Numerical investigation on tortuosity of transport paths in cement-based materials

Xiao-Bao Zuo,Wei Sun,Zhi-Yong Liu,Yu-Juan Tang 사단법인 한국계산역학회 2014 Computers and Concrete, An International Journal Vol.13 No.3

Based on the compositions and structures of cement-based materials, the geometrical models of the tortuosity of transport paths in hardened cement pastes, mortar and concrete, which are associated withthe capillary porosity, cement hydration degree, mixture particle shape, aggregate volume fraction andwater-cement ratio, are established by using a geometric approach. Numerical simulations are carried out to investigate the effects of material parameters such as water-cement ratio, volume fraction of the mixtures, shape and size of aggregates and cement hydration degree, on the tortuosity of transport paths in hardened cement pastes, mortar and concrete. Results indicate that the transport tortuosity in cement-based materials decreases with the increasing of water-cement ratio, and increases with the cement hydration degree, the volume fraction of cement and aggregate, the shape factor and diameter of aggregates, and the material parameters related to cement pastes, such as the water-cement ratio, cement hydration degree and cement volume fraction, are the primary factors that influence the transport tortuosity of cement-based materials.

Modeling of diffusion-reaction behavior of sulfate ion in concrete under sulfate environments

Xiao-Bao Zuo,Wei Sun,Hua Li,Yu-Kui Zhao 사단법인 한국계산역학회 2012 Computers and Concrete, An International Journal Vol.10 No.1

This paper estimates theoretically the diffusion-reaction behaviour of sulfate ion in concrete caused by environmental sulfate attack. Based on Fick’s second law and chemical reaction kinetics, a nonlinear and nonsteady diffusion-reaction equation of sulfate ion in concrete, in which the variable diffusion coefficient and the chemical reactions depleting sulfate ion concentration in concrete are considered, is proposed. The finite difference method is utilized to solve the diffusion-reaction equation of sulfate ion in concrete, and then it is used to simulate the diffusion-reaction process and the concentration distribution of sulfate ion in concrete. Afterwards, the experiments for measuring the sulfate ion concentration in concrete are carried out by using EDTA method to verify the proposal model, and results show that the proposed model is basically in agreement with the experimental results. Finally, Numerical example has been completed to investigate the diffusion-reaction behavior of sulfate ion in the concrete plate specimen immersed into sulfate solution.

Modeling of ion diffusion coefficient in saturated concrete

Xiao-Bao Zuo,Wei Sun,Cheng Yu,Xu-Rong Wan 사단법인 한국계산역학회 2010 Computers and Concrete, An International Journal Vol.7 No.5

This paper utilizes the modified Davis model and the mode coupling theory, as parts of the electrolyte solution theory, to investigate the diffusivity of the ion in concrete. Firstly, a computational model of the ion diffusion coefficient, which is associated with ion species, pore solution concentration, concrete mix parameters including water-cement ratio and cement volume fraction, and microstructure parameters such as the porosity and tortuosity, is proposed in the saturated concrete. Secondly, the experiments, on which the chloride diffusion coefficient is measured by the rapid chloride penetration test,have been carried out to investigate the validity of the proposed model. The results indicate that the chloride diffusion coefficient obtained by the proposed model is in agreement with the experimental result. Finally, numerical simulation has been completed to investigate the effects of the porosity, tortuosity, water-cement ratio, cement volume fraction and ion concentration in the pore solution on the ion diffusion coefficients. The results show that the ion diffusion coefficient in concrete increases with the porosity, water-cement ratio and cement volume fraction, while we see a decrease with the increasing of tortuosity. Meanwhile, the ion concentration produces more obvious effects on the diffusivity itself, but has almost no effects on the other ions.

Syntheses, Structures, and Luminescence Properties of Two Copper(II) Complexes Constructed by Rigid Bis(triazole) and Nitrogen-containing Carboxylic Acid Ligands

Xiao-Xiao,Zuo-Xi Li,Bao-yi Yu,Guang-hua Cui 대한화학회 2015 Bulletin of the Korean Chemical Society Vol.36 No.7

Two new copper(II) complexes with the formulas [Cu(btb)(pydc)(H2O)]n (1) and [Cu(btb)0.5(nph)(H2O)]n (2) have been synthesized under hydrothermal conditions by employing a rigid bis(triazole) ligand (btb = 4,4′-bis(1,2,4-triazolyl-1-yl)-biphenyl) and two carboxylic acids mixed ligands (H2pydc = pyridine-2,5-dicarboxylic acid and H2nph = 3-nitrophthalic acid). The copper atoms present different environments, with a tetragonal pyramidal geometry in 1 and octahedral configuration in 2. Complex 1 displays a dinuclear cluster, which is further packed into a 2D supramolecular layer by classical OH · · · O hydrogen bonds. Complex 2 possesses a trinodal 3,4,4-connected 3D framework with a rare sqc69 topology. In addition, thermal stability and luminescence property were investigated.

Numerical investigation on gypsum and ettringite formation in cement pastes subjected to sulfate attack

Xiao-Bao Zuo,Jia-Lin Wang,Wei Sun,Hua Li,Guang-Ji Yin 사단법인 한국계산역학회 2017 Computers and Concrete, An International Journal Vol.19 No.1

This paper uses modelling and experiment to perform a quantitative analysis for the gypsum and ettringite formations in cement pastes subjected to sulfate attack. Firstly, based on Fick’s law and chemical reaction kinetics, a diffusion model of sulfate ions in cement pastes is proposed, and then the model of the gypsum and ettringite formations is established to analyze its contents in cement pastes with corrosion time. Secondly, the corrosion experiment of the specimens with cement pastes immersed into 2.5%, 5.0% and 10.0% Na2SO4 solutions are carried out, and by using XRD-Rietveld method, the phases of powder samples from the specimens are quantitatively analyzed to obtain the contents of gypsum and ettringite in different surface depth, solution concentration and corrosion time. Finally, the contents of gypsum and ettringite calculated by the models are compared with the results from the XRD experiments, and then the effects of surface depth, corrosion time and solution concentration on the gypsum and ettringite formations in cement pastes are discussed.

Numerical analysis of concrete degradation due to chloride-induced steel corrosion

Ayinde, Olawale O.,Zuo, Xiao-Bao,Yin, Guang-Ji Techno-Press 2019 Advances in concrete construction Vol.7 No.4

Concrete structures in marine environment are susceptible to chloride attack, where chloride diffusion results in the corrosion of steel bar and further lead to the cracking of concrete cover. This process causes structural deterioration and affects the response of concrete structures to different forms of loading. This paper presents the use of ABAQUS Finite Element Software in simulating the processes involved in concrete's structural degradation from chloride diffusion to steel corrosion and concrete cover cracking. Fick's law was used for the chloride diffusion, while the mass loss from steel corrosion was obtained using Faraday's law. Pressure generated by steel corrosion product at the concrete-steel interface was modeled by applying uniform radial displacements, while concrete smeared cracking alongside the Extended Finite Element Method (XFEM) was used for concrete cover cracking simulation. Results show that, chloride concentration decreases with penetration depth, but increases with exposure time at the concrete-steel interface. Cracks initiate and propagate in the concrete cover as pressure caused by the steel corrosion product increases. Furthermore, the crack width increases with the exposure time on the surface of the concrete.

Investigation on alkalinity of pore solution and microstructure of hardened cement-slag pastes in purified water

Hu, Ya-Ru,Zuo, Xiao-Bao,Li, Xiang-Nan,Jiang, Dong-Qi Techno-Press 2021 Advances in concrete construction Vol.12 No.6

To evaluate the influence of slag on the alkalinity of pore solution and microstructure of concrete, this paper performs a leaching experiment on hardened cement-slag pastes (HCSP) slice specimens with different slag content in purified water. The pH value of pore solution, average porosity, morphology, phase composition and Ca/Si of HCSP specimens in the leaching process are measured by solid-liquid extraction, saturated-dried weighing, scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) and X-ray diffraction (XRD). Results shows that the addition of slag can mitigate an increase in porosity and a decrease in Ca/Si of HCSP in the leaching process. Besides, an appropriate slag content can improve the microstructure so as to obtain the optimum leaching resistance of HCSP, which can guarantee the suitable alkalinity of pore solution to prevent a premature corrosion of reinforced bar. The optimum slag content is 40% in HCSP with a water-binder ratio of 0.45, and an excessive slag causes a significant decrease in the alkalinity of pore solution, resulting in a loss of protection on reinforced bar in HCSP.

Modeling of time-varying stress in concrete under axial loading and sulfate attack

Guang-Ji Yin,Xiao-Bao Zuo,Yu-Juan Tang,Olawale Ayinde,Dong-Nan Ding 사단법인 한국계산역학회 2017 Computers and Concrete, An International Journal Vol.19 No.2

This paper has numerically investigated the changes of loading-induced stress in concrete with the corrosion time in the sulfate-containing environment. Firstly, based on Fick’s law and reaction kinetics, a diffusion-reaction equation of sulfate ion in concrete is proposed, and it is numerically solved to obtain the spatial and temporal distribution of sulfate ion concentration in concrete by the finite difference method. Secondly, by fitting the existed experimental data of concrete in sodium sulfate solutions, the chemical damage of concrete associated with sulfate ion concentration and corrosion time is quantitatively presented. Thirdly, depending on the plastic-damage mechanics, while considering the influence of sulfate attack on concrete properties, a simplified chemo-mechanical damage model, with stress-based plasticity and strain-driven damage, for concrete under axial loading and sulfate attack is determined by introducing the chemical damage degree. Finally, an axially compressed concrete prism immersed into the sodium sulfate solution is regarded as an object to investigate the time-varying stress in concrete subjected to the couplings of axial loading and sulfate attack.

Numerical simulation on the coupled chemo-mechanical damage of underground concrete pipe

Xiang-nan Li,Xiao-bao Zuo,Yu-xiao Zou,Yu-juan Tang 국제구조공학회 2023 Structural Engineering and Mechanics, An Int'l Jou Vol.86 No.6

Long-termly used in water supply, an underground concrete pipe is easily subjected to the coupled action of pressure loading and flowing water, which can cause the chemo-mechanical damage of the pipe, resulting in its premature failure and lifetime reduction. Based on the leaching characteristics and damage mechanism of concrete pipe, this paper proposes a coupled chemo-mechanical damage and failure model of underground concrete pipe for water supply, including a calcium leaching model, mechanical damage equation and a failure criterion. By using the model, a numerical simulation is performed to analyze the failure process of underground concrete pipe, such as the time-varying calcium concentration in concrete, the thickness variation of pipe wall, the evolution of chemo-mechanical damage, the distribution of concrete stress on the pipe and the lifetime of the pipe. Results show that, the failure of the pipe is a coupled chemo-mechanical damage process companied with calcium leaching. During its damage and failure, the concentrations of calcium phase in concrete decrease obviously with the time, and it can cause an increase in the chemo-mechanical damage of the pipe, while the leaching and abrasion induced by flowing water can lead to the boundary movement and wall thickness reduction of the pipe, and it results in the stress redistribution on the pipe section, a premature failure and lifetime reduction of the pipe.