Jinyang Jiang

Effect of functional chemical admixtures on the performance of cement asphalt mortar used in ballastless track

Chemical admixtures are of paramount importance to the performance of modern cement based composites. In this paper, we performed a series of tests to investigate the effects of chemical admixtures on the cement asphalt mortar (CA mortar), i e, compressive strength, frost resistance, permeability, fatigue resistance, pore structure and microstructure. In particular, two types of chemical admixtures were tested, i e, defoamer (tributyl phosphate (TBP)) and polycarboxylate superplasticizer (PS). The results indicate that the addition of TBP and PS eliminates big bubbles and promotes small non-connected pores forming in matrix. Besides, an optimum dosage of TBP and PS may be determined with respect to the frost resistance, permeability and fatigue resistance of CA mortar. Further elaborative discussions are presented as well as experimental evidences from mercury intrusion porosimetry, scanning electron microscopy and energy dispersive spectroscopy.

Fractal Modeling of Pore Structure and Ionic Diffusivity for Cement Paste

Pore structure in cement based composites is of paramount importance to ionic diffusivity. In this paper, pore structure in cement paste is modeled by means of the recently proposed solid mass fractal model. Moreover, an enhanced Maxwell homogenization method that incorporates the solid mass fractal model is proposed to determine the associated ionic diffusivity. Experiments are performed to validate the modeling, that is, mercury intrusion porosimetry and rapid chloride migration. Results indicate that modeling agrees well with those obtained from experiments.

Chloride ion transport in fly ash mortar under action of fatigue loading

In order to study the chloride ion transport performance in fly ash addition mortar, a new method, in which the fatigue loading and chloride diffusion are undertaken simultaneously, was developed. This method realizes coupling the fatigue damage process and the process of chloride transporting of fly ash mortar. The transport performance of chloride in fly ash mortar specimens was studied under different stress levels. Moreover, the effect of fly ash content on transport performance of chloride ion in mortar was investigated. AE (Acoustic Emission) and SEM were used to acquire the damage distribution of mortar specimens under action of fatigue load. The results show that the diffusion coefficient of chloride in mortar specimens increases with stress level of fatigue loading. The addition of fly ash can mitigate the penetration of chloride ion. The results of microcrack 3D location acquired by AE, accompanied with crack characterizing from SEM, indicate that the damage degree of mortar specimen increases with stress level of fatigue loading. Furthermore, higher damage degree of mortar leads to more the chloride ion content in the sample.

Modeling of Ionic Diffusivity for Cement Paste with Solid Mass Fractal Model and Lattice Boltzmann Method

AbstractIonic diffusivity is of critical importance to the service life of cementitious composites. In this paper, ionic diffusivity of cement paste is modeled with the solid mass fractal model and...

Coupling Mechanism of Saturated Concrete Subjected to Simultaneous Fatigue Loading and Freeze-thaw Cycles

The coupling mechanism of saturated concrete subjected to simultaneous 4-point fatigue loading and freeze-thaw cycles was, for the first time, experimentally studied by strain technology. The coupling strain, temperature strain and fatigue strain of concrete specimens were measured at the same time from one sample with stain analysis method and the relationship among these three kinds of strains was studied by fitting data to present coupling mechanism at macro level. The results showed that there was no interaction between fatigue strain and temperature strain and the coupling strain could be written by linear superposition of temperature strain and fatigue strain.

Effect of chromium micro-alloying on the corrosion behavior of a low-carbon steel rebar in simulated concrete pore solutions

A new low-cost corrosion-resistant rebar (HRB400R) was designed and fabricated by chromium micro-alloying. The effects of Cr on the passivation and corrosion behavior of this rebar in the simulated concrete pore solutions were studied systematically, and its improved corrosion resistance was revealed. In the Cl--free saturated Ca(OH)2 solution, the HRB400R rebar presented nearly the same passive film and similar passivation ability compared to the common carbon steel rebar. In the long-term immersion corrosion test in the Cl--contained Ca(OH)2 solution, the HRB400R rebar presented improved corrosion resistance and obvious longer passivation-maintaining period. Micro-alloying of Cr element in the rebar matrix enhanced its corrosion resistance against Cl--attack and retarded the corrosion initiation in the matrix. In the alkaline NaCl salt spraying test, the HRB400R rebar also presented obviously lower mass-loss rate. The enrichment of Cr element in the rust layer improved its retardant effect to the penetration of aggressive medium, and decreased the corrosion propagation rate of the rebar.

Simulation of the cracking and ablation behavior of ferro-siliceous and siliceous nuclear sacrificial concretes

We investigated the simulation of the cracking and ablation behavior of ferro-siliceous and siliceous nuclear sacrificial concretes. To this end, four type of sacrificial concretes were fabricated, i e, the ferro-siliceous (F) and siliceous (S) plain concretes, and the polypropylene fiber reinforced concretes of the above two (FF, SF). The cracking and ablation behaviors of the sacrificial concretes were investigated by simulation tests, and the simulated elevated temperature was obtained by means of thermite powder. The number and the width of the cracks were compared and the pore size distribution of sacrificial concretes was measured. In addition, the interface and chemical composition of melt at different positions were analyzed, and the ablation depth of the sacrificial concrete crucibles was also measured. It was found that the siliceous concrete shows to be more prone to cracking than the ferro-siliceous concrete due to the higher content of fly ash and lower water to binder ratio; though the ablation depth of siliceous concrete is found to be slightly larger, no clear difference can be detected for the basemat ablation rate.

Multi-scale modeling of the ionic diffusivity of cement-based materials

A new multiscale numerical approach was presented to predict the ionic diffusivity of cement based materials, which incorporated the lattice Boltzmann method, the conjugate gradient method, and the random walk method. In particular, the lattice Boltzmann method was applied to model the ionic diffusion in pore space of cement paste, while the upscaling of effective ionic diffusivity from cement paste (mortar) to concrete was processed by means of the conjugate gradient method and the random walk method. A case study was then presented, i e, the chloride diffusivity of concrete affected by sand content and gravel content. It is shown that the results of numerical prediction agree well with those of experimental measurements adopted from literatures. The multiscale numerical approach provides a prior assessment of ionic diffusivity for cement based materials from a microstructural basis.

Influence of Concrete Properties on Molten Core-Concrete Interaction: A Simulation Study

In a severe nuclear power plant accident, the molten core can be released into the reactor pit and interact with sacrificial concrete. In this paper, a simulation study is presented that aims to address the influence of sacrificial concrete properties on molten core-concrete interaction (MCCI). In particular, based on the MELCOR Code, the ferrosiliceous concrete used in European Pressurized Water Reactor (EPR) is taken into account with respect to the different ablation enthalpy and Fe2O3 and H2O contents. Results indicate that the concrete ablation rate as well as the hydrogen generation rate depends much on the concrete ablation enthalpy and Fe2O3 and H2O contents. In practice, the ablation enthalpy of sacrificial concrete is the higher the better, while the Fe2O3 and H2O content of sacrificial concrete is the lower the better.

Quantitative calculation of hydration products for binary slag-Portland cement system

The reaction models and the quantitative calculation on the volume fraction of hydration products for binary ground granulated blast-furnace slag (GGBFS) cement system are presented, in which two important factors are taken into account, i e, the reactivity of GGBFS influenced by its chemical compositions and the partial replacement of aluminum phase in calcium silicate hydrate (C-S-H) gel. A simplified treatment is further suggested towards the quantification. In particular, when the replacement level of GGBFS is lower than 70%, the ratio of calcium over silica (C/S) is set at 1.5 or at 1.2 otherwise. The validity of the proposed model is addressed in terms of the contents of calcium Portlandite and non-evaporable water.