Structures In Chloride Environments Research Articles

Design Method for Concrete Mix Proportion Based on Indices of Strength and Durability of Concrete Exposed to Chloride Attack

In the current design code, concrete mix proportions are not based on durability parameters that can quantitatively design the requirement of service life for chloride-exposed structures. Multifactor calculation models of durability index are developed, and the replacement of mineral admixtures in concrete is determined according to predetermined concrete strength and durability requirements. The chloride diffusion coefficient and aging factor of concrete are chosen as index parameters for the resistance of concrete to chloride salts. A multifactor model of the chloride diffusion coefficient of concrete is then established by regression analysis based on the experimental data, and explicit expressions for the water-to-binder ratio and replacement of admixture were determined by solving the multivariate nonlinear equation system. By calculating the average compressive strength and chloride diffusion coefficient from their specified values, the mix proportion of concrete is determined. The superiority of the proposed method is demonstrated by the comparison with the current code. Lastly, an orthogonal experiment was conducted to demonstrate the validity of the proposed method for designing concrete mix proportions by referencing engineering practices. The results show that major parameters of concrete mix proportion can be determined quantitatively from structural design parameters such as compressive strength, chloride diffusion coefficient, and aging factor, satisfying the requirements of ultimate bearing capacity and service life for concrete structures in chloride environments, as demonstrated in this paper.

A Numerical Study on Chloride Diffusion in Cracked Concrete

The cracks in concrete are a fast transport path for chlorides and influence the service life of concrete structures in chloride environments. This study aimed to reveal the effect of crack geometry on chloride diffusion in cracked concrete. The chloride diffusion process in cracked concrete was simulated with the finite difference method by solving Fick’s law. The results showed that the apparent chloride diffusivity was lower in more tortuous cracks, and the cracks with more narrow points also showed lower apparent chloride diffusivity. For tortuous cracks, a higher crack width meant relatively more straight cracks, and consequently, higher apparent chloride diffusivity, while a lower crack width resulted in more tortuous cracks and lower apparent chloride diffusivity. The crack depth showed a more significant influence on the chloride penetration depth in cracked concrete than crack geometry did. Compared with rectangular and V-shaped cracks, the chloride diffusion process in cracked concrete with a tortuous crack was slower at the early immersion age. At the same crack depth, the crack geometry showed a marginal influence on the chloride penetration depth in cracked concrete during long-term immersion.

Effect of water‐to‐cement ratio on service life of reinforced concrete structures in chloride environment

AbstractCracking of concrete induced by rebar corrosion is a main cause for deterioration of reinforced concrete structures especially for those exposed to chloride environment. In all of the parameters influencing the service life of concrete structures, water‐to‐cement ratio is a very important one for it determines the resistance of concrete against chloride aggression and corrosion‐induced damage. In this paper, the parameters in corrosion process influenced by water‐to‐cement ratio of concrete (w/c) are screened all‐round which include concrete tensile strength ft, elastic modulus Ec, porous zone thickness δ0, corrosion current density icorr, critical chloride content Ccr, and chloride diffusion coefficient Dcl. The influences of w/c on the chloride penetration stage and the corrosion damage stage are studied. The first stage is expressed by Fick's second law, and the second stage is calculated using a commonly used analytical model, i.e. the thick‐walled cylinder model. It is found that with the increase of w/c, the change of some parameters (ft↑, Ccr↑, Dcl↓, icorr↓) will induce increase of the service life, while the change of the others (Ec↑, δ0↓) will induce decrease of the service life. The diffusion stage and damage stage both shorten with the increase of w/c, and thus the total service life decreases with the increase of w/c. These results show that the increase of w/c presents a negative effect on the service life of concrete structures in total. In addition, with the increase of w/c, the service life for various concrete cover thicknesses and rebar diameters decrease in a similar tendency. For probability analyses, it is found that the probability density curves under different w/c are almost symmetric but show a little left‐skewed, and the total service life for various w/c fluctuates about 0.6 to 1.0 year.

A Theoretical Framework for Service Life Prediction of Reinforced Concrete Structures in Chloride Environment using Load Factors

Service life modeling of reinforced concrete structures in a chloride environment is mainly performed without considering the loading effects. Different loading effects can produce different service life results. This study presents a theoretical framework for the modeling of the service life of reinforced concrete structures in a chloride environment using loading factors, showing that, depending on the loading nature (either compression or tension), different diffusion results could be obtained. This paper also highlights various approaches to service life modeling, such as the deterministic, probabilistic, and semi-probabilistic, which consider different ways to estimate the service life of reinforced concrete structures in chloride environments. The importance of various distributions for the input parameters in the chloride ingress modeling was examined. The proposed framework includes a procedure to estimate the probability of concrete failure in chloride environments.

Experimental and numerical study on seismic performance of corroded steel frames in chloride environment

To investigate the seismic behavior of corroded steel frames in chloride environment, accelerated corrosion tests and lateral low cyclic loading tests were carried out on four steel frames. Firstly, a deterioration model of mechanical property with mass loss rate for corroded Q235B steel was proposed. Then, the failure process and mechanism of test steel frames were observed, the strain developments of potential plastic hinge sections at beam ends and column bases were analyzed, and the effects of corrosion level on the hysteretic curves, bearing capacity, stiffness degradation, ductility and energy dissipation capacity were studied in detail. The test results indicated that in relatively slight corrosion level, all specimens exhibited a hybrid yield dissipation mechanism, with the “beam end - column bottom - beam end” sequence of the plastic hinges occurring. However, an increase in corrosion level tended to accelerate plastic hinge formation and aggravate structural damage degree. Additionally, with the increase of corrosion level, the lateral bearing capacity reduced, stiffness degradation intensified, ductility and energy dissipation capacity decreased. Base on test data, the structural performance levels and performance index quantified thresholds of corroded steel frame were preliminarily determined. Moreover, a finite element model adopting a time-varying constitutive model of corroded steel and a stress triaxiality dependent failure criterion, was established using ABAQUS, and then verified to be valid by comparing the numerical and experimental results. The research results lay an experimental and theoretical foundation for the seismic performance evaluation of in-service steel frame structures in chloride environment.

Modeling of the chloride diffusivity of ultra-high performance concrete with a multi-scale scheme

The chloride diffusivity of ultra-high performance concrete (UHPC) is the key parameter that determines the service life of UHPC structures in chloride environments. In this study, a multi-scale model, which considered the structural information at micro and meso-scale, was proposed to predict macro property, i.e. chloride diffusivity, of UHPC. At micro-scale, the chloride diffusivity of UHPC paste was predicted based on an extended HYMOSTRUC3D model and a finite element method. At meso-scale, Anm model and finite element method were applied to calculate the chloride diffusivity of UHPC mortar. At macro-scale, the chloride diffusivity of UHPC was determined with a two phase model. The chloride diffusivity at a lower scale was used as input to predict the chloride diffusivity at the higher scale. Non-steady chloride diffusion tests were performed to validate the proposed model. The results showed that the proposed multi-scale model gave a little higher chloride diffusivity of UHPC than experiment results. However, the proposed model is still helpful for the durability design of UHPC structure in chloride environments.

Effect of Aggregate of Electric Arc Furnace Slag on Durability of Reinforced Concrete Structures in Chloride Environment (Part I)

Durability of reinforced concrete structures including electric arc furnace slag as aggregate in chloride environment is a significant topic to enable their performance stable in their service period. In this study, chloride penetration in recycled concretes including gradation of fine and coarse aggregate that is electric arc furnace (EAF) slag is tested and compared with that of reference concrete including just natural aggregate gradation. To observe chloride penetration effect, concrete samples are exposed to a saline attack firstly; and then, total chloride content and chloride penetration profiles are tested. The Part I of the study argues the effect of aggregate of electric arc furnace slag on durability of reinforced concrete structures in chloride environment in the Introduction and Background section and the Materials and Methods section. Second part of the study is going to present the Results and Discussions section and the Conclusions section. Both part have the Abstract section and Reference section differently each other.

Effect of Aggregate of Electric Arc Furnace Slag on Durability of Reinforced Concrete Structures in Chloride Environment (Part II)

Second part of the study presented argues the effect of aggregate of electric arc furnace slag on durability of reinforced concrete structures in chloride environment based on the Results and Discussions section and the Conclusions section. Both part have the Abstract section and Reference section differently each other. Results display that the EAF slag aggregate is favorable because of increasing the durability against chloride damage and of stoping the penetration of free chloride measured with method of the colorimetric test as well as wet chemical analysis. However, the study also answers that the wet chemical analysis is still the best method to measure the penetration of free chloride in concretes.

Chloride diffusivity and service life prediction of fatigue damaged RC beams under seawater wet-dry environment

The effects of low fatigue load level with lower limit of 0.1 Pu and upper limit of 0.2 Pu or 0.3 Pu on chloride diffusivity of RC beams were experimentally studied after 100 times of seawater wet-dry cycles, where Pu is the static ultimate strength of the RC beams and the cycles of fatigue loading was 200,000. Based on the test results, residual service life was also discussed here. Results show that, chloride diffusivity was accelerated as fatigue load level increased. When the load level was 0.2, based on Fick’s 2nd law, chloride diffusion coefficient of tensile concrete was higher than that of compressive concrete, but the result was reverse for the load level of 0.3. Furthermore, based on the Monte-Carlo Method, the predicted residual service life in one-dimensional state of chloride diffusion would be shortened by 85 percent when the fatigue load level was 0.2, and by 95 percent when fatigue load level was 0.3, compared with that of no fatigue damaged RC beam. The predicted service life in two-dimensional state was about half of that in one-dimensional state. It was concluded that fatigue damage has a more considerable effect on shortening service life of RC structures in chloride environment.

Need for Further Development in Service Life Modelling of Concrete Structures in Chloride Environment

Designing concrete structures for a very long service life may have considerable economic and societal benefits including minimized material consumption over the long term, thus contribute to more sustainable solutions. However, such long service lives require determination and extrapolation of environmental loadings and material durability performance over a long period, as well as reliable and operational models for service life predictions. Codes and standards give deem-to-satisfied recommendations for intended working (service) life up to about 100 years. However, if higher working lives of 200 and 300 years are specified, as for monumental buildings, bridges and other important infrastructures, more in-depth service life predictions are required. This paper focuses on durability and service life predictions for reinforced concrete structures for working (service) life requirements above 100 years.

Durability of Steel Fibres Reinforcement Concrete Beams in Chloride Environment Combined with Inhibitor

This paper presented the effect of the combination of an inhibitor and steel fibre reinforced concrete (SFRC) for concrete structures in chloride environments. Twelve beams were cast and tested to study their flexural behavior. The morphology of steel surfaces using the inhibitor after observing the scanning electron microscope showed a low layer of corrosion products. The steel surface immersed in the inhibitor free solution was seen to have been subject to chloride ions attacks as shown in this study. The interest to the field of the present study is the relatively higher durability of the performance when using the inhibitor. Crack width and crack spacing for beams under the same load showed that the use of SFRC with the inhibitor for concrete structures in chloride environments must have transferred tension across cracks that led to reducing crack spacing without any chloride ions attack.

Effect of metakaolin on the chloride ingress properties of concrete

Research and practical experience have shown that partial replacement of cement by metakaolin improves concrete durability as a result of the refinement of the pore structure. While much research has been presented on concrete performance with metakaolin, it is scarce concerning the transport properties of chlorides in concrete with metakaolin, in natural conditions (i.e., un-accelerated). This study determines the chloride diffusion coefficients for several concretes with vary levels of cement replacement with metakaolin, and compares these results with chloride migration coefficients obtained from accelerated laboratory testing. In this study, two cement types (CEM I 42,5R and CEM IV/A 42,5) and two cement contents levels where used with metakaolin replacement levels varying from 10-20%. Concretes where tested for fresh properties, compressive strength, electrical resistivity, chloride ingress characteristics (natural diffusion and migration), and mercury intrusion porosimetry. The results show improved strength, durability properties and chloride penetration resistance of concretes with metakaolin. Furthermore, the use of metakaolin in fly ash concrete improves the early age performance of the concrete (<90 days), counteracting the delay in strength and durability gain typically associated with fly ash concrete. The results obtained from this study fulfil the lack of critical input for service life design models of reinforced concrete structures in chloride environments, with emphasis on concretes with metakaolin replacement.

A review of the chloride transport properties of cracked concrete: experiments and simulations

Chloride transport property is very important for the durability and service life of reinforced concrete structures subjected to marine environments and de-icing salt. In reality, for different reasons, concrete structures are frequently cracked, and cracks can alter the chloride transport properties of concrete. Recently, several studies have been conducted by both experiment and simulation on the influence of cracks on the chloride transport properties of concrete. The aim of this paper is to review these research efforts. The experimental methods and simulation approaches on the chloride transport properties of cracked concrete are introduced. Detailed discussions on the findings from these experimental and simulation studies are given. The chloride transport properties of cracked concrete are influenced by various factors, such as crack geometry, concrete composition, and load condition. Research in this area is still on-going, and many problems need to be settled before proposing reliable models for predicting the service life of real cracked concrete structures in chloride environments. Hence, some further research topics are recommended. The influences of other factors, such as carbonation, freeze-thaw, fatigue, and saturation degree, on the transport properties of cracked concrete should be revealed.

Chloride binding isotherm from migration and diffusion tests

Chloride binding is often described by chloride binding isotherm, which is closely related to the service life of concrete structures in chloride environments. Many methods have been proposed to determine chloride binding isotherm. Compared to other methods, chloride binding isotherms obtained directly from nonsteady-state diffusion tests seem closer to the reality. We studied the chloride binding isotherm from both nonsteady-state electrical-accelerated migration and diffusion tests at different temperatures. Twelve concrete mixes with different supplementary cementing materials and water-to-binder ratios of 0.35, 0.48 and 0.6 were cast for study. The specimens after diffusion (or migration) tests were sliced layer by layer, and acid-soluble and free chloride contents of each layer were measured. A chloride binding isotherm was obtained from one specimen. Experimental results indicated that electrical voltage had a slight effect on the chloride binding isotherm of concrete. Temperature had a positive effect on chloride binding. The higher the water-to-binder ratio was, the higher the chloride binding was.

Corrosion-Induced Cover Cracking: New Test Data and Predictive Models

Corrosion, especially in chloride environments, can contribute to cracking of reinforced concrete (RC) structures. In this paper, accelerated corrosion tests of reinforced concrete slabs were conducted to develop a new empirical model to predict the timing of corrosion-induced cover cracking for RC structures in chloride environments. The accelerated corrosion experimental program measured concrete cover cracking for RC slab specimens based on various concrete covers, concrete tensile strengths, and reinforcing bar diameters. A correction factor for concrete confinement is proposed to account for bars located at edges and corners. A rate of loading correction factor is applied to allow the cracking times for accelerated corrosion rates to be adjusted for the lower corrosion rates generally found in RC structures. A statistical analysis of model accuracy is used to account for variabilities between experimental data and model predictions. The new model is shown to be particularly sensitive to cover and reinforcing bar diameter.

An overview and sensitivity study of a multimechanistic chloride transport model

Service life prediction models have been developed in order to ensure adequate durability of reinforced concrete structures in chloride environments. Many of the models currently available are overly simplistic, assuming chloride ingress occurs solely by diffusion and that boundary conditions and concrete properties (i.e., diffusivity) remain constant with time. This paper describes a recently developed model that considers multimechanistic transport, chemical binding, and the time-dependent nature of concrete properties. The results of a sensitivity analysis carried out on eight of the input parameters in the model are also reviewed.

Concrete durability: estimation of chloride concentration during design life

Values of the coefficient of chloride diffusion D, as measured by the classical diffusion test and by a penetration test, are compared. It is shown that D cannot be determined reliably from penetration tests. Two sets of nomograms are developed for the design of concrete structures in chloride environments. The proposed nomograms can cope with a wide range of concrete variables: a basis is provided for their extension to include parameters not studied in the present investigation.