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The objective of this paper is to present the methodology used to validate the threshold levels of durability indicators from data acquired from existing reinforced concrete structures that may present corrosion problems resulting from carbonation or chloride ingress. This study was conducted in the framework of the French National Project PERFDUB in close collaboration with the MODEVIE ANR project. Durability tests form the basis of a performance‐based approach for the durability of concrete structures, and in order to set the threshold level for each of these durability indicators, it is important to take into account the experience feedback that can be obtained from the actual behavior of existing structures. This paper presents the investigations carried out on eight reinforced concrete structures, all of them having an age well in excess of 50 years, and the main data gathered on some relevant durability and lifetime indicators. Based on all these data, predictive models of carbonation and chloride penetration are applied in order to validate the threshold levels to be adopted for the durability indicators, and several other conclusions are drawn. [ABSTRACT FROM AUTHOR]

Existing structures often no longer meet the demands formulated in contemporary design codes with respect to structural safety and serviceability. This occurs, for instance, if the loads on existing structures, like traffic loads on bridges, are larger than assumed in the original design. A second potential reason is that structures are subject to deterioration, like alkali–silica reaction within the concrete or corrosion of the reinforcement due to chloride attack or carbonation. A third possible reason is that in recent codes, additional criteria have been introduced based on new theories and/or negative experiences with older structures. The new fib Model Code for Concrete Structures 2020 will be valid both for the design of new structures and the assessment of existing structures. This paper shows how the design and assessment of concrete structures are integrated into this new code concept. [ABSTRACT FROM AUTHOR]

In order to realize the probability prediction of concrete carbonation depth, this paper proposed a probabilistic prediction model of carbonation depth based on 433 sets of existing experimental data. First, considering the influence of water‐binder ratio, fly ash content, stress level, and carbonation time, a deterministic prediction model of carbonation depth was established based on stepwise regression method. Then, based on the established deterministic prediction model, considering the influence of aleatory and epistemic uncertainties, a probabilistic prediction model of carbonation depth was established, and the posterior distributions of the parameters in the probabilistic prediction model were updated iteratively by using Bayesian theory and Markov chain Monte Carlo method, and the influence of standard deviation of prior distribution of the parameters in probabilistic prediction model on their posterior distributions was discussed. Finally, the prediction accuracy of the probabilistic prediction model was verified by comparing with another 127 sets of existing experimental data and the predicted values of the deterministic prediction model, and the effect of the standard deviations of prior distribution of the parameters on the prediction accuracy of the probabilistic prediction model was analyzed. The results showed that the prediction accuracy of the proposed probabilistic prediction model of carbonation depth after three updates was higher than that of the deterministic prediction model. When the standard deviation of prior distributions of the parameters in probabilistic prediction model was 100, the proposed probabilistic prediction model after three updates had the highest prediction accuracy. Under the tensile stress state of concrete, the prediction accuracy of the probabilistic prediction model was improved by 26.76% compared with the deterministic prediction model, and when the concrete under compressive stress state, the prediction accuracy of the probabilistic prediction model was 44.07% higher than that of the deterministic prediction model. [ABSTRACT FROM AUTHOR]

The paper describes the main mechanisms of carbonation and chloride penetration into concrete and how the rate of penetration can be measured with laboratory tests. It is also discussed why nondestructive test (NDT) methods for measuring directly the resistance of aged field concrete to carbonation or chloride penetration are not available. Only destructive tests are applicable which, given the very high‐spatial variability encountered, become prohibitive in terms of costs and damage. The only way to assess the resistance of aged field concrete to carbonation or chloride penetration is through surrogate tests measuring variables that are linked to it. There are two standardized test methods with potential as surrogate tests, as they are fast and entirely nondestructive: the coefficient of air permeability kT ("Torrent" method) and the electrical resistivity ρ ("Wenner" method). Data obtained on‐site with both methods are presented, confirming their potential to assess the resistance of aged field concrete to carbonation or chloride penetration. [ABSTRACT FROM AUTHOR]

This paper reviews the long-term properties of recycled aggregate concrete (RAC), including long-term strength, shrinkage, creep, carbonation resistance, antifreeze resistance, impermeability, abrasion resistance, alkaline aggregate reactions, sulphate corrosion and fatigue behaviour. Most studies have shown that the long-term properties of RAC are inferior to those of natural aggregate concrete (NAC), and some researchers have observed that the long-term properties are better than those of NAC. RAC's long-term properties are affected by many factors such as recycled coarse aggregate (RCA) replacement percentage, water-cement ratio, mineral admixtures and mix proportions. The long-term properties of RAC can be improved through better control of these factors. This paper will be helpful for a comprehensive understanding of and further research on RAC, and provides an important basis and references for the engineering applications of RAC. [ABSTRACT FROM AUTHOR]

At present, prescriptive regulations with regard to concrete cover and composition are applied to provide sufficient durability of reinforced concrete members under exposure conditions with different degrees of severity. In view of current knowledge on deterioration mechanisms and their modelling, it is planned to change from these deemed-to-satisfy specifications to a performance-based design approach in future standards. In such specifications, concrete durability design is based on the statistically characterized performance of concrete, determined in standardized tests with respect to defined classes of concretes with similar performance. This paper presents the results of a study in which concrete mixes were tested and analysed with respect to their carbonation resistance. Compositions with similar performance are grouped into carbonation resistance classes. These classes are described statistically and requirements for performance testing are given. In addition, composition requirements are introduced in order to determine concrete performance depending on mix composition prescriptively. Finally, an example is given for the assessment of concrete performance with regard to carbonation. This work was carried out at the request of JWG under CEN TC 250/SC2 and CEN TC 104/SC1 as an input and starting point for the ongoing committee work to implement the methodology from the fib Model Code for Concrete Structures 2010 in the next generation (2021) of European concrete standards. [ABSTRACT FROM AUTHOR]

The paper reports on a benchmark of European deemed-to-satisfy rules for exposure class XC (carbonation exposed structural members). The benchmark of the descriptive rules was carried out following the probabilistic design approach for carbonation-induced corrosion developed in [1] and adopted in fib bulletin 34: Model Code for Service Life Design (2006) [2] and fib Model Code for Concrete Structures 2010 [3], respectively. To perform a representative study, three groups of European countries were selected, representing different parts of Europe, south (Spain, Portugal), middle (Netherlands, Great Britain and Germany) and northern Europe (Denmark, Norway). Reliability ranges for carbonation-induced depassivation of rebar were calculated for 'favourable' and 'unfavourable' design situations in exposure classes XC2, XC3 and XC4 [4]. In each design situation the deemed-to-satisfy rules of selected countries were followed. The probabilistic calculations were mainly based on short-term carbonation data. However, some calculations were also based on long-term observation. The latter was implemented for independent validation purposes. The calculated reliability ranges are very broad and in some 'unfavourable' situations, the deemed-to satisfy requirements do not guarantee the required limit state (LS) arget reliabilities for the particular exposure. In 'favoura-ble' situations less stringent demands would have been sufficient. [ABSTRACT FROM AUTHOR]

Understanding and predicting concrete carbonation are significant in designing durability and maintaining the service life of reinforced concrete structures. However, this purpose can hardly be reached because of complex carbonation mechanisms depending on various variables such as cement content (C), fly ash content (FA), water content (W), concentration of CO2, relative humidity (RH), temperature (T°C), and exposition time (Time). This investigation proposes a machine learning (ML) approach including eight ML algorithms such as four single ML models: XGB, GB, RF, and SVM, and four hybrid ML models: XGB_RRHC, GB_RRHC, RF_RRHC, and SVM_RRHC for investigating and predicting concrete carbonation depth containing fly ash. To achieve this purpose, a dedicated database consisting of 688 samples and seven input variables is built, and the performance of eight machine learning models is compared. Single ML model Extreme Gradient Boosting (XGB) using the default hyperparameters exhibited the highest performance with R2 = 0.9770, RMSE = 2.2725 and MAE = 1.5218. Shapley Additive exPlanations (SHAP) identifies the most influential feature and order of feature effect on concrete carbonation depth. The first four important features can be sorted in order: time of exposition > cement content > water content > CO2 concentration. Moreover, at a higher value of exposition time, water content, CO2 concentration, fly ash content, temperature and relative humidity, the carbonation depth of concrete increases. Using high content of cement can reduce the carbonation depth of concrete. [ABSTRACT FROM AUTHOR]

This article addresses the applicability of experimental methods for the evaluation of concrete durability on localized structural weaknesses posed by joints. Three different types of exposure are analyzed: chloride penetration, carbonation, and freeze–thaw attack. First, established experimental methods for the characterization of the durability of plain concrete are presented and their general applicability on concrete joints is discussed. The experimental program focuses on the execution of selected tests on laboratory specimens with and without joints. Indicator tests for both chloride penetration and carbonation are generally feasible, but require attention to specimen preparation. Rapid chloride migration yielded inconclusive results, while uniaxial chloride diffusion coefficient of concrete joints cannot be determined because of the areal extent of profile grinding. For the same reason, the characterization of the resistance to freeze–thaw cycles through surface deterioration is not applicable for concrete joints. The structural damage was instead successfully qualified through direct tension tests. [ABSTRACT FROM AUTHOR]

Research argues in favor of using recycled aggregates in structural concrete. However, lack of specific clauses in standards for recycled aggregate concrete design hinders the use of recycled aggregate concrete by the industry. Recycled aggregate concrete‐specific design clauses are hindered because conventional research must be complemented with experiments directed towards stochastic modeling, so that reliability concepts may evaluate whether the probabilities of failure of design comply with the expectations of Society or whether specific design rules are needed. This article compares the within‐batch variability of carbonation under accelerated conditions of four natural aggregate concrete and four analogue recycled aggregate concrete mixes and analyses the reliability of Eurocode 2 cover design for carbonation exposure. In general, the incorporation of coarse recycled concrete aggregates worsened the mean durability behavior of concrete, including resistance to carbonation. However, the variability of carbonation depth was not meaningfully affected by the recycled aggregates used (coarse recycled aggregates produced from concrete waste) and the reliability of concrete cover design only decreased due to the effect of the recycled aggregates on the mean resistance to carbonation. A 5 mm increase of concrete cover was found to more than offset the detrimental influence of full incorporation of recycled aggregates on the reliability of concrete cover design. [ABSTRACT FROM AUTHOR]

This study assessed the performance of self‐compacting rubberized concrete against carbonation and chloride penetration. In addition to fresh‐state and compressive strength tests, durability of the studied mixes was evaluated by measuring carbonation and chloride penetration depths, total porosity, and pores'size distribution. It was found that an increase in rubber content leads to a decrease in compressive strength. However, stadequate strength for structural applications was achieved up to an incorporation of 15% rubber by volume. As for durability, the presence of rubber was found to enhance performance against carbonation, but to worsen the resistance to chloride ion penetration. [ABSTRACT FROM AUTHOR]

With an increase in CO2 emission, corrosion of reinforcement as a result of carbonation has become more significant to researchers besides corrosion due to chloride ion ingress. Many structures are exposed to coupled effects of carbonation and chloride attack; therefore, investigating the effect of this combined attack on the durability of concrete is a vital subject. Also, the use of supplementary cementitious materials (SCMs) has become more common due to their contribution to concrete's properties. In this study, the combined effect of chloride attack and carbonation on the durability of mortars containing SCMs was studied. Two natural pozzolans (Pumice and Trass) were used as cement replacements in mortar mixtures with three w/b ratios of 0.485, 0.44, and 0.4. Four exposure conditions, namely normal (immersed in lime‐saturated water), chloride attack, carbonation, and cyclic carbonation and chloride attack were considered. Compressive strength, carbonation and chloride penetration depth, capillary water absorption, surface electrical resistivity, and chloride diffusion tests were performed. The results indicate that while carbonation depth is reduced in the combined attack environment, it imposed a more severe corrosive condition in comparison to pure immersion in saline water by increasing the diffusion coefficient up to 500%. Also, addition of pozzolans showed the potential to reduce the diffusion coefficient of the mortars as much as 76% pointing out that mortars without pozzolans were more vulnerable to this combined attack environment. Furthermore, cement replacement with Trass and reduction of w/b showed to be beneficial in reducing the destructive effects of the combined environment. [ABSTRACT FROM AUTHOR]

Carbonation resistance of fly ash concrete (FAC) and recycled aggregate concrete (RAC) has been extensively tested in the past decade, but there are still no recommendations for their application in codes of practice. Therefore, the main objective of this study was to determine the minimum concrete cover depth of RAC and FAC exposed to carbonation to satisfy the required service life. In this study, the relationship between concrete compressive strength at 28 days (fcm) and inverse effective carbonation resistance (R−1ACC) was established. The analysis was carried out on all data available in the literature in order to establish this relationship for FAC and RAC made with different replacement percentages. After establishing a relationship between R−1ACC and fcm, a prediction of the service life was made using the fib‐Bulletin 34 (2006, Model code for service life design (1st ed.). Lausanne: International Federation for Structural Concrete (fib)) prediction model for all exposure classes to carbonation. For same concrete class, it can be concluded that in the case of RAC there was no significant difference in the duration of the service life compared with natural aggregate concrete, even at 100% replacement of natural aggregate with recycled ones. For concrete up to 35% of fly ash in total cementitious materials, concrete cover for satisfying the required service life of 50 years was 40% higher compared with Portland cement concrete, and more than two times higher in case of concrete with 40–70% fly ash. [ABSTRACT FROM AUTHOR]

The effect of incorporation of recycled powder and CO2 absorption of concrete was not considered in the previous life cycle assessment studies. This study evaluated the carbon emission of recycled concrete with different replacement rate of recycled powder and recycled coarse aggregate, taking the carbon absorption in storage and service phase into consideration. The carbon absorption model of recycled concrete with recycled powder was optimized and validated by rapid carbonization tests. It is found that concrete with recycled powder has favorable carbon absorption capacity and can greatly reduce carbon emission. The carbon emission of recycled concrete with recycled powder decreased 12.4–52.2% when compared to normal concrete, and with the increase in the replacement of recycled materials, this advantage becomes more obvious. The evaluation results show that the raw material production and transportation phase are the two main contributors to the carbon emission of recycled concrete, accounting for 67.03–85.81% and 4.01–15.27%, respectively. The evaluation results also show the compensation benefits in storage and service phase from carbon absorption account for 1.58–26.93%. Meanwhile, the long‐distance delivery of natural coarse aggregates makes it possible for recycled concrete to be widely applied. [ABSTRACT FROM AUTHOR]