The variability and uncertainty associated with chloride thresholds for corrosion initiation in reinforced concrete structures can be partly explained by the presence of localized defects and imperfections along steel-concrete interface such as elongated cracks, pores, gaps, crevices, and mill scale. It has been demonstrated in prior research that pore solution in these imperfections might be different from that of the bulk pore solution, and this difference may create the necessary conditions for the breakdown of the passive film. These studies showed that the chemistry of the pore solution, in particular pH and Cl - /OH - , within localized defects provide more favorable conditions for depassivation than the bulk concrete pore solution. Local acidification and increase in Cl - /OH - within these defects were observed, albeit to different degrees. Defect geometry has been found to be a critical parameter affecting local acidification and the increase in Cl - /OH - . However, chemical composition of the pore solution, reactions between corrosion products and the ionic species in the pore solution and changes in transport properties within the defect due to accumulation of solid corrosion product have also been shown to affect the process. Due to these complications, it is challenging to study these effects experimentally, hence numerical investigation techniques are required. In order to better understand the processes that take place within local defects along the steel-concrete interface a reactive-transport modeling framework is developed. The model incorporates finite element analysis (FEM) with thermodynamic/kinetic modeling of cementitious systems. The FEM module is responsible for modeling multiphysics phenomena such as corrosion, mass transport, heat transfer, phase flow and kinetics, while thermodynamic module is used to model chemically complex reaction computations based on Gibbs Energy Minimization (GEM) theory. A non-iterative operator splitting technique in a time marching scheme is used for uncoupling the multiphysics phenomena from reaction equations. The framework is able to analyze complex chemical systems including processes that take place in cementitious materials and as a result of corrosion reactions. The presentation will highlight the main components of developed framework and demonstrate its functionality though case studies and hypothetical simulations.
The transformation of 2-line ferrihydrite to goethite from supersaturated solutions at alkaline pH >= 13.0 was studied using a combination of benchtop and advanced synchrotron techniques such as X-ray diffraction, thermogravimetric analysis and X-ray absorption spectroscopy. In comparison to the transformation rates at acidic to mildly alkaline environments, the half-life,t_1/2, of 2-line ferrihydrite reduces from several months at pH = 2.0, and approximately 15 days at pH = 10.0, to just under 5 hours at pH = 14.0. Calculated first order rate constants of transformation, k, increase exponentially with respect to the pH and follow the progression log_10 k = log_10 k_0 + a*pH^E3. Simultaneous monitoring of the aqueous Fe(III) concentration via inductively coupled plasma optical emission spectroscopy demonstrates that (i) goethite likely precipitates from solution and (ii) its formation is rate-limited by the comparatively slow re-dissolution of 2-line ferrihydrite. The analysis presented can be used to estimate the transformation rate of naturally occurring 2-line ferrihydrite in aqueous electrolytes characteristic to mine and radioactive waste tailings as well as the formation of corrosion products in cementitious pore solutions.
Abstract This study examines the performance of cementitious systems made using clinker that is typically used to make Type II/V cement, limestone, and supplementary cementitious materials (SCMs). The porosity, formation factor, and pore connectivity of mortars are examined. The mortars are made with ordinary portland cement (OPC), OPC+limestone (LS), and portland limestone cement (PLC) with and without typical commercial SCMs (silica fume, fly ash, and slag). The porosity of both the PLC and OPC+LS mortars is approximately 4 % higher than the porosity of commercial OPC (which typically contains 2–3 % interground limestone) mortar. The porosity of PLC+SCM and OPC+LS+SCM mortars is also 2–6 % higher than the porosity of commercial OPC+ SCM mortars. The mixtures containing SCMs with alumina showed less of an increase in porosity because the limestone reacted with alumina to form carbo-aluminate reaction products. Despite the increase in porosity, there is no statistically significant difference between the formation factor of the PLC, OPC+LS, and OPC mortars without SCM. The PLC+SCM, OPC+LS+SCM, and OPC+SCM mortars had a higher formation factor than the plain OPC/PLC/OPC+LS mortars because of pore refinement. Pore refinement is also observed in PLC and OPC+LS mortars containing SCMs with alumina. The results of this study indicate that PLCs (ASTM C595/C595M-20, Standard Specification for Blended Hydraulic Cements) can be used as a direct replacement for OPCs (ASTM C150/C150M-20, Standard Specification for Portland Cement) without any significant reduction in performance as related to transport.
ASTM C1556-11a [ASTM (2016). Standard Test Method for Determining the Apparent Chloride Diffusion Coefficient of Cementitious Mixtures by Bulk Diffusion] provides a systematic procedure to determine the apparent diffusion coefficient of concrete, which is generally used to assess concrete's ability to resist chloride ingress and to make service life predictions. Although ASTM C1556-11a and other similar tests are widely used, they are time-consuming and expensive to perform. The formation factor can be used as an alternative to rapidly assess the transport properties of concrete; however, it does not consider chloride binding. In this paper, a theoretical relationship is developed to relate the formation factor, chloride binding parameters, and apparent chloride diffusion coefficient. A simplified equation based on this theoretical approach is proposed as an alternative to estimate the apparent chloride diffusion coefficient. The chloride profiles obtained using the apparent diffusion coefficients and surface chloride concentrations from the theoretical approach and ASTM C1556-11a were compared. Combining the formation factor with chloride binding properties provides an alternative to ASTM C1556-11a for determination of the apparent chloride diffusion coefficient and surface chloride concentration.
Abstract The passivity of iron in alkaline media enables the use of carbon steel as reinforcement in concrete, which makes up the majority of modern infrastructure. However, chlorides, mainly from deicing chemicals or marine salts, can break down the iron passive film and cause active corrosion. Despite recent advances in nanoscale characterization of iron passivity, significant gaps exist in our understanding of the dynamic processes that lead to the chloride-induced breakdown of passive films. In this study, chloride-induced depassivation of iron in pH 13.5 NaOH solution is studied using reactive force field molecular dynamics. The depassivation process initiates by local acidification of the electrolyte near the film surface, followed by iron dissolution into the electrolyte, and iron vacancy formation in the passive film. Chlorides do not penetrate the passive film, but mainly act as a catalyst for the formation of iron vacancies, which diffuse toward the metal/oxide interface, suggesting a depassivation mechanism consistent with the point-defect model.
