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This research was conducted to develop a model to estimate the effective diffusion coefficient of substances in concrete considering with spatial properties of each composition material. In this model, concrete was assumed to be composed of cement paste, interfacial transition zone and aggregate. Proposed model could appropriately evaluate the effe...
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It was experimentally founded, that the size and the topology of the active additives particles in the composition of the new kinds of high performance concretes is a very important parameter (as its chemical activity), especially if the combination of these additives enables realization of nonlinear phenomena. The sample which is combination of hi...
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... Yoon [34] and co-workers reported that the chloride diffusion coefficient increased significantly with w AC when the width was in the range of 30-200 µm, but when the width exceeds 200 µm, it was almost a constant as in free solution. The chloride diffusion coefficient in free solution is about 2.03 × 10 −9 m 2 /s [35]. In this paper, based on experimental data available in refs. ...
Chloride ion attack is a major cause of concrete durability problems, and existing studies have rarely addressed the effects of damage zones. In this paper, an improved mesoscale model including five phases was constructed using the finite element software ABAQUS to study the diffusivity of chloride ions in cracked concrete. It was found that the damage zone is negligible when the crack width is less than 50 μm, while the width and depth of the damage zone are about 15 times the crack width and 15% of the crack depth when the crack is greater than 50 μm. The results show that the diffusion of chloride is greatly influenced by the crack width, while it is little-influenced by the crack shape. Low water–cement ratio and adequate hydration of the concrete are also key factors affecting chloride diffusion. In contrast, regular rounded aggregates have a positive effect on reducing chloride diffusion compared to irregularly shaped aggregates, and this effect becomes weaker with increasing service time. In addition, the protective layer can effectively prevent the diffusion of chloride in concrete. Therefore, when designing marine concrete, efforts should be made to ensure that the concrete has a low water–cement ratio, adequate hydration, less cracking and a protective layer.
... On the one hand, the introduction of aggregate increases the curvature of the chloride's transport path and slows down the transport of chloride in concrete; on the other hand, the introduction of aggregate generates a transition zone between aggregate and mortar, which opens new channels for the transport of chloride in concrete and accelerates the transport of chloride in concrete. Yang and Cho [9], Care [10], and Kato and Uomoto [11] studied the transport of chloride in the transition zone by accelerated tests. Garboczi and Bentz [12] viewed concrete as a three-phase composite consisting of aggregate, mortar, and transition zone, and theoretically investigated the difusion coefcient of chloride in concrete. ...
Chloride diffusivity is the most crucial factor for evaluating durability and predicting service life of concrete structures exposed to chloride environment. In the present paper, concrete is considered as a random heterogeneous composite of three phases: the aggregates, the matrix, and the interfacial transition zones (ITZ) between them. A mesoscopic model has been established based on the random aggregate model and a numerical model for calculating the chloride diffusivity has been proposed. The influences of aggregate size and the properties of the interfacial transition zones on chloride diffusivity of concrete are also discussed. The chloride diffusivity is size dependent because aggregates are randomly distributed in concrete. The chloride diffusivity under different sizes is analyzed by numerous numerical simulations, and the representative volume element (RVE) of chloride penetration into concrete is researched by the statistical method.
... It should be noted that the instantaneous diffusion coefficient discussed above is different to the diffusion coefficient normally obtained from the solution of the error function complement (erfc) to a one-dimensional chloride profile obtained from dust drillings of concrete exposed to chloride. This is generally referred to as the effective diffusion coefficient, D eff (m 2 /s), (Delagrave et al. 1996;Kato and Uomoto 2005;Patel et al. 2018) or when chloride binding is included, referred to as the apparent diffusion coefficient. For clarity, Fig. 12 displays the relationship between the instantaneous and the effective diffusion coefficient, D i and D eff , respectively, which can be related through the following relationship (Pack et al. 2010;McCarter et al. 2015c), ...
The electrical properties of porous systems are intimately linked to mass transport and flow processes such as diffusion and permeability and offer a simple testing methodology for assessing those properties which are responsible for the durability and long-term performance of construction materials. In the current study, electrical impedance spectra for concretes containing both plain and blended Portland cement binders were obtained over a period of 360 days. In-situ impedance measurements were used to accurately identify the bulk resistance (hence evaluation of resistivity) of the concretes and the optimum frequency range for bulk resistance measurements. The bulk resistivity was normalised by that of the pore-fluid resistivity obtained from computer simulations and the results indicated that the pore-fluid resistivity decreased only marginally with time once the hydration process had advanced beyond 28 days. It is shown that the normalised resistivity – termed the Formation Factor – displayed a continual increase with time, highlighting on-going hydration/pozzolanic reaction and pore structure refinement over the entire test period. This was particularly evident for the slag concretes. Using the normalisation process, a simple approach is presented to evaluate the effective diffusion coefficient of the concretes and a durability/performance classification system, based on the Formation Factor, is presented.
For the reinforced concrete structure in high latitude marine environment, the influence of freeze-thaw on chloride ion transport has been paid more and more attention. This paper intends to explore whether there is any other unknown coupling mode, by an alternating experiment of rapid freeze-thaw and chloride diffusion. For the purpose of data comparison, the tests of pure immersion diffusion are also carried out. It is found that the transport rate of chloride ions in the alternating test is significantly higher than that in the control groups; and the boundary concentration increases with the number of freeze-thaw cycles. The result indicates that there exists another influence mechanism of freeze-thaw on the transport of chloride ions, in addition to increasing the diffusion coefficient. Then, a new hypothesis mode is proposed and its mechanism is explained from the micro scale, i.e. the phase transition of pore solution during the freeze-thaw process can drive the migration of chloride ions dissolved in it.
Concrete structures in coastal areas are regularly subjected to the coupling of sustained loads and a corrosive environment of chloride salts. With the influence of the coupling effect, structural cracks and steel bar corrosion will occur and severely deteriorate the mechanical properties and durability of reinforced concrete (RC) structures. In this study, 11 RC beams were subjected to a coupled sustained load and salt environment. The generation and development of cracks, the corrosion rate of the steel bar, and the residual bearing capacity of tested beams were all considered. The results showed that cracks developed continually under the coupling of sustained loads and chloride corrosion. No obvious correlation was observed between the sustained loads and the residual bearing capacity reduction of the beams within a certain range. However, the residual bearing capacity of the beams decreased with increased loading time. The chloride ion content of the crack area at different depth was larger than that of the non-crack area, and show positive correlation with the crack width.
The aggregate composed primarily of CaO, Al2O3（CaO・Al2O3 aggregate）generates hydroculmite by hydrating with calcium hydroxide which is a cement hydrate, and reacting and generate Friedel salt when a chloride ion acts. When the calcium aluminate is used for concrete as aggregate, a hydration was occurred at the aggregate surface. As a result, the possibility that a transition zone of concrete was modified by using the CaO・Al2O3 aggregate was suggested. In this research, physical properties and substance penetration property of the concrete which using with calcium aluminate aggregate were examined. Regarding the strength development property, it was confirmed that the concrete using with CaO・Al2O3 aggregate showed higher compressive strength compare with the concrete using with natural aggregate but there was not confirmed interrelation between compressive strength and absorption property of concrete. This was considered to be caused by generation of Hydroculmite and Friedel’s salt which were generated by the reaction of CaO・Al2O3 aggregate. By the result of SEM-EDS, an aggregate interface became the dense with the concrete using the CaO・Al2O3 aggregate in comparison with concrete using the natural aggregate, and it was confirmed that a different hydrate was generated inside and outside the outer layer of the CaO・Al2O3 aggregate.
This paper presents new models for computing the chloride-induced corrosion period based on combining Monte Carlo simulation and statistical analysis methods. The study is divided into two parts. The first part utilized Monte Carlo simulation to predict the probability of corrosion (1-200 years) for concrete containing metakaolin (MK) with different mixture proportions and concrete covers. Also, the effect of different mixture parameters (percentage of MK, binder content, and water-to-binder ratio) on the probability of corrosion was studied in this part. The second part adopted statistical analysis to develop models predicting the chloride-induced corrosion period (time to reach 10% probability of corrosion initiation) in various MK mixtures and identified the most significant factors affecting this period. Design charts were also developed using statistical analysis to facilitate and simplify the prediction of the chloride-induced corrosion period. The results showed that the probability of corrosion decreased as the percentage of MK increased. Also, using a lower W/B ratio or higher binder content in MK mixtures improved the effectiveness of MK to reduce the probability of corrosion. The results also showed that the most significant factor affecting the chloride-induced corrosion period was found to be MK replacement, W/B ratio, and binder content respectively, in order of significance. The developed models and design charts in this paper will help designers and engineers to better understand the influence and the importance of various mix design parameters of MK mixtures on the chloride-induced corrosion period estimation.
Cracks within concrete accelerate the ingress of chloride ions, causing acceleration of the degradation of the mechanical performance of reinforced concrete (RC) structures. The effects of cracks should therefore be considered in the durability design of RC structures. However, the available test observations show large discrepancies regarding the quantitative effect of cracks on chloride diffusivity in concrete, and the corresponding developed empirical formulas have some limitations. In the present study, a unified quantitative relation between the chloride diffusion coefficient in cracks and crack width was developed. The parameters in the proposed formula have their own physical meanings, and the corresponding curve of the formula is smooth without an inflection point. The rationality and accuracy of the developed simplified formula were verified through comparisons with test data. Furthermore, chloride diffusivity in cracked concrete was studied and simulated based on the developed simplified formula, and the effects of crack width and crack depth were examined. The simulation results indicate that chloride diffusivity in cracked concrete is significantly affected by both crack width and crack depth. Moreover, the chloride diffusion behaviour in higher strength concrete with lower porosity is much more sensitive to cracks than normal- and lower strength concretes.
Hydration mechanism of calcium aluminate aggregate was examined. In order to confirm the hydration property of calcium aluminate aggregate, hydrate in the paste specimen which was mixed grinded calcium aluminate aggregate, Ca(OH)2 and water was confirmed by XRD. And also hydrate in the paste which was added CaCl2 to above paste was confirmed. The hydrates in the mortar with calcium aluminate aggregate were confirmed by XRD and TG-D TA, and these hydrates were observed by SEM image. In addition, the hydrates were confirmed by EDS and same examinations were carried out for the mortar with natural sand for comparison. Author confirmed that grinded calcium aluminate powder reacts with Ca(OH)2 which is provided as a cement hydrate and it generates hydrocalumite. Furthermore, there was a property to change to Friedel's salt when a chloride ion acted. When the calcium aluminate is used for mortar as fine aggregate, a hydration was occurred at the aggregate surface and it was confirmed to generate hydrocalumite and hydrogarnet in the aggregate surface section. As a result, the possibility that a transition zone of mortar was modified by using the calcium aluminate aggregate was suggested. In the mortar which after salt water immersion, chloride ion penetration depth of mortar with calcium aluminate aggregate was smaller than ordinary mortar. And the hydrate formed layer was confirmed at the fine aggregate surface and its Ca/Al or Cl/Al molar ratio was similar to Friedel's salt. And hydrates which Ca/Al molar ratio is 1.5 to 2.0 were also confirmed on the surface of calcium aluminate aggregate.
Chloride diffusion coefficient depends on many variables including concrete quality, environmental conditions, and time. In this investigation, the concrete quality and time were studied while maintaining the environmental conditions constant. Fifty-three concrete mixtures were tested based on a refined statistical analysis. Enhanced response surface method (RSM) was used to present the most significant factors affecting the chloride diffusion at different ages. The tested mixtures contained various water-to-binder (W/B) (ratios 0.3–0.5), metakaolin (MK) replacement (0–25%), and total binder content (350–600 kg/m3). Bulk diffusion test was adopted for two years to determine the time-dependent coefficient _m_ of chloride diffusion for all mixtures based on the error function solution to Fick's law. This coefficient was calculated based on two different bulk diffusion test methods: total and average methods. Design charts were developed to facilitate the optimization of mixture proportions for designers/engineers. The investigation also included some experimental relationships between the rapid chloride permeability test (RCPT), chloride diffusion coefficient, and compressive strength results. The results showed that the values of the chloride diffusion indicated a general reduction from 28 days to 760 days of testing. As the percentage of MK or binder content increased or as the W/B ratio decrease, the chloride diffusion reduction coefficients, _m_total and _m_avr, were found to increase. Based on the analysis of variance (ANOVA) from the statistical model, MK was found to be the most significant factor affecting the chloride diffusion at late ages (360 and 720 days), while the W/B ratio was the most significant factor affecting early ages of chloride diffusion (28 and 90 days). The developed models and design charts in this paper can be of special interest to designers/engineers by aiding prediction of service life of concrete containing MK.
