Xingguo Feng

An Elastoplastic Damage Constitutive Model for Cementitious Materials under Wet-Dry Cyclic Sulfate Attack

The mechanical properties of cement mortars subjected to wet-dry cyclic sulfate attack were studied by the compression strength test. The results showed that the ultimate compressive strength increased with number of cycles at the initial stage. However, after a certain time, it started to decrease with further increases in the number of cycles. Moreover, the concentration of the sodium sulfate solution proved to be an important factor affecting the ultimate compressive strength. Based on continuum damage mechanics theory, an elastoplastic damage constitutive model is presented to describe the mechanical behavior of cementitious materials under compressive stress. The results obtained agree well with the experimentally observed elastic, plastic, and damage characteristics of cement mortars under compressive stress.

The mechanical properties of coastal soil treated with cement

The influences of cement type, cement content, and curing time on the unconfined compression strength (UCS) of soil-cement were investigated. The influence of groundwater on UCS of soil-cement was also studied. The experimental results indicate that the soil treated with high grade cement presents a higher UCS. Additionally, the UCS of soil-cement presents linearly increased with the cement content. A logarithm correlation between UCS and curing time presents to forecast the strength development. Compared with the UCS of samples immersed in distilled water, those immersed in groundwater present a higher value.

The influence of plastic deformation on the structure of passive films on carbon steel in simulated pore solution

The process of passivation of carbon steel when experiencing plastic deformation in simulated pore solution has been studied using electrochemical tests and atomic force microscopy (AFM). The polarization results show that the activity of the carbon steel increased with increasing degree of deformation. Before the passive films were ruptured, the heavily deformed samples presented a high open circuit potential (OCP). On the other hand, the pitting incubation time decreased with increasing plastic deformation. The Mott-Schottky results suggested that the high deformation caused the passive films to be heavily doped. In addition, the space charge layers of passive films were thinned when the plastic deformation increased. The AFM observations indicated that the passive films become more inhomogeneous as the deformation increased. These results demonstrate that passive films on the deformed carbon steel become unstable when the plastic deformation increases.

The influence of Ce(NO3)3·6H2O on the inhibitive effect of Ca(NO2)2 in simulated concrete pore solution

The effects of cerium nitrite on corrosion behaviors of carbon steel in simulated concrete pore solutions were studied with the methods of linear polarization, electrochemical impedance spectroscopy and surface analysis. In pore solutions in the presence of Ce(NO3)3·6H2O, the corrosion potential, polarization resistance and impedance of carbon steel obviously increased in contrast to the situation in the absence of cerium salts. The pore solution with [NO2−] / [Cl−] = 0.3 and 0.1% Ce(NO3)3·6H2O, carbon steel shows better corrosion resistance than that in the pore solution with [NO2−] / [Cl−] = 0.6, which indicates that a small amount of Ce(NO3)3·6H2O in pore solutions can effectively promote passivation of the steel and reduce the threshold [NO2−] / [Cl−] ratio for corrosion control. The surface layer formed in cerium salt containing pore solutions is more compact and smooth and 1.36%Ce is examined on the sample surface. The addition of 0.1% Ce(NO3)3· 6H2O in pore solutions can decrease the corrosion rate of steel in pore solutions and has little influence on pH change of the solutions. However, more cerium nitrate addition above 0.1% may result in pH decrease of the solution.

Galvanic Corrosion Between AISI304 Stainless Steel and Carbon Steel in Chloride Contaminated Mortars

Galvanic corrosion between AISI304 stainless steel and carbon steel in mortars has been studied by open circuit potential, electrochemical impedance spectroscopy, and galvanic coupling currents. The corrosion state of the coupling rebar was also examined after removing the mortars. The results suggest that the carbon steel presents a high risk of galvanic corrosion in the initial stage when it is electrically coupled with stainless steel in the chloride-contaminated mortars. However, for the stainless steel promotes the passivation of carbon steel, the risk of galvanic corrosion on carbon steel significantly decreased as the time extends. In addition, the water-cement ratio of mortars does not have a dramatic influence on the galvanic corrosion between stainless steel and carbon steel in the chloride-contaminated environment.

Effect of Strain on Passivation of Stainless Steel in a Simulated Concrete Pore Solution

Passivation behavior of a deformed stainless steel in a simulated concrete pore solution was studied by means of open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and Mott-Schottky plots. The results show that with the increasing strain, the OCP and im- pedance of the steel decreased, whereas the concentration of oxygen vacancy in passive films in- creased. This result suggests that the passivation of the stainless steel is degraded by the increasing strain. In addition, the difference in passivation between the deformed samples would not be dimin- ished with the increasing immersion time.

Tensile Strength and Oxide Analysis of Carbon Steel in Concrete Exposed in Atmospheric Environment for 53 Years

The tensile strength of a corroded rebar in a 53-year-old concrete structure was studied. The microstructure of the metallic substrate, the fracture surface, and the corrosion product layers were investigated. Metallographic observation results showed that the carbon steel was constituted of ferrite and some pearlite. The tensile test results indicated that the corroded rebar presented low strength and elongation. In addition, the fracture surface of the rebar in the tensile test displayed dimple fracture behavior. The Raman spectroscopy results indicated that corrosion products at the general corrosion zone were obviously different from those at the localized corrosion zone. The rust layer at the general corrosion zone was composed of goethite (α-FeOOH), magnetite (Fe3O4), and hematite (α-Fe2O3), while that of the pitting zone was made of feroxyhyte (δ-FeOOH), goethite (α-FeOOH), and hematite (α-Fe2O3). However, the general tendencies that the corrosion products were constituted of a mix of oxides and hydroxides, the oxides mainly existed in the internal part and the hydroxides more presented in the external layer were observed.