Lattice modeling of rapid chloride migration in concrete

Test methods which use external voltage are commonly used to assess resistance of concrete to chloride ion penetration. In order to facilitate fast chloride ingress, electrical voltage (typically 10–60 V) is applied across the concrete specimen. These methods have also been used on microcracked and cracked specimens in order to study the influence of cracking on chloride ingress. Chloride migration transport mechanism is fundamentally different from the diffusion process usually occurring in practice. To study the behavior during the test, a model is proposed, based on the transport lattice modeling framework. First, the accuracy and computational aspects of the proposed model are discussed. Then, the model is applied to study the transport in heterogeneous concrete (i.e. on the meso-scale). Also, chloride migration in microcracked, notched, and cracked concrete is simulated. The findings show that the proposed model can successfully reproduce experimentally observed behavior.     

Prediction of the hydraulic diffusivity from pore size distribution of concrete

A model is presented in this work through which variation of hydraulic diffusivity of concrete with relative water content can be obtained from pore size distribution as an input. The specific water capacity and hydraulic conductivity of concrete are expressed in terms of pore size characteristics, considering laminar flow due to capillary suction through tortuous elliptic tubes, oriented equally in three orthogonal directions. Hydraulic diffusivity being the ratio of hydraulic conductivity and specific water capacity is thus expressed in terms of pore size characteristics. The input pore size distributions have been determined experimentally for normal strength concrete mixes through mercury intrusion porosimetry. Using the model the variation of hydraulic diffusivity with relative water content is determined for three cases viz. 1) ideal continuous wetting, 2) ideal continuous drying and 3) random access of pores by water. These results are then compared with an experimentally obtained variation.     

Prediction of convective transport within unsaturated concrete utilizing pore size distribution data

The durability of concrete structures is often compromised by physical and chemical interaction with the external environment that leads to ongoing maintenance and, in the worst cases, can lead to reduced structural integrity and consequent asset replacement. Concrete is a porous material and most field-exposed concrete is partially saturated with water. Where the concrete is unsaturated and there is no external water pressure acting on a concrete surface, the primary mechanisms of transport into concrete are convective-diffusion ingress (i.e. uptake of water and water-borne agents due to capillary attraction). This paper assesses capillarity and outlines a predictive model of the uptake of water by concrete based on analysis of the pore size distribution. It is acknowledged that concrete has a multitude of internal pores with a broad range of lengths and cross-sectional shapes, surface roughness, tortuosities, random meeting and divergence with adjacent pores, microcracks and fractures, and variable pore-water chemical composition, however the prediction model shows reasonable agreement with water sorptivity test data.     

内掺硅烷乳液憎水剂对混凝土性能的影响

通过强度测试、水化热实验、热重分析、毛细吸水实验以及压汞实验研究了内掺硅烷乳液憎水剂对混凝土强度以及毛细吸水性能的影响规律及其机理。结果表明:高掺量硅烷乳液一定程度上抑制水泥水化,从而导致混凝土早龄期抗压强度和劈裂抗拉强度下降以及28d养护后水泥净浆试块中毛细孔的体积增大。毛细吸水实验表明:28d龄期混凝土毛细吸水过程的初始吸水速率、二次吸水速率以及总吸水量因硅烷乳液的引入而显著降低;硅烷乳液通过与水泥水化产物反应在毛细孔孔壁形成一层烷基疏水层,使混凝土毛细孔壁性质由亲水性转变为憎水性,进而显著降低混凝土的吸水速率和总吸水率;高水灰比混凝土的毛细孔体积较大,因此硅烷乳液对其影响更为明显。     

Probabilistic evaluation of initiation time in RC bridge beams with load-induced cracks exposed to de-icing salts

In this study, a reliability-based method for predicting the initiation time of reinforced concrete bridge beams with load-induced cracks exposed to de-icing salts is presented. A practical model for predicting the diffusion coefficient of chloride ingress into load-induced cracked concrete is proposed. Probabilistic information about uncertainties related to the surface chloride content and the threshold chloride concentration has been estimated from a wide review of previous experimental or statistical studies. Probabilistic analysis to estimate the time to corrosion initiation with/without considering the effect of the load-induced cracks on the chloride ingress into concrete has been carried out. Results of the analysis demonstrate the importance of considering the effect of the load-induced cracks for correct prediction of corrosion initiation in RC bridge beams exposed to chlorides.     

Reliability analysis for the existing reinforced concrete pile corrosion of bridge substructure

This study investigated the failure probability for existing reinforced concrete (RC) pile corrosion due to carbonation, chloride ion ingress and sulfate attack combined with the load-carrying capacity for bridge substructures at different amounts of service times. The general corrosion resulted from carbonation, the pitting corrosion occurred from chloride ion ingress, and the chemical mechanism due to sulfate attack to the existing RC piles of a bridge substructure is described. Both the theory of structural joint failure probability and the measurement techniques of carbonation depth, chloride ion ingress depth, and sulfate expansion are also provided. An existing RC bridge pile with a service time of 60 years is offered as an example for illustrating the structural joint failure probability theory. The structural joint failure probability of existing RC bridge piles due to the combination of carbonation, chloride ion ingress, sulfate attack, and load-carrying capacity at a service time of 60 years had lower and upper bound values of .440382 and .502755, respectively. These results are provided as a critical decision-making for the repair, strengthening, or demolition of existing RC bridge substructure pile foundations.     

PEMFC带沟槽气体扩散层内传输特性孔隙网络模拟

采用三维孔隙网络模型计算了不同沟槽参数下气体扩散层(GDL)的液态水突破压力、毛细压力分布、气体扩散率和液相相对渗透率随饱和度变化,并从孔隙尺度角度探究了沟槽的作用机制。研究结果表明：沟槽改变了GDL的毛细压力分布,提供了液态水直接传输路径并优化了GDL内氧气和液态水的分布,从而提高了氧气有效扩散率。沟槽位置对氧气传输有明显影响,对液相传输的影响取决于是否形成贯穿GDL的传输路径;沟槽加深,氧气和液态水传输性能增强,沟槽穿透GDL时传输性能达到最佳;沟槽变宽,液相传输性能增强,氧气传输性能在低饱和度范围内先增强后减弱。综合各因素,给出了氧气和液态水传输性能最优时的沟槽参数。     

Observation of the redistribution of nanoscale water filled porosity in cement based materials during wetting

Spatially resolved GARField NMR has been used to follow the ingress of water into previously dried Portland cement concrete and mortar samples. It is shown that the amount of capillary water in the surface layers of different samples after 1 day of capillary absorption exceeds the amount found before drying but that over the subsequent 7 days the amount of capillary water decreases once more, even though the external source of water is maintained. The hydrate gel pore water was additionally tracked in the mortars. It shows complementary behaviour. The data is discussed in terms of C–S–H swelling.     

Porosity of recycled concrete with substitution of recycled concrete aggregate: An experimental study

In this paper, we present the experimental analysis of samples of recycled concrete (RC) with replacement of natural aggregate (NA) by recycled aggregate originating from concrete (RCA). The results of the tests of mechanical properties of RC were used for comparison with tests of mercury intrusion porosimetry (MIP), in which the distribution of the theoretical pore radius, critical pore ratio, the surface area of the concrete, threshold ratio and average pore radius were studied at ages of 7, 28 and 90 days. The results showed some variation in the properties of the RC with respect to ordinary concrete. Porosity increases considerably when NA is replaced by RCA. Additionally, a reduction in the mechanical properties of the RC is seen compared with ordinary concrete when porosity increases.     

Capillary suction and diffusion model for chloride ingress into concrete

A numerical approach, named TransChlor, is proposed to simulate transport phenomena of various substances in concrete. This approach is a theoretical model based on finite elements and finite differences methods. The model consists of coupled nonlinear partial differential equations based on Fick's diffusion law and on kinematics equations. Simulation results from a parametrical study highlight the influence of microclimatic conditions, exposure to deicing salts and concrete cover permeability and thickness on chloride ingress in concrete. The results show that the chloride ion concentration increases quickly in concrete cover when a structure is exposed to deicing salts at a mountainous location; whereas permeability of concrete cover is an insignificant parameter when the concrete is in direct or splash water contact.     

Modeling of chloride diffusivity coupled with non-linear binding capacity in sound and cracked concrete

The objective of this research is to establish a model that can predict chloride transport phenomena in sound and cracked concrete. The chloride diffusivity is formulated based on computed micro-pore structure, which considers tortuosity and constrictivity of porous network as reduction factors in terms of complex micro-pore structure and electric interaction of chloride ions and pore wall. In the real environment, concrete structures are not always crack-free, therefore chloride transport in cracked concrete is also simulated by section large void spaces in a control volume to represent the crack and by proposing a model of chloride diffusivity through the cracked region The proposed models are implemented into a finite-element computational program DuCOM, which simulates the early-age development process of cementitious materials. The calculated concentration profiles of total chloride ions are verified through a comparison with experiments results.     

Predicting carbonation in early-aged cracked concrete

Carbonation in cracked concrete is considered as one of major deteriorations accelerating steel corrosion in reinforced concrete structures. For durable concrete structures, it is necessary to control crack in concrete through crack resistance evaluation for early-aged concrete structures, but often unavoidable cracks in early-aged concrete may occur. These cracks become a main path for CO₂ penetration inside concrete so that the carbonation is accelerated in cracked concrete.In this study, an analytical technique for carbonation prediction in early-aged cracked concrete was developed for considering both CO₂ diffusion of pore water in sound concrete and in cracked concrete. Then, characteristics of diffusivity on the carbonation in early-aged concrete are studied through finite element analysis implemented with the so-called multi-component hydration heat model and micro-pore structure formation model. The carbonation behaviour in sound concrete and cracked concrete are also simulated by using the derived diffusivity with consideration of reaction with dissolved CO₂. Finally, numerical results obtained for cracked concrete made with 3 different W / C ratios (45%, 55%, and 65%) with different crack widths were compared with experimental results.     

Evaluation of silicon-based strengthening agents on porous limestones

Deteriorated porous bioclastic limestones from the archaeological area of Aptera, Crete, Greece, were treated with silicon-based strengthening products, tetraethoxysilanes, elastified silicic acid ethyl esters, an aqueous colloidal dispersion of silica particles, as well as an oligomeric siloxane water repellent. Changes in color, water vapor permeability, water absorption by capillary suction, porosity and pore size distribution were evaluated after treatment. Energy dispersive X-ray fluorescence and Fourier transform infrared spectroscopy indicate the penetration depth of the applied products. The effectiveness of strengthening was also evaluated by measuring the tensile strength. The pore size distribution was insignificantly modified decreasing only the ratio of coarse pores. The applied products penetrated up to a depth of 30 mm. All the studied products create linkages across the interface between calcite and the alkoxysilanes-derived gels, due to the hydroxyl groups of the quartz included in the limestones. Better reinforcing results were obtained by applying the elastified silicic acid ethyl ester. The elastified silicic acid ethyl ester confers chromatic stability on the stone, shows insignificant modification of the water vapor permeability and reduces the absorbed quantity of water.     

Electrochemical impedance spectroscopy for studying passive layers on steel rebars immersed in alkaline solutions simulating concrete pores

Present paper deals with the use of the electrochemical impedance spectroscopy to identify different processes in the passive layer growth over steel rebar surface immersed in an alkaline media simulating the concrete pore solution. Two cases have been considered: a passive layer spontaneously grown in a high alkaline media and a passive layer assisted by the application of an anodic potential in the same media. The application of electric equivalent circuits allows distinguishing between the different mechanisms occurring in this passive layer when grows in different conditions. An electric equivalent circuit with two RC loops connected in parallel is often used for fitting the EIS diagrams obtained for spontaneous growth of passive layers in the alkaline solution simulating the concrete pores. However, when the passive layer is formed under anodic polarization, a Warburg element must be introduced in the equivalent circuit. According to the Point Defect Model (PDM), this Warburg element is allocated to the transport of oxygen vacancies through the passive layer, which concentration changes with the potential.     

Investigation of concrete exposed to dual sulfate attack

Durability of concrete exposed to sulfates has primarily been studied on specimens fully-submerged in sulfate solutions. However, field experience shows that concrete exposed to sulfates can suffer from surface scaling above ground level due to physical attack. This damage has often been ignored and even confused with chemical sulfate attack. In this study, concrete partially-immersed in sulfate solutions and exposed to cyclic temperature and relative humidity was explored. Results show that concrete can experience dual sulfate attack. The lower immersed portion can suffer from chemical sulfate attack, while the upper portion can be vulnerable to physical attack. Lowering the water-to-binder ratio and moist-curing reduced surface scaling above the solution level, since the volume of pores was decreased. Although partial replacement of cement with pozzolans also decreased the pore volume, surface scaling increased due to increased proportion of small diameter pores and associated growth of capillary suction and surface area for evaporation.     

Porengrössenverteilung von Mörteln nach lagerung im wasser und in einer chloridlösung

For the purpose of the investigation of relations between the chloride diffusion into concrete and the pore structure of the hardened cement paste, the porosity of cement mortars were examined. The mortar specimens of four cements with a graded blast-furnace slag content (0 to 79 % by mass) prepared at w/c ratios 0,50 and 0,70 were stored in a concentrated NaCl solution and water respectively up to 2 years. The pore size distributions of the mortars were determined by means of mercury intrusion porosimetry and water absorption. The samples stored in water showed a relationship between the percentage of small (gel) as well as great (capillary) pores and the slag content of the cement and the w/c ratio of the mortar. After 6 and 12 months, the storing in the chloride solution effected an alteration of the pore structure reducing the total and gel pore volume and increasing the capillar pore volume. Later, the gel pore fraction was near to that of the specimens stored in water.     

The influence of longitudinal cracks on the corrosion protection afforded reinforcing steel in high performance concrete

It is almost impossible to produce crack-free concrete and, therefore, codes of concrete structural design (such as ACI 318 [ACI 318, Building Code Requirements for Structural Concrete, American Concrete Institute: Farmington Hills, MI, USA]) take cracking into account and relate permissible crack widths to exposure conditions. Chloride ingress is significantly enhanced by cracks because the ions can penetrate the concrete cover from the walls of the crack as well as from the outer surface of the concrete [P. P. Win, M. Watanabe, and A. Machida, Penetration profile of chloride ion in cracked reinforced concrete. Cement and Concrete Research, 2004. 34(7): p. 1073–1079]. Thus, while the chlorides reach the steel very rapidly directly through the crack, they also reach adjacent areas of steel more rapidly than in uncracked concrete.The objective of the project was to ascertain whether high performance concrete (HPC) could provide superior protection to Ordinary Portland Cement Concrete (OPCC) even when the structure was cracked parallel to the reinforcing bars (longitudinal cracks) while being exposed to a de-icing salt environment. Two HPCs, as specified by the Ministry of Transportation of Ontario (MTO) [MTO, SSP 904 S13 High Performance Concrete, Amendment to OPSS 904 Construction Specification for Concrete Structures. 1995, Ontario Provincial Standard Specification] were tested: both used blended silica fume cement (Canadian Type 10 E-SF) and had 25% replacement of the cement by either fly ash or ground granulated blast furnace slag. A Class C-2 ordinary Portland cement concrete was used as control.In the case of cracks parallel to the rebar, HPC does not appear to have any beneficial influence on the corrosion of bars, which is not surprising in view of the fact that the whole length of the bar is directly exposed to the environment via the crack. In contrast, HPC has been found to provide better protection for steel exposed to transverse cracks than does OPCC. However, the benefits of HPC are not as great as they are for sound (uncracked) concrete. The reasons for the better protection are (i) the greater resistance of HPC to chloride penetration from the walls of the crack, (ii) its greater tendency to crack healing and (ii) the different crack path in HPC.     

Microstructural analysis of plain and reinforced mortars under chloride-induced deterioration

This paper reports the results of microstructural analysis of plain and steel-reinforced mortar specimens deteriorated by chlorides that were admixed or introduced through chloride ingress. The electrical properties of mortars were measured and their microstructural characteristics were investigated using quantitative image analysis techniques. The influence of chloride ions on mortar microstructure are discussed in terms of hydration and corrosion products. The research reveals that chlorides will induce changes in the chemical compositions and morphology of cement hydration products, and thereby exert influence on ion transport in the mortar specimens. The electrical properties of plain and reinforced mortars are not only related to the presence of chlorides in the pore system but are also influenced by the pore structure characteristics. The cementitious matrix undergoes certain alterations in conditions of the combined effects of: cement hydration, chloride ion transport and chemical binding mechanisms. To this end the pore structure characteristics appeared to be a significantly contributing factor in the process of chloride-induced corrosion in reinforced cement-based materials.