Nanguo Jin

Modeling of Chloride Ions Diffusion in Cracked Concrete

In the saline environment, chloride ions diffusion depth in concrete is very important for predicting the service life of concrete structures. Moreover, concrete structures are not always crack-free. Cracks with different widths and depths will reduce the cover thickness and accelerate the migration of chloride ions. So, it’s significant to develop a model to predict the chloride diffusion depth in cracked concrete. In this research, the whole chloride diffusion process was divided into two parts: chloride diffusion in sound concrete and in crack. The solution concentration gradient existed in the crack obviously, and it changed the chloride concentration along the crack surface continually. Considering four factors that will influence the chloride diffusion in sound concrete, the diffusion model based on Fick’s second law was modified. And a FEM analysis on chloride diffusion in cracked concrete was performed with the new model in ANSYS. A good agreement can be found in comparison the numerical solution with test results. It proves that the new mathematic model is within a good accuracy in predicting the chloride diffusion depth in cracked concrete.

Chloride penetration in concrete under marine atmospheric environment – analysis of the influencing factors

Abstract Marine atmospheric exposure conditions provide a severe environment for reinforced concrete structures, mainly due to the occurrence of chloride-induced reinforcement corrosion. This procedure was influenced by many parameters related to the concrete properties and to the environmental condition. In this paper, cubic concrete specimens with 150-mm edge, different types of cementitious material and different strengths, were arranged on a structure, which exposed them to a natural marine atmospheric environment. The purpose was to evaluate quantitatively the influence of different exposure conditions on the durability of concrete, measured in terms of chloride penetration into concrete. Both relative humidity (RH) and temperature were monitored in the experiment. The results indicated that the surface RH and temperature of the concrete were much different from that of the air. The diffusion coefficient and surface chloride concentration were time- and location-dependent, and were influenced by the RH, temperature, and the concrete strength. The results also indicate that error results would be made when using constant diffusion coefficient and surface chloride concentration with the air RH and temperature to predict long-term chloride penetration.

Risk Analysis of Tunnel Lining Using Fault Tree Theory and Entropy Measurement

Risk analysis of tunnel lining is a vital and complex problem. This paper presents a metrology for risk analysis of tunnel lining by employing Fault Tree Theory and Entropy Measurement. Some risk factors of tunnel lining are first identified. A lining fault tree analysis model is then established. Next, the entropy and entropy weight are applied to the risk analysis of tunnel lining. Finally, a definition of Lining Risk Entropy is put forward and the computational method and appraisal criterion are discussed.

Study on the electrical properties of young concrete

The process of hydration and solidification of young concrete has significant effect on the long term strength and durability of concrete. The electrical property of concrete provides a direct and practical method for monitoring and investigating the hydration process of young concrete. This study developed an advanced system for measurement of electrical parameters, used to study the electrical properties of young concrete. The test results provided the electric parameters for concretes with different water binder ratios and different mineral admixture incorporations. The variations and characteristics of the measured electrical parameters were closely related to the physical and chemical properties of young concrete. These parameters were used to analyze and study the hydration process of young concrete.

DAMAGE BASED CONSTITUTIVE MODEL FOR PREDICTING THE PERFORMANCE DEGRADATION OF CONCRETE

An anisotropic elastic-damage coupled constitutive model for plain concrete is developed, which describes the concrete performance degradation. The damage variable, related to the surface density of micro-cracks and micro-voids, and represented by a second order tensor, is governed by the principal tension strain components. For adequately describing the partial crack opening/closure effect under tension and compression for concrete, a new suitable thermodynamic potential is proposed to express the state equations for modeling the mechanical behaviors. Within the frame-work of thermodynamic potential, concrete strain mechanisms are identified in the proposed anisotropic damage model while each state variable is physically explained and justified. The strain equivalence hypothesis is used for deriving the constitutive equations, which leads to the development of a decoupled algorithm for effective stress computation and damage evolution. Additionally, a detailed numerical algorithm is described and the simulations are shown for uni-axial compression, tension and multi-axial loadings. For verifying the numerical results, a series of experiments on concrete were carried out. Reasonably good agreement between experimental results and the predicted values was observed. The proposed constitutive model can be used to accurately model the concrete behaviors under uni-axial compression, tension and multi-axial loadings. Additionally, the presented work is expected to be very useful in the nonlinear finite element analysis of large-scale concrete structures.

A New Graphical Quantitative Evaluation Method of Concrete Cracks Based on Digital Image Technology and Fractal Theory

Based on digital image technology, a graphical extraction method of concrete cracks was presented. With the help of fractal theory, there is a characteristic parameter algorithmic method of calculating area, fractal dimension, length, width of concrete cracks and using MTLAB as an application program calculating the characteristic parameters. The cracks’ test analysis in concrete flat slab shows, the graphical extraction method of cracks has advantages of preciseness, convenience and quickness. The method of calculating characteristic parameter has high degree of precision. It works well of describing the irregular characteristic of concrete cracks at early stage with fractal theory which gives useful tools for quantitative study of concrete cracks.

Numerical Experiment on Flexural Properties of Steel Fiber Reinforced Concrete Beam

Based on the laboratory experiment of steel fiber reinforced concrete (SFRC), a total of 10 specimens have been designed according to the aggregate interface condition, steel fiber ratios and different-strength concrete. These specimens were computed by MFPA2D in micro-structure level. Comparing the result of SFRC with plain concrete beams, this paper validated the positive effect of steel fiber in crack-resistant and the flexural strength improvement of SFRC beam, analyzed the sensitivity of steel fiber to concrete with different strength, and provided the stress field and elastic modulus images. The numerical results gave the whole failure process of crack initiation, propagation, and interpenetrated. These results provide useful information toward understanding the flexural behavior of SFRC and have great significance to comprehend the flexural failure process of SFRC beam.