Papers by Author: Nan Guo Jin

Abstract: In this work, the correlation of mechanical and microstructure development of early-age concrete was studied through electrical resistivity measurement. The inner enhancement of hydration products other than evolution of pore structure was identified as a vital factor for microstructural development of early-age concrete, which embodies in the increase of tortuosity a and significantly contributes to the growth of mechanical properties. Finally, a correlation between compressive strength and two crucial microstructural parameters (effective porosity ϕeff and tortuosity a) was built for the three mixes of concrete studied in this paper.

Abstract: Based on the MFPA^2D software system and test method of the wedge splitting fracture in the Lab., a numerical model was established. The numerical experiments on wedge splitting fracture with plain concrete and SFRC (steel fiber reinforced concrete) were carried on. The calculation results about plain concrete with different dimensions and ligament length proved the numerical model is effectively on numerical calculation, and have a good agreement with the results of the test results. Then, the process of crack initiation, propagation of SFRC specimens with different dimensions and fitting formula about size effect were given. The results indicated that the size effect existed in the splitting fracture energy of SFRC.

Abstract: Boric acid (H₃BO₃) is often used for cooling the heat in nuclear power plant. Though the drastic measures have been taken, the potential leakage still can not be fully eliminated. Therefore, an attempt to understand the durability mechanism of RC subjected to boric acid is of vital importance. In this paper, specimens cured in a standard moist room at a temperature of 20 ±3 °C after 28days were immersed in three different concentration of boric solution (2000ppm, 8000ppm, 30000ppm) artificially simulated to accelerate the corrosion of acid environment. The hydration products are investigated. Microstructure of reinforced concrete on the surface, 2cm and 4cm away from the surface at ages of 60, 90 and 150 days are examined using X-ray diffraction (XRD) and scanning electron microscope (SEM) respectively. Results indicate that main hydration product is Ca(BO₂)₂,boric acid corrosion to concrete surface can only be detected from the specimens in 30000ppm solutions at the ages of 150 days and its effect is slight. However, the inner part of the concrete was not affected. The results of XRD coincide with those of SEM.

Abstract: Cement concrete has an increasing utilization in highway construction. However, the lower flexural tensile strength and abrasion-resistance restricted the development of concrete pavement with heavy traffic. Hence, It has highlighted the need for research on high performance concrete (HPC). The design of specific concrete normally depends on the mixing of mineral admixtures in concrete mixture. This paper presents the first experimental study on strength and abrasion-resistance enhancement admixture (SAEA) mixed with slag powder, steel slag powder and silica fume. The optimal proportion of three mineral admixture and reasonable mixed amount of SAEA were then given, and the mechanism of SAEA reacted in concrete was analyzed based on sol-gel theory. Lastly, the effect of concrete mixed with SAEA was validated and the micro-structure transformation of specimens was given by SEM.

Abstract: Reaches on durability of concrete has been made great advances over the past decades. However, little works have been done on the corrosion of reinforced concrete exposed to boric acid in cooling pond of heating exchange system of nuclear power plant. The purpose of this study is to experimentally investigate the behavior of reinforced concrete in such circumstance. Tests were carried out on specimens cured in a standard moisture room at a temperature of 20±3°C. Three different concentration of boric solution (2000ppm, 8000ppm, 30000ppm) were simulated to accelerate the corrosion. The compressive and splitting tensile strength,elastic modulus of concrete , mass loss rate of steel bar, bond strength and flexural bearing capacity of reinforced concrete at 60, 90, 120, 150 and 180 days were obtained respectively. The results indicate that the presence of boric acid below 30000ppm does not significantly affect serviceability of concrete. The corrosion is slight for the specimens with crack less than 0.4 mm under flexural loads. Boric acid almost has no effect on the corrosion of the steel bar and bond strength.

Abstract: Based on the project requirement of a nuclear power plant, using the method of free shrinkage test and ring cracking test, shrinkage cracking resistance property of grade C35 and C50 self-compacting concrete (SCC) with the optimized mix proportion was studied. The test results between SCC and ordinary vibrated concrete afforded by nuclear power plant project with the same grade were compared. The results indicated that the shrinkage cracking resistance property of SCC is prior to the ordinary vibrated concrete with the same strength grade. This kind of SCC can satisfy the nuclear power plant project, also the test results can be a reference of SCC design theory.

Abstract: As a supporting member of concrete structures, the reliability of the columns is of critical importance. To have models targeted to estimating the residual load-bearing capacity of corroded reinforced concrete (R.C.) columns so that inspection procedures and maintenance interventions can be well conducted, a time-dependent capacity model for corroded R.C. column is presented. The model was based on equilibrium equations on which load-bearing capacity of R.C. column at given time (year) can be predicted considering the overall deteriorations of rebar, concrete section, mechanical properties and bond behavior between corroded reinforcement and concrete. The model was verified by published literature test results of corroded R.C. columns served in atmosphere environment for years. The comparison of the model predictions with the test results shows the validity of the model. In the end, considerations for use of the model were suggested.

Abstract: By method of artificial simulating acid environment to celebrating corrosion, the effects of boric acid on the physical properties and performance of reinforced concrete were investigated. The experimental study of concrete in nature and in various boric solutions (2000ppm, 8000ppm, 30000ppm) was carried out. The compressive strength, splitting tensile strength and elastic modulus of concrete were measured. The weight losses of steel bars and bond strength of reinforced concrete were calculated to analysis the influence of boric acid on reinforced bars. The ingredient change of concrete submerged in boric acid solution was studied with X-ray diffraction analysis. The experiments show that, in general, the mechanical strength of corrupted concrete increases with time. While comparing to the specimen curried in nature environment, the fundamental properties of concrete in boric acid solution decrease slightly. In this research, the influence of boric acid on the corrosion of the steel bars and the interacting between the concrete and the steel bars are insignificant. Three kinds of borides are detected in the X-ray diffraction analysis. It can be concluded from the research that boric acid will not generate apparent influence on the mechanics performance of concrete and steel bars. The same conclusion can be drawn on the bond properties between concrete and steel bars. Generally, corrosiveness of boric acid to reinforced concrete is quite weak.

Abstract: Taking free shrinkage as the key parameter in evaluating cracking of concrete, the cracking properties of self-compacting concrete with strength grade of C35 and C50 were investigated based on ring and slab restraint tests. Meanwhile, the ultimate tensile strain of self-compacting concrete was studied by using flexural test. Experimental results show that the shrinkage of self-compacting concrete is lower than that of normal concrete. Self-compacting concrete cracks later than normal concrete with the same strength grade. Although the ultimate flexural tensile strains of self-compacting concrete in 7 d and 28 d are a little bit lower than that of the normal concrete, it can still be concluded from the research that the cracking property of self-compacting concrete can be greatly improved by using proper mix proportion.

Abstract: In this paper, the development of cubic compressive strength of concrete with the age was studied first. Four kinds of concrete, which were made from the mixes by using 0%, 20%, 30% and 40% Composite Mineral Admixture (CMA) as the replacement of the cement for each kind of concrete, were used in the study. Test results showed that the concrete with 30% CMA had the best properties compared with others both under standard-curing condition and steam-curing condition. Furthermore, the normal concrete (without CMA ) and the high performance concrete (with 30% CMA) were chosen as the main materials in comparative tests to study the effect of curing conditions on the property of high performance concrete, such as the basic mechanical properties, the property against chloride penetration, and the property against carbonization. The results indicated that the curing temperature would increase the concrete strength in early age but not in long term. The curing condition would affect the property of concrete against carbonization but not the property of concrete against chloride penetration obviously.