrete) with two chloride contents are compared with three low-cost and low-Ni austenoferritic stainless steels and with the conventional carbon steel. The lower chloride contamination selected in this research, was enough to cause the corrosion in the active state of the carbon steel reinforcements, whereas the highest one exceeded the expected contamination in the natural environments, including sea media. The metallic materials remaining in the passive state can be considered, from the point of view of corrosion resistance, adequate as reinforcements in the RCS.     

Chloride-induced corrosion products of steel in cracked-concrete subjected to different loading conditions

This project focused on examining the composition and distribution of chloride-induced corrosion products at the rebar-concrete interfaces and on crack surfaces in reinforced ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) subjected to different loading conditions. The results indicated that, regardless of the type of loading, there was a larger distribution of corrosion products along the rebar surface in the HPC than in the OPCC. Also, dynamic loading caused a greater detachment of the aggregate-paste bond in OPCC than static loading. The opening and closing of the cracks in salt solution under dynamic load forced corrosion products to flow from the rebar-concrete interface into the cracks in both OPCC and HPC. As a result, corrosion products diffused from the crack into the cement paste in the dynamically loaded OPCC but remained in the cracks in the dynamically loaded HPC, where they induced branched cracks. The mill-scale on the rebar was also evaluated before and after being embedded in concrete and was observed to be porous allowing ingress of species from the cement paste.     

Comparison between surface and bulk hydrophobic treatment against corrosion of galvanized reinforcing steel in concrete

The effectiveness of bulk hydrophobic treatment against corrosion of galvanized steel reinforcement in concrete specimens with w/c = 0.45 and w/c = 0.75 was compared with that of surface treatment, even in the presence of cracks 0.5 and 1 mm wide in the concrete cover. In this case surface hydrophobic treatments were applied both before and after cracking as a preventive and a restorative method against reinforced concrete deterioration, respectively. The obtained results in terms of water absorption, electrochemical measurements, chlorides penetration, and visual observations carried out on reinforced concrete specimens during the exposure to wet–dry cycles in 10% NaCl solution showed that bulk hydrophobization is the most effective treatment in improving the corrosion resistance of galvanized steel reinforcements in concrete also in the presence of cracks. Surface hydrophobization is very effective just in the first few exposure cycles to the aggressive environment and when used as a restorative method which is able to cancel the deleterious effect of cracks only 0.5 mm wide.     

The use of migrating corrosion inhibitors to repair motorways' concrete structures contaminated by chlorides

The effectiveness of migrating corrosion inhibitors (MCIs) and repair mortars against rebar corrosion was studied in concrete specimens made by ordinary Portland cement with w/c ratio equal to 0.6 and containing 1 wt.% of chlorides. An alkanolamine-based inhibitor was tested in addition with a common mortar and two repair mortars. Electrochemical techniques, measurements of corrosion potential and electrochemical impedance spectroscopy (EIS) were used to determine the corrosion behaviour of the specimens when a cell containing a 3.5% NaCl solution was applied on the rehabilitation mortar. Mercury intrusion porosimetry (MIP) was also used for the characterisation of repair mortars' total porosity and a chemical analysis was made to determine the amount of chlorides penetrated in the mortar layers and in the concrete substrate. Results demonstrate that the simultaneous use of the alkanolamine-based inhibitor with a good barrier coating offers protection against rebar corrosion and allows rehabilitation of deteriorated concrete structures.     

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.     

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.     

Concrete cover cracking with reinforcement corrosion of RC beam during chloride-induced corrosion process

This paper deals with the evolution of the corrosion pattern based on two beams corroded by 14 years (beam B1CL1) and 23 years (beam B2CL1) of conservation in a chloride environment. The experimental results indicate that, at the cracking initiation stage and the first stage of cracking propagation, localized corrosion due to chloride ingress is the predominant corrosion pattern and pitting corrosion is the main factor that influences the cracking process. As corrosion cracking increases, general corrosion develops rapidly and gradually becomes predominant in the second stage of cracking propagation. A comparison between existing models and experimental results illustrates that, although Vidal et al.'s model can better predict the reinforcement corrosion of beam B1CL1 under localized corrosion, it cannot predict the corrosion of beam B2CL1 under general corrosion. Also, Rodriguez's model, derived from the general corrosion due to electrically accelerated corrosion experiments, cannot match natural chloride corrosion irrespective of whether corrosion is localized or general. Thus, for natural general corrosion in the second stage of cracking propagation, a new model based on the parameter of average steel cross-section loss is put forward to predict steel corrosion from corrosion cracking.     

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.     

Corrosion of steel bars in OPC mortar exposed to NaCl, MgCl2 and CaCl2: Macro- and micro-cell corrosion perspective

In North America, corrosion of the steel rebar commonly occurs due to chloride attack from deicing salts. In Canada, based on the severity and temperature of the ambient environment, three different deicing salts, or combination of them, are used: NaCl, MgCl₂ and CaCl₂. In this paper, the effect of each of these salts on the corrosion of steel rebar and their impact on the durability of the mortar have been investigated. The results show that CaCl₂ has the most negative effect on the steel and, in high concentrations, on the integrity of the mortar. MgCl₂ also deteriorates the mortar if used in high concentration, while NaCl has no apparent effect on mortar durability even in high concentration.     

Corrosion by chlorides in reinforced concrete: Determination of chloride concentration threshold by impedance spectroscopy

Chloride penetration in reinforced concrete induces depassivation of the steel rebars and initiation of the corrosion process leading to degradation of the structure. The coupling of “low-frequency” impedance response with SEM observations and multielementary analyses emphasized that the strong decrease of the capacitive part is related to the corrosion initiation. This experimentally determined incubation period is used in an electrodiffusion model based on Fick's second law to quantify the chloride concentration threshold responsible for corrosion initiation on the reinforcing steel surface. This work thus allowed quantifying the incubation period and the critical chloride concentration, referred to in Tuutti's diagram [K. Tuutti, Corrosion of steel in concrete, CBI Research Report no. 4.82, Swedish Cement and Concrete Research Institute, Stockholm, Sweden, 1982].     

The corrosion pattern of reinforcement and its influence on serviceability of reinforced concrete members in chloride environment

This paper deals with two corroded reinforcement concrete beams, which have been stored under sustained load in a chloride environment for 14 and 23 years respectively. The evolution of corrosion pattern of reinforcement and its influence on serviceability are studied. In chloride-induced corrosion process, corrosion cracking affects significantly the corrosion pattern. During the corrosion cracking initiation period, only local pitting corrosion occurs. At early stage of cracking propagation, localized pitting corrosion is still predominant as cracks widths are very small and cracks are not interconnected, but a general corrosion slowly develops as the cracks widen. At late cracking stage, interconnected cracking with wide width develops along large parts of the beam leading to a general corrosion pattern. Macrocells and microcells concepts are used for the interpretation of the results.Mechanical experiments and corrosion simulation tests are performed to clarify the influence of this corrosion pattern evolution on the serviceability of the beams (deflection increase). Experimental results show that, when the corrosion is localized (early cracking stage), the steel–concrete bond loss is the main factor affecting the beams serviceability. The local cross-section loss resulting from pitting attack does not significantly influence the deflection of the beam. When corrosion is generalized (late cracking stage), as the steel–concrete bond is already lost, the generalized steel cross-section reduction becomes the main factor affecting the beams serviceability. But, at this stage, the deflection increase is slower due to the low general corrosion rate.     

Characterization of solid embeddable reference electrodes for corrosion monitoring in reinforced concrete structures

Metal-metal oxide (MMO), graphite and laboratory-made Ag/AgCl electrodes were electrochemically characterized to be used as reference electrodes embedded in concrete structures. Electrodes were studied in both, aqueous solutions of pH ranging from 7 to 13.5 and embedded into cement mortars; and the electrochemical studies were carried out in the absence and presence of chloride ions. Potential evolution, polarization behaviour, galvanostatic pulse response and impedance characteristics of the electrodes were carried out in aqueous solutions. Besides, the electrochemical stability of the electrodes embedded in mortar was studied for an exposure period of 2 years. It was found that the MMO pseudo-reference electrode is pH-sensitive, the graphite pseudo-reference electrode is oxygen sensitive and the Ag/AgCl pseudo-reference electrode is chloride sensitive. In spite of the fact that any of them can be used to determine the corrosion rates of rebars because they do not depend on the absolute potential and/or the long-term stability of the reference electrode when using traditional electrochemical techniques, long-term drifts in the electrode potentials may lead to misinterpretations of the rebar state. In this context graphite electrodes are recommended because they provide conservative results regarding the active/passive state of the rebars.     

Cohesive fracture model for functionally graded fiber reinforced concrete

A simple, effective, and practical constitutive model for cohesive fracture of fiber reinforced concrete is proposed by differentiating the aggregate bridging zone and the fiber bridging zone. The aggregate bridging zone is related to the total fracture energy of plain concrete, while the fiber bridging zone is associated with the difference between the total fracture energy of fiber reinforced concrete and the total fracture energy of plain concrete. The cohesive fracture model is defined by experimental fracture parameters, which are obtained through three-point bending and split tensile tests. As expected, the model describes fracture behavior of plain concrete beams. In addition, it predicts the fracture behavior of either fiber reinforced concrete beams or a combination of plain and fiber reinforced concrete functionally layered in a single beam specimen. The validated model is also applied to investigate continuously, functionally graded fiber reinforced concrete composites.     

Experimental and numerical study of electrochemical chloride removal from brick and concrete specimens

The electrochemical technique for chloride extraction (desalination) was applied in galvanostatic mode to cylindrical brick and concrete specimens with a steel bar as reinforcement placed in the centre. The specimens were initially contaminated by immersion in a solution of 35 g/l NaCl. Based on the Nernst-Planck equations, a numerical model was developed considering the interactions between the various ionic species in the pore solution. The model makes it possible to predict the evolution of the chloride profile with time. The numerical and experimental results are compared and the model parameters discussed.