Service Life Of Reinforced Concrete Structures Research Articles

Effect of carbon dots with different sizes on chloride binding of cement

Carbon dots (CDs) as a novel and green nanomaterial exhibit a promising application in enhancing the chloride binding capacity of cement to extend the service life of reinforced concrete (RC) structures. However, their size influence on the enhancement of chloride binding is still a research gap, enormously hindering the further employment of CDs toward cement system. Herein, their size-induced effect on chloride binding actions of cement is comprehensively unveiled using CDs with various sizes for the first time. Specifically, three kinds of CDs with different sizes are controllably prepared through simply altering the calcinating time of citric acid. And the relationship between sizes of CDs and chloride binding capacity of cement is confirmed through the equilibrium tests. More importantly, the mechanism for the effect of CDs sizes on chloride binding is reasonably proposed according to the alteration analyses of phase compositions. The consequences demonstrate that the chloride binding capacity in cement modified by CDs with average sizes of 1.73, 5.79, and 12.37 nm increases by 41%, 20%, and 14% respectively, compared with that of the blank group after 28-d immersion in 3 mol/L NaCl solution. This testifies that the chloride binding capacity of CDs-incorporated cement is negatively correlative with CDs sizes. The related mechanism is that CDs with smaller sizes can provide more nucleation sites to promote cement hydration, further advancing the production of C–S–H gels and monosulfate (Ms), thus more markedly enhancing the physical adsorption and chemical binding of chloride ions in cement pastes. This work would guide the preparation of CDs with more suitable sizes to improve the chloride binding ability of cement, thus anticipated to extend the service life of RC structures in coastal environment, ultimately reducing the CO2 emission from cement industry.

Service life prediction and time-variant reliability of reinforced concrete structures in harsh marine environment considering multiple factors: A case study for Qingdao Bay Bridge

Chloride attack is the primary cause of performance deterioration and life attenuation of reinforced concrete (RC) structures in marine environments. In an attempt to accurately predict the reliability and service life of RC structures under the co-effects of multiple factors, a comprehensively modified chloride-transport-based model is proposed in this paper. Based on Fick's second law, a multifactor integrated deterministic chloride transport model is developed considering the time dependence, concrete mix proportion, chloride binding capacity, freeze–thaw cycles, sustained load, temperature, humidity and concrete deterioration. Based on extensive literature review combined with the results of multitudinous long-term field exposure tests used in the durability design of Hong Kong-Zhuhai-Macao Bridge and Shenzhen-Zhongshan Bridge, the probability distribution types and parameter values of the governing variables are determined. Taking Qingdao Bay Bridge as an example, the time-variant reliability index, failure probability and reliability-index-based corrosion initiation are determined by using the equivalent normalization method (JC method) and Monte Carlo simulation technique for different environmental zones including atmospheric/splash/tidal/submerged zones. The sensitivity of the reliability index to the control variables and the effects of age index, chloride binding capacity, numbers of freeze–thaw cycles, stress level and concrete deterioration on the reliability index are quantitatively investigated. The results show that the time-variant reliability of different environmental zones is sequenced as: atmospheric zone > submerged zone > tidal zone > splash zone; the sensitivity of reliability to the governing variables sequenced as: concrete cover thickness > surface chloride concentration > chloride diffusion coefficient > critical chloride concentration; the time dependence of the diffusion coefficient has a significant effect on the time to corrosion initiation and cannot be ignored in the predictive model for both the high-performance or ordinary concrete; the number of freeze–thaw cycles and the tensile stress level are relatively insensitive but substantially detrimental to the reliability of corrosion initiation.

Corrosion damage detection in reinforced concrete using Rayleigh wave-based method

Corrosion of reinforcing bars is a dominant cause of degradation of reinforced concrete (RC) structures. Over time, expansive products of corrosion build up tensile stresses within the concrete, leading to cracks on the exterior surface and spalling of concrete cover. To extend the service life of RC structures, a reliable non-destructive inspection method is required to detect and evaluate corrosion-induced internal cracks in concrete before spalling, so that timely repair can be conducted. Ultrasonic Rayleigh waves have been employed to detect near-surface cracks in concrete based on their transmission coefficient in frequency domain or their time-of-flight information. However, due to heterogeneity of concrete materials, the ill-defined Rayleigh wave component and the sensor coupling on the rough surface of concrete have a significant influence on accuracy of the method. In this study, an improved method using Rayleigh waves has been investigated to detect corrosion-induced internal cracks in concrete. In the method, energy spectrum of the Rayleigh wave is analysed using the continuous wavelet transform and correlated to locations of internal cracks. This method is not sensitive to surface roughness of concrete members and does not need accurate identification of the Rayleigh wave components. Performance of the method was examined using corroded RC specimens with different cover depths and aggregate sizes. As a comparison, electrochemical tests were performed to map the corrosion current density. The test results showed very good agreement with the corrosion damage map generated from the proposed Rayleigh wave-based method.

Influence Of Marine Environment Exposure On The Engineering Properties Of Steel-concrete Interface

Aims: A detailed and reformed service life prediction model needs to be developed by considering the non-uniform distribution of the porous zone and the non-uniform distribution of the corrosion products layer. Background: The microstructure of the steel-concrete interface (SCI) plays an important role in corrosion initiation and concrete cover cracking. The porous zone around SCI is one of the vital engineering properties that influence the service life of corroding reinforced concrete structures in service life prediction models. Objective: The SCI properties are sensitive to the sample preparation technique of reinforced concrete (RC) samples for studying with the aid of scanning electron microscopy (SEM). A simple step-wise sample preparation technique of RC samples for SEM analysis is proposed where there is minimal damage to the properties of SCI. The development, distribution, and propagation of corrosion products at SCI are investigated for RC samples exposed to the marine environment for different exposure periods. The service life of RC structures was assessed through experimentally determined porous zone thickness (PZT) values. Assuming a uniform and constant value of PZT and uniform distribution of corrosion products around SCI might lead to variation or misinterpretation of the service life of structures. The same is explored in the present study. Methods: In this research investigation, backscattered electron images were obtained for the analysis of porous zone thickness around SCI. The distribution and propagation of corrosion products around SCI were investigated for different mineral admixed reinforced concrete samples exposed to the marine environment. Also, porous zone thickness values were used experimentally measured, and the time from corrosion initiation to corrosion cracking was estimated using a service life prediction model. Results: Results show that porous zone thickness is not uniform around SCI. Once the corrosion is initiated, the corrosion products accumulate in the SCI's porous region. Further, the non-uniform porous zone thickness directly influenced the non-uniform distribution of corrosion products. Assuming a constant or uniform porous zone thickness and uniform distribution of corrosion products around SCI leads to misinterpretation of the service life of corroding reinforced concrete structures. Conclusion: The porous zone thickness values around the steel-concrete interface and corrosion current density play an important role in predicting the service life of reinforced concrete structures exposed to the marine environment.

РЕМОНТНЫЕ СТРОИТЕЛЬНЫЕ РАСТВОРЫ, ПОЛУЧЕННЫЕ ПЛАЗМЕННОЙ МОДИФИКАЦИЕЙ И ВИХРЕВОЙ АКТИВАЦИЕЙ

Introduction: The service life of reinforced-concrete structures can be increased with the use of effective repair compositions obtained by activating the original components. Purpose of the study: We aimed to develop effective repair compounds obtained by activating the original components. Methods: To process the original components, low-temperature non-equilibrium plasma (LTNP) and electromagnetic activation in a vortex layer device were used. In the course of the study, we used polypropylene, steel, glass, and basalt fiber and fiber made of structured ferromagnetic microwire. Electron microscopy and X-ray diffraction analysis were applied. Results: It was established that the combined use of the above methods for the modification of raw components makes it possible to improve the strength of these materials by more than 50%, which is due to the characteristics of structure formation in the developed compositions. For instance, LTNP increases the amount of portlandite and reduces the main phases of cement stone— C3S and β-C2S, and vortex activation contributes to an increase in the total number of crystalline phases. Quartz powder particles processed in an electromagnetic mill are characterized by layered structure, high surface roughness, large developed cracking, as well as inclusions as a result of impact action. All that improves the physical and mechanical properties of the resulting repair compositions by an average of 20%. Repair compositions additionally treated with plasma modification feature new hydrated formations on quartz grains.

Evaluation of carbonation coefficient of reinforced concrete structures

ABSTRACT With the advancement of modern construction technology, reinforced concrete (RC) has become the most widely used composite construction material. Corrosion of embedded reinforcement bars in RC structures, which is essentially the result of carbonation and chloride attack, is regarded as one of the weaknesses. Hence, many research studies have been conducted to evaluate the effect of corrosion on the service life of RC structures. However, so far no proper equation has been derived to predict the extent of carbonation-induced corrosion. Identifying this research gap, the present study intends to derive an equation for the coefficient of carbonation (K), which is the governing measure of the rate of carbonation. This paper identifies concrete grade, exposed CO2 gas concentration, and exposed Relative Humidity (RH) level as the major parameters affecting the carbonation process. The behaviour of each factor along with K was identified by conducting a modified accelerated carbonation test (ACT). These experimentally derived K values were statistically analysed under the principle of multiple linear regression, to derive an equation for predicting K values for RC structures. Based on the results, this paper proposes two user-friendly mathematical models covering the entire environmental RH range to predict the K values of RC structures.

Effect of exposure temperatures on chloride penetration resistance of concrete incorporating polypropylene Fibers, silica fume and metakaolin

• The diffusion resistance of silica fume (SF) concrete was 1.4–7.3 less than OPC concrete. • The diffusion resistance of metakaolin (MK) concrete was 1.2–3.9 less than OPC concrete. • The effect of 10% SF on migration resistance was almost similar to that of 15% MK. • Models to predict the service life of RC structures in hot environments were developed. Rapid degradation of reinforced concrete (RC) has become a serious concern due to its reduction of life span and high unexpected maintenance cost. Chloride penetration and subsequent corrosion are the primary causes of such concrete deterioration. This study investigated the effects of polypropylene fibers, metakaolin and silica fume on chloride diffusion and migration resistance of concrete at different exposure temperatures. Eight concrete mixes were prepared using a water/binder ratio of 0.38, cement replacement of 0, 5, 10, and 15 % by either silica fume (SF) or metakaolin (MK) with zero or 0.6 kg/m 3 polypropylene fibers (PPFs). Furthermore, concrete resistance to chloride diffusion at four different exposure temperatures (23, 38, 53 and 68 °C) and chloride migration at 23 °C was evaluated. The results showed that both pozzolanic admixtures, SF and MK, significantly reduced apparent chloride diffusion coefficient (D a ) and chloride migration coefficient (D nssm ). However, PPFs concretes incorporating MK showed inferior chloride diffusion than those containing SF. D a for PPFs concretes incorporating SF and MK were 1.4–7.3 and 1.2–3.9 times less than Type I cement concrete, respectively. While D nssm for PPFs concretes incorporating SF and MK were 1.7–6.3 and 1.5–3.2 times less than Type I cement concrete, respectively. The results also showed that the effect of 15 % MK on D nssm and D a was almost similar to that of 10 % SF. The D a increased slightly at exposure temperatures of 23 and 38 °C. However, at higher exposure temperatures of 53 and 68 °C, a significant increase in D a was noted. Eight models were developed using regression analysis and were validated by comparing the predicted apparent chloride diffusion coefficient values with the measured ones. The validation of the developed models showed that the predicted D a values were within ± 7 % error margin. The developed chloride diffusion models in the current study allow us to predict and quantify the long life of future realizations of RC structures in marine hot environments.

A time-dependent diffusive model for the simulation of chloride penetration in concrete considering the effect of natural salt freeze-thaw cycles

In marine cold regions, reinforced concrete (RC) structures are affected by the coupling action of chloride erosion and freeze-thaw (FT) cycles. The coupling effects will lead to the concrete deterioration, accelerate the chloride diffusion process, and significantly reduce the service life of RC structures. In this investigation, a chloride diffusive model considering the time-varying effects of FT damage and temperature was proposed to quantify the diffusion of chloride in concrete under the action of natural salt freeze-thaw cycles (SFTCs) or under the action of alternating environment, in which the salt FT cycling environment and the pure chloride diffusion environment appear alternately. In addition, a natural salt FT cycling experimental method was put forward, which can simulate the actual salt FT cycling process of concrete in tidal zone. Chloride content and the dynamic elastic modulus (DEM) of specimen were determined after corresponding exposure duration, i.e., 30 d, 60 d, 80 d, 100 d and 120 d. Meanwhile, during the exposure time, the temperature at different positions (air temperature, water temperature, central temperature of the specimen in atmospheric area, tidal zone, and underwater area) was monitored continuously. The results show that the maximum number of FT cycles in this experiment is 36, and the average FT minimum temperature is −4.3 °C. The cycle of FT action is 24 h, and the ratio of freezing time and thawing time is about 5:1. After the initiation of FT action, the DEM decreases linearly with exposure time, with maximum loss of about 3% in one winter. Compared with the previously reported rapid FT cycling test data, the loss of DEM caused by one rapid FT cycle is about 9.3 times of that caused by one natural salt FT cycle. Besides, the mass loss of concrete under natural SFTCs for one winter in Tianjin area is about 0.3%. And chloride profiles in concrete deepened with exposure duration. Subsequently, the proposed model was used to simulate the diffusion of chloride in concrete under the experimental conditions. The chloride contents simulated by the proposed model agree well with the test results, showing that this model is applicable to predict the diffusion of chloride in concrete exposed to the action of alternating environment.

Effect of cracks on the service life of RC structures exposed to chlorides

To move towards a more sustainable concrete, the enhancement of its durability is strongly encouraged and, dealing in particular with reinforced concrete (RC), this mainly means to prevent the damage due to environmental actions, e.g. due to chloride-induced corrosion. Therefore, there is the need of models aimed at designing durable structures. Usually the service life design models consider concrete in uncracked condition. In real structures, however, several phenomena can generate cracks on concrete surface, leading to an acceleration of the corrosion of steel rebar. A number of studies have been recently carried out in order to evaluate the influence of cracks on reinforced concrete durability in chloride-contaminated environment, however the knowledge of the effect of cracks on the initiation and propagation periods is still lacking. Furthermore, few studies have considered additional protection strategies, such as the use of stainless steel rebar. In this work, experimental results are presented concerning the influence of cracks on the service life of reinforced concrete structures in order to evaluate if cracks lead to an earlier corrosion initiation induced by chlorides. Prismatic specimens, reinforced with carbon steel and 304L stainless steel bars, were longitudinally cracked and exposed to ponding with 3.5% NaCl solution. The monitoring of corrosion behaviour showed that when cracks reached the steel surface corrosion initiated immediately.

Numerical and Empirical Models for Service Life Assessment of RC Structures in Marine Environment

The service life prediction of reinforced concrete (RC) structures in marine environment is essential in structural repair and health monitoring. In this paper, a numerical model for predicting the service life of reinforced concrete is first developed which considering the time-varying boundary of chloride concentration, critical chloride concentration and density of corrosion current. Based on the model, the effects of water–cement ratio, reinforcement diameter, concrete cover thickness and critical chloride ion concentration on the service life and deterioration duration of RC structures are investigated. The key factors affecting the service life of reinforced concrete structures are determined. More importantly, based on regression analysis, a new simplified empirical model for predicting the service life of RC structures is also developed. It provides a fast assessment tool for practical engineers. Both the numerical model and empirical model validated are suitable for practical engineering applications. The results show that with the increase of water–cement ratio, the service life of reinforced concrete structure decreases exponentially. And with the increase of the thickness of the concrete cover, the service life, deterioration duration, and safety reserve increase linearly. However, the influence of the diameter of the reinforcing bar on the service life can be ignored.

Probabilistic prediction and calibration for residual shear strength of corroded reinforced concrete columns

Degradation of shear strength for reinforced concrete (RC) columns caused by steel corrosion is one of the prominent factors for shortening the entire service life of RC structures. In this work, probabilistic prediction and calibration methods for residual shear strength of corroded RC columns were developed by combining theoretical analysis with available experimental data. Deterministic shear strength degradation model accounting for the cross-sectional area reduction and mechanical properties degradation of steel reinforcement and concrete was derived first based on the truss and arch mechanisms. Then probabilistic prediction model was proposed by introducing probabilistic model parameters and systematic error. Meanwhile, probabilistic model parameters regarding the influences of prior variation coefficients and update times were investigated using the Bayesian theory. Moreover, probabilistic method was implemented to calibrate seven available deterministic shear strength models based on probability density function and confidence interval. Results indict that the proposed model can not only quantify the probabilistic characteristics of shear strength and predict the residual shear strength for any given corrosion level and guarantee rate, but also provide a suitable approach for calibrating the applicability of conventional deterministic shear strength models.

Effect of rust inhibitors and surface strengthening materials on service life of marine RC structures

Purpose The purpose of this paper is to compare the effect of rust inhibitors and surface strengthening materials on the service life of RC structures in tropical marine environments and ultimately to provide basis and recommendations for the durability design of reinforced concrete (RC) structures. Design/methodology/approach Slag concrete specimens mixed with four kinds of rust inhibitors and coated with four kinds of surface strengthening materials were corroded by seawater exposure for 365 days, and the key parameters of chloride ion diffusion were obtained by testing. Then a new service life prediction model, based on the modified model for chloride ion diffusion and reliability theory, was applied to analyze the effect of rust inhibitors and surface strengthening materials on the service life of RC structures in tropical marine environments. Findings Rust inhibitors and surface strengthening materials can effectively extend the service life of RC structures through different effects on chloride ion diffusion behavior. The effects of rust inhibitors and surface strengthening materials on the service life extension of RC structures adhered to the following trend: silane material > cement-based permeable crystalline waterproof material > hydrophobic plug compound > spray polyurea elastomer > water-based permeable crystalline waterproof material > calcium nitrite > preservative > amino-alcohol composite. Originality/value Using a new method for predicting the service life of RC structures, the attenuation law of the service life of RC structures under the action of rust inhibitors and surface strengthening materials in tropical marine environments is obtained.

Design Service Life of RC Structures with Self-Healing Behaviour to Increase Infrastructure Carbon Savings.

Corrosion of reinforced concrete (RC) structures costs the UK GBP 23b annually and is one of the main durability problems contributing to the development of rust, spalling, cracking, delamination, and structural deterioration. This paper intends to demonstrate the benefit of using tailored self-healing bacteria-based concrete for RC corrosion minimisation and service life increase. The purpose was to evaluate the enhancement in the lifespan of the structure exposed to a harsh marine microenvironment by utilising a probabilistic performance-based method. Comparison is made with the performance of a commercially available solution and in terms of embodied carbon impact. Three different concretes, using CEM I 52.5N, CEM II/A-D, and CEM III/A, were tested with and without an iron-respiring bioproduct (BIO) and an added admixture corrosion inhibitor (AACI). Results show that bioproduct significantly contributes to service life increase of RC structures with CEMIII/A. The repair solution with self-healing behaviour not only increases RC service life, but also enables us to decrease the required cover thickness from 60 mm to 50 mm in an XS2 chloride environment. In both XS2 and XS3 environments, a comparison of CEMIII/A+BIO and CEMII/A-D+AACI concrete shows the benefit of using bioproduct in corrosion inhibition context, besides contributing to an embodied carbon reduction of more than 20%.

Methods of accelerating chloride-induced corrosion in steel-reinforced concrete: A comparative review

Rebar corrosion has been recognized as a major durability issue of the reinforced concrete (RC) structures over recent decades, prompting the necessity to study the corrosion behavior of steel in concrete during the service life. To estimate service life of RC structure, the corrosion of reinforcing steel is generally accelerated by means of the impressed current technique in limited available time. In the literature, several accelerated laboratory corrosion methods have been proposed, however the results varied greatly as chloride concentrations and testing conditions were different. Researchers are unsure which testing methods can be used to give reliable results to simulate the real-life corroded RC structure. This review paper aims to systematically compare and evaluate different methods of accelerating chloride-induced corrosion of steel reinforcement in concrete. First, the accelerated methods categorized into three groups namely, the impressed current (IC), the chloride ion diffusion (CID) and the artificial climate environment (ACE) are briefly discussed. Then, the key influencing factors (i.e. current density, salt concentration, service load) affecting the corrosion results in different methods are compared and evaluated. Finally, the corrosion products and acceleration method are comprehensively discussed. By this comparative review, it is believed that researchers can have a better understanding of different accelerated corrosion methods along with the influencing factors. So they could select or even develop the most suitable techniques to yield reliable corrosion test results.

Impact of sustained load on damage characteristics of reinforced concrete beams under the combined action of salt freeze-thaw cycles and corrosion

A large number of reinforced concrete (RC) structures in cold regions are attacked by the combined action of sustained load, freeze-thaw cycles and steel corrosion. The combined effects accelerate the deterioration process of RC and significantly reduce the service life of RC structures. This paper presents an investigation of the damage characteristics of RC beams under combined action of salt freeze-thaw cycles (SFTCs), load and corrosion. By using the large artificial climate chamber and the RC beam reverse loading device, three RC beams were subjected to SFTCs and corrosion tests under different load levels, and one beam was subjected to corrosion tests under normal temperature without load. The test results showed that the second-order natural frequency can be used as an index to evaluate the freeze-thaw damage of RC beams. The higher the load level, the greater the amount of concrete spalling and the faster the second-order natural frequency decreases. Affected by the freeze-thaw damage, the surface of test beams is not prone to rust crack, but the stress cracks become wider with the increase of SFTCs. In addition, freeze-thaw damage aggravates the non-uniformity of longitudinal reinforcement corrosion, accelerates the corrosion ratio of reinforcement. With the increase of load level, the average corrosion ratios of longitudinal rebar and stirrups increase, and the local corrosion of longitudinal reinforcement is more serious. The coupling effect is obvious when the load, SFTCs and corrosion work together, and the evaluation of the damage characteristics of concrete members should pay attention to the interaction between various factors.

Experimental and numerical study of an electromagnetic sensor for non-destructive evaluation of steel corrosion in concrete

Corrosion of steel is one of the major causes for the degradation of reinforced concrete (RC) structures. In this regard, the assessment of reinforcement corrosion is critical in evaluating the service life of RC structures. The present study aimed to develop an electromagnetic-based apparatus which can be externally positioned on RC structures, to detect and monitor the corrosion of embedded steels. Experiments and numerical simulations based on finite element method (FEM) were carried out to verify the effectiveness of the external electromagnetic (EM) sensor. Different degrees of corrosion for steel in concrete specimens were obtained by impressing current method. Experimental results showed a linear relationship between the mass loss of corroding steel and the change of magnetic flux density sensed by the external EM sensor. Compared with the numerical results obtained from FEM simulations, a good accuracy was exhibited in estimating the mass loss of corroding steel in concrete by the change of magnetic flux density. Numerical simulations were further conducted to investigate the effects of the dimension and position of steel, the distance between testing surfaces of the EM sensor, and the magnitude of magnetic field intensity. As a non-destructive technique, a promising application of the external EM sensor was demonstrated in terms of evaluating the corrosion degree of steel in concrete, with the feasibility to increase its sensitivity by controlling the driving current and coil turns.

Experimental Contribution Concerning the Effect of Carbonation Reaction on the Oxygen Permeability of Concrete

Reinforced concrete (RC) structures are prone to steel corrosion that affects their service life. The two main processes, involved in the destruction of the steel-concrete self-protection are the carbonation and the penetration of chlorides. Understanding the behavior of these degrading phenomena is of paramount importance to enhance the field of predicting the service life of RC structures. In this investigation, a total number of 54 cylindrical specimens (150×100 mm2) and 54 cubic specimens (100×100 mm2) from six different concrete mixtures covering a range of water-to-cement ratio (w/c) between 0.76 and 0.36 was tested. The effect of the carbonation reaction on the concrete oxygen permeability (assessed by a recent method), compressive strength, and weight has been investigated. The obtained data were analyzed to establish correlations to predict the oxygen permeability of non-carbonated concrete with concrete characteristics such as w/c ratio and compressive strength. Test results show good correlations. The results of the accelerated carbonation test showed that compressive strength increases with carbonation. Concerning the effect of carbonation reaction on concrete oxygen permeability which is the main goal of this paper, test results showed that carbonation considerably affects the concrete permeability. This result was observed for all concrete mixtures. Moreover, the relationship between carbonation depth and concrete oxygen permeability in ordinary concrete differed than high performance concrete. Furthermore, the increase of the oxygen permeability observed after the carbonation has been justified by the analysis of the Scanning Electron Microscopy images that show the formation of larger pores in comparison with non-carbonated concrete.

Assessing the influence of fibers on the flexural behavior of reinforced concrete beams with different longitudinal reinforcement ratios

AbstractThe use of fibers in reinforced concrete (RC) beams mainly improves both the bearing capacity and the cracking control. In this way, positive effects on the service life of RC structures can be expected. In this paper, the fiber influence on the flexural behavior of RC beams with different longitudinal reinforcement ratios (0.5% ≤ ρs ≤ 1.2%) is analyzed by testing small‐scale RC beams. Concretes incorporating 0, 25 and 50 kg/m3 of steel, 6 and 12 kg/m3 of glass macrofibers, and 5 and 10 kg/m3 of polymer macrofibers were studied. Crack and deflection control, as well as bearing capacity and crack localization were evaluated for a broad range of fiber‐reinforced concrete (FRC) toughness. It is verified that fibers, in the longitudinal reinforcement ratio considered, improve the bending behavior at serviceability limit state (SLS) and ultimate limit state (ULS) of RC beams, without limiting the structure ductility. It was also confirmed the philosophy of the fib Model Code 2010, such that FRC can be considered as a composite material where performance parameters govern its mechanical behavior. Finally, the several data available allowed to deeply analyze fib Model Code 2010 formulations (mean crack spacing and flexural bearing capacity) and to propose modifications where needed.

Effect of surface preparation on corrosion of steel rebars coated with cement-polymer-composites (CPC) and embedded in concrete

Nowadays, Cement-Polymer-Composites are widely used to coat steel rebars to delay the initiation of corrosion in reinforced concrete (RC) structures. However, Cement-Polymer-Composite (CPC) coating is sometimes inadequately applied on rusted steel and can lead to premature under film/crevice corrosion. This paper investigates the effect of such inadequate applications of CPC coating and premature corrosion on the service life of RC structures. For this, maximum surface chloride concentrations, diffusion coefficients, and chloride thresholds were determined by a one-year-long laboratory study on the specimens obtained from a 6-year-old bridge and prepared in the laboratory. Studies found that the chloride threshold of inadequately coated steel rebar (i.e., coating on ‘as received’ surface) is significantly lower than that of the adequately coated steel rebars (i.e., coating on ‘sandblasted’ surface). Also, the corrosion initiation time for systems with inadequately coated steel rebar was about 50% less than that of the systems with adequately coated steel rebars. The corrosion mechanisms were justified with micrographs. It is recommended not to use CPC coated steel rebars if adequate surface preparation (say, cleaning/sandblasting) is not implemented.

Carbonation modelling for cement-based materials considering influences of aggregate and interfacial transition zone

The influences of aggregate and interfacial transition zone (ITZ) on the carbonation resistance of cement-based materials cannot be ignored. Based on Fick's second law, a CO2 diffusion model considering the influences of aggregate and ITZ is proposed in this work. Subsequently, the computing methods of corresponding parameters in this model are presented. A CO2 reaction model based on the law of conservation of mass is also proposed. The reliability of theoretical modelling was further verified by comparisons with experimental data from the literature. Finally, this model was used to study the concentration distribution of CO2 in C30 and C50 concrete with and without load effects. The results showed that the carbonation prediction method considering the influences of aggregate and ITZ was capable of handling the diffusivity of CO2 in cement-based materials. This theoretical model can predict the carbonation depth values with reasonable accuracy. Therefore, this model can be used as a practical tool considering the existence of aggregate and ITZ as a significant factor to evaluate the durability of concrete. Thus, it can help to design and predict the service life of reinforced-concrete structures.