Carbonation-induced Corrosion Research Articles

Transport properties of high-volume fly ash concrete: Capillary water sorption, water sorption under vacuum and gas permeability

Abstract Studying concrete’s resistance to carbonation-induced corrosion usually involves exposing the material to CO2 for quite some time. To estimate the performance of high-volume fly ash (HVFA) concrete more quickly, two key properties governing this process can be studied, namely water penetrability and gas permeability. With respect to HVFA mixtures optimized for usage in an environment exposed to carbonation with wetting and drying, we adopted the latter approach. This paper presents a full assessment of concrete mixtures with varying fly ash amounts. A 50% fly ash mixture by mass with a binder content of 400 kg/m3 and a water-to-binder ratio of 0.4 had a lower capillary water uptake (−32.6%), water sorption under vacuum (−10.7%) and gas permeability (−78.9%) than a proper reference normally used in this environment. The fly ash applied had an excellent quality regarding loss on ignition (3.5%) and fineness (19% retained on a 45 μm sieve).

Service life prediction of a concrete bridge structure subjected to carbonation

Carbonation-induced corrosion in concrete may often occur in a high carbon dioxide environment. In this study, the risk of carbonation of a concrete bridge in an urban area was evaluated by measuring the carbonation rate and concrete cover depth in three different parts: the sound, cracked and construction joint parts of cover concrete. The average carbonation rate was ordered by the sound > joint > cracked parts, and the concrete cover depth measured by an ultrasonic detector indicated the slightly greater value than the designed one (50.0 mm). Then, the carbonation-free service life at the depth of the steel was calculated, based on in situ information, by the safety factor method and the Monte Carlo simulation. The service life calculated by the two methods was mostly identical. The sensitivity of the carbonation rate and concrete cover depth to the time to carbonation at the depth of the steel was mathematically determined.

Bayesian Belief Network to Assess Carbonation-Induced Corrosion in Reinforced Concrete

Corrosion of reinforcing steel is a major cause of deterioration of reinforced concrete structures. Two primary causes leading to corrosion of reinforcing steel are, respectively, carbonation and chloride contamination of the concrete cover. Estimating the time to corrosion initiation through modeling enables a qualitative and quantitative assessment of the influence of several variables on the mechanism involved. However, many of these variables are subject to stochastic uncertainty, which can be represented through either frequentist or Bayesian probability. This paper proposes a probabilistic-based model for carbonation-induced corrosion based on a Bayesian belief network. The proposed model is compared with a more traditional probabilistic-based technique, Monte Carlo simulation. Both methodologies are illustrated through two case studies and similarities and differences are highlighted.

Effect of global climatic change on carbonation progress of concrete

In the recent years, global warming has dramatically increased the atmospheric carbon-dioxide (CO 2) concentration and temperature. As a consequence of this, carbonation has become one of the most critical durability issues for concrete structures in urban environment. In this study, the climate scenario IS92a recommended by Intergovernmental Panel on Climate Change (IPCC) is used for evaluating the effect of CO 2 concentration on carbonation of concrete. A modified mathematical equation, based on Fick's 1st law of diffusion, is used to evaluate CO 2 diffusion coefficient of concrete. The required cover depth of concrete is estimated by using the applicative methods of reliability and stochastic concepts to take microclimatic conditions into consideration. The tolerance of cover depth should be considered in order to prevent carbonation-induced corrosion. From the relationship between the weight loss of reinforcement and corrosion current density for a given time, the tolerance of cover depth to prevent carbonation-induced corrosion is suggested. It was observed that corrosion occurs when the distance between carbonation front and reinforcement bar surface (the uncarbonated depth) is <5 mm.

Performance of a Penetrating Corrosion Inhibitor in Concrete Affected by Carbonation-Induced Corrosion

The performance of an organic, penetrating corrosion inhibitor in reducing the rate of corrosion and delaying the onset of corrosion in carbonated concrete was investigated. Laboratory trials were undertaken on specimens treated with the corrosion inhibitor before and after accelerated carbonation. Corrosion monitoring was undertaken for 11 months measuring corrosion rate, rebar potential, and resistivity. Results indicate that the penetrating corrosion inhibitor is capable of reducing corrosion rates and delaying the onset of corrosion under carbonation conditions. The effectiveness of the inhibitor was also assessed on site structures exhibiting severe carbonation-induced corrosion. Corrosion monitoring on site was done for a period of nine months. Findings from site studies confirmed that surface application of the corrosion inhibitor significantly reduced the corrosion rate of reinforcement embedded in carbonated concrete when compared with untreated elements.

A Computational Framework for Risk Assessment of RC Structures Using Indicators

The present article starts out by proposing a framework for risk assessment of RC structures utilizing condition indicators. Thereafter, the various building stones of the suggested framework are described. This description includes a summary of the basis for the probabilistic modeling of the initiation phases of chloride-induced corrosion of concrete structures. Furthermore, a probabilistic modeling of condition indicators regarding the condition state of concrete structures is proposed whereby information available at the design stage of concrete structures as well as information obtained through in-service inspections may be utilized for the purpose of reliability updating. Finally, it is described how the probability of localized and spatially distributed degradation of different degrees can be assessed and examples are given on how the various indicators may be used for the purpose of updating the statistical characteristics of the future degradation of RC structures. The presented framework forms a consistent basis for risk assessment of concrete structures subject to chloride-induced corrosion. It can easily be adopted to other degradation phenomena such as carbonation-induced corrosion and it forms a good basis for the development of efficient approaches to Asset Integrity Management of RC structures.

Theoretical considerations on the supposed linear relationship between concrete resistivity and corrosion rate of steel reinforcement

Traditionally, the assessment of service life of steel reinforced concrete structures has been focused on the prediction of the time required to achieve a transition from passive to active corrosion rather than to accurately estimate the subsequent corrosion rates. However, the propagation period, i.e. the time during which the reinforcing steel is actively corroding, may add significantly to the service life. Consequently, ignoring the propagation period may prove to be a conservative approach. On the other hand the prediction of the corrosion rate may result in a very complex task in view of the electrochemical nature of corrosion and the numerous parameters involved. In order to account for the various influences an essentially empirical model has been introduced in which the electrolytic resistivity of the concrete environment serves as the major parameter. This model will be discussed for carbonation-induced corrosion based on the commonly accepted theory of aqueous corrosion. An alternative model for microcell corrosion is proposed which is based on the commonly accepted view that anodic and cathodic sites are microscopic and their locations change randomly with time. In line with this view electrolytic resistivity can be incorporated and thus may play a significant role in the kinetics of the corrosion process. For a wide range of corrosion current densities the relationship between corrosion current density, log(icorr), and concrete resistance, log(Rcon), can then be approximated by an almost ideal linear relationship. Assuming a fixed geometrical arrangement of anodic and cathodic sites on the steel surface, this linear relationship is also valid for concrete resistivity, ρcon. However, from the theoretical treatment of the electrochemical processes underlying reinforcement corrosion it becomes evident that a linear relationship between corrosion current density and concrete resistivity does not necessarily imply that concrete resistance is dominating the overall corrosion cell resistance. In most cases a significant portion of the driving voltage of the corrosion cell will be consumed by the transfer of electrical charge involved in cathodic reactions, i.e. cathodic activation control will dominate.

The effect of elevated initial temperatures on durability index values of plain and corex slag concretes

The initial maximum placing temperature of concrete is usually limited by specifications, which might result in the concrete supplier having to implement cooling measures in hot climates. These limitations were originally based upon work that examined the influence of elevated temperature on the compressive strength of concrete. The question arises as to whether these limiting values are appropriate for the transport properties of concrete, which largely govern its durability. This was investigated using the durability index test approach developed in South Africa. It was found that, while there was on occasions a statistically significant influence on the test results at an elevated initial temperature of 37°C, this effect was usually of no practical importance. One exception was the gas permeability of corex slag concretes (relating to resistance to carbonation), which led to a decrease in performance at 37°C that was both statistically significant and of practical importance. Notwithstanding this, the results indicate that carbonation-induced corrosion is unlikely provided a cover of at least 30 mm is achieved.

Measurement of loss of steel from reinforcing bars in concrete using linear polarisation resistance measurements

The measured weight loss data for a number of mild steel bars contained in Portland Cement concrete, together with predicted weight loss values monitored using potentiostatically controlled linear polarisation resistance (LPR) measurements are reported. Three sets of reinforced concrete specimens, each containing electrically isolated mild steel bars, were subjected to either • chloride-induced corrosion, • carbonation-induced corrosion • a control nitrogen rich environment with minimal corrosion. Each set of specimens was initially exposed to a 22-hour dry, 2-hour wet controlled environment for a duration of between 1026 and 1085 days. This was later changed to a 6-day dry, 1 day wet cycle for the carbonation exposure specimens after the initial set of gravimetric testing. The weight loss for each bar due to corrosion was recorded. Instantaneous LPR measurements were also taken on each bar at regular intervals throughout the exposure period. These resistance measurements were then integrated over the exposure period to estimate total weight loss. The results show that weight loss evaluated from experimental LPR measurements gives a significant over-estimate of the weight losses measured gravimetrically.

建設材料 コンクリートの欠陥部に生じる中性化による鉄筋腐食の形態と速度

The objective of this study was to analyze comprehensively the corrosion cell formation pattern (macro-cell and micro-cell) and the corrosion rate induced by carbonation in concrete with joints or cracks. First, mortar tests were performed using specimens with model defects that simulated cold joints or bending cracks. Herein three important factors that caused steel corrosion were investigated in detail. These factors were: 1) carbonized location, 2) supply of water and oxygen, and 3) water-cement ratio of mortar. Next, concrete tests were performed using specimens with cold joints and bending cracks. The results obtained in this study were as follows; a) 27 cases of which the carbonized location, supply of water and oxygen, and the water-cement ratio of mortar differed were experimented. As results, the corrosion cell formation pattern and the corrosion rate were comprehensively analyzed based on unified conditions. b) In the case of carbonation-induced corrosion with defects, macro-cell corrosion would be formed regardless of the water-cement ratio. Also, the corrosion rate decreased with the decreasing of water-cement ratio. c) The result b) was confirmed not only in mortar tests but also in concrete tests. d) According to above results, it could be said that the low water cement ratio was excellent in the durability for the steel corrosion, when concrete with defects received the influence of carbonation.

An accelerated carbonation procedure for studies on concrete

Carbonation of the concrete leading to reduced alkalinity around the steel is one of the main reasons for the corrosion of reinforced concrete. Studies of carbonation-induced corrosion and of rehabilitation methods, such as electrochemical re-alkalization, require the convenient preparation of realistically large specimens of carbonated concrete in a sufficiently short time. This paper describes a rapid method of preparing carbonated concrete by exposing concrete, which has been dried to an internal relative humidity of 60%, to a pure atmosphere of carbon dioxide gas at 15 bar pressure (1500 kPa). The pressure chamber used can accommodate specimens up to 150 mm diameter or 100 × 100 mm section and such specimens can be fully carbonated in 2 weeks, much more quickly than by other methods. Carbonation increases the electrical resistivity and strength of the concrete and reduces the water absorption. Optical and electron microscopical investigations on the carbonated concrete confirm that the microstructure is no different from that produced in concrete by carbonation under natural exposure. The accelerated carbonation method can be used for development work on materials and repair methods and has been used by the authors in preparing carbonated concrete specimens for re-alkalization tests.

A study of carbonation-induced corrosion

This study is concerned with measurements of corrosion in steel rods embedded at 4, 8, 12 and 20 mm from the exposed surface of concrete dried and carbonated uniaxially for 0·4, l·5 and 4 years. Corrosion was stimulated by exposing all specimens to a moist environment for a prescribed period prior to extraction and cleaning of the rods for gravimetric corrosion measurements. Corrosion was mainly controlled by the unneutralized remainder, i.e. the depth of reinforcement cover minus the depth of carbonation. Corrosion increased as the unneutralized remainder reduced from about +5 mm to −10 mm. The relationship between corrosion and the unneutralized remainder was not significantly affected by the depth of cover, the water/cement ratio, the period of curing or the exposure conditions during carbonation. However, corrosion seemed to be increased when there was 50% ground granulated blast-furnace slug in the cement; in contrast 30% pulverizedfuel ash or 5% ground limestone had little effect.